DENTAL  DEPARTMENT 


THE     MICEOSCOPE 


FRONTISPIECE. 
1 


X140 


X138 


THE 


MICROSCOPE 


ITS    REVELATIONS 

BY    THE    LATE 

WILLIAM  B.  CARPENTEK,  C.B.,  M.D.,  LL.D.,  F.R.S. 
EIGHTH     EDITION 

IN   WHICH   THE    FIRST   SEVEN   AND   THE    TWENTY-THIRD    CHAPTERS    HAVE 

BEEN   ENTIRELY    REWRITTEN,    AND   THE    TEXT   THROUGHOUT 

RECONSTRUCTED,    ENLARGED,    AND   REVISED 

BY  THE   REV. 

W.  H.  DALLINGER,  D.Sc.,  D.C.L.,  LL.D.,  F.R.S.,  &c. 


Printed 


WITH  XXII  PLATES  AXD  NEARLY  NINE   HUNDRED  WOOD  EXGRAVIXGS 

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COLLEGE   OF    DENTISTRY 

UNIVERSITY  OF  CALIFORNIA 

PHILADELPHIA 
P.    BLAKISTON'S    SON    &    CO 

1012    WALNUT    STKEET 

"  .1901         •  ;;' 

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PREFACE 


ALTHOUGH  no  changes  of  so  important  a  character  as  those  which 
distinguished  the  Tilth  Edition  of  this  book  from  the  editions 
that  had  preceded  it  have  been  necessitated,  yet  a  thorough  and 
complete  revision  of  the  entire  text  has  been  made,  and  everything 
of  importance  to  Microscopy  which  has  transpired  in  the  interval 
has  been  noted.  This  applies  to  the  theory  of  the  microscope  as 
well  as  to  its  use. 

We  have  adopted  a  classification  of  microscopes  that  we  hope 
may  be  of  value  to  many  in  the  purchase  of  a  stand,  especially  as  we 
also  point  out  with  pleasure  the  great  and  successful  efforts  which 
English,  Continental,  and  American  makers  have  made  within  the 
last  few  years  to  supply  good  and  useful  microscopes  at  a  greatly 
reduced  price. 

Invaluable  aid  and  suggestion  have  been  given  me  by  my  friend 
MR.  E.  M.  NELSON,  ex-President  of  THE  ROYAL  MICROSCOPICAL 
SOCIETY,  to  whom  my  thanks  are  due.  MR.  ARTHUR  BOLLES 
LEE  has  rendered  unique  service  in  the  section  dealing  with  the 
Preparation  and  Mounting  of  Objects  ;  and  to  PROF.  E.  CROOKSHANK 
I  am  indebted  for  valuable  and  useful  help.  In  the  matter  of  the 
Application  of  the  Microscope  to  Geological  Investigation  the 
REV.  PROF.  T.  BONNEY,  F.R.S.,  has  been,  fortunately,  my  valued  co- 
adjutor. On  the  subjects  of  Micro-crystallisation,  Polarisation,  and 
Molecular  Coalescence,  I  have  received  the  expert  advice  and  help 
of  MR.  W.  J.  POPE,  F.I.C.,  F.C.S.,  &c.,  Chemist  to  the  Goldsmiths' 
Technical  Institute,  whose  large  practical  knowledge  of  this  depart- 
ment of  chemistry  is  widely  known. 

For  the  valued  help  of  PROF.  A.  W.  BENNETT,  M.A.,  B.Sc., 
Lecturer  on  Botany  at  St.  Thomas's  Hospital,  -and  of  PROF.  F. 
JEFFREY  BELL,  M.A.,  Professor  of  Comparative  Anatomy  and 
Zoology,  King's  College,  London,  I  have,  as  in  the  former  Edition, 
to  make  my  appreciative  acknowledgments. 

It  is  hoped  that  this  Edition  may,  as  its  predecessors  have  done, 
prove  of  practical  help  to  many  in  understanding  the  scientific  use 
of  the  microscope. 

W.  H.  DALLINGER. 

LONDON  :  MARCH  1901. 


ERRATUM.— Page  333,  eleventh  line  from  the  bottom,  read  'Plate  IV.'  not  III. 


PREFACE 

TO 
THE     SEVENTH    EDITION 


THE  use  of  the  Microscope,  both  as  an  instrument  of  scientific  research 
and  as  a  means  of  affording  pleasure  and  recreative  instruction,  has 
become  so  widespread,  and  the  instrument  is  now  so  frequently  found 
in  an  expensive  form  capable  of  yielding  in  skilled  hands  good 
optical  results,  that  it  is  eminently  desirable  that  a  treatise  should 
be  within  the  reach  of  the  student  and  the  tiro  alike,  which  would 
provide  both  with  the  elements  of  the  theory  and  principles  involved 
in  the  construction  of  the  instrument  itself,  the  nature  of  its  latest 
appliances,  and  the  proper  comditions  on  which  they  can  be  em- 
ployed with  the  best  results.  Beyond  this  it  should  provide  an 
outline  of  the  latest  and  best  modes  of  preparing,  examining,  and 
mounting  objects,  and  glance,  with  this  purpose  in  view,  at  what  is 
easily  accessible  for  the  requirements  of  the  amateur  in  the  entire 
organic  and  inorganic  kingdoms. 

This  need  has  been  for  many  years  met  by  this  book,  and 
its  six  preceding  editions  have  been  an  extremely  gratifying  evidence 
of  the  industry  and  erudition  of  its  Author.  From  the  beginning 
it  opened  the  right  path,  and  afforded  excellent  aid  to  the  earnest 
amateur  and  the  careful  student. 

But  the  Microscope  in  its  very  highest  form  has  become — so  far 
at  least  as  objectives  of  the  most  perfect  construction  and  greatest 
useful  magnifying  power  are  concerned — so  common  that  a  much 
more  accurate  account  of  the  theoretical  basis  of  the  instrument 
itself  and  of  the  optical  apparatus  employed  with  it  to  obtain  the 
best  results  with  *  high  powers '  is  a  want  very  widely  felt. 

The  advances  in  the  mathematical  optics  involved  in  the  con- 
struction of  the  most  perfect  form  of  the  present  Microscope  have 
been  very  rapid  during  the  last  twenty  years  ;  and  the  progress  in 
the  principles  of  practical  construction  and  the  application  of  theory 


PREFACE   TO   THE   SEVENTH   EDITION  vii 

has,  even  since  the  last  edition  of  this  book  was  published,  been  so 
marked  as  to  produce  a  revolution  in  the  instrument  itself  and  in  its 
application.  The  new  dispensation  was  dimly  indicated  in  the  last 
edition  ;  but  it  has  effected  so  radical  a  change  in  all  that  apper- 
tains to  Microscopy  that  a  thorough  revision  of  the  treatment  of 
this  treatise  was  required.  The  great  principles  involved  in  the 
use  of  the  new  objectives  and  the  interpretation  of  the  images  pre- 
sented by  their  means,  are  distinct  and  unique  ;  and  unless  these  be 
clearly  understood  the  intelligent  use  of  the  finest  optical  appliances 
now  produced  by  mathematical  and  practical  optics  cannot  be 
brought  about.  They  have  not  rendered  the  use  of  the  instrument 
more  difficult — they  have  rather  simplified  its  employment,  provided 
the  operator  understand  the  general  nature  and  conditions  on 
which  his  Microscope  should  be  used.  If  the  modern  Microscope  be, 
as  a  mechanical  instrument  with  its  accompanying  optical  apparatus, 
as  good  as  it  can  be,  a  critical  image — a  picture  of  the  object  having 
the  most  delicately  beautiful  character — is  attainable  with  'low 
powers '  and  *  high  powers '  alike.  Microscopists  are  no  longer 
divisible  into  those  who  work  with  '  high  powers '  and  those  who 
work  with  'low  powers.'  No  one  can  work  properly  with  either 
if  he  does  not  understand  the  theory  of  their  construction  and  the 
principles  upon  which  to  interpret  the  results  of  their  employment. 
If  he  is  familiar  with  these  the  employment  of  any  range  of  magni- 
fying power  is  simply  a  question  of  care,  experiment,  and  practice  ; 
the  principles  applicable  to  the  one  are  involved  in  the  other.  Thus, 
for  example,  a  proper  understanding  of  the  nature  and  mode  of 
optical  action  of  a  *  sub-stage  condenser  '  is  as  essential  for  the  very 
finest  results  in  the  use  of  a  1-inch  object-glass  as  in  the  use  of  a 
2  mm.  with  N.A.  1'40  or  the  2'5  mm.  with  N.A.  T60,  while  it 
gives  advantages  not  otherwise  realisable  if  the  right  class  of  con- 
denser used  in  the  right  way  be  employed  with  the  older  ^th  inch 
or  ^th  inch  achromatic  objectives,  and  especially  the  T^th  inch 
and  ^o*h  inch  objectives  of  Powell  and  Lealand,  of  N.A.  1'50. 
Without  comparing  the  value  of  the  respective  lenses,  the  best 
possible  results  in  every  case  will  depend  upon  a  knowledge  of  the 
nature  of  the  instrument,  the  quality  of  the  condenser  required  by 
it,  and  its  employment  upon  right  principles. 

This  is  but  one  instance  out  of  the  whole  range  of  manipulation 
in  Microscopy  to  which  the  same  principles  apply. 

Jn  its  present  form,  therefore,  a  treatise  of  this  sort,  preserving 
the  original  idea  of  its  Author  and  ranging  from  the  theory  and 
construction  of  the  Microscope  and  its  essential  apparatus,  embracing 
a  discussion  of  all  their  principal  forms,  and  the  right  use  of  each,  and 
passing  to  a  consideration  of  the  best  methods  of  preparation  and 


viii  PKEFACE   TO   THE   SEVENTH  EDITION 

mounting  of  objects,  and  a  review  of  the  whole  Animal,  Vegetable, 
and  Inorganic  Kingdoms  specially  suited  for  microscopic  purposes, 
must  be  essentially  a  cyclopaedic  work.  This  was  far  more  possible 
to  one  man  when  Dr.  Carpenter  began  his  work  than  it  was  even 
when  he  issued  his  last  edition.  But  it  is  practically  impossible 
now.  It  is  with  Microscopy  as  with  every  department  of  scientific 
work — we  must  depend  upon  the  specialist  for  accurate  knowledge. 

In  the  following  pages  I  have  been  most  generously  aided.  In 
no  department,  not  even  that  in  which  for  twenty  years  I  have 
been  specially  at  work,  have  I  acted  without  the  cordial  interest, 
suggestion,  and  enlightenment  afforded  by  kindred  or  similar  workers. 
In  every  section  experts  have  given  me  their  unstinted  help. 
To  preserve  the  character  of  the  book,  however,  and  give  it  homo- 
geneity, it  was  essential  that  all  should  pass  through  one  mind  and 
be  so  presented.  My  work  for  many  years  has  familiarised  me, 
more  or  less,  with  every  department  of  Microscopy,  and  with  the 
great  majority  of  branches  to  which  it  is  applied.  I  have  therefore 
given  a  common  form,  for  which  I  take  the  sole  responsibility, 
to  the  entire  treatise.  The  subject  might  have  been  carried  over 
ten  such  volumes  as  this  ;  but  we  were  of  necessity  limited  as 
to  space,  and  the  specific  aim  has  been  to  give  such  a  condensed 
view  of  the  whole  range  of  subjects  as  would  make  this  treatise 
at  once  a  practical  and  a  suggestive  one. 

The  first  five  chapters  of  the  last  edition  are  represented  in  this 
edition  by  seven  chapters  ;  the  whole  matter  of  these  seven  chapters 
has  been  re- written,  and  two  of  them  are  on  subjects  not  treated  in 
any  former  edition.  These  seven  chapters  represent  the  experience 
of  a  lifetime,  confirmed  and  aided  by  the  advice  and  practical  help 
of  some  of  the  most  experienced  men  in  the  world,  and  they  may  be 
read  by  any  one  familiar  with  the  use  of  algebraic  symbols  and  the 
practice  of  the  rule  of  three.  They  are  not  in  any  sense  abstruse, 
and  they  are  everywhere  practical. 

In  the  second  chapter,  on  The  Principles  and  Theory  of  Vision 
with  the  Compound  Microscope,  so  much  has  been  done  during  the 
past  twenty  years  by  Dr.  ABBE,  of  Jena,  that  my  first  desire  was  to 
induce  him  to  summarise,  for  this  treatise,  the  results  of  his  twenty 
years  of  unremitting  and  marvellously  productive  labour.  But  the 
state  of  his  health  and  his  many  obligations  forbade  this ;  and  at 
length  it  became  apparent  that  if  this  most  desirable  end  were  to 
be  secured,  I  must  re-study  with  this  object  all  the  monographs  of 
this  author.  I  summarised  them,  not  without  anxiety  ;  but  that  was 
speedily  removed,  for  Dr.  ABBE,  with  great  generosity,  consented  to 
examine  my  results,  and  has  been  good  enough  to  write  that  he  has 
*  read  [my]  clear  expositions  with  the  greatest  interest ; '  and,  after 


PREFACE   TO   THE   SEVENTH  EDITION  IX 

words  which  show  his  cordial  friendliness,  he  says  :  '  I  find  the  whole 
.  .  .  much  more  adequate  to  the  purposes  of  the  book  than  I  should 
have  been  able  to  write  it.  ...  I  feel  the  greatest  satisfaction  in 
seeing  my  views  represented  in  the  book  so  extensively  and  inten- 
sively.' 

These  words  are  more  than  generous ;  but  I  quote  them  here 
in  order  that  the  reader  may  be  assured  of  the  accuracy  and 
efficiency  of  the  account  given  in  the  following  pages  of  the  invalu- 
able demonstrations,  theories,  and  explanations  presented  by  Dr. 
ABBE  on  the  optical  principles  and  practice  upon  which  the  recent 
improvement  in  the  construction  of  microscopical  lens  systems  IK  is 
so  much  depended. 

It  will  not  be  supposed  that  I  implicitly  coincide  with  every 
detail.  Dr.  ABBE  is  too  sincere  a  lover  of  independent  judgment 
to  even  desire  this.  But  it  was  important  that  his  views  as  such 
should  be  found  in  an  accessible  English  form  ;  in  that  form  I  have 
endeavoured  to  present  them  ;  and  in  the  main  there  can  be  no 
doubt  whatever  that  these  teachings  are  absolutely  incident  with 
fact  and  experience.  In  details,  as  may  appear  here  and  there  in 
these  pages,  especially  where  it  becomes  a  question  of  practice,  I  may 
differ  as  to  method,  and  even  interpretation,  from  this  distinguished 
master  in  Mathematical  Optics.  But  our  differences  in  no  way  affect 
the  great  principles  he  has  enunciated  or  the  comprehensive  theory 
of  microscopical  vision  he  has  with  such  keen  insight  laid  down. 

In  preparing  the  remainder  of  the  seven  new  chapters  of  this 
book  I  have  sought  and,  without  hesitancy,  obtained  advice  and 
the  advantage  of  the  support  of  my  own  judgment  and  experience 
from  many  competent  men  of  science,  who  have  shown  a  sincere 
interest  in  my  work  and  have  aided  me  in  my  endeavours.  But 
first  on  the  list  I  must  place  my  friend  Mr.  E.  M.  NELSON.  Our 
lines  of  experience  with  the  Microscope  have  run  parallel  for  many 
years,  although  the  subjects  of  our  study  have  been  wholly  different ; 
but  the  advantages  of  his  suggestion,  confirmation,  and  help  have 
been  of  constant  and  inestimable  value  to  me.  He  placed  his  know- 
ledge, instruments,  and  experience  at  my  disposal,  fully  and  without 
limit  or  condition  ;  and  his  exceptional  skill  in  Photo-micrography 
has  enabled  me  to  add  much  to  the  value  of  this  book. 

To  Count  CASTRACANE  I  am  indebted  for  valuable  suggestions 
regarding  the  Diatomacese,  to  be  used  at  my  discretion  ;  to  Dr. 
VAN  HEURCK  I  am  also  under  much  obligation  for  his  courtesy  in 
preparing  Plate  XI.  of  this  book,  giving  some  of  his  photo-micro- 
graphic  work  with  the  new  object-glass  of  2'5  mm.  N.A.  1'60. 
The  full  description  of  this  plate  is  given,  with  some  critical  remarks, 
in  the  General  Description  of  Plates.  To  the  late  and  deeply 


X  PKEFACE   TO   THE   SEVENTH   EDITION 

lamented  Dr.  H.  B.  BRADY,  F.R.S.,  I  am  under  obligation  for 
valuable  suggestions  regarding  the  Foraminifera. 

From  Dr.  HUDSON  I  have  received  cordial  aid  in  dealing  with 
his  special  subject,  the  Rotifera ;  and  to  Mr.  ALBERT  MICHAEL  I  am 
under  equal  obligation  for  his  assistance  in  regard  to  the  Acarina. 

Mr.  W.  T.  SUFFOLK  gave  me  his  most  welcome  judgment  and 
advice  regarding  my  chapter  on  Mounting,  and  I  received  also  the 
suggestions  of  Mr.  A.  COLE  with  much  pleasure  and  advantage. 
I  have  received  help  from  Dr.  A.  HILL,  of  Downing  College, 
Cambridge,  and  from  Professor  J.  N.  LANGLEY,  of  Trinity  College, 
Cambridge — from  both  of  whom  special  processes  of  preparation 
for  histological  work  were  sent. 

Mr.  FRANK  CRISP,  with  characteristic  generosity,  aided  me  much 
by  suggestions  of  special  and  practical  value  ;  and  Mr.  JOHN  MAYALL, 
jun.,  the  present  Secretary  of  the  Royal  Microscopical  Society,  has 
been  untiring  in  his  willingness  to  furnish  the  aid  which  his  influence 
was  able  to  secure. 

To  Professor  W.  HICKS,  F.R.S.,  Principal  of  Firth  College, 
Sheffield,  I  am  indebted  for  the  revision  of  special  sheets  ;  so  also  I 
owe  acknowledgments  to  Dr.  HENRY  CLIFTON  SORBY,  F.R.S.,  and  to 
Dr.  GROVES,  as  well  as  to  others,  whose  suggestions,  advice,  or  con- 
firmation of  my  judgments  have  been  much  esteemed  ;  and  prominent 
amongst  these  are  Professor  ALFRED  W.  BENNETT,  B.Sc.,  and  Professor 
F.  JEFFREY  BELL,  M.A.,  whose  constant  advice  in  their  departments 
of  Biology  I  have  received  throughout ;  while  in  Micro-geological 
subjects  I  have  been  aided  by  the  suggestions  and  experience  of 
Professor  J.  SHEARSON  HYLAND,  D.Sc. 

It  will  be  observed  that  every  endeavour  has  been  made  to 
bring  each  of  the  many  subjects  discussed  in  this  book  into  conformity 
with  the  most  recent  knowledge  of  experts.  Many  of  the  sections, 
in  fact,  have  been  wholly  rewritten  and  illustrated  from  new  and 
original  sources  ;  this  may  be  seen  in  the  sections  on  the  History 
as  well  as  the  Construction  and  Use  of  the  Microscope  and  its  appli- 
ances, as  also  in  those  on  Diatomaceae,  Desmids,  Saprophytes, 
Bacteria,  Rotifera,  Acarina,  and  in  the  chapters  on  Microscopic 
Geology  and  Mineralogy.  To  the  same  end  nineteen  new  plates 
have  been  prepared  and  300  additional  woodcuts,  many  of  which 
are  also  new,  and  for  the  use  of  the  majority  of  those  which  are 
not  so,  I  am  indebted  to  the  Editors  and  Secretary  of  the  Royal 
Microscopical  Society. 

There  certainly  never  was  a  time  when  the  Microscope  was  so 
generally  used  as  it  now  is.  With  many,  as  already  stated,  it  is  simply 
an  instrument  employed  for  elegant  and  instructive  relaxation  and 
amusement.  For  this  there  can  be  nothing  but  commendation,  but  it  is 


PREFACE   TO   THE   SEVENTH  EDITION  xi 

desirable  that  even  this  end  should  be  sought  intelligently.  The  social 
influence  of  the  Microscope  as  an  instrument  employed  for  recreation 
and  pleasure  will  be  greater  in  proportion  as  a  knowledge  of  the 
general  principles  on  which  the  instrument  is  constructed  are  known, 
and  as  the  principles  of  visual  interpretation  are  understood.  The 
interests  of  these  have  been  specially  considered  in  the  following 
pages ;  but  such  an  employment  of  the  Microscope,  if  intelligently 
pursued,  often  leads  to  more  or  less  of  steady  endeavour  on  the  part 
of  amateurs  to  understand  the  instrument  and  use  it  to  a  purpose 
in  some  special  work,  however  modest.  This  is  the  reason  of  the 
great  increase  of  '  Clubs '  and  Societies  of  various  kinds,  not  only  in 
London  and  in  the  provinces,  but  throughout  America ;  and  these 
are  doing  most  valuable  work.  Their  value  consists  not  merely  in  the 
constant  accumulation  of  new  details  concerning  minute  vegetable 
and  animal  life,  and  the  minute  details  of  larger  forms,  but  in  the 
constant  improvement  of  the  quality  of  the  entire  Microscope  on  its 
optical  and  mechanical  sides.  It  is  largely  to  Amateur  Microscopy 
that  the  desire  and  motive  for  the  great  improvements  in  object-glasses 
and  eye-pieces  for  the  last  twenty  years  are  due.  The  men  who  have 
compared  the  qualities  of  respective  lenses,  and  have  had  specific  ideas 
as  to  how  these  could  become  possessed  of  still  higher  qualities,  have 
been  comparatively  rarely  those  who  have  employed  the  Microscope 
for  professional  and  educational  purposes.  They  have  the  rather 
simply  used — employed  in  the  execution  of  their  professional  work 
— the  best  with  which  the  practical  optician  could  supply  them. 
It  has  been  by  amateur  microscopists  that  the  opticians  have  been 
incited  to  the  production  of  new  and  improved  objectives.  But  it 
is  the  men  who  work  in  our  biological  and  medical  schools  that 
ultimately  reap  the  immense  advantage — not  only  of  greatly  im- 
proved, but  in  the  end  of  greatly  cheapened,  object-glasses.  It  is 
on  this  account  to  the  advantage  of  all  that  the  amateur  micro- 
scopist  should  have  within  his  reach  a  handbook  dealing  with  the 
principles  of  his  instrument  and  his  subject. 

To  the  medical  student,  and  even  to  the  histologist  and  patho- 
logist, a  treatise  which  deals  specifically  with  the  Microscope,  its 
principles,  and  their  application  in  practice,  cannot  fail,  one  may 
venture  to  hope,  to  be  of  service. 

This  book  is  a  practical  attempt — the  result  of  large  experience 
and  study — to  meet  this  want  in  its  latest  form ;  and  I  sincerely 
desire  that  it  may  prove  useful  to  many. 

W.  H.  DALLINGER. 

LONDON:  1891. 


CONTENTS 


CHAPTEK  PAGE 
I.    ELEMENTARY   PRINCIPLES    OF   MICROSCOPICAL    OPTICS             .            .  1 
II.    THE     PRINCIPLES      AND     THEORY     OF    VISION    WITH      THE     COM- 
POUND  MICROSCOPE 36 

III.  THE   HISTORY   AND   DEVELOPMENT    OF   THE    MICROSCOPE       .            .  117 

IV.  ACCESSORY   APPARATUS        .            .            , 270 

V.   OBJECTIVES,    EYE-PIECES,    THE    APERTOMETER     .            .            .  353 

vi.  PRACTICAL    MICROSCOPY:    MANIPULATION   AND   PRESERVATION 

OF   THE    MICROSCOPE 397 

VII.    PREPARATION,    MOUNTING,    AND    COLLECTION    OF    OBJECTS  .            .  438 

VIII.   MICROSCOPIC    FORMS    OF   VEGETABLE    LIFE — THALLOPHYTES           .  530 

IX.    FUNGI 633 

X.    MICROSCOPIC    STRUCTURE    OF   THE    HIGHER   CRYPTOGAMS                 .  665 

XI.    OF   THE    MICROSCOPIC    STRUCTURE    OF    PHANEROGAMIC     PLANTS  .  684 

XII.    MICROSCOPICAL   FORMS    OF   ANIMAL   LIFE — PROTOZOA             .            .  726 

XIII.   ANIMALCULES — INFUSORIA   AND    ROTIFERA             ....  753 

XIV.    FORAMINIFERA   AND    RADIOLARIA          ......  795 

XV.    SPONGES   AND    ZOOPHYTES 855 

XVI.    ECHINODERMA 884 

XVII.    POLYZOA   AND    TUNICATA 904 

XVIII.    MOLLUSCA   AND    BHACHIOPODA 919 

XIX.   WORMS .            .            .  943 

XX.    CRUSTACEA 957 

XXI.   INSECTS   AND   ARACHNIDA 972 

XXII.   VERTEBRATED    ANIMALS 1017 

XXIII.    APPLICATION    OF   THE    MICROSCOPE    TO    GEOLOGICAL    INVESTIGA- 
TION       1066 

XXIV.    CRYSTALLISATION,    POLARISATION,    MOLECULAR   COALESCENCE       .  1094 

INDEX .  1137 


EXPLANATION    OF   PLATES 


FKtfNTISPJECE 

Fig.  1.    x  6  diameters.     Horizontal  and  transverse  section  of  an  orbitolite. 

Fig.  2.  An  imperfect  or  uncritical  image  of  the  minute  hairs  on  the  lining 
membrane  of  the  extremity  of  the  proboscis  of  the  blow-fly  x  510  diams.,  taken 
with  a  Zeiss  apochromatic  ^-inch  objective  of  '95  N.A.  x  3  projection  eye-piece ; 
but  it  was  illuminated  by  a  cone  of  small  angle,  viz.  of  0-1  N.A.,  and  illustrates 
the  unadvisability  of  small  cones  for  illumination. 

The  first  obvious  feature  in  the  picture  is  the  doubling  of  the  hairs  which 
are  out  of  focus ;  but  the  important  difference  lies  in  the  bright  line  with  a 
dark  edge  round  the  hairs  which  are  precisely  in  focus.  This  is  a  diffraction 
effect  which  is  always  present  round  the  outlines  of  every  object  illuminated 
by  a  cone  of  insufficient  angle.  Experiment  shows  that  this  diffraction  line 
always  ceases  to  be  visible  when  the  aperture  of  the  illuminating  cone  is  equal 
to  about  two-thirds  the  aperture  of  the  objective  used :  but  it  will  become 
again  distinctly  apparent  when  the  aperture  of  the  cone  is  reduced  less  than 
half  that  of  the  objective. 

Fig.  3.  x  510  diams.  A  correct  or  critical  image  of  the  minute  hairs  on  the 
lining  membrane  of  the  extremity  of  the  blow-fly's  proboscis.  In  this  picture 
the  focus  has  been  adjusted  for  the  long  central  hair.  It  will  be  observed  that 
this  hair  is  very  fine  and  spinous  ;  it  has  not  the  ring  socket  which  is  common 
to  many  hairs  on  insects,  but  grows  from  a  very  delicate  membrane,  which  in 
the  balsam  mount  is  transparent.  This  photograph  was  taken  with  a  Zeiss 
apochromatic  £  of  -95  N.A.  x  3  projection  eye-piece.  The  illumination  was 
that  of  a  large  solid  axial  cone  of  '65  N.A.  from  an  achromatic  condenser,  the 
source  of  light  being  focussed  on  the  object. 

Fig.  4.  Section  of  cerebellum  of  a  lamb,  x  77  diams.,  by  apochromatic  1-inch 
•3  N.A.  This  preparation  was  courteously  supplied  to  the  present  Editor  by  Dr. 
Hill,  whose  imbedding  and  staining  processes  for  these  tissues  it  beautifully 
illustrates. 

Fig.  5.  Amphipleura  pellucida  x  1860  diams.,  by  apochromatic  ^  1'4  N.A. 
illuminated  by  a  very  oblique  pencil  in  one  azimuth  along  the  valve. 

Fig.  6.  A  hair  of  Polyxenus  lagurus,  a  well-known  and  excellent  test  object 
for  medium  powers  x  490  diams.  by  apochromatic  %  '95  N.A. 

Fig.  7.  A  small  vessel  in  the  bladder  of  a  frog,  prepared  with  nitrate  of 
silver  stain,  showing  endothelium  cells,  x  40  diams.,  by  Zeiss  A.  -2  N.A.  This 
object  has  been  photographed  for  the  purpose  of  exposing  the  fallacy  which 
underlies  the  generally  accepted  statement  that  '  low-angled '  glasses  are  the 
most  suitable  for  histological  purposes.  The  supposition  that  it  is  so  has 
been  founded  on  the  fact  that  the  penetration  of  a  lens  varies  inversely  as  its 
aperture ;  therefore,  it  is  said,  a  '  low-angled '  glass  is  to  be  preferred  to  a 
wide-angled  one,  because  '  depth  of  focus,'  which  is  supposed  to  enable  one  to 
see  into  tissues,  is  the  end  in  view. 

On  carefully  examining  this  figure  it  will  be  noticed  that  it  is  almost 
impossible  to  trace  the  outline  of  any  particular  endothelium-cell  because  its 
image  is  confused  with  that  of  the  lower  side  of  the  pipe.  In  a  monocular 
microscopical  image  a  perspective  view  does  not  exist ;  it  is  better,  therefore,  to 
use  a  wide-angled  lens,  and  so  obtain  a  clear  view  of  a  thin  plane  at  one  time, 
and  educate  the  mind  to  appreciate  solidity  by  means  of  focal  adjustment.  It 
will  be  admitted  that  unless  one  approaches  fig.  7  with  a  preconceived  idea  of 


xiv  EXPLANATION   OF  PLATES 

what   an   endothelium-cell   is  like,  the  knowledge  gained  of  it  will  be  small 
indeed. 

Fig.  8  represents  the  same  structure,  x  138  diams.,  by  an  apochrornatic 
i  -65  N.A.  Here  only  the  upper  surface  of  the  pipe  is  seen,  so  that  the  out- 
line of  the  endothelium-cells  can  be  clearly  traced.  The  circular  elastic  tissue 
is  also  displayed.  There  is,  moreover,  an  increased  sharpness  over  the  whole 
picture,  due  to  the  greater  aperture  of  the  objective. 

PLATE    I 

Fig.  1.  The  inside  of  a  valve  of  Pleurosigma  angulatum,  showing  a 
'  postage-stamp  '  fracture,  x  1750  diams.,  with  an  apochromatic  ^  1'4  N.A.  by 
Mr.  T.  F.  Smith,  and  illustrating  his  view  of  the  nature  of  the  Pleurosigma 
valve. 

Fig.  2.  The  outside  of  a  valve  of  Pleurosigma  angulatum,  showing  a  dif- 
ferent form  of  structure,  x  1750  diams.,  with  an  apochromatic  ^  1-4  N.A.  by 
Mr.  T.  F.  Smith.  These  two  photo-micrographs  demonstrate  the  existence  of  at 
least  two  layers  in  the  angulatum. 

Fig.  3.  Coscinodiscus  asteromphalus,  x  110  diams.,  with  an  apochromatic 
1-inch  -3  N.A. 

Fig.  4.  A  portion  of  the  preceding,  x  2000  diams.,  to  show  the  lacework 
inside  the  areolations.  This  lacework  is  believed  to  be  a  perforated  structure, 
as  a  fracture  passes  through  the  markings.  In  the  central  areolation  there 
are  forty-six  smaller  perforations  surrounded  by  a  crown  of  fifteen  larger  ones.1 
Photographed  with  an  apochromatic  £  1-4  N.A. 

Fig.  5.  Aulacodiscus  Kittonii,  x  270,  by  an  apochromatic  1-inch  -3  N.A. 

Fig.  6.  A  small  portion  in  the  centre  of  an  Aulacodiscus  Sturtii,  x  2000, 
by  an  apochromatic  ^  T4  N.A.  Broadly  speaking,  the  difference  between  the 
Coscinodisci  and  the  Aulacodisci  lies  in  the  fact  that  in  the  former  the 
secondary  structure  is  inside  the  primary,  while  in  the  latter  it  is  exterior  to  it. 
This  definition,  however,  is  not  strictly  accurate,  as  it  is  believed  that  the  fine 
perforated  structure  covers  the  entire  valve,  it  being  only  optically  hidden  by 
the  primary  structure. 

The  whole  of  these  demonstrations  were  photographed  for  the  present 
Editor  by  his  friend  E.  M.  Nelson,  Esq.,  and  have  been  reproduced  from  the 
negatives  by  a  process  of  photo- printing. 

PLATE   II.     (Facing  p.  274) 

ARRANGEMENT    OF    THE    MICROSCOPE    WITH    A    STAND   FOR    THE    MICROMETER 
EYE-PIECE,    TO    SECURE    STEADINESS    AND    ACCURACY    IN  MEASUREMENT 

PLATE   III.     (Facing  p.  286) 

ARRANGEMENT    OF     THE    MICROSCOPE     AND    ACCESSORIES    FOR    THE     EMPLOY- 
MENT   OF    THE    CAMERA   LUCIDA 

PLATE   IV.     (Facing  p.  334) 

THE    METHOD    OF    USING    THE    SILVER    SIDE    REFLECTOR    OR  PARABOLOID 

PLATE   V.     (Facing  p.  410) 

METHOD    OF   USING    DIRECT    TRANSMITTED    LIGHT    WITHOUT    THE 
EMPLOYMENT    OF    THE    MIRROR 

PLATES  II.  to  V.  are  engraved  from  photographs,  taken  at  the  request  of 
the  Editor  by  Mr.  E.  M.  Nelson,  from  the  arranged  instruments. 

1  A  section  of  this  diatom  will  be  found  in  the  Transactions  of  the  County  of 
Middlesex  Natural  History  Society  for  1889,  Plate  I.  fig.  2. 


EXPLANATION  OF  PLATES  XV 

PLATE    VI.     (Facing  p.  550) 

SEXUAL    GENERATION    OF   VOLVOX    GLOBATOR.       (After    Cohn) 

Fig.  1.  Sphere  of  Volvox  globator  at  the  epoch  of  sexual  generation  :  a, 
sperm-cell  containing  cluster  of  antherozoids ;  a2,  sperm-cell  showing  side- 
view  of  discoidal  cluster  of  antherozoids ;  a3,  sperm  cell  whose  cluster  has 
broken  up  into  its  component  antherozoids ;  a4,  sperm-cell  partly  emptied  by 
the  escape  of  its  antherozoids;  66,  flask-shaped  germ-cells  showing  great 
increase  in  size  without  subdivision ;  62,  6'2,  germ-cells  with  large  vacuoles  in 
their  interior ;  63,  germ-cell  whose  sh^,pe  has  changed  to  the  globular. 

Fig.  2.  Sexual  cell,  a,  distinguishable  from  sterile  cells,  6,  by  its  larger 
size. 

Fig.  3.  Germ-cell,  with  antheroids  swarming  over  its  endochrome. 

Fig.  4.  Fertilised  germ-cell,  or  oosphere,  with  dense  envelope. 

Fig.  5.  Sperm-cell,  with  its  contained  cluster  of  antherozoids,  more 
enlarged. 

Figs.  6,  7.  Liberated  antherozoids,  with  their  flagella. 

PLATE   VII.     (Facing  p.  553) 

OSCILLARIACE^E    AND    SCYTONEMACE^ 

Fig.  1.  Lyngbya  fcstuarii,  Lieb.   x   160. 

Fig.  2.  Spirulina  Jenneri,  Ktz.    x  400. 

Fig.  3.  Tolypoilirix  cirrhosa,  Carm.    x  400. 

Fig.  4.  Oscillaria  insignis,  Thw.    x  400. 

Fig.  5.  O.  Frolichii,  Ktz.    x  400. 

Fig.  6.  O.  tenerrima,  Ktz.    x  400. 
These  figures  are  after  Cooke. 

PLATE    VIII.     (Facing  p.  554) 

DESMIDIACEJE,    RIVULARIACEJS,   AND    SCYTONEMACE^ 

Fig.  1.  Zygosperm  of  Micrasterias  denticulata,  Breb.     (After  Ealfs.) 

Fig.  2.  Cosmarium  Brebissonii,  Men.     (After  Cooke.) 

Fig.  3.  Euastrum  pectinatum,  Breb.     (After  Kalfs.) 

Fig.  4.  Zygosperm  of  Staurastrum  hirsutum,  Breb.     (After  Ealfs.) 

Fig.  5.  S.  gracile,  Ealfs.     (After  Cooke.) 

Fig.  6.  Xanthidium  aculeatum,  Ehrb.     (After  Ealfs.) 

Fig.  7.  Rivularia  dura,  Ktz.     (After  Cooke.) 

Fig.  8.  B.  diira,  Ktz.    x  400.     (After  Cooke.) 

Fig.  9.  Seytonema  natans,  Breb.    x  400.     (After  Cooke.) 

Fig.  10.  Stanrastrum  hirsutum,  Breb.     (After  Cooke.) 

PLATE   IX.     (Facing  p.  580) 

DESMIDIACE^ 

Fig.  1.  Micrasterias  crux-melitensis,  Ehrb.     (After  Cooke.) 

Fig.  2.  Closterium  setaceum,  Ehrb.     (After  Cooke.) 

Fig.  3.  Desmidium  Swartzii,  Ag.     (After  Cooke.) 

Fig.  4.  Penium  digitus,  Ehrb.     (After  Cooke.) 

Fig.  5.  P.  digitus,  Ehrb.  (transverse  view). 

Fig.  6.  Spirotcenia  condensata,  Breb.     (After  Cooke.) 

Fig.  7.  Docidium  baculum,  Breb.     (After  Cooke.) 

Fig.  8.  Gonatozygon  Brebissonii,  De  Bary,  conjugating.     (After  Cooke.) 

PLATE   X.     (Facing  p.  593) 

PLEUROSIGMA   ANGULATUM 

This  is  a  direct  photo-micrograph,  taken  by  Dr.  E.  Zeiss,  as  magnified  4900 
diameters.  We  direct  attention  specially  to  it  as  giving  evidence  of  the  pre- 
sence (however  originated)  of  the  intercostal  markings,  which  may  be  seen 
with  considerable  clearness  on  the  right-hand  side  of  the  midrib  and  in  the 
middle  of  the  valve. 


xvi  EXPLANATION   OF   PLATES 


PLATE   XI.     (Facing  p.  594) 

This  plate  has  a  twofold  purpose.  It  is  designed,  first,  to  justify  the 
opinions  held  by  Dr.  Henry  varn  Heurck  upon  the  structure  of  the  valves  of 
diatoms,  and  also  to  show  how  the  usual  microscopical  tests  present  them- 
selves when  examined  with  the  new  objective  with  N.A.  1-60,  lately  constructed 
by  the  firm  of  Zeiss.  This  objective  is  believed  by  Dr.  van  Heurck  to  realise 
what  he  considers  the  highest  results  of  photographic  optics,  which  in  his 
judgment  could  only  be  surpassed  by  finding  a  new  immersion  liquid  of  still 
higher  refractive  index  presenting  all  the  necessary  qualities,  and  which  at  the 
same  time  would  not  affect  the  very  delicate  flint  of  which  it  is  necessary  to 
make  the  front  lens  of  this  objective.  This  medium  he  hopes  may  be  some  day 
realised.  Unfortunately,  up  to  this  time,  no  indication  permits  us  to  foresee 
the  discovery  of  the  liquid  desired. 

The  following  is  the  way  in  which  Dr.  Henry  van  Heurck  summarises  his 
ideas  upon  the  structure  of  the  valve : — 

1.  The  valve  of  diatoms  '  is  formed  by  two  membranes  or  thin  plates  and 
by  an  intermediate  septum.     By  this  he  understands  a  plate  pierced  with 
openings.      The  superior  membrane,  often  very  delicate,   may   be   destroyed 
in  the  treatment  by  acids  in  the  washings,  by  rubbing,  &c.     It  is  possible  also 
that  it  sometimes  only  exists  in  a  very  rudimentary  state.     The  majority  of 
the  students  of  diatoms  agree  in  believing  that  these  membranes  may  be  suf- 
ficiently permeable  to  permit  of  exchange  by  endosmose  between  the  contents 
of  the  valve  and  the  surrounding  outer  water,  but  that  these  membranes  have  no 
real  openings  so  long  as  the  diatom  is  living  and  intact. 

2.  When  the  openings  of  the  septum  are  disposed  in  alternate  rows,  then 
they  take  an  hexagonal  form.     When  in  perpendicular  rows  then  the  openings 
are  square  or  elongated.     The  hexagonal  form,  which  is  besides  so  frequent  in 
nature,  seems  to  be  the  typical  form  of  the  openings  of  the  septum,  and  it 
is  found  most  frequently  when  the  valve  is  large,  destitute  of  consolidated 
sides,  and  must  offer  resistance  to  outside  agents.     Even  in  the  forms  of  the 
square  openings  we  see  very  frequently  deviations  and  returns  to  the  hexagonal 
type  upon  certain  parts  of  the  valve.     It  is  possible  that  the  septa  may  be 
sometimes  composed  of  many  layers,  placed  one  above  another,  formed  succes- 
sively and  closely  united  ;  but  up  to  this  time  we  have  no  proof  of  it,  neither 
have  we  met  with  any  form  presenting  layers  placed  one  above  another. 

Such,  in  brief,  is  the  view  held  by  Dr.  van  Heurck  as  an  interpretation  of 
our  present  knowledge  of  the  structure  of  the  valve  of  the  diatoms.  We  give 
now  a  description  of  the  objects  represented  on  the  plate. 

Figs.  1,  2,  3.  Amphipleura  pellucida,  Kiitz,  1  and  2,  valve  resolved  into 
pearls.  Fig.  2  x  2000  diams.  Fig.  1  x  3000  diams.  Fig.  3.  Valve  resolved 
in  strice  at  about  2300  diams. 

Fig.  4.  Amphipleura  Lindheimeri,  Gr.,  x  2500  diams. 

Fig.  5.  Pleurosigma  angulatum,  in  hexagons,  x  (about)  10,000  diams. 

Fig.  6.  Idem  x  2000  diams.,  illusory  pearls  which  are  formed  by  the  angles 
of  the  hexagonal  cells  when  the  focussing  is  not  perfect. 

Fig.  7.  The  nineteenth  band  of  Nobert's  test  plate.  This  photo-micro- 
graph has  been  made  exceptionally  with  the  apochromatic  i  Of  1-4  N.A. 
The  lines,  being  traced  upon  a  cover  in  crown-glass,  the  objective  of  N.A.  l-f> 
cannot  be  used  here. 

Fig.  8.  Surirella  gemma,  Ehrb.  x  (about)  1000  diams. 

Fig.  9.  Van  Heurckia  crassinervis,  Breb.  (Frustulia  saxonica,  Eabh)  x  2000 
diams. 

All  the  photo-micrographs  (except  fig.  7)  have  been  done  with  the  new  J_- 
inch  N.A.  1-60  of  MM.  Zeiss. 

These  micro-photographs  have  been  produced  by  sunlight  in  a  monochro- 
matic form,  the  special  compensating  eye-piece  12,  and  the  Abbe  condenser  of 
N.A.  1*6. 

1  '  The  Structure  of  the  Valve  of  Diatoms  '  in  Records  of  the  Belctian  Society, 
v.  xiii.  1890. 


EXPLANATION   OF    PLATES  xvii 

Covers  and  slides  in  flint  of  1-72 ;  diatoms  in  a  medium  2-4. 

We  are  bound,  however,  to  note  that  the  condenser  used  is  not  corrected  in 
any  way  ;  its  aberrations  are  enormous.  Although  the  highest  admiration  must 
be  expressed  for  the  skill  exercised  by  Dr.  van  Heurck  in  these  remarkable 
photo-micrographs,  and  the  highest  esteem  for  his  courtesy  to  the  present  Editor 
in  supplying  them,  it  must  not  be  forgotten  that  Dr.  van  Heurck  was  obliged 
to  employ  an  imperfect  condenser — a  condenser  absolutely  unconnected— and 
although  we  can  testify  to  the  high  quality  and  fine  corrections  of  at  least  one 
of  the  lenses  of  N.A.  1-6,  we  are  convinced  that  much  of  its  real  perfection 
in  image-forming  is  destroyed  by  uncorrected  sub-stage  illumination.  Upon 
the  corrections  and  large  aplanatic  a'rea  presented  by  the  condenser  and  its 
careful  and  efficient  employment  depends  entirely  the  nature  of  the  image 
presented  by  the  finest  objective  ever  constructed ;  and  as  the  perfection  of  the 
objective,  with  a  high  amplification  and  a  great  aperture,  is  more  nearly 
approached,  the  more  dependent  are  we  upon  perfect  corrections  in  the  con- 
denser to  bring  out  the  perfect  image-forming  power  of  the  objective.  No 
image  formed  by  such  an  objective  as  that  possessing  N.A.  1-60  can  be  consi- 
dered reliable  until  a  condenser  corrected  for  all  aberrations  like  the  objective 
itself  is  produced  ;  and  so  convinced  are  we  of  the  possible  value  of  this  objec- 
tive that  we  trust  its  distinguished  deviser  and  maker  may  be  soon  induced  to 
produce  the  condenser  referred  to. 

If,  then,  by  the  aid  of  the  chemist  we  can  discover  media  which  will  be 
of  sufficiently  high  refractive  index,  and  still  tolerant  or  non-injurious  to 
organic  tissues  immersed  in  it,  a  new  line  of  investigation  may  be  open  to 
histology  and  pathology. — W.  H.  D. 


PLATE   XII.     (Facing  p.  597) 
ARACHNOIDISCUS  jAPONicus.     (After  B.  Beck) 

The  specimens  attached  to  the  surface  of  a  seaweed  are  represented  as 
seen  under  a  £th  objective,  with  Lieberkiihn  illumination :  A,  internal 
surface ;  B,  external  surface  ;  C,  front  view,  showing  incipient  subdivision. 

PLATE   XIII.     (Facing  p.  651) 

BACTERIA,    SCHIZOMYCETES,    OR   FISSION    FUNGI 

1.  Cocci  singly  and  varying  in  size.  2.  Cocci  in  chains  or  rosaries  (strepto- 
coccus). 3.  Cocci  in  a  mass  (staphylococcus).  4  and  5.  Cocci  in  pairs 
(diplococcus).  6.  Cocci  in  groups  of  four  (merismopedia).  7.  Cocci  in  packets 
(sarcina).  8.  Bacterium  termo.  9.  Bacterium  termo  x  4000  (Ballinger  and 
Drysdale).  10.  Bacterium  septicamia  hamorrhagica.  11.  Bacterium  pneu- 
monia crouposa.  12.  Bacillus  subtilis.  13.  Bacillus  murisepticus.  14. 
Bacillus  diphtheria.  15.  Bacillus  typhcsus  (Eberth).  16.  Spirillum  undula 
(Cohn).  17.  Spirillum  volutans  (Cohn).  18.  Spirillum  cholera  Asiatics. 
19.  Spirillum  Obermeieri  (Koch).  20.  Spirochceta  plicatilis  (Fliigge).  21. 
Vibrio  rugula  (Prazmowski).  22.  Cladothrix  Forsteri  (Cohn).  23.  Cladothrix 
dichotoma  (Cohn).  24.  Monas  Okenii  (Cohn).  25.  Nonas  Warmingii  (Cohn). 
26.  Ehabdomonas  rosea  (Cohn).  27.  Spore-formation  (Bacillus  alvei).  28. 
Spore-formation  (Bacillus  anthracis}.  29.  Spore-formation  in  bacilli  cultivated 
from  a  rotten  melon  (Frankel  and  Pfeiffer).  30.  Spore-formation  in  bacilli 
cultivated  from  earth  (Frankel  and  Pfeiffer).  31.  Involution-form  of  Crenothnx 
(Zopf).  32.  Involution-forms  of  Vibrio  serpens  (Warming).  33.  Involution- 
forms  of  Vibrio  rugula  (Warming).  34.  Involution-forms  of  Clostridium 
polymyxa  (after  Prazmowski).  35.  Involution-forms  of  Spirillum  cholera 
Asiatica.  36.  Involution-forms  of  Bacterium  aceti  (Zopf  and  Hansen). 
37.  Spirulina-form  of  Beggiatoa  alba  (Zopf).  3j3.  Various  thread-forms  of 
Bacterium  mcrismopedioides  (Zopf).  39.  False-branching  of  Cladothrix  (Zopf). 


xviii  EXPLANATION   OF  PLATES 

PLATE   XIV.     (Facing  p.  664) 

PURE-CULTIVATIONS    OF   BACTERIA 

Fig.  1.  In  the  depth  of  Nutrient  Gelatine.  A  pure-cultivation  of  Koch's 
comma-bacillus  (Spirillum  cholera  Asiatics)  showing  in  the  track  of  the 
needle  a  funnel-shaped  area  of  liquefaction  enclosing  an  air-bubble,  and  a 
white  thread.  Similar  appearances  are  produced  in  cultivations  of  the  comma- 
bacillus  of  Metchnikoff. 

Fig.  2.  On  the  surface  of  Nutrient  Gelatine.  A  pure-cultivation  of  Bacillus 
typhosus  on  the  surface  of  obliquely  solidified  nutrient  gelatine. 

Fig.  3.  On  the  surface  of  Nutrient  Agar-agar.  Pure-cultivation  of  Bacillus 
indicus  on  the  surface  of  obliquely  solidified  nutrient  agar-agar.  The  growth 
has  the  colour  of  red  sealing-wax,  and  a  peculiar  crinkled  appearance.  After 
some  days  it  loses  its  bright  colour  and  becomes  purplish,  like  an  old  cultiva- 
tion of  Micrococcus  prodigiosus. 

Fig.  4.  On  the  surface  of  Nutrient  Agar-agar.  A  pure -cultivation  obtained 
from  an  abscess  (Staphylococcus  pyogenes  aureus). 

Fig.  5.  On  the  surface  of  Nutrient  Agar-agar.  A  pure-cultivation  obtained 
from  green  pus  (Bacillus  pyocyaneus).  The  growth  forms  a  whitish,  transparent 
layer,  composed  of  slender  bacilli,  and  the  green  pigment  is  diffused  throughout 
the  nutrient  jelly.  The  growth  appears  green  by  transmitted  light,  owing  to 
the  colour  of  the  jelly  behind  it. 

Fig.  6.  On  the  surface  of  Potato.  A  pure-cultivation  of  the  bacillus  of 
glanders  on  the  surface  of  sterilised  potato. 

PLATE  XV.     (Facing  p.  756) 

COMPLETE  LIFE-HISTORIES  OF  TWO  SAPROPHYTES 

(Drawn  from  nature  by  Dr.  Dallinger) 

PLATE  XVI.     (Facing  p.  763) 

The  various  stages  of  the  development  of  the  nucleus  in  two  saprophytic 
organisms,  as  studied  with  recent  homogeneous  and  apochromatic  objectives 
both  in  the  several  stages  of  fission  and  genetic  fusion,  indicating  karyoki- 
nesis,  and  proving,  as  established  in  detail  by  the  text,  that  all  the  steps  in 
the  cyclic  changes  of  these  unicellular  forms  are  initiated  in  the  nucleus  before 
being  participated  in  by  the  whole  body  of  the  organism.  (Drawn  from  nature 
by  Dr.  Dallinger.) 

PLATE  XVII.     (Facing  p.  792) 

ROTIFERS 

Fig.  1.  Floscularia  campanulata. 
Fig.  2.  Steplwnoceros  EichJiornii 
Fig.  3.  Melicerta  ringens. 
Fig.  4.  Pedalion  mirum  (side  view). 
Fig.  5.  P.  mirum  (dorsal  view,  showing  muscles). 
Fig.  6.  Copeus  cerbems  (side  view). 

Fig.  7.  Philodina  aculeata  (side  view,  corona  expanded). 
Fig.  8.  Male  of  Pedalion  mirum. 

All  these  figures,  save  fig.  2,  are  reduced  to  scale  from  the  beautiful  plates 
in  Hudvson  and  Goss's  Rotifera. 

PLATE    XVIII.     (Facing  p.  797) 

FORAMINIFERA 

Fig.  1.  Miliolina  seminulum  (a  and  b,  lateral  aspects). 

Fig.  2.  Alveolina  Boscii  (a,  lateral  aspect ;  6,  longitudinal  section). 


EXPLANATION   OF   PLATES  xix 

Fig.  3.  Astrorhiza  limicola  (a,  lateral  aspect ;  b,  portion  of  the  test  more 
highly  magnified,  showing  structure). 

Fig.  4.  Haliphysema  Tumanoiviczii,  showing  the  pseudo-polythalamous  foot. 

Fig.  5.  Ibid,  (group  of  specimens  in  situ). 

Fig.  6.  Haplopliragmium  agglutinans  (a,  lateral  aspect;  b,  longitudinal 
section). 

Fig.  7.  H.  nanum  (a,  superior  aspect ;  6,  peripheral  aspect). 

Fig.  8.  Tcxtularia  gramen  (a,  lateral  aspect ;  6,  oral  aspect). 

Fig.  9.  T.  gramen  (peripheral  aspect). 

Fig.  9a.  Pavonina  flabelliformis  (a,  lateral  aspect ;  6,  oral  aspect). 

Fig.  10.  Bulminia  spinulosa. 

Fig.  11.  Chilostoniella  ovoidea  (a  and  b,  lateral  aspects ;  c,  specimen 
mounted  in  Canada  balsam  and  seen  with  transmitted  light). 

PLATE  XIX.     (Facing  p.  799) 

FORAMINIFE  RA 

Fig.  12.  Lagena  sulcata. 
Fig.  13.  L.  sulcata. 
Fig.  14.  L.  sulcata. 

Fig.  15.  L.  sulcata  (a,  lateral  aspect;  b,  oral  aspect). 
Fig.  16.  Nodosaria  raplianus. 
Fig.  17.  Cnstellaria  calcar  (a,  b,  c,  lateral  aspects). 
Fig.  18.  Ramulina  globulifera. 
Fig.  19.  R.  globulifera. 

Fig.  20.  Globigerina  bulloides  (var.  triloba,  pelagic  specimen). 
Fig.  21.  G.  bulloides  (a  6,  c,  adult  typical  shell). 
Fig.  22.  Eotalia  Beccarii. 
Fig.  23.  Polystomella  craticulata. 

Fig.  24.  Amphistcgina  Lessonii  (a,  superior  lateral  aspect ;  6,  inferior  lateral 
aspect ;  c,  peripheral  aspect). 

Fig.  25.  Nunimulites  Icvvigata  (b,  lateral  aspect ;  c,  vertical  section). 
Fig.  26.  Portion  of  Orbitoides  nummulitica. 


PLATES   XX,   XXI,   XXII 

ACARINA 

All  the  figures,  except  fig.  4,  Plate  XXII.,  are  copied  from  plates  drawn  by 
Mr.  A.  D.  Michael,  F.L.S.,  &c.  by  the  kind  permission  of  the  respective 
societies  that  published  them.  Figs.  1  to  6,  Plate  XX.,  and  1  to  3,  Plate 
XXI.,  are  from  '  British  Oribatidee,'  published  by  the  Bay  Society ;  fig.  7,  Plate 
XX.,  from  the  '  Journal  of  the  Linnean  Society  ;  '  fig.  4,  Plate  XXI.,  and  fig.  3, 
Plate  XXII.,  from  the  '  Journal  of  the  Koyal  Microscopical  Society  ; '  fig.  5,  Plate 
XXL,  and  figs.  1  and  2,  Plate  XXII.,  from  the  '  Journal  of  the  Quekett  Micro- 
scopical Club.'  Fig.  4,  Plate  XXII.,  is  drawn  after  Fiirstenberg  by  the  Editor. 


PLATE     XX.     (Facing  p.  1008) 

ORIBATID^E 

Fig.  1.  Anatomy  of  Nothrus  theleproctus  (male,  dorsal  aspect,  x  about  60). 
The  dorsal  portion  of  the  chitinous  exo-skeleton,  and  the  fat  and  muscles 
which  underlie  it,  have  been  removed  from  the  abdomen.  The  internal  organs 
are  shown  protruding,  as  they  usually  do  when  the  creature  is  opened,  as 
though  they  were  too  large  to  be  contained  in  the  ventral  exo-skeleton.  Part 
of  the  oesophagus  is  seen  at  the  top  (the  brain  having  been  removed).  The 
preventricular  glands  (brown)  lie  on  each  side  of  the  oesophagus.  The  ventri- 
culus  is  coloured  pink  ;  part  of  it  and  the  whole  of  the  ceeca  are  covered  with 


XX  EXPLANATION   OF   PLATES 

botryoidal  tissue  (yellow).  The  testes  (white  shaded  with  blue)  show  at  the 
sides  protruding  from  beneath  the  alimentary  canal. 

Fig.  2.  Hoplophora  magna  (female,  lateral  aspect,  x  about  50).  The  chitin 
at  the  side  and  the  fatty  tissue  and  muscles  have  been  removed.  Alimentary 
canal  pink  ;  caeca  of  the  ventriculus  spotted ;  preventricular  glands  brown  ; 
supercoxal  gland  white ;  its  vesicles  yellow ;  expulsory  vesicle,  between 
supercoxal  and  ovaries,  grey  ;  ovary  and  oviducts  white  shaded  with  blue  and 
yellow.  The  genital  and  anal  plates  are  open,  and  the  genital  suckers  pro- 
truding. One  maxilla,  white,  is  seen  between  the  legs. 

Fig.  3.  Tegeocranus  latus  (female,  dorsal  aspect,  x  about  55).  Dorsal 
exo-skeleton,  fatty  tissue,  and  muscles  removed.  Same  colours  as  before. 
Brain  (between  preventricular  glands)  blue  grey.  Mandibles  seen  from  above 
and  behind,  their  retractor  muscles  cut  short.  The  tracheae,  which  are  present 
in  this  species,  are  seen  proceeding  to  their  stigmata  in  the  acetabula  of  the 
legs. 

Fig.  4.  Female  genital  organs  of  Cepheus  tegeocranus  (  x  about  25),  Vigt. 
Central  Ovary,  oviducts  with  eggs,  vagina,  and  ovipositor. 

Fig.  5.  The  same  of  Damceus  geniculatus  (  x  about  20).  The  genital  plates 
and  the  muscles  and  tendons  which  move  them,  arid  the  genital  suckers,  are 
shown. 

These  two  figures  are  reduced  from  the  originals. 

Fig.  6.  Nymph  (active  pupal  stage)  of  Tegeocranus  hericius  (  x  about  100) 
(carrying  its  cast  dorsal  skins). 

TYROGLYPHIDJE 

Fig.  7.  Hypopial  (travelling)  nymph  of  Rhizoglyphus  Robini  (ventral 
aspect,  x  100). 

PLATE  XXI.     (Facing  p.  1010) 

ORIBATIDjE 

Fig.  1.  Leiosoma  palmicinctum  (  x  about  40). 

Fig.  2.  Nymph  of  same  species,  fully  grown  ( x  about  55).  The  central 
ellipse  with  the  innermost  set  of  scales  attached  is  the  cast  larval  dorsal 
abdominal  skin.  The  other  rows  of  scales  belong  to  the  successive  nympha- 
skins. 

Fig.  3.  One  of  the  scales  more  highly  magnified. 

CHEYLETID^l 

Fig.  4.  Rostrum  and  great  raptorial  palpi,  with  their  appendages  of  Ghey- 
letus  venustissimus  (  x  about  150). 

MYOBIID.E 

Fig.  5.  Myobia  chiropteralis  (female,  x  about  125). 

PLATE  XXII.     (Facing  p.  1012) 

Claw  of  first  leg  of  same  species,  being  an  organ  for  holding  the  hair  of 
the  bat. 

GAMASID^E 

Fig.  2.  Gamasus  terribilis  (male,  x  30).    A  species  found  in  moles'  nests. 

ANALGIN.E 

Fig.  3.  Freyana  heteropus  (male,  x  about  95,  a  parasite  of  the  cormorant). 
Fig.  4.  Sarcoptes  scabiei  (the  itch  mite,  x  about  150,  adult  female). 


PLATE    1, 


X1750 


X1750 


X270 


X2000 


Collotype  Ptg.  Co.,  282  High  Holborn,  W.C, 


COLLEGE    OF    DENTISTRY 
UNIVERSITY  OF  CALIFORNIA 


THE    MiCBOSCOPE 


CHAPTER   I 

ELEMENTARY  PRINCIPLES   OF   MICROSCOPICAL   OPTICS 

To  be  tlie  owner  of  a  well-chosen  and  admirably  equipped  miwo- 
scope,  and  even  to  have  learnt  the  general  purpose  and  relations  of 
its  parts  and  appliances,  is  by  no  means  to  be  a  master  of  the  in- 
strument, or  to  be  able  to  employ  it  to  the  full  point  of  its 
efficiency  even  with  moderate  magnifying  powers.  It  is  an  instru- 
ment of  precision,  and  both  on  its  mechanical  and  optical  sides 
requires  an  intelligent  understanding  of  pi  inciples  before  the  best 
optical  results  can  be  invariably  obtained. 

We  may  be  in  a  position,  with  equal  facility,  to  buy  a  high-class 
microscope  and  a  high-class  harp  ;  but  the  mere  possession  makes 
us  no  more  a  master  of  the  instrument  in  the  one  case  than  the 
other.  An  intelligent  understanding  and  experimental  training  are 
needful  to  enable  the  owner  to  use  either  instrument.  In  the  case 
of  the  microscope,  for  the  great  majority  of  purposes  to  which  it  is 
applied  in  science,  the  amount  of  study  and  experimental  training 
needed  is  by  comparison  incomparably  less  than  in  the  case  of  the 
musical  instrument.  But  the  amount  required  is  absolutely  essen- 
tial, the  neglect  of  it  being  the  constant  cause  of  loss  of  early  enthu- 
siasm and  not  infrequent  total  failure. 

In  the  following  pages  we  propose  to  treat  the  elementary 
principles  of  the  optics  of  the  microscope  in  a  practical  manner,  not 
merely  laying  down  dogmatic  statements,  but  endeavouring  to  show 
the  student  how  to  demonstrate  and  comprehend  the  application  of 
each  general  principle.  But  in  doing  this  we  are  bound  to  re- 
member a  large  section  of  the  readers  who  will  employ  this  treatise, 
and  to  so  treat  the  subject  that  all  the  examples  given,  or  that  may 
be  subsequently  required  by  the  ordinary  microscopist,  may  be 
worked  out  with  no  heavier  demand  upon  mathematics  than  the 
employment  of  vulgar  fractions  and  decimals. 

In  like  manner,  although  we  shall  again  and  again  employ  the 
trigonometrical  expression  '  sine,'  its  use  will  not  involve  a  mathe- 
matical knowledge  of  its  meaning.  The  sines  of  angles  may  be 

B 


2       ELEMENTARY   PRINCIPLES   OF  MICROSCOPICAL   OPTICS 

found  by  published  tables.  A  table  to  quarter  degrees  is  given  in 
Appendix  A  of  this  book,  which  will,  in  the  majority  of  cases, 
suffice ;  it  is  not  difficult  to  find  such  tables  as  may  be  required.1 

Of  course  it  is  more  than  desirable  that  the  microscopist  should 
have  good  mathematical  knowledge  ;  but  there  are  many  men  who 
desire  to  obtain  a  useful  knowledge  of  the  principles  of  elementary 
optics  who  are  without  time  or  inclination,  or  both,  to  obtain  the 
large  mathematical  knowledge  required. 

Now,  just  as  a  man  who  is  without  any  accurate  knowledge  of 
astronomy  or  .mathematics  may  find  time  from  a  sun-dial  by  applying 
the  equation  of  time  taken  from  a  table  in  an  almanac,  so  by  the 
use  of  a  table  of  sines  the  microscopist  may  reach  useful  and  reliable 
results,  although  he  may  have  no  clear  knowledge  of  trigonometry, 
physical  optics,  nor  the  mathematical  proof  of  formulae. 

All  microscopes,  whether  simple  or  compound,  in  ordinary  use 
depend  for  their  magnifying  power  upon  the  ability  possessed  by 
lenses  to  refract  or  bend  the  light  which  passes  through  them.  Re- 
fraction acts  in  accordance  with  the  two  following  laws,  viz.  : — 

1.  A  ray  which  in  passing  from  a  rare  medium  into  a  denser 
medium  makes  a  certain  angle  with  the  normal,  i.e.  the  perpendicu- 
lar to  the  surface  or  plane  at  which  the  two  media  join,  will,  on 
entering  the  denser  medium,  make  a  smaller  angle  with  the  normal. 
Conversely,  a  ray  passing  out  from  a  dense  medium  into  a  rarer  one, 
making  a  certain  angle  with  the  normal,  will,  on  emergence  from 
the  dense  medium,  make  a  greater  angle  with  the  normal. 

The  ray  in  one  medium  is  called  the  incident  ray,  and  in  the 
other  medium  the  refracted  ray. 

The  incident  and  refracted  rays  are  always  in  the  same  plane. 

2.  The  sine  of  the  angle  of  incidence  divided  by  the  sine  of  the 
angle  of  refraction  is  a  constant  quantity  for  any  two   particular 
media. 

When  one  of  the  media  is  air  (accurately  a  vacuum)  the  ratio  of 
these  sines  is  called  the  absolute  refractive  index  of  the  medium. 
As  every  known  medium  is  denser  than  a  vacuum,  it  follows  that 
the  angle  of  the  refracted  ray  in  that  medium  will  be  less  than  the 
angle  of  the  incident  ray  in  a  vacuum  ;  consequently,  the  absolute 
refractive  index  of  any  medium  is  greater  than  unity. 

Further,  the  absolute  refractive  index  for  any  particular  sub- 
stance will  differ  according  to  the  colour  of  the  ray  of  light  employed. 
The  refraction  is  least  for  the  red,  and  greatest  for  the  violet.  The 
difference  between  these  refractive  values  determines  what  is  called 
the  dispersive  power  of  the  substance. 

This  will  be  understood  by  fig.  1.  Let  I  0,  a  ray  of  light  travel- 
ling in  air,  meet  the  surface  A  B  of  water  at  the  point  C.  Through 
C  draw  N  N7  at  right  angles  to  the  surface  of  the  water  A  B.  The 
line  N  Nr  is  called  the  normal  to  the  surface  A  B.  The  ray  I C  will 
not  continue  its  path  through  the  water  in  a  straight  line  to  Q  ;  but, 
because  water  is  denser  than  air,  it  will  be  bent  to  R,  that  is 
towards  Nr.  The  whole  course  of  the  ray  will  be  I  C  R,  of  which 
the  part  I  C  is  called  the  incident  ray,  and  C  R  the  refracted  ray. 
1  Vide  Chambers's  Mathematical  Tables. 


THE   LAW   OF   SINES  3 

The  angle  I  C  makes  with  the  normal  N  Nr,  viz.  I  C  N,  is  called  the 
angle  of  incidence  ;  and  the  angle  R  C  makes  with  the  normal  N'  N, 
viz.  R  C  N',  is  called  the  angle  of  refraction. 

Conversely,  if  a  my  R  C,  travelling  in  water,  meet  the  surface  of 
air  A  B  in  the  point  C,  it  will  not  continue  in  a  straight  line,  but 
will  be  bent  to  the  point  I  farther  away  from  N.  Thus,  when  a 
ray  passes  from  a  rarer  to  a  denser  medium  it  is  bent  or  refracted 
towards  the  normal,  and  when  it  passes  out  of  a  dense  medium  into 
a  rarer  one  it  is  bent  or  refracted  away  from  the  normal. 

Further,  if  the  shaded  portion  of  the  figure  were  glass  instead  of 
water,  the  refracted  ray  R  C  would  be  bent  still  nearer  N',  and, 
conversely,  if  the  ray  passed  out  of  glass  into  air,  it  would  be  more 


FIG.  1. — The  refraction  of  light.     The  law  of  sines. 

bent  away  from  the  normal  than  if  it  had  passed  out  of  water  into 
air. 

The  angle  of  incidence  I C  N  is  connected  with  the  angle  of  re^ 
fraction  RON'  (as  stated  above)  by  what  is  known  as  Snell's  Law  of 
Sines.  The  constant  relation  between  the  two  sines  for  two  specific 
media  is  called  the  refractive  index  of  the  medium,  and  is  usually 
indicated  in  problems  by  the  symbol  p. 

This  law,  stated  with  reference  to  the  figure,  would  be : 

1-  =  u  =  the  refractive  index  of  water, 
sine  R  C  .N 

In  I  C  take  any  point,  P,  and  from  P  draw  P  T  perpendicular 
to  NX'.  Similarly  in  RC  take  any  point,  F,  and  draw  FH  per- 
pendicular to  N  N7. 

B2 


4       ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL   OPTICS 

-p)  rp  "C1  T  r 

Now,  as  sine   I  C  N  =     -^  and  sine  R  C  N'  =       ^,    then,    by 
PC  r  C 

FT 

Snell's  law,  -—  -=  fj.. 

FIT 

As  any  points  may  be  taken  in  I  C  and  R  C  if  the  points  had  been 
more  judiciously  selected,  we  might  have  greatly  simplified  the  above 
expression.  Thus,  if  we  take  two  other  points,  K  and  E,  such  that 
K  C  =  E  C,  and  draw  the  perpendiculars  as  before,  we  shall  have 

K^S 

sine  I  C  N  =  ?-?  and  sine  R  C  N'  =  ?  ^,  and  therefore    JL£  =  „. 
Iv  C  Ju  C 


But  as  K  C  =  E  C  by  construction,  we  can  write  K  C  for  E  C 
KS 


jr  & 

thus  :    —  —  =  fj..     K  C  is  cancelled,  which  leaves          = 
ED  ED 


As  p  can  be  experimentally  determined  for  any  two  particular 
media,  it  follows  that  if  one  of  the  other  terms  is  known,  then  the 
remaining  term  can  be  found.  Thus,  if  /i  and  the  angle  of  incidence 
are  known,  the  angle  of  refraction  can  be  found  ;  and  if  /n  and  the 
angle  of  refraction  are  known,  the  angle  of  incidence  can  be  found. 
The  unknown  quantity  can  be  found  either  geometrically  or  by  cal- 
culation when  the  other  two  terms  are  given. 

It  will,  of  course,  be  understood  that,  for  the  same  medium  in 
every  case,  a  red  ray  would  be  bent  or  refracted  less  than  a  violet 
ray.  The  value  therefore  of  p.  for  a  red  ray  will  be  less  than  that  of 
//  for  a  violet  ray.  As  a  practical  illustration  :  The  refractive  in- 
dex for  a  red  ray  in  crown  glass  is  1'5124  =  ^u,  and  for  a  violet  rav 
is  1-5288  =  /,  the  difference  being  /  —  /u  =  '0164. 

The  refractive  index  for  a  red  ray  in  dense  flint  glass  is  1-7030 
=  /u,  and  for  a  violet  ray  is  1-7501  =  //,  the  difference  being  n'  —  fj. 
=  •0471. 

Consequently  there  will  be  a  greater  difference  between  the  bend- 
ing of  the  refracted  red  and  violet  rays  in  the  case  of  dense  flint  than 
in  the  case  of  crown  glass,  the  angle  of  the  incident  ray  with  the 
normal  being  the  same  in  either  case. 

Where  air  (more  correctly  a  vacuum)  is  not  one  of  the  media, 
then  the  refractive  index  is  called  the  relative  refractive  index. 

The  normal  to  a  plane  surface  is  always  the  perpendicular  to  it  ; 
the  normal  to  a  spherical  surface  is  the  radius  of  curvature.  The 
angle  of  the  incident  ray  and  the  angle  of  the  refracted  ray  are 
always  measured  with  the  normal,  and  not  with  the  surface. 

Fig.  2  a,  bj  shows  the  normals  A,  B  to  both  a  plane  and  a 
spherical  surface,  C  D. 

In  the  case  of  the  spherical  surface,  B  is  the  centre  of  curvature,  E  F 


PKOBLEMS  ON  REFRACTIVE  INDEX 


a, 

A 


is  the  incident  ray  in  air,  F  G  the  refracted  ray  in  crown  glass.     The 
angle  A  FE  is  the  angle  of  incidence,  BFG  the  angle  of  refraction. 
Sine  A  F  E  divided  by  sine  B  F  G  is  equal  to  the  refractive  in- 
dex of  air  into  crown  glass,  or,  in  other  words,  the  absolute  refractive 
index  of  crown  glass,  /u  ;  thus  in  this  particular  case  : 
(Problem)  I.  : 

sin  AJFJ^_  sin  45°  _  -707  _  3^  _ 
sin  B  F  G  ~~  sin~280  ~~  -472"  ~  2"  =  ^' 

This  problem,  however,  is  not  actually  needed  by  the  reader  of 
this  book,   for  a  table  of 
absolute  refractive  indices 
is  given  in  Appendix  B. 

It  will  be  clear  from 
the  above  that  when  the 
refractive  index,  absolute 
or  relative,  of  a  ray  from 
any  first  medium  is  given, 
the  refractive  index  from 
the  second  to  the  first  may 
be  found. 

Thus,  the  absolute  re- 
fractive index  p  from  air 
into  glass  being  given  as 

'-,  find  nf,  the  refractive 

index  from  glass  into  air. 

(Problem)  II.  : 
,       1        1       2 

**        ~  =o  =  .r 
H       6       3 

2 

When  the  absolute 
refractive  indices  of  any 
two  media  are  given,  the 
relative  refractive  indices 
between  the  media  can  be 
found. 

Thus,  the  absolute  re- 
fractive index  p  of  crown 
glass  is  1-5,  and  the  ab- 
solute refractive  index  // 
of  flint  glass  is  1*6  ;  find  the  relative  refractive  index  //'  from  crown 
to  flint. 


G- 


FIG.  2. — The  normals  to  a  plane  and  a  curved 
surface. 


(Problem)  III.  : 


£  =  HJ  = 

JL  1*5 


The  relative  refractive  index 
by  (problem)  ii.  : 

" 


1-066 


from  flint  to  crown  is  determined 


=  -938. 


6       ELEMENTARY  PKINCIPLES   OF   MICROSCOPICAL  OPTICS 

Let  us  now  suppose  that  in  fig.  2  the  ray  is  travelling  in  the  op- 
posite direction,  G  F  in  the  denser  medium  will  now  be  the  incident 
ray,  and  F  E  in  the  rarer  medium  will  be  the  refracted  ray.  Now, 
if  the  angle  B  F  G  be  increased,  the  angle  A  F  E  will  also  be  in- 


FIG.  3.— The  phenomenon  of  total  reflexion.     (From  the  '  Forces  of  Nature,' 
published  by  Macmillan.) 

creased  in  a  greater  proportion,  and  the  ray  F  E  will  approach  the 
surface  F  D. 

When  F  E  coincides  with  F  D,  G  F  is  said  to  be  incident  at  the 
critical  angle  of  the  medium.  When  this  critical  angle  is  reached, 
none  of  the  incident  light  will  pass  out  of  the  denser  medium,  but  it 


PROBLEMS   ON   REFRACTIVE   INDEX  7 

will  be  totally  reflected   from  the  surface  C  D  back  into  the  denser 
medium. 

A  simple  illustration  of  this  is  shown  in  fig.  3.  It  represents 
a  glass  of  water  so  held  that  the  surface  of  the  water  is  above  the 
eye.  If  we  look  obliquely  from  below  at  this  surface,  it  appears 
brighter  than  polished  silver,  and  an  object  placed  in  the  wTater  has 
the  upper  portion  of  it  brightly  reflected. 

The  action  on  all  light  incident  on  C  D  in  the  denser  medium 
(fig.  2)  at  an  angle  greater  than  the  critical  angle  is  precisely  the 
same  in.  fact  as  if  C  D  were  a  silvered  mirror. 

A  critical  angle  can  only  exist  in  a  denser  medium,  for  obviously 
there  can  be  no  critical  angle  in  the  rarer  medium,  since  a  ray  of 
any  angle  of  incidence  can  enter. 

When  the  relative  or  absolute  refractive  index  of  the  denser 
medium  is  given,  the  critical  angle  for  that  medium  can  be  found, 
thus  :  The  absolute  refractive  index  of  water  is  1-33  =  /i ;  find  its 
critical  angle  0. 

(Problem)  IV. :     ^  g  =1=_1_  .75  . 

fj,     1*33 
0  =48£°  (found  by  table). 

So  the  sine  of  the  critical  angle  is  the  reciprocal  of  the  refractive 
index. 

The  connection  between  the  path  of  an  incident  ray  in  a  first 
medium  and  its  refracted  ray  in  a  second  medium  is  established  by 
the  formula 

fj.  sin  (f>  =  p,'  sin  <^/, 

where  /z  is  the  absolute  refractive  index  of  the  first  medium,  0  the 
angle  of  the  incident  ray  in  it,  fj'  the  absolute  refractive  index  of 
the  second  medium,  and  <//  the  angle  of  the  refracted  ray  in  it. 

The  angle  <j>  =  45°  of  the  incident  ray  in  the  first  medium  A  F  E 

q 

(fig.  2)  and  /u  =  1,  p'  =      the  absolute  refractive  indices  of  both  the 

media,  air  and  glass  respectively,  being   given,   find  <^r,  the  angle  of 
the  refracted  ray  in  glass. 
(Problem)  V.  1  : 

Sin  f  =  M_sin_0=rl    x  sin45°  =  l__x  j707=  .4?1  . 

n'  1'5  1*5 

<//  =  28°  (found  by  table). 

To  put  another  case.     Suppose  the  angle  <//  =  28°  (fig.  2,  B  F  G) 
is  given  ;  find  0,  the  refractive  indices  remaining  the  same  as  before. 
(Problem)  Y.  2  : 

Sin  ^  _/ *inf_l-5  X  sin28°_l-5   x  -741_.70ftfi 

0  =  45°  (found  by  table). 

Now,  suppose  the  A  side  of  C  D  (fig.  2)  is  crown  glass,  ^  =  1'5, 
and  the  B  side  of  C  D  is  flint  glass,  /  =  1'6.  The  angle  of  the 
incident  ray  A  F  E  <f>  =  45°,  find  the  angle  of  the  refracted  ray  tf  or 
BFG. 


8       ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL  OPTICS 


(Problem)  V.  3  : 


1-5  x  sin  45     1-5x707     1-0605 


1-6 


1-6 


I 

=  •663; 


'=41^°  (found  by  table). 


As  a  final  instance.  Suppose  the  ray  to  be  travelling  in  the 
opposite  direction,  so  that  G  F  is  the  incident  ray  and  B  F  G,  or 
<^/— 41l°?  be  given?  the  media  being  the  same  as  in  the  last  case, 
//=l-6  and  /*=l-5,  find  </>,  or  the  angle  of  the  refracted  ray. 


(Problem)  V.  4 : 


Sin  0= 


u!  sin 


1-6  sin  41V     1-6  x -663 


1-5 


1-5 


0=45°  (found  by  table). 


The  importance  of  the  prism  in  practical  optics  is  well  known. 

Its  geometrical  form  in  per- 
spective and  in  section  is  shown 
in  fig.  4. 

By  means  of  the  above  pro- 
blems and  their  solutions  we 
are  now  able  to  trace  the  diver- 
gence of  a  ray  through  a  prism. 
In  fig.  5  let  ABC  repre- 
sent a  prism  of  very  dense  nint 
glass  whose  absolute  refractive 
indices  /x/  for  red  light  is  1*7, 
and  ft"  for  blue  light  is  175. 
Let  the  refracting  angle  B  A  C 
of  the  prism  =50°,  and  let  the 
angle  of  incidence  of  a  ray  of 
white  light  I)  E=45°  =0  in 
air,  /x=  1 .  The  dotted  lines  show 
the  normals.  Then  by  (problem) 
v.  1  for  red  light  we  have  for 
the  angle  of  refraction  0'. 


FIG.  4. — The  geometrical  form  of  the  prism. 
(From  the  '  Forces  of  Nature.') 


sn      = 


•404; 


/A  sin  0     1  sin  45°     707 
~~f~        ~TT~    =  1T7  = 
0'=  241°  (found  by  table). 

And  for  Uue,  light : 

,,_/Ksin_0__l  sin  45°_7p7_ 
fj.ff  1-75      ~~175~~ 

0"=23f°  (found  by  table). 

Now,  for  the  red  ray  draw  E  F  (fig.  5),  24^°  to  the  normal,  and 
let  it  meet  the  other  side  of  the  prism  A  C  in  F.  At  F  draw 
another  normal. 

On  the  scale  of  our  diagram  it  is  not  possible  to  draw  two  lines 
E  F,  one  for  the  red  ray  and  the  other  for  the  blue,  for  they  are  too 
close  together,  their  angular  divergence  being  only  j°.  But  by 


PATH   OF  LIGHT   THKOUGH  A  PJRISM  9 

measurement  it  will  be  found  that  E  F  makes,  with  the  normal  at 
F,  an  angle  9'  of  25^°,  and  for  the  blue  ray  an  angle  <£"  of  26J°. 

It  should  be  remembered,  however,  that  if  the  refracting  angle 
of  the  prism  is  known,  there  is  no  necessity  for  this  measurement, 
because  it  is  always  the  difference  between  this  and  the  angle  of 
refraction  before  determined,  thus  50°  — 24^°= 25  V>. 


R 

B~  C  ^V 

FIG.  5. — Diagram  of  deviation  of  luminous  ray  by  a  prism. 

This  ray  E  F  now  becomes  the  incident  ray  on  the  surface  A  C  ; 
and  as  the  angle  it  makes  with  the  normal  at  F  is  known,  and  as 
the  refractive  indices  remain  the  same,  we  can,  by  (problem)  v.  2, 
find  the  angles  of  refraction  for  each  colour. 

If  we  take  red  light : 


,  the  angle  of  refraction  =47°  (found  by  table). 


If  we  take  blue  light  : 


sn 


-75  sin  26J°     175  x  '442 

__    L=-    T__=.m. 

<j),  the  angle  of  refraction  =  50  J°  (found  by  table). 

This  dispersion  can  now  be  represented  in  the  diagram,  seeing 
that  it  amounts  to  3|°. 

In  optics  it  is  convenient  to  use  an  expression  to  measure  the 
dispersive  power  of  diaphanous  substances,  which  does  not  depend 
on  the  refracting  angle  of  the  prism  employed.  Further,  in  order 
that  various  substances  may  be  compared,  their  dispersive  powers 
are  all  measured  with  reference  to  a  certain  selected  ray.  (For  this 
purpose  the  bisection  of  the  D  or  sodium  lines  is  the  point  in  the 
spectrum  often  chosen.) 

In  the  crown  and  flint  glasses  mentioned  on  page  4  the  dispersion 
between  the  lines  C  and  F,  in  the  spectrum,  referred  to  the  bisection 
of  the  sodium  lines  D,  is  as  follows.  Crown  glass  :  —  refractive  index 
bisection  of  lines  D,  l'5179=/u;  line  F,  1'52395=//;  line  C, 
l'51535=/x//.  Then  the  dispersive  power  to 

1-52395  -1-51  535     -0086 

=<01661- 


uf—uf' 


_l       1-5179  -1 


-5T79 


10      ELEMENTARY   PRINCIPLES   OF   MICROSCOPICAL   OPTICS 


The  values  of  the  same  lines  for  the  flint  glass  are  as  follows  : 
D,  1-7174=^;  F,  1-73489=M';  0,  1-71055=/*". 
u'-ii"      1-73489-1-71055     -02434 


-  1    "1-7174    -1 


•7174 


-==•0339. 


So  the  dispersive  power  of  the  flint  between  the  lines  C  and  F  is 
slightly  more  than  twice  that  of  the  crown  for  the  same  region  of 
the  spectrum.  In  the  above  formula  the  expression  /u'  —  /*"  is  usually 


written  b  u  ;    in  full  it  is  therefore 


Having  thus  traced  a  ray 
experimentally  through  a 
prism,  our  next  step  is  to  show 
that  a  convex  lens  is  only  a 
curved  form  of  two  suck  prisms 
with  their  bases  in  contact, 
as  is  shown  in  A,  fig.  6,  , 
where  the  curved  line  shows 
the  lenticular  character  and 
the  shaded  elements  the  two 
prisms.  A  concave  lens  is  in 
effect  two  prisms  reversed, 
that  is,  with  their  apices  in 
contact,  as  in  B,  fig.  6,  where, 
again,  the  curved  line  shows 
the  form  of  the  lens  and  the 


— 
/i-  1 


FIG.  6. — Convex  and  concave 
lenses  are  related  to  the 
prism. 


FIG.  7. — Proof  that  a  lens  may  be  considered 
as  an  assemblage  of  prisms.  (From  the 
'  Forces  of  Nature.') 


shaded  parts  its  relation  to  a  pair  of  prisms.  The  fact  that  a  lens  is, 
in  effect,  as  such,  but  an  assemblage  of  superposed  prisms  is  seen  in 
fig.  7,  the  refracting  angle  of  the  prism  being  more  acute  as  the 
principal  axis  is  approached,  and  the  deviation  being  greater  as  the 
angle  is  more  obtuse. 

In  fig.  8  let  0  P  be  the  axis  in  each  case ;  then,  from  what  we 
have  seen,  it  is  manifest  that  rays  parallel  to  the  axis  falling  on  the 
prisms  with  their  bases  in  contact  and  acting  like  a  convex  lens  will 
be  refracted  towards  the  axis  O  P.  But  in  the  other  case,  where 
the  prisms  have  their  apices  together,  as  in  fig.  9,  acting  as  a  con- 
cave lens,  the  light  is  refracted  away  from  the  axis  O  P. 


ACTION   OF   A   PAIR   OF   PRISMS 


II 


H    ^ 


G 


FIG.  8. — Action  of  a  pair  of  prisms  with  their  bases  in  contact  on 
parallel  light. 


O 


L 


FIG.  9. — Action  of  a  pair  of  prisms  with  their  apices  in  contact  on 
parallel  light. 


12      ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL   OPTICS 

It  must,  however,  be  understood  that  there  is  a  very  important 
difference  between  the  action  of  spherical  lenses,  which  is  dm  to  the 
different  positions  of  the  normals. 

In  the  prisms  (figs.  8,  9)  the  incident  surface  A  B  is  a  plane  ; 
and  as  the  normals  are  perpendicular  to  it,  they  must  be  parallel  to 
one  another,  whether  near  the  base  or  near  the  apex.  Thus  the 
normal  at  E  is  parallel  to  the  normal  at  K  ;  therefore,  whatever 
angle  D  E  makes  with  the  normal  at  E,  H  K  will  make  a  similar 
angle  with  the  normal  at  K,  because  the  normals  are  parallel  and  the 
incident  rays  are  parallel. 

But  in  the  case  of  a  spherical  lens  the  normals  are  radii ; 
parallelism  is  therefore  impossible,  and  parallel  incident  rays  will 
not  make  equal  angles  with  them,  and  so  the  refracted  rays  will  not 
be  parallel. 

This  explains  how  it  is  that  when  rays  parallel  to  the  axis  fall 
on  the  prism  (see  fig.  8)  those  which  pass  through  the  prisms  near 
their  bases  cut  the  axis  nearer  the  prisms  than  those  which  pass 
through  near  the  apex. 

But  in  a  convex  lens  the  reverse  takes  place ;  the  rays  passing 
through  near  the  middle  of  the  lens  cut  the  axis  farther  from  the 
lens  than  those  which  pass  through  the  edge  of  the  lens.  The 
typical  form  of  a  biconvex  or  magnifying  lens  is  shown  in  fig.  10, 


FIG.  10. — Front  and  edge  views  of  a  biconvex  lens. 
(From  the  '  Forces  of  Nature.') 

both  in  perspective,  as  seen  from  the  edge,  and  with  a  full  view  of 
the  disc ;  while  the  various  forms  which  for  various  optical  purposes 
are  given  to  lenses  is  shown  in  figs.  11  and  12. 

Now,  if  we  study  the  four  following  figures,  we  shall  see  the 
principal  action  of  lenses  on  light  incident  on  their  surfaces.  Fig. 
1 3  shows  that  if  a  radiant  is  placed  at  the  principal  focus  of  a  con- 
verging lens,  the  rays  are  rendered  parallel ;  conversely,  if  parallel 
rays  fall  on  a  converging  lens,  they  are  brought  to  a  principal  focus 
or  point  upon  the  axis. 

Fig.   14  shows  that  if  a  radiant  be  placed  beyond  the  principal 


THE   FOCI   OF   LENSES  13 

focus  of  a  converging  lens,  the  rays  are  brought  to  a  focus  beyond 
the  principal  focus  on  the  other  side  of  the  lens.  The  nearer  the 
radiant  is  to  the  principal  focus,  the  farther  away  will  be  its  conjugate 
focus  from  the  other  principal  focus.  In  other  words,  there  are  two 
points  in  the  axis  such  that  if  the  object  is  one  point  its  focus  will 
be  the  other  ;  these  are  reciprocal  one  to  the  other.  These  points, 


FIG.  11. — Biconvex,  plano-convex, 
and  converging  meniscus  lenses. 
(From  the  '  Forces  of  Nature.') 


FIG.  12. — Biconcave,  plano-concave, 
and  diverging  meniscus  lenses. 
(From  the  '  Forces  of  Nature.') 


the  focal  distances  of  which  can  always  be  calculated,  are  known  as 
conjugate  foci. 

Should  the  radiant  be  at  a  distance  from  the  principal  focus  equal 
to  the  focal  length  of  the  lens  (i.e.  twice  the  focal  length  from  the 
lens),  then  its  conjugate  will  be  at  the  same  distance  from  the  focus 


FIG.  13. — A  radiant  at  the  principal  focus  of  a  biconvex  tens  makes  the  refracted 

rays  parallel. 


FIG.  14. — A  radiant  placed  beyond  the  principal  focus  causes  rays  to  converge 
beyond  the  principal  focus  on  the  other  side  of  the  lens. 

on  the  other  side  of  the  lens  (i.e.  twice  the  focal  length  from  the  lens). 
In  other  words,  when  the  object  and  its  image  are  equidistant  011 
either  side  of  the  lens,  they  are  equal  to  each  other  in  size,  and 
are  four  times  the  focal  length  of  the  lens  apart. 


14      ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL  OPTICS 

This  law  forms  a  ready  means  of  determining  the  focal  length  of 
a  lens.  An  object  is  placed  in  front  of  a  lens,  and  the  distances 
between  this  object  and  the  lens  and  a  screen  to  receive  the  image 
of  the  object  are  so  adjusted  that  the  image  of  the  object  becomes  equal 
in  size  to  the  object  itself.  The  distance  of  the  object  from  the  screen 
divided  by  4  gives  the  focal  length  of  the  lens. 

If  a  radiant  be  placed  between  a  lens  and  its  principal  focus,  the 
rays  on  the  other  side  of  the  lens  are  still  divergent,  and  will  never 
meet  in  a  focus  on  that  side.  This  is  seen  in  fig.  1 5  ;  but  if  they  are 
traced  backwards,  as  in  the  dotted  lines  of  fig.  15,  they  will  then 


FIG.  15. — Kays  diverge  when  a  radiant  is  placed  between  a  lens  and  its 
principal  focus.     Focus  of  divergent  rays  is  virtual. 

meet  in  a  point.  This  is  called  the  virtual  conjugate  focus  of  the 
radiant.  The  principal  focus  of  a  concave  (or  diverging)  lens  is 
shown  in  fig.  16.  It  will  be  seen  that  the  principal  focus  is  not 
real  but  virtual.1  Parallel  rays  falling  on  a  concave  lens  are  rendered 


FIG.  16. — '  Virtual '  focus  of  concave  lens. 

divergent  on  the  other  side  of  the  lens,  and  consequently  can  never 
come  to  a  focus.  But  if  we  trace  these  divergent  rays  backwards, 
as  in  the  dotted  lines  of  fig.  16,  we  find  that  they  meet  in  a  point, 
and  this  point  is  called  the  virtual  principal  focus  of  the  lens. 

It  will  be  manifest  that  since  the  rays  in  passing  through  lenses  of 
various  kinds  are  unequally  refracted  they  cannot  all  meet  exactly  in  a 
single  focal  point.  This  gives  rise  to  what  is  a  most  important  feature 
in  the  behaviour  of  lenses,  which  is  known  as  spherical  aberration. 

Figs.  17  and  18  show  the  refraction  of  rays  of  monochromatic 
1  A  real  image  can  be  received  on  a  screen,  but  a  virtual  image  cannot. 


SPHERICAL  ABERRATION 


light  parallel  to  the  axis  falling  on  a  plano-convex  lens  of  crown 
glass.  These  figures  illustrate  :  (1)  Longitudinal  spherical  aberration 
and  (2)  the  focal  length  of  a  plano-convex  lens  and  the  point  from 
which  it  is  measured. 

(1)  In  regard  to  the  former  it  will  be  seen  that  the  longitudinal 
spherical  aberration  is  greatest  in  fig.  17,  where  the  parallel  rays 
of  light  fall  upon  the  plane  surface,  and  least  where,  as  in  fig.  18, 
they  fall  upon  the  spherical  surface.  For  spherical  aberration  is  the 


F     F  F 


ft2 


ft,1 


FIG.  17. — Spherical  aberration. 


distance  of  the  focus  for  any  ray  passing  through  a  lens  from  the 
principal  Jocus  of  that  lens. 

Thus  in  figs.  17,  18,  the  spherical  aberration  is  F  F'  for  the  rays 
R2  R2,  and  F  F"  for  the  rays  R1  R1,  and  the  difference  between  the 


Fig.  18. — Spherical  aberration. 

spherical  aberration  of  the  rays  R1  R1  and  that  of  the  rays  R2  R2  is 
F  F"  —  F  F,  which  is  V  F". 

Thus  F  F  and  F  F'  in  (fig.  17),  S/=  -  |  -    |* ;  F  F  and  F  F' 

J 

7      ?/2 
in  (fig.  18)  c/= •  Sy,  where  £/  signifies  the  distances  FFr, 

F  F"  respectively,  y  the  distance  from  the  axis  where  the  incident 
ray  enters  the  lens,  and  f  the  focus. 

(2)  In  regard  to  the  focal  length  of  a  plano-convex  lens,  it  may 
be  incidentally  noted  that  the  focal  length  in  fig.  1 7  is  twice  the  radius, 
measured  from  the  vertex  A,  that  is,  A  F.  But  in  fig.  18  it  is  twice 
the  radius  measured  from  the  point  A  ;  that  is,  the  point  F  is  distant 
from  the  lens  twice  the  radius  less  two-thirds  the  thickness  of  the  lens. 

It  will  be  seen,  then,  that  the  amount  of  spherical  aberration  is 
due  to  the  shape  of  the  lens,  and  is  least  in  a  biconvex  lens,  when  the 
radii  of  curvature  are  in  the  proportion  of  6  :  1 ,  when  the  more  curved 
surface  faces  the  incident  light.  But  when  the  lens  is  turned  round, 
so  that  the  other  side  faces  the  incident  light,  the  spherical  aberration 
reaches  a  maximum. 

It  would  be  well  for  the  student  who  desires  to  become  familiar 
with  these  facts,  without  attempting  any  profound  mathematical 


1  6      ELEMENTARY  PRINCIPLES  .OF   MICROSCOPICAL   OPTICS 

grasp  of  them,  to  draw  such  a  lens,  and  trace  the  paths  of  two  rays 
through  it,  one  near  the  axis,  the  other  near  the  edge  ;  then  do  the 
same  with  the  lens  reversed. 

Formula  for  spherical  aberration  :  1 


f        If      lr>  f          r'        f      r' 

where  f  =  principal  focal  length  ;    y  =  semi  -aperture  ;    p  =  refr. 
index  ;  and  r,  —  r',  radii. 

Q 

In  an    equi-convex    of    crown,  where    p  =     ,  r  =  —  r'  =  /, 

sf-  -5  .  >>* 
f       3    /• 

o  /• 

In  a  plano-convex  of  crown,  where  /z  =  -,  —  r'  =  oo,   r  =  •'-, 

7     ?/2 
I  f  =  —  -  •  ~  .     Here  parallel  rays  are  incident    on  the   convex 

6      / 
surface.     But  when  parallel  rays  are  incident  on  the  plane  surface, 

H  =  0,  r  =  co,  —  r'  =  -,  $f=  —      •  '     ;    consequently  the  sphe- 

Z  JL  2      j 

rical  aberration  is  four  times  as  great  (see  figs.  17  and  18). 

When  —  r'  —  oo,  and  p  =  1'69,  the  plano-convex  becomes  the 
form  of  minimum  aberration. 

q 

In  a  crossed  2  biconvex  lens,  where   —  rf  =  6  r,   and  p  =     , 

15     ?/2 
Sf=  —     ;  •  -f1  the   parallel   rays    being    incident    on    the    more 

curved  surface. 

Formula  for  finding  the  principal  focus  F  of  a  lens  equivalent  to 
two  other  lenses  whose  foci  are  f,  f  and  their  distance  apart  d  : 
1  __  1       J__     _c£_ 

F""/V     ff1' 

In  figs.  5,  8,  and  9  we  see  that  when  the  incident  ray  D  E  con- 
sists of  white  light,  the  colours  of  which  it  is  composed  are  unequally 
refracted  ;  the  two  extremes,  R  (red  light)  and  Y  (violet  light),  being 
bent  in  different  directions,  the  other  colours  lying  between  them 
in  their  proper  order. 

This  unequal  refraction  of  the  different  colours  takes  place  in 
like  manner  in  spherical  lenses,  and  it  is  then  known  as  chromatic 
aberration. 

The  effect  of  this  upon  the  action  of  a  lens  is  that,  if  parallel  white 
light  fall  upon  a  convex  surface,  the  most  refrangible  of  its  component 
rays  (which,  as  we  have  seen,  is  the  violet)  will  be  brought  to  a  focus 
at  a  point  somewhat  nearer  the  lens  than  the  principal  focus  ;  and 
the  red  ray,  having  the  least  refrangibility,  will  be  brought  to  a  focus 
at  a  point  farther  from  the  lens  than  its  principal  focus,  which  is,  in 
effect,  the  mean  of  the  chromatic  foci. 

1  Encyclopedia  Brit.  vol.  xvii. 

2  A  biconvex  lens  is  said  to  be  '  crossed  '  when  the  radii  of  its  surfaces  are  in  the 
proportion  of  1  :  6. 


HOW   ACHROMATISM   MAY   BE   OBTAINED  17 

This  will  be  fully  understood  by  the  aid  of  fig.  19. 

The  white  light,  A  A",  falling  on  the  peripheral  portion  of  the  lens, 
is  so  far  dispersed  or  decomposed  that  the  violet  rays  are  brought  to 
a  focus  at  C,  and,  crossing  there,  diverge  again  and  pass  on  towards 
F  F  ;  whilst  the  red  rays  are  not  brought  to  a  focus  until  they  reach 
the  point  D,  crossing  the  divergent  violet  rays  at  E  E.  The  foci  of 
the  intermediate  rays  of  the  spectrum  (indigo,  blue,  green,  yellow, 
and  orange)  are  intermediate  between  these  two  extremes.  The 
distance  C  D,  limiting  the  violet  and  the  red,  is  termed  the  longitu- 
dinal chromatic  aberration  of  the  tens. 

If  the  image  be  received  upon  a  screen  placed  at  C,  violet  will 
predominate,  and  will  be  surrounded  by  a  prismatic  fringe  in  which 
blue,  green,  yellow,  orange,  and  red  may  be  distinguished.  If,  on 
the  other  hand,  the  screen  be  placed  at  D,  the  image  will  have  a 


FIG.  19. — Chromatic  aberration. 

predominantly  red  tint,  and  will  be  surrounded  by  a  series  of 
coloured  fringes,  in  inverted  order,  formed  by  the  other  rays  of  the 
spectrum  which  have  met  and  crossed. 

The  line  E  E  joins  the  points  of  intersection  between  the  red 
and  the  violet  rays  which  marks  the  mean  focus,  or  the  point  where 
the  dispersion  of  the  coloured  rays  will  be  least. 

The  axial  ray  undergoes  neither  refraction  nor  dispersion,  and 
the  nearer  the  rays  are  to  the  axial  the  less  dispersion  do  they 
undergo.  Similarly,  when  the  refraction  of  the  rays  is  greatest  at 
the  periphery  of  a  lens,  there  the  dispersion  will  be  most.  Hence 
the  peripheral  portions  of  unconnected  lenses  are  stopped  out,  and 
the  centre  only  often  used  that  the  chromatic  aberration  may  be 
reduced  to  a  minimum. 

Manifestly,  therefore,  the  correction  or  neutralisation  of  this 
chromatic  aberration,  which  is  known  in  optics  as  achromatism,  is  a 
matter  of  the  first  moment.  Multiplied  colour  foci  between  C  and 
D  (fig.  19)  make  a  perfect  optical  image  impossible. 

It  is  a  question  of  interest  and  importance  to  the  microscopist  to 
know  how  achromatism  is  obtained. 

In  a  prism  the  amount  of  dispersion  or  unequal  bending  of 
E,  and  V  (fig.  5)  depends  on  two  things :  (1)  the  nature  of  the 
glass  of  which  the  prism  is  composed,  and  (2)  the  refracting  angle 
B  AC. 

If,  for  example,  another  prism  were  taken,  made  of  a  different 
kind  of  glass,  possessing  only  half  the  dispersive  power  of  that  in 
the  figure,  but  with  the  angle  BAG  50°,  as  in  this  case,  the  separa- 

c 


1 8      ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL   OPTICS 


tion  of  R  and  Y  would  only  be  half  as  great  as  that  effected  by  the 
prism  in  the  figure. 

Then  if  another  prism  were  made  of  the  same  material  as  that 
assumed  in  fig.  5,  but  with  only  half  the  refracting  angle,  viz.  25°, 
the  dispersion  between  R  and  Y  would  also  be  but  half  that  repre- 
sented. Also  a  prism  having  50°  of  refracting  angle  gives  the  same 
amount  of  dispersion  as  that  from  a  prism  of  25°  of  refracting  angle, 
but  of  twice  its  dispersive  power. 

Under  these  conditions,  when  one  prism,  exactly  like  another  in 
angle  and  dispersive  power,  is  placed  close  to  it  in  an  inverted 
position,  the  dispersion  of  the  first  prism  is  entirely  neutralised  by 
that  of  the  second  because  it  is  precisely  equal  in  amount  :md 

opposite  in  power. 
This  will  be  under- 
stood by  a  glance  at 
fig.  20.  But  it  will 
be  seen  that  not  only 
is  dispersion  reversed, 
but  refraction  also 
is  neutralised,  the 
emergent  ray  being 
parallel  to  the  in- 
cident ray.  Therefore 
the  equal  and  inverted 
system  of  prisms  can 
be  of  no  possible  use 
to  the  practical  opti- 
cian in  the  correc- 
tion of  lenses  because 
the  convergence  and  divergence  of  rays  are  both  essential  to  the 
construction  of  optical  instruments.  The  dispersion,  in  fact,  must 
be  destroyed  without  neutralising  all  the  refraction. 

Suppose  we  take  a  prism  with  an  angle  of  50°,  composed  of  glass 
having  a  certain  dispersive  power,  and  invert  next  it  a  prism  of  25° 
angle,  composed  of  glass  having  twice  the  dispersive  power  of  the 
former.  Dispersion  will  be  manifestly  destroyed,  because  it  is  equal 
in  amount  and  opposite  in  nature  to  that  possessed  by  the  prism  of 
50°  ;  but  the  prism  with  an  angle  of  25°  will  not  neutralise  all  the 
refraction  effected  by  the  prism  of  50°. 

These  conditions  plainly  suggest  the  solution  of  the  problem,  for 
part  of  the  convergence  is  maintained  while  the  whole  of  the 
dispersion  is  destroyed. 

The  spherical  lenses  which  answer  to  these  prisms  are  a  crown 
biconvex,  fitting  into  a  flint  plano-concave  of  double  the  dispersive 
power 

It  has  been  pointed  out  above  that  all  the  other  colours  lie  in 
their  proper  order  between  the  rays  R  and  Y  (fig.  5).  Let  us  select 
one,  green,  and  represent  it  by  G.  Now  if  G  lies  midway  between 
R  and  Y  in  the  prism  of  50°  of  angle,  and  also  between  R  and  Y  in 
the  prism  of  25°  of  angle,  its  dispersion  will  also  be  neutralised. 
This  means  that  when  the  dispersion  between  the  three  colours  in 


FIG.  20. — Recomposition  of  light  by  prisms.     (From 
tlie  'Forces  of  Nature.') 


ACHROMATIC    OBJECTIVES  19 

one  kind  of  glass  is  proportional  to  their  dispersion  in  the  other, 
then  when  any  two  are  destroyed  the  third  is  destroyed  with  them. 
This  unfortunately  is  not  the  case  in  practice,  because  two  kinds  of 
glass  having  proportional  dispersion  powers  cannot  be  obtained. 
This,  however,  is  what  really  happens.  G  may  lie  midway  between 
R  and  Y  in  one  kind  of  glass,  but  in  the  other  it  may  lie,  for 
instance,  much  nearer  R,  say  a  third  instead  of  half  the  distance 
of  R  from  Y.  If  now  the  dispersion  of  R  Y  be  destroyed,  G  will 
be  left  outstanding.  If  a  different  angle  of  prism  be  chosen,  so  that 
R  and  G  are  neutralised,  then  Y  must  be  left  outstanding. 

This  want  of  proportion  in  the  dispersion  of  the  various  colours 
of  the  spectrum  in  two  kinds  of  glass  is  termed  the  irrationality  of 
the  spectrum,  and  the  colour  or  colours  left  outstanding  in  a  corrected 
combination  of  lenses  is  known  as  the  secondary  spectrum. 

In  some  subsequent  pages  we  shall  have  to  call  attention  to  the 
manufacture  in  Germany  of  some  new  vitreous  compounds  by  the 
combination  of  which  with  fluor  spar  the  secondary  spectrum  has 
been  removed  from  microscope  objectives,  and  an  apochromatic 
system  of  construction  has  been  introduced. 

Meanwhile,  we  may  remember  that  it  has  only  been  in  compa- 
ratively recent  times  that  the  construction  of  achromatic  object- 
classes  for  microscopes  has  been  brought  about,  but  the  gradual 
enlargement  of  aperture  and  the  greater  completeness  of  the  cor- 
rections soon  after  the  discovery  of  achromatism  rendered  sensible 
an  imperfection  in  the  performance  of  these  lenses  under  certain 
circumstances,  which  had  previously  passed  unnoticed,  and  Andrew 
Ross  made  the  important  discovery  that  the  use  of  cover-glass  in 
mounting  minute  objects  introduced  aberration,  and  that  a  very 
obvious  difference  exists  in  the  precision  of  the  image,  according  as 
it  is  viewed  with  or  without  a  covering  of  thin  glass,  an  object- 
glass  which  may  be  perfectly  adapted  to  either  of  these  conditions 
being  sensibly  defective  under  the  other. 

He  also  devised  the  means  of  correcting  this  error,  and  published 
his  device  in  vol.  li.  of  '  Transactions  of  the  Society  of  Arts  '  for  1837. 

Fig.  21  will  illustrate  the  effect  produced  on  the  corrections  of 
an  object-glass  by  the  interposition  of  a  cover-glass  between  the 
object  and  the  objective. 

The  rays  radiating  from  the  object  O  in  every  direction  fall  upon 
the  cover-glass  C  C  (/j  =  1*6).  On  tracing  two  definite  rays,  such 
as  O  A  and  O  B,  it  will  be  found  that  they  will  be  refracted  to  R 
and  P  (shown  by  the  dotted  lines  of  the  figure).  On  their  emergence 
into  air  they  will  be  again  refracted  in  a  direction  parallel  to  their 
first  path,  and  will  enter  the  front  lens  of  the  objective  at  the 
points  M  and  N. 

Now  as  M  R  and  N  P,  produced,  meet  in  Y,  it  follows  that,  so 
far  as  the  objective  is  concerned,  the  rays  M  R,  N  P  might  have 
diverged  from  the  point  Y. 

Similarly,  by  tracing  two  of  the  less  divergent  rays  from  O  they 
will  be  made  by  the  refraction  of  the  cover-glass  to  appear  as  if 
they  diverged  from  X.     Therefore,  in  consequence  of  the  cover-glass 
the  objective  has  to  deal  with  rays  radiating  apparently  from  two  dis- 
ci 


20      ELEMENTARY   PRINCIPLES   OF   MICROSCOPICAL   OPTICS 


tinct  points,  X  and  Y.  If  there  were  no  cover-glass  all  the  rays  would 
diverge  from  O,  and  then  the  objective  would  require  to  be  perfectly 
aplanatic.  This  word  (derived  from  a  =  privative,  and  irAapaw,  to 
\vander,  i.e.  free  from  wandering  or  error)  means,  as  used  by  opticians, 


FIG.  21. — The  effect  produced- by  a  cover-glass  on  the  corrections  of  an 
object-glass. 

that  all  the  rays  passing  through  a  lens  system  are  brought  to  an  identi- 
cal conjugate  focus,  as  shown  in  fig.  22.  But  as  affected  by  the  cover- 
glass  the  marginal  rays  diverge,  apparently,  from  a  focus,  nearer  tJie 
objective  than  the  central  rays  ;  therefore  the  objective,  to  meet  this 
condition,  must  be  what  is  called  under -corrected  ;  a  condition  pre- 
sented in  fig.  23,  so  as  to  focus  both  these  points  at  once.  Here  the 


FIG.  22. — Aplanatic  system. 


FIG.  23.— Under-corrected  system. 


curvature  of  the  surface  of  the  crown  lens  being  increased,  the  Hint 
plano-concave  is  not  sufficiently  powerful  to  neutralise  all  the 
spherical  aberration  of  the  crown.  As  a  consequence  the  peripheral 
rays  are  brought  to  a  focus  at  F,  while  the  central  rays  pass  on  to 
F.  This  is  what  is  meant  by  *  under-correction '  in  an  object-glass. 

In  fig.  24  the  reverse  condition 
is  presented,  for  the  incident  curve 
of  the  crown  lens  has  been  flattened, 
while  that  of  the  flint  has  been 
deepened,  which  increases  the  cor- 
rective power  of  the  flint,  and  thus 
destroys  the  balance  of  the  com- 
bination in  other  directions.  The  rays  passing  through  the  periphery 
of  the  combination  will  be  brought  to  a  focus  F',  while  the  central 
rays  will  be  focussed  at  F.  This  is  what  is  known  as  over -correction. 


FIG.  24.— Over-corrected  system. 


COLLAR   CORRECTION — FOCI   OF   LENSES  21 

An  aplanatlc  objective  can  be  made  into  an  under-corrected 
objective  by  (1)  causing  the  back  lenses  of  ivhich  it  is  composed 
to  approach  the  front  lens.  This  is  the  device  of  Andrew  Ross,  and 
is  now  effected  l  by  means  of  a  special  '  collar '  arrangement,  which, 
by  the  action  of  a  screw,  approximates  or  separates  the  suitable 
lenses.  But  for  this  a  special  device  is  needed  for  each  objective. 
(2)  The  result  can  moreover  be  secured  by  causing  the  eye-piece  to 
approach  the  objective,.  This  of  course  is  accomplished  by  the  use  of 
the  draw-tube,  and  must  be  employed  with  objectives  having  rigid 
mounts. 

Closing  lenses,  that  is,  bringing  them  together,  whether  in  the 
objective  itself  or  in  the  microscope  as  a  whole,  by  shortening  the 
distance  between  the  eye-piece  and  the  objective,  under -corrects  the 
objective,  that  is,  gives  negative  aberration  ;  while  the  separation  of 
lenses  over-corrects  or  gives  positive  aberration. 

In  using  the  collar  correction  l  for  a  longer  body  or  a  thicker 
cover-glass  the  collar  adjustment  must  be  moved  so  as  to  cause  the 
back  lenses  of  the  objective  to  approach  the  front  lens,  while  for  a 
shorter  body  or  a  thinner  cover-glass,  the  adjustment  must  be  moved 
so  as  to  cause  their  separation. 

In  correcting  by  tube  length  for  a  thicker  cover  shorten  the  tube, 
and  for  a  thinner  one  lengthen  it. 

For  the  benefit  of  those  who  aim  at  work  with  lenses,  that  is 
such  as  may  be  compassed  with  the  aid  of  the  most  elementary 
mathematics,  it  may  be  well  to  indicate  a  simple  method  for  the 
deduction  of  the  foci  of  plano-convex  and  biconvex  lenses. 

In  fig.  17  the  focus  is  twice  the  radius  measured  from  the  vertex 
A,  that  is,  A  F.  But  in  fig.  18  it  is  twice  the  radius  measured 
from  the  point  A,  that  is,  the  point 
F  is  distant  from  the  lens  twice  the 
radius  less  two-thirds  the  thickness 
of  the  lens. 

Similarly,  in  fig.  25,  the  focus 
of  a  biconvex  lens  is  measured  from 
the  point  A  ;  in  other  words,  F  is 
distant  from  the  lens  the  length  of 
the  radius  less  one-sixth  the  thick-  FIG.  25.— The  focus  of  a  convex  lens 
ness  of  that  lens  (nearly). 

Formula  relating  to  a  biconvex  lens. — Where  P  is  one  focus,  Pr  its 
conjugate,  F  principal  focus  (solar  focus,  or  that  for  a  very  distant 
object),  R  radius  of  curvature  for  one  surface,  R'  for  the  other 
surface,  p  the  refractive  index  of  the  medium,  then 

1       1 


FIG.  25A. — Focus  of  a  concave  lens. 
1  See  Chapter  V. 


22      ELEMENTARY   PRINCIPLES   OF  MICROSCOPICAL  OPTICS 

Also,  if  x  is  the  distance  of  a  focus  from  F,  the  principal  focus, 
and  y,  the  distance  of  its  conjugate  from  F',  the  other  principal 
focus  on  the  other  side,  then 


or, 


In  an  equiconvex  lens  of  crown  glass  if  /u  =  l'5,  F=  radius  of 
curvature.  But  in  a  plano-convex  lens  of  crown  glass  if  ^  =  1>5, 
F=  twice  the  radius  of  curvature. 

In  the  above  formula  the  thickness  of  the  lens  has  been  neglected. 
In  thick  lenses,  however,  its  effect  must  not  be  disregarded,  even  if 
only  approximate  results  are  required.  A  very  approximate  deter- 
mination of  the  principal  focal  length  of  an  equiconvex  lens  measured 
from  the  surface  may  be  made  by  subtracting  from  the  result 
obtained  by  the  foregoing  formulae  one-sixth  of  the  thickness  of  the 
lens.  (See  fig.  25.) 

Examples.  —  Equiconvex  lens  of  crown  glass  /u=l'5,  r=^,  thick- 
ness=J.  By  above  formula  F=^.  Subtracting  from  this  one- 
sixth  of  the  thickness  of  the  lens,  we  get  F=^  as  the  distance 
between  the  focus  and  the  surface  of  the  lens.  This  is  only  -^l^  inch 
from  the  truth.  If  the  lens  were  a  sphere  it  would  be  necessary  to 
subtract  J  of  its  thickness. 

In  the  case  of  a  plano-convex  lens  the  principal  focus  on  the 
convex  side  is  equal  to  twice  the  radius  as  above,  but  on  the  plane 
side  two-thirds  of  the  thickness  of  the  lens  must  be  subtracted  from 
it. 

In  a  hemispherical  lens  of  crown  gla-ss  ^  =  1'5,  radius  =]>,  thick  - 
ness=^,  the  principal  focus  on  the  convex  side  will  be  one  inch 
from  the  curved  surface  and  on  the  plane  side  §  inch  from  the  plane 
surface. 

In  an  equiconcave  lens  the  foci  are  virtual  and  are  crossed  over  ; 
thus,  the  lens  in  fig.  25  A  is  equiconcave,  the  focus  F,  instead  of  being 
measured  from  A  to  the  right  hand,  must  be  measured  to  the  left 
hand;  consequently,  f  of  the  thickness  must  be  subtracted  from 
the  focal  length  in  order  to  determine  the  distance  of  F  from  the 
surface  of  the  lens. 

A  plano-concave  lens  follows  the  plano-convex,  but  the  foci  are 
virtual  and  crossed  over.  From  the  principal  focus  on  the  curved 
side  subtract  |  of  the  thickness,  and  from  that  on  the  plane  side 
subtract  the  whole  thickness  of  the  lens. 

Examples.  —  Equiconcave  of  dense  flint  /x  =  l'75,  radius  =  —  ^, 
thickness  J,  F  by  formula  =  —  £•  ;  subtract  from  this  }  of  the  thick- 
ness of  the  lens,  we  obtain—  |,'  which  is  only  1-Jir  inch  too  short. 

Plano-concave  of  dense  flint  /*  =  1*75,  radius=  —  J,  thickness  J, 
F  by  formula=  -  f  ,  subtract  from  this  the  thickness  of  the  lens. 
Then  F=  —  -^  ;  this  is  the  focal  distance  from  the  plane  side.  For 
the  focal  distance  from  the  curved  side  subtract  f  of  the  thickness, 
then  F=  —  (;£  ,  which  is  ^¥  inch  too  long. 

The  pi*incipal  focus  of  a  combination  of  two  or  more  lenses,  whose 


THE  FORMATION  OF  A  'REAL  IMAGE'         23 

principal  foci  and  distances  are  known,  can  be  found  from  the  formula 
-  +  ,=  7-  by  assigning  for  the  value  of  p  the  distance  of  the  prin- 
cipal focus  of  the  first  lens  from  the  second,  and  so  on. 

Example. — Parallel  rays  fall  on  an  equiconvex  lens  of  four  inches 
focus.  Two  inches  from  this  lens  is  another  equiconvex  lens  of 
three  inches  focus.  Find  the  distance  of  the  focal  point  from  this 
last  lens,  to  which  the  rays  will  be  brought.  It  is  evident  that  the 
rays  would  be  brought  by  the  first  lens  to  a  focus  two  inches  behind 
the  second  if  it  were  not  there.  This  point,  which  is  negative  with 
regard  to  the  second  lens,  must  be  taken  as  the  value  of  p  in  the 
formula.  We  have,  therefore  : 


-2 


3 


6 


Hitherto  our  attention  has  been  confined,  in  studying  the  action 
of  lenses,  to  the  manner  in  which  they  act  upon  a  bundle  of  parallel 
rays,  or  upon  a  pencil  of  rays  issuing  from  a  radiant  point.  More- 
over, we  have  considered  this  point  as  situated  in  the  line  of  axis. 
But  the  surface  of  every  luminous  body  may  be  regarded  as  compre- 
hending an  infinite  number  of  such  points,  from  every  one  of  which 
a  pencil  of  rays  proceeds,  to  be  refracted  in  its  passage  through  the 
lens  according  to  the  laws  enunciated.  In  this  way  a  complete 
image,  i.e.  picture  of  the  object,  will  be  formed  upon  a  suitable 
surface  placed  in  the  position  of  the  focus. 

There  are  two  kinds  of  image  formed  by  lenses,  a  real  image  and 
a  virtual  image. 

1.  The  formation  of  a   real   image  means  the  production  of  a 


K 


FIG.  26. — The  formation  of  a  real  image. 


picture  by  a  lens,  or  a  combination  of  lenses,  which  can  be  thrown 
upon  a  screen ;  such  are  the  images  of  a  projection  lantern  and  the 
image  produced  by  the  camera  upon  the  focussing  glass.  The  manner 
in  which  this  takes  place  will  be  understood  by  reference  to  fig.  26, 
where  A  B  is  an  object  placed  beyond  P,  the  principal  focus  of  the 
aplanatic  combination.  From  every  point  of  A  B  are  rays  radiating 
at  every  possible  angle.  Let  AF  and  AH  be  two  such  rays 
radiating  from  the  point  A.  Now  if  the  refraction  of  these  rays  be 


24      ELEMENTARY   PRINCIPLES   OF  MICROSCOPICAL   OPTICS 

traced,  in  the  manner  already  indicated,  through  the  aplanatic  com- 
bination, it  will  be  found  that  the  rays  which  before  immergence 
were  diverging  are  by  the  refraction  of  the  combination  on  emer- 
gence rendered  converging.  Thus  the  ray  F  C  meets  H  C  at  the 
point  C.  The  point  C  is  called  the  conjugate  focal  point  of  A,  and 
wherever  there  is  a  focal  point  there  will  be  an  image.  Therefore, 
at  C,  there  will  be  an  image  of  A.  In  the  same  manner  the  rays 
issuing  from  every  point  along  A  B  may  be  traced,  and  will  be  found 
to  have  each  one  its  respective  conjugate  lying  on  C  D,  so  the  con- 
jugate of  B  is  at  D.  Hence  it  is  at  once  manifest  that  an  inverted 
conjugate  image  of  the  object  A  B  is  formed  at  C  D.  Further,  it 
will  be  noticed  that,  although  the  object  is  straight,  the  image  of  it 
is  curved  towards  the  lens. 

If  the  object  A  B  had  been  curved,  so  that  it  presented  a  convex 
aspect  to  the  lens,  then  its  conjugate  image  CD  would  have  been 
more  curved  ;  but  if  A  B  had  been  slightly  concave  towards  the  lens, 
then  its  conjugate  would  have  been  straight. 

As  before  stated,  the  point  C  has  been  determined  by  tracing 
the  refraction  of  two  rays,1  A  F  and  A  H,  through  the  lens.  Another 
method  is,  however,  often  employed. 

In  every  lens  there  is  a  point  which  is  called  its  optical  centre. 
This  point  is  such  that  any  ray,  wThich  in  its  refraction  through  the 
lens  passes  through  this  point,  will  emerge  in  a  direction  parallel  to 
its  path  before  immergence.  Now  as  lenses  for  graphic  and  theoreti- 
cal purposes  are  often  assumed  to  be  of  insensible  thickness,  it  has 
become  the  practice  to  draw  any  ray  passing  through  the  optical 
centre  of  the  lens  a  straight  line.  Obviously,  if  the  lens  has  sensible 
thickness  the  ray  cannot  be  considered  a  straight  line,  and  in  the 
microscope,  where  the  lenses  are  very  thick  in  proportion  to  the 
length  of  their  foci,  this  method  will  lead  to  much  error.  Of  course, 
in  those  cases  where  it  can  be  taken  as  a  straight  line,  it  saves  the 
trouble  of  computing  a  second  ray  to  intersect  the  first,  as  any  ray 
intersecting  the  straight  line  will  determine  a  conjugate  focal  point. 

In  the  upper  part  of  fig.  26  the  two  rays,  A  F  and  A  H,  are 
traced  through  the  lens  to  determine  the  point  C,  but  in  the  lower 
part  of  the  figure  only  the  ray  B  K  is  traced,  and  the  intersection  of 
this  ray  by  the  straight  line  B  D  passing  through  the  optical  centre 
gives  the  point  D. 

2.  An  image  is  said  to  be  virtual  when  it  cannot  be  received  on 
a  screen.  Fig.  27  shows  how  a  virtual  image  is  formed.  The 
letters  are  the  same  as  in  the  preceding  figure,  so  as  to  show  the 
analogy  between  the  two.  The  fundamental  difference  between 
this  figure  and  the  last  is  that  the  object  A  B  is  placed  between  P,  the 
principal  focus,  and  the  lens. 

We  have  already  seen  from  fig.  15  that  when  a  radiant  is  placed 
before  a  converging  lens,  and  nearer  to  it  than  its  principal  focus, 
the  rays  emerging  from  the  lens  are  still  divergent  even  after  their 
refraction  through  the  lens ;  consequently  they  will  never  intersect, 

1  In  the  majority  of  the  preceding  diagrams  the  drawing  has  represented  the 
facts  accurately  ;  in  this  instance  they  are  diagrammatic,  the  size  of  admissible  illus- 
trations making  an  accurately  traced  ray  impossible. 


FORMATION   OF   A    :  VIRTUAL  IMAGE'  25 

and  as  there  is  no  focal  point,  there  can  be  no  screen  image. 
Thus  two  rays  radiating  from  the  point  A  of  the  object  A  B  fall  011 
the  lens  and  are  refracted  in  the  directions  A  F,  AH:  these  are 
divergent  and  will  never  meet ;  but  if  the  human  eye  is  placed  near 
the  lens,  so  that  it  can  receive  the  rays  F  and  H,  the  rays  will  be 
converged  by  the  lens  of  the  eye,  and  will  be  brought  to  a  focal 
point  in  the  retina. 

Similarly,  from  every  point  in  A  B  there  will  be  a  corresponding 
retinal  point.  Now  if  we  produce  F  and  H  backwards  (see  the 
dotted  lines  in  the  figure)  we  shall  .find  that  they  intersect  at  the  point 
C.  As  the  rays  F  and  H  are  precisely  identical  with  rays  which 
would  have  diverged  from  the  point  C  had  it  been  an  entity,  the 
retinal  image  therefore  will  be  an  image  of  a  non-existent  picture 
CD. 

The  method  of  drawing  this  is  exactly  similar  to  that  of  the 


FIG.  27. — The  formation  of  a  '  virtual  image. 

preceding  figure.  The  rays  A  F  and  A  H  are  traced  through  the 
lens,  and  their  prolongation  backwards  (see  the  dotted  lines  in  the 
figure)  gives  the  point  C.  Also,  as  in  the  preceding  figure,  any 
point  of  the  picture  can  be  found  by  tracing  one  ray,  such  as  K ; 
then  the  intersection  of  its  backward  prolongation  with  a  straight 
line  joining  B  with  the  optical  centre,  produced,  will  give  D. 

The  points  C  and  D  are  called  the  virtual  conjugate  foci  of  A 
and  B  respectively.  In  mathematical  optics  it  appears  as  a  negative 
quantity  which  satisfies  an  equation,  and  is  a  sort  of  metaphysico- 
mathematical  truth.  In  this  case  the  virtual  image  is  convex 
towards  the  lens. 

Fig.  27  illustrates  the  action  of  a  simple  microscope.  The  object 
itself  is  not  seen,  but  the  picture  presented  to  the  eye  is  an 
enlarged  ghost  of  it.  As  some  eyes  can  take  in  rays  of  less  diverg- 
ence than  others,  it  might  happen  that  the  rays  C  F,  C  H,  were  too 
divergent  for  the  observer's  eyesight,  in  which  case  the  lens  would 


26      ELEMENTARY   PRINCIPLES   OF   MICROSCOPICAL   OPTICS 

have  to  be  withdrawn  from  the  object.  Similarly,  if  the  observer 
were  short-sighted,  the  lens  must  be  placed  nearer  the  object  to 
render  the  rays  more  divergent.  Dr.  Abbe  points  out 1  that  the 
generally  adopted  notion  of  a  'linear  amplification  at  a  certain 
distance '  is,  in  fact,  a  very  awkward  and  irrational  way  of  defining 
the  '  amplifying  power  '  of  a  lens  or  a  lens-system. 

In  the   formula  N=       the  amplification  of  one  and  the  same 

system  varies  with  the  length  of  /,  or  the  l  distance  of  vision,'  and 
an  arbitrary  conventional  value  of  I  (i.e.  10  inches,  or  250  mm.) 
must  be  introduced  in  order  to  obtain  comparable  figures.  The 
actual  '  linear  amplification '  of  a  system  is,  of  course,  different  in 


FIG.  28. — The  amplifying  power  of  a  lens. 

the  case  of  a  short-sighted  eye,  which  projects  'the  image  at  a  dis- 
tance of  100  mm.,  and  a  long-sighted  one,  which  projects  it  at 
1000  mm.  Nevertheless,  the  '  amplifying  power  '  of  every  system  is 
always  the  same  for  both,  because  t/ie  short-sighted  and  the  long-sighted 
observers  obtain  the  image  of  the  same  object  under  the  same  visual 
angle,  and  consequently  the  same  real  diameter  of  the  retinal  image. 
That  this  is  so  will  be  seen  from  fig.  28,  where  the  thick  lines  show 
the  course  of  the  rays  for  a  short-sighted  eye,  and  the  thin  lines  for 
a  long-sighted  one,  the  eye  in  each  case  being  supposed  at  the  pos- 
terior principal  focus  of  the  system. 

The  other  generally  adopted  expression  of  the  power  by  N  =  — 

may  be  put  on  a  somewhat  more  rational  basis  than  is  generally 
done  by  defining  the  length  I  (10  inches)  not  as  *  distance  of  distinct 
vision,'  but  rather  as  '  distance  of  projection  of  the  image.'  As  far 
as  ;  distinct  vision '  is  assumed  for  determining  the  amplification, 
the  value  of  N  has  no  real  signification  at  all  in  regard  to  an  observer 

1  Journ.  B.M.S.  vol.  iv.  ser.  ii.  p.  348. 


AMICI'S   USE    OF    'IMMERSION'   LENSES  2/ 

who  obtains  distinct  vision  at  50  inches  instead  of  10  inches,  and,  in 
fact,  many  microscopists  declare  the  ordinary  figures  of  amplification 
to  be  useless  for  them  because  they  cannot  observe  the  image  at  the 
supposed  distance.  It  appears  as  if — and  many  have  this  opinion — 
the  performance  of  the  microscope  in  regard  to  magnification 
depended  essentially  on  the  accommodation  of  the  observer's  eye. 
This  misleading  idea,  resulting  from  the  common  expression,  is 
eliminated  by  defining  the  10  inches  merely  as  the  distance  from  the 
eye  at  which  the  image  is  measured — whether  it  be  a  distinct  or  an 
indistinct  image.  For,  if  an  obsef  ver,  owing  to  the  accommodation 
of  his  eye,  obtains  a  distinct  image  at  a  distance  of  10  feet,  I  may 
nevertheless  assume  a  plane  at  a  distance  of  10  inches  from  the  eye 
on  which  the  distant  image  is  virtually  projected,  and  measure  the 
diameter  of  that  projection.  Now  this  diameter  is  strictly  the  same 
as  the  diameter  of  that  image,  which  another  observer  would 
really  obtain  with  distinct  vision  at  that  same  distance  of  10 
inches. 

The  only  difference  is  that  in  the  former  case  we  must  take  the 
centres  of  the  circles  of  indistinctness  instead  of  the  sharp  image- 
points  in  the  latter  case.  If  the  conventional  length  of  £=10  inches 
is  interpreted  in  this  way  (as  distance  of  projection,  independently 
of  distinct  vision)  the  absurdity  at  least  of  a  real  influence  of  the 
accommodation  on  the  power  of  a  microscope  is  avoided.  It  becomes 
obvious  that  for  long-sighted  and  for  short-sighted  eyes  the  same  N 
must  indicate  the  same  visual  angle  of  the  enlarged  objects,  or  the 
same  magnitude  of  the  retinal  image,  because  it  indicates  the  same 
diameter  of  the  projection  at  10  inches  distance. 

It  was  long  since  pointed  out  by  Amici,  that  the  introduction  of 
a  drop  of  water  between  the  front  surface  of  the  objective,  and 
either  the  object  itself  or  its  covering  glass,  would  diminish  the  loss 
of  light  resulting  from  the  passage  of  the  rays  from  the  object  or  its 
covering  glass  into  air,  and  then  from  air  into  the  object-glass. 
This,  which  is  known  as  '  water  immersion,'  was,  however,  first  sug- 
gested by  Sir  D.  Brewster  in  1813.  But  it  is  obvious  that  when  the 
rays  enter  the  object-glass  from  water  instead  of  from  air,  both  its 
refractive  and  its  dispersive  action  will  be  greatly  changed,  so  as  to 
need  an  important  constructive  modification  to  suit  the  new  condi- 
tion. This  modification  seems  never  to  have  been  successfully 
effected  by  Amici  himself;  and  his  idea  remained  unfruitful  until  it 
was  taken  up  by  Hartnack,  who  showed  that  the  application  of  what 
is  now  known  as  the  immersion  system  to  objectives  of  high  power 
and  large  aperture  is  attended  with  many  advantages  not  otherwise 
attainable.  For,  as  already  pointed  out,  the  loss  of  light  increases 
with  the  obliquity  of  the  incident  rays  ;  so  that  when  objectives  of 
very  wide  aperture  are  used  '  dry,'  the  advantages  of  its  increase  are 
in  great  degree  nullified  by  the  reflection  of  a  large  proportion  of 
the  rays  falling  very  obliquely  upon  the  peripheral  portion  of  the 
front  lens.  When,  on  the  other  hand,  rays  of  the  same  obliquity 
enter  the  peripheral  portion  of  the  lens  from  water,  the  loss  by  re- 
flection is  greatly  reduced,  and  the  benefit  derivable  from  the  large 
aperture  is  proportionately  augmented.  Again,  the  'immersion 


28      ELEMENTARY   PRINCIPLES   OF   MICROSCOPICAL   OPTICS 

system  '  allows  of  a  greater  working  distance  between  the  objective 
and  the  object  than  is  otherwise  attainable  with  the  same  extent  of 
aperture  ;  and  this  is  a  great  advantage  in  manipulation.  Further, 
the  observer  is  rendered  less  dependent  upon  the  exactness  in  the 
correction  for  the  thickness  of  the  covering  glass,  which  is  needed 
where  objectives  of  large  aperture  are  used  '  dry ; '  for  as  the 
amount  of '  negative  aberration  '  is  far  smaller  when  the  rays  which 
emerge  from  the  covering  glass  pass  into  water  than  when  they  pass 
into  air,  variations  in  its  thickness  produce  a  much  less  disturbing 
effect.  And  it  is  found  practically  that  '  immersion '  objectives 
can  be  constructed  with  magnifying  powers  sufficiently  high,  and 
apertures  sufficiently  large,  for  the  majority  of  the  ordinary  pur- 
poses of  scientific  investigation,  without  any  necessity  for  cover-ad- 
justment ;  being  originally  adapted  to  give  the  best  results  with  a 
covering  glass  of  suitable  thinness,  and  small  departures  from  this 
in  either  direction  occasioning  comparatively  little  deterioration  in 
their  performance.  But  beyond  all  these  reasons  for  the  superiority 
of  the  '  immersion  system '  is,  as  will  be  presently  seen,  the  fact  that 
it  admits  into  the  lens  a  larger  number  of  '  diffraction  spectra  '  than 
can  be  possibly  admitted  by  a  lens  working  in  air  ;  and  upon  this 
depends  the  perfect  presentation  of  the  image. 

The  immersion  system  has  still  more  recently  been  advanced  upon 
by  the  application  of  a  principle  which  lies  at  the  root  of  the  optical 
interpretation  of  the  images  which  modern  lenses  present,  and 
which  has  greatly  increased  the  value  of  the  microscope  as  a  scientific 
instrument.  It  is  an  improvement  that  primarily  depends  upon  a 
correct  theoretical  understanding  of  the  principles  of  the  construction 
of  microscopical  lenses,  and  the  interpretation  of  the  manner  in 
which  the  image  is  realised  by  the  observer.  The  late  Mr.  Tolles 
was  the  first  to  adopt  this  system,  as  we  point  out  subsequently  ; 
but  it  is  to  Professor  Abbe  we  are  indebted  for  its  practical  appli- 
cation, through  whom  it  is  now  known  as  the  homogeneous  system. 
The  word  '  homogeneous '  was,  however,  first  applied  to  microscope 
lenses  by  Tolles  (1871),  as  may  be  seen  in  the  following  passage.  .  .  . 
'  two  hemispherical  lenses  balsam-cemented,  with  a  diatom  or  other 
small  object  at  the  centre,  together  constituting  a  nearly  homo- 
geneous transparent  globe'  (M.  M.  J.,  vol.  vi.  p.  214).  'The  idea 
of  realising  the  various  advantages  of  such  '  a  system  by  constructing 
a  certain  class  of  homogeneous  objectives  had,  Professor  Abbe 
says,  l  'for  sometime  presented  itself  to  his  mind.'  'The  matter 
assumed,  however,  subsequently,  a  different  shape  in  consequence 
of  a  suggestion  made  by  Mr.  John  Ware  Stephenson,  ...  of 
London,  who  independently  discovered  the  principle  of  homogeneous 
immersion.'  2 

This  method  consists  of  the  replacement  of  water  between  the 
covering  glass  of  the  mounted  object  and  the  front  surface  of  the 
object-glass  by  a  liquid  having  the  same  refractive  and  dispersive 
power  as  crown  glass.  With  such  a  fluid  taking  the  place  of  air,  it 

1  On  '  Stephenson's  System  of  Homogenous  Immersion  for  Microscopic  Objec- 
tives'  (Abbe),  Jo-urn.  R.M.S.  vol.  ii.  1879,  p.  257. 

2  Ibid. 


ABBE'S  CONSTRUCTION  OF  FIRST  HOMOGENEOUS  OBJECTIVE    29 

follows  that  the  correction  collar,  though  still  a  refinement  and  aid 
in  the  attainment  of  the  finest  critical  images,  would  be  a  necessity 
no  more. 

The  desirability  of  the  construction  of  a  combination  of  lenses 
which  would  satisfy  these  conditions  was  urged  by  Mr.  Stephenson 
upon  Professor  Abbe,  and  he  secured  the  profound  knowledge, 
which,  as  a  mathematical  optician  he  possessed,  for  the  complete  and 
practical  solution  of  the  problems  involved,  and  the  production  of  a 
remarkable  series  of  lenses,  marking  a  distinct  epoch  in  the  progress 
of  theoretical  and  practical  optics. 

He  had,  in  fact,  as  we  have  hinted,  already  approached  the  con- 
sideration of  the  subject  from  another  point  of  view,  believing  that 
petrographic  work — the  study  of  thin  sections  of  mineral  substances — 
could  be  far  more  efficiently  accomplished  by  the  use  of  homo- 
geneous lenses.  But  in  the  new  aspect  in  which  the  problem  was 
presented  by  Mr.  Stephenson  it  carried  with  it  new  interest  to  Abbe, 
not  only  as  promising  to  largely  dispense  with  the  '  correction  collar,' 
but  also  io  greatly  enlarge  the  '  numerical  aperture,'  and  therefore 
secure  a  greater  resolving  power  in  the  objective. 

One  of  the  difficulties  was  to  find  a  suitable  fluid  to  meet  the 
necessities  as  to  refraction  and  dispersion.  But  after  a  long  series 
of  experiments  Professor  Abbe  found  that  oil  of  cedar  wood  so 
nearly  corresponds  with  crown  glass  in  these  respects  that  it  served 
the  purpose  w^ell. 

The  result  of  Abbe's  calculations  based  on  Mr.  Stephenson's  sug- 
gestion was  the  construction  by  Carl  Zeiss  of  a  TVth  with  a  N.A.1 
of  1*25  of  fine  quality,  and  still  higher  promise,  and  subsequently 
of  a  ^th  and  a  TVth  in.  objective  of  a  like  character. 

It  may  be  well  to  note  that  Amici  suggested  the  use  of  oil 
instead  of  water  prior  to  1850,  and  Mr.  Wenham  again  revived 
the  suggestion  in  1870.2  But  neither  of  these  is  in  even  a  remote 
sense  an  anticipation  of  the  '  homogeneous  system  '  of  lenses  as  we 
now  understand  it.  The  'oil  immersion'  in  both  instances  was  an 
expedient.  The  principle  011  which  the  construction  carried  out  by 
Professor  Abbe  depended  was  the  '  optical '  principle  that  a  medium  of 
high  refractive  power  gives  an  aperture  greatly  in  excess  of  the 
maximum  (180°)  of  a  dry  lens  ;  while  Abbe's  explanation,  propounded 
in  1874,  of  the  important  bearing  which  the  diffraction  pencils  have 
on  the  formation  of  the  microscopic  image  makes  the  resolving 
power  of  the  object-glass  dependent  upon  the  diffraction  pencils  that 
are  taken  up  by  it. 

All  this  was  unknown  or  unadmitted  by  those  who  had  previously 
suggested  oil  as  an  immersion  medium,  which  leaves  the  homogeneous 
system  as  now  employed  wholly  dependent  upon  the  principles 
enunciated  by  Abbe,  arising  from  the  practical  suggestion  of  Stephen- 
son  and  resulting  in  the  beautiful  object-glasses  of  Abbe  and  Zeiss, 
although  it  is  best  just  to  remember  that  Tolles  always  maintained 
that  his  immersion  objectives  had  a  greater  aperture  than  180°  air 

1  The  meaning  of  this  expression  will  be  found  on  p.  49,  but  the  whole  of  Chap.  II. 
must  be  carefully  read. 

2  Monthly  Micro.  Jonrn.  vol.  iii.  p.  303. 


30      ELEMENTARY  PRINCIPLES   OF'  MICROSCOPICAL   OPTICS 

angle.  Dr.  Royston-Pigott  constructed  the  first  aperture  table 
giving  the  relative  values  of  dry,  water,  and  homogeneous  (nascent 
pencil)  immersion  objectives  ;  it  is  given  in  M.  M.  J.,  vol.  iv.  p.  26, 
(1870). 

One  of  the  essential  advantages  of  this  system,  beyond  those 
stated,  is  that  by  the  suppression  of  spherical  aberration  in  front  of 
the  objective,  facilities  are  afforded  for  correcting  objectives  of  great 
numerical  aperture,  both  in  theory  and  practice,  that  reduce  it  to 
the  level  of  the  problem  of  correcting  objectives  of  moderate  '  angle.' 
As  a  result,  stimulated  by  the  manifest  advantage  to  be  obtained 
and  the  wants  of  those  engaged  in  actual  research,  Messrs.  Powell  & 
Lealand,  of  London,  very  soon  made  a  .r-th  incft  and  a  sV*n  inc^ 
objective  on  the  homogeneous  principle,  with  numerical  apertures 
respectively  of  1*38,  and  during  the  year  1885  produced  lenses  of  an 
excellence  impossible  to  any  previous  system  of  -^th  inch,  TVfch  inch, 
and  -gVkh  incn  power?  having  respectively  numerical  apertures  of 
1-50,  while  1'52  is  the  theoretical  maximum. 

The  use  of  a  '  correction  collar  '  in  homogeneous  object-glassee 
has  been  dispensed  with,  correction  being  obtained  by  alteration  of 
the  tube  length  solely,  but  this  must  also  be  aided  in  endeavour- 
ing to  secure  the  most  perfect  '  critical  images  '  by  a  body-tube  pro- 
vided with  rack  and  pinion  motion ;  this  should  be  of  the  best 
quality,  and  if  the  object-glass  is  of  perfect  construction  and  of  latest 
form  (apochromatic,  q.v.),  results  never  before  attainable  can  be  got 
with  comparative  ease. 

With  such  evidence  of  advance  in  the  optical  construction  of 
microscopes,  dependent  apparently  on  such  accessible  conditions,  the 
question  of  what  is  possible  in  the  future  of  the  instrument  no  doubt 
obtrudes  itself;  that,  however,  can  only  be  considered  as  having 
application  to  the  area  of  our  present  knowledge  and  resources.  It 
is  impossible  to  forecast  the  future  agencies  which  may  be  at  the 
disposal  of  the  practical  optician.  To  photograph  stars  in  the  im- 
measurable amplitudes  of  space,  absolutely  invisible  to  the  human 
eye,  however  aided,  was  hardly  within  the  purview  of  the  astronomers 
of  a  quarter  of  a  century  ago  ;  that  there  may  be  energies  and 
methods  discoverable  by  man  that  will  open  up  possibilities  to  the 
eager  student  of  the  minute  in  nature  which  will  just  as  widely 
overstep  our  present  methods  of  optical  demonstration,  there  can  be 
little  reason  to  question.  But  it  is  no  doubt  true  that  with  the  in- 
struments and  media  now  at  the  disposal  of  the  practical  optician 
no  indefinite  and  startling  advance  in  microscopic  optics  is  to  be 
looked  for.  The  *  atom  '  is  infinitely  inaccessible  with  any  conceiv- 
able application  of  all  the  resources  within  our  reach.  But  optical 
improvement  of  great  value,  bringing  nature  more  and  more  nearly 
and  accurately  within  our  ken  and  reducing  more  and  more  certainly 
the  interpretation  of  the  most  difficult  textures  and  constructions 
in  the  minutest  accessible  tissue  to  an  exact  method,  is  certainly 
within  our  sight  and  reach.  It  is  not  a  small  matter  that  the  homo- 
geneous lenses  were,  in  a  comparatively  short  period  of  time,  carried 
from  a  N.A.  of  1'25  to  1'50  ;  and  this  carried  with  it  the  capacity 
theoretically  indicated. 


NEW   VITBEOUS   OPTICAL   COMPOUNDS  31 

High  refractive  media  can  greatly  reduce  the  value  of  even  the 
wave-length  of  light,  and  what  is  possible  in  the  production  of  vitreous 
combinations,  refractive  fluid  media,  and  mounting  substances  we 
may  not  forecast ;  but,  judging  from  the  past,  we  have  by  no  means 
reached  their  limit.  At  the  same  time,  it  may  be  remembered  that 
photo-micrography,  by  constantly  covering  a  wider  area  of  applica- 
tion with  its  ever  increasingly  delicate  and  subtle  methods,  is 
more  penetrating  in  the  revelation  of  structure  than  the  human 
eye. 

It  may  be  taken  for  granted  that- in  the  present  state  of  optical 
mathematics  the  opticians,  English,  Continental,  and  American, 
have  given  up  the  quest  of  many  things  fruitlessly  sought.  Empty 
amplification  is  a  folly  of  lenses  of  the  past.  Magnification  without 
concurrent  disclosure  of  detail  is  of  no  more  scientific  value  for  the 
.disclosure  of  structure  than  the  projection  of  the  photo-micrograph 
by  an  electric  arc  upon  a  screen  would  be.  What  is  needed  is  an 
ever-increasing  exactitude  in  the  formation  of  the  dioptrical  image. 
The  imperfection  of  this  at  the  focal  point  springs  from  two  causes  : 
one,  as  we  have  just  demonstrated,  arises  from  the  residual  spherical 
and  chromatic  aberrations,  the  other  takes  origin  in  the  want  of  homo- 
geneity, absolute  precision  of  curve,  and  perfect  centering  of  the  system 
of  lenses  in  a  combination.  This  causes  the  cone  of  rays  proceeding 
from  the  object  to  unite,  not  in  perfect  image  points,  but  in  *  light 
surfaces  of  greater  or  less  extent — circles  of  dissipation  ' — which 
limits  the  distinctness  of  minute  details.  It  is  the  faults  of  the  ob- 
jective that  in  practice  are  alone  important,  and  with  the  crown  and 
flint  glass  commonly  at  the  disposal  of  the  optician  there  are  two 
great  drawbacks  to  perfection,  or  rather  to  an  approximation  to  it. 

1.  The  first  arises  from  the  unequal  course  of  the  dispersion  in 
crown  and  flint  glass,  already  described,  which  makes  it  impossible 
to  unite  perfectly,  with  the  properties  they  possess,  all  the  coloicred 
rays  in  an  image.     Absolute  achromatism  cannot  by  their  means  be 
attained,  the  dispersion  at  different  parts  of  the  spectrum  being  so 
greatly  disproportional.     It  has  never  been  possible  to  unite  more 
than  two  different  colours  of  the  spectrum.     The  rest,  in  spite  of  all 
effort,  deviate  and  form  the  secondary  spectrum,  leaving,  in  the  very 
finest  lenses,  circles  of  dispersion  not  to  be  excluded. 

2.  The  second  defect  arises  in  the  impossibility  of  correcting  by 
means  of  ordinary  crown  and  flint  glass  the  spherical  aberration  for 
more  than  one  colour.     If  the  spherical  aberration  be  removed  as 
far  as  may  be  for  the  centre  of  the  spectrum,  there  remains  under- 
correction  for  the  red,  and  over-correction  for  the  blue  and  violet 
rays,  presenting  a  want  of  balance  between  the  chromatic  corrections 
for    the  central   and    marginal    zones  of  the  objective.     Although 
perfect  chromatic  corrections  for  the  central  rays  may  be  effected, 
giving  images  of  great  beauty,  the  chromatic  over-correction  for  the 
peripheral  rays  with  oblique  illumination  will  show  the  borders  of 
the  image  with  distinct  chromatic  fringes. 

To  compensate  these  aberrations  in  the  construction  of  an  object- 
glass,  what  is  needed  is  a  vitreous  material  applicable  to  optical 
purposes  possessed  of  such  properties  that  a  relatively  smaller  re- 


32      ELEMENTARY   PRINCIPLES   OF  MICROSCOPICAL   OPTICS 

fractive  index  could  be  united  with  a.  higher  dispersive  power,  or  a 
higher  refractive  index  with  a  relatively  lower  dispersive  power. 
By  proper  combination  of  such  materials,  if  they  be  provided  with 
ordinary  crown  and  flint  glass  to  partly  remove  the  chromatic  and 
spherical  aberrations  independently  of  each  other,  and  so  to  obey 
the  conditions  on  which  the  removal  of  the  chromatic  difference 
depends,  these  aberrations  could  be  compensated. 

All  this  was  seen  and  fully  demonstrated  and  set  forth  by  Abbe 
as  far  back  as  1876,1  and  he  pointed  out  that  the  further  perfecting 
of  the  microscope  in  its  dioptrical  working  wras  dependent  on  the 
art  of  glass  making ;  the  production,  that  is  to  say,  of  vitreous 
compounds  possessing  different  relations  of  refractive  and  disper- 
sive power  by  [means  of  which  the  secondary  spectrum  could  be 
removed. 

For  practical  purposes  the  matter  was  in  abeyance  until  1881, 
but  since  that  time  Dr.  Schott  and  Professor  Abbe,  with  the  active 
co-operation  of  the  optical  workshops  of  Zeiss,  undertook  the 
laborious  and  prolonged  investigation  into  the  improvement  of 
optical  glass,  to  which  we  have  alluded  ;  the  result  has  been  the 
production  of  '  crown '  and  *  flint '  glass  possessing  exactly  the 
qualities  foreshown  as  indispensable  by  Abbe. 

By  chemical,  physical,  and  optical  research  of  a  most  laborious 
nature,  and  by  spectrometric  observations  of  numerous  experimental 
fusions  systematically  carried  out  with  a  large  variety  of  chemical 
elements,  the  relation  between  the  vitreous  products  and  their 
chemical  composition  has  been  more  closely  investigated. 

In  the  crown  and  flint  glass  produced  up  to  the  time  of  these 
investigations,  the  uniformity  of  property  arose  from  the  relatively 
small  number  of  materials  employed.  Aluminium  and  thallium, 
with  silica,  alkali,  lime,  and  lead,  formed  the  limit.  By  the  use  of 
more  chemical  elements,  especially  phosphoric  and  boric  acid  as  the 
essential  constituents  of  glass  fluxes  in  the  place  of  silica  alone,  flint 
and  crown  glass  have  been  produced  in  which  the  dispersion  in  the 
different  parts  of  the  spectrum  is  nearly  proportional ;  so  that  in 
achromatic  combinations  it  is  now  a  question  of  detail  and  practical 
optics  to  eliminate  almost  entirely  the  secondary  spectrum.  It  is 
unfortunate,  nevertheless,  that  a  large  number  of  these  glasses, 
especially  those  of  most  value  to  the  optician,  have  proved  to  be  so 
unstable  in  their  composition  that  opticians  refrain  from  using  them. 
It  may  be  hoped  that  further  experiment  and  research  will  greatly 
reduce  this  defect.  On  the  other  hand,  the  kinds  of  glass  which 
can  be  used  for  optical  purposes  have  been  so  increased  in  variety 
that,  while  the  mean  index  of  refraction  is  constant,  considerable 
variations  can  be  given  to  the  dispersion  or  to  the  refractive  index 
while  the  dispersion  remains  constant.  A  high  index  of  refraction 
is  no  longer  of  necessity  accompanied  by  a  high  dispersion  in 
flint  glass,  but  may  be  retained  in  crown  glass  with  a  low  degree 
of  dispersion. 

The  practical  consequence  of  this  is  that  both  the  imperfections 

1  Hoffman,  A.  W.,  Bericlit  uber  die  wissensclia ft lichen  Apparate  anf  der  Lon- 
doner Internationalen  AussteUnng  im  Jalire  1876. 


ADVANTAGES   OF   APOCHEOMATIC    OBJECT-GLASSES         33 

inalienable  from  an  objective  constructed  of  ordinary  crown  and 
flint  glass,  can  be,  and  have  been,  eliminated,  and  the  secondary 
.spectrum  annulled  ;  it  is  removed  and  reduced  to  a  residue  of 
chromatism  of  a  tertiary  character,  while  the  chromatic  difference 
of  spherical  aberration  can  be  eliminated  or  completely  corrected 
for  two  different  colours  of  the  spectrum  at  once,  and  therefore 
practically  for  all. 

In  the  lenses  formed  of  the  crown  and  flint  glass  as  used  prior 
to  the  new  German  glass,  we  were  .provided  with  what  (in  com- 
parison with  non-achromatised  lenses)  were  called  '  achromatic ; ' 
but  in  the  new  system  of  lenses,  which  may  be  '  dry '  or  '  homogeneous,' 
we  have  so  great  a  freedom  from  colour  defect  as  to  admit  of  their 
being  designated  apochromatic  lenses  (a=privative  ;  gjHtyifi&seoloiir  ; 
a7ro=from,  away  from  ;  xpc!>/ia=colour). 

The  practical  advantages  obtained  by  this  system  of  object-glass 
construction  are  so  great  as  in  delicate  researches  to  be  invaluable — 
provided  always  that  the  work  in  all  its  details  is  of  the  most  perfect 
kind.  The  accidental  juxtaposition  of  lenses  of  the  required  curves, 
and,  relatively,  even  the  careful  selection  of  lenses  not  homogeneously 
related  to  each  other  by  a  unity  of  purpose  and  work  on  the  part  of 
the  practical  optician,  cannot  yield  perfect  results.  '  Division  of 
labour  '  is  not  compatible  with  perfect  results  in  the  making  and 
building  up  of  an  apochromatic  lens;  and  therefore,  in  their  best 
form,  these  objectives  must  apparently  command  a  high  price.  But, 
given  such  an  object-glass — which  is  the  production  of  a  thoroughly 
competent  practical  optician — and  its  advantages,  theoretical  and 
practical,  are  great. 

1 .  The  aperture  of  the  objective  can  be  utilised  to  its  full  extent. 
In  the  best  of  the  older  object-glasses   at  least  one-tenth   of  the 
available  aperture  was  useless  ;  the  inalienable  defect  in  the  con- 
vergence of  the  rays  prevented  a  proper   combined  action  of  the 
outermost  zone  and  the  central  parts  of  the  aperture,  and  therefore 
by  those  objectives  it  has  never  been  possible  to  realise  the  amount l 
of  resolving  power  indicated  by  theory  with  a  given  aperture.     But 
in  a  well -constructed  apochromatic  objective — the  secondary  spec- 
trum being  removed,  and  the  spherical  aberration  being  uniformly 
corrected  for  different  parts  of  the  spectrum — there  is  a  practically 
perfect  focal  concentration  of  the  rays  in  the  image. 

2.  Increase  of  magnifying  power  by  means  of  specially  constructed 
eye-pieces  is  also  a  most  important  feature  of  objectives  of  this  class. 
The  result  of  this  is  that  great  magnifying  power  can  be  obtained 
by  objectives  of  relatively  large  focal   lengths.     We    have  always 
maintained  the  utility  of  high  eye-piecing  under  proper  conditions, 
and  with  suitable  apertures  and  fine  corrections  in  the  objective  ; 
the  physical  brightness,  wre  learn  from  Abbe,  in  every  case  depends 
only  upon  the  aperture  and  the  total  magnifying  power ;  and  it  is 
of  no  moment  in  what  way  the  latter  is  produced — by  means  of  focal 
length  of  the  objective,  length  of  tube,  and  focal  length  of  eye-piece. 

1  Excepting  when  resolution  is  effected  by  light  of  extreme  obliquity.  If  the 
outermost  zone  of  the  objective  is  corrected  alone,  and  that  only  be  employed,  at  that 
limit  equally  good  resolution  may  be  accomplished. 

D 


34     ELEMENTARY  PRINCIPLES   OF  MICROSCOPICAL  OPTICS 

But  he  has  further  shown  us  1  that  with  the  best  objectives  of  the 
old  construction,  and  with  large  apertures,  the  limits  of  a  completely 
satisfactory  clearness  of  image  are  reached  when  the  swper-amplifiea- 
tion  is  four-  to  six-fold  ;  that  is,  when  the  total  magnifying  power  of  the 
objective  and  eye-piece  together  is  four  to  six  times  as  great  as  that 
obtained  with  the  objective  when  used  by  itself  as  a  magnifying 
lens.  On  the  other  hand,  with  apochromatic  objectives  the  available 
super-amplification — even  with  the  greatest  apertures — is  at  least 
twelve-  to  fifteen-fold,  and  considerably  higher  with  medium  and  low 
objectives. 

3.  Achromatism  touches  almost  an  ideal  point  in  these  objectives. 
The  images  are  practically  free  from  colour  over  the  entire  area. 
This  is  of  great  value  in  photo-micography.     The  correction  errors 
of  the  ordinary  achromatic  systems   are    much    more   powerful    as 
disturbing  influences  than  in  ordinary  observation  with  the  eye. 

4.  In  spite  of  the  removal  of  the  secondary  spectrum  certain 
colour  deviations  of  a  tertiary  nature  remained,  and  are  inevitable 
in  all  objectives  of  great  aperture  in  which  the  front  lens  cannot  be 
made  achromatic  by  itself.     With  ordinary  achromatic  objectives, 
from  the  properties  of  the  glass  used,  the  amount  of  this  is  very  un- 
equal in  the  central  and  peripheral  parts,  but  in  the  apochromatic 
object-glass  it  is  approximately  constant  for  all  parts  of  the  opening, 
and  therefore  it  allows  of  correction  by  the  eye-piece,   a  special   con- 
struction  possessing  equal   but   opposite  differences  of  magnifying 
power  for  different  colours.     The  eye-piece  is  so  constructed  as  to 
completely  secure  the  desired  result,  and,  as  we  have  stated  above, 
images  free  from  colour  are  obtained. 

5.  The  classification  of  the  eye-pieces  for  this  system  of  objectives 
has  been  established  by  Abbe,  and  depends  on  the  increase  in  the 
total  magnifying  power  of  the  microscope  obtained  by  means  of  the 
eye-piece  as  compared  with  that  given  by  the  objective  alone.     The 
number  which  denotes  how  many  times  an  eye-piece  increases  the 
magnifying  power  of  the  objective,  when  used  with  a  given  body- 
tube,  gives  the  proper  measure  of  the  eye -piece  magnification,  and 
at  the  same  time  the  figures  for  rational  numeration.2 

From  their  properties  these  are  known  as  '  compensating  eye- 
pieces.' 

The  following  is  a  fair  typical  selection  of  the  objectives  and 
eye-pieces  furnished  from  the  workshops  of  Carl  Zeiss,  of  Jena,  on 
this  important  system,  viz. : 

1  '  On  the  Relation  of  Aperture  to  Power,'  Journ.  B.M.S.  1883,  p.  803. 

2  '  On  Improvements  of  the  Microscope  with  the  aid  of  new  kinds  of  optical  glass  ' 
(Abbe),  Journ.  B.M.S.  1887,  p.  25  et  seq. 


APOCHROMATIC   LENSES 


35 


Apochromatic  Objectives. 


— 

Numerical 
aperture 

Equivalent 
focus  in 
mm. 

Initial 
magnification 

English 
equivalent 
focus  in 
inches 

0-30 

24-0 
16-0 

10-5 
155 

1 

1 

Dry  series    .         .  • 

0-65 

>12-0 

8-0 

21 

31 

1 

0-95 

6-0 
4-0 
3-0 

42 

63 

83 

j 

Water  immersion   . 

1-25 

2-5 

100 

A 

Homogeneous 

1-30 

3-0 
2-0 
15 

83 
125 
167 

* 

1-40 

30 

20 

83 
125 

I 

Compensating  Eye-pieces  for  English  Bodies. 
2         4         8         12         18         27 

It  is  of  interest  to  note  that  Messrs.  Powell  and  Leal  and  have 
since  produced  a  remarkable  lens  on  the  same  system,  having  a  N.A. 
of  1'50,  with  a  power  of  -^th  of  an  inch.  Object-glasses  are  also  now 
made  by  other  makers,  English,  European,  and  American,  those 
having  fluorite  in  them  being  termed  apochromatic,  while  others 
made  of  new  kinds  of  glass  are  called  semi-apochromatic.  Semi- 
apochromats  are  being  daily  improved,  so  much  so  that  some  recent 
objectives  nearly  equal  apochromatic  objectives  themselves. 


T)  2 


36  VISION  WITH   THE    COMPOUND   MICROSCOPE 


CHAPTER  II 

THE  PRINCIPLES  AND  THEORY  OF  VISION  WITH  THE 
COMPOUND  MICROSCOPE- 

WE  are  now  prepared  to  enter  upon  the  application  of  the  optical 
principles  which  have  been  explained  and  illustrated  in  the  foregoing 
pages  to  the  construction  of  microscopes.  These  are  distinguished 
as  simple  and  compound,  each  kind  having  its  peculiar  advantages 
to  the  student  of  nature.  Their  essential  difference  consists  in  this, 
that  in  the  former,  the  rays  of  light  which  enter  the  eye  of  the 
observer  proceed  directly  from  the  object  itself,  after  having  boon 
subjected  only  to  a  change  in  their  course,  as  we  have  shown  by 
fig.  26,  which  fully  explains  the  action  of  the  simple  lens  ;  whilst  in 
the"  compound  microscope  an  enlarged  image  of  the  object  is  formed 
by  one  lens,  which  image  is  magnified  to  the  observer  by  another, 
as  if  he  were  viewing  the  object  itself.  In  the  compound  micro- 
scope not  less  than  two  lenses  must  be  employed :  one  to  form  the 
enlarged  image  of  the  object,  immediately  over  which  it  is  placed, 
and  hence  called  the  object-glass  ;  whilst  the  other  again  magnifies 
that  image,  and,  being  interposed  between  it  and  the  eye  of  the 
observer,  is  called  the  eye-glass.  A  perfect  object-glass,  as  we  have 
seen,  must  consist  of  a  combination  of  lenses,  and  the  eye-glass  is 
best  combined  with  another  lens  interposed  between  itself  and  the 
object-glass,  the  two  together  forming  what  is  termed  an  eye-piece. 
The  compound  microscope  must  be  the  subject  of  careful  and  de- 
tailed consideration  ;  but  it  must  be  remembered  that  the  shorter 
the  focus  of  the  simple  magnifying  lens,  the  smaller  must  be  the 
diameter  of  the  sphere  of  which  it  forms  part ;  and,  unless  its 
aperture  be  proportionately  reduced,  the  distinctness  of  the  image 
will  be  destroyed  by  the  spherical  and  chromatic  aberrations  neces- 
sarily resulting  from  its  high  curvature.  Yet  notwithstanding  the 
loss  of  light  and  other  drawbacks  attendant  on  the  use  of  single 
lenses  of  high  power,  they  proved  of , great  value  to  the  older  micro-, 
scopists  (among  whom  Leeuwenhoek  should  be  specially  named),  on 
account  of  their  freedom  from  the  errors  to  which  the  compound 
microscope  of  the  old  construction  wras  necessarily  subject  ;  and  the 
amount  of  excellent  work  done  by  means  of  them  surprises  every  one 
who  studies  the  history  of  microscopic  inquiry.  An  important  im- 
provement on  the  single  lens  was  introduced  by  Dr.  Wollaston,  who 
devised  the  doublet,  still  known  by  his  name,  which  consists  of  two 
plano-convex  lenses,  whose  focal  lengths  are  in  the  proportion  of  one 
to  three  or  nearly  so,  having  their  convex  sides  directed  towards 


PRINCIPLES   AND   THEORY   OF   MICROSCOPIC    VISION        37 

the  eye,  and  the  lens  of  shortest  focal  length  nearest  the  object.  In 
Dr.  Wollastori's  original  combination  no  perforated  diaphragm  (or 
'  stop ')  was  interposed,  and  the  distance  between  the  lenses  was  left 
to  be  determined  by  experiment  in  each  case.  A  great  improvement 
was  subsequently  made,  however,  by  the  introduction  of  a  '  stop  ' 
between  the  lenses,  and  by  the  division  of  the  power  of  the  smaller 
lens  between  two  (especially  when  a  very  short  focus  is  required),  so 
as  to  form  a  triplet,  as  first  suggested  by  Mr.  Holland.1  When 
combinations  of  this  kind  are  well  constructed,  both  the  spherical 
and  the  chromatic  aberrations  are  so  much  reduced  that  the  angle 
of  aperture  may  be  considerably  enlarged  without  much  sacrifice  of 
distinctness  ;  and  hence  for  all,  save  very  low  powers,  such  *  doublets  ' 
and  '  triplets '  are  far  superior  to  single  lenses.  These  combinations 
took  the  place  of  single  lenses  among  microscopists  (in  this  country 
at  least),  who  were  prosecuting  minute  investigations  in  anatomy 
and  physiology  prior  to  the  vast  improvements  effected  in  the  com- 
pound microscope  by  the  achromatisation  of  its  object-glasses. 

Another  form  of  simple  magnifier,  possessing  certain  advantages 
over  the  ordinary  double-convex  lens,  is  that  commonly  known  by 
the  name  of  the  '  Coddington  '  lens.2  The  first  idea  of  it  was  given 
by  Dr.  Wollaston,  who  proposed  to  apply  two  plano-convex  or  hemi- 
spherical lenses  by  their  plane  side,  with  a  '  stop '  interposed,  the 
central  aperture  of  which  should  be  equal  to  one-fifth  of  the  focal 
length.  The  great  advantage  of  such  a  lens  is,  that  the  oblique 
pencils  pass,  like  the  central  ones,  at  right  angles  to  the  surface,  so 
that  they  are  but  little  subject  to  aberration.  The  idea  was,  how- 
ever, greatly  improved  upon  by  Sir  D.  Brewster,  who  pointed  out 
that  the  same  end  would  be  much  better  answered  by  taking  a 
sphere  of  glass,  and  grinding  a  deep  groove  in  its  equatorial  part, 
which  should  be  then  filled  with  opaque  matter,  so  as  to  limit 
the  central  aperture  ;  in  other  words,  Brewster  made  Wollaston's 

r  no-convex  lenses  hemispheres.  Such  a  combination  gives  a 
ge  field  of  view,  admits  a  considerable  amount  of  light,  and 
is  equally  good  in  all  directions ;  but  its  power  of  definition 
is  by  no  means  equal  to  that  cf  an  achromatic  lens,  and  its 
working  distance  is  inconveniently  small.  This  form  is  chiefly 
useful,  therefore,  as  a  hand-magnifier,  in  which  neither  high  power 
nor  perfect  definition  is  required,  its  peculiar  qualities  rendering 
it  superior  to  an  ordinary  lens  for  the  class  of  objects  for  which 
a  hand-magnifier  of  medium  power  is  required.  Many  of  the 
magnifiers  sold  as  '  Coddington '  lenses,  however,  are  not  really 
portions  of  spheres,  but  are  manufactured  out  of  ordinary  double- 
convex  lenses,  and  are  therefore  destitute  of  the  special  advantages 
of  the  real  '  Coddington.'  The  '  Stanhope  '  lens  somewhat  resembles 
the  preceding  in  appearance,  but  differs  from  it  essentially  in 
properties.  It  is  nothing  more  than  a  double-convex  lens,  having 
two  surfaces  of  unequal  curvatures,  separated  from  each  other  by  a 

1  Transactions  of  the  Society  of  Arts,  vol.  xlix. 

2  This  name,  however,  is  most  inappropriate,  since  Mr.  Coddington  neither  was, 
nor  ever  claimed  to  be,  the  inventor  of  the  mode  of  construction  by  which  this  lens 
is  distinguished. 


38  VISION  WITH   THE   COMPOUND   MICROSCOPE 

considerable  thickness  of  glass,  the  distance  of  the  two  surfaces  from 
each  other  being  so  adjusted  that  when  the  more  convex  is  turned 
towards  the  eye  minute  objects  placed  on  the  other  surface  shall  be 
in  the  focus  of  the  lens.  This  is  an  easy  mode  of  applying  a  rather 
high  magnifying  power  to  scales  of  butterflies'  wings,  and  other 
similar  flat  and  minute  objects,  which  will  readily  adhere  to  the 
surface  of  the  glass ;  and  it  also  serves  to  detect  the  presence  of  the 
larger  animalcules  or  of  crystals  in  minute  drops  of  fluid,  to  exhibit 
the  '  eels  '  in  paste  or  vinegar  &c.  A  modified  form  of  the  '  Stan- 
hope '  lens,  in  which  the  surface  remote  from  the  eye  is  plane  instead 
of  convex,  lias  been  brought  out  in  France  under  the  name  of 
'  Stanhoscope,'  and  has  been  especially  applied  to  the  enlargement  of 
minute  pictures  photographed  011  its  plane  surface  in  the  focus  of  its 
convex  surface.  A  good  '  Stanhoscope,'  magnifying  from  100  to  150 
diameters,  is  a  very  convenient  form  of  hand-magnifier  for  the 
recognition  of  diatoms,  infusoria,  tfcc.,  all  that  is  required  being  to 
place  a  minute  drop  of  the  liquid  to  be  examined  on  the  plane 
surface  of  the  lens  and  then  to  hold  it  up  to  the  light.  But  no  hand 
lenses  we  have  yet  seen  will  compare  with  the  Steinheil  '  loups '  of 
six  and  ten  diameters  made  by  Zeiss,  and  Keichart's  pocket  loups. 
For  the  ordinary  purposes  of  microscopic  dissection  single  lenses 
of  from  3  inches  to  1  inch  focus  answer  very  well.  But  when  higher 
powers  are  required,  and  when  the  use  of  even  the  lower  powers  is 
continued  for  any  length  of  time,  great  advantage  is  derived  from 
the  employment  of  achromatic  combinations,  now  made  expressly 
for  this  purpose  by  several  opticians.  The  Steinheil  combinations 
give  much  more  light  than  single  lenses,  with  much  better  definition, 
a  very  flat  field,  longer  working  distance  (which  is  very  important 
in  minute  dissection),  and,  as  a  consequence,  greater  '  focal  depth  ' 
or  '  penetration,'  i.e.  a  clearer  view  of  those  parts  of  the  object 
which  lie  above  or  below  the  exact  local  plane.  And  only  those 
who  have  carried  on  a  piece  of  minute  and  difficult  dissection 
through  several  consecutive  hours  can  appreciate  the  advantage  in 
comfort  and  in  diminished  fatigue  of  eye  which  is  gained  by  the 
substitution  of  one  of  these  achromatic  combinations  for  a  single 
lens  of  equivalent  focus,  even  where  the  use  of  the 
former  reveals  no  detail  that  is  not  discernible  by  the 
latter. 

Although  not  strictly  its  position,  it  is  convenient 
here  to  refer  to  what  is  known  as  the  '  Briicke  lens  ; ' 
it  is  much  used  on  the  Continent,  but  does  not  ap- 
pear in  any  English  treatise  we  have  seen.  It  has 
two  achromatic  lenses  for  the  objective,  and  a  concave 
eye  lens.  It  is  illustrated  in  fig.  29. 

To  remedy  the  inconvenience  of  the  lens  being  too 

close  to  the    object    in  all   but  low    powers,    Charles 

FIG  29— The    Chevalier,  in  his  'Manuel  du    Micrographe'   (1839), 
Briicke  lens,    proposed  to  place  above  a  doublet  a  concave  achro- 
matic lens,  the  distance  of  which  could  be  varied  at 
pleasure.  The  effect  of  this  combination  is  to  increase  the  magnifying 
power  and  lengthen  the  focus.     Thus  arranged,  this  instrument  will 


COMPOUND   MICEOSCOPE 


39 


be  the  most  powerful  of  all  simple  microscopes,  and  the  space 
available  for  scalpels,  needles,  &c.  will  be  much  greater  than 
with  a  doublet  alone.  The  further  the  concave  lens  is  removed  from 
the  latter,  the  greater  will  be  the  amplification.1  Even  in  this, 
however,  Chevalier  had  been  anticipated  by  Professor  Joblot  in 
1718. 

This  combination,  applied  to  lenses  for  examining  the  eye  and 
skin,  allows  the  use  of  doublets  which  leave  a  considerable  distance 
above  the  object,  and  it  is  this  idea  which  has  governed  the  con- 
struction of  the  Briicke  lens.  .f 

*  The  lens  has  a  very  long  focus,  -and  the  construction  is  that  of 
the  Galileo  telescope  as  applied  to  opera-glasses,  but  the  amplifica- 
tion of  the  objective  is  much  greater  than  that  usually  obtained  in 
opera-glasses.  The  focus  is  about  6  cm.,  and  the  power  three  to 
eight  times.  The  latter  power  is  obtained  by  lengthening  the  tube, 
by  which  means  the  distance  between  the  two  lenses  is  much 
enlarged,  and  the  amplification  increased  without  inconveniently 
modifying  the  focus.' 

This  lens  may  be  used  in  place  of  the  body  of  a  compound 
microscope,  wlien.  it  is  desired  to  dissect  or  to  find  small  objects,  or 
it  can  be  adapted  to  a  simple  microscope  or  lens-holder,  with  from 
3  to  8  cm.  between  the  object  and  objective.  But  the  Briicke  lens, 
like  the  Galilean  opera  glass,  has  a  very  small  field. 

Compound  microscope. — The  compound  microscope,  in  its  most 
simple  form,  consists  of  only  two  lenses,  the  object-glass  and  the 
eye-glass,  and  is  a  Keplerian  telescope  adapted  for  viewing  very  near 
objects.  The  former  receives  the  light-rays  direct  from  the  object 
brought  into  near  proximity  to  it,  and  forms  an  enlarged  but  inverted 
and  reversed  image  at  a  greater  distance  on  the  other  side  ;  whilst  the 
latter  receives  the  rays  which  are  diverging  from  this  image,  as  if 
they  proceeded  from  an  object  actually  occupying  its  position  and 
enlarged  to  its  dimensions,  and  brings  these  to  the  eye,  so  altering 
their  course  as  to  make  that  image  appear  far  larger  to  the  eye,  pre- 
cisely as  in  the  case  of  the  simple  microscope.  It  is  obvious  that, 
in  the  use  of  the  very  same  lenses,  a  considerable  variety  of  magnify- 
ing power  may  be  obtained  by  merely  altering  their  position  in  regard 
to  each  other  and  to  the  object.  For  if  the  eye-glass  be  carried  farther 
from  the  object-glass,  whilst  the  object  is  approximated  nearer  to  the 
latter,  the  image  will  be  formed  at  a  greater  distance  from  the  object- 
glass,  and  the  dimensions  of  the  magnified  image  will  consequently 
be  augmented  ;  whilst,  on  the  other  hand,  if  the  eye-glass  be  brought 
nearer  to  the  object-glass,  and  the  object  removed  further  from  it, 
the  distance  of  the  image  from  the  object-glass  will  be  less  than  it 
was  before,  and  the  dimensions  of  the  magnified  image  will  be 
correspondingly  diminished.  The  amplification  may  also  be  varied 
by  altering  the  magnifying  power  of  the  eye-pieces.  In  practice, 
variations  in  power  must  be  obtained  by  altering  either  the  objective 
or  the  eye-piece,  or  both,  and  the  use  of  the  draw-tube  for  this 
purpose  must  be  altogether  abandoned,  because  objectives  are 

1  Kobin,  C.,  Traite  du  Microscope  et  des  Injections,  2nd  ed.  8vo.  pp.  33,  34. 
Paris,  1887. 


40  VISION    WITH   THE   COMPOUND   MICROSCOPE 

corrected  for  a  certain  length  of  draw-tube,  and,  in  order  that  they 
may  work  efficiently,  that  definite  length  of  draw-tube  must  be 
maintained. 

In  general  it  is  not  advisable  to  use  with  an  achromatic  objective 
a  greater  super-amplification  than  can  be  obtained  with  a  10-power 
eye-piece,  or  with  an  apochromatic  objective  that  yielded  with  a 
12  or  18  power  one. 

We  shall  facilitate  the  comprehension  by  the  student  of  the 
principles  of  the  modern  form  of  a  compound  microscope  by  means 
of  fig.  30.  In  this  figure  the  optical  portion,  that  is,  the  objective  and 
eye-piece,  are  drawn  to  the  full  size,  but  the  distance  between  these 
has,  from  the  exigencies  of  space,  been  much  curtailed.  A  low- 
power  objective  has  been  specially  chosen  for  simplicity,  and  a  com- 
pensating eye-piece  (vide  Chapter  Y.)  has  been  introduced  to  show 
its  form  and  mode  of  action. 

The  objective  is  a  copy  of  an  old  Ross  1-inch  of  1856.  The 
incident  front  (that  is,  the  lens  on  which  the  incident  beams  from 
the  object  first  strike)  is  a  convex  of  long  radius ;  the  incident  sur- 
face of  the  flint  lens  of  the  back  combination  is  a  concave  of  very 
long  radius,  being  in  fact  about  twenty  inches. 

The  object  F  has  only  rays  drawn  from  one  side  in  order  that 
a  clearer  perception  of  the  path  of  the  rays  may  be  seen.  This  pair 
of  rays  passes  from  the  arrow  (object)  through  the  combination  of 
lenses  forming  the  objective,  giving  an  inverted  real  image  at  A  B. 
This  image,  in  fact,  has  a  convex  curve  towards  the  eye-piece :  this 
is  a  position  that  will  tend  to  increase  the  curvature  of  the  virtual 
image  C  D  given  by  the  eye-piece,  the  inverted  image  (A  B)  at  the 
diaphragm  of  the  eye-piece  being  the  subject  of  still  further  and 
often  great  magnification. 

In  addition  to  the  two  lenses  of  which  the  compound  microscope 
may  be  considered  to  essentially  consist,  it  was  soon  found  needful 
to  introduce  another  lens,  or  a  combination  of  lenses,  between  the 
object-glass  and  the  image  formed  by  it,  the  purpose  of  this  being 
to  change  the  course  of  the  rays  in  such  a  manner  that  the  image 
may  be  formed  of  dimensions  not  too  great  for  the  whole  of  it  to 
come  within  the  range  of  the  eye-glass.  As  it  thus  allows  more  of 
the  object  to  be  seen  at  once,  it  has  been  called  the  field-glass ;  but 
it  is  now  usually  considered  as  belonging  to  the  ocular  end  of  the 
instrument,  the  eye-glass  and  the  field-glass  being  together  termed 
the  eye-piece,  or  ocular.  Various  forms  of  this  eye-piece  have  been 
proposed  by  different  opticians,  and  one  or  another  will  be  preferred 
according  to  the  purpose  for  which  it  maybe  required.  That  which, 
until  the  construction  of  the  compensation  eye-pieces  by  Abbe,  was 
considered  the  most  advantageous  to  employ  with  achromatic  object- 
glasses,  to  the  performance  of  which  it  is  desired  to  give  the  greatest 
possible  effect,  was  termed  the  Huyghenian,  having  been  employed 
by  Huyghens  for  his  telescopes,  although  without  the  knowledge  of 
all  the  advantages  which  its  best  construction  renders  it  capable  of 
affording.  This  eye-piece,  with  others,  will  be  considered  in  detail 
in  the  chapter  (v.)  given  in  part  to  their  consideration ;  but  this 
eye-piece  consists  of  two  plano-convex  lenses,  with  their  plane  sides 


FIG.  30.— Path  of  a  ray  of  light  through  a  modern  combination  of  lenses  for 
compound  microscope. 


42  VISION  WITH   THE    COMPOUND   MICROSCOPE 

towards  the  eye.  A  '  stop '  or  diaphragm,  B  B,  must  be  placed 
between  the  two  lenses,  in  the  visual  focus  of  the  eye-glass,  which 
is,  of  course,  the  position  wherein  the  image  of  the  object  will  be 
formed  by  the  rays  brought  into  convergence  by  their  passage 
through  the  field-glass.  Huyghens  devised  this  arrangement  merely 
to  diminish  the  spherical  aberration  ;  but  it  was  subsequently  shown 
by  Boscovich  that  the  chromatic  dispersion  was  also  in  great  part 
corrected  by  it.  With  the  apochromatic  lenses  of  the  highest  and 
best  quality  (see  Chapter  Y.)  no  amount  of  obtainable  eye-piecing,  if 
it  be  of  the  *  compensation  '  form,  can  break  down  the  image.  The 
editor  has  tried  in  vain  to  break  down  the  image  formed  by  a 
24  mm.,  a  12  mm.,  a  6  mm.,  and  a  4  mm.,  all  dry  apochromatics 
by  Zeiss,  and  especially  with  a  Jth  by  Powell  and  Lealand.  It 
is,  however,  a  matter  of  moment  and  interest  to  note  that  with 
good  objectives  of  the  ordinary  achromatic  construction  of  large 
N.A.  the  compensating  eye-pieces  give  better  results  than 
Huyghenian. 

But  of  the  old  form  of  achromatic  object-glass  it  is  true  of  the 
majority  that  they  will  not  bear  high  eye-piecing.  '  B,'  1  ^  inch  in 
focus,  is  a  convenient  and  useful  eye-piece  for  viewing  large  flat 
objects,  such  as  transverse  sections  of  wood  or  of  echinus-spines, 
under  low  magnifying  powers.  A  flat  large  field  may  be  obtained 
by  means  of  a  Kellner  ;  but,  on  the  other  hand,  there  is  a  very 
serious  falling  off  of  defining  power,  which  renders  the  Kellner  eye- 
piece unsuitable  for  objects  presenting  minute  structural  details  ; 
and  it  is  an  additional  objection  that  the  smallest  speck  or 
smear  upon  the  surface  of  the  field-glass  is  made  so  unplea- 
santly obvious  that  the  most  careful  cleansing  of  that  surface  is 
required  every  time  that  this  eye-piece  is  used.  Hence  it  is 
better  fitted  for  the  occasional  display  of  objects  of  the  character' 
already  specified  than  for  the  scientific  requirements  of  the  working 
microscopist. 

A  '  positive '  or  Ramsden's  eye-piece — in  which  the  field-glass, 
whose  convex  side  is  turned  upwards,  is  placed  so  much  nearer  the 
eye-glass  that  the  image  formed  by  the  objective  lies  below  instead 
of  above  it — is  sometimes  used  for  the  purpose  of  micrometry,  a 
divided  glass  being  fitted  in  the  exact  plane  occupied  by  the  image, 
so  that  its  scale  and  the  image  are  both  magnified  together  by  the 
lenses  interposed  between  them  and  the  eye.  The  same  end,  how- 
ever, is  also  attained  with  the  Huyghenian  eye-piece,  and  it  is 
doubtful  if  any  advantage  is  gained  by  the  Ramsden  in  microscope 
work.  The  compensating  eye-piece  is  also  used  in  conjunction 
with  the  micrometer. 

Aperture  in  microscopic  objectives  and  the  principles  of  micro- 
scopic vision. — It  is  now  of  the  utmost  moment  that  we  should 
understand  clearly  the  meaning  and  importance  of  'aperture'  in 
microscopic  objectives,  and  by  that  means  be  led  to  a  perception  of 
the  principles  of  the  most  recent  and  only  rational  theory  of  micro- 
scopic vision.  Within  the  last  twenty-five  years  this  entire  subject 
has  undergone  a  rigorous  and  exhaustive  reinvestigation  by  one 
of  the  most  competent  and  masterly  mathematical  and  practical 


THE   FORMATION    OF   MICROSCOPIC   IMAGES  43 

opticians  in  the  world,  Professor  Abbe  of  Jena  ;  and,  as  a  result, 
some  of  the  judgments  and  opinions,  as  well  as  what  were  supposed 
to  be  established  truths,  depending  apparently  upon  the  simplest 
principles,  and  not  believed  to  be  open  to  change,  have  been  shown 
to  be  absolutely  without  foundation ;  while  principles  hitherto  quite 
unknown  and  unsuspected  have  been  shown  to  operate  and  to  rest 
on  clearly  demonstrable  mathematical  and  physical  bases.  The 
result  has  been  a  complete  revolution  of  what  were  held  to  be 
fundamental  principles  of  microscopic  optics  and  the  theory  of 
vision  with  microscopic  object-gl;i- 

Professor  Abbe  contends  that  one  of  the  foremost  errors  relates 
to  the  mode  in  which  microscopic  images  are  formed.  It  was 
assumed  that  their  formation  took  place  on  ordinary  dioptric 
principles.  As  the  camera  or  the  telescope  formed  images,  so  it 
was  assumed  that  the  image  in  the  compound  microscope  was 
brought  about.  The  delicate  and  complex  structure  of  an  insect's 
scale  or  of  a  diatom  were  believed  to  form  their  images  according  to 
the  same  precise  dioptric  law^s  by  which  the  image  of  the  moon  or 
Mars  is  formed  in  the  telescope.  Hence  it  was  taken  for  granted 
that  every  function  of  the  microscope  was  determined  by  the  geo- 
metrically traceable  relations  of  the  refracted  rays  of  light.  We 
would  nevertheless  remark  that  visibility  of  detail  in,  for  example, 
the  moon  depends  on  the  aperture  of  the  telescope ;  of  course,  what 
is  known  as  its  '  aperture '  is  simply  estimated  by  the  diameter  of 
the  object-glass,  but  accuracy  appears  to  require  that  n  sin  u  =  a 
ought  to  be  applied  to  the  telescope.  In  practice  the  diameter  is 
taken  conventionally  for  the  sake  of  simplicity,  as  it  makes  no 
numerical  difference,  because  the  sines  of  small  angles  such  as  are 
dealt  with  in  the  telescope  are  proportional  to  the  angles  themselves. 
The  microscope,  on  the  other  hand,  deals  with  large  angles ;  con- 
sequently the  sine  cannot  be  dispensed  with. 

But  Professor  Abbe  argues  that  a  close  examination  in  theory 
and  practice  of  the  conditions  of  vision  with  microscopic  objectives 
shows  that  such  an  estimate  of  aperture  is  wholly  wrong  in  prin- 
ciple. The  front  lens  of  a  -^g-in.  objective  may  be  no  more  than  the 
sVth  of  an  inch  in  diameter,  while  a  3-in.  objective  may  have  a 
diameter  of  half  an  inch.  Yet  it  is  the  smaller  lens  that  has  by  far 
the  larger  '  aperture.' 

Light  is  dispersed  from  every  point  on  the  surface  of  an  object 
in  all  directions  up  to  180°.  Only  an  extremely  narrow  pencil  of 
this  can  be  received  by  the  human  eye,  a  large  pencil  of  light 
emanating  from  the  object  being  lost  on  each  side  of  what  the  eye 
receives.  The  apparent  problem  of  practical  optics  is  to  be  able, 
by  means  of  lenses,  to  gather  up  and  bring  to  a  focus  as  many  of  the 
unadmitted  rays  as  possible.  The  general  manner  in  which  lenses 
act  in  doing  this  we  have  endeavoured  in  an  elementary  manner  to 
show. 

Soon  after  achromatic  object-glasses  were  first  made,  Dr.  Goring 
found  that  the  markings  on  special  objects — such  as  the  scales  of 
the  wings  of  insects — could  be  seen  by  some  object-glasses,  while 
with  others,  although  the  magnifying  power  was  equal,  it  was  im- 


44  VISION  AVITH  THE   COMPOUND  MICROSCOPE 

possible  to  discern  them.  In  every  case  the  greater  '  angle '  was 
shown  to  possess  the  greater  '  resolving  '  or  delineating  power  ;  and 
this  led  to  the  important  conclusion  that  power  of  '  resolution '  in  a 
lens  was  dependent  upon  '  angular  aperture.' 

This,  however,  was  at  a  time  when  only  '  dry '  objectives  were  in 
use  ;  the  immersion  and  homogeneous  systems,  as  we  use  them,  were 
unknown. 

But  (as  we  shall  subsequently  see),  even  with  objectives  employed 
only  with  air,  the  angle  of  the  radiant  pencil  did  not  afford  a  true 
comparison  ;  when  immersion  objectives  were  introduced — objectives 
in  which  water  or  cedar  oil  replaced  the  air  between  the  objective 
and  the  upper  surface  of  the  cover  of  the  mounted  object — the 
use  of  angles  of  aperture  became  in  the  utmost  degree  misleading  ; 
for  different  media  with  different  refractive  indices  were  employed, 
and  the  angle  of  the  radiant  pencil  was  supposed  not  only  to  admit 
of  a  comparison  of  two  apertures  in  the  same  medium,  but  also  to 
be  a  standard  of  comparison  when  the  media  were  different.  It 
was,  in  short,  believed  that  an  angle  of  180°  in  air  represented 
a  large  excess  of  aperture  in  comparison  with  96°  in  water  and 
82°  in  balsam  or  oil,  denoting,  in  reality,'  what  was  believed 
to  be  the  maximum  aperture  of  any  kind  of  objective,  which 
could  not,  it  was  held,  be  exceeded,  but  only  equalled,  by  180° 
in  water  or  oil ;  in  other  words,  that  a  radiant  pencil  has  exactly 
the  same  value,  when  the  angles  are  equal,  no  matter  what  the 
refractive  index  of  the  medium  through  which  the  pencil  might 
be  passing. 

But  to  a  thorough  physical  and  mathematical  study  of  the  ques- 
tion such  as  that  in  which  Professor  Abbe  engaged,  it  soon  became 
apparent  that  even  in  the  same  medium  the  only  exact  method  of 
comparison  for  objectives — when  the  fundamental  phenomena  of 
optics  (which  the  older  opticians  had  disregarded)  were  taken  into 
account — was  not  a  comparison  by  the  angles  of  the  radiant  pencils 
only,  but  a  comparison  by  their  sines;  while,  when  the  media  arc 
different,  the  indices  of  those  media  would  be  found  to  form  an 
essential  factor  in  the  problem  ;  for  an  angle  of  1 80°  in  air  is  equal 
to  96°  in  water  or  82°  in  oil ;  hence  three  angles  might  all  have  the 
same  number  of  degrees  and  yet  denote  different  values,  according 
as  they  were  in  air,  water,  or  oil. 

Thus  there  might  be  large  divergence  of  aperture  in  twTo  or 
more  cases  while  the  angle  was  identical,  and  from  this  the  greatest 
confusion  was  not  only  possible  but  was  realised. 

A  solution  of  the  difficulty  was  (as  we  have  indicated  above) 
discovered  by  Professor  Abbe ;  and  it  is  to  Mr.  Frank  Crisp's  lucid 
exposition  of  Abbe's  elaborate  monographs  that  the  English  student 
is  immensely  indebted.1 

The  definition  of  '  aperture '  in  its  legitimate  sense  of  '  opening ' 
is  shown  by  Abbe  to  be  obtained  when  we  compare  the  diameter  of 

1  '  On  the  Estimation  of  Aperture  in  the  Microscope '  (Abbe),  Jo  urn.  R.M.S. 
ser.  ii.  vol.  i.  388  ;  '  Notes  on  Aperture,  Microscopical  Vision,  and  the  Value  of  wide- 
angled  Immersion  Objectives,'  ibid.  803  ;  '  The  Aperture  of  Microscope  Objectives,' 
English  Mechanic. 


HOW    'APERTURE'   IS   OBTAINED  45 

the  pencil  emergent  from  the  objective  with  the  focal  length  of  that 
objective. 

It  will  be  desirable  to  explain  somewhat  more  in  detail  how 
this  conclusion  is  arrived  at,  as  given  in  Professor  Abbe's 
papers. 

Taking  in  the  first  case  a  single-lens  microscope,  the  number  of 
rays  admitted  within  one  meridional  plane  of  the  lens  evidently  in- 
creases as  the  diameter  of  the  lens  (all  other  circumstances  remaining 
the  same),  for  in  the  microscope  we  have  at  the  back  of  the  lens  the 
same  circumstances  as  are  in  front  in  the  case  of  the  telescope.  The 
larger  or  smaller  number  of  emergent  rays  will  therefore  be  properly 
measured  by  the  clear  diameter ;  and,  as  no  rays  can  emerge  that 
have  not  first  been  admitted,  this  must  also  give  the  measure  of  the 
admitted  rays. 

Suppose  now  that  the  focal  lengths  of  the  lenses  compared  are 
not  the  same — what,  then,  is  the  proper  measure  of  the  rays 
admitted  ? 

If  the  two  lenses  have  equal  openings  but  different  focal  lengths, 
they  transmit  the  same  number  of  rays  to  equal  areas  of  an  image 
at  a  definite  distance,  because  they  would  admit  the  same  number  if 
an  object  were  substituted  for  the  image — that  is,  if  the  lens  were 
used  as  a  telescope -objective.  But  as  the  focal  lengths  are  different, 
the  amplification  of  the  images  is  different  also,  and  equal  areas  of 
these  images  correspond  to  different  areas  of  the  object  from  which 
the  rays  are  collected.  Therefore  the  higher-power  lens,  with  the 
same  opening  as  the  lower  power,  will  admit  a  greater  number  of 
rays  in  all  from  the  same  object,  because  it  admits  the  same  number 
as  the  latter  from  a  smaller  portion  of  the  object.  Thus,  if  the  focal 
lengths  of  two  lenses  are  as  2  :  1 ,  and  the  first  amplifies  N  diameters, 
the  second  will  amplify  2  N  with  the  same  distance  of  the  image,  so 
that  the  rays  which  are  collected  to  a  given  field  of  1  mm.  diameter 

of  the  image  are  admitted  from  a  field  of  ^-    mm.    in  the  first  case 

and  of  9i>j-mm<  *n  *ne  second.     Inasmuch  as  the  'opening'  of  the 

objective  is  estimated  by  the  diameter  (and  not  by  the  area),  the 
higher -power  lens  admits  twice  as  many  rays  as  the  lower  power, 
because  it  admits  the  same  number  from  a  field  of  half  the  diameter, 
and  in  general  the  admission  of  rays  with  the  same  opening 
but  different  powers  must  be  in  the  inverse  ratio  of  the  focal 
lengths. 

In  the  case  of  the  single  lens,  therefore,  its  aperture  must  be 
determined  by  the  ratio  between  the  clear  opening  and  the  focal 
length,  in  order  to  define  the  same  thing  as  is  denoted  in  the  telescope 
by  the  absolute  opening. 

Consider  now  the  compound  objective — the  most  important  case 
in  the  microscope.  What  is  the  opening  of  this  composite  system  ? 
We  must  adhere  to  the  diameter  of  the  admitted  cone  at  that  plane 
where  it  has  its  ultimate  maximum  value,  which  is  obviously  the 
diameter  of  the  pencil  at  its  emergence,  from  the  system,  or,  practi- 


46  VISION   WITH   THE    COMPOUND   MICROSCOPE 

cally,  the  clear,  effective  diameter  of  the  back  lens.  The  emergent 
pencil  from  a  microscope -objective  converging  to  a  relatively  distant 
focus  has  its  rays  approximately  parallel,  and  the  conditions  are 
once  more  similar  to  those  of  the  telescope-objective  on  the  side  of 
the  object.  The  diameter  of  this  emergent  pencil,  whether  it  emerges 
from  a  single  lens  or  from  a  composite  system,  must  therefore  always 
have  the  same  signification.  The  influence  of  the  power  on  focal 
length  also  remains  the  same  as  in  the  case  of  the  single  lens.  An 
objective  with  a  focal  length  equal  to  half  that  of  another  admits, 
with  the  same  linear  opening,  twice  as  many  rays  as  the  latter, 
because  the  amplification  of  the  image  at  one  and  the  same  distance 
is  doubled,  and  the  same  number  of  rays  consequently  are  admitted 
by  the  higher  power  from  a  field  of  half  the  diameter.  And  this 
will  hold  good  whether  the  medium  around  the  object  is  the  same 
in  the  case  of  both  objectives  or  different ;  for  an  immersion  system 
and  a  dry  system  always  give  the  same  amplification  when  the  focal 
length  is  the  same. 

Thus  we  arrive  at  the  general  proposition  for  all  kinds  of  objec- 
tives. First,  when  the  power  is  the  same,  the  admission  of  rays 
varies  with  the  diameter  of  the  pencil  at  its  emergence.  Secondly, 
when  the  powers  are  different  the  same  admission  requires  different 
openings  in  the  proportion  of  the  focal  lengths,  or,  conversely,  with 
the  same  opening  the  admission  is  in  inverse  proportion  to  the  focal 
length — that  is,  the  objective  which  has  the  wider  pencil  relatively 
to  its  focal  length  has  the  larger  aperture. 

Thus  we  see  that,  just  as  in  the  telescope  the  absolute  diameter 
of  the  object-glass  defines  the  aperture,  so  in  the  microscope  the 
ratio  between  the  utilised  diameter  of  the  back  lens  and  the  focal 
length  of  the  objective  defines  its  aperture. 

This  definition  is  clearly  a  definition  of  aperture  in  its  primary 
and  only  legitimate  meaning  as  '  opening  ' — that  is,  the  capacity  of 
the  objective  for  admitting  rays  from  the  object  and  transmitting 
them  to  the  image ;  and  it  at  once  solves  the  difficulty  which  has 
always  been  involved  in  the  consideration  of  the  apertures  of 
immersion  objectives. 

So  long  as  the  angles  were  taken  as  the  proper  expression  of 
aperture,  it  was  difficult  for  those  who  were  not  well  versed  in 
optical  matters  to  avoid  regarding  an  angle  of  180°  in  air  as  the 
maximum  aperture  that  any  objective  could  attain.  Hence,  water- 
immersion  objectives  of  96°  and  oil-immersion  objectives  of  82° 
were  looked  upon  as  being  of  much  less  aperture  than  a  dry  objective 
of  180°,  whilst,  in  fact,  they  are  all  equal,  that  is,  they  all  transmit 
the  same  rays  from  the  object  to  the  image.  Therefore,  180°  in 
water  and  180°  in  oil  are  unequal,  and  both  are  much  larger  aper- 
tures than  the  180°  which  is  the  maximum  that  the  air  objective  can 
transmit. 

If  we  compare  a  series  of  dry  and  oil-immersion  objectives,  and, 
commencing  with  very  small  air-angles,  progress  up  to  180°  air- 
angle,  then  taking  an  oil-immersion  of  82°  and  progressing  again  to 
1 80°  oil-angle,  the  ratio  of  opening  to  power  progresses  continually 


COMPARISON   OF   OBJECTIVES    OF  THE   SAME   POWER       47 


also,  and  attains  its  maximum,  not  in  the  case  of  the  air-angle  of 
180°  (when  it  is  exactly  equivalent  to  the  oil-angle  of  82°),  but  is 
greatest  at  the  oil-angle  of  180°. 

If  we  assume  the  objectives  to  have  the  same  power  throughout, 
we  get  rid  of  one  of  the  factors  of  the  ratio,   and  we  have  only  to 
compare  the  diameters  of  the  emergent  beams,  and  can  represent 
their  relations  by  diagrams.      Fig.  31  illustrates  five  cases  of  different 
apertures   of    J-in.    objectives — 
viz.   those    of  dry   objectives    of 
60°,  97°,    and  180°  air-angle,  a' 
water-immersion  of  180°  water- 
angle,    and    an  oil -immersion  of 
1 80°  oil-angle .     The  inner  dotted 
circles    in    the    two   latter  cases 
are   of    the   same   size   as    that 
corresponding   to    the  180°  air- 
angle. 

A  dry  objective  of  the  full 
maximum  air-angle  of  180°  is 
only  able  (whether  the  first  sur- 
face is  plane  or  concave)  to  utilise 
a  diameter  of  back  lens  equal  to 
twice  the  focal  length,  while  an 
immersion  lens  of  even  only  100° 
(in  glass)  requires  and  utilises  a 
larger  diameter,  i.e.  it  is  able 
to  transmit  more  rays  from  the 
object  to  the  image  than  any 
dry  objective  is  capable  of  trans- 
mitting. Whenever  the  angle  of 
an  immersion  lens  exceeds  twice 
the  critical  angle  for  the  immer- 
sion-fluid, i.e.  96°  for  water  or 
82°  for  oil,  its  aperture  is  in  ex- 
cess of  that  of  a  dry  objective  of 
180°. 

Having  settled  the  principle, 
it  was  still  necessary,  however, 
to  find  a  proper  notation  for  com- 
paring apertures.     The  astrono- 
mer can  compare  the  apertures  of  his  various  telescopes  by  simply 
expressing  them  in  inches ;  but  this  is  obviously  not  available  to 
the  microscopist,  who  has  to  deal   with   the  ratio  of  two  varying 
quantities. 

Professor  Abbe  here  again  conferred  a  boon  upon  microscopists  by 
his  discovery  (in  1873,  independently  confirmed  by  Professor  Helm- 
holtz  shortly  afterwards)  that  a  general  relation  existed  between  the 
pencil  admitted  into  the  front  of  the  objective  and  that  emerging 
from  the  back  of  the  objective,  so  that  the  ratio  of  the  semi -diameter 
of  the  emergent  pencil  to  the  focal  length  of  the  objective  could  be 


© 


Numerical  Aperture 

1-52 
=  180°  oil-angle. 


Numerical  Aperture 

1-33 
=  180°  water-angle. 


Numerical   Aperture 

1-00 

=  180°  air-angle 
=  96°  water-angle 
=  82°  oil-angle. 


Numerical   Aperture 

•75 
=  97°  air-angle. 


Numerical  Aperture 

•50 
=  60°  air-angle. 


FIG.  31. — Eelative  diameters  of  the  (uti- 
lised) back  lenses  of  various  dry  and 
immersion  objectives  of  the  same 
power  (|)  from  an  air-angle  of  60°  to 
an  oil-angle  of  180°. 


48 


VISION   WITH   THE    COMPOUND   MICEOSCOPE 


expressed  by  the  sine  of  half  the  angle  of  aperture  (u)  l  multiplied 
by  the  refractive  index  of  the  medium  (n)  in  front  of  the  objective, 
or  n  sin  u  (n  being  !•()  for  air,  1'33  for  water,  and  1'5  for  oil  or 
balsam). 


FIG.  32.— Illustration  of  the  law  of  consequence  for  aplanatic  systems. 

Let  0  and  O*  (fig.  32)  be  the  conjugate  aplanatic  foci  of  a  wide- 
angled  system  ;  u,  U  the  angles  of  inclination  of  any  two  rays  admitted 

1  In  the  original  translation  of  the  papers  of  Professor  Abbe  from  German  into 
English  the  German  mathematical  symbols  have  been  retained.     In  the  summary  of 


N.A.  of  objective -/x  sin  <£_l-5x  -573  =  - 


oil  ju^  1-5  \35° 
glass  ju  =1-5   \ 


ar  n*= 


N.A.  of  condenser =M*  sin  «*- 1-0  x  -86  =  -86. 


M  sin  M  =  l-5x'573='86=N.A.of  objective. 


35°/   Oil 71  =  1-5  Angular  aperture  of 

objective  =  35°  + 

35°=70 in 


,•    glass  ?i  =  f-5 


which  is  equiva- 
lent to  the  angular 
of    the 
=  60°  + 
60°  =120°  in  air. 


'=  1-0  x  -86  =  -86  =  N.A.  of  condenser. 


Fre.  Al.— Identity  of  n  sin  u  (German  math,  form)  with  p.  sin  $  (English).    Also  N.A.  and 
angular  aperture. 


Abbe's  theories  and  demonstrations  presented  in  the  following  pages  the  Editor  has 
scarcely  felt  justified  in  altering  this,  especially  as  the  German  form  of  symbol  ob- 


KELATIVE   APERTUEES 


49 


from  the  radiant,  and  u*,  U*  the  angles  of  the  same  rays  on  their 
emergence  ;  then  we  shall  have  always 

sin  U*  :  sin  u*  :  :  sin  U  :  sin  u  ; 

sin  U*     sin  u* 

or,  _^^=_  --  =  const.  =c; 

sin  U        sin  u 

that  is,  the  SMWS  of  the  angles  of  the  conjugate  rays  on  both  sides  of 
an  aplanatic  system  always  yield  one  and  the  same  quotient  c,  what- 
ever rays  may  be  considered,  so  long  as  the  same  system  and  the 
same  foci  are  in  question. 

This  proposition  holds  good  for  'every  arrangement  of  media,  and 
refracting  surfaces  that  may  go  to  the  composition  of  the  system,  and 
for  every  position  of  object  and  image.  It  is  the  law  upon  which  de- 
pends the  delineation  of  an  image  by  means  of  wide-angled  pencils. 

When,  then,  the  values  in  any  given  cases  of  the  expression 
n  sin  u  (which  is  known  as  the  '  numerical  aperture  '  and  expressed 
by  N.A.)  has  been  ascertained,  the  objectives  are  instantly  compared 
as  regards  their  aperture,  and,  moreover,  as  180°  in  air  is  equal  to 
1-0  (since  w=l-0  and  the  sine  of  half  180°  or  90°=1'0),  we  see  with 
equal  readiness  whether  the  aperture  of  the  objective  is  smaller  or 
larger  than  that  corresponding  to  180°  in  air. 

Thus,  suppose  we  desire  to  compare  the  relative  aperture  of  three 

tains  in  our  Universities,  and  is  thoroughly  understood  amongst  University  men. 
But  to  those  unaccustomed  to  mathematical  formulae  confusion  might  easily  arise 
from  the  juxtaposition  of  different  symbols  meaning  precisely  the  same  thing.  To 
meet  the  possible  necessity  of  these  this  footnote  is  inserted  with  an  accompanying 
diagram  to  illustrate  the  identity  of  '  n  sin  u  '  with  '  fj.  sin  <J>.' 

The  student  who  has  mastered  Snell's  Law  of  Sines,  given  and  illustrated  on  p.  8 
(fig.  1),  will  by  a  glance  at  the  figure  Al  on  p.  48  understand  the  meaning  and  import- 
ance of  the  expression  '  N.A.'  (numerical  aperture)  and  at  the  same  time  will  grasp 
wherein  it  differs  from  '  angular  aperture  '  (q.v.).  He  will  also  perceive  how  it  comes 
to  pass  that  an  angular  aperture  of  70°  in  glass  is  equivalent  to  an  angular  aperture  of 
120°  in  air. 

In  the  figure  the  upper  hemispherical  lens  represents  the  front  of  a  homogeneous 
immersion  objective.  It  is  supposed  to  be  focussed  on  an  object  in  contact  with  the 
lower  side  of  a  cover-glass.  Between  the  plane  front  of  the  lens  and  the  upper  surface 
of  the  cover-glass  is  a  drop  of  oil  of  cedar-wood,  whose  refractive  index  is  1*5,  being 
thus  identical  with  the  cover-glass  and  the  front  lens. 

It  is  understood  that  no  slip  is  used,  and  that  there  is  nothing  between  the  object 
and  the  front  lens  of  the  condenser. 

In  this  case  the  axis  A  B  is  the  normal  (p.  5,  fig.  2)  ;  on  the  left-hand  side  there 
is  a  ray  which  makes  an  angle  of  35°  with  the  normal  in  glass  issuing  into  air  on  the 
right-hand  side  of  the  normal.     By  Snell's  formula  (p.  3)  — 
fj.  sin  <f>  =  /a'  sin  </>'  ; 
sin*'=   ^gH^  =  l-5x'573  =  .86; 

fj.  I'O 

<£'  =  60°  (from  Table  I.) 

Therefore  the  ray  on  emerging  from  the  under  surface  of  the  cover-glass  will  make 
an  angle  of  60°  from  the  normal. 

The  dotted  lines  show  the  path  of  the  ray  where  the  German  symbols  are  used. 
n  sin  u—n*  sin  u* 


sin  u*=  _ 

n*  1-0 

?,*  =  60°  (from  Table  I.) 

Numerical  aperture,  therefore,  is  the  sine  of  half  the  angular  aperture  multiplied 
by  the  refractive  index  of  the  medium. 

It  will  be  observed  that  the  rays  passing  through  the  oil  of  cedar  enter  the  front 
lens  without  refraction  ;  this  is  due  to  the  fact  that  the  media  in  which  the  rays  are 
travelling  are  of  the  same  refractive  index,  i.e.  they  are  homogeneous. 

E 


50  VISION  WITH   THE    COMPOUND   MICROSCOPE 

objectives,  one  a  dry  objective,  the  second  a  water-immersion,  :md 
the  third  an  oil-immersion.  These  would  be  compared  on  the 
angular  aperture  view  as,  say,  74°  air-angle,  85°  water -angle,  and 
118°  balsam-angle;  so  that  a  calculation  must  be  worked  out  to 
arrive  at  a  due  appreciation  of  the  actual  relation  between  them. 
Applying,  however,  'numerical'  aperture,  which  gives  -60  for  the  dry 
objective,  '90  for  the  water-immersion,  and  1'30  for  the  oil-immersion, 
their  relative  apertures  are  immediately  appreciated,  and  it  is  seen,  for 
instance,  that  the  aperture  of  the  water-immersion  is  somewhat  less 
than  that  of  a  dry  objective  of  180°,  and  that  the  aperture  of  the 
oil-immersion  exceeds  that  of  the  latter  by  30  per  cent. 

When  these  considerations  have  been  appreciated,  the  advantage 
possessed  by  immersion  in  comparison  with  dry  objectives  is  no 
longer  obscured.  Instead  of  this  advantage  consisting  merely  in 
increased  working  distance  or  absence  of  correction-collar,  it  is  seen 
that  a  wide-angled  immersion  objective  has  a  larger  aperture  than 
a  dry  objective  of  the  maximum  angle  of  180° ;  so  that  for  any  of 
the  purposes  for  which  aperture  is  desired  an  immersion  must 
necessarily  be  preferred  to  a  dry  objective. 

1.  There  exists  then  a  definite  ratio  between  the  linear  opening 
and  the  focal  length  of  a  system,  which  must  be  entirely  indepen- 
dent of  the  composition  and  arrangement  of  the  system,  and  solely 
determined   by   the   above-mentioned   aperture    equivalent   of  the 
admitted  cone  of  rays.     When  the  equivalent  is  the  same  we  have 
always  the  same  proportion  of  opening  to  focal  length,  whatever  may 
be  the  particular  arrangement  of  refracting  media  in  the  system. 

2.  If  the  objectives  whose  apertures  are  compared  work  in  the 
same  medium,  and  admit  angles  of,  say,  60°,  90°,  180°,  their  aper- 
tures are  not  in  the  ratios  of  those  numbers,  but  are  as  '50,  '70,  and 
I'O.    The  180°,  for  instance,  does  not  represent  three  times  the  aper- 
ture of  the  60°,  but  twice  only. 

3.  If  the  objectives  work  in  different  media,  as  air  and  oil,  the 
latter  may  have  an  aperture  exceeding  that  of  a  dry  objective  of 
180°  angle.     For  with  the  dry  objective  the  refractive  index  (n)  and 
the  sine  of  half  the  maximum  angle  (u)  both=l,  so  that  n  sin   n 
=  1  also,  whilst  with  the  immersion  objective  n  is  greater  than  1  (say 
1*5  for  oil),  and  the  angle  u  may  therefore  be  much  less  than  in  the 
case  of  the  dry  objective,  and  yet  the  value  of  the  expression  n  sin  // 
(i.e.  the  aperture)  may  be  greater  than  I'O. 

The  two  latter  deductions  are  so  directly  opposed  to  what  was 
accepted  by  the  older  opticians  and  microscopists  that  a  closer  if 
brief  consideration  of  some  of  the  points  which  bear  upon  this  branch 
of  the  subject  may  here  be  serviceably  summarised. 

Take,  first,  the  case  of  the  medium  being  the  same. 

Difference  of  aperture  involves  a  different  quantity  of  liyld  ad- 
mitted to  the  objective  provided  all  other  circumstances  are  equal. 
Hence  the  question  of  aperture  leads  to  the  consideration  of  i\\e  photo- 
metrical  equivalent  of  different  apertures  or  aperture  angles.  It  is 
not  of  the  essence  of  the  problem,  but  it  affords  an  additional  illus- 
tration of  numerical  aperture,  and  is  thus  of  great  service  in  its 
exposition.  It  is  manifest  that  aperture  cannot  be  based  on  quantity 


UNEQUAL   INT  HNS  IT  V    OF   KMITTKI)    LIMIT 


of  light  alone — more  light  can  always  be  obtained  in  the  image  by 
t  li i-o \vin_i:  more  upon  the  object — but  no  increase  in  the  amount  of  illu- 
mination ran  make  a  <hy  lens  equal  in  per formance  a s  re^an Is  aperture 
to  a  wide-angled  immersion  lens. 

The  popular  notion  of  a  pencil  of  light  may  be  illustrated  by 
fig.  33,  which  assumes  that  there  is  equal  intensity  of  emission  in 
all  directions,  and  that  the  intensity  of  a  portion  of  the  pencil  taken 
close  to  the  perpendicular  is  identical  with  that  of  another  portion 
of  equal  angular  extension,  but  morp  removed  from  the  perpendicular. 
On  this  view,  therefore,  the  quantity  of  light  contained  in  any  given 
pencils  may  be  compared  by  simply  comparing  the  contents  of  the 
>olid  cones. 

When,  however,  aperture  is  considered,  and  the  values  of  n  sin  u 
are  worked  out  for  particular  cases,  they  are  seen  to  differ  from 


FIG.  33. — Diagram  showing  erroneous  inference  as  to  the  intensity  of  emitted  rays. 

those  obtained  by  estimating  in  the  above  manner  the  amount  of 
light  in  the  solid  cones,  and  some  perplexity  naturally  arises  from 
the  supposition  that  the  measure  of  the  aperture  of  the  objective  does 
not  correspond  to  that  of  the  quantity  of  light  which  it  admits. 

All  this  arises  from  the  mistaken  assumption  that  a  luminous 
pencil  is  properly  represented  by  fig.  33. 

In  the  last  century  (1760)  Bouguet1  and  Lambert2  established 
the  important  fact  that 
with  any  -urtace  of  uni- 
form I'ti'I'nif.ioii  the  inten- 
sity of  the  emitted  rays  is 
not  the  same  in  all  direc- 
tions. The  {tower  of  emis 
sion  and  the  intensity  of 
the  rays  (i.e.  the  quantity 
of  li«;lit  emanating  from 
a  given  surface-element 
within  a  cone  of  given 
narrow  angle)  varies  in 

the  proportion    of  the  co-  ,- 

1  ...  /.IT  FIG.  34. — The  intensity  of  emitted  rays  is  not  the 

sine  ot  the  angle  of  obliqui-  same  m  an  directions. 

ty  under   which  the  ray  is 

emitted  ;   iii  other  words,  in  the  proportion  of  the  cosine,  of  the  angle 

of  deflection  from  the  perpendicular  to  the  luminous  surface  under 


Traitt  d'Optique  sur  la  Gradation  de  la  Luinii-re,  1760. 
-'  '        1760. 


E2 


52  VISION  WITH  THE   COMPOUND   MICROSCOPE 

which  the  ray  is  sent  out.  The  rays  are  more  intense  in  proportion 
as  they  are  inclined  to  the  surface  which  emits  them,  so  that  a  pencil 
varies  in  proportion  as  it  is  taken  close  to  or  is  removed  from  the 
perpendicular.  A  pencil  is  not,  therefore,  correctly  represented  by 
fig.  33,  but  by  fig.  34,  the  density  of  the  rays  decreasing  continuously 
from  the  vertical  to  the  horizontal. 

Owing  to  the  different  emission  in  different  directions,  the  quan- 
tities of  light  emitted  by  an  element  in  the  same  medium  in  cones  of 
different  angle  such  as  w  and  w',  fig.  35,  are  not  in  the  ratio  of  the 
solid  cones,  as  would  be  the  case  with  equal  emission,  but  in  the 
ratio  of  the  squares  of  the  sines  of  the  semi-angles  so  that  the  squares 
of  the  sines  of  the  semi -angles  constitute  the  true  measure  of  the 
quantity  of  light  contained  in  any  solid  pencil. 

When,  therefore,  the  medium  is  the  same,  it  is  seen  that  there 


FIG.  35. — The  unequal  emission  of  rays. 

is  no  contradiction  between  the  measure  of  the  aperture  of  an  ob- 
jective (n  sin  u)  and  that  of  the  quantity  of  light  admitted  by  it 
the  latter  being  (n  sin  u)2. 

The  simplest  experimental  proof  of  the  unequal  emission  in 
different  directions  will  be  found  in  the  fact  that  the  sun,  the  moon 
the  porcelain  globe  of  a  lamp  or  any  other  bright  spherical  object 
with  so-called  uniform  radiation  in  all  directions,  is  seen  projected 
as  a  surface  of  equal  brightness.  If  there  were  equal  intensity  of 
emission  in  all  directions,  what  would  be  the  necessary  result? 
Compare  two  equal  portions  of  the  surface,  one,  a  (fig.  36),  perpen- 
dicular to  the  line  of  vision,  and  the  other,  6,  greatly  inclined. 
Every  infinitesimal  surface-element  of  b  sends  to  the  pupil  of  the  eye 
a  cone  of  the  same  angle  u',  as  a  similar  point  of  a  (the  slight  differ- 
ence of  the  distance  from  the  eye  being  disregarded).  If  the  in- 
tensity of  the  rays  were  equal  as  supposed,  the  whole  area  b  would 
send  to  the  eye  the  same  quantity  of  light  as  the  equal  area  a, 
since  both  areas  contain  exactly  the  same  number  of  elements.  But 
the  whole  quantity  of  light  from  b  would  be  projected  upon  a 
smaller  area  of  the  retina  than  that  from  a  (as  b  appears  under  a 
smaller  visual  angle,  being  diminished  according  to  the  obliquity,  or 


KADIATION   OF  LIGHT   IN   DIFFEEENT  MEDIA 


53 


as  1  :  cos  w).  Consequently,  if  the  assumption  were  true,  b  must 
appear  to  be  brighter  than  a,  and  the  sphere  would  show  increasing 
brightness  from  the  centre  to  the  circumference.  Close  to  the 
margin  the  increase  ought  to  be  very  rapid,  and  the  brightness  a 
large  multiple  of  that  at  the  centre. 

This,  as  is  well  known,  is  not  the  case,  the  projection  of  the 
sphere  showing  equal  brightness.  The  quantity  of  light,  therefore, 
emitted  from  b  within  a  given  small 
solid  cone  uf  in  an  oblique  direction 
must  be  less  than  that  which  is  emit- 
ted from  a  within  an  equal  solid  cone 
u  in  a  perpendicular  direction,  and  the 
intensity  of  the  rays  must  decrease  in 
the  proportion  of  1  :  cos  w  when  the 
obliquity  w  increases. 

As  then  in  one  and  the  same 
medium  the  number  of  rays  conveyed 
by  a  pencil  and  the  photometrical 
quantity  of  light  are  proportional,  this 
theorem  of  Lambert,  established  for 
more  than  a  century,  is  sufficient  of 
itself  to  overthrow  the  very  basis  of 
the  angular  expression  of  aperture, 
and  to  prove  that,  when  we  are  dealing 
with  one  and  the  same  medium  only, 
the  angle  is  not  the  sufficient  expres- 
sion, but  that  it  is  the  sine  of  the  semi- 
angle  which  must  be  taken. 

We  may  pass  now  to  the  case  of  the 
media  being  different,  as  air  and  oil, 
and  comparing  the  aperture  of  a  dry 
objective  of  180°  with  that  of  an  oil-im- 
mersion objective  of  100°,  the  values  of 
n  sin  u  (or  the  '  numerical '  aperture) 
give  1'Oforthe  former  and  1*17  for  the 
latter,  which  is  therefore  represented  to 
have  a  larger  aperture  than  a  dry  ob- 
jective of  the  greatest  possible  angle. 

In  this  case  also  considerable  per- 
plexity has  arisen.  It  has  been  assumed 
that  the  total  amount  of  light  emitted 
from  a  radiant  point  under  a  given 
fixed  illumination  must  be  the  same, 
whether  the  point  is  in  air,  water,  or  oil,  and  that  that  being  so, 
the  180°  admitted  by  the  dry  objective  must  represent  a  maximum 
quantity  of  light,  a  *  whole  '  which  cannot  be  exceeded,  but  only 
equalled,  by  a  water-  or  oil-immersion  objective.  The  numerical 
aperture  notation  giving  figures  in  excess  of  I'O  (which  represents 
180°  in  air)  is  consequently  supposed  to  be  clearly  erroneous  and 
misleading.  Here  the  whole  difficulty  lies  in  the  absolutely  false 
assumption  that  there  is  identity  of  radiation  in  different  media. 


FIG.  36. — Diagram  of  a  bright 
spherical  object  emitting 
light. 


54  VISION  WITH  THE   COMPOUND  MICROSCOPE 

In  1864  R.  Clausius  established,  by  distinguished  research,  the  propo- 
sition l  that  the  power  of  emission  of  a  body — in  regard  to  heat  as  well 
as  light — is  not  the  same  in  different  media,  but  varies  in  the  ratio 
of  the  squares  of  the  refractive  indices,  so  that  the  whole  emitted 
light  from  any  surface-element  of  a  self-luminous  body  is  increased 
in  the  proportion  of  1  :  n2  when  this  body  is  brought  from  air  into  a 
denser  medium  of  refractive  index  n.  If  a  glowing  body  at  a  con- 
stant temperature,  such  as  a  bar  of  iron,  could  be  immersed  in  a 
medium  of  1'5  refractive  index  in  such  a  way  that  the  surface  were 
in  optical  contact  with  the  medium,  and  the  eye  of  the  observer  im- 
mersed likewise  in  suitable  conditions,  the  body  would  be  seen  brighter 
in  all  directions  in  the  proportion  of  9  :  4  than  it  appeared  in  air. 

The  whole  hemisphere  of  radiation  in  air  is  indeed  less  than  the 
whole  hemisphere  of  radiation  in  water  or  oil,  as  the  squares  of  the 
refractive  indices  of  the  media,  viz.  as  I'O,  1'77,  and  2*25. 

Thus  it  is  seen  that  the  quantity  of  light  emitted  from  an  object 
under  a  given  illumination  is  not  measured  by  the  angle  of  the 
emitted  cone  at  the  radiant,  nor  can  it  be  measured  in  any  way  by 
means  of  the  angle  alone.  The  quantity  depends  under  all  circum- 
stances on  the  product  of  the  sine  of  the  semi-angle  and  the  refractive 
index  of  the  medium  in  which  the  object  is  luminous,  and  is  expressed 
by  the  square  of  this  product,  or  by  the  square  of  the  '  numerical 
aperture '  of  the  pencil. 

It  thus  follows  that  the  estimation  of  the  quantity  of  light  is 
found  to  be  in  complete  accordance  with  the  expression  of  aperture? 
We  are  now  prepared  to  advance  to  another  point.  It  was  a 
view  very  commonly  held  until  recently,  that  the  superiority  of  im- 
mersion objectives  over  dry  ones  was  confined  to  the  case  of  the 
former  being  used  with  balsam-mounted  objects. 

If  wTe  have  a  pencil  in  air,  say  170°,  as  shown  in  fig.  37,  a  dry 
objective  of  large  aperture  will  be  able  to  admit  it.  If,  however,  the 
object  is  in  balsam,  as  in  fig.  38,  it  is  no  longer  possible  for  so  large 
a  pencil  to  emerge  from  the  balsam.  The  rays  shown  by  the  dotted 


FIG.  37. 

lines  in  fig.  38  will  be  totally  reflected  by  the  cover-glass,  and  only 
those  within  a  smaller  angle  of  82°  will  pass  out.  Although  these 
are  expanded  into  180°  on  emerging  into  air,  of  which  the  objective 
takes  up  170°,  yet  this  170°  contains,  it  is  supposed,  less  light  than 
the  170°  in  fig.  37,  as  it  has  been  'diluted'  by  the  refraction. 

1  '  Ueber  die  Concentration  von  Wa'rme-  und  Lichtstrahlen  &c.'     Fogg.  Annalen 
d.  Physik,  cxxi.  1864. 

2  Fig.  A  2  gives  a  good,  practical  illustration  of  the  relative  illuminating  power  of 
objectives  of  varying  apertures,  and  at  the  same  time  affords  a  simple  explanation 
of  the  reason  why  (n  sin  u}2  is  a  measure  of  this  illuminating  power.     Let  the  circles 
A  and  B  represent  the  backs  of  two  objectives  of  the  same  power  but  of  different 
apertures  ;  then  the  radii  C  D  and  E  F  will  represent  the  angle  n  sin  u  (or  fj.  sin  (/>) 
in  fig.  Al  (p.  48,  note).     Now  because  the  areas  of  circles  are  to  one  another  in  the 


RADIATION   IN   AIR  AND   BALSAM 


55 


A  dry  objective  was  therefore  supposed  to  be  placed  at  a  disad- 
vantage when  used  upon  balsam-mounted  objects,  its  aperture  being- 
supposed  to  be  '  cut  down '  by  the  balsam,  and  the  advantage  of  the 
immersion  objective  was  considered  to  rest  on  the  fact  that  it 
restored,  in  the  case  of  the  balsam-mounted  object,  the  same  condi- 
tions as  subsisted  in  the  case  of  the  dry-mounted  object,  allowing  as 
large  (but  no  larger)  an  aperture  to  be  obtained  with  the  former 
object  as  is  obtained  by  the  dry  objective  with  the  latter. 

The  error  here  lies  in  the  assumption  of  the  identity  of  radiation 
in  air  and  balsam.  If  there  were  in  fact  any  such  identity,  the 

170°   IN 


AIR 


FIG.  38. 

conclusion  above  referred  to  would,  of  course,  be  correct,  for  if  in 
fig.  37  the  air  pencil  of  170°  was  identical  with  the  balsam  pencil  of 
1 70°  (shown  by  the  dotted  lines  in  fig.  38),  there  would  necessarily 
be  a  relative  loss  of  light  in  the  latter  case  in  consequence  of  so 
much  of  the  pencil  being  reflected  back  at  the  cover-glass. 

When,  however,  the  increase  of  radiation  with  the  increase  in 
the  refractive  index  of  the  medium  is  recognised,  the  mistake  of  the 
preceding  view  is  appreciated.  The  170°  in  air  of  fig.  37  is  not 
equal  to,  but  much  less  than,  the  170°  in  balsam  of  fig.  38,  and  not- 
withstanding that  a  great  part  of  the  latter  does  not  reach  the 

proportion  of  the  squares  of  their  radii,  it  follows  that  if  we  designate  the  radius  by 
n  sin  u  (or  p  sin  <f>),  the  area  of  the  circle  A  will  be  to  the  area  of  the  circle  B  as  the 
square  of  the  radius  of  A  is  to  the  square  of  the  radius  of  B,  or  as  (n  sin  u)'*  is  to 
(n'  sin  u')2. 


sin. 

Area, 

propo7^io7iaZ  to 


FIG.  A  2.— The  backs  of  two  objectives  of  the  same  power  but  different  apertures. 

The  student  will  observe  that  the  radius  of  B  is  twice  that  of  the  radius  of  A  ; 
consequently  the  area  of  B  will  be  four  times  as  great  as  that  of  A ;  which  means 
that,  since  the  numerical  aperture  of  the  objective  B  is  twice  as  great  as  that  of  the 
objective  A,  its  illuminating  power  will  be  four  times  as  great. 


56  VISION   WITH  THE   COMPOUND   MICROSCOPE 

objective  in  consequence  of  total  reflection,  yet  the  remainder  (80°) 
which  does  reach  it  is  the  exact  equivalent  of  the  air-pencil  of  fig.  37, 
the  two  air-pencils  of  170°  being  in  all  respects  identical. 

The  immersion  objective,  therefore,  which  is  able  to  receive  the 
whole  balsam  pencil  of  170°  (dotted  lines  in  fig.  38),  takes  up  ;i 
greater  quantity  of  light  than  the  air  pencil  of  fig.  37,  and  so  not 
merely  equals  the  dry  objective  but  surpasses  it. 

Let  it  be  specially  noted  that  in  dealing  with  the  quantity  of 
light  in  connection  with  aperture,  the  idea  has  not  been  that  we  have 
been  engaged  with  what  is  in  any  sense  essential,  but  to  remove  a 
difficulty  felt  by  many.  It  must  be  clear  to  all  that  if  a  greater 
aperture  signified  nothing  more  than  a  greater  quantity  of  light,  that 
is  to  say,  if  there  were  no  such  specific  difference  of  the  rays  which 
can  be  utilised  by  different  apertures,  as  we  have  demonstrated 
above,  the  whole  question  would  be  of  quite  subordinate  interest. 

Another  subject  requiring  some  further  elucidation  here  is  the 
'  different  angular  distribution  of  the  rays  in  different  media.'  The 
essence  of  the  idea  of  'aperture'  is  relative  opening.  However 
defined,  its  significance  can  only  be  appreciated  by  taking  into 
account  the  image-forming  pencil  emergent  from  the  objective,  and 
the  change  in  its  diameter  consequent  upon  the  admission  of  different 
cones  of  light.  This  diameter  affords  a  visible  indication  of  the 
number  of  rays  (not  mere  quantity  of  light  photometrically,  which 
can  be  readily  varied)  which  are  collected  to  a  given  area  of  the 
image,  and  which  must  have  been  gathered  in  by  the  lens  from  the 
conjugate  area  of  the  object.  If  the  diameter  of  the  emergent  pencil 
is  seen  to  be  increased,  whilst  the  amplification  of  the  image  and  the 
focal  length  are  unchanged,  it  is  clear  that  the  objective  must  have 
admitted  more  rays  from  every  element  of  the  object  because  it  has 
collected  more  to  every  element  of  an  equally  enlarged  image.  Mani- 
festly we  get  an  accurate  measure  of  what  is  admitted  into  an  objective 
by  being  able  to  estimate  what  it  emits.  It  is  physically  impossible 
that  a  system  of  lenses  should  emit  more  light  than  it  has  taken  in. 

Hence  *  aperture '  means  the  greater  or  less  capacity  of  objectives 
for  gather ing-in  rays  from  luminous  objects. 

When  the  admitted  pencil  is  in  the  same  medium,  we  see  the 
additional  portions  of  the  solid  cone  from  the  radiant,  which  corre- 
spond to  the  additional  portions  of  the  enlarging  opening.  But  if  in 
any  other  case  (e.g.  where  the  medium  is  different)  we  see  that  a 
certain  solid  cone,  A,  from  a  radiant  is  transmitted  through  a  certain 
opening,  a,  and  that  another  solid  cone  of  rays,  B,  cannot  be  trans- 
mitted through  the  same  opening,  a,  but  requires  a  wider  one,  /?, 
whilst  all  other  circumstances,  except  those  of  the  radiant,  have 
remained  the  same,  we  can  only  conclude  that  the  pencil  B  must 
contain  rays  which  are  not  contained  in  A,  even  if  the  admitted  cone 
is  not  increased  in  size.  For  the  additional  portion  (ft  — a)  of  the 
wider  opening,  /3  conveys  rays  to  the  image  wrhich  are  certainly  not 
conveyed  by  the  smaller  opening  a.  From  the  radiant  only  can  this 
surplus  come,  and  the  pencil  B  which  requires  the  additional  opening 
must  embrace  more  rays,  even  if  it  should  not  be  of  greater  angle. 

A  given  objective  may,  in  fact,  collect  the  rays  from  a  radiant  in 


EADIATION   IN  AIR  AND   BALSAM  57 

air  almost  to  the  entire  hemisphere,  and  it  then  utilises  a  definite 
opening  double  its  focal  length.  But  when  the  radiant  is  in  balsam 
(without  any  other  alteration),  the  same  opening  is  seen  to  be  utilised 
by  the  rays  which  are  within  a  smaller  cone  of  not  more  than  82°. 
and  rays  which  are  outside  this  cone  require  a  surplus  opening  which 
is  never  required  for  rays  in  air. 

This  holds  good  whether  there  be  refraction  or  no  refraction  at 
the  front  surface  of  the  system  ;  the  difference  is  based  solely  on  the 
difference  of  the  medium.  Consequently  we  arrive  at  the  conclusion 
that  the  solid  cone  of  82°  in  balsam  embraces  the  same  rays  which, 
in  air,  are  embraced  by  the  whole  hemisphere,  and  every  wider  cone 
in  balsam  exceeding  the  82°  conveys  more  rays  from  the  object  than 
are  admitted  by  the  whole  hemisphere  of  radiation  in  air. 

It  follows,  therefore,  that  the  same  rays  which  in  air  are  spread 
over  the  whole  hemisphere  are  closed  together  or  compressed  in 
balsam  within  a  narrower  conical  space  of  41°  around  the  perpen- 
dicular, and  all  rays  which  travel  in  balsam  outside  this  cone  con- 
stitute a  surplus  of  new  rays,  which  are  never  met  with  in  air — that 
is,  are  not  emitted  ivhen  the  object  is  in  air.  The  loss  which  takes 
place  in  the  latter  case  can  never  be  compensated  for  by  increase  of 
illumination  because  the  rays  which  are  lost  are  different  rays 
physically  to  those  obtained  by  any  illumination,  however  intense,  in 
a  medium  like  air. 

In  the  paper  of  Professor  Abbe  there  is  an  elaborate  and  careful 
elucidation  of  this  change  in  the  angular  distribution  of  the  radiating 
light  when  the  medium  is  changed ;  but  to  Mr.  Crisp's  paper  on  the 
same  subject,  giving  an  exposition  and  simplification  of  Abbe's  de- 
monstration, the  novice  will  look  with  the  utmost  profit.1  The 
following  extract  will  give  clearness  and  emphasis  to  the  above 
deductions  of  Abbe  : — 

'  If  we  take  the  case  of  refraction,  then  one  of  the  most  funda- 
mental of  optical  principles  shows  that  the  same  rays  which  in  air 
occupy  the  whole  hemisphere 
are  compressed  in  a  medium  of 
higher  refractive  index  within 
a  smaller  angle,  viz.  twice  the 

critical    angle.       If  in   fig.    39      gg  SLJD& 

the  object  is  illuminated  by  an 
incident  cone  of  rays  of  nearly        FIG.  39.— Comparative  compression  of 
82°    within    the    slide,    and    the  light  rays  in  two  different  media. 

slide  has  air  above  in  the  first 

case  and  oil  in  the  second,  it  is  obvious  that  the  same  ray  which  is 
incident  on  the  object  at  nearly  41°  will  always  emerge  in  air  at  an 
angle  of  nearly  90°  (a1),  and  in  oil  at  nearly  41°  (a"),  so  that  the 
same  rays  which  in  air  are  expanded  over  the  whole  hemisphere  are 
compressed  into  82°  in  oil,  and,  therefore,  rays  beyond  82°  in  oil 
must  represent  surplus  rays  in  excess  of  those  found  in  the  air- 
hemisphere. 

;  If,  on  the  other  hand,  the  case  of  diffraction  is  considered,  then 
Fraunhofer's  law  shows  that  the  same  diffracted  beams  which  in  air 
1  Journ.  B.M.S.  ser.  ii.  vol.  i.  p.  303. 


58  VISION  WITH  THE   COMPOUND  MICROSCOPE 

occupy  the  whole  hemisphere  (fig.  40)  are  in  oil  compressed  within 
an  angle  of  82°  round  the  direct  beam  (fig.  41),  so  that  there  is  room 
for  additional  beams.' 

The  unequal  equivalent  of  equal  angles  becomes,  therefore,  a  de- 


FIG.  40.— Diffracted  beams  in  air. 


FIG.  41. — Diffracted  beams  in  oil. 


monstrated  truth — a  truth  which  is  capable  of  experimental  proof  by 
every  owner  of  a  fair  microscope. 

Any  one  possessing  a  dry  object-glass  of  an  aperture  of  170°,  for 
example,  may  readily  do  so.     In  this  case,  a,  a,  fig.  42,  will  represent 


FIG.  42. 

the  pencil  radiating  from  an  object  in  air,  and  capable  of  being 
taken  up  by  that  objective.  This  pencil,  on  its  emergence  from  the 
back  lens  of  the  combination,  will  present  a  diameter  somewhat  less 
than  twice  the  focal  length  of  the  objective  presented  in  fig.  43. 
But  let  the  object  be  now  placed  in  Canada  balsam  and 
covered  in  the  usual  way ;  the  angle  of  the  pencil,  by 
the  greater  refractive  power  of  the  medium,  will  be  de- 
monstrably  reduced  to  80°,  as  shown  in  fig.  44.  But  it 
will  be  found,  on  examination  of  the  etnergent  pencil 
from  the  back  lens,  that  this  pencil  occupies  exactly  the 
same  diameter  (fig.  43)  as  before.  The  medium  in  which 
the  object  is  has  not,  of  course,  altered  the  power  of  the 
and  since  the  diameter  of  the  emergent  pencil  is  the  same 
in  both  cases,  the  ratio  of  '  opening '  to  focal  length,  which  is  the 
aperture,  is  the  same  also.  Hence  it  is  seen  in  the  simplest  way 
that  different  angles  in  media  of  different  refractive  indices  may 

170°  IN    AIR 


FIG.  43. 


objective 


FIG.  44. 


in  different  media  denote 


denote  equal  apertures,  and  equal  angle 
different  apertures. 

That  '  immersion  '  objectives  may  have  greater  apertures  than 
the  maximum  attainable  by  a  dry  objective  is  capable  of  equally 
simple  proof  by  accessible  experiment. 

If  an  oil-immersion  objective  of  122°  balsam  angle  be  taken,  and 
so  illuminated  that  the  whole  aperture  is  filled  with  the  incident  rays, 
and  if  we  use  first  an  object  mounted  in  air,  we  really  find  that  we 


DIFFRACTION  RAYS  AEE   IMAGE-FOEMING 


59 


have  a  dry  object-glass  of  nearly  1 80°  angular  aperture.  This  is  readily 
seen  by  fig.  45.  By  the  arrangement  presented  in  the  figure  the  cover- 
glass  is  practically  the 
first  surface 
jective,  for 

the 

and 

FRONT  LENS 


OBJECT  IN  AfR 
SLIDE. 


IMMERSION  FLUID 
COVER  CLASS 


FIG.  45. — Diagram  illustrating  difference  of  emerging 
pencil  without  and  with  balsam. 


of  the  ob- 
the    front 

lens,  the  immersion 
Huid,  and  the  cover- 
glass  are  all  homo- 
geneous, and  of  the 
same  refractive  index, 
and  consequently  they 
form  a  front  lens  of 
extra  thickness.  When 
the  object  is  close  to 
the  cover-glass  the  pencil  radiating  from  it  will  be  very  nearly  180°, 
and  the  emergent  pencil  will  be  seen  to  utilise  so  much  of  the  back 
lens  of  the  combination  as  is  equal  to  twice  the  focal  length  of  the 
objective,  as  shown  in  the  intwr  circle  of  fig.  46. 

If  now  we  run  Canada  balsam  beneath  the  cover-glass  so  as  to 
immerse  the  object,  the  pencil  taken  up  by  the  objective  is  no  longer 
1 180°,  but  only  122°  ;  but  in  spite  of  that  the  diameter 
of  the  emergent  pencil  is  larger  than  it  was  when  the 
angle  of  the  pencil  was  180°  in  air,  and  is  represented 
by  the  outer  circle  in  fig.  46.  In  both  these  cases 
the  power  is  identical ;  it  follows,  therefore,  that  the 
greater  diameter  of  the  emergent  pencil  from  the 
back  of  the  combination  denotes  the  greater  aperture 
of  the  immersion  objective  over  that  of  the  dry  one, 
although  it  possessed  an  angle  of  180°.  From  this  escape  is  impos- 
sible, and  it  is  for  this  reason  that  opticians  make  the  back  lenses  of 
their  immersion  object-glasses  larger  than  those  of  dry  ones  of  the 
same  power. 

Many  further  illustrations  might  be  given,  but  none  affording 
greater  facility  than  the  following,  viz.  :  *  Select  a  good  specimen  of 
Amphipleura  pellucida  and  use  oblique  illumi- 
nation, bringing  out  clearly  the  striation. 

On  removing  the  eye-piece,  placing  the 
on  the  air-image  of  the  diatom,  and 
looking  down  on  the  lens,  the  direct  incident 
beam  will  be  seen  emerging  as  a  bright  spot, 
and  exactly  opposite  and  close  to  the  margin 
a  faint  bluish  light  (see  fig.  47).  If  now  a 
small  piece  of  paper  is  placed  on  the  back  lens 
of  the  objective  so  as  to  just  cover  up  the  blue  I 
light,  and  the  eye-piece  is  replaced,  the  diatom 
is  still  visible,  but  all  the  striation  which  was 
imaged  by  the  blue  marginal  light  has  entirely 
disappeared.  The  latter  must  therefore  consist 
of  image -forming  rays.' 

Enough  has  thus  been  advanced  to  enable  the  student  of  even 
the  elementary  principles  of  modern   object-glass    construction    to 


FIG.  46. 


pupil 
lookir 


on  removing  eye- 
piece when  A. pellu- 
cida has  been  resol- 
ved, showing  spot  of 
bright  light  and  faint 
bluish  spot  opposite. 


60  VISION  WITH  THE   COMPOUND   MICROSCOPE 

demonstrate  for  himself  that  immersion  lenses  not  only  possess  an 
excess  of  aperture  over  dry  lenses,  but  that  the  rays  so  in  excess  are 
image  -  for  ming . 

The  refractive  indices  of  (cedar)  oil,  water,  and  air  are  respec- 
tively 1*52,  1'33,  and  I'O.  'Angular  aperture'  claimed  that  the 
angles  of  the  admitted  pencils  to  lenses  of  these  three  constructions 
expressed  equal  '  apertures.'  But  this  is  a  fallacy,  now  so  palpable, 
but  which  has  exerted  an  influence  so  deterrent  on  the  progress 
of  the  construction  of  our  higher  object-glasses  and  condensers, 
that  its  final  disappearance  as  an  unjustified  assumption  which  had 
crept  into  the  area  of  theoretical  and  practical  optics,  unverified  by 
facts  and  devoid  of  the  wedding  garment  of  deduction,  is  a  triumph 
which  will  make  the  name  of  Abbe  long  and  gratefully  remem- 
bered. 

The  principle  upon  which  increase  of  numerical  aperture  gives 
increased  advantage  to  an  object-glass  manifestly  needs  careful 
study  and  elucidation.  We  have  but  to  refer  to  the  best  work  done 
by  those  who  have  employed  the  microscope  to  any  scientific  purpose 
for  the  past  fifty  years  to  discover  that  there  has  been  an  admission, 
which  has  steadily  strengthened,  that  by  enlargement  of  aperture  an 
increase  in  the  efficiency  of  the  objective,  when  well  made,  was 
inevitable.  During  the  last  thirty-five  years  this  has  been  especially 
manifest.  To  increase  the  aperture  of  an  objective  under  the  name 
of  greater  *  angle '  has  been  the  special  aim  of  the  optician  and  the 
constant  and  increasing  desire  of  all  workers  with  moderate  and 
high  powers. 

The  true  explanation  of  this  is  quite  independent  of  any  con- 
sideration of  apertures  in  excess  of  the  maximum  in  air,  and  indeed 
of  the  whole  question  of  immersion  objectives.  The  old  view  that 
all  high  and  excellent  results  depended  on  the  angle  at  which  the 
light  emerged  from  the  object,  involving  some  assumed  property  of 
a  special  kind  in  the  obliquity  as  such,  has  been  most  tenaciously 
held ;  but  it  is  an  x  in  the  problem  which  has  not  only  never  been 
demonstrated,  but  the  scientific  explanations  of  all  the  optical 
properties  of  lens  combinations  in  the  formation  of  images  by  means 
of  numerical  aperture,  prove  that  it  is  hopeless  to  attempt  to  attach 
any  value  to  angle  as  angle. 

About  thirty  years  ago  it  presented  itself  to  Professor  Abbe  as 
a  problem  worthy  of  most  careful  inquiry  as  to  wrhy  great  '  angle ' 
or  obliquity  as  such  gave  to  objectives  an  enhanced  capacity  in  the 
disclosure  of  obscure  structure.  The  first  step  was  a  consideration 
of  the  grounds  on  which  the  theory  of  the  value  of  angle  of  aperture 
rested.  But  no  such  basis  was  found  to  exist ;  no  investigation  of 
the  question  had  been  made.  It  was  demonstrated  that  a  pencil  of 
170°  would  show  minuter  structure  than  one  of  80°  in  the  same 
medium ;  and  from  this  a  generalisation  had  been  made  that  upon 
the  obliquity  of  the  *  angle '  of  light  depended  the  delineating  power. 
It  was  taken  as  a  self-evident  proposition  that  the  formation  of  the 
image  in  the  microscope  took  place  in  every  particular  according  to 
the  same  dioptric  laws  by  ivhich  images  are  formed  in  the  telescope, 
and  it  was  tacitly  taken  for  granted  that  every  function  of  the 


'ANGLE'   AS   SUCH   OF  NO   VALUE  6 1 

microscope  was  determined  by  the  geometrically  traceable  relations 
of  the  refracted  rays  of  light. 

A  prolonged  course  of  able  and  exhaustive  experiments  con- 
ducted by  Abbe  showed  that,  whilst  the  old  view  held  good  in 
certain  cases  capable  of  definite  verification,  yet  that  the  vast 
majority  of  objects,  and  especially  those  with  which  the  highest 
qualities  of  an  objective  are  called  into  operation,  the  production  of 
the  microscopic  image  is  wholly  and  absolutely  dependent,  not  upon 
the  obliquity  of  the  rays  to  the  object,  as  it  had  been  so  long  and  so 
stoutly  maintained,  but  upon  their  obliquity  to  the  axis  of  the  micro- 


Such  coarse  objects  as  require  only  a  few  degrees  of  aperture 
to  disclose  them  are  dependent  on  '  shadow  effects ; '  but  when 
extremely  minute  and  delicate  structures  are  to  be  disclosed  small 
apertures  are  absolutely  useless,  and  mere  increase  of  obliquity  of 
pencil  as  such  is  powerless  to  alter  the  result.  It  can  be  effected 
only  by  increased  numerical  aperture,  showing  that  the  greater 
obliquity  of  the  rays  incident  on,  or  remitted  from,  the  object  is  not, 
and  cannot  be,  of  itself  an  element  in  the  superior  optical  perform- 
ance of  greater  aperture.  If  it  were  so,  all  the  results  of  increased 
aperture  would  be  secured  by  inclining  the  object  to  the  axis  of  the 
microscope ;  but  it  may  be  readily  tested  that  when  a  given  object 
cannot  be  'resolved,'  or  its  structure  delineated,  by  an  objective  with 
an  aperture  of  80°  in  the  ordinary  position,  but  can  be  resolved  in 
the  ordinary  position  by  an  objective  with  an  aperture  of  90°,  no 
inclination  of  the  object  to  the  axis  of  the  instrument  will  enable  the 
objective  of  80°  to  do  the  work  easily  done  by  one  of  90°.  This 
may  be  tested  by  any  one  possessing  the  instruments. 

As  a  matter  of  fact,  this  so-called  but  imaginary  *  angular  grip  ' 
is  greater  in  a  wide-angled  dry  lens  than  in  one  of  90°  balsam-angle, 
and  it  is  certainly  cut  down  more  and  more  when  with  one  and  the 
same  objective  preparations  are  observed  in  water,  balsam,  and 
cedar  oil  successively.  If  now  the  angles  qua  angles  are  effective 
in  any  way,  something  must  be  lost  by  change  of  angle  in  this  direc- 
tion, and  something  ought  to  be  gained  by  change  in  the  reverse 
direction,  other  conditions  remaining  the  same.  It  is  needless  to 
say  that  all  experience  and  the  entire  course  of  proof  and  reasoning 
given  above  are  diametrically  opposed  to  such  conclusions. 

Similarly  it  will  be  manifest  that  the  conception  that  '  solid 
vision '  or  perspective  effect  in  a  microscopic  image  is  one  of  the 
consequences  of  oblique  *  angular  '  illumination  is  equally  invalid. 
It  assumes  that  the  different  perspective  views  of  a  preparation 
examined  with  the  microscope,  which  correspond  to  the  different 
obliquities,  produce  the  same  effects  as  if  they  were  seen  separately 
by  different  eyes,  as  in  the  case  with  the  binocular  microscope.  In 
reality,  whenever  we  have  the  advantage  of  solid  vision,  owing  to  a 
different  perspective  projection  of  different  images,  in  the  microscope 
or  otherwise,  this  is  solely  because  the  different  images  are  seen  by 
different  eyes.  In  microscopic  vision  there  is  no  difference  of  pro- 
jection connected  with  different  obliquities  ;  in  the  binocular  micro- 
scope there  is  a  diversity  of  images  which  are  depicted  by  pencils  of 


62  VISION  WITH  THE   COMPOUND  MICROSCOPE 

different  obliquities  at  the  object  which  is  a  certain  kind  of  perspec- 
tive difference  ;  but  the  above  and  other  observations  and  experiments 
show  that  even  here  there  is  essential  divergence  from  the  conditions 
of  ordinary  vision. 

It  is  thus  plain  that  whenever  aperture  is  effective  in  delineation 
the  mode  in  which  it  becomes  so  is  not  by  means  of  the  obliquity  of 
the  rays  to  the  object ;  while  it  has  already  been  shown  that 
increase  of  light,  always  concomitant  with  the  use  of  immersion 
objectives,  is  a  relative  advantage,  but  no  part  of  the  explanation  of 
the  superior  action  of  the  combination  of  lenses.  Angle  is  demon - 
strably  not  the  true  basis  for  the  comparison  of  objectives ;  it  fails 
in  regard  to  aperture  in  general,  so  far  as  it  has  relation  to  opening  ; 
it  fails  equally  in  regard  to  the  number  of  rays  and  the  quantity  of 
light  admitted  to  the  system  of  lenses  ;  while  its  failure  in  regard  to 
the  delineating  power  of  objectives  is  everywhere  seen  and  admitted. 

At  the  same  time  it  is  plain  that  the  cause  of  increased  power  of 
performance  in  the  objective  is  directly  connected  with  the  larger 
opening  or  '  aperture  '  of  the  immersion  and  homogeneous  systems. 
In  other  words,  it  becomes  clear  that  something  is  admitted  into  the 
objectives  with  greater  apertures  which  contributes  to  the  formation 
of  an  image,  such  as  objectives  of  lesser  aperture  cannot  form 
because  their  '  openings '  or  '  apertures  '  cannot  admit  that  *  some- 
thing.' 

What  this  is  becomes  explicable  by  the  researches  of  Abbe.  It 
is  demonstrated  that  microscopic  vision  is  sui  generis.  There  is, 
and  can  be,  no  comparison  between  microscopic  and  macroscopic 
vision.  The  images  of  minute  objects  are  not  delineated  microscopi- 
cally by  means  of  the  ordinary  laws  of  refraction  ;  they  are  not 
dioptrical  results,  but  depend  entirely  on  the  laws  of  diffraction. 
These  come  within  the  scope  of  and  demonstrate  the  undulatory 
theory  of  light,  and  involve  a  characteristic  change  which  material 
particles  or  fine  structural  details,  in  proportion  to  their  minuteness, 
effect  in  transmitted  rays  of  light.  The  change  consists  generally 
in  the  breaking  up  of  an  incident  ray  into  a  group  of  rays  with 
large  angular  dispersion  within  the  range  of  which  periodic  alterna 
tions  of  dark  and  light  occur. 

If  a  piece  of  wire  be  held  in  a  strong  beam  of  divergent  light  so 
that  its  shadow  fall  upon  a  white  surface,  the  shadow  wrill  not  be 
sharp  and  black,  but  surrounded  by  luminous  fringes  having  the 
colours  of  the  spectrum,  and  the  centre,  where  the  black  shadow  of 
the  wire  should  be,  is  a  luminous  line,  as  if  the  wire  were  transparent. 
This  phenomenon,  as  is  generally  known,  is  due  to  the  inflection  of 
the  diverging  rays  on  either  side  of  the  wire.  The  inflected  rays,  in 
passing  over  one  edge  of  the  wire,  meet  the  rays  inflected  by  the 
other  edge  and  *  interfere,'  producing  alternate  increase  and  diminu- 
tion of  amplitude  of  oscillation  or  undulatory  intensity,  and  giving 
rise  to  coloured  fringes  if  white  light  is  used,  and  if  homogeneous 
light  be  employed  giving  origin  to  alternate  bands  of  light  and  dark, 
the  centre  always  being  luminous. 

Again,  if  a  disc  perforated  with  a  very  small  hole  in  the  centre 
be  held  in  a  pencil  of  diverging  light,  those  undulations  which  pass 


DIFFE ACTION  PHENOMENA  63 

directly  through  the  aperture  interfere  with  those  passing  obliquely 
at  the  edge  of  the  disc  and  produce,  at  certain  distances,  a  dark 
spot,  at  other  distances  increased  brightness,  on  that  part  of  the 
shadow  which  is  opposite  the  aperture  in  the  disc  ;  so  that  light  is 
supplanted  by  darkness,  and  darkness  changed  to  light,  by  the  discord 
or  concord  of  the  luminous  waves. 

Independently  of  all  experiment,  the  first  principles  of  undulatory 
optics  lead  to  these  experimental  conclusions.  The  laws  of  recti- 
linear propagation  of  the  luminous  rays  of  reflection  and  refraction 
are  not  absolute  laws.  They  arise  from,  and  depend  upon,  a  certain 
relation  between  the  wave-lengths  and  the  absolute  dimensions  of 
the  objects  by  which  the  waves  are  intercepted,  or  reflected,  or 
refracted. 

Taking  as  illustrative  the  waves  of  sound,  an  acoustic  shadow  is 
only  produced  if  the  obstacle  be  many  times  greater  than  the  length 
of  the  sound-waves.  If  the  obstacle  is  reduced,  the  waves  pass  com- 
pletely round  it  and  there  is  no  shadow,  or  if  the  notes  are  of  higher- 
pitch,  so  that  the  waves  are  reduced,  a  smaller  obstacle  than  before 
will  produce  the  shadow.  In  the  case  of  light  there  are  similar 
phenomena.  If  the  obstacles  to  the  passage  of  light  be  large  in 
comparison  with  the  wave-lengths,  shadow  effects  result ;  but  if  the 
linear  dimensions  of  objects  are  reduced  to  small  multiples  of  the 
wave-lengths  of  light,  all  shadows  or  similar  effects  of  solidity  must 
cease.  As  in  the  instances  given  above,  light  and  dark,  or  maxima 
and  minima  of  luminosity,  interchange  their  normal  positions  by 
harmony  or  disharmony  of  luminous  waves. 

It  is  then  by  means  of  diffraction  phenomena  that  Abbe  is 
enabled  to  explain  the  formation  of  the  images  of  objects  containing 
delicate  strife  or  structure,  and  requiring  large  apertures  for  their 
complete  or  approximate  delineation.  In  the  interests  of  this  ex- 
position we  must  here  for  a  moment  diverge  on  slightly  personal 
grounds.  It  has  been  the  good  fortune  of  the  present  editor  to  obtain 
the  courteous  consent  of  Dr.  Abbe  to  read  and  criticise  the  whole 
of  the  present  chapter ;  however  careful  and  earnest  a  student  of 
such  complex  and  original  work  as  Dr.  Abbe  has  done  and  recorded 
in  German  and  English  during  the  last  thirty  years  or  more,  it  is 
impossible  to  be  wholly  satisfied  with  the  most  sympathetic  and 
sincere  desire  to  give  such  work  a  popular  form  unless  it  should 
have  been  perused  and  accepted  by  the  author.  Dr.  Abbe  has  read 
the  entire  chapter,  and,  with  many  generous  words  besides,  relieves 
the  editor  in  his  consciousness  of  great  responsibility  by  saying  that 
he  distinctly  approves  of  the  'lively  interest  and  care  which  (the 
present  editor)  has  bestowed  on  the  exposition  of  his  (Dr.  Abbe's) 
views,'  and  that  he  feels  '  the  greatest  satisfaction  in  seeing  (his) 
views  represented  ...  so  extensively  and  intensively.' 

But  beyond  this,  an  original  worker  like  Dr.  Abbe  would  almost 
inevitably  find,  in  the  course  of  years,  reason  for  slight  verbal  and 
other  more  serious  modifications  of  his  inferences,  explanations,  and 
views ;  and  the  editor  has  great  satisfaction  in  being  able  to  put 
these  modifications  where  they  occur,  with  the  approval  of  Dr.  Abbe. 
In  the  expositions  of  Dr.  Abbe's  views  on  the  diffraction  theory 


64  VISION  WITH  THE   COMPOUND   MICEOSCOPE 

of  microscopic  vision  given  up  to  this  time,  it  has  been  usual  to 
state  that  he  held  and  taught  that  the  microscopic  image  consists  of 
two  superimposed  images,  each  having  a  distinct  character  as  well 
as  a  different  origin,  and  capable  of  being  separated  and  examined 
apart  from  each  other.  The  one  called  the  '  absorption  image  '  is  a 
similitude  of  the  object  itself,  an  image  of  the  main  outlines  of  the 
larger  parts ;  but  by  the  other  image  all  minute  structures,  striation, 
and  delicate  complexity  of  detail  whose  elements  lie  so  close  together 
as  to  occasion  diffraction  phenomena  can  alone  be  formed,  because 
these  could  not  be  geometrically  imaged.  So  that  in  the  case  of  an 
object  with  lines  closer  than  the  ^sVo  °^  an  incn  apart,  the  image 
seen  by  the  eye  is  formed,  not  simply  by  the  central  dioptric  beam, 
but  by  the  joint  action  of  that  and  the  superimposed  diffraction 
images,  and  their  exact  union  in  the  upper  focal  plane  of  the  objective. 
The  first  of  these  was  held  to  be  a  negative  image,  representing 
geometrically  the  constituent  parts  of  the  object ;  but  the  second 
was  considered  a  positive  image  because  it  delineates  structure,  the 
parts  of  which  appear  self-luminous  on  account  of  the  diffraction 
phenomena  which  they  cause.  It  was  this  '  diffraction  image '  that 
was  said  to  be  the  instrument  of  what  has  so  long  been  known  as 
the  *  resolving  '  power  of  lenses. 

But  Dr.  Abbe,  with  the  full  light  of  further  investigation  and 
experience,  does  not  hesitate  to  modify  this  explanation.  He  says : 
'  I  no  longer  maintain  in  principle  the  distinction  between  the 
"  absorption  image  "  (or  direct  dioptrical  image)  and  the  "  diffraction 
image,"  nor  do  I  hold  that  the  microscopical  image  of  an  object 
consists  of  two  superimposed  images  of  different  origin  or  different 
mode  of  production. 

*  This  distinction,  which,  in  fact,  I  made  in  my  first  paper  of  1873, 
arose  from  the  limited  experimental  character  of  my  first  researches 
and  the  want  of  a  more  exhaustive  theoretical  consideration  at  that 
period.  I  was  not  then  able  to  observe  in  the  microscope  the  dif- 
fraction effect  produced  by  relatively  coarse  objects  because  my 
experiments  were  not  made  with  objectives  of  sufficiently  long  focus  ; 
hence  it  appeared  that  coarse  objects  (or  the  outlines  of  objects 
containing  fine  structural  details)  were  depicted  by  the  directly 
transmitted  beam  of  light  solely,  without  the  co-operation  of  diffracted 
light. 

'  My  views  on  this  subject  have  undergone  important  modifica- 
tions. Theoretical  considerations  have  led  me  to  the  conclusion 
that  there  must  always  be  the  same  conditions  of  the  delineation  as 
long  as  the  objects  are  depicted  by  means  of  transmitted  or  reflected 
light,  whether  the  objects  are  of  coarse  or  very  fine  structure. 
Further  experiments  with  a  large  microscope,  having  an  objective 
of  about  twelve  inches  focal  length,  have  enabled  me  to  actually 
observe  the  diffraction  effect  and  its  influence  on  the  image,  viewing 
gratings  of  not  more  than  forty  lines  per  inch.1 

1  Diffraction  effects  may  be  observed  without  a  microscope  ;  they  can  be  easily 
demonstrated  by  observing  a  lamp- flame  through  a  linen  pocket-handkerchief  or  a 
fine  gauze  wire  blind.  This  can  be  done  readily  by  placing  the  eye  close  to  the  linen 
or  wire. 


RECENT  MODIFICATIONS    OF   ABBE'S   VIEWS  65 

•  My  present  views  may  be  thus  expressed  :  With  coarse  objects 
the  diffracted  (bent  off)  rays  belonging  to  an  incident  ray  or  pencil 
are  all  confined  within  a  very  narrow  angular  sy)ace  around  that 
Incident  ray,  and  do  not  appear  separated  from  this  except  with 
objectives  of  very  long  focus.  The  whole  of  such  a  narrow  diffraction 
pencil  is  consequently  always  admitted  to  the  objective  together  with 
the  direct  (incident)  beam,  whatever  may  be  the  direction  of  inci- 
dence, axial  or  oblique.  According  to  the  proposition  of  p.  72  (1) 
the  image  is  in  this  case  strictly  similar  to  the  object,  i.e.  the  effect 
is  the  same  as  if  we  had  a  direct  delineation  by  the  incident  cones 
of  light  alone,  and  as  if  the  image  did  not  depend  at  all  upon  the 
diffractive  action  of  the  object. 

'  If  we  have  a  preparation  like  a  diatom — a  relatively  coarse 
object,  including  fine  structural  details — or  another  preparation  con- 
taining coarse  elements  and  fine  ones  in  juxtaposition,  the  total 
diffraction  effect  may  be  separated  (theoretically  and  practically) 
into  two  parts  :  (1)  that  which  depends  on,  or  corresponds  with,  the 
coarse  object  (e.g.  the  outlines  of  the  diatom)  or  to  the  coarse 
elements  ;  and  (2)  that  depending  upon,  or  resulting  from,  the  fine 
structural  detail  or  the  minute  elements.  The  foregoing  consideration 
applies  to  (1)  :  this  constituent  part  of  the  total  diffraction  pencil 
of  the  preparation  which  is  admitted  to  the  objective  completely, 
independently  of  the  limiting  action  of  the  lens  opening,  and  hence 
the  corresponding  parts  of  the  object  (outlines  £c.)  are  depicted  as 
if  there  were  a  direct  delineation,  i.e.  in  perfect  similarity — even 
with  low  apertures.  Those  diffracted  rays  within  the  whole  diffrac- 
tion pencil  which  are  due  to  the  minute  elements  are  strongly 
deflected  from  the  incident  beams  to  which  they  belong/  1 

According  to  the  less  or  greater  aperture  of  the  objective  and 
the  axial  or  oblique  incidence  of  the  illuminating  pencil  or  cone, 
this  part  of  the  total  diffraction  pencil  will  be  subject  to  a  more  or 
less  incomplete  admission  to  the  objective,  and  the  corresponding 
image  will  therefore  show  the  characteristic  traces  of  the  diffraction 
image,  that  is  to  say,  change  of  aspect  with  different  apertures  and 
different  illumination,  dissimilarity  to  the  real  structure,  and  so 
forth.  Thus  we  have  practically,  in  most  cases,  a  composition  of 
the  microscopical  image,  consisting  of  two  superimposed  images  of 
different  behaviour.  But  the  difference  is  not  to  be  considered  one 
of  principle,  so  far  as  the  production  of  the  image  is  concerned  ;  for 
it  depends  solely  upon  the  different  angular  expression  of  the  diffrac- 
tion fans  resulting  from  coarse  and  from  extremely  fine  elements.2 

Resuming,  then,  our  illustration  of  diffraction  phenomena  as 
applied  to  the  theory  of  microscopic  vision,  we  would  point  out  that 
perhaps  the  most  serviceable  illustration  for  our  purpose  is  a  plate 
of  glass  ruled  with  fine  parallel  lines.  If  the  flame  of  a  candle  be  so 
placed  that  its  image  may  be  seen  through  the  centre  of  the  plate,  this 

1  Letter  from  Dr.  Abbe. 

2  Thus  it  appears  that  both  the   '  absorption  image  '  and  the  '  diffraction  image  ' 
are  now  held  to  be  equally  of  diffraction  origin ;  but,  whilst  a  lens  of  small  aperture 
would  give  the  former  with  facility,  it  would  be  powerless  to  reveal  the  latter  because 
of  its  limited  capacity  to  gather  in  the  strongly  deflected  diffraction  rays  due  to  the 
minuter  elements. 


66 


VISION   WITH  THE    COMPOUND   MICROSCOPE 


central  image  will  be  clear  and  uncoloured,  but  it  will  be  flanked  011 
either  side  by  a  row  of  coloured  spectra  of  the  flame  which  are  fainter 
and  more  dim  as  they  recede  from  the  centre  :  fig.  48  illustrates  this. 
A  similar  phenomenon  may  also  be  produced  by  dust  scattered 
over  a  glass  plate  and  by  other  objects  whose  structure  contains  very 
minute  particles,  the  light  suffering  a  characteristic  change  in  pass 

ing  through  such  objects,  that  change 
consisting  in  the  breaking  up  of  a 
parallel  beam  of  light  into  a  group  of 
rays  diverging  with  wide  angle,  and 
forming  a  regular  series  of  maxima  and 
minima  of  intensity  of  light  due  to  difference  of  phase  of  vibration. 

In  the  same  way  in  the  microscope  the  diffraction  pencil  origi- 
nating from  a  beam  incident  upon,  for  instance,  a  diatom  appears 
as  a  fan  of  isolated  rays,  decreasing  in  intensity  as  they  are  further 
removed  from  the  direction  of  the  incident  beam  transmit  ted  through 
the  structure,  the  interference  of  the  primary  waves  giving  a  number 
of  successive  maxima  of  light  with  dark  interspaces. 

With  daylight  illumination  if  a  diaphragm  opening  be  interposed 
between  the  mirror  and  a  plate  of  ruled  lines  placed  upon  the  stage, 
the  appearance  shown  in  fig.  49  will  be  observed  at  the  back  of  the 

objective  on  removing  the  eye-piece  and 
looking  down  the  tube  of  the  microscope. 
The  central  circle  is  an  image  of  the  dia- 
phragm opening  produced  by  the  direct, 
so-called  non-diffracted  rays,  while  those 
on  either  side  are  the  diffraction  images 
produced  by  the  rays  which  are  bent  off 
from  the  incident  pencil.  In  homogene- 
ous light  the  central  and  lateral  images 
agree  in  size  and  form,  but  in  white  light 
the  diffracted  images  are  radially  drawn 
out  with  the  outer  edges  red  and  the 
inner  blue  (the  reverse  of  the  ordinary 
spectrum),  forming,  in  fact,  regular  spec- 
tra, the  distance  separating  each  of  which  varies  inversely  as  the 
closeness  of  the  lines,  being,  for  instance,  with  the  same  objective 
twice  as  far  apart  when  the  lines  are  twice  as  close. 

The  formation  of  the  microscopical  image  is  explained  by  the 
fact  that  the  rays  collected  at  the  back  of  the  objective,  depicting 
there  the  direct  and  spectral  images  of  the  source  of  light,  reach  in 
their  further  course  the  plane  which  is  conjugate  to  the  object,  and 
give  rise  there  to  an  interference  phenomenon  (owing  to  the  connec- 
tions of  the  undulations),  this  interference  effect  giving  the  ultimate 
image  which  is  observed  by  the  eye-piece,  and  which  therefore 
depends  essentially  on  the  number  and  distribution  of  the  diffracted 
beams  which  enter  the  objective. 

It  would  exceed  the  limits  and  the  object  of  this  handbook  to 
attempt  a  theoretical  demonstration  of  the  action  of  diffraction 


FIG.  49. 


so  as 


spectra  in  forming  the  images  of  fine  structure  and  striation 

to  afford  '  resolution.'     Those  who  desire  to  pursue  this  part  of  the 


DIFFRACTION   EXPERIMENTS 


67 


subject  may  do  so  most  profitably  by  the  study  of  the  only  book  in 
our  language  that  deals  exhaustively  with  the  theory  of  modern 
microscopical  optics,  viz.  the  translation  of  Naegeli  and  Schwendener's 
'  Microscope  in  Theory  and  Practice,'  translated  and  placed  within 
the  reach  of  English  microscopists  by  the  joint  labour  of  Mr.  Frank 
Crisp  and  Mr.  John  Mayall,  jun.  The  experimental  proof  of  the 
diffraction  theory  of  microscopic  vision  lies  within  the  range  of  our 


000000000 
0    0   o  0   0 


FIG.  50. — Diffraction  grating 


FIG.  51. — Diffraction  image  at  back 
of  lens  without  eye-piece. 


purpose,  and  the  following  experiments  will  suffice  to  show  those  who 
possess  the  instruments,  and  desire  the  evidence,  that  to  the  action 
of  diffraction  spectra  we  are  indebted  for  microscopical  delineation. 

The  first  experiment  shows  that  with,  for  instance,  the  central 
beam,  or  any  one  of  the  spectral  beams  alone,  only  the  contour  of 
the  object  is  seen,  the  addition  of  at  least  one  diffraction  spectrum 
being  essential  to  the  visibility  of  the  structure. 

Fig.  50  shows  the  appearance  presented  by  an  object  composed 
of  wide  and  narrow  lines  ruled  on  glass  when  viewed  in  the  ordinary 
way  with  the  eye-piece  in  place,  and  fig.  51  the  appearance  presented 
at  the  back  of  the  objective  wrhen  the  eye-piece  is  removed,  the 


FIG.  52. 


FIG.  58. 


spectra  being  ranged  on  either  side  of  the  central  (white)  image,  and 
at  right  angles  to  the  direction  of  the  lines  ;  in  accordance  with  theory, 
they  are  farther  apart  for  the  fine  lines  than  for  the  wide  ones. 

If  now,  by  a  diaphragm  at  the  back  of  the  objective,  like  fig.  52, 
we  cover  up  all  the  diffraction-spectra,  allowing  only  the  direct  rays 
to  reach  the  image,  the  object  will  appear  to  be  wholly  deprived  of 


68 


VISION    WITH   THE    COMPOUND   MICROSCOPE 


fine  details,  only  the  outline  remaining,  and  every  delineation 
of  minute  structure  disappearing  just  as  if  the  microscope  had  sud- 
denly lost  its  optical  power  (see  fig.  53). 

This  illustrates  a  case  of  the  obliteration  of  structure  by  obstruct- 
ing the  passage  of  the  diffraction-spectra  to  the  eye-piece. 

The  second  experiment  shows  how  the  appearance  of  fine  structure 
may  be  created  by  manipulating  the  spectra. 


FIG.  54. 


FIG.  55. 


If  a  diaphragm  such  as  that  shown  in  fig.  54  is  placed  at  the  back 
of  the  objective,  so  as  to  cut  off  each  alternate  one  of  the  upper  row 
of  spectra  in  fig.  50.  that  row  will  obviously  become  identical  with 
the  lower  one,  and  if  the  theory  holds  good,  we  should  find  the  ima.yv 
of  the  upper  lines  identical  with  that  of  the  lower.  On  replacing 
the  eye -piece  we  see  that  it  is  so  :  the  upper  set  of  lines  are  doubled 
in  number,  a  new  line  appearing  in  the  centre  of  the  space  between 
each  of  the  old  (upper)  ones,  arid  upper  and  lower  sets  having  become 
to  all  appearance  identical  (fig.  55). 

In  the  same  way,  if  we  stop  off  all  but  the  outer  spectra,  as  in  fig. 
56,  the  lines  are  apparently  again  doubled,  and  are  seen  as  in  fig.  57. 


FIG.  56. 


FKI.  57. 


A  case  of  apparent  creation  of  structure  similar  in  principle  to 
the  foregoing,  though  more  striking,  is  afforded  by  a  network  of 
squares,  such  a,s  fig.  58,  having  sides  parallel  to  the  page,  which  gives 
the  spectra  shown  in  fig.  59,  consisting  of  vertical  rows  for  the 
horizontal  lines  and  horizontal  rows  for  the  vertical  ones.  But  it 
is  readily  seen  that  two  diagonal  rows  of  spectra  exist  at  right 


DIFFRACTION  EXPERIMENTS 


69 


angles  to  the  two  diagonals  of  the  squares,  just  as  would  arise  from 
sets  of  lines  in  the  direction  of  the  diagonals,  so  that  if  the  theory 
holds  good  we  ought  to  find,  on  obstructing  all  the  other  spectra  and 


FIG.  58. 


fo   o   o  o  ox 

o  o  O  o  o 

\o  o  o  o  o/ 

o  CL 

- --— 

FIG.  59. 


allowing  only  the  diagonal  ones  to  pass  to  the  eye-piece,  that  the 
vertical  and  horizontal  lines  have  disappeared,  and  two  new  sets  of 
lines  at  right  angles  to  the  diagonals  have  taken  their  place. 


FIG.  61. 


On  inserting  the  diaphragm,  fig.  60,  and  replacing  the  eye-piece, 
we  find,  in  the  place  of  the  old  network,  the  one  shown  in  fig.  61, 


/ 

x  x  x  x  x  x 

x 


x  x  x  x  x 

X  X  X  X  X/ 


FIG.  62. 


FIG.  63. 


the  squares  being,  however,  smaller  in  the  proportion  of  1  :  V  2,  as 
they  should  be  in  exact  accordance  with  theory. 

An    object    such   as    Pleurosigma    angulatum,    which    gives    six 


70  VISION   WITH   THE   COMPOUND  MICKOSCOPE 

diffraction  spectra  arranged  as  in  fig.  62,  should,  according  to  theory, 
show  markings  in  a  hexagonal  arrangement.  For  there  will  be  one 
.set  of  lines  at  right  angles  to  b  a  e,  another  set  at  right  angles  to 
c  af,  and  a  third  at  right  angles  to  g  a  d.  These  three  sets  of  lines 
will  obviously  produce  the  appearance  shown  in  fig.  63. 

A  great  variety  of  other  appearances  may  be  produced  with  this 
same  arrangement  of  spectra.  Any  two  adjacent  spectra  with  the 
central  beam  (as  be  a)  will  form  equilateral  triangles  and  give 
hexagonal  markings.  Or  by  stopping  off  all  but  gee  (or  b  df)  we 
again  have  the  spectra  in  the  form  of  equilateral  triangles  ;  but  as 
they  are  now  further  apart,  the  sides  of  the  triangles  in  the  two 
cases  being  as  V  3  :  1,  the  hexagons  will  be  smaller  and  three  times 
as  numerous.  Their  sides  will  also  be  arranged  at  a  different  angle 
to  those  of  the  first  set.  The  hexagons  may  also  be  entirely 
obliterated  by  admitting  only  the  spectra  g  c  or  g  f  or  bf,  etc.,  when 
new  lines  will  appear  parallel  at  right  angles  or  obliquely  inclined 
to  the  median  line. 

By  varying  the  combinations  of  the  spectra,  therefore,  different 
figures  of  varying  size  and  positions  are  produced,  all  of  which  cannot 
of  course  represent  the  true  structure. 

In  practice,  indeed,  it  has  been  proved  that  if  the  position  and 
relative  intensity  of  the  spectra,  as  found  in  any  particular  case,  be 
given,  what  the  resultant  image  will  be  can  be  reached  by  mathe- 
matical calculations  wholly,  and  with  an  exactness  that  may  even  to 
some  extent  transcend  the  results  of  previous  observation  on  the 
actual  image  of  the  object  whose  spectra  formed  the  mathematician's 
data. 

If  P.  angulatum  be  illuminated  by  central  light  transmitted 
from  an  achromatic  condenser,  and  examined  by  means  of  a  homo- 
geneous lens  of  large  aperture,  Mr.  Stephenson  points  out T  that 
under  ordinary  conditions  it  would  show,  on  withdrawing  the  eye- 
piece and  looking  down  the  tube,  one  bright  central  light  from  the 
lamp  with  six  equidistant  surrounding  diffraction  spectra,  produced 
by  the  lines  ('  if,  indeed,  lines  they  be  ')  in  the  object  itself.  But  let 
a  stop  made  of  black  paper,  which  entirely  excludes  the  central  beam 
of  light,  be  placed  at  the  back  of  the  objective  and  close  to  the  pos- 
terior lens  ;  in  the  stop  let  six  marginal  openings  be  made  through 
which  the  diffraction  spectra  may  pass.  On  examining  the  image 
we  find  that  in  lieu  of  the  ordinary  hexagonal  markings  the  valve 
appears  of  a  beautiful  blue  colour  on  a  black  ground,  and  covered 
with  circular  spots,  clearly  defined,  and  admitting  of  the  use  of  deep 
eye-pieces. 

This  is  precisely  what  we  learn  from  Abbe  that  the  diffraction 
theory  involves.  In  support  of  this,  the  philosophical  faculty  of  the 
University  of  Jena  had  proposed  as  a  question  to  the  mathematical 
students  the  effect  produced  in  the  microscope  by  these  interference 
phenomena.  One  problem  was  that  of  the  appearance  produced  by 
six  equidistant  spectra  in  a  circle  ;  they  correspond  precisely  with 
the  spectra  of  P.  angulatum,  as  accessible  to  us  with  our  present 
numerical  aperture  ;  and  the  diagram  of  the  diffraction  image,  de- 
1  Journ.  E.M.S.  vol.  i.  1878,  p.  186. 


PLEUROSIGMA  ANGULATUM  71 

duced  from  theory,  of  what  spectra  of  the  given  position  and  inten- 
sity of  the  proposed  data  should  give  is  seen  in  fig.  64.  But  what 
seems  quite  as  much  to  the  purpose  is,  that  Dr.  Zeiss  has  produced  a 
fine  photograph  of  P.  anyulaiwm,  given  in  Plate  X.,  where  it  will 
he  seen  that  the  details  shown  in  fig.  64  appear. 

Let  it  be  clearly  understood  that  this  does  not  pretend  to  be  an 
interpretation  of  the  markings  of  the  diatom  ;  it  is  only  held  by 
Abbe  to  be  an  accurate  indication  by  calculation  of  what  image  the 
tfiven  diffraction  spectra  should  produce.  An  optical  glass  and 
media  for  '  mounting  '  and  '  immersion  '  of  immensely  greater  refrac- 
tive and  dispersive  indices — at  present  wholly  inaccessible  to  us — 
must,  he  contends,  be  found  and  employed  before  all  the  diffraction 
spectra  of  P.  angulatum  could  be  admitted  to  form  its  absolute  and 


PIG.  64. 

complete  '  diffraction  image  ; '  but  from  such  spectra  as  the  objective 
employed  can  admit,  it  is  maintained  by  Abbe  that  the  mathe- 
matician can  accurately  show  what  the  image  will  be.  In  the  case  of 
P.  angulatum  theory  indicated  the  optical,  but  not  necessarily  the 
structural  existence  *  of  smaller  markings,  shown  in  fig.  64,  between 
the  circular  spots.  These  had  not  been  before  seen  by  observers  ;  and 
the  mathematician  who  made  the  calculation  (Dr.  Eichhorn)  had  never 
seen  the  diatom  under  the  microscope  ;  but  when  Mr.  Stephenson 
re-examined  the  object — stopping  out  the  central  beam  as  above 
described  and  allowing  the  six  spectra  only  to  pass — he  saw  the 
small  markings,  and  showed  them  at  a  meeting  of  the  Royal  Micro- 
scopical Society  to  many  experts  who  were  there.  They  were  small 
and  faint,  and  no  doubt  purely  optical ;  and,  we  learn  from  experiment, 
may  readily  escape  observation ;  but  by  careful  investigation  they 

1  Con/.  Abbe's  recent  note,  pp.  72  et  seq. 


72  VISION   WITH   THE    COMPOUND   MICROSCOPE 

are  as  present  to  the  observer  as  they  are  capable  of  being  demon- 
strated by  calculation  to  the  mathematician. 

Clearly,  then,  on  these  assumptions  and  with  all  other  considera- 
tions put  aside,  our  finest  homogeneous  objectives  of  greatest  aper- 
ture inevitably  fail  to  reveal  to  us  the  real  structure  of  the  finer 
kinds  of  diatom  valves.  We  learn  that  dissimilar  structures  will 
give  identical  microscopical  images  when  the  difference  of  their 
diffractive  effect  is  removed,  and  conversely  similar  structures  may 
give  dissimilar  images  when  their  diffractive  images  are  made 
dissimilar.  A  purely  dioptric  image  answers  point  for  point  to  the 
object  on  the  stage,  and  therefore  enables  a  safe  inference  to  be 
drawn  as  to  the  true  nature  of  that  object  ;  but  the  diffraction  or 
interference  images  of  minute  structure  stand  in  no  direct  relation 
to  the  nature  of  the  object,  and  are  not  of  necessity  conformable  to 
it.  As  Dr.  Abbe  has  already  insisted,  minute  structural  details  arc 
not  imaged  by  the  microscope  geometrically  or  dioptrically  and  can- 
not be  interpreted  as  images  of  material  forms,  but  only  as  signs  of 
material  differences  of  composition  of  the  particles  composing  the 
object,  so  that  nothing  more  can  safely  be  inferred  from  the  image 
as  presented  to  the  eye  than  the  presence  in  the  object  of  such 
structural  peculiarities  as  will  produce  the  specific  diffraction  pheno- 
mena on  which  the  images  depend.1 

It  follows,  therefore,  that  the  larger  the  number  of  diffracted  rays 
admitted  into  the  objective  the  greater  is  the  similarity  between  the 
image  and  the  object.  But  carefully  observe — 

(1)  Perfect  similarity  between  these  depends  always  on  the  ad- 
mission to,  and  utilisation  by,  the  optical  combination  of  the  ivhole  of 
the  diffracted  rays  which  the  structure  is  competent  to  emit. 

For  the  same  reason  the  diffraction  fan  of  isolated  corpuscles  or 
Jlagella  in  a  clear  field  must  be  exactly  identical  to  that  of  equal  - 
sized  holes  or  slits  of  equal  shape  in  a  dark  background,  and  theory 
shows  that  there  must  be  a  continuous  and  nearly  uniform  dissipa- 
tion of  diffracted  light  over  the  whole  hemisphere,  provided  the 
diameter  of  the  object  is  a  small  fraction  of  the  wave-length  of  light ; 
and  this  would  be  so  even  in  a  medium  of  highest  known  refractive 
index.  Stick  isolated  objects  can  be  seen,  however  minute  they  may 
bz  ;  it  is  merely  a  question  of  contrast  in  the  distribution  of  light,  of 
good  definition  in  the  objective,  and  of  sensibility  of  the  retina. 
The  diffraction  theory  does  not  put  a  limit  to  visibility  with  micro- 
scopic objectives ;  it  simply  proves,  in  theory  and  practice,  what  is 
the  limit  of  visible  separation  in  fine  striation  and  structure. 

In  the  visible  flagellum  of  Bacterium  termo  only  a  fraction  of  a 
wave-length  in  diameter  appears  as  of  considerably  increased  dia- 
meter, even  with  a  very  wide  aperture.  The  image  seen  is  that  of 
another  thread,  the  composition  of  which  theory  can  be  employed  to 

1  See  Abbe's  note,  p.  65.  But  we  cannot  pass  over  in  this  connection  the 
remarkable  paper  in  the  Journ.  Quekett  Club,  ser  ii.  vol.  iv.  on  the  '  Sub-stage 
Condenser,'  by  Mr.  Nelson.  His  photo-micrographs  illustrating  the  mutable  diffrac- 
tion effects  of  the  '  small  cone  '  of  oblique  illumination,  as  distinct  from  a  '  solid  central 
cone,'  and  the  curious  '  ghostly '  diffraction  images  of  the  former,  as  distinct  from  the 
immutable  diffraction  images  of  the  latter,  deserve  careful  consideration.  From 
p.  125  of  the  paper  this  matter  is  carefully  discussed. 


SIX   EQUIDISTANT   SPECTRA    AS   A   DIFFRACTION  PROBLEM     73 

compute,  which  would  give  an  exactly  similar  diffraction  fan,  but 
abruptly  broken  off  at  the  limit  of  the  aperture.  Theory  shows  that 
a  thread-shaped  object  which  could  yield  such  a  particular  diffraction 
effect  must  (without  considering  other  differences)  be  greater  in 
breadth  than  another  one  yielding  the  full  continuous  dissipation  of 
light ;  in  other  words  the  actual  thread,  so  inconceivably  fine, 
belonging  to  the  Bacterium  has  produced  a  i  diffraction  effect ' 
through  the  microscope,  resulting  in  the  appearance  of  a  thread 
which  is  the  '  diffraction  image.'  But  this  latter  is  greater  in  width 
than  the  actual  thread  or  protoplasmic  fibre  would  be  could  it  be 
seen  directly  without  the  aid  of  diffraction. 

(2)  Whenever  a  portion  of  the  total  diffraction  fan  appertaining 
to  a  given  structure  is  lost,  the  image  wTill  be  more  or  less  incomplete 
and  dissimilar  from  the  object ;    and    in  general  the  dissimilarity 
will  be  the  greater  the  smaller  the  fraction  of  light  admitted.     With 
structures  of  every  kind  (regular  and  irregular)  the  image  will  lose 
more  and  more  the  indications  of  the  minuter  details,  as  the  peri- 
pheral (more  deflected)  rays  of  the  diffraction  pencil  are  more  and 
more  excluded.     The  image  then  becomes  that  of  a  different  structure, 
namely,   of  one  the  ivhole  of  whose  diffracted  beams  would  (if  it 
physically  existed)  be  represented  by  the  utilised  diffracted  beams  of 
the  structure  in  question. 

At  this  place  it  is  suitable  to  point  out  that  Dr.  Abbe  em- 
phasises to  the  present  editor  the  importance  of  interpreting  the 
'  intercostal  points '  shown  by  Mr.  Stephenson  in  P.  angulatum 
(fig.  64)  as  not  a  revelation  of  real  structure.  '  The  fact  is  that  the 
image,  which  is  obtained  by  stopping  off  the  direct  beam,  will  be 
more  dissimilar  from  the  real  structure  than  the  ordinary  image. 
It  has  already  been  shown  that  the  directly  transmitted  ray  is  a 
constituent  and  most  essential  part  of  the  total  diffraction  pencil 
appertaining  to  the  structure  ;  it  is  the  central  maximum  of  this 
pencil.  If  this  be  stopped  off  a  greater  part  of  the  total  diffraction 
pencil  is  lost  than  otherwise,  and  the  image,  consequently,  is  a  more  in- 
complete one,  and  therefore  more  dissimilar  than  the  ordinary  image. 

'  The  interest  of  the  experiment  in  question  is  consequently  confined 
to  two  points,  viz. — 

i.  '  It  is  an  exemplification  of  the  general  proposition  that  the 
same  object  affords  different  inages  if  different  portions  of  the  total 
diffraction  fan  are  admitted  to  the  objective. 

ii.  '  The  image  in  question  show^s  to  the  observer  what  would  be 
the  true  aspect  of  that  structure  which  will  split  up  an  incident  beam 
of  light  into  six  isolated  maxima  of  second  order  of  equal  intensity, 
suppressing  totally  the  (central)  maximum  of  the  first  order,  as 
fig.  65 ;  a  structure  of  such  a  particular  and  unusual  diffraction  effect 
is  theoretically  possible,  although  it  may  be  probably  impossible  to 
realise  it  practically.  Mr.  Stephenson's  experiment  shows,  in  fact, 
the  true  projection  of  the  hypothetical  structure. 

(3)  '  As  long  as  the  elements  of  a  structure  are  large  multiples  of 
the  wave-length  of  light,  the  breaking  up  of  the  rays  by  diffraction 
is  confined  to  smaller  and  smaller  angles  ;  that  is,  all  diffracted  rays 
of  perceptible  intensity  will  be  comprised   within  a  narrow  cone 


74 


VISION   WITH   THE    COMPOUND   MICROSCOPE 


around  the  direction  of  the  incident  beam  from  which  they  originate. 
In  such  a  case  even  small  apertures  are  capable  of  admitting  the 
whole.  The  images  of  such  coarse  objects  will  therefore  be  always 
perfectly  similar  to  the  object,  and  the  result  of  the  interference 
effect  is  the  same  as  if  there  were  no  diffraction  at  all,  and  the 
object  were  a  self-luminous  one. 

(4)  '  When  the  elements  of  a  structure  are  reduced  in  diameter  to 
smaller  and  smaller  multiples  of  the  wave-length  which  corresponds 
to  the  medium  in  which  the  object  is,  the  diffraction  pencil  originating 
from  an  incident  beam  has  a  wider  and  wider  angular  expansion 
(the  diffracted  rays  are  further  apart)  ;  and  when  they  are  reduced 

to  only  a  few  wave-lengths,  not  even 
the  hemisphere  can  embrace  the  whole 
diffraction  effect  which  appertains  to 
the  structure.  In  this  case  the  whole 
can  only  be  obtained  by  shortening 
the  wave-length,  i.e.  by  increasing 
the  refractive  index  of  the  surround- 
ing medium  to  such  a  degree  that  the 
linear  dimensions  of  the  elements  of 
the  object  become  a  large  multiple  of 
the  reduced  wave-length.  With  very 
minute  structures,  the  diffraction  fan 
which  can  be  admitted  in  air,  and 
even  in  water  or  balsam,  is  only  a 
greater  or  less  central  portion  of  the 
whole  possible  diffraction  fen  corre- 
sponding to  those  structures,  and  which 
could  be  obtained  if  they  were  in  a 
medium  of  much  shorter  wave-length. 
Under  these  circumstances  no  objec- 
tive, however  wide  may  be  its  aperture, 
can  yield  a,  complete  or  strictly  similar 
image! 

It  is  at  points  of  such  extreme 
delicacy  and  moment  as  this  that  the 
diffraction  hypothesis  of  Dr.  Abbe  is 
so  liable  to  misapprehension  and  mis- 
interpretation, and  a  further  note  from 
him  relating  to  the  dissimilarity  of  the 
image  in  the  case  of  incomplete  admis- 
sion of  the  diffraction  pencil  will  be  of 
great  value  here. 

i.  'In  the  case  of  regular  periodic 
structures  (i.e.  equidistant  striae,  rows  of  apertures, '  dots,'  and  so  forth) 
the  distance  of  the  lines  apart  is,  even  with  an  incomplete  admission 
of  the  diffracted  light,  always  depicted  correctly ;  that  is  to  say,  the 
number  of  the  lines  per  inch  is  never  changed,  provided  the  direct  beam 
(i.e.  the  central  maximum  of  the  diffraction  fan)  is  admitted  to  the 
objective  and  at  least  one  of  the  next  diffracted  rays,,  or,  in^other  words, 
one  of  the  next  maxima  of  second  order.  The  range  of  dissimilarity 


DIFFRACTION   THEOKY   UNIVERSALLY   APPLICABLE         75 

is  in  this  case  confined  to  the  proportion  between  the  bright  and  the 
dark  interspaces  of  the  striation  and  to  the  appearance  of  the  con- 
tours of  the  striae. 

'  If  not  more  than  the  said  two  rays  of  the  total  diffraction  fan  are 
admitted,  the  dark  and  the  light  intervals  are  always  shown  of 
approximately  equal  breadth,  even  if  the  real  proportion  of  both 
intervals  differs  much  from  1:1;  and  the  dark  and  bright  striae  show 
always  gradually  increasing  and  decreasing  brightness ;  in  other 
words,  want  of  distinct  contours. 

'  This  phenomenon  shows  the  typical  picture  of  every  regular 
striation  for  the  depiction  of  which  riot  more  than  two  diffraction 
rays  can  be  utilised.  For  example^  Amphipleura  pellucida,  or  any 
other  striation  which  is  near  to  the  Iftnit  of  resolution  for  the  optical 
system  in  use,  and,  therefore,  even  with  oblique  light,  brings  only 
one  diffracted  beam  into  the  objective. 

ii.  '  Whenever  a  structure  gives  rise  to  a  diffraction  fan  of  con- 
siderable angular  extension,  the  observation  with  a  central  incident 
beam  or  axial  light  may  lose  a  greater  or  smaller  portion  of  the 
wrhole  diffracted  light  if  the  angular  expansion  of  the  fan  extends  to 
the  aperture  of  the  objective  in  use.  But  oblique  illumination  must 
always  involve  a  loss,  and  this  loss  is  not  confined  to  the  external 
(peripheral)  rays  of  the  diffraction  pencil  (as  is  the  case  in  central 
light),  but  the  portion  lost  will  more  and  more  extend  to  one  full 
half  of  the  whole  when  the  obliquity  is  gradually  increased  to  the 
utmost  limit,  so  that  the  direct  beam  touches  the  edge  of  the  aper- 
ture. It  follows  that  the  images  which  are  obtained  with  oblique 
light  will  always  be  incomplete  and  not  similar  to  a  geometrical 
projection  of  the  object ;  and  generally  (though  not  always)  more  dis- 
similar than  those  by  central  light  in  regard  to  the  minuter  details. 

'  Strictly  similar  images  cannot  be  expected,  except  with  a  central 
illumination  with  a  narrow  incident  pencil,  because  this  is  the 
necessary  condition  for  the  possible  admission  of  the  whole  of  the 
diffracted  light.' 

Let  it  be  noted  that  these  principles  of  the  diffraction  theory  of 
microscopical  vision  relate  to  structures  of  all  kinds,  whatever  may 
be  their  physical  and  geometrical  composition.  Irregular  structures, 
isolated  elements  of  any  shape,  equally  produce  diffraction  effects, 
observed  either  by  transmitted  or  reflected  light,  and  being  either 
transparent,  semitransparent,  or  opaque. 

The  value  of  a  =  n  sin  u  indicates  the  number  of  rays  which 
an  objective  can  admit ;  the  aperture  equivalent  measures  the  very 
essence  of  microscopical  performance.  It  measures  the  degree  in 
which  a  given  objectiA^e  is  competent  to  exhibit  a  true,  complete 
delineation  of  structures  of  given  minuteness,  and  conversely  the 
proportion  of  a  in  different  objectives  is  the  exact  measure  of  the 
different  degree  of  'minuteness  of  structural  details  which  they  can 
reach,  either  with  perfect  similarity  of  the  image  or  with  an  equal 
degree  of  incompleteness  of  the  image,  provided  that  the  purely 
dioptrical  conditions  are  the  same. 

'  Resolving  '  power  is  thus  a  special  function  of  aperture.  The 
limit  of  visible  separation  in  delicate  structure  and  striation  is 


7  6  VISION   WITH   THE    COMPOUND  MICROSCOPE 

determined  by  the  fact  that  no  resolution  can  be  effected  unless  at 
least  two  diffraction  pencils  are  admitted,  and  the  admission  of  these 
we  have  seen  is  absolutely  dependent  on  the  aperture  of  the  objective. 

The  rule  given  by  Professor  Abbe  for  determining  the  greatest 
number  of  lines  'per  inch  which  can  be  resolved  by  oblique  light  will 
be  found  (taking  any  given  colour  as  a  basis)  to  be  equal  to  twice 
the  number  of  undulations  in  an  inch  multiplied  by  the  numerical 
aperture. 

To  those  who  have  studied  this  subject  it  will  be  seen  that  the 
'  numerical  aperture  '  here  takes  the  place  of  what  was  formerly  the 
*  sine  of  half  the  angle  of  aperture  ; '  and  it  has  the  effect  of  giving 
the  proposition  a  broader  generality.  By  using  the  '  sine  of  half 
the  angle  of  aperture,'  the  proposition  is  only  true  with  the  addition 
that  the  number  of  undulations  be  calculated  from  the  wave-length 
within  the  special  medium  to  which  the  angle  of  aperture  relates. 

In  introducing  the  numerical  aperture  instead  of  the  sine  of  the 
angle,  the  latter  (the  sine)  is  increased  in  the  proportion  of  1  :  n 
(n  standing  for  the  index  of  the  medium),  and  that  has  the  same 
effect  as  increasing  the  other  factor  the  number  of  undulations. 

What  the  colour  employed  should  be  is  only  capable  of  individual 
determination,  since  the  capacity  for  appreciating  light  varies  with 
different  individuals. 

If,  for  instance,  we  take  '43/u  in  the  solar  spectrum  as  being 
sufficiently  luminous  for  vision,  we  find  the  maximum — so  far  as 
seeing  is  concerned — to  be  118,000  to  the  inch  (the  object,  in  this 
case,  being  in  air)  ;  but  as  the  non -luminous  chemical  rays  remain 
in  the  field  after  the  departure  of  the  visible  spectrum,  a  photo- 
graphic image  of  lines  much  closer  together  might  be  produced. 


AIR 


FIG.  HO. 

This  important  subject  can  scarcely  be  considered  complete,  even 
in  outline,  without  a  brief  consideration,  in  their  combined  relations, 
of  apertures  in  excess  of  180°  in  air  and  the  special  function  these 
apertures  possess. 

1 .  Suppose  any  object  composed  of  minute  elements  in  regular 
arrangement,  such  as  a  diatom  valve  ;  and,  to  confine  the  considera- 
tion to  the  most  simple  case,  suppose  it  illuminated  by  a  narrow 


APPLICATION    OF   THE   DIFFRACTION   THEOKY 


77 


axial  pencil  of  incident  rays.  If  this  object  is  observed  in  air,  the 
radiation  from  every  point  of  the  object  is,  in  consequence  of  the 
diffraction  effect,  composed  of  an  axial  pencil  S,  fig.  66  (the  direct 
continuation  of  the  incident  rays),  and  a  number  of  bent-off  pencils, 
S1?  S2,  •  •  .  surrounding  S.1 

If,  now,  instead  of  air,  the  object  is  immersed  in  a  medium  of 
greater  refractive  index,  n,  it  results  from  Fraunhofer's  formula  that 
the  sine  of  the  angle  of  deflection  of  the  first,  second,  .  .  .  bent-off 


FIG.  67. 

brain  is  reduced  in  the  exact  proportion  of  «,  and  the  angle  is  re- 
duced also — that  is,  the  whole  fan  of  the  diffracted  rays  is  contracted 
in  comparison  with  its  extension  in  air.  Fig.  67  will  represent  the 
case  of  the  same  object  in  oil. 

If  now  any  dry  objective  (with  a  given  angular  semi-aperture  u) 
is  capable  of  gathering-in  from  air  the  first,  or  the  first  and  second, 
diffraction  beams  on  every  side  of  the  axial  pencil,  another  objective 
with  a  more  dense  front  medium  of  the  refractive  index,  n,  will  be 
capable  of  admitting,  from  within  the  dense  medium,  exactly  the  same 
beams  (no  more  and  no  less),  if  its  angular  semi-aperture,  v,  is  less 
than  u  in  the  proportion  : 

sin  v  :  sin  u  =  1  :  n, 

or 

n  sin  v  =  sin  u, 

all  other  circumstances — object  and  illumination — remaining  the 
same. 

For  example,  a  diatom  for  which  the  distance  of  the  stria?  is  0'6  /j, 
will  give  the  first  bent-off  beam  of  green  light  (/\  =  -55/z)  in  air  under 
an  angle  of  66 '5°.  This  will  be  just  admitted  by  a  dry  objective  of 
133°  angular  aperture.  In  balsam  (n  =  l'5)  the  same  pencil  will 
be  deflected  by  37'5°  only,  and  would  be  admitted,  therefore,  by  an 
objective  of  not  more  than  75°  balsam-angle.  Again,  if  the  distance 
of  the  lines  should  be  greater,  as  1'2/u,  the  second  deflected  beam 

1  In  figs.  66,  67,  and  68  S4  and  S6  are  supposed  to  be  identical  with  the  surfaces, 
but  are  drawn  at  a  slight  inclination  to  them  for  the  purpose  of  clearness  in  the  dia- 
grams. 


7  8  VISION   WITH   THE   COMPOUND   MICROSCOPE 

would  be  emitted  in  air  under  an  angle  of  66'5°,  but  in  balsam  the 
third  would  attain  the  same  obliquity.  Whilst  now  the  dry  objective 
of  133°  air-angle  cannot  admit  more  than  the  two  first  diffraction 
beams  on  each  side  of  the  axis,  the  immersion  of  133°  balsam-angle 
is  capable  of  admitting  from  balsam  three  on  each  side  under  exactly 
the  same  illumination.1 

It  follows,  therefore,  that  a  balsam-angle  of  75°  denotes  the  same 
aperture  as  the  larger  air-angle  of  133°,  and  a  balsam-angle  of  133° 
a  much  greater  aperture  than  an  air-angle  of  the  same  number  of 
degrees,  and  in  general  two  apertures  of  different  objectives  must  be 
equal  if  the  sines  of"  the  semi-angles  are  in  the  inverse  ratio  of  the 
refractive  index  of  the  medium  to  which  they  relate — or,  which  is  the 
same  thing,  if  the  product  of  the  refractive  index  multiplied  by  sine 
of  the  angular  semi-aperture  (n  sin  u)  yields  the  same  value  for  both, 
i.e.  if  they  are  of  the  same  numerical  aperture. 

2.  Suppose  the  same  object  to  be  observed  by  a  dry  objective 
of  a  given  air-angle,  at  first  in  air  uncovered,  and  then  in  balsam 
protected  by  a  cover-glass.  The  first  case  would  be  represented  by 


A  i it 


FIG.  68. 

fig.  66,  and  the  second  by  fig.  68.  As  we  have  seen,  the  group  of 
diffracted  beams  from  the  object  in  balsam  is  contracted  in  com- 
parison to  that  in  air  in  the  ratio  of  the  refractive  index.  But 

1   The  following  are  the  actual  angles  represented  in  the  diagrams,  viz.  : 

(Striae  =  2'2  /*,  wave-length  A  =  '55  /JL,  medium  air  n  =  l.) 

51  =  14°  30' 

52  =  30°  0' 
83=48°  36' 
S4  =  90°  0'. 

Strise  =  2'2  /A,  wave-length  A  =  '55  ju,  medium  balsam  n  —  1*5.) 

51  =  9°  36' 

52  =  19°28 

53  =  30°0' 

54  =  41°  48' 

55  =  56°  26' 
8,5  =  90°  0'. 


HOMOGENEOUS    VERSUS   DRY   OBJECTIVES  79 

according  to  the  law  of  refraction,  this  group,  on  passing  to  air  by 
the  plane  surface  of  the  covering-glass,  is  spread  out — the  sines  of 
the  angles  being  compared — in  the  ratio  of  the  same  refractive  index. 
Consequently  the  various  diffraction  pencils,  the  first,  second,  .  .  . 
on  every  side,  after  their  transmission  into  air,  have  exactly  the 
same  obliquity  which  they  have  in  the  case  of  direct  emission  in 
air  from  an  uncovered  object. 

If  now  any  dry  objective  of,  say,  133°  air-angle  is  capable  of 
admitting  a  certain  number  of  these  pencils  from  the  uncovered 
object,  it  will  admit  exactly  the  came  pencils  from  the  balsam- 
mounted  object.  The  contracted  cone  in  balsam  of  75°  angular 
aperture  embraces  all  rays  which  ark  ^emitted  in  air  within  a  cone  of 
133°. 

The  aperture  of  an  objective  is  not,  therefore,  cut  down  by 
mounting  the  object  in  a  dense  medium,  for  no  ray  which  could  be 
taken  in  from  the  uncovered  object  is  lost  by  the  balsam-mounting. 

3.  A  comparison  of  figs.  66,  67,  and  68  will  show  that  a  cone  of 
82°  within  the  balsam  medium  embraces  all  the  diffracted  rays 
which  are  emitted  from  the  object  in  air  or  transmitted  from  balsam 
to  air.  This,  however,  is  not  the  totality  of  rays  which  are  emitted 
in  the  balsam.  The  formula  of  Fraunhofer  shows  that  the  number 
of  the  emitted  beams  is  greater  in  balsam  than  in  air  in  the  same 
ratio  as  the  refractive  index. 

A  structure  the  distance  of  whose  elements  equals  2'2^t  emits  in 
balsam  six  distinct  beams  on  each  side  of  the  direct  beam,  but  in  air 
only  four  (see  figs.  66,  67,  and  68);  the  fifth  and  sixth  are  completely 
lost  in  air.  A  dry  objective  of  an  angular  aperture  closely  approaching 
180°  wTill  not  even  take  in  the  fourth  deflected  beam,  as  this  is  de- 
flected at  an  angle  of  90°.  But  any  immersion-glass  of  a  balsam- 
angle  slightly  exceeding  82°  will  take  in  the  fourth,  and  if  the 
balsam-angle  should  exceed  112°  it  will  take  in  the  fifth  beam  also, 
provided  the  object  is  in  balsam,  and  in  optical  continuity  with  the 
front  of  the  lens. 

Thus,  again,  it  is  seen  (as  was  before  shown  by  the  purely  dioptric 
method)  that  an  immersion  objective  of  balsam-angle  exceeding  82° 
has  a  wider  aperture  than  any  dry  objective  of  maximum  angle  can 
have,  for  it  is  capable  of  gathering  in  from  objects  in  a  dense  medium 
rays  which  are  not  accessible  to  an  air-angle  of  180°. 

It  is,  then,  by  the  above  facts  and  reasoning,  placed  beyond  all 
dispute — 

1 .  That  a  wide-angled  '  immersion  '  or  '  homogeneous '  objective 
possesses  an  aperture  in  excess,  of  180°  '  angular  aperture '  in  air  ; 

2.  That  the  great  value  of  this — always  manifest  practically — is 
fully  accounted  for  and  explained  by  the  diffraction  theory  of  micro- 
scopic vision  ;  and 

3.  That  *  dry  '  objectives,  so  far  as  regards  the  perfect  delineation 
of  very  minute  structures,  can  only  be  considered  as  representing  an 
imperfect  phase  of  construction.     When  made  by  the  best  hands, 
with  every  precaution  and  care  employed  to  secure  the  best  possible 
corrections,  their  defects  do  not  lie  in  the  direction  of  the  presen- 
tation of  false  or    even    partially  erroneous  and    distorted  images. 


80  VISION   WITH   THE   COMPOUND   MICROSCOPE 

Their  defects  are  their  inevitable  incapacity  to  open  up  details  in 
structure  that  can  be  disclosed  with  relative  ease  by  the  inclusion 
into  an  oil  immersion,,  and  especially  an  '  apochromatic '  objective  of 
great  aperture,  of  the  all-revealing  diffraction  beams  excluded  by  the 
dry  lens  of  equivalent  power. 

With  dry  objectives  splendid  results  have  been  attained  both  in 
low  and  high  power  work  ;  but  all  the  latter  is  being  advanced  upon 
by  revision  with  lenses  of  greater  aperture  in  a  striking  manner. 
For  twenty  years  we  have  been  urging  our  best  English  microscope 
makers  to  enlarge  the  *  angle '  of  our  objectives,  and  employing 
them  from  a  1-inch  to  a  5\rinch  focus.  We  have  seen  them 
advance  from  dry  to  water  immersion,  and  from  this  to  oil  ;  from 
v^-inch,  a  TjVinch,  and  a  -10-inch  of  N.A.  O95  each,  and  re- 
spectively to  water  immersions  of  N.A.  1'04  and  then  to  *  oil 
immersions'  or  homogeneous  lenses  of  N.A.  1'38  for  the  ^5 -inch 
and  5Vinch  respectively,  and  ultimately  by  a  ^L-inch  with  N.A. 
of  1'50;  and  from  that  we  have  progressed  to  the  apochromatic 
objectives  with  compensating  eye-pieces. 

Now  the  objectives  with  wThich  the  earlier  work  done  by  the 
present  editor  and  his  colleague,  Dr.  Drysdale,  was  effected — to 
which  allusion  is  made  only  as  being  the  instance  with  which  we 
have  most  practical  familiarity — are  still  in  our  possession  ;  what 
was  revealed  by  them  fifteen,  twelve,  or  ten  years  ago  we  can 
exactly  repeat  to-day  ;  and  in  the  general  features  of  the  work — in 
the  broad  characteristics  of  the  life  histories  of  the  saprophytic 
organisms,  minute  as  they  are,  revision  with  objectives  of  IS".  A.  1*50 
and  other  lenses  of  the  best  English  and  German  makers,  reveals  no 
positive  error,  even  in  the  minutest  of  the  details  then  discovered  and 
delineated.  But  the  later  lenses  of  great  aperture  and  beautiful 
corrections  have  opened  up  structure  absolutely  invisible  before. 

Thus,  for  example,  a  minute  oval  organism  from  the  •jnnnr^1  to 
the  5-oVoth  °f  an  illch  *n  l°ng  diameter  was  known  to  possess  a 
distinct  nucleus ;  the  long  diameter  of  this  was  from  the  ^th  to  the 
rUth  of  the  diameter  of  the  whole  body  of  the  organism.  In  the  ol  >s<  no- 
vations of  fifteen  to  twenty-five  years  since  the  cyclic  changes  of  the 
entire  organism  were  clearly  visible  and  constantly  observed ;  but 
of  the  nucleus  nothing  could  be  made  out  save  that  it  appeared  to 
share  the  changes  in  self-division  and  genetic  reproduction,  initiated 
by  the  organism  as  a  whole.  But  by  lenses  of  N.A.  1'50  and  the 
finest  apochromatic  objectives  of  Zeiss,  especially  a  most  beautifully 
-corrected  3  mm.  and  2  mm.,  structure  of  a  remarkable  kind  has 
been  demonstrated  in  the  nucleus,  and  it  has  been  shown  that  the 
initiation  of  the  great  cyclic  changes  takes  place  in  the  nucleus^  and 
is  then  shared  in  by  the  organism  as  a  w-hole.  In  short,  we  have 
discovered  as  much  concerning  the  *  inaccessible '  nucleus — which 
may  be  not  more  than,  say,  a  twelfth  of  the  long  diameter  of  the 
whole  organism — by  means  of  lower  poivers,  but  greater  apertures,  as 
we  were  able  to  find  concerning  the  complete  body  of  the  saprophyte 
with  dry  objectives. 

But  in  spite  of  these  facts  there  is  a  certain  class  of  even  high 
power  work  in  biology  from  which  the  dry  lens  can  never  be  dis- 


DRY    OBJECTIVES   OF   GREAT   VALUE   STILL  8 1 

missed.  It  must  always  be  an  indispensable  instrument  in  a  large 
part  of  the  work  done  in  the  study  of  the  life  history  of  active 
living  organisms ;  and  whatever  accessories  in.  research  on  such 
subjects  be  employed,  the  main  path  of  accurate  and  well  correlated 
discovery  must  be  by  ultimate  and  consecutive  reference  to  the 
changes  of  the  lii'iiiy  organism.  But  we  cannot  with  any  certainty 
do  this  with  either  a  water  immersion  or  a  homogeneous  objective. 
With  an  active  organism  under  investigation,  we  desire,  as  far  as 
practicable,  to  limit  the  area  of  its  excursions ;  a  cover-glass  of  not 
more  than  four-tenths  or  a  quarter  of  an  inch  in  diameter  is  large 
enough  when  objectives  from  a  ^  inch  to  a  -^  inch  are  used, 
or  when  the  recent  2  mm.  objective*  with  27  eye -piece  is  employed. 
To  have  oil  or  water  on  the  top  of  the  cover,  between  it  and  the 
front  lens  of  the  objective  combination,  is,  with  almost  inevitable 
certainty,  sooner  or  later,  in  following  the  object  with  counter  move- 
ments of  the  stage,  to  reach  the  edge  of  the  cover,  and  cause  the  oil 
or  water  above  to  mingle  by  capillarity  with  the  minute  drop  of  fluid 
under  observation,  and  thus  to  involve  the  whole  in  catastrophe. 

To  do  the  main  work  of  studying  consecutively  the  life  history 
of  unknown  organisms,  dry  objectives  will  and  must  be  used  ;  but 
in  all  cases  such  work  must  be  supplemented  by  the  use  of  objectives 
of  great  aperture.  The  details  and  relations  of  minute  structure 
must  be  studied  in  one  field,  and  their  general  origin  and  sequences 
in  another.  The  latter  will  be  '  continuous,'  the  former  will  be 
employed  as  necessity  indicates.  The  diffraction  theory  of  micro- 
scopic vision  does  not  invalidate,  but  in  reality,  under  definable 
conditions,  directs  the  employment  of  '  narrow '  apertures.  All 
depends  on  the  minuteness  of  microscopic  detail.  The  law  has  been 
enunciated  above  :  the  minuter  the  dimensions  of  the  structural 
elements,  the  wider  must  be  the  aperture  :  the  larger  the  details  of 
ultimate  structure,  the  narrower  the  aperture  that  will  suffice.  This 
is  true  in  regard  to  objects  of  every  kind  ;  there  is  no  variation  in 
the  conditions  of  microscopical  delineation. 

The  men  engaged  in  microscopical  research  have  different  aims, 
nay,  the  same  worker  at  different  times  differs  in  the  object  pursued. 
^  Ultimate  structure '  is  not  the  one  consideration  of  the  micro- 
scopist ;  he  often,  as  indicated  above,  has  to  take  a  comprehensive 
view  of  the  whole  object  or  objects  of  his  research,  apart  from  the 
most  complex  and  delicate  details. 

It  is  folly  to  suppose  that  because  great  apertures  have  been 
proved  theoretically  and  practically  to  be  able  to  open  out  minute 
structure  so  perfectly,  therefore  there  is  no  raison  d'etre  for  small 
apertures.  LOWT  amplification  cannot  render  distinctly  visible  de- 
tails beyond  a  certain  limit  of  minuteness,  and  wide  apertures 
cannot  be  utilised  unless  there  is  a  concurrent  linear  amplification 
of  the  image  which  is  competent  to  exhibit  to  the  eye  the  smallest 
dimensions  which  are  by  optical  law  u-lthin  the  reach  and  grasp  of 
such  an  aperture. 

In  the  same  way  great  amplification  will  be  useless  if  we  have 
small  apertures  which  delineate  details  of  dimensions  only  capable 
of  being  distinctly  seen  in  an  image  of  much  lowTer  amplification. 

G 


82  VISION   WITH   THE    COMPOUND   MICROSCOPE 

It  will  be  '  empty  amplification/  because  there  is  nothing  in  the 
image  which  requires  so  much  power  for  distinct  recognition.  If 
the  power  be  deficient,  aperture  will  not  avail ;  if  the  aperture  be 
wanting,  nothing  is  gained  by  high  power.  If  the  angular  aperture 
of  the  microscope  is  such  that  the  delineation  of  fine  lines,  whose 
interspaces  are  one  micron^  is  just  possible,  it  is  fruitless  labour  to 
increase  the  amplification  beyond  what  we  know  to  be  sufficient  for 
their  observation.  We  potentially  differentiate  what  we  are  power- 
less to  see. 

Thus  it  may  be  inferred  from  the  diffraction  theory,  as  such,  that 
wide  aperture  should  accompany  high  amplification,  and  moderate 
aperture  be  the  accompaniment  of  low  or  moderate  amplification. 
We  have  observed  with  great  regret  that  students  at  our  Biological 
Schools  in  these  days  of  low-priced  objectives  frequently  abandon  a 
fairly  good  ^-inch  objective  of  suitable  numerical  aperture,  and 
obtain  in  its  place  a  |  inch  or  T\T  inch  with  scarcely  any  increase  of 
numerical  aperture,  merely  for  the  ease  with  which  amplification  is 
effected.  But  it  would  be  well  to  remember  that  high  amplification 
effects  nothing  unless  accompanied  by  suitably  widened  aperture. 

The  circumstances  on  which  what  has  been  called  'penetration  ' 
in  objectives  is  dependent  will  be  shortly  considered  ;  2  it  may  be 
stated  here  that  theory  and  experience  alike  show  that '  penetration ' 
is  reduced  with  increasing  aperture  under  one  and  the  same  ampli- 
fication. As  we  have  indicated,  there  are  many  subjects  of  study 
and  research  presented  to  the  biologist  for  which  he  needs  as  much 
'  penetration '  as  possible.  This  is  always  the  case  where  the  recog- 
nition of  solid  forms — as  the  infusoria,  for  example — is  important. 
A  fair  vision  of  different  planes  at  once  is  required.3  Indeed  the 
greater  part  of  all  morphological  work  is  of  this  kind  ;  here,  then, 
in  the  words  of  Abbe,  '  a  proper  economy  of  aperture  is  of  equal 
importance  with  economy  of  powTer.'  4 

Whenever  the  depth  of  the  object  or  objects  under  observation 
is  not  very  restricted,  and  for  the  purposes  of  observation  we  require 
depth  dimension,  IOWT  and  moderate  powers  must  be  used ;  'and  no 
greater  aperture  should  therefore  be  used  than  is  required  for  the 
effectiveness  of  these  pow-ers — an  excess  in  such  a  case  is  a  real 
damage.'  5 

Moreover,  in  biological  work — constant  application  of  the  instru- 
ment to  varied  objects — lenses  of  moderate  aperture  and  suitable 
power  facilitate  certainty  of  action  and  conserve  labour.  Increase 
of  aperture  involves  a  diminished  working  distance  in  the  objective, 
and  it  is  inseparable  from  a  rapid  increase  of  sensibility  of  the 
objectives  for  slight  deviations  from  the  conditions  of  perfect  cor- 
rection. If  it  be  not  necessary  to  encounter  the  possible  difficulties 
these  things  involve,  to  do  so  is  to  lose  valuable  moments.  These 
difficulties,  of  course,  are  diminished  by  the  use  of  homogeneous,  and 

1  A  micron  is  M^ToVo  mm-  Vide  Journ.  E.M.S.  1888,  pp.  502  and  526;  and 
Nature,  vol.  xxxviii.  pp.  221,  244.  -  See  p.  .So. 

•"  Abbe's  explanation  of  the  reason  of  the  non-stereoscopic  perception  of  these  is. 
given  (see  pp.  93  et  seq.}. 

*  '  The  Relation  of  Aperture  to  Power,'  Journ.  P. M.S.  series  ii.  vol.  ii.  i>.  304. 

*  Ibid. 


PENETRATING  POWER   IN   OBJECTIVES  83 

especially  apochromatic  objectives,  but  even  with  these  they  are 
not,  in  practice,  eliminated  where  the  best  results  are  sought. 

Employ  the  full  aperture  suitable  to  the  power  used.  This  is  the 
practical  maxim  taught  in  effect  by  the  Abbe  theory  of  microscopic 
vision. 

It  has  been  suggested  that  all  objectives  be  made  of  relatively 
wide  apertures,  and  that  they  be  *  stopped  down  '  by  diaphragms 
when  the  work  of  '  lower  apertures '  has  to  be  done.  But  this  is 
not  a  suggestion  that  commends  itself  to  the  working  biologist.  If 
there  were  no  other  defects  in  such  a  method,  the  fact  that  the 
working  distance  remains  unaltered  would  be  fatal ;  and  we  may 
safely  adopt  the  statement  of  Abbe,1  <that  '  scientific  work  with  the 
microscope  will  always  require,  not  only  irigh  power  objectives  of 
the  widest  attainable  apertures,  but  also  carefully  finished  lower 
powers  of  small  and  very  moderate  apertures. 

We  complete  this  section  with  a  table  of  numerical  apertures, 
which  will  be  found  on  the  following  page.  As  already  stated,  the 
resolving  powers  are  exactly  proportional  to  the  numerical  apertures, 
and  the  expressions  for  this  latter  will  allow  the  resolving  power  of 
different  objectives  to  be  compared,  not  only  if  the  medium  be  the 
same  in  each,  but  also  if  it  be  different.  The  resolving  power  for  an 
objective,  when  illuminated  by  a  ^  solid  axial  cone  of  white  light,  is 
found  by  multiplying  its  X.A.  by  70,000,  and  for  monochromatic 
blue-green  light  (Gifford's  screen)  by  80,000.2 

The  first  column  gives  the  numerical  apertures  from  "40  to  1*52. 
The  second,  third,  and  fourth,  the  air-,  water-,  and  oil-  (or  balsam-) 
angles  of  aperture,  corresponding  to  every  '02  of  N.A.  from  47°  air- 
angle  to  180°  balsam-angle.  The  theoretical  resolving  power  in 
lines  to  the  inch  is  shown  in  the  sixth  column ;  the  line  E  of  the 
spectrum  about  the  middle  of  the  green  (X  =  0'5269/u)  being  taken. 

The  column  giving  '  illuminating  power,'  we  have  already  seen, 
is  of  less  importance ;  while  it  must  be  borne  in  mind  in  using 
the  column  of  *  penetrating  power '  that  several  data  besides. 

-  go  to  make  up  the  total  depth  of  vision  with  the  microscope. 
a 

Penetrating  Power  in  Objectives, — Intelligibility  and  sequence, 
more  than  custom,  suggest  the  consideration  of  this  subject  at  this 
point.  The  true  meaning  and  real  value  of  *  depth  of  focus,'  or  what 
is  known  as  '  penetrating  power/  follows  logically  upon  the  above 
considerations. 

That  quality  in  an  objective  which  was  supposed  to  endow  it 
with  a  capacity  of  visual  range  in  a  vertical  direction,  that  is,  in  the 
direction  of  the  axis  of  vision,  has  been  called  *  penetration,'  it 
being  supposed  that  by  this  '  property '  parts  of  the  object  not  in 
the  focal  plane  could  be  specially  presented,  so  as  to  enable  their 
perspective  and  other  relations  with  what  lies  precisely  in  the  focal 
plane  to  be  clearly  traced  out. 

Concerning  the  manner  in  which  this  quality  of  the  objective 
operated,  there  have  been  most  diverse  opinions ;  indeed,  the  whole 

1  '  The  Kelatioii  of  Aperture  to  Power,'  Journ.  B.M.S.  series  ii.  vol.  ii.  p.  309. 
3  Journ.  R.M.S.  (1893),  p.  17. 

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mat ter  was  involved  in  obscurity.  The  remarkable  insight,  and 
learning  of  Professor  Abbe  have,  however,  found  for  this  important 
subject  a  sound  scientific  basis. 

The  delineation  of  solid  objects  by  a  system  of  lenses  is  by 
virtue  of  the  most  general  laws  of  optical  delineation,  subject  to  a 
peculiar  disproportion  in  amplification.  The  linear  amplification  of 
the  depth-dimenxum  is,  when  both  the  object  and  the  image  are  in 
the  same  medium  (air),  found  to  be  always  equal  to  the  square  of 
the  linear  amplification  of  the  dimensions  at  right  angles  to  the 
optical  axis  ;  but  if  the  object  be  in  a  more  highly  refracting  medium 
than  air,  it  is  equal  to  this  square  divided  by  the  refractive  index 
of  the  medium.  In  proportion  to  the  lateral  amplification  there  is 
a  progressive,  and  with  high  powers  a  rapidly  increasing,  over- 
amplification  of  the  depth  of  the  three-dimensional  image.  If  a 
transverse  section  of  an  object  is  magnified  100  times  in  breadth  the 
distance  between  the  planes  of  parts  lying  one  behind  the  other  is 
magnified  10,000  times  at  the  corresponding  parts  011  the  axis  when 
the  object  is  in  air,  7500  times  when  it  is  in  water,  and  6600  time- 
when  it  is  in  Canada  balsam. 

This  excessive  distortion  in  the  case  of  high  amplifications  is  not, 
however,  of  itself  so  complete  a  hindrance  to  correct  appreciation  of 
solid  forms  in  the  microscopical  image  as  at  first  appears.  The 
appreciation  of  solid  form  is  not  a  matter  of  sensation  only  ;  it  is  a 
mental  act — a  conception — and,  therefore,  the  peculiarity  of  the 
optical  image,  however  great  the  amplification,  would  not  prevent 
the  conception  of  the  solidity  of  the  object  so  long  as  salient  points 
for  the  construction  of  a  three-dimensional  image  wrere  found.  But 
for  this  the  solid  object,  as  such,  must  be  simultaneously  visible  ; 
a  single  layer  of  inappreciable  depth  can  convey  no  conception  of 
the  three  space  dimensions  possessed  by  the  object. 

Owing  to  the  disproportion!  amplification  of  the  depth-dimension 
normal  to  the  action  of  optical  instruments,  the  visual  space  of  the 
microscope  loses  more  and  more  in  depth  as  the  amplification  increases, 
and  thus  constantly  approximates  to  a  bare  horizontal  section  of  the 
object. 

The  visual  space,  which  at  one  adjustment  of  the  focus  is  plainly 
visible,  is  made  up  of  two  parts,  the  limits  of  which  as  regards  the 
depth  are  determined  in  a  very  different  mariner. 

First,  the  accommodation  of  the  eye  embraces  a  certain  depth, 
different  planes  being  successively  depicted  with  perfect  sharpness 
of  image  on  the  retina,  whilst  the  eye,  adjusting  itself  by  conscious 
or  unconscious  accommodation,  obtains  virtual  images  of  greater  or 
less  distance  of  vision.  This  depth  of  accommodation,  which  pla.vs 
the  same  part  in  microscopical  as  in  ordinary  vision,  is  wholly 
determined  by  the  extent  of  power  in  this  direction  possessed  by  the 
particular  eye,  the  limits  being  the  greatest  and  the  least  distance 
of  distinct  vision.  Its  exact  numerical  measure  is  the  difference 
between  the  reciprocal  values  of  these  two  extreme  distances.  The 
depth  of  distinct  vision  is  directly  proportional  to  this  numerical 
equivalent  of  the  accommodation  of  the  eye,  directly  proportional 
to  the  refractive  medium  of  the  object,  and  inversely  proportional 
to  the  square  of  the  amplification  when  referred  always  to  the  same 


PRINCIPLES   OF   STEREOSCOPIC   VISION  89 

image-distance.  For  example,  a  moderately  short-sighted  eye  sees 
distinctly  at  l-~>(>  mm.  as  its  shortest  distance,  and  at  300  mm.  as 
its  longest  distance;  then  the  numerical  equivalent  of  the  extent 
of  accommodation  would  be  equal  to  -jj^y  mm.;  the  calculation  for 
an  object  in  air  would  give  a  depth  of  vision  by  accommodation 
amounting  to 

2-08  mm.  with  10  times  amplification 

0-23  „        30 

0-02  „      100 

0-0023       „      300 

0-00021     „    1000 

0-00002     „     3000 

These  figures  are  modified  by  the  medium  in  which  the  object  is 
placed  and  by  the  greater  or  less  shortness  and  length  of  vision. 

Secondly,  the  perception  of  depth  is  assisted  by  the  insensi- 
bility of  the  eye  to  small  defects  in  the  union  of  the  rays  in  the  optic- 
image,  and  therefore  to  small  circled  of  confusion  in  the  visual  image. 
Transverse  sections  of  the  object  which  are  a  little  above  and  below 
the  exact  focal  adjustment  are  seen  without  prejudicial  effects.  The 
total  effect  so  obtained  is  the  so-called  penetration  or  depth  of  focus 
of  an  objective.  This  may  be  determined  numerically  by  defining 
the  allowable  magnitude  of  the  circles  of  confusion  in  the  micro- 
scopical image  by  the  visual  angle  under  which  they  appear  to  the  eye. 
It  is  found  that  one  minute  of  arc  denotes  the  limit  of  sharply 
defined  vision,  two  to  three  minutes  for  fairly  distinct  vision,  and  five 
to  six  minutes  the  limits  of  vision  only  just  tolerable.  This  being 
determined,  the  focal  depth  depends  only  011  the  refractive  index  of 
the  medium  in  which  the  object  is  placed,  the  amplification,  and  the 
angle  of  aperture,  and  it  is  directly  proportional  to  the  refractive 
index  of  the  object  medium,  and  inversely  proportional  to  the 
•  numerical  aperture  '  of  the  objective,  as  also  to  the  first  power  of 
the  amplification.  These  assume  the  visible  angle  of  allowable 
indistinctness  to  be  fixed  at  5',  the  aperture  angle  of  the  image- 
forming  pencils  to  be  60°  in  air  ;  the  depth  of  focus  of  an  object  in 
tit i"  will  then  be — 

0-073  mm  for  10  times  amplification 

0-024  30 

0-0073  100 

0-0024  300 

0-00073  1000 

0-00024  3000 

I5y  limiting  or  enlarging  the  allowable  magnitude  of  indistinctness 
in  the  image  we  correspondingly  modify  these  figures,  as  we  should 
do  with  media  of  different  refractive  indices  and  increased  aperture  - 
angle. 

It  is  plain,  then,  that  the  actual  depth  of  vision  must  always  be 
the  exact  sum  of  the  accommodation  depth  and  focal  depth.  The 
former  expresses  the  object  space  through  which  the  eye  by  the  play 
of  accommodation  can  penetrate  and  secure  a  sharp  image  ;  the  latter 
gives  the  amount  by  which  this  object-space  is  extended  in  its 
limits — reckoning  both  from  above  and  below — because  without  per- 
fect sharpness  of  image  there  is  still  a  sufficient  distinctness  of  vision. 


VISION    WITH   THE   COMPOUND    MICROSCOPE 


A.s    the   amplification   increases   tin-   over-amplification    of  the 

depth  dimension  presents  increasingly  unfavourable  relation  between 
the  depth  and  width  of  the  object-space  accessible  to  acconnnodat  ion. 
When  low  powers  are  employed  we  have  relatively  great  depth  of 
vision,  because  we  have  large  accommodation-depth  ;  but  as  we  pass 
to  medium  powers,  the  accommodation-depth  diminishes  in  rapid 
ratio,  becoming  equal  to  only  a  small  depth  of  focus;  while  when 
the  magnifying  power  is  greatly  increased  the  accommodation-depth 
is  a  factor  of  no  moment,  and  we  have  vision  largely,  indeed  almost 
wholly,  dependent  on  depth  of  focus. 

The  following  table  shows  the  total   depth  of  vision  from  ten  to 
3,000  time.; 


Amplification 

10 

30 

100 

300 
1000 

3000 


Aocommoda-  I 
tiou  Depth     I   Focal  Depth 


Depth  of  Vision,  Ratio  of  Dei  >th 

Accommodation  of  Vision  to 

Depth,  and  Diameter  of 

Focal  Depth  Field 


nun. 
25-0 

8-3 
2-5 

0-83 
0-25 

0-083 


2-08 
0-23 
0-02 

0-0023 
0-00021 

0-00002 


mil). 
0-073 

0-024 
0-0073 

0-0024 
0-00073 

0-00024 


mm. 
2-153 


0-254 
0-0273 

0-0047 
0-00094 

0-00026 


1 

11-6 
1 


1 
170-6 

1 

263 
^L 

819 


It  has  been  pointed  out  by  Abbe  that  this  over-amplification  of 
depth-dimension,  though  it  limits  the  direct  appreciation  of  sol  id 
forms,  yet  is  of  great  value  in  extending  the  indirect  recognition  of 
space  relations.  When  with  increase  of  magnifying  power  the  depth 
of  the  image  becomes  more  and  more  flattened,  the  images  of  different 
planes  stand  out  from  each  other  more  perfectly  in  the  same  ratio. 
and  in  the  same  degree  are  clearer  and  more  distinct.  With  an 
increase  of  amplification  the  microscope  acquires  increasingly  the 
property  of  an  optical  microtome,  which  presents  to  the  observer's 
eye  sections  of  a  fineness  and  sharpness  which  would  be  impossible 
to  a  mechanical  section.  It  enables  the  observer,  by  a  series  of 
adjustments  for  consecutive  planes,  to  construe  the  solid  forms  of 
the  smallest  natural  objects  with  the  same  certainty  as  he  is 
accustomed  to  see  with  the  naked  eye  the  objects  with  which  it  is 
concerned.  This  is  a  large  advantage  in  the  general  scientific  u<e 
of  the  instrument;  a  greater  gain,  in  fact,  than  could  be  expected 
from  the  application  of  stereoscopic  observation. 

Stereoscopic  Binocular  Vision. — This  subject  has  been  elaborately 
considered  and  partially  expounded  and  demonstrated  by  Pro fessor 
Abbe  ;  his  exposition  differs  in  some  important  particulars  from 
that  of  the  original  author  of  this  book,  but  in  its  present  incomplete 


STEREOSCOPIC   BINOCULAR   VISION  91 

forms  it  appears  to  the  editor  to  be  the  wiser  way  to  allow  Dr.  Car- 
penter's treatment  of  the  subject  to  stand,  ami  to  place  below  it  as 
complete  a  digest  of  Professor  Abbe's  theory  and  explanation  of  the 
same  subject  as  the  data  before  us  will  admit. 

The  admirable  invention  of  the  stereoscope  by  Professor  Wheat- 
stone  has  led  to  a  general  appreciation  of  the  value  of  the  cotr/oi at 
use  of  both  eyes  in  conveying  to  the  mind  a  notion  of  the  solid  form* 
of  objects,  such  as  the  use  of  either  eye  singly  does  not  generate  with 
the  like  certainty  or  effectiveness ;  and  after  several  attempts, 
which  were  attended  with  various  degrees  of  success,  the  principle  of 
the  stereoscope  has  now  been  applied  to  the  microscope,  with  an 
advantage  which  those  only  can  truly  estimate  who  (like  the  Author) 
have  been  for  some  time  accustomed  to  work  with  the  stereoscopic 
binocular1  upon  objects  that  are  peculiarly  adapted  to  its  powers. 
As  the  result  of  this  application  cannot  be  rightly  understood  with- 
out some  knowledge  of  one  of  the  fundamental  principles  of  binocular 
vision,  a  brief  account  of  this  will  be -here  introduced.  All  vision 
depends  in  the  first  instance  on  the  formation  of  a  picture  of  the 
object  upon  the  retina  of  the  eye,  just  as  the  camera  obscura  forms 
a  picture  upon  the  ground  glass  placed  in  the  focus  of  its  lens.  But 
the  two  images  that  are  formed  by  the  two  eyes  respectively  of  any 
solid  object  that  is  placed  at  no  great  distance  in  front  of  them  are 
far  from  being  identical,  the  perspective  projection  of  the  object 
varying  with  the  point  of  view  from  which  it  is  seen.  Of  this  the 
reader  may  easily  convince  himself  by  holding  up  a  thin  book  in 
such  a  position  that  its  back  shall  be  at  a  moderate  distance  in  front 
of  the  nose,  and  by  looking  at  the  book,  first  with  one  eye  and  then 
with  the  other  ;  for  he  will  find  that  the  two  views  he  thus  obtains 
are  essentially  different,  so  that  if  he  were  to  represent  the  book  as 
he  actually  sees  it  with  each  eye,  the  two  pictures  would  by  no 
means  correspond.  Yet  on  looking  at  the  object  with  the  two  eyes 
conjointly,  there  is  no  confusion  between  the  images,  nor  does  the 
mind  dwell  on  either  of  them  singly  ;  but  froin  the  blending  of  the 
two  a  conception  is  gained  of  a  solid  projecting  body,  such  as  could 
only  be  otherwise  acquired  by  the  sense  of  touch.  Now  if,  instead 
of  looking  at  the  solid  object  itself,  we  look  with  the  right  and  left 
eyes  respectively  at  pictures  of  the  object,  corresponding  to  those 
which  would  be  formed  by  it  on  the  retina?  of  the  twro  eyes  if  it  were 
placed  at  a  moderate  distance  in  front  of  them,  and  these  visual 
pictures  are  brought  into  coincidence,  the  same  conception  of  a  solid 
projecting  form  is  generated  in  the  mind,  as  if  the  object  itself  were 
there.  The  stereoscope — whether  in  the  forms  originally  devised  by 
Professor  Wheatstone  or  in  the  popular  modification  long  subse 
quently  introduced  by  Sir  D.  Brewster — simply  serves  to  bring  to 
the  two  eyes,  either  by  reflexion  from  mirrors  or  by  refraction 
through  prisms  or  lenses, 'the  two  dissimilar  pictures  which  would 
accurately  represent  the  solid  object  as  seen  by  the  two  eyes  respec- 

1  It  has  become  necessary  to  distinguish  the  binocular  microscope  which  gives 
true  stereoscopic  effects  by  the  combination  of  two  dissimilar  pictures  from  a 
binocular  which  simply  enables  us  to  look  with  both  eyes  at  images  which  are 
essentially  identical  (p.  106). 


92  VISION   WITH   THE    COMPOUND   M1CKOSCOPE 

tively,  these  being  thrown  on  the  two  retina'  in  the  precise  positions 
they  would  have  occupied  if  formed  there  direct  from  the  solid 
object,  of  which  the  mental  image  (if  the  picture  li;i\ •»•  been  correctly 
taken)  is  the  precise  counterpart.  Thus  in  fig.  69  the  upper  pair  of 
pictures  (A,B)  when  combined  in  the  stereoscope  suggest  the  idea  of 
&  projecting  truncated  pyramid,  with  the  small  square  in  the  centre 
and  the  four  sides  sloping  equally  away  from  it ;  whilst  the  combi- 
nation of  the  lower  pair,  C,  D  (which  are  identical  with  the  upper. 
but  are  transferred  to  opposite  sides),  no  less  vividly  brings  to  tin- 
mind  the  visual  conception  of  a  receding  pyramid,  still  with  the  si  MM  11 
square  in  the  centre,  but  the  four  sides  sloping  equally  towards  it. 

Thus  we  see  that  by  simply  crossing  the  pictures  in  the  stereo- 
scope, so  as  to  bring  before  each  eye  the  picture  taken  for  the  other, 
a  *  con  version  of  relief '  is  produced  in  the  resulting  solid  image, 
the  projecting  parts  being  made  to  recede  and  the  receding  part> 
brought  into  relief.  In  like  manner,  when  several  objects  are  com- 


FIG.  «9. 


bined  in  the  same  crossed  pictures,  their  apparent  relative  distances- 
are  reversed,  the  remoter  being  brought  nearer  and  the  nearer 
carried  backwards  ;  so  that  (for  example)  a  stereoscopic  photograph 
representing  a  man  standing  in  front  of  a  mass  of  ice  shall,  by  the 
crossing  of  the  pictures,  make  the  figure  appear  as  if  imbedded  in  the 
ice.  A  like  conversion  of  relief  may  also  be  made  in  the  case  of 
"actual  solid  objects  by  the  use  of  the  pseudoscope,  an  instrument 
devised  by  Professor  Wheatstone,  which  has  the  effect  of  reversing 
the  perspective  projections  of  objects  seen  through  it  by  the  two 
eyes  respectively  ;  so  that  the  interior  of  a  basin  or  jelly-mould  is 
made  to  appear  as  a  projecting  solid,  whilst  the  exterior  is  made  to 
appear  hollow.  Hence  it  is  now  customary  to  speak  of  stereoscopic 
vision  as  that  in  which  the  conception  of  the  true  natural  relief  of  an 
object  is  called  up  in  the  mind  by  the  normal  combination  of  the 
two  perspective  projections  formed  of  it  by  the  right  and  left  eyes 
respectively  ;  whilst  by  pseudoscopic  vision  we  mean  that  '  conver- 
sion of  relief  '  which  is  produced  by  the  combination  of  two  reversed 


CARPENTER'S    I'.    ABBE'S   VIEW   OF   STEREOSCOPIC   VISION     93 

perspective  projections,  whether  these  be  obtained  directly  from  the 
object  (as  by  the  pseudoecope)  or  from  'crossed'  pictures  (as  in  the 
stereoscope).  It  is  by  no  means  every  solid  object,  however,  or  everv 
pair  of  stereoscopic  pictures  which  can  become  the  subject  of  this 
conversion.  Tlie  decree  of  facility  with  which  the  *  converted  '  form 
can  be  apprehended  by  the  mind  appears  to  have  great  influence  on 
the  readiness  with  which  the  change  i*  produced.  And  while  there 
are  some  objects — the  interior  of  a  plaster  mask  of  a  face,  for  ex- 
ample— which  can  always  be  *  converted  '  (or  turned  inside  out)  at 
once,  there  are  others  which  resist  such  conversion  with  more  or  less 
of  persistence.1 

Xo\v  it  is  easily  shown  theoretically  that  the  picture  of  any 
projecting  object  seen  through  the  microscope  with  only  the  riyht- 
hand  half  of  an  objective  having  an  even  moderate  angle  of  aperture, 
must  differ  sensibly  from  the  picture  of  the  same  object  received 
through  the  left  hand  of  the  same  objective  ;  and,  further,  that  the 
difference  between  such  pictures  must  increase  with  the  angular 
aperture  of  the  objective.  This  difference  may  be  practically  made 
apparent  by  adapting  a  '  stop'  to  the  objective  in  such  a  manner  as 
to  cover  either  the  right  or  the  left  half  of  its  aperture,  and  then  by 
carefully  tracing  the  outline  of  the  object  as  seen  through  each  half. 
But  it  is  more  satisfactorily  brought  into  view  by  taking  two  photo- 
graphic pictures  of  the  object,  one  through  each  lateral  half  of  the 
objective  ;  for  these  pictures  when  properly  paired  in  the  stereo- 
scope ii'ive  a  magnified  image  in  relief,  bringing  out  on  a  large  scale 
the  solid  form  of  the  object  from  which  they  were  taken.  What  is 
needed,  therefore,  to  give  the  true  stereoscopic  power  to  the  micro- 
scope is  a  means  of  so  bisecting  the  cone  of  rays  transmitted  by  the 
objective  that  of  its  two  lateral  halves  one  shall  be  transmitted  to 
the  right  and  the  other  to  the  left  eye.  If,  however,  the  image  thus 
formed  by  the  right  half  of  the  objective  of  a  compound  microscope 
were  seen  by  the  right  eye,  and  that  formed  by  the  left  half  were 
seen  by  the  left  eye,  the  resultant  conception  would  be  not  stereo- 
scopic but  pseudoscopic,  the  projecting  parts  being  made  to  appear 
receding,  and  vice  versa.  The  reason  of  this  is,  that  as  the  microscope 
itself  reverses  the  picture,  the  rays  proceeding  through  the  right  and 
the  left  hand  halves  of  the  objective  must  be  made  to  cross  to  the 
left  and  the  right  eyes  respectively,  in  order  to  correspond  with  the 
direct  view  of  the  object  from  the  two  sides ;  for  if  this  second 
reversal  does  not  take  place,  the  effect  of  the  first  reversal  of  the 
images  produced  by  the  microscope  exactly  corresponds  with  that 
produced  by  the  '  crossing '  of  the  pictures  in  the  stereoscope,  or  by 
that  reversal  of  the  two  perspective  projections  formed  direct  from 
the  object,  which  is  effected  by  the  pseudoscope.  It  was  from  a 
want  of  due  appreciation  of  this  principle  (the  truth  of  which  can 
now  be  practically  demonstrated)  that  the  earlier  attempts  at  pro- 
ducing a  stereoscopic  binocular  microscope  tended  rather  to  produce 
a  '  pseudoscopic  conversion  '  of  the  objects  viewed  by  it  than  to 
represent  them  in  this  true  relief. 

1  For  a  fuller  discussion  of  this  subject  see  the  Author's  Mental  Physiology, 
S§  1CH-170. 


94  VISION  WITH   THE   COMPOUND   MICROSCOPE 

In  contradistinction  to  this  explanation  of  binocular  vision  Dr. 
Abbe,  as  we  have  seen,  has  demonstrated  that  oblique  vision  in  the 
microscope  is  wholly  unlike  ordinary  vision  ;  there  is,  in  fact,  no 
perspective.  The  perspective  shortening  of  lines  and  surfacfs  1>\ 
oblique  projection  is  entirely  lost  in  the  microscope,  and,  as  a  con- 
sequence, it  is  contended  that  the  special  dissimilarity  which  is  the 
raison  d'etre  of  ordinary  stereoscopic  effects  does  not  exist,  but  that. 
an  essentially  different  mode  of  dissimilarity  is  found  between  the 
two  pictures.  The  outline  or  contour  of  a  microscopic  object  is 
unaltered,  whether  viewed  by  an  axial  or  an  oblique  pencil ;  there  is 
no  foreshortening,  there  is  simply  lateral  displacement  of  the  images 
of  consecutive  layers.  But  Abbe  contends  that,  whilst  the  manner  in 
which  dissimilar  pictures  are  formed  in  the  binocular  microscope  is 
different  from  that  by  which  they  are  brought  about  in  ordinary 
stereoscopic  vision,  yet  the  activities  of  the  brain  and  mind  by  which 
they  are  so  blended  as  to  give  rise  to  sensations  of  solidity,  depth. 
and  perspective  are  practically  identical. 

The  fact  that  lateral  displacements  of  the  image  are  seen  in  the 
microscope  depends  on  a  peculiar  property  of  microscopic  amplifica- 
tion, which  is  in  strong  contrast  to  the  method  of  ordinary  vision. 
It  depends  entirely  on  the  fact,  enunciated  above,  that  the  amplifi- 
cation of  the  depth  is  largely  exaggerated.  Hence  solid  vision  in 
the  binocular  microscope  is  confined  to  large  and  coarse  objects,  the 
dimensions  of  which  are  large  multiples  of  the  wave-length.  It 
therefore  follows  that  when  moderate  or  large  apertures  have  to  be 
employed — that  is  to  say,  whenever  delineation  requires  the  employ- 
ment of  oblique  rays — the  elements  of  the  object  are  no  longer 
depicted  as  solid  objects  seen  by  the  naked  eye  or  through  the  telescope 
would  be  depicted  ;  nevertheless  the  brain  arranges  them  so  that 
the  characteristics  of  solid  vision  are  still  presented. 

Professor  Abbe  demonstrates  l  that  in  an  aplanatic  system  pencils 
of  different  obliquities  yield  identical  images  of  every  plane  object, 
or  of  a  single  layer  of  a  solid  object.  This  is  true  however  large  the 
aperture  may  be. 

This  carries  with  it,  as  we  have  said,  a  total  absence  of  perspec- 
tive and  an  essential  geometrical  difference  between  vision  with  the 
binocular  microscope  and  vision  with  the  unaided  eye. 

An  object,  not  quite  flat,  as  a  curved  diatom,  when  observed  with 
an  objective  of  wide  aperture  will  present  points  of  great  indistinct- 
ness. This  has  been  by  some  supposed  to  arise  from  the  assumption 
that  there  was  a  dissimilarity  between  the  images  formed  by  the 
axial  and  oblique  pencils  ;  but  this  is  not  so.  It  is  wholly  expli- 
cable by  the  fact  that  the  depth  of  the  object  is  too  great  for  the 
small  depth  of  vision  attendant  upon  a  large  aperture. 

It  will  be  seen,  then,  that  so  long  as  the  depth  of  the  object  is 
within  the  limits  of  the  depth  of  vision,  corresponding  to  the  aperture 
and  amplification  in  use,  we  obtain  a  distinct  parallel  projection  of 
all  the  successive  layers  in  one  common  plane  perpendicular  to  the 
axis  of  the  microscope — a  ground  plan,  as  it  were,  of  the  object. 
Manifestly,  then,  since  depth  of  vision  decreases  with  increasing 
1  Joitrn.  If. M.S.  series  ii.  vol.  iv.  pp.  21-24. 


ABBE   ON   STEREOSCOPIC    VISION  95 

aperture,  good  delineation  with  these  must  be  confined  to  thinner 
objects  than  can  be  successfully  employed  with  objectives  of  narrow 
apertures. 

Stereoscopic  vision  with  the  microscope,  therefore,  is  due  solely 
to  difference  of  projection  exhibited  by  the  different  parallactic  dis- 
placements of  the  images  of  successive  layers  on  the  common  ground 
plane  and  to  the  perception  of  depth,  not  to  the  delineation  of  the 
plane  layers  themselves.  For,  if  there  were  dissimilar  images  per- 
ceptible at  different  planes,  the  ouf-of-focus  layers  must  appear  con- 
fu.M-d  and  no  vision  of  depth  would  be  possible. 

Now  stereoscope  vision  requires,  as  shown  by  Dr.  Carpenter,  that 
the  delineating  pencils  shall  be  so  divided  that  one  portion  of  the 
admitted  cone  of  light  is  conducted  to  one  eye  and  another  portion  to 
the  other  eye.  If  this  division  of  the  image  is  effected  in  a  symmetri- 
cal way.  the  cross  section  of,  e.g.,  a  circle  must  be  reduced  to  two 
semicircles  representing  one  of  these  two  arrangements  seen  in 
o  and  P,fig.  70. 


0 


FIG.  70. 


Dr.  Abbe's  theory  is  that  the  only  condition  necessary  for  ortho- 
scopic  effect  in  any  binocular  system  is  that  these  semicircles  oi- 
l-heir equivalents  should  be  depicted  according  to  diagram  O,  fig.  70, 
and  for  pseudoscopic  effect  according  to  diagram  P  in  the  same  figure  ; 
and  he  demonstrates  that  all  other  circumstances,  such,  e.g.,  as  the 
crossing  of  the  images,  are  wholly  immaterial. 

Orthoscopic  vision  is  always  obtained  when  the  right  half  of  the 
right  pupil  and  the  left  half  of  the  left  pupil  only  are  employed ; 
pseudoscopic  vision  in  the  opposite  conditions.  '  It  is  quite  indif- 
ferent whether  the  effect  is  obtained  with  crossing  or  non-crossing 
ray>.  whether  the  image  be  erect,  or  inverted,  or  semi-inverted,  and 
whatever  may  be  components  of  the  optical  arrangement.' 

The  observant  reader  will  perceive  that  it  is  at  this  point  that 
there  is  a  radical  divergence  from  the  interpretation  given  by  Dr. 
Carpenter,  who,  as  we  have  seen  above,  insisted  that  orthoscopic 
vision  is  not  to  be  obtained  in  a  binocular  with  non-erecting  eye-pieces 
unless  the  axes  of  the  two  halves  of  the  admitted  cone  cross  each 
other. 

Of  course  we  must  keep  clearly  before  us  the  fact  that  in  micro- 
scopic vision  it  is  not  the  object  but  its  virtual  image  only  that  we 
see.  This  apparently  solid  image  is  placed  in  the  binocular  micro- 
scope under  circumstances  similar  to  those  of  common  objects  in 
ordinary  vision.  Clearly,  then,  it  is  the  perspective  projections  of 
this  image  which  require  to  be  compared  to  the  projections  of  solid 
objects  in  ordinary  vision,  in  respect  to  which  the  criteria  of  ortho- 
scopic and  pseudoscopic  vision  have  been  defined.  But  it  can  be 
geometrically  demonstrated  that  right-eye  perspective  of  the  ap- 
parently solid  image  is  always  obtained  from  the  right-hand  portion  of 
the  emergent  pencils,  left-eye  perspective  from  the  left-hand  portion  ; 


96  VISION  WITH   THE   COMPOUND   MICROSCOPE 


FIG.  71. 


BINOCULAR  MICEOSCOPES 


97 


and  it  is  quite  immaterial,  as  regards  this  result,  which  portion  of  the 
emergent  rays  is  admitted  by  tJie  right  or  the  left  part  of  the  objective. 

The  manner  in  which  the  delineating  pencils  are  transmitted 
through  the  system  may  he  such  as  to  require  crossing  over  of  the 
rays  from  the  right-hand  half  of  the  objective  to  the  left  eye-piece, 
and  vice  versa.  But  it  is  not  essential  to  binocular  effect.  In  the 
Weiiham  and  Xacliet  binocular  (pp.  98,  99)  crossing  over  is  required 
because  the  inversion  of  the  pencils  js  not  changed  by  two  reflexions. 
If  the  delineating  pencils  ha  ve  l.een  reflected  uneven  number  of  times 
in  the  same  plane,  it  will  be  necessary  for  the  rays  to  cross  ;  but  if 
they  have  been  reflected  an  odd  number  of  times,  it  is  not  only  un- 
necessary, but  is  destructive  of  orthoscopic  effect,  provided  ordinary 
eye-pieces  (non-erecting)  are  employed.  Hence  in  the  Stephenson  bi- 
nocular it  is  not  only  not  required,  but  would  give  pseudoscopic  effect. 

Principal  Forms  of  Binocular  Microscopes.— The  first  binocular 
of  a,  practical  character  was  the  arrangement  of  Professor  J.  L. 
lliddell,  of  Xew  Orleans.  It  was  devised  in  1851  and  constructed  in 
1852,  and  a  description  of  its  nature  and  its  genesis  was  given  by 
him  in  the  second  volume  of  the  first  series  of  the  '  Quarterly  Journal 
of  Microscopical  Science'  in  the  year  1854.1 

A  representation  of  his  original  instrument  is  presented  in  fig.  71 , 
and  the  arrangement  of  the  prisms  by  which  the  binocular  effect  was 
obtained  is  shown  in  fig.  72. 

It  will  be  seen  that  the  pencil  of  rays  emerging  from  the  back 
lens  of  the  combination  I  is  divided  into  two,  each  half  passing  re- 
spectively into  the  right  and  left  prisms ;  the  path  of  the  rays  is 
indicated  at  a,  b,  c,  d,  the  object  being  at  o. 

To  secure  coincidence  of  the  images  in  the  field  of  view  for 
varying  widths  between  the  eyes  Professor  Riddell  devised  (1)  a 
means  of  regulating  the  inclination  of  the  prisms  by  mounting  them 
in  hinged  frames,  so  that,  while  their  lower  edges,  near  a,  fig.  72, 
remain  always  in  parallel  contact,  the  inclination  of  the  internal 
reflecting  surfaces  can  be  varied  by  the  action  of  the  milled  head  in 
front  of  the  prism  box  ;  (2)  the  lower  ends  of  the  binocular  tubes 
are  connected  by  travelling  sockets,  moving  on  one  and  the  same 
axis,  on  which  are  cut  corresponding  right-  and  left-handed  screws, 
so  that  the  width  of  the  tubes  may  correspond  with  that  of  the 
prisms  ;  and  (3)  the  upper  ends  of  the  tubes  are  connected  by  racks, 
one  acting  above  and  the  other  below  the  same  pinion,  so  that  right- 
aiid  left-handed  movements  are  communicated  by  turning  the  pinion. 

This  instrument  could  only  be  used  in  a  vertical  position,  as 
shown  in  the  figure  (71).  The  two  prisms  in  fig.  72  correct  the  in- 
version of  the  image  in  a  lateral  direction,  two  more  prisms  are 
needed  to  correct  the  inversion  in  the  vertical  direction.  These 
Professor  lliddell  placed  above  the  eye  caps,  but  now  they  are  placed 
immediately  above  the  binocular  prisms,  fig.  78. 

This  system  of  binocular  excited  much  interest  in  England  im- 
mediately after  its  publication,  and  Mr.  Wenham  in  London  and 
MM.  Nachet,  of  Paris,  soon  suggested  and  devised  a  variety  of 
binocular  systems. 

A    P.  13. 

H 


98 


VISION   WITH   THE   COMPOUND   MICROSCOPE 


Nachet's  Binocular  was  early  in  the  field,  but  was  riot  a 
practical  construction  on  account  of  the  parellelism  of  its  tubes,  ;nul 
is  not  now  advocated  by  its  inventor  or  adopted  by  opticians  of  any 
country. 

Wenham's  Stereoscopic  Binocular. — All  these  objections  are 
overcome  in  the  admirable  arrangement  devised  by  the  ingenuity  of 

Mr.  Wenham,  in  1860  (Trans.  Microscopi 
cal  Soc.  of  London,  vol  i.  N.S.  p.  15),  in 
whose  binocular  the  cone  of  rays  pro- 
ceeding upwards  from  the  objective  is 
divided  by  the  interposition  of  a  prism 
of  the  peculiar  form  shown  in  fig.  7.'!.  BO 
placed  in  the  tube  which  carries  the  objec- 
tive (figs.  74,  75,  a),  as  only  to  interrupt 
one  half,  a  c,  of  the  cone,  the  other  half, 
a  6,  going  on  continuously  to  the  eye- 
piece of  the  principal  or  right-hand  body, 
R,  in  the  axis  of  which  the  objective  is 
placed.  The  interrupted  half  of  the  cone 
(figs.  73,  74,  a),  on  its  entrance  into  the 
prism,  is  scarcely  subjected  to  -mv  refrac- 
tion, since  its  axial  ray  is  perpendicular 
to  the  surface  it  meets ;  but  within  the  prism  it  is  subjected  to  two 
reflexions  at  b  and  c,  which  send  it  forth  again  obliquely  in  the  line 

K 


FIG.  73. — Wenham's  prism 
(1860). 


FIG.  74.  FIG.  75. 

Wenham's  stereoscopic  binocular  microscope  (1860). 

(I  towards  the  eye-piece  of  the  secondary  or  left-hand  body  (fig.  74. 
L) ;  and  since  at  its  emergence  its  axial  ray  is  again  perpendicular 


WENHAM'S   BINOCULAR  PRLSM  99 

to  the  surface  of  the  glass,  it  suffers  110  more  refraction  on  passing 
out  of  the  prism  than  on  entering  it.  By  this  arrangement  the 
image  received  by  the  right  eye  is  formed  by  the  rays  which  have 
ji;i>sed  through  the  left  half  of  the  objective,  and  have  come  on 
without  any  interruption  whatever  ;  whilst  the  image  received  by 
the  left  eye  is  formed  by  the  rays  which  have  passed  through  the  right 
half  of  the  objective,  and  have  been  subjected  to  two  reflexions  within 
the  prism,  passing  through  only  fcooj&urfhces  of  glass.  The  adjustment 
for  the  variation  of  distance  between  the  axes  of  the  eyes  in  different 
individuals  is  made  by  drawing  out  or  pushing  in  the  eye-pieces,  which 
are  moved  consentaneously  by  means  of  a  milled  head,  as  shown  in 
fig.  75.  Now,  although  it  may  be  objected  to  Mr.  Wenham's  method 
(1)  that,  as  the  rays  which  pass  through  the  prisin  and  are  obliquely 
reflected  into  the  secondary  body  traverse  a  longer  distance  than 
those  which  pass  on  uninterruptedly  into  the  principal  body,  the 
picture  formed  by  them  will  be  somewhat  larger  than  that  which 
is  formed  by  the  other  set ;  but  this  can  be  easily  compensated  for 
by  (a)  altering  the  power  of  one  of  the  eye-pieces,  (b)  by  increasing  the 
tube  length  of  the  direct  tube  ;  and  (2)  that  the  picture  formed  by  the 
rays  which  have  been  subjected  to  the  action  of  the  prism  must  be 
inferior  in  distinctness  to  that  formed  by  the  uninterrupted  half  of 
the  cone  of  rays ;  these  objections  are  found  to  have  no  practical 
weight.  For  it  is  well  known  to  those  who  have  experimented 
upon  the  phenomena  of  stereoscopic  vision  (1)  that  a  slight  differ- 
ence in  the  size  of  the  two  pictures  is  no  bar  to  their  perfect  com- 
bination ;  and  (2)  that  if  one  of  the  pictures  be  good,  the  full  effect 
of  relief  is  given  to  the  image,  even  though  the  other  picture  be 
faint  and  imperfect,  provided  that  the  outlines  of  the  latter  are 
sufficiently  distinct  to  represent  its  perspective  projection.  Hence 
if,  instead  of  the  two  equally  half -good  pictures  which  are  obtainable 
by  MM.  Nachet's  original  construction,  we  had  in  Mr.  Wenham's 
one  good  and  one  indifferent  picture,  the  latter  would  be  decidedly 
preferable.  But,  in  point  of  fact,  the  deterioration  of  the  second 
picture  in  Mr.  Wenham's  arrangement  is  less  considerable  than 
that  of  both  pictures  in  the  original  arrangement  of  MM.  Nachet; 
so  that  the  optical  performance  of  the  Wenham  binocular  is  in  every 
way  superior.  It  has,  in  addition,  these  further  advantages  over 
the  preceding :  First,  the  greater  comfort  in  using  it  (especially  for 
some  length  of  time  together),  which  results  from  the  convergence 
of  the  axes  of  the  eyes  at  their  usual  angle  for  moderately  near 
objects ;  secondly,  that  this  binocular  arrangement  does  not  necessi- 
tate a  special  instrument,  but  may  be  applied  to  any  microscope 
which  is  capable  of  carrying  the  weight  of  the  secondary  body,  the 
prisin  being  so  fixed  in  a  movable  frame  that  it  may  in  a  moment 
be  taken  out  of  the  tube  or  replaced  therein,  so  that  when  it  has 
been  removed  the  principal  body  acts  in  every  respect  as  an  ordinary 
microscope,  the  entire  cone  of  rays  passing  uninterruptedly  into  it ; 
and  thirdly,  that  the  simplicity  of  its  construction  renders  its  de- 
rangement almost  impossible.1 

1  The  Author  cannot  allow  this  opportunity  to  pass  without  expressing  his  sense 
of  the  liberality  with  which  Mr.  Wenham  freely  presented  to  the  public  this  im- 

H  2 


100 


VISION  WITH  THE    COMPOUND   MICKOSCOPE 


tephenson  (1870). 


Stephenson's  Binocular,  —  A  new  form  of  stereoscopic  binocular 
has  been  introduced  by  Mr.  Stephenson,1  which  has  certain  dis- 
tinctive features,  and  at  the  time  Mr.  Stephenson  devised  it  he  was 
entirely  unaware  that  any  part  of  the 
method  he  employed  had  been  used  by 
another.  He  had,  however,  independently 
conceived  Riddell's  device  for  dividing  the 
beam  as  a  part  of  his  very  ingenious  in- 
strument. This  he  discovered  and  acknow- 
ledged about  three  years  after  the  full  de- 
scription and  completion  of  his  binocular.  - 
The  cone  of  rays  passing  upwards  from  the 
object-glass  meets  a  pair  of  prisms  (A  A, 
fig.  76)  fixed  in  the  tube  of  the  microscope 
immediately  above  the  posterior  combina- 
tion of  the  objective,  so  as  to  catch  the 
light-rays  on  their  emergence  from  it  ; 
these  it  divides  into  two  halves  and  be- 
haves as  described  in  the  Riddell  prisms, 
which,  in  fact,  they  are.  As  the  cone  of 
rays  is  equally  divided  by  the  two  prisms, 
«•>«>  itHtwo  halves  are  similarly  acted  on, 
the  two  pictures  are  equally  illuminated, 
and  of  the  same  size  ;  wrhile  the  close  ap- 
proximation of  the  prisms  to  the  back  lens  of  the  objective  enables 
even  high  powers  to  be  used  with  very  little  loss  of  light  or  of 
definition,  provided  that  the  angles  and  surfaces  of  the  prisms  are 

worked  with  exactness  ;  and  as  the  tw<  > 
bodies  can  be  made  to  converge  at  a 
smaller  angle  than  in  the  Wenhain  ar- 
rangement, the  observer  looks  through 
them  with  more  comfort.  But  Mr.  Ste- 
phenson's ingenious  arrangement  islial  >1  e 
to  the  great  drawback  of  not  being 
convertible  (like  Mr.  Wenham's)  into 
an  ordinary  monocular  .by  the  with- 
drawal of  a  prism,  so  that  the  use  of 
this  form  of  it  will  be  probably  re- 
stricted to  those  who  desire  to  work 
with  a  binocular  when  employing  high 
powers. 

But  one  of  the  greatest  advantages 
attendant   on   Mr.    Stephenson's   con- 

struction is  its  capability  of  being  combined  with  an  erectiny 
arrangement,  which  renders  it  applicable  to  purposes  for  which 
the  Wenhain  binocular  cannot  be  conveniently  used.  By  the  in- 
terposition of  a  plane  silvered  mirror,  or  (still  better)  of  a  reflecting 

portant  invention,  by  which,  there  can  be  no  doubt,  he  might  have  largely  pro- 
fited if  he  had  chosen  to  retain  the  exclusive  right  to  it. 

1  Monthly  Microscopical  Journal,  vol.iv.  (1870),  p.  61,  and  vol.  vii.  (1872),  p.  167. 

-  Ibid.  vol.  x.  p.  41. 


FIG.   77. — Stephenson's  erecting 
prism  (1870). 


STEPHENSON'S  BINOCULAR  IOI 

prism  (fig.  77),  above  the  tube  containing  the  binocular  prisms, 
each  half  of  the  cone  of  rays  is  so  deflected  that  its  image  is  reversed 
vertically,  the  rays  entering  the  prism  through  the  surface  C  B,  being 
reflected  by  the  surface  A  B,  so  as  to  pass  out  again  by  the  surface 
A  C  in  the  direction  of  the  dotted  lines.  Thus  the  right  and  the  left 
half-cones  are  directed  respectively  into  the  right  and  the  left 
I  todies,  which  are  inclined  at  a  convenient  angle,  as  shown  in  fig.  78  ; 
so  that — the  stage  being  horizontal—the  instrument  becomes  a  most 
useful  compound  dissecting  microscope,  and  as  thus  arranged  by 
Swift,  with  well  adjusted  rests  for  the  hands,  has  but  few  equals  for 
the  purposes  of  minute  dissections  and  delicate  mounting  operations  ; 
indeed,  the  value  of  the  erecting  binocular  consists  in  its  applica- 
bility to  the  picking  out  of  very  minute  objects,  such  as  Diatoms, 
Polycystina,  or  Foraminifera, 
and  to  the  prosecution  of 
minute  dissections,  especially 
when  these  have  to  be  carried  on 
in  fluid.  No  one  who  has  only 
thus  worked  monocularly  can 
appreciate  the  guidance  derivable 
from  binocular  vision  when  once 
the  habit  of  working  with  it  has 
been  formed. 

Tolles's  Binocular  Eye-piece. 
An  ingenious  eye-piece  has  been 
constructed  by  Mr.  Tolles  (Boston, 
U.S.A.),  which,  fitted  into  the 
body  of  a  monocular  microscope, 
converts  it  into  an  erecting  stereo-  FIG.  78.— Stephenson's  erecting 

scopic  binocular.    This  conversion  binocular  (1870). 

is  effected  by  the  interposition 

of  a  system  of  prisms  similar  to  that  originally  devised  by  MM. 
Nachet,  but  made  on  a  larger  scale,  between  an  'erector'  (re- 
sembling that  used  in  the  eye-piece  of  a  day-telescope)  and  a  pair 
of  ordinary  Huyghenian  eye-pieces,  the  central  or  dividing  prism 
being  placed  at  or  near  the  plane  of  the  secondary  image  formed  by 
the  erector,  while  the  two  eye-pieces  are  placed  immediately  above 
the  two  lateral  prisms,  and  the  combination  thus  making  that 
division  in  the  pencils  forming  the  secondary  image  which  in  the 
Nachet  binocular  it  makes  in  the  pencils  emerging  from  the  objective. 
As  all  the  image-forming  rays  have  to  pass  through  the  two  surfaces 
of  four  lenses  and  two  prisms,  besides  sustaining  two  internal  re- 
flexions in  the  latter,  it  is  surprising  that  Professor  H.  L.  Smith,  while 
admitting  a  loss  of  light,  should  feel  able  to  speak  of  the  definition 
of  this  instrument  as  not  inferior  to  that  of  either  the  Wenham  or 
the  Nachet  binocular.  It  is  obviously  a  great  advantage  that  this 
eye-piece  can  be  used  with  any  microscope  and  with  objectives  of 
high  power  ;  but  as  its  effectiveness  must  depend  upon  extraordinary 
accuracy  of  workmanship  its  cost  must  necessarily  be  great.1 

1  See  American  Journal  of  Science,  vol.  xxxviii.  (1864),  p.  Ill,  and  vol.  xxxix. 
(1865),  p.  212;  and  Monthly  Microsc.  Journal,  vol.  vi.  (1871),  p,  45. 


IO2 


VISION   WITH   THE   COMPOUND   MICROSCOPE 


A  form  of  this  binocular  eye-piece  was  made  by  Professor  Abbe 
with  the  ingenuity  and  thoroughness  characteristic  of  the  firm  of 
Zeiss ;  but  in  spite  of  its  beauty  as  an  optical  instrument,  and  its  use- 
fulness as  applicable  to  any  tube,  and  especially  the  shorter  tubes 
to  which  the  Wenham 
binocular  could  not  well 
apply,  the  double  image 
in  the  right-hand  tube 
was  most  conspicuously 
apparent,  greatly  inter- 
fering with  the  perfection 
of  the  stereoscopic  image. 
On  this  account  chiefly  it 
has  not  come  into  general 
use.  We  are  nevertheless 
indebted  to  the  firm  of 
Zeiss  for  the  introduction 
of  a  very  satisfactory 
form  of  binocular  instru- 
ment, of  which  we  can 
speak  with  unconditional 
praise.  It  is  designated 
as  Greenough's  binocular 
microscope,  and  we  can 
confidently  affirm  that 
it  furnishes  an  accurate 
solid  and  withal  an  erect 
image,  so  that  for  all  the 


FIG.  79. — Greenough's  binocular  microscope  (1897). 

purposes  for  which  the  use  of  the  binocular  is  at  present  desirable  it  ac- 
complishes what  is  sought,  and  will  be  found  invaluable  for  zoologists, 
botanists,  and  embryologists.  The  microscope  is  shown  in  fig.  79, 


GKEENOUGH'S  BINOCULAK  MICROSCOPE 


103 


IB 


and  has  been  constructed  by  means  of  a  combination  of  Porro  prisms 
with  a  compound  microscope  of  the  usual  optical  type  ;  it  possesses 
many  of  the  advantages  of  the  compound  micro- 
scope, but  inevitably  loses  light  by  the  passing  of 
the  ray  through  so  many  prisms,  yet  by  means  of 
the  Porro  prisms  the  inverted  image  is  rendered 
erect.  This  may  be  practically  illustrated  by  fig. 
80,  which  shows  that  the  rays  of  light  in  passing 
from  the  object  to  the  eye  undergo* four  succes- 
sive reflexions  at  the  surfaces  of  the  prisms  and 
emerge  from  the  last  prism  with  undiminished 
intensity.  The  prisms,  it  will  be  seen,  have  the 
effect  of  erecting  the  inverted  image  formed 
by  the  object-glass.  But  in  this  microscope 
binocular  vision  is  obtained,  not  as  in  the  usual 
form  of  binocular  microscope,  by  the  subsequent 
division  of  a  pencil  of  light  passing  through 
one  object- (/lass  ;  but  two  complete  microscopes, 
each  having  its  own  objective  and  eye-pieces, 
are  simultaneously  directed  upon  the  object. 
This  secures  perfect  stereoscopic  (orthomorphic)  j 

vision,    but   of  course    no    power  higher   than  if 

H  inch  can  be  employed.     The  path  of  the  rays  U 

is  more  clearly  seen  in  fig.  81,  giving  a  diagram  FIG.  80. — Showing  the 
by  Mr.  Nelson  with  one  of  the  prisms  turned      £?&*££  SST 
round  90°  to  make  clearer  the  action   of   the      rays  (1894). 
prisms  on  the  ray.     It  is  well  to   note   that, 

when  two  of  these  erectors  with  a  double  objective  binocular  are 
used,  the  distance  between  the  eyes  can  be  compensated  for  by 
merely  turning  the  erector  adaptors  round  in  the  microscope  tube. 

This  method  of  erection,  which  is  both  valuable  and  practical,  was 
first  described  in  Zahn's  *  Oculus  Artificialis  '  (1702),  only  reflectors 
were  used  instead  of  prisms,  but  the  path  of  the  rays  is  diverted  in 
precisely  the  same  way  as  with  the  Porro  prisms. 

The  stereoscopic  binocular  is  put  to  its  most  advantageous  use 
when  applied  either  to  opaque  objects  of  whose  solid  forms  we  are 
desirous  of  gaining  an  exact  appreciation  or  to  transparent  objects 
which  have  such  a  thickness  as  to  make  the  accurate  distinction 
between  their  nearer  and  their  more  remote  planes  a  matter  of  im- 
portance. All  stereoscopic  vision  with  the  microscope,  so  far  as  it 
is  anything  more  than  mere  seeing  with  two  eyes,  depends,  as  already 
seen,  exclusively  upon  the  unequal  inclination  of  the  pencils  which 
form  the  two  images  to  the  plane  of  the  preparation,  or  the  axis  of 
the  microscope.  By  uniform  halving  of  the  pencils — whether  by 
prisms  above  the  objective  or  by  diaphragms  over  the  eye-pieces — 
the  difference  in  the  directions  of  the  illumination  in  regard  to  the 
preparation  reaches  approximately  the  half  of  the  angle  of  aperture 
of  the  objective,  provided  that  its  whole  aperture  is  filled  with  rays. 
By  the  one-sided  halving  we  have  been  considering,  the  direct  image 
is  produced  by  a  pencil  the  axis  of  which  is  perpendicular  to  the 


104 


VISION'   WITH   THE    COMPOUND   MICROSCOPE 


plane  of  the  preparation,  and  the  deflected  image  by  one  whose  axis 

is  inclined  about  a  fourth  of  the  angle  of  aperture. 

With    low    powers,    which    allow    of  a    relatively    considerable 

depth-perspective,  the  slight  difference  of  inclination,  which  remain.-- 

in  the  latter  case,  is  quite  sufficient  to 
v  _  s  produce  a  very  marked  difference  in 
the  perspective  of  the  successive  layer.- 
in  the  images.  But  with  high  powers 
the  difference  in  the  two  images  does 
not  keep  pace  —  even  when  both  eye- 
pieces are  ha,lf  covered  —  with  the  in- 
crease of  the  angle  of  aperture,  so  long 
as  ordinary  central  illumination  is 
used.  For  in  this  case  the  incident 
pencil  does  not  fill  the  whole  of  the 
opening  of  the  objective,  but  only  a 
relatively  small  .central  part,  which, 
as  a  rule,  does  not  embrace  more  than 
40°  of  angle,  and  in  most  cases  can- 
not embrace  more  without  the  clear 
ness  of  the  microscopic  image  being 
affected  and  the  focal  depth  also  being 
unnecessarily  decreased.  But  as 
those  parts  of  the  preparation  which 
especially  allow  of  solid  conception 
are  always  formed  by  direct  trans 
mittedraysin  observation  with  trans- 
mitted light,  it  follows  that  under 
these  circumstances  the  difference  of 

the 


turned  through  an  angle  of  90°  to  whole    aperture  -angle    of  the   objec- 
make  the  path  of  the  rays  clearer,  tive,  but  on  the  much  smaller  angle 

of   the  incident   and   directly   trans- 

mitted pencils,  which  only  allow  of  relatively  small  differences 
of  inclination  of  the  image-forming  rays  to  the  preparation.  It  is 
evident,  however,  that  wThen  objectives 
of  short  focus  and  correspondingly  large 
angle  are  used,  a  considerably  greater 
differentiation  of  the  two  images  with  re- 
spect to  parallax  can  be  produced  if,  in 
place  of  one  axial  illuminating  pencil,  two 
pencils  are  used  oppositely  inclined  to  the 
axis  in  such  a  way  that  each  of  the 
images  is  produced  by  one  of  the  pencils. 
This  kind  of  double  illumination,  though 
it  cannot  be  obtained  by  the  simple 
mirror,  can  be  easily  produced  1  >y  using 
with  the  condenser  a  diaphragm  with  two 

openings  (fig.  82),  placed  in  the  diaphragm  stage  under  the  con- 
denser. We  then  have  it  in  our  power  to  use,  at  pleasure,  pencils 
of  narrower  or  wider  aperture  and  of  greater  or  less  inclination 


POWELL  AXD    LE ALAND'S   HIGH-POWER   BINOCULAR       105 

towards  the  axis  by    making  the  openings  of  different  width  and 
different  distance  apart. 

With  diaphragms  of  this  form  (which  can  easily  be  made  out  of 
cardboard)  the  larger  aperture  angles  of  high-power  objectives  may 
be  made  use  of  to  intensify  the  stereoscopic  effect  without  employing 
wide  pencils,  which  are  prejudicial  both  as  diminishing  the  clem-ne» 
of  the  image  and  the  focal  depth. 

Of  course  with  this  method  of  illumination  both  eye-pieces  must 
be  half  covered  in  order  that  one  image  may  receive  light  only  from 
one  of  the  two  illuminating  cones,  and  the  other  only 
from  the  other.     The  division  of  light  in  both  the  aper- 
ture-images will  then  be  as  shown  in  fig.  83  ;  and  it  is 
evident   that  in  this  case  the  brightness  of  the  image  for 
both  eyes  together  is  exactly  the  same  as  would  be  given       FIG.  88. 
by  one  of  the  two  cones  alone  without  any  covering. 

The  method  of  illumination  here  referred  to — which  was  origi- 
nally recommended  by  Mr.  Stephenson  for  his  binocular  microscope — 
has.  in  fact,  proved  itself  to  be  by  far  the  best  when  it  is  a  question 
of  using  higher  powers  than  about  300  times.  It  necessarily  requires 
very  well  corrected  and  properly  adjusted  objectives  if  the  sharpness 
of  the  image  is  not  to  suffer  ;  but  if  these  conditions  are  satisfied  it 
yields  most  striking  stereoscopic  effects,  even  with  objectives  of  2  mm. 
and  less  focal  length,  provided  the  preparation  under  observation 
presents  within  a  small  depth  a  sufficiently  characteristic  structure. 

Non-Stereoscopic  Binoculars. — The  great  -  comfort  which  is  ex- 
perienced by  the  microscopist  from  the  conjoint  use  of  both  eyes  has 
led  to  the  invention  of  more  than  one  arrangement  by  which  this 
comfort  can  be  secured  when  those  high  powers  are  required  which 
cannot  be  employed  with  the  ordinary  stereoscopic  binocular.  This 
is  accomplished  by  Messrs.  Powell  and  Lealand  by 
taking  advantage  of  the  fact  already  adverted  to,  that 
when  a  pencil  of  rays  falls  obliquely  upon  the  sur- 
face of  a  refracting  medium  a  part  of  it  is  reflected 
without  entering  that  medium  at  all.  In  the  place 
usually  occupied  by  the  Weiiham  prism,  they  in- 
terpose an  inclined  plate  of  glass  with  parallel  sides, 
through  which  one  portion  of  the  rays  proceeding  up- 
wards from  the  whole  aperture  of  the  objective  pas,-e> 
into  the  principal  body  with  very  little  change  in  its 
course,  whilst  another  portion  is  reflected  from  its  sur 
face  into  a  rectangular  prism  so  placed  as  to  direct  it 
obliquely  upwards  into  the  secondary  body  (fig.  84). 
Although  there  is  a  decided  difference  in  brightness  be- 
tween the  two  images,  that  formed  by  the  reflected  rays 
being  the  fainter,  yet  there  is  marvellously  little  loss  of  FIG.  81.  (1865.) 
definition  in  either,  even  when  the  50th  of  an  inch  objec- 
tive is  used.  The  disc  and  prism  are  fixed  in  a  short  tube,  which  can 
be  readily  substituted  in  any  ordinary  binocular  microscope  for  the 
one  containing  the  Wenham  prism.  Other  arrangements  were  long 
since  devised  by  Mr.  Wenham,1  and  subsequently  by  Dr.  Schroder, 
1  Transactions  of  the  Microsc.  Soc.  N.S.  vol.  xiv.  (1866),  p.  105. 


IO6  VISION  WITH  THE    COMPOUND   MICROSCOPE 

for  securing  binocular  vision  with  the  highest  powers.  We  have  used 
the  latter  of  these  with  perfect  satisfaction,  but  all  that  is  required 
is  at  the  disposal  of  the  student  in  the  arrangement  of  Powell  and 
Lealand. 

To  those  who  have  used  these  forms  of  binocular  habitually  it 
has  been  a  frequent  source  of  surprise  and  perplexity  that,  although 
theoretically  such  a  form  as  that  of  Powell  and  Lealand's  is  non- 
stereoscopic,  yet  objects  studied  with  high  powers  have  appeared  as 
if  in  relief,  and  the  effect  upon  the  mind  of  stereoscopic  vision  has 
been  distinctly  manifest.  The  Editor  was  conscious 
of  this  for  many  years  in  the  use  of  the  Powell  and 
Lealand  form,  with  even  the  ^V^h  of  an  inch  power 
of  the  achromatic  construction .;  at  the  time  he  inter- 
preted it  as  a  conceptual  effect ;  but  it  always  arose 
when  the  pupils  fell  upon  the  outer  halves  of  the 
Ramsden  circles.  The  explanation,  Dr.  A.  C. 
Mercer  considers,1  is  due  to  Abbe.  Since  (fig.  85) 
when  the  eye-pieces  are  at  such  a  distance  apart  that 
the  Ramsden  circles  correspond  exactly  with  the 
pupils  of  the  eyes,  centre  to  centre,  the  object  appears 
flat.  But  if  the  eye-pieces  be  racked  down,  so  as 
FIG.  85.  to  be  nearer  together,  the  centres  of  the  pupils  fall 

upon  the  outer  halves  of  the  Ramsden  circles  and  we 
have  the  conditions  of  orthoscopic  effect ;  while  if  they  be  racked  up 
so  as  to  be  more  separated,  the  centres  of  the  pupils  fall  on  the  inner 
halves,  and  we  have  pseudoscopic  effect. 

The  Optical  Investigations  of  Gauss. — Before  leaving  this  section 
of  our  subject,  in  which  we  have  endeavoured,  with  as  much  clear- 
ness as  we  could  command,  to  enable  the  general  reader  to  com- 
prehend with  intelligibility  the  principles  of  theoretical  and  applied 
optics  as  they  relate  to  the  microscope,  we  believe  we  shall  serve 
the  higher  interests  of  microscopy,  and  the  wants  or  desires  of  the 
more  advanced  microscopical  experts,  if  we  endeavour  to  present  in 
a  form  either  devoid  of  technicality  or  with  inevitable  technicalities 
explained  a  general  outline  and  then  an  application  of  the  famous 
dioptric  investigations  of  Gauss,  an  eminent  German  mathematician, 
who,  amongst  many  bther  brilliant  labours  in  applied  mathematics, 
expounded  the  laws  of  the  refraction  of  light  in  the  case  of  a  co-axi«l 
system  of  spherical  surfaces,  having  media  of  various  refractive  in- 
dices lying  between  them. 

Although  the  assumptions  upon  which  the  formula?  of  Gauss 
rest  are  not  coincident  with  the  conditions  presented  by  the  lens- 
combinations  which  are  employed  in  the  construction  of  modern 
objectives  of  great  aperture,  the  results,  nevertheless,  furnish  an 
admirable  presentation  of  the  path  of  the  rays  and  the  positions  of 
cardinal  points,  even  in  the  microscope  as  we  know  and  use  it. 

We  remember  that  the  microscope  is  largely  used  in  England 
and  America  by  men  who  can  only  employ  it  in  their  more  or  less 
brief  recessions  from  professional  and  commercial  pursuits,  but  who 
often  employ  it  with  enthusiasm  and  intelligent  purpose.  Much 

1  Journ.  H.M.S.  ser.  ii.  vol.  ii.  p.  271. 


DIOPTRIC   INVESTIGATION   BY   GAUSS 


107 


scientific  work  may  be  done  by  such  men,  and  it  will  promote  the 
accomplishment  of  this,  in  our  judgment,  if  the  frequently  expressed 
desire  be  met  which  will  enable 
such  students  to  understand  in  a 
general   but   thoroughly   intelli- 
gent manner   the    principles  in- 
volved  in    the    employment    of 
systems  of  lenses. 

Many  such  either  have  scanty 
knowledge  of  algebra,  or  in  the 
continuous  pressure  of  other 
rl.tims  have  lost  much  that  they 
once  possessed.  We  believe  that 
in  these  cases  the  following  ex- 
position of  the  dioptric  system 
of  Gauss,  with  a  following  ex- 
ample worked  out  in  full  and 
with  every  step  made  clear,  will 
be  of  real  and  practical  value. 
Without  some  intelligible  under- 
standing of  the  ultimate  prin- 
ciples of  the  microscope  no  re- 
sults of  the  highest  order  can,  at 
least  with  moderate  and  high- 
power  lenses  of  the  best  modern 
construction,  be  anticipated.  On 
this  ground  we  commend  the 
study  to  the  earnest  reader. 

Let  R  N,  S  N'  (fig.  86)  be 
the  spherical  surfaces  of  a  lens 
of  density  greater  than  air,  and 
let  P  R  S  p  be  the  course  of  a 
ray  of  light  passing  through  it ; 
C,  C',  the  centres  of  the  spherical 
surfaces. 

Let  PR,  R  S  be  produced 
to  meet  the  perpendiculars 
through  C  and  Cr  in  A  and  A'. 

Let  C  R=r,  C'  S=/,2  //,== 
index  of  refraction  out  of  air 
into  the  medium.  N  N'=tZ,  the 
thickness  of  the  lens.  N  R=£>, 
N'  S=6'.  These  may  be  con- 
sidered as  straight  lines. 

Let  the  equation  to  P  R  be 
y-b=m(x— ON)  .  .  (1) 

Let  the  equation  to  R  S  be 
y— b=m'(x— ON)  .  .  (2) 

1  This  figure  is  greatly  exaggerated  for  the  sake  of  clearness. 

2  If  either  of  the  curvatures  be  turned  in  the  opposite  direction  the  sign  of  the 
•corresponding  r  must  be  changed. 


108  VISION    WITH   THE    COMPOUND   M1CKOSCOPE 

or,  y—b'=in'(x—OX')  .     (X) 

Let  the  equation  to  Sp  be  y  -b'= in' '(,»•  —  O  X')  .          .     (4) 

From  (2)  and  (3) 

b'-b=m'  (ON'-ON)=m'X'X  =  /y/^   .  .     (">) 

Xo\v  sin  C  R  A=/^  .  sin  ORB; 

^  .  sin  C  A  R=Ju  .  V*  .  sin  C  B  R. 

Now  C  A  and  C  B  are  the  values  of  //  in  equations  (1)  and  (2) 
when  #=0  C ; 

.•;  C  A=&-fw(OC-OX)=£+wr; 

and  similarly  C  B= ?;  +  ?/&'  »• ; 

/.  (b  +  mr)  sin  C  A  R=/w,  (b+m1  r)  sin  C  B  R. 
Now  CAR,  GBR  do  not  in  general  differ   much   from    each 
other,  so  that  for  a  first  approximation  we  may  consider  them  to  be 
equal. 

.'.  b-\-m  r=p(b-}-mf  r),  i.e.  p  mf=.in — —      .  b. 


r 


Let  =u  ;  then  u.  m'=im  —  b  n 

r  " 

Similarly,  sin  C'  S  B'=M  .  sin  C'  S  A/  ; 

or,  .  sin  C'  B'  S=  .  sin  C'  A' S ; 

and,  as  before, 


Cr  ~B'=b'  +  m"<>'/,  C'  A'=b'  +  m'  r'  from  equations  (4)  and  (3) ; 
.-.  as  before  we  may  take 


Let  £_    =  %',  then  /u  m'  =  m"—  b'  ,,'      .  .     (7> 

C1  /K\  1      /r»\          I/  7      ,     W  -  &**    7          7      /  ^W\  ?>''  ^ 

From  (5)  and  (6)     ?/=/>+  -          «=;6  I  j  _        14. 

/i  V  P/M 

.,.        j  /_x      „  /  .  7     /  /i        (Z?fc\      in  d  id 

„     this  and  (7)  m"  =  umf  +  bn'  (1  —          14-  — 

V  A*  /  /" 

and  fi-om  (6)        =m-?y  //  +  /;  v//     (\  - 


d 


Assume 

d  d  u  duf  <1  n  u' 

-  =h    [  —  —  =  g  1  +      -=/,  w  —  n  —  =/.' 

P  H  V  P 


then 


Now  let  X,  Y  be  the  coordinates  of  P,  the  point  from  which  the 
ray  of  light  proceeds  ; 

then  by  (1)  6=Y—  w  (X-O  N)  ; 


DIOPTRIC   INVESTIGATION  BY   GAUSS 
substituting  in  (8) 
whence 


109 


bf=y  Y  +  -m  (h—g  .  X  —  O  X)  ; 
/„."=£  Y  +  m  (l—k  X=OX)  ; 


"—  &Y 


—  «(X—  ON) 


Now    substituting    in    (4)    the    equation    to    the    refracted    ray 
becomes 


or  by  (8) 

" 


First:     If    X    be    taken    sucli    that    Z—  /.-  (X-ON)=1,    i.e. 
X  =  O  X  -  ]  "-^=0  E,  suppose  ; 
tli  en  when 

;«-0  X'-A  +  r/  ^~  ]  =(.)  X'+  ]  7'7=O  Er, 


suppose, 

;  ^ 

y=Yr,  or  P  Mild  y>  are  equally  distant  from  the  axis. 
Also,  if  Y  =  0,  >j  =  0  ;  or  if  a  ray  proceed  from  E,  it  will  after 

refraction  pass  through  E'.  Also  m  =  -  —    -.        l  =  w",  that  is, 

I  —  «/'  (-X-  —  O-N  ) 

the  ray  will  be  equally  inclined  to  the  axis  before  and  after  refrac- 
tion. 

E  and  E'  are  called  the  '  principal  points.' 

du1 

O  E  =  O  X  —        -  =  O  X  +  • 


,  d  u  id 

u'  —  a  — 


—  —    O  JS     "i  //  \  7 


du' 

\  7  / 

'(  —  auw 


OE/=OX'+ 


'  ~ 


u  —  u  — 


d  u 


' 


Secondly  :   If  iti"  =  0,  or  the  ray  be  parallel  to  the  axis  after 
refraction,  we  have  from  (8) 

b  =  —  -  m,  and  the  equation  to  the  incident  ray  becomes 

K 

y  +     m  =  in  (./•  -  (  )  X),  or  .//  =  in  I  x  -  O  N  -  ^  }  ; 


10  VISION   WITH  THE   COMPOUND   MICROSCOPE 

dvf 


=  O  F,  suppose. 

If  in  =  0,  or  the  ray  be  parallel  to  the  axis  before  refraction.  \\c 
have  from  (8) 

£/  _  g  5  _.  j  m/r,  and  the  equation  to  the  refracted  ray  becomes 
y  -  SL  m"  =  m"  (x  -  0  NO,  or  y  =  m"  (./•  -  0  N'  +  fy  ; 


.-.  when  y  =  0,  x  =  0 N'  -  ?  =  ON'- 

=  O  F',  suppose. 
F  and  F'  are  called  the  '  focal  points.' 


du 


OF  =  ON+     -7-7-^    \        ,       , 

H  (uf  —  u)  —  duu 

OF'  =  ON' .  ,  ^~/"/ 

/u  (ur  —  u)  —  duu' 

The  focal  distance  -/=OF-OE  =  OE'-OF' 
= Af 1 

k 


Similarly,  it  may  be  shown  that  if  there  be  two  lenses,  and  sub- 
script numbers  refer  to  the  first  and  second  lens  respectively, 
E,  Er,  F,  F'  refer  to  the  entire  system,  and  if 
3  =  OE2-OE/, 

«i  =  —  ^  =  P\  (™\  —  H\)  —  d\  u\  u\', 
J\ 

V2=   —  *?•  =  /"2  (««2;  —  ^2)  —  ^2  ««2  W2^» 
/2 


Un  C>  Vi 


We  are  now  prepared  to  wo?*^  o««^  a?i  example  of  the  Gauss  system 
by  tracing  a  ray  through  two  or  more  lenses  on  an  axis,  showing  h<>\\ 
any  conjugate  may  be  found  through  two  or  more  lenses  on  that  axis.1 

1  Remembering  our  object,  and  the  assumed  conditions  of  some  for  whom  we 
write,  we  do  not  hesitate  to  preface  this  with  the  following  notes  to  remind  the 
reader  of  the  sense  attached  to  certain  mathematical  expressions. 

c»  means  infinity.  A  plane  surface  of  a  lens  is  considered  a  spherical  surface  of 
an  infinite  radius.  Any  number  divided  byco  =  0  any  number  divided  byO  =  oo; 


K.\ AMPLE   AFTER   GAUSS  III 

The  Gauss  system  of  tracing  a  ray  through  two  or  more  lenses 
on  an  axis  illustrated  by  means  of  a  worked- out  example. 

Two  lenses,  1  and  2,  fig.  87,  or  an  axis  ./•  //  are  given.  No.  1  is 
;i  double  convex  of  crown  ^  inch  thick,  the  refractive  index  p.  being 
:;.  the  radius  of  the  surface  A  i>  ;|  and  that  of  B  1  inch.  No.  2  lens 
i>  a  ]  ilano-concave  of  flint  y1,,  inch  thick,  the  refractive  index  /z  being 
?,  the  radius  of  the  Miriace  ('  i>  »,  ;ind  the  surface  I)  is  plane.  The 
distance  between  the  lenses,  that  is>  from  B  to  C  measured  on  the 
axis,  is  |  inch.  The  problem  is  to  firtd  the  conjugate  focus  of  any 
given  point  V. 

In  order  to  accomplish  this  two  points  have  first  to  be  found  with 
regard  to  each  lens.  These  points  are  called  principal  points  (see 
PP',  QQ'  in  fig.  87).  When  the  radii  of  curvature  r  and  r',  d, 
the  thickness,  and  pl  /*2,  the  refractive  indices  of  the  respective 
lenses,1  are  known,  the  distance  of  these  points  from  the  vertices,  i.e. 
the  points  where  the  axis  cuts  the  surfaces  of  the  lens,  can  be  found. 
Thus  by  applying  Professor  Fuller's  formulae  to  lens  1  the  distance  of 
P  from  the  vertex  A  can  be  determined — seep.  115  (i) — similarly  P' 
from  B — p.  115  (ii).  In  the  same  way  the  points  QQ'  from  C  and  I) 
in  lens  2  can  be  measured  off — (v)  (vi),  pp.  115,  116. 

It  must  be  particularly  noticed  that  in  measuring  off  any  dis- 
tance if  the  number  is  +  it  must  be  measured  from  left  to  right, 
and  if  —  from  right  to  left.  Thus  in  (i)  p.  115  because  the  sign  of 
•158  is  +  P  lies  '158  of  an  inch  to  the  right  of  A.  And  in  (ii) 
because  '21  is  —  P'  lies  -21  of  an  inch  to  the  left  of  B.  The  same 
rule  applies  to  the  radii ;  thus  the  radius  of  A,  being  measured  from 
the  vertex  to  the  centre  or  from  left  to  right,  is  +  ;  but  the  radius 
of  B,  being  measured  from  the  vertex  to  its  centre  or  from  right  to 
left,  is  — .  Similarly  with  the  concave  surface,  C  being  measured 
from  right  to  left  is  — . 

In  both  the  examples  before  us  the  points  PP',  QQ'  fall  inside 

any  number  multiplied  by  0  =  0.    cx>  plus,  or  minus,  or  multiplied  by  any  number  is 

still  cc. 

The  following  are  the  rules  for  the  treatment  of  algebraical  signs  : 

In  the  multiplication  or  division  of  like  signs  the  result  is  always  plus;  but  if 

the  signs  are  dissimilar  it  is  always  minus. 

In  addition,  add  all  the  terms  together  that  have  a  plus  sign  ;  then  all  the  terms 

with  a  minus  sign  ;  subtract  the  less  from  the  greater  and  affix  the  sign  of  the 

greater.     Example : 

+  3-4  +  2-5= -4. 

In  subtraction  change  the  sign  of  the  term  to  be  subtracted  and  then  add  in 
accordance  with  the  previous  rule.  Example  : 

-3 
+  2 
-5 

An  example  occurs  in  the  annexed  equations  (x)  and  (xi),  p.  116,  of  -•*•  —  =  +  r 
but  then  the  +  is  changed  into  a  -  by  the  negative  sign  in  front  of  the  fraction. 
In  (xii),  p.  116,  however,  there  being  a  +  in  front  of  the  fraction,  the  result  remains- 
positive. 

1  In  the  worked-out  example  no  distinction  has  been  made  between  the  r,  r1  of 
one  lens  and  the  r,  r'  of  the  other  lens,  as  well  as  of  /j.  and  d,  because  when  the 
principal  points  and  focal  length  are  determined  for  one  lens  those  expressions  are 
not  again  needed,  so  the  same  letters  with  different  values  assigned  to  them  may  be 
equally  well  used  for  the  next  lens.  Too  manj'  different  terms  are  apt  to  confuse 
the  student,  while  those  who  are  familiar  with  mathematical  expressions  will  under- 
stand the  arrangement. 


112  VISION   WITH  THE   COMPOUND  MICROSCOPE 

their  respective  lenses,  but  it  does  not  follow  that  they  will  do  so  in 
every  instance.  In  some  forms  of  menisci,  for  example,  they  will  fall 
outside  the  lens  altogether. 

With  regard  to  the  focus  of  the  lens  it  follows  the  same  rule  ; 
thus,  f  in  lens  1  is  measured  to  the  left  from  P,  and/'  to  the  right 
fromP';  similarly  in  lens  2,  f"  is  measured  to  the  right  from  (t). 
a  nd/'"  to  the  left  from  Q'. 

Having  determined  the  focal  length  of  each  lens,  the  distance 
between  the  right-hand  principal  point  of  the  first  lens  P'  and  tin- 
left-hand  principal  point  of  the  second  lens  Q  must  next  be  found.  It 
manifestly  is  the  distance  of  B  from  P'  +  the  distance  B  C  between 
the  lenses,  Q  being  at  the  point  C.  Therefore, 


When  these  three  data  have  been  obtained  —  that  is,  the  focal 
length  of  each  lens,  and  the  distance  between  them  —  we  are  in  a 
position  to  apply  the  formulae  (ix)  and  (x),  p.  116,  to  find  the  principal 
points  E  and  E'  of  the  combination. 

In  selecting  the  value  of  the  focus  to  be  put  into  the  equations 
for  both  lenses,  the  last  must  be  taken,  that  is,  in  lens  1  (iv)  or 
+  •947,  and  in  lens  2  (viii),  or  -1-875. 

It  will  be  noticed  that  the  value  of  E  being  negative,  it  will  be 
measured  '314  inch  to  the  left  from  P.  Similarly,  E'  is  measured 
•622  inch  to  the  left  from  Q'. 

<j>  also  is  1-28  to  the  left  from  E,  and  0'  1-28  to  the  right  from  E'. 

These  four  points,  E  E'  and  0  0r,  are  called  the  cardinal  points 
of  the  combination. 

Here  it  must  be  observed  that  in  this  work  it  has  been  necessary 
for  want  of  space  to  restrict  the  problem  to  dry  lenses,  that  is.  to 
those  cases  where  the  ray  emerges  from  the  combination  into  air.  the 
same  medium  in  which  it  was  travelling  on  immergence.  It  is  on 
that  account  that  the  values  of  <j>  and  (j>f  are  the  same. 

Having  now  obtained  the  four  cardinal  points,  we  may  at  once 
proceed  to  find  the  conjugate  of  x. 

Let  x  equal  the  distance  of  the  point  x  from  the  focal  plane  <j>. 
and  y  the  distance  of  its  conjugate  from  <f>f.  Then  by  formula  (xiii) 

.I-//  =  f2,  and  as  x  =  1  inch,  y  =  ]  '     l84  =  1-6384. 

This  numerically  determines  the  position  of  the  conjugate  plane. 
If  the  rays  incident  on  the  combination  are  parallel,  then  ,/:=  x. 

and  //  =  ™  =  0,  which  means  that  y  is  coincident  with  $'  '. 

The  following  is  the  graphic  method  of  finding  the  conjugate  of 
V.  From  V,  fig.  87,  draw  a  line  parallel  to  the  axis  to  meet  E',  and 
from  the  point  where  it  meets  E'  draw  a  line  through  N,  the  point 
where  </>'  cuts  the  axis,  to  W. 

From  V  draw  another  line  through  M,  the  point  where  0  cuts 
the  axis,  to  meet  E,  and  from  the  point  where  it  meets  E  draw  a 
line  parallel  to  the  axis,  cutting  the  other  line  in  W.  W  will  be  tin- 
conjugate  of  V,  which  was  required. 


A   PEACTICAL   EXAMPLE   AFTEK    GAUSS 


If  it  is  required  to  find 
the  conjugate  of  a  ray  pass- 
ing through  three  lenses  on 
an  axis,  two  of  the  lenso 
must  be  combined  and  their 
four  cardinal  points  found. 

The  principal  points 
and  the  focal  length  of  the 
third  lens  must  then  be 
calculated,  and  then  com- 
bined in  their  turn  by 
formula  (ix),  (x),  (xi),  and 
(xii),  p.  116,  with  the  car- 
dinal points  of  the  double 
combination.  8  is  taken  as 
the  distance  of  the  first 
principal  point  of  the  com- 
bination, nearest  the  third 
lens,  to  the  second  principal 
point  of  the  lens,  nearest 
the  combination.  A  fresh 
set  of  cardinal  points  is  de- 
termined in  this  manner 
for  the  three  lenses. 

So  also  with  four  Irn.-o  : 
the  cardinal  points  of  each 
pair  being  found,  they  are 
combined  by  the  same 
formulae,  and  new  cardinal 
points  for  the  whole  com- 
bination of  four  lenses  arc 
obtained.  Similarly,  the 
cardinal  points  of  five,  six, 
or  any  number  of  lenses 
can  be  found  and  the  con- 
jugate of  any  point  localised. 

Finally,  no  one  need  be 
discouraged  by  the  appear- 
ance of  the  length  of  the 
calculation  ;  the  example  is 
given  in  full,  so  that  any 
one  acquainted  only  with 
vulgar  fractions  and  deci- 
mals can  work  it,  or  any 
other  similar  problem,  out. 

In  lens  No.  1,  for  in- 
stance, the  numerators  of 
the  fractions  are  all  very 
simple,  and  the  denomina- 
tors of  the  four  equations 
are  all  alike  ;  so,  too,  in 


v 


114  VISION  WITH   THE   COMPOUND   MICROSCOPE 

the  equations  for  No.  2  and  in  those  for  both  lenses.  Further,  /  is 
the  same  as/",/"7  as  /",  and  <£7  as  (/>.  Hence  the  problem  is  much 
shorter  than  it  looks. 

If  the  conjugate  of  a  point  on  the  axis  is  only  required,  and  if 
the  principal  points  and  foci  of  each  lens  have  been  determined,  it 
will  not  be  necessary  to  enter  into  the  further  calculation  to  find  E. 
E'  and  </>,  <£7,  the  cardinal  points  of  the  combination, 

The  method  of  procedure  is  as  follows  :  If  x  is  the  given  point. 
its  distance  from/,  the  focus  of  lens  No.  1,  must  first  be  measured. 
Call  this  distances.  Then  the  distance  of  o  its  conjugate  from  the 
other  focus,  /',  supposing  lens  No.  2  to  be  removed,  can  be  found  by 
formula 

f'2 

o  x  =  /2,  o  =  -L-, 
/2  =  -897,  '  x  =  1-65; 


.. 

This  is  the  distance  from./'  to  o. 

As  the  distance  from  x  to/  is  positive,  the  distance  between  /' 
and  o  is  also  positive  ;  so  o  is  to  the  right  of/7. 

Before  proceeding  it  will  be  as  well  to  examine  other  possible 
cases  which  might  occur. 

Suppose  that  x  was  at  the  point  /  then  x  would  equal  0,  and 
0=00  ;  that  is,  o  would  lie  at  an  infinite  distance  from  /'.  If,  on 
the  other  hand,  the  point  x  was  to  the  right  of/  x  would  be  ne.y;a- 
tive,  and  o  would  be  also  negative,  because  /2  is  always  positive  ; 
o  would  then  be  measured  off  to  the  left  of  /',  and  the  conjugate 
would  be  virtual.  This  means  that  there  will  be  no  real  image'. 
because  the  rays  will  be  divergent  on  the/7  side  of  the  lens,  as  if 
they  had  come  from  some  focus  on  the  /  side  of  the  lens.  But  to 
return.  The  point  o  having  been  found  to  be  the  conjugate  of  x, 
due  to  the  sole  influence  of  No.  1  lens,  we  have  next  to  measure  the 
distance  between  o  and/",  and,  by  applying  the  same  formula,  find 
the  distance  of  its  conjugate  from  /7",  owing  to  the  exclusive  effect 
of  No.  2  lens  now  replaced.  This  distance  of"  may  be  found  thus  : 


P7  /"=P7  B  +  B  C  +  Q/"=-21  +'25  +  1-875=2-335  ; 
p/  f»—  p/  0=o/"=2-335—  1-49  =  -845. 

Calling  this  distance  O,  then,  by  formula  y  0=/"  2,  we  shall  find 

f"  2       3'515 

the  distance  of  y  from/7",  which  we  shall  call  y.     y=  ='->.., 

O  *o4o 

=4*  16,  which  is  positive  ;  therefore  y  lies  4'  16  inches  from/'77  to  the 
right  hand,  y  is  therefore  the  conjugate  of  x,  due  to  the  influence 
of  both  lenses  1  and  2.  Similarly,  the  conjugate  of  any  point  on  the 
axis  may  be  found  through  any  number  of  lenses. 

Lens  No.  1  :  Data.  —  Radius  A  =  '-  =  /•  ;  radius  B  =  —  1  =  r'  ; 
foci,//7;    thickness  =     =  d  ;   ^=1    P  =  principal  point  mea- 


A   PRACTICAL   EXAMPLE   AFTER    GAVSS 
stu-ed  from  A  ;  P'= principal  point  measured  from  B 

_?.     ,      fcl      I--.!- 

—  o  '    '6 ~, r~  9' 


/       \     3  /     1     2\  7      j        ,       1     2  1  1 

t*'—  Wlas-,   /  _   —     )=  —  -   ;   d  u  u   =      v      v  _ 

2\     2     3J  4'    >  2X3  x  ~2~   ~  6' 


7     1          19 
ft  (w/  _  M)  -  d  ^6  M'  =  -  4  +  g=  -12=  —1-583  ; 

1         1 

du'  2X~2  3 

P=A  +  —  j—.  -  c  -  -,  -  .=  A  4-         in 
^ 


=A  +  -158        .....      (i) 
1     2 

vv 

du  _  =B,2  _  ?_K_4 
»)—  rftttt'  19  ]{) 

12 

=B--21          .  .  (ii) 

3 

^_  _P,-2  __  P_18 

w)_^fcw/-  19-         19 

~12 

=p_-947         .....    (iii) 
3 

//__p/_  ^  _  p/  _    ^    _  P'.L 

,,  (M>-_  M)  _^  ^S>-  1  9  -      *  19 

""12 

=F  +  -947        .....    (iv) 
9 

Ze^is  JVb.  2  :  Z>ata.  —  Radius  C  =  —  b=r  ;    radius  D  =  «  =  /; 

o 

1  8 

foci,  f",f"']   thickness  *=-J-Q=  d  ;  ^  =  -=  ;     Q  =  principal   point 

measured  from  C  ;  Q'=  principal  point  measured  from  D. 

8       1  8  -  1 

=_8;^=^1=^  = 


=_ 
r  9  15  ?  oc 


64  fi4 

u  u>  =75-o  =  75='853  ; 

.       (v) 


, 
—  it)—  d  u  u'         r  64 

75 

i  2 


Il6  VISION   WITH  THE   COMPOUND   MICROSCOPE 

I  8^ 

du  rox~i.~,        i 

Q'  =  1>  +  W(^)-^-'        +        1        =1>-ir, 

75 

=  1)— -0625 (vi) 

_8 

H  (id — u) — d  uuf  8 

75 

_8 

n  8  U 

=Q/  ~"u(uf—u)—duufSi^  "~64=Q  .     "8 

75 
=Q'_l-875    . 

Jioth  Lenses.— Distance  apart  =  B  C  =^='25  ;  F  Q  =  -21  +  '25 

=  -46  ==$;/"=  focus  of  No.  1  lens  =  -947  ;  /'  =  focus  of  Xo.   2 
leiis=  —  1-875. 

F  =  P  4       _J/_     =  P  4-          '46  X  -947          =  p            43(> 
'   _  a           r  -947  _  1-875  —  -46                —  1  -388 
=  p_-314 (ix) 

=  O'-    -46  X  -  1>875 
-I       V        -947  — 1-875  —  -46 


"947  x  - 


/•  +  /"'  -a  -947  -  1-875  --4C) 


1-6384 

_:_=  1'6384 


CHAPTER     III 

THE  HISTORY  AND  DEVELOPMENT   OF  THE   MICROSCOPE 

THE  historic  progression  of  the  modern  microscope  from  its  earliest 
inception  to  its  most  perfect  form  is  not  only  full  of  interest,  but  is 
also  full  of  the  most  valuable  instruction  to  the  practical  micro- 
scopist.  In  regard  to  the  details  of  this,  our  knowledge  has  been 
greatly  enriched  during  recent  years.  The  antiquarian  knowledge 
and  zeal  in  this  matter  possessed  by  Mr.  John  Mayall,  jun.,  and 
the  unique  and  valuable  collection  of  microscopes  made  by  Frank 
Crisp,  Esq.,  LLB.,  ranging  as  they  do  through  all  the  history  of 
the  instrument,  from  its  earliest  employment  to  its  latest  forms, 
have  furnished  us  with  a  knowledge  of  the  details  of  its  history  not 
possessed  by  our  immediate  predecessors. 

We  may  obtain  much  insight  into  the  nature  of  what  is  indis- 
pensable and  desirable  in  the  microscope,  both  on  its  mechanical  and 
optical  sides,  by  a  thoughtful  perusal  of  these  details.  It  will  do 
more  to  enable  the  student  to  infer  what  a  good  microscope  should 
be  than  the  most  exhaustive  account  of  the  varieties  of  instrument 
at  this  time  produced  by  the  several  makers  (always  well  presented 
in  their  respective  catalogues)  can  possibly  do.  Availing  ourselves 
of  the  material  placed  at  our  disposal  by  the  generosity  of  these 
gentlemen,  we  shall  therefore  trace  the  main  points  in  the  origin 
and  progress  of  the  microscope  as  we  now  know  it. 

Mr.  Mayall 1  gives  what  we  must  consider  unanswerable  reasons 
for  looking  upon  the  microscope,  '  as  we  know  and  employ  it,'  as  a 
strictly  modern  invention.  Its  occurrence  at  the  period  when  the 
spirit  of  modern  scientific  research  was  asserting  itself,  and  when 
the  necessity  for  all  such  aids  to  physical  inquiry  and  experimental 
research  was  of  the  highest  value,  is  as  striking  as  it  is  full  of 
interest. 

It  may  be  held  as  fairly  established  that  magnifying  lenses  were 
not  known  to  the  ancients,  the  simplest  optical  instruments  as  we 
understand  them  having  no  place  in  their  civilisation. 

A  large  number  of  passages  taken  from  ancient  authors,  and 
having  an  apparent  or  supposed  reference  to  the  employment  of 
magnifying  instruments,  have  been  collected  and  carefully  criticised, 
with  the  result  that  all  such  passages  can  be  explained  without  in- 
volving this  assumption. 

We  learn  from  Pliny  the  elder  and  others,  that  crystal  globes 
filled  with  water  were  employed  for  cauterisation  by  focussing  the 

1  Cantor  Lectures  on  the  Microscope,  1886,  p.  1. 


I  1 8      THE   HISTORY  AND  DEVELOPMENT  OF  THE   MICROSCOPE 

sun's  rays  as  a  burning-glass,  and  that  these  were  used  to  produce 
ignition ;  but  there  is  no  trace  of  suggestion  that  these  refracting 
globes  could  act  as  magnifying  instruments. 

Seneca  (*  Qusest.  Nat.'  i.  6,  §  5)  states,  however,  that  '  letters 
though  small  and  indistinct  are  seen  enlarged  and  more  distinct- 
through  a  globe  of  glass  filled  with  water.'  He  also  states  that 
*  fruit  appears  larger  when  seen  immersed  in  a  vase  of  glass.'  But 
he  only  concludes  from  this  that  all  objects  seen  through  water 
appear  larger  than  they  are. 

In  like  manner  it  could  be  shown  that  Archimedes,  Ptolemy, 
and  others  had  no  knowledge  of  the  principles  on  which  refraction 
took  place  at  curved  surfaces. 

Nor  is  there  any  ancient  mention  of  spectacles  or  other  aids  to 
vision.  Optical  phenomena  were  treated  of ;  Aristotle  and  the  Greek 
physician  Alexander  dealt  with  myopy  and  presbyopy ;  Plutarch 
treated  of  myopy,  and  Pliny  of  the  sight.  But  no  allusion  is  made 
to  even  the  most  simple  optical  aids ;  nor  is  there  any  reference  to 
any  such  instruments  by  any  Greek  or  Roman  physician  or  author. 
In  the  fifth  century  of  the  Christian  era  the  Greek  physician  Actius 
says  that  myopy  is  incurable;  and  similarly  in  the  thirteenth 
century  another  Greek  physician,  Actuarius,  says  that  it  is  an  in- 
firmity of  sight  for  which  art  can  do  nothing.  But  since  the  end  of 
the  thirteenth  century,  which  is  after  the  invention  of  spectacles, 
they  are  frequently  referred  to  in  medical  treatises  and  other  works. 

If  we  turn  to  the  works  of  ancient  artists  we  find  amongst  their 
cut  gems  some  works  which  reveal  extreme  minuteness  of  detail  and 
delicacy  of  execution,  and  some  have  contended  that  these  could 
only  have  been  executed  by  means  of  lenses.  But  it  is  the  opinion 
of  experts  that  there  is  no  engraved  work  in  our  national  collection 
in  the  gem  department  that  could  not  have  been  engraved  by  a 
qualified  modern  engraver  by  means  of  unaided  vision ;  and  in 
reference  to  some  very  minute  writing  which  it  was  stated  by  Pliny 
that  Cicero  saw,  Solinus  and  Plutarch,  as  well  as  Pliny,  allude  to  these 
marvels  of  workmanship  for  the  purpose  of  proving  that  some  men 
are  naturally  endowed  with  powers  of  vision  quite  exceptional  in 
their  excellence,  no  attempt  being  made  to  explain  their  minute 
details  as  the  result  of  using  magnifying  lenses. 

These  and  many  other  instances  in  which  reference  to  lenses 
must  have  been  made  had  they  existed  or  been  known  are  con- 
clusive ;  for  it  is  inconceivable  that  even  simple  dioptric  lenses,  to 
say  nothing  of  spectacles,  microscopes  and  telescopes,  could  have 
been  known  to  the  ancients  without  reference  to  them  having  been 
made  by  many  writers,  and  especially  by  such  men  as  Galen  and 
Pliny. 

The  earliest  known  reference  to  the  invention  of  spectacles  is 
found  in  a  manuscript  dating  from  Florence  in  1299,  in  which  the 
writer  says, '  I  find  myself  so  pressed  by  age  that  I  can  neither 
read  nor  write  without  those  glasses  they  call  spectacles,  lately  in- 
vented, to  the  great  advantage  of  poor  old  men  when  their  sight 
grows  weak.'  x  Giordano  da  Rivalto  in  1305  says  that  the  invention 

1  Smith's  Optics,  Cambridge,  1738,  2  vols.  ii.  pp.  12,  13. 


A  'LENS'  FROM  SARGON'S  PALACE          119 

of  spectacles  dates  back  'twenty  years,'  which  would  be  about  1285. 
It  is  now  known  that  they  were  invented  by  Salvino  d'Armato  degli 
Armati,  a  Florentine,  who  died  in  1317.  He  kept  the  secret  for 
profit,  but  it  was  discovered  and  published  before  his  death.  But 
there  is  a  singular  evidence  that  a  lens  used  for  the  purpose  of 
magnification  was  in  existence  as  early  as  between  1513  and  1520, 
for  at  that  time  Raphael  painted  a  portrait  of  Pope  Leo  X.  which 
is  in  the  Palazzo  Pitti,  Florence.  In  this  picture  the  Pope  is  drawn 
holding  a  hand  magnifier,  evidently  ^intended  to  examine  carefully 
the  pages  of  a  book  open  before  him.  But  no  instruments  com- 
parable to  the  modern  telescope  and  microscope  arose  earlier  than 
the  beginning  of  the  seventeenth  century  and  the  closing  years  of 
the  sixteenth  century  respectively. 

It  is,  of  course,  known  that  there  is  in  the  British  Museum  a 
remarkable  piece  of  rock  crystal,  which  is  oval  in  shape  and  ground 
to  a  plano-convex  form,  wrhich  was  found  by  Mr.  Layard  during  the 
excavations  of  Sargon's  Palace  at  k 

Ximroud,  and  which  Sir  David 
Brewster  believed  w7as  a  lens  de- 
signed for  the  purpose  of  magni- 
fying. If  this  could  be  established 
it  would  of  course  be  of  great 
interest,  for  it  has  been  found 
possible  to  fix  the  date  of  its  pro- 
duction with  great  probability  as 
not  later  than  721-705  B.C. 

A  drawing  of  this  *  lens '  in  two 
aspects  is  shown  in  figs.  88  and  89, 
and  we  spent  some  hours  in  the 
careful  examination  of  this  piece 
of  worked  rock  crystal,  which  by 
the  courtesy  of  the  officials  we  were 
permitted  to  photograph  in  various  FIG.  89.— An  Assyrian  '  lens '  (?). 
positions,  and  we  are  convinced 

that  its  lenticular  character  as  a  dioptric  instrument  cannot  be  made 
out.  There  are  cloudy  striae  in  it,  which  would  prove  fatal  for 
optical  purposes,  but  would  be  even  sought  for  if  it  had  been  intended 
as  a  decorative  boss ;  while  the  grinding  of  the  '  convex '  surface 
is  not  smooth,  but  produced  by  a  large  number  of  irregular  facets, 
making  the  curvature  quite  unfit  for  optical  purposes.  In  truth, 
it  may  be  fairly  taken  as  established  that  there  is  no  evidence  of  any 
kind  to  justify  us  in  believing  that  lenses  for  optical  purposes  were 
knowTn  or  used  before  the  invention  of  spectacles. 

From  the  simple  spectacle-lens,  the  transition  to  lenses  of  shorter 
and  shorter  focus,  and  ultimately  to  the  combination  of  lenses  into 
a  compound  form,  would  be — in  such  an  age  as  that  in  which  the 
invention  of  spectacles  arose — only  a  matter  of  time.  But  it  is 
almost  impossible  to  fix  the  exact  date  of  the  production  of  the  first 
microscope,  as  distinguished  from  a  mere  magnifying  lens. 

There  is  nevertheless  a  consent  on  the  part  of  those  best  able 
to  judge  that  it  must  have  been  between  1590  and  1609  ;  while  it  is 


120      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

probable  (but  by  no  means  certain)  that  Hans  and  Zacharias  Jaiisson. 
spectacle  makers,  of  Middelburg,  Holland,  were  the  inventors.  But 
it  would  appear  that  the  earliest  microscope  was  constructed  for 
observing  objects  by  reflected  light  only. 

At  the  Loan  Collection  of  Scientific  Instruments  in  London  in 
1876  an  old  microscope,  which  had  been  found  at  Middelburg,  was 
shown,  which,  Professor  Harting  considered,  might   possibly  have 
been  made  by  the  Janssens.     It  is  drawn  in  fig.  90,  and  consists  of 
a  combination  of  a  convex  object-lens  and  a  convex  eye- 
lens,  which  form  was  not  published  as  an  actual   con- 
struction until  1646  by  Fontana,  which,  as  Mr.  Mayall 
points  out,  does   not   harmonise   with   the   assumption 
that  this  instrument   was   constructed   by   one    of  the 
Janssens. 

It  is  strictly  a  compound  microscope,  and  the  dis- 
tance between  the  lenses  can  be  regulated  by  two 
draw-tubes.  There  are  three  diaphragms,  and  the  eye- 
lens  lies  in  a  wood  cell,  and  is  held  there  by  a  wire  ring 
sprung  in.  The  object-lens,  a,  is  loose  in  the  actual 
instrument,  but  was  originally  fixed  in  a  similar  way 
to  b. 

It  cannot  be  an   easy   task — if  it   be   even   a    pos- 
'    sible  one — to  definitely  determine  upon  the  actual  indi- 
JanssenV   vidual  or  individuals   by  whom   the   compound    micro- 
compound    scope    was    first    invented.       Recently    some    valuable 
evidence  has  been  adduced  claiming  its  sole   invention 
for  Galileo.    In  a  memoir  published  in  1888  l  Professor 
•G.  Govi,  who  has  made  the  question  a  subject  of  large  and  continuous 
research,  certainly  adduces  evidence  of  a  kind  not  easily  waived. 

Huyghens  and,  following  him,  many  others  assign  the  invention 
of  the  compound  microscope  to  Cornelius  Drebbel,  a  Dutchman,  in 
the  year  1621  ;  but  it  has  been  suggested  that  he  derived  his  know- 
ledge from  Zacharias  Janssen  or  his  father,  Hans  Janssen,  spectacle 
makers,  in  Holland  about  the  year  1590;  while  Fontana,  a  Nea- 
politan, claimed  the  discovery  for  himself  in  1618.  It  is  said  that 
the  Janssens  presented  the  first  microscope  to  Charles  Albert,  Arch- 
duke of  Austria ;  and  Sir  D.  Brewster  states,  in  his  *  Treatise  on  the 
Microscope,'  that  one  of  their  microscopes  which  they  presented  to 
Prince  Maurice  was  in  1617  in  the  possession  of  Cornelius  Drebbel, 
then  mathematician  to  the  Court  of  James  I.,  where  'he  made 
microscopes  and  passed  them  off  as  his  own  invention.' 

Nevertheless  we  are  told  by  Viviani,  an  Italian  mathematician, 
in  his  '  Life  of  Galileo,'  that  '  this  great  man  was  led  to  the  discovery 
of  the  microscope  from  that  of  the  telescope,'  and  that  'in  1612  he 
sent  one  to  Sigismund,  King  of  Poland.' 

We  now  receive  evidence  through  the  researches  of  Govi  that 
the  invention  was  solely  due  to  Galileo  in  the  year  1610.  Professor 
Govi  understands  by  '  simple  microscope '  an  instrument  '  consisting 
of  a  single  lens  or  mirror,'  and  by  '  compound  microscope '  one  '  con- 

1  Atti  R.  Acad.  Sci.  Fis.  Nat.  Napoli,  vol.  ii.  series  ii.  '  II  microscopic  composto 
inventato  da  Galileo,'  Journ.  R.M.S.  Pt.  IV.  1889,  p.  574. 


DID    GALILEO   INVENT   THE    COMPOUND   MICROSCOPE?       121 

si  sting  of  several  lenses  or  a  suitable  combination  of  lenses  and 
mirrors.' 

In  a  pamphlet  published  in  1881,  treating  of  the  invention  of 
the  binocular  telescope,  Govi  pointed  out  that  Chorez,  a  spectacle 
maker,  in  1625,  used  the  Dutch  telescope  as  a  microscope,  and  stated 
that  with  it  '  a  mite  appeared  as  large  as  a  pea ;  so  that  one  can 
distinguish  its  head,  its  feet,  and  its  hair — a  thing  which  seemed  in- 
credible to  many  until  they  witnessed  it  with  admiration.' 

To  this  quotation  he  added  : — 

*  This  transformation  of  the  telescope  into  a  microscope  (or,  as 
opticians  in  our  own  day  would  say,  into  a  Briicke  lens)  was  not  an 
invention  of  the  French  optician.     Galileo  had  accomplished  it  in  the 
year  1610,  and  had  announced  it  to  the  learned  by  one  of  his  pupils, 
John  Wodderborn,  a  Scotchman,  in  a  work  which  the  latter  had 
just  published  against  the  mad  "  Peregrinazione  "  of  Horky.     Here 
are  the  exact  words  of  Wodderborn  (p.  7) : — 

*  Ego  nunc  admirabilis  huius  perspicilli  perfectiones   explanare 
no  conabor  :  sensus  ipse  iudex  est  integerrimus  circa  obiectum  pro- 
prium.     Quid  quod  eminus  mille  pavssus  et  ultra  cum  neque  videre 
iudicares  obiectum,  adhibito  perspicillo,  statim  certo  cognoscas,  esse 
hunc  Socratem  Sophronici  filium  venientem,  sed  tempus  nos  docebit 
et  quotidianae  nouarum  rerum  detectiones  quam  egregie  perspicillum 
suo  fungatur  munere,  nam  in  hoc  tota  omnis  instrument!  sita  est 
pulchritude. 

*  Audiueram,  paucis  ante  diebus  authorem  ipsum  Excellentissimo 
D.  Cremonino  purpurato  philosopho  varia  narrantem  scitu  dignissima 
et  inter  caetera  quomodo  ille  minimorum  animantium  organa  motus, 
et  sensus  ex  perspicillo  ad  vnguem  distinguat ;   in  particulari  autem 
de  quodam  insecto  quod  utrumque  habet  oculum  membrana  crassius- 
cula  vestitum,  quae  tamen  septe  foraminibus  ad  instar  larvae  ferreae 
militis  cataphracti  terebrata,  viam  praebet  speciebus  visibilium.     En 
tibi  [so  says  "Wodderborn  to  Horky]  nouum  argumentum,  quod  per- 
spicillum per  concentrationem   radiorum  multiplicet  obiectu  ;    sed 
audi  prius   quid  tibi  dicturus  sum :    in  caeteris  animalibus  eiusdem 
magnitudinis,  vel  minoris,  quorum  etiam  aliqua  splendidiores  habent 
oculos,  gemini  tantum  apparent  cum  suis  superciliis  aliisque  partibus 
annexis.' 

To  this  Govi  adds  :— 

'  I  have  wished  to  quote  this  passage  of  Wodderborn  textually, 
so  that  the  honour  of  having  been  the  first  to  obtain  from  the  Dutch 
telescope  a  compound  microscope  should  remain  ^*ith  Galileo,  which 
the  latter  called  occhialino,  and  that  the  glory  of  having  reduced  the 
Keplerian  telescope  to  a  microscope  (in  1621)  should  rest  with 
Drebbel.  The  apologists  of  the  Tuscan  philosopher,  by  attributing 
to  him  the  invention  of  the  microscope  without  specifying  with  what 
microscope  they  were  dealing,  defrauded  Drebbel  of  a  merit  which 
really  belongs  to  him  ;  but  the  defenders  of  Drebbel  would  act  un- 
justly in  depriving  Galileo  of  a  discovery  which  incontestably  was  his.'  • 

I  turn  now  to  Wodderborn's  account,  published  in  1610  (the 
date  of  the  dedication  to  Henry  Wotton,  English  Ambassador  at 
Venice,  is  October  16,  1610),  which  reads  thus  : — 


122      THE   HISTOEY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

'  I  will  not  now  attempt  to  explain  all  the  perfections  of  this 
wonderful  occhiale  \  our  sense  alone  is  a  safe  judge  of  the  things 
which  concern  it.  But  what  more  can  I  say  of  it  than  that  by 
pointing  a  glass  to  an  object  more  than  a  thousand  paces  off,  which 
does  not  even  seem  alive,  you  immediately  recognise  it  to  be 
Socrates,  son  of  Sophronicus,  who  is  approaching?  But  time  and 
the  daily  discoveries  of  new  things  will  teach  us  how  admirably  the 
glass  does  its  work,  for  in  that  alone  lies  all  the  beauty  of  that 
instrument. 

'  I  heard  a  few  days  back  the  author  himself  (Galileo)  narrate  to 
the  Most  Excellent  Signor  Cremonius  various  things  most  desirable 
to  be  known,  and  amongst  others  in  what  manner  he  perfectly  dis- 
tinguishes with  his  telescope  the  organs  of  motion  and  of  the  senses 
in  the  smaller  animals;  and  especially  in  a  certain  insect  wrhich  lias 
each  eye  covered  by  a  rather  thick  membrane,  which,  however,  per- 
forated with  seven  holes,  like  the  visor  of  a  warrior,  allows  it  sight. 
Here  hast  thou  a  new  proof  that  the  glass  concentrating  its  rays 
enlarges  the  object ;  but  mind  what  I  am  about  to  tell  thee,  viz.  in 
the  other  animals  of  the  same  size  and  even  smaller,  some  of  which 
have  nevertheless  brighter  eyes,  these  appear  only  double  with  their 
eyebrows  and  the  other  adjacent  parts.' 

After  reading  this  document  Govi  judges  that  it  is  impossible  ta 
refuse  Galileo  the  credit  of  the  invention  of  a  compouiid  microscope 
in  1610,  and  the  application  of  it  to  examine  some  very  minute 
animals ;  and  if  he  himself  neither  then  nor  for  many  years  after 
made  any  mention  of  it  publicly,  this  cannot  take  away  from  him  or 
diminish  the  merit  of  the  invention. 

It  is  not  to  be  believed,  however,  that  Galileo  after  these  first 
experiments  quite  forgot  the  microscope,  for  in  preparing  the 
'  Saggiatore '  between  the  end  of  1619  and  the  middle  of  October, 
1622,  he  spoke  thus  to  Lotario  Sarsi  Segensano  (anagram  of  Oratio 
Grassi  Salonense) : — 

*  I  might  tell  Sarsi  something  new  if  anything  new  could  be  told 
him.  Let  him  take  any  substance  whatever,  be  it  stone,  or  woody 
or  metal,  and  holding  it  in  the  sun  examine  it  attentively  and  he 
will  see  all  the  colours  distributed  in  the  most  minute  particles,  and 
if  he  will  make  use  of  a  telescope  arranged  so  that  one  can  see  very 
near  objects,  he  will  see  far  more  distinctly  what  I  say.' 

It  will  not  therefore  be  surprising  if,  in  1624  (according  to 
some  letters  from  Rome,  written  by  Girolamo  Aleandro  to  the 
famous  M.  de  Peiresc),  two  microscopes  of  Kuffler,  or  rather  Drebbel, 
having  been  sent  to  the  Cardinal  of  S.  Susanna,  who  at  first  did  not 
know  how  to  use  them,  they  were  shown  to  Galileo,  who  was  then 
in  Rome,  and  he,  as  soon  as  he  saw  them,  explained  their  use,  as 
Aleandro  writes  to  Peiresc  on  May  24,  adding,  '  Galileo  told  me 
that  he  had  invented  an  occhiale  which  magnifies  things  as  much 
as  50,000  times,  so  that  one  sees  a  fly  as  large  as  a  hen.' 

This  assertion  of  Galileo,  that  he  had  invented  a  telescope  which 
magnified  50,000  times,  so  that  a  fly  appears  as  big  as  a  hen, 
must,  without  doubt,  be  referred  to  the  year  1610,  and  from  the 
measure  given  of  the  amplification  by  the  solidity  or  volume  the 


GALILEO'S   'OCCHIALE'  123 

linear  amplification  (as  it  is  usually  expressed  now)  would  have 
been  equal  to  something  less  than  the  cubic  root  of  50,000 — that  is, 
about  3(5 — and  that  is  pretty  fairly  the  relative  size  of  a  fly  and 
a  hen. 

Aleandro's  letter  of  May  24  (1624)  does  not  state  at  what  time 
Calili'o  saw  the  telescope  and  explained  the  use  of  it,  but  another 
letter  of  Faber's  to  Can,  amongst  the  autograph  letters  in  the 
possession  of  D.  B.  Boiicompagni,  says  (May  11) :  'I  was  yesterday 
evening  at  the  house  of  our  Signer- Galileo,  who  lives  near  the 
Madalena  ;  he  gave  the  Cardinal  di  Zoller  a  magnificent  eye-glass 
for  the  Duke  of  Bavaria.  1  saw  a  fly  which  Signor  Galileo  him- 
self showed  me.  I  was  astounded,  and  told  Signor  Galileo  that  he 
was  another  creator,  in  that  he  shows  things  that  until  now  we 
did  not  know  had  been  created.'  So  that  even  on  May  10,  1624, 
Galileo  had  not  only  seen  the  telescope  of  Drebbel,  and  explained 
the  use  of  it,  but  had  made  one  himself  and  sent  it  to  the  Duke  of 
Bavaria. 

We  lack  documents  to  show  how  this  microscope  of  Galileo  was 
made,  that  is,  whether  it  had  twro  convergent  lenses  like  those  of 
Drebbel.  A  letter  of  Peiresc  of  March  3,  1624,  says  that  'the 
effect  of  the  glass  is  to  show  the  object  upside  down  .  .  .  and  so 
that  the  real  natural  motion  of  the  animalcule,  which,  for  example, 
goes  from  east  to  west,  seems  to  go  contrariwise,  that  is,  from  west 
to  east,'  or  whether  it  was  not  rather  composed  of  a  convex  and  a 
concave  lens,  like  that  marie  earlier  by  him,  and  used  in  1610,  and 
then  almost  forgotten  for  fourteen  years. 

It  is,  however,  very  probable  that  this  last  was  the  one  in 
question,  for  Peiresc,  answering  Aleandro  on  July  1,  1624,  wrote  : — 
'  But  the  occhiale  mentioned  by  Signor  Galileo,  which  makes  flies 
like  hens,  is  of  his  own  invention,  of  which  he  made  also  a  copy 
for  Archduke  Albert  of  pious  memory,  which  used  to  be  placed  on 
the  ground,  where  a  fly  would  be  seen  the  size  of  a  hen,  and  the 
instrument  was  of  no  greater  height  than  an  ordinary  dining-room 
table.'  Which  description  answers  far  better  to  a  Dutch  tele- 
scope used  as  a  microscope,  in  the  same  way  exactly  as  Galileo 
had  used  it,  rather  than  to  a  microscope  with  two  convex 
lenses. 

One  cannot  find  any  further  particulars  concerning  Galileo's 
occhialini  (so  he  had  christened  them  in  the  year  1624),  either  in 
Bartholomew  Imperiali's  letter  of  September  5,  1624,  in  which  he 
thanks  Galileo  for  having  given  him  one  in  every  way  perfect,  or  in 
that  of  Galileo  to  Cesi  of  September  23,  1624,  accompanying  the 
gift  of  an  occhialino,  or  in  Federico  Cesi's  answer  of  October  26,  or 
in  a  letter  of  Bartholomeo  Balbi  to  Galileo  of  October  25,  1624, 
which  speaks  of  the  longing  with  which  Balbi  is  awaiting  '  the  little 
occhiale  of  the  new  invention,'  or  in  that  of  Galileo  to  Cesar  Marsili 
of  December  17  in  the  same  year,  in  which  Galileo  says  to  the 
learned  Bolognese  'that  he  would  have  sent  him  an  occhialino  to 
see  close  the  smallest  things,  but  the  instrument  maker,  who  is 
making  the  tube,  has  not  yet  finished  it.'  This,  however,  is  how 
Galileo  speaks  of  it  in  his  letter  to  Federico  Cesi,  written  from 


124      THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

Florence  on  September  23,  1624,  more  than  three  months  after  his 
departure  from  Rome  : — 

1 1  senrl  your  Excellency  an  occhialino,  by  which  to  see  close  the 
smallest  things,  which  I  hope  may  give  you  no  small  pleasure  and 
entertainment,  as  it  does  me.  I  have  been  long  in  sending  it.  because 
I  could  not  perfect  it  before,  having  experienced  some  difficulty  in 
finding  the  way  of  cutting  the  glasses  perfectly.  The  object  must 
be  placed  on  the  movable  circle  which  is  at  the  base,  and  moved  to 
see  it  all,  for  that  which  one  sees  at  one  look  is  but  a  small  part. 
And  because  the  distance  between  the  lens  and  the  object  must  In- 
most exact,  in  looking  at  objects  which  have  relief  one  must  be  able 
to  move  the  glass  nearer  or  further,  according  as  one  is  looking  at 
this  or  that  part ;  therefore  the  little  tube  is  made  movable  on  its  stand 
or  guide,  as  we  may  wish  to  call  it.  It  must  also  be  used  in  very 
bright,  clear  weather,  or  even  in  the  sun  itself,  remembering  that  the 
object  must  be  sufficiently  illuminated.  I  have  contemplated  very 
many  animals  with  infinite  admiration,  amongst  which  the  flea  is 
most  horrible,  the  gnat  and  the  moth  the  most  beautiful ;  and  it  was 
with  great  satisfaction  that  I  have  seen  how  flies  and  other  little 
animals  manage  to  walk  sticking  to  the  glass  and  even  feet  upward*. 
But  your  Excellency  will  have  the  opportunity  of  observing  thousands 
and  thousands  of  other  details  of  the  most  curious  kind,  of  which  I 
beg  you  to  give  me  account.  In  fact,  one  may  contemplate  endlessly 
the  greatness  of  Nature,  and  how  subtilely  she  works,  and  with  what 
unspeakable  diligence. — P.S.  The  little  tube  is  in  two  pieces,  and 
you  may  lengthen  it  or  shorten  it  at  pleasure.' 

It  would  be  very  strange,  knowing  Galileo's  character,  that  in 
1624,  and  after  the  attacks  made  on  him  for  having  perhaps  a  little 
too  much  allowed  the  Dutch  telescope  to  be  considered  his  invention, 
he  should  have  been  induced  to  imitate  Drebbel's  glass  with  the  two 
convex  lenses,  and  have  wished  to  make  them  pass  as  his  own  invention, 
whilst  he  had  always  used,  and  continued  to  use  to  the  end  of  his  days, 
telescopes  with  a  convex  and  a  concave  lens  without  showing  that 
he  had  read  or  in  the  least  appreciated  the  proposal  made  by  Kepler, 
ever  since  1611,  to  use  two  convex  glasses  in  order  to  have  telescopes 
with  a  large  field  and  more  powerful  and  convenient. 

In  any  case  it  is  impossible  to  form  a  decided  opinion  on  such  a 
matter,  the  data  failing  ;  but  the  very  fact  that  from  1624  onwards 
Galileo  thought  no  more  of  the  occhialino  (probably  because  he  found 
it  less  powerful  and  less  useful  than  the  occhiale  of  Drebbel),  as  he 
had  not  occupied  himself  with  it  or  had  scarcely  remembered  it  from 
the  year  1610  to  1624,  seems  sufficient  to  show  that  the  occhialino, 
like  the  microscope  of  1610,  was  a  small  Dutch  telescope  with  two 
lenses,  one  convex  and  one  concave,  and  not  a  reduced  Kepleriari 
telescope  like  that  invented  by  Drebbel  in  1621. 

The  name  of  microscope,  like  that  of  telescope,  originated  with 
the  Academy  of  the  Lincei,  and  it  was  Giovanni  Faber  who  invented 
it,  as  shown  by  a  letter  of  his  to  Cesi,  written  April  13,  1625,  and 
which  is  amongst  the  Lincei  letters  in  the  possession  of  D.  B.  Bon- 
compagni.  Here  is  the  passage  in  Faber's  letter  : — 

'  I  only  wish  to  say  this  more  to  your  Excellency,  that  is,  that 


GALILEO   THE   INVENTOE   OF   THE   MICROSCOPE   IN   1610     125 

you  will  glance  only  at  what  I  have  written  concerning  the  new  in- 
ventions of  Signor  Galileo ;  if  I  have  not  put  in  everything,  or  if 
anything  ought  to  be  left  unsaid,  do  as  best  you  think.  As  I  also 
mention  his  new  occhiale  to  look  at  small  things  and  call  it  micro- 
scope, let  your  Excellency  see  if  you  would  like  to  add  that,  as  the 
Lyceum  gave  to  the  first  the  name  of  telescope,  so  they  have  wished 
to  give  a  convenient  name  to  this  also,  and  rightly  so,  because  they 
are  the  first  in  Rome  who  had  one.  As  soon  as  Signor  Rikio's 
epigram  is  finished,  it  may  be  printed  the  next  day ;  in  the  mean- 
while I  will  get  011  with  the  rest.  I  humbly  reverence  your  Excel- 
lency.— From  Rome,  April  13,  1625.  Your  Excellency's  most 
humble  servant,  GIOVAXNI  FABER  (Lynceo).' 

The  Abbe  Rezzi,  in  a  work  of  his  on  the  invention  of  the  micro- 
scope, thought  that  he  might  conclude  from  the  passage  of 
Wodderborn,  reproduced  above,  that  Galileo  did  not  invent  the  com- 
pound microscope,  but  gave  a  convenient  form  to  the  simple  micro- 
scope, and  in  this  way  as  good  as  invented  it,  for  the  Latin  word  used 
by  Wodderborn,  perspicillum, i  signified  at  that  time,  it  is  clear,'  Rezzi 
says,  '  no  other  optical  instrument  than  spectacles  or  the  telescope, 
never  the  microscope,  of  which  there  is  no  mention  whatever  in  any 
book  published  at  that  time,  nor  in  any  manuscript  known  till  then.' 

But  Rezzi  was  not  mindful  that  on  October  16,  1610,  the  date 
of  Wodderborn's  essay,  the  name  of  microscope  had  not  yet  been 
invented,  nor  that  of  telescope,  which,  according  to  Faber,  was  the 
idea  of  Cesi,  according  to  others  of  Giovanni  Demisiano,  of 
Cephalonia,  at  the  end,  perhaps,  of  1610,  but  more  probably  at  the 
time  of  Galileo's  journey  to  Rome  from  March  29  to  June  4,  1611. 
If,  therefore,  the  word  microscope  had  not  yet  been  invented,  and 
if  the  telescope,  or  the  occhiale  as  it  was  then  called,  was  by  all 
named  perspictttum,  one  cannot  see  why  Wodderborn's  perspicillum 
cannot  have  been  a  cannocchiale  (telescope)  smaller  than  the  usual 
ones,  so  that  it  could  easily  be  used  to  look  at  near  objects,  but  yet 
a  cannocchiale  with  two  lenses,  one  convex  and  one  concave,  like  the 
others,  and,  therefore,  a  real  compound  microscope,  although  not 
mentioned  by  that  name  either  by  Wodderborn  or  others.  And, 
besides  that,  how  could  it  be  that  Wodderborn  beginning  to  treat 
'  admirabilis  huius  perspicilli,'  that  is,  of  the  telescope  in  the  first 
line,  should  then  have  called  perspicillum  a  single  lens  in  the  eleventh 
line  of  the  same  page  ?  Rezzi's  mistake  is  easily  explained,  remem- 
bering that  he  had  not  under  his  eyes  Wodderborn's  essay,  but  only 
knew  a  brief  extract  reported  by  Yenturi. 

It  thus  appears  as  in  the  highest  degree  probable  that  Galileo, 
in  1610,  was  the  inventor  of  the  compound  microscope  ;  it  was 
subsequently  invented,  or  introduced,  and  zealously  adopted  in 
Holland;  and  when  Dutch  invention  penetrated  into  Italy  in  1624 
Galileo  attempted  a  reclamation  of  his  invention  (w^hich  was  undoubt- 
edly distinct  from  that  of  Drebbel) ;  but  as  these  were  not  warmly 
seconded  and  responded  to  abroad  he  allowed  the  whole  thing  to 
pass.  Nevertheless  the  facts  Govi  gives  are  as  interesting  as  they 
are  important. 

In  regard  to  the  discovery  of  the  simple  lens  Govi  points  out 


126      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 


that  after  the  year  1000,  minds  having  reopened  to  hope  and  in- 
tellects to  study,  there  began  to  dawn  some  light  of  science,  so  that 
in  1276  a  Franciscan  monk,  Roger  Bacon,  of  Ilchester,  in  his  '  Opus 
Majus,'  dedicated  and  presented  by  him  to  Clement  IV.,  could  show 
many  marvellous  things,  and  amongst  these  the  efficacy  of  crystal 
lenses,  in  order  to  show  things  larger,  and  in  this  wise  he  says  make 
of  them  '  an  instrument  useful  to  old  men  and  those  whose  sight  is 
weakened,  who  in  such  a  way  will  be  able  to  see  the  letters  suf- 
ficiently enlarged,  however  small  they  are.'  As  long  as  no  documents 
anterior  to  him  are  discovered,  Roger  Bacon  may  be  considered  the 
first  inventor  of  convergent  lenses,  and  therefore  of  the  simple  micro- 
scope, however  small  the  enlargement  by  his  lenses  may  have  been. 
As,  however,  that  man  of  rare  genius,  the  initiator  of  experi- 
mental physics,  had  brought  on  himself  the 
hatred  of  his  contemporaries,  they  kept  him 
for  many  years  in  prison,  then  shut  him  up 
in  a  convent  of  his  order  to  the  end  of  his 
long  life  of  nearly  eighty  years.  His  writings 
had  to  be  hidden,  at  least  those  treating  on 
natural  science,  to  save  them  from  destruc- 
tion, and  so  the  invention  of  lenses,  or  the 
knowledge  of  their  use  to  enlarge  images  and 
to  alleviate  the  infirmities  of  sight,  remained 
unknown  or  forgotten  in  the  pages  of  the 
famous  '  Opus  Majus,'  which  only  came  to 
light  in  1733  by  the  care  of  Samuel  Jebb,  a 
learned  English  doctor. 

A  Florentine,  by  name  Salvino  degli 
Armati,  at  the  end  of  the  thirteenth  century 
(?  1280)  (in  Bacon's  lifetime),  had  therefore 
the  glory  of  inventing  spectacles,  and  it  was 
a  monk  of  Pisa,  Alexander  Spina,  who  sud- 
denly charitably  divulged  the  secret  of  their 
construction  and  use. 

Perhaps  Salvino  degli  Armati  and  Spina 
really  discovered  more  than  Roger  Bacon  had 
discovered ;  that  is,  they  found  out  the  use 
of  converging  lenses  for  long-sighted  people,  and  of  diverging  lenses 
for  short  sight,  whilst  the  English  monk  had  only  spoken  of  the 
lenses  for  long  sight,  and  perhaps  they  added  to  this  first  inven- 
tion the  capability  of  varying  the  focal  lengths  of  the  lenses  accord- 
ing to  need,  and  the  other  of  fixing  them  on  to  the  visor  of  a  cap  to 
keep  them  firm  in  front  of  the  eyes,  or  to  fasten  them  into  two, 
circles  made  of  metal,  or  of  bone  joined  by  a  small  elastic  bridge 
over  the  nose.  However  it  may  be,  the  discovery  of  spectacles,  or, 
as  it  may  be  called,  of  the  simple  microscope,  may  be  equally  divided 
betwen  Roger  Bacon  and  Salvino  degli  Armati,  leaving  especially  to 
the  latter  the  invention  of  spectacles. 

The  earliest  known  illustration  of  a  simple  microscope  is  given 
by  Descartes  in  his  '  Dioptrique'  in  1637  :  fig.  91  reproduces  it.  It 
is  practically  identical  with  one  devised  by  Lieberkiihn  a  century 


FIG.  91. — Descartes'  simple 
microscope  with  reflector 
(1687). 


'  GALILEO'S  '  AND   CAMPANI'S  MICROSCOPES 


127 


after  and  shown  on  p.  139.  A -lens  is  mounted  in  a  central  aperture 
in  a  polished  concave  metal  reflector.  Descartes  apparently  devised 
another  and  much  more  pretentious  instrument,  but  it  appears  im- 
practicable and  could  never 
have  existed  save  as  a  sugges- 
tion. But  he  appears  to  have 
been  the  first  to  publish  'figures 
>and  descriptions  for  grinding 
;tnd  polishing  lenses. 

In  the  Museo  di  Fisica  there 


FIG.  93. — Campani's  microscope  (1660)? 

are  two  small  microscopes  which 
it  is  affirmed  have  been  handed 
down  from  generation  to  gene- 
ration since  the  dissolution  of 
the  Accademia  del  Cimento  in 
1667,  with  the  tradition  of 
having  been  constructed  by 
Galileo.  They  are  shown  in 
fig.  92,  but  from  the  superiority  of  construction  of  these  instru- 
ments it  is  very  improbable  that  they  belong  to  the  days  of  Galileo, 
who  died  in  1642;  and  there  is  a  specially  interesting  compound 


FIG.  92.— Galileo's  microscopes. 
?  Campani  or  later. 


128      THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 


microscope,  by  Giuseppe  Campani,  which  was  published  first  in  1686, 
which  is  presented  in  fig.  93  ;  its  close  similarity  to  *  Galileo  micro- 
scopes'  is  plainly  apparent,  making  it  still  more  improbable  that 
these  could  be  given  a  date  prior  to  1642. 

In  a  journal  of  the  travels  of  M.  de  Monconys,  published  in 
1665,  there  is  a  description  of  his  microscope  which  is  of  much 
interest.  He  states  that  the  distance  from  the  object  to  the  first 
lens  is  one  inch  and  a  half ;  the  focus  of  the  first  lens  is  one  inch  ; 
the  distance  from  the  first  lens  to  the  second  is  fifteen 
inches  ;  the  focus  of  the  second  lens,  one  inch  and  a  half; 
distance  from  the  second  to  the  third,  one  inch  and 
eight  lines  ;  the  focus  of  the  third  lens,  one  inch  and 
eight  lines  ;  and  the  distance  from  the  eye  to  the  third 
lens,  eight  lines. 

This  would    form  the  data  of  a   pr.ictic.il   com- 
pound microscope  with  a  field  lens ;    and  as  Mon- 
conys had  this  instrument  made  in   1660  by  the 
'  son-in-law  of  Yiselius,'  it  becomes  probable  in  a 
very  high  degree  that  to  him  must  be  attributed 
the  earliest  device  of  a  microscope  with  a  field- 
lens. 

In    1665    Hooke   published   his    '  Micro- 
graphia,'  giving  an  account  and  a  figure  of 
his    compound    microscope.       He    adopted 
the  field-lens  employed  by  Monconys  and 
gives  details  as  to  the  mode  and  object 


FIG.  94. — Hooke's  compound  microscope  (1665). 


DIVINTS    o  IMPOUND   MICKOSCOPE 


129 


of  its  employment,  which  are  at  once  interesting  and  instruc- 
tive ;  for  they  show  quite  clearly  that  it  was  not  employed  by  him 
to  correct  the  spherical  aberration  of  the 
eye-lens,  but  merely  to  increase  the  size  of 
the  field  of  view.  He  tells  us  that  he  used 
it  '  only  when  he  had  occasion  to  see  much 
of  an  object  at  once.  .  .  .  But  whenever 
I  had  occasion  to  examine  the  sinal}  parts 
of  a  body  more  accurately  I  took  out  the 
middle  glass  (field-lens)  and  only  made  use 
of  one  eye-glass  with  the  object-glass.' 

Fig.  94  is  a  reproduction  of  the  original 
drawing,  and  the  general  design  appears 
to  be  claimed  by  Hooke.  There  is  a  ball- 
and-socket  movement  to  the  body,  of 
which  he  writes  :  '  On  the  end  of  this  arm 
(I),  which  slides  on  the  pillar  C  C)  was 
a  small  ball  fitted  into  a  kind  of  socket 
F,  made  in  the  side  of  the  brass  ring  <  >. 
through  which  the  small  end  of  the  tube 
was  screwed,  by  means  of  which  contri- 
vance I  could  place  and  fix  the  tube  in 
whatsoever  posture  I  desired  (which  for 
many  observations  was  exceedingly  neces- 
sary), and  adjusted  it  most  exactly  to  any 
object.' 

It  need  hardly  be  remarked  that,  useful 
as  the  ball-and-socket  joint  is  for  many 
purposes  in  microscopy,  it  is  not  advan- 
tageously employed  in  this  instrument. 

Hooke  devised  the  powerful  illuminat- 
ing arrangement  seen  in  the  figure,  and 
employed  a  stage  for  objects  based  on  a 
practical  knowledge  of  what  was  required. 
He  described  a  useful  method  of  estimat- 
ing magnifying  power,  and  was  an  in- 
dustrious, wide,  and  thoroughly  practical 
ol>.>prver.  But  he  worked  without  a 
mirror,  and  the  screw-focussing  arrange- 
ment seen  in  the  drawing  must  have  been 
as  troublesome  as  it  was  faulty.  But  as  a 
microscopist,  Hooke  gained  a  European 
fame,  and  gave  a  powerful  stimulus  to 
microscopy  in  England. 

In  1668  a  description  was  published 
in  the  *  Giornale  dei  Letterati  '  of  a  com- 
pound microscope  by  Eustachio  Divini, 
which  Fabri  had  previously  commended. 
It  was  stated  to  be  about  16^  inches 
high,  and  adjustable  to  four  different 
lengths  by  draw-tubes,  giving  a  range  of 


FIG.  95. — Divini's  compound 
microscope  (1668). 


130     THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 


magnification  from  41  to  143  diameters.  Instead  of  the  usual  bi- 
convex eye-lens,  two  plano-convex  lenses  were  applied  with  their 
convex  surfaces  in  contact,  by  which  he  claimed  to  obtain  a  much 
flatter  field.  Mr.  Mayall  found  in  the  Museo  Copernicano  at  Rome 
a  microscope  answering  so  closely  to  this  description  that  he  does 
not  hesitate  to  refer  its  origin  to  Divini.  He  made  the  sketch  of 

it  given  in  fig.  95. 
But  the  optical  con- 
struction had  been 
tampered  with  and 
could  not  be  esti- 
mated. 

Cherubin  d'Orleai  is 
published,  in  1671,  a 
treatise  containing  a 
design  for  a  micro- 
scope, of  which  fig. 
96  is  an  illustration. 
The  scrolls  were  of 
ebony,  firmly  at- 
tached to  the  base 
and  to  the  collar 
encircling  the  fixed 
central  portion  of  the 
body-tube.  An  ex- 
terior sliding  tulte 
carried  the  eye-piece 
above  on  the  fixed 
tube,  and  a  similar 
sliding  tube  carried 
the  object-lens  below. 
these  sliding  tul  x  -s 
serving  to  focus  the 
image  and  regulate 
(within  certainlimits) 
the  magnification. 
He  also  suggested  a 
screw  arrangement 
to  be  applied  beneath 
the  stage  for  focus- 
sing. He  devised,  or 
recommended,  seve- 
ral combinations  of 
lenses  for  the  optical 
part  of  the  micro- 
scope, and  refers  to  combinations  of  three  or  four  separate  lenses, 
by  which  objects  could  be  seen  erect,  which  he  considered  '  much  to 
be  preferred.' 

He  also  invented  a  binocular  form  of  microscope  and  published 
it  in  his  work,  '  La  Vision  Parfaite,'  in  1677.  It  consisted  of  two 
compound  microscopes  joined  together  in  one  setting,  so  as  to  be 


FIG.  96. — Cherubin  d'Orleans'  compound  microscope 

(1671). 


EARLY   BINOCULAR   MICROSCOPE  131 

applicable  to  both  eyes  at  once ;  a  segment  of  each  object-lens 
(supposed  to  be  of  one-inch  focus)  was  ground  away  to  allow  the 
convergent  axes  starting  from  the  two  eyes  to  meet  at  about  16 
inches  distance  at  the  common  focus.  Mechanism  was  provided  for 
regulating  the  width  of  the  axes  to  correspond  with  the  observer's  eyes. 


Fig.  97,  showing  the  optical  construction,  is  copied  from  the 
original  diagram  ('  La  Vision  Parfaite,'  tab.  i.  fig.  2,  p.  80).  Accord- 
ing to  the  arrangement  of  the  lenses  as  shown  in  the  figure  a  pseudo- 
stereoscopic  image  would  have  been  obtained. 

A  drawing  of  this  binocular,  as  known  to  Zahn,  was  given  in 
the  first  edition  of  his  '  Oculus  Artificialis  '  in  1685  (Fundamen  III. 
p.  233),  and  is  reproduced  in  fig.  98. 

K  2 


132     THE   HISTOEY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 


In  1672  Sir  Isaac  Newton  communicated  to  the  Royal  Society 
a  note  and  diagram  for  a  reflecting  microscope  ;  we  have,  however, 
no  evidence  that  it  was  ever  constructed.  But  in  1673  Leemven 
hoek  began  to  send  to  the  Royal  Society  his  microscopical  discoveries 
Nothing  was  known  of  the  construction  of  his  instruments,  except 
that  they  were  simple  microscopes,  even  down  to  so  late  a  period  as 
1709.  We  know,  however,  that  his  microscopes  were  mechanically 
rough,  and  that  optically  they  consisted  of  simple  bi-convex  lenses, 
with  worked  surfaces  mounted  between  two  plates  of  thin  metal 
with  minute  apertures  through  which  the  objects  were  directly  seen. 
At  his  death  Leeuwenhoek  bequeathed  a  cabinet  of  twenty-six  of  his 
microscopes  to  the  Royal  Society  ;  unhappily,  they  have  mysteriously 

_^^  disappeared.     But  Mr.  May 

ISjjV  all  was  enabled  to  figure  one 
"""''*  lodged  in  the  museum  of  the 
Utrecht  University,  which  i> 
given  in  figs.  99  and  KM)  in 
full  size.  The  lens  is  seen  in 
the  upper  third  of  the  plate. 
It  has  a  J-inch  focus.  The 
object  is  held  in  front  of  the 
lens,  on  the  point  of  a  short 
rod,  with  screw  arrange- 
ments for  adjusting  the 
object  under  the  lens. 

Many  modifications  of 
this  and  the  preceding  in- 
struments are  found  with 
some  early  English  forms, 
but  no  important  construc- 
tive or  optical  modification 
immediately  presents  itself. 
But  some  ingenious  arrange- 
ments are  found  in  the 
simple  microscopes  de v  i  s»  •  d 
by  Musschenbroek  in  the 
early  years  of  the  eighteenth 
century. 

Grindl  figured  a  microscope  in  hip  '  Micrographia  Nova '  in 
1687,  in  which  optical  modifications  arise.  Divini  had,  as  was 
stated,  combined  two  plano-convex  lenses,  with  their  convex  surfaces 
facing,  to  form  an  eye-piece  :  this  idea  was  carried  further  in  1 668 
by  a  London  optician,  who  used  two  pairs  of  these  lenses  ;  Grindl 
did  this  also,  but  in  addition  he  used  two  similar  (but  smaller)  lenses 
in  the  same  manner  as  an  objective.  The  form  of  the  microscope 
itself  was  copied  from  that  of  Cherubiii  d'Orleans  (fig.  97),  but  was 
modified  by  the  application  of  an  external  screw. 

In  1691  Bonannus  modified  preceding  arrangements  by  devising 
a  means  of  clipping  the  object  between  two  plates  pressed  away  from 
the  object-lens  by  a  spiral  spring,  the  focussing  being  then  effected 
by  a  '  screw-barrel.' 


FIG.  99.  FIG.  100. 

Leeuwenhoek's  microscope  (1C>7:1  . 


134     THE   HISTORY  AND  DEVELOPMENT   OF  THE   MICROSCOPE 

This  system  of  focussing  was  employed  in  a  more  practical  form 
by  Hartsoeker  in  1694  ami  was  adopted  by  Wilson  in  1702.  It 
became  a  very  popular  form  for  the  microscope  in  the  eighteenth 
century. 

We  are  indebted  to  Bonamms  also  for  originating  a  horizontal 
form  of  microscope,  which  is  interesting  and  which,  in  a  drawing  of 
the  instrument,  is  shown  to  possess  a  sub-stage  compound  condenser 
fitted  with  focussing  arrangements  for  illuminating  transparent 
objects.  There  was  great  convenience  in  using  the  microscope  in  a 
horizontal  position  with  a  lamp  and  condenser  in  the  same  axi>. 
especially  as  all  the  compound  microscopes  previously  constructed 
had  been  employed  vertically,  or  had  been  directed  towards  the  sky 
for  purposes  of  illumination.  Remarkably  crude  as  the  mechanism 
appears,  it  is  a  very  early  instance  of  the  use  of  what  has  become — 
though  slowly  and  late  on  the  continent — a  now  universally  acknow- 


FIG.  102. — Hartsoeker's  simple  microscope  (1694). 

ledged  optical  arrangement  indispensable  for  the  best  results,  viz.  a 
compound  condenser  fitted  with  focussing  mechanism  for  illuminating 
transparent  objects.  The  picture  of  the  entire  instrument  is  shown 
in  fig.  101. 

In  Hartsoeker's  microscope  'the  lens-carrier  A  B,  fig.  102  (on 
which  the  cell  P,  containing  the  lens,  is  screwed),  screws  into  the 
body  O  C,  Q  Dat  0  Q  ;  the  thin  brass  plates  E  and  F  fit  within  the 
body,  the  portions  cut  out  allowing  them  to  slide  on  the  short  pillars 
O  C  and  Q  D,  and  the  spiral  spring  pressing  them  towards  C  D  ; 
the  object-slides,  or  an  animalcule  cage  G  H  (hinged  at  a  b  to  allow 
the  lid  G  to  fit  into  H,  enclosing  the  objects  between  strips  of  talc), 
slide  between  the  plates  E  and  F  when  in  position,  and  the  "  screw- 
barrel  "IK  fits  into  the  screw-socket  C  D  and  regulates  the  focus- 
sing ;  a  condensing  lens,  N,  fits,  on  a  second  "  screw-barrel,"  L  M, 
which  is  applied  in  the  screw-socket  of  I  K.  This  arrangement  of 


HAKTSOEKER'S    MICROSCOPE 


135 


the  condenser  is  better  than  the  plan  adopted  by  Wilson,  as  it  allows 
the  illumination  to  be  focussed  on  the  object  independently  of  the 
focal  adjustment  of  the  object  to  the  magnifying  lens  ;  whereas  in 
Wilson's  microscope,  the  condenser  being  mounted  in  I  K,  without 
facility  of  adjustment,  remained  at  a  fixed  distance  from  the  object, 
and  hence  the  control  of  the  illumination  was  very  limited.' 

Another  microscope  dated  1702  is  shown  in  fig.  103  as  drawn  by 
/aim  in  his  '  Oculus  Artificialis.'  Fig.  103  presents  a  back  view  of 
it  and  shows  an  oval  wooden  plate  £  on  the  other  side  of  this  is  a 

similar  plate  which  holds  the  lens 
in  such  a  position  that  it  is  oppo- 
site the  aperture  A.  Between  the 
two  plates  there  is  a  rotary  multiple 
object  holder  shown  in  fig.  103A  M 
N,  the  object  being  inserted  in  the 
apertures  in  the  circumference  of 
the  disc.  Focussing  is  accomplished 


FIG.  103  (1702). 


FIG.  103A  (1686). 


by  means  of  the  milled  head  B  which  is  attached  to  a  screw  regulating 
the  distance  between  the  two  plates,  one  of  which  carries  the  lens, 
the  other  the  rotary  object  holder.  The  point  worthy  of  note  in 
this  instrument  is  the  rotating  wheel  of  graduated  diaphragms 
A,  C,  D,  E,  placed  on  the  side  away  from  the  lens.  This  is  the  first 
instance  of  a  useful  appliance  surviving  in  our  present  microscopes. 
In  Harris's  l  Lexicon  Technicum '  (1704,  2  vols.  fol.),  under  the 
word  microscope,  Marshall's  compound  microscope  (fig.  104)  is 
described  and  figured.  Several  important  innovations  in  micro- 


IOHN  MARSHALL'S 

New  Invented 

DOUBLEMICROSCOPE, 

For  Viewing  the 
ClRCULATIONoF  the  BLOOD 

Made  &?  Sold  bv  him  af  <he  Archimedes  &S 
Golden  SpccOaclea  in  Lucivsue  Street. 


TUa.  du 


\v 


FIG.  104  (1704>. 


HERTEL'S   ^IICKOSCOPE 


137 


FIG.  105. — Hertel's  microscope  (1716). 


138    THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 

scopical  construction  were  here  embodied.  (1)  A  fine -adjustment 
screw  F  is  connected  with  the  sliding  socket  E,  supporting  the  arm 
D,  in  which  the  body-tube  is  screwed ;  the  focussing  could  thus  be 
controlled  in  a  far  more  effective  manner  than  by  any  system  pre- 
viously applied  to  a  large  microscope.  The  previous  systems  involved 
the  direct  movement  of  the  body-tube  either  by  rotating  in  a  screw- 
socket  (as  in  Hooke's)  or  by  sliding  in  a  cylindrical  socket  (as  in 
Divini's  and  Cherubin's) ;  in  a  few  instances  the  object  was  moved 


FIG.  106.— M.  Joblot's  microscope  (1718). 


in  relation  to  the  object-lens,  but  all  these  plans  were  more  or  less 
defective,  especially  with  microscopes  of  large  dimensions.  Marsha  1 1  's 
system  was  a  distinct  mechanical  improvement,  for  the  object  could 
now  be  viewed  during  the  actual  process  of  focussing,  as  the  image 
would  remain  steadily  in  the  field.  (2)  A  fork,  N  N,  is  here  applied 
with  a  thumb-screw  clamp,  O,  on  the  pillar  itself.  (3)  Hooke's  ball- 
and-socket  joint,  which  was  applied  to  the  arm  I,  is  here  shifted  to 
the  lower  end  of  the  pillar,  where  it  would  give  the  movements  of 
inclination  to  the  whole  microscope  instead  of  to  the  body  tube  only, 


DE.   LIEBERKUHN'S   MICROSCOPE 


139 


as  in  Hooke's ;  the  ball  L  could  be  tightly  clamped  by  the  screw 
collar  M,  in  which  slots  were  cut  to  give  spring.  (4)  A  condensing 
lens  on  jointed  arms  appears  ;  this  probably  was  the  first  application 
of  such  adjustments  to  the  con- 
denser. From  the  singular  posi- 
tion of  the  candle  beneath  the 
condenser,  we  may  infer,  without 

doubt,  that  the  mirror  was  still  II  A%,    ^bA  I A 

unknown  as  a  microscopical  ac-  ,• 
cessory  in  England. 

In  fact,  in  no  microscope  up 
to  this  time  has  there  been  any   FIG.  107.— Lieberkiihn's  microscope  (1739). 
trace  of,  or  reference  to,  a  mirror  ; 

but  in  1716  Hertel  employed  it  and  introduced  some  other  consider- 
able modifications.  The  *  general  appearance  of  the  instrument  as 
originally  figured  by  Hertel  is  given  in  fig.  105.  Not  only  have  we 
the  mirror  below  the  stage,  but  also  above 
the  stage  a  concave  metal  mirror  reflecting 
light  through  a  condenser  on  the  object, 
while  the  stage  has  focussing  movement  by 
the  right-hand  ornamental  'butterfly'  nut, 
and  is  capable  of  movement  to  and  from  the 
pillar  by  the  middle  nut,  and  also  of  rotary 
movement  by  the  left-hand  nut.  These  two 
last  movements  form  what  is  now  known  as 
a  'mechanical  stage.'  The  body-tube  is 
hinged  and  is  inclined  by  a  screw-sector 
mechanism.  A  distinct  advance  on  the  simple 
microscopes  which  had  preceded  it  was  made 
by  one  devised  by  M.  Joblot,  and  illustrated 
in  fig.  106.  The  ornamental  plate  holds  the 
lens,  the  focus  being  adjusted  by  the  nut  and 
screw ;  the  plate  next  to  the  ornamental  one 
is  a  concentric  rotary  stage,  of  good  mechanical 
quality.  The  tube  A  was  called  by  Joblot 
'  the  Canon,'  and  was  lined  with  black  cloth 
or  velvet,  and  has  a  diaphragm  at  each  end. 
These  diaphragms  are  movable,  which  was 
practically  a  considerable  optical  benefit. 

In  1738  Dr.  N.  Lieberkiihn  devised, 
what  had  been  employed  in  principle  by 
Descartes  a  century  before,1  the  instrument 
that  has  ever  since  been  known  by  his  name, 
and  which  is  still  of  considerable  value  to  the 
microscopist.  Fig.  107  is  a  reproduction 

from  the  earliest  drawing  known  of  Lieber-      pIG  IQS Culpeper  and 

kiilm's  microscope.     A  A  is  a  concave  mirror  Scarlet's  microscope  (1738). 
of  silver  ;  from  its  form  the  light  is  reflected 

from  it  to  a  focus  on  the  object  C.     The  mirror  is  pierced  in  the 
centre  at  B,  and  the  lens,  or  object-glass,  is  inserted  and  adjusted, 

'  See  pp.  126-7. 


140     THE   HISTORY   AND  DEVELOPMENT   OF  THE   MICROSCOPE 


the  eye  being  placed  behind  in  the  direction  D  at  any  point  the 
single  lens  or  a  combination  might  require. 

Culpeper  and  Seal-let's  microscope  requires  a  note,  and  is  illus- 
trated in  fig.  108.  It  was  inappropriately  designated  a  '  reflecting ' 
microscope,  but  this  arose  merely  from  the  fact  that  it  was  the  first 
English  model  which  employed  an  illuminating  mirror.  It  was, 
however,  a  dioptric,  not  a  catoptric  instrument,  and  is  figured  in 
Dr.  Smith's  '  Opticks,'  1738. 

'  A  Pocket  Reflecting  Microscope '  was  figured  by  Benjamin 
Martin  in  his  '  Micrographia  Nova'  in  1742,  having  the  interesting 
feature  of  a  micrometer  eye-piece  depending  on  a  screw  with  a  certain 
number  of  threads  to  the  inch,  and  by  which  accurate  measurements 
could  be  taken.  It  was  called  a  reflecting  microscope  because  it  had 
a  mirror  fitted  into  its  cylindrical  base;  but  it  was,  in  reality,  a 
compound  refracting  form,  and  appears  to  have  a  good  claim  to  have 

been  the  original  from  wrhence  the 
modern  'drum'  microscopes  were 
taken. 

Wilson  devised  a  simple 
'  screw-barrel '  microscope  in  1702, 
and  Baker  describes  and  figures 
in  1742  the  Wilson  model 
mounted  on  a  scroll  standard  and 
with  a  mirror  mounted  on  the  l>a-e 
in  a  line  with  the  optic  axis.  I-':,-'. 
109  reproduces  the  drawing  of 
Adams. 

But  Martin  originated  a  lai-^e 
number  of  improvements  both  in 
the  optical  arrangements  and  the 
mechanism  of  the  microscope,  and 
was  an  excellent  maker.  He  ap- 
plied rack-and-pinion  focussing  ad- 
justments, to  the  compound  micro- 
scope he  added  inclining  move- 
ments to  the  pillar  carrying  the 
stage  and  mirror,  and  he  furnished 
the  stage  with  rectangular  movements. 

It  was  to  this  maker  that  the  late  Professor  Quekett  was 
indebted  for  an  early  microscope,  of  which  he  evidently  to  the  last 
thought  highly,  and  which  was  subsequently  purchased  by  the  Royal 
Microscopical  Society.  A  drawing  of  this  instrument  is  given  in  fig. 
110,  and  should  be  described  in  Quekett's  own  words.  He  says: 
'  It  stands  about  two  feet  in  height,  and  is  supported  on  a  tripod 
base,  A;  the  central  part  or  stem,  B,  is  of  triangular  figure,  having 
a  rack  at  the  back,  upon  which  the  stage,  0,  and  frame,  D,  support- 
ing the  mirror,  E,  are  capable  of  being  moved  up  or  down.  The 
compound  body,  F,  is  three  inches  in  diameter  ;  it  is  composed  of 
two  tvibes,  the  inner  of  which  contains  the  eye-piece,  arid  can  be 
raised  or  depressed  by  rack  and  pinion,  so  as  to  increase  or  diminish 
the  magnifying  power.  At  the  base  of  the  triangular  bar  is  a  cradle 


FIG.  109. — Wilson's  simple  microscope 
on  scroll  standard  (as  made  by 
Adams,  1746). 


( >  r  K  K  KIT'S   MICROSCOPE 


joint,  G,  by  which  the  instrument  can  be  inclined  by  turning  the 
screw-head,  H  [connected  with  an  endless  screw  acting  upon  a  worm- 
wheel].  The  arm,  I,  supporting  the  compound  body,  is  supplied 
witli  a  rack  and  pinion,  K,  by  which  it  can  be  moved  backwards  and 
forwards,  and  a  joint  is  placed  below  it,  upon  which  the  body  can  be 
turned  into  a  horizontal  position  ;  another  bar  carrying  a  stage  and 
mirror  can  be  attached  by  the  screw.  L  X,  so  as  to  convert  it  hit  >  a 
horizontal  microscope. 
The  stage,  0,  is  provided 
with  all  the  usual  appa- 
ratus for  clamping  ob- 
jects, and  a  condenser 
can  be  applied  to  its 
under  surface  ;  the  stage 
itself  may  be  removed, 
the  arm,  P,  supporting 
it,  turned  round  011  the 
pivot  C,  and  another 
stage  of  exquisite  work- 
manship placed  in  its 
>tead,  the  under  surface 
of  which  is  shown  at  Q. 
'  This  stage  is  strictly 
a  micrometer  one,  hav- 
ing rectangular  move- 
ments and  a  fine  ad- 
justment, the  move- 
ments being  accom- 
plished by  fine- threat  led 
screws,  the  milled  lira<l> 
of  which  are  graduated. 
'The  mirror,  E,  is  a 
double  one,  and  can  be 
raised  or  depressed  by 
rack  and  pinion ;  it  is 
also  capable  of  removal, 
and  an  apparatus  for 
holding  large  opaque 
objects,  such  as  minerals, 
can  be  substituted  for  it. 
The  accessory  instru- 
ments are  very  numer- 
ous, and  amongst  the  Flo>1]0._Martin'B  large  universal  microscope  as  used 
more  remarkable  may  by  Quekett  (1780). 

be  mentioned  a  tube,  M, 

containing  a  speculum,  which  can  take  the  place  of  the  tube,  R,  and  so 
form  a  reflecting  microscope.  The  apparatus  for  holding  animalcules 
or  other  live  objects,  which  is  represented  at  S,  as  well  as  a  plate  of 
glass  six  inches  in  diameter,  with  four  concave  wells  ground  in  it, 
can  be  applied  to  the  stage,  so  that  each  well  may  be  brought  in 
succession  under  the  magnifying  power.  The  lenses  belonging  to 


142     THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

this  microscope  are  twenty-four  in  number ;  they  vary  in  focal 
length  from  four  inches  to  one-tenth  of  an  inch ;  ten  of  them  are 
supplied  with  Lieberkiihns.  A  small  arm,  capable  of  carry  ing  single 
lenses,  can  be  applied  at  T,  and  when  turned  over  the  stage  the  in- 
strument becomes  a  single  microscope  ;  there  are  four  lenses  suitable 
for  this  purpose,  their  focal  length  varying  from  -^th  to  ^th  of  an 
inch.  The  performance  of  all  the  lenses  is  excellent,  and  no  pains 
appear  to  have  been  spared  in  their  construction.  There  are 
numerous  other  pieces  of  accessory  apparatus,  all  remarkable  for  the 
beauty  of  their  workmanship.'  l 

Benj.  Martin  not  only  in  this  way  greatly  advanced  the 
mechanical  arrangements  of  the  microscope,  but  he  improved  the 
optical  part.  He  used  a  Huyghenian  eye-piece  on  the  telescope 
formula,  where  the  focus  of  the  eye-lens  was  that  of  the  field-lens 
3,  and  the  distance  between  them  2  ;  but  instead  of  employing  a 
single  eye-lens  he  broke  it  up  into  two  of  equal  foci,  that  nearest  the 
eye  being  a  '  crossed  '  lens,  and  the  other  a  plano-convex,  the  steeper 
convexities  of  these  lenses  being  towards  each  other.  In  addition  to  this 
lie  placed  at  a  short  distance  above  the  nose-piece  an  equi-convex  lens 
of  5J  inches  focus  ;  this  acted  as  a  back  lens  to  all  the  objectives, 
so  that  when  an  objective  was  changed  it  was  really  only  the  front 
lens  of  a  compound  objective  that  was  altered. 

Cuff  designed  and  made  a  microscope,  in  1744,  which  Baker 
figured  and  described  in  his  *  Employment  for  the  Microscope '  in 
1753,  which  possessed  several  conveniences  and  improvements.  Xot 
the  least  of  these  is  that  which  gives  greater  delicacy  to  the  fine  ad- 
justment than  is  found  in  any  preceding  model.  It  was  subse- 
quently further  improved  by  the  addition  of  a  cradle  joint  at  the 
bottom  of  the  pillar  by  Adams.  Cuff  also  designed  a  simple  form  of 
micrometer. 

There  were  three  designs  of  microscopes  by  George  Adams,  of 
London,  in  1746  and  1771,  which  have  many  points  of  interest,  but 
scarcely  contribute  enough  of  distinctive  improvement  to  the  modern 
forms  of  the  microscope  to  detain  us  long.  That  designed  in  1771  is 
figured  in  the  Adams  '  Micrographia  Illustrata,'  and  is  reproduced 
in  fig.  111. 

In  this  instrument  Adams  claims  to  have  embodied  a  number  of 
improvements  on  all  previous  constructions.  He  applied  '  two  eye- 
glasses at  A,  a  third  near  B,  and  a  fourth  in  the  conical  part  between 
B  and  C,'  by  which  he  increased  '  the  field  of  view  and  of  light ; ' 
draw-tubes  were  at  A  and  B,  by  which  these  lenses  could  be  separated 
more  or  less,  but  the  probability  is  very  great  that  these  were 
simply  copied  from  the  improvements  of  a  like  kind  devised  by  B. 
Martin  and  described  above.  He  also  arranged  the  object-lenses,  or 
*  buttons,'  a  and  b,  to  be  combined  ;  seven  *  buttons  '  were  provided, 
'  also  six  silver  specula  ['  Lieberkiihns ']  highly  polished,  each  having 
a  magnifier  adapted  to  the  focus  of  its  concavity,  one  of  which  is 
represented  at  e,'  and  the  '  buttons '  could  also  be  used  with  *  any 
one  of  these  specula '  by  means  of  the  adapter,  d. 

1  A  Practical  Treatise  on  the  Use  of  the  Microscope,  3rd.  ed.  London,  1855,  8vo, 
pp.  25,  26. 


THE  VARIABLE    MICROSCOPE 

By  George  Adams  J{?6o.JFl&t  StrettJ^ONDON 


: cccoo 


FIG.  Ill  (1771). 


144     THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

The  body-tube,  ABC,  with  its  arm,  F  (in  which  it  screwed  at/), 
and  stem  attachment  with  the  fine  adjustment  were  clearly  modified 
from  a  design  which  Cuff  originated.  The  large  ivory  head,  T, 
actuated  a  pinion  and  rack  for  raising  or  depressing  the  body-attach 
ment  on  the  stem,  but  as  there  was  only  one  slide  the  rack  work 
could  not  be  used  unless  the  fine  adjustment  was  first  put  out  of  action 
by  unclamping  it.  The  stage  and  mirror  were  adjustable  on  the  stem . 
The  large  ratchet-wheel  controlled  by  the  pinion-handle,  S, gave  the 
required  inclination  to  the  stem. 

Nos.  1  and  2  were  ivory  and  irlass  'sliders'  for  objects,  to  be 
applied  in  the  spring-stage  No.  3  fitting  at  T  ;  the  *  hollow  at  K  [No. 
3]  is  to  receive  the  glass  tube  No.  10.'  No.  4  was  a  diaphragm  called 
a  cone,  from  its  conical  shape  ;  this  was  invented  by  Baker  in  1743, 
and  was  used  in  all  microscopes  up  to  about  1820,  when  the  wheel  of 
diaphragms  was  re-invented  by  Mons.  Le  Baillif  of  Paris  fitting  in 
the  lower  end  of  No.  3,  *  to  exclude  some  part  of  the  light  which 
is  reflected  from  the  mirror  Q.'  The  forceps,  No.  5,  could  be  placed 
*  in  one  of  the  small  holes  near  the  extremities  of  the  stage,  or  in  the 
socket,  R,  at  the  end  of  the  chain  of  balls  No.  6.'  No.  6  was  an  arm 
composed  of  a  series  of  ball-and-socket  joints,  similar  to  the  system 
employed  by  Musschenbroek,  by  Joblot,  and  by  Lyonet,  and  was  in- 
tended to  be  applied  at  W,  when  the  stage  was  removed.  No.  7  wa* 
a  box  of  ivory  in  which  discs  of  talc  and  brass  rings  were  packed  : 
No.  8,  a  hand-magnifier  ;  No.  9,  a  sliding  arm  lens-carrier  fitting  on 
Z,  when  the  instrument  was  required  to  be  used  as  a  simple  micro- 
scope ;  No  11,  a  rod  of  wire  with  spiral  at  the  end  for  picking  up 
soft  objects  from  bottles  <fcc. ;  and  No.  12,  an  ivory  disc,  black  on 
one  side  and  white  on  the  other,  fitting  at  T,  to  carry  opaque 
objects. 

To  use  the  instrument  as  a  simple  microscope  the  body-tube. 
ABC,  was  removed  from  the  ring,  F  ;  the  lens-carrier,  No.  !>.  was 
placed  on  Z,  and  a  lens  with  reflector,  E,  screwed  in  the  ring,  c ; 
the  ball-and-socket  arm,  No.  6,  was  applied  at  W,  by  the  part  X, 
and  the  object  held  by  either  of  the  forceps  could  be  turned  and 
viewed  as  desired.  For  dissections  <tc.  the  stage  could  be  screwed 
on  at  F,  and  a  glass  plate  applied  at  T. 

One  of  the  best  examples  of  this  design  has  a  nose-piece  with  a 
slide  carrying  three  objectives — one  of  the  first  arrangements  of 
'  triple  nose-piece,'  or,  indeed,  of  changing  nose-piece  for  objectives 
(as  distinguished  from  simple  lens-carriers)  that  have  been  met  with. 
A  microscope  devised  by  Dellebarre  was  made  the  subject  of  a 
special  report  to  the  'Academic  des  Sciences'  in  June  1777,  but 
there  is  nothing  in  it  deserving  special  consideration  in  comparison 
with  contemporary  or  even  anterior  forms  as  bearing  upon  the  evo- 
lution of  the  microscope  as  we  now  know  it.  In  fact,  up  to  the  time 
when  achromatism  exerted  so  powerful  an  influence  upon  the  form 
and  construction  of  the  instrument,  there  is  no  microscope  that  calls 
for  further  consideration  save  one — by  an  English  maker  named 
Jones — it  was  called  Jones's  *  Most  Approved  Compound  Microscope 
and  Apparatus,'  and  although,  in  principle,  it  does  not  differ  from 
Adams's  instrument,  fig.  Ill,  it  yet  presented  differences  of  detail. 


JONES'S  MICROSCOPE 


Its  date  was  1798,  and  is  seen  in  fig.  112,  which  is  taken  from  the 
original  figure  in  Adams's  *  Essays  011  the  Microscope.' 

The  base  is  a  folding  tripod,  and  the  stem  inclines  upon  a 
compass-joint  on  the  top  of  the  pillar.  Mr.  Mayall  justly  remarks 
that  this  was  the  best  system  devised  up  to  this  date.  The  arm 
carrying  the  body- 
tube  can  be  rotated 
on  the  top  of  the  limb 
E,  and  is  also  pro- 
vided with  a  rack  and 
pinion  D.  An  extra 
carrier,  W,  is  pro- 
vided for  special  pur- 
poses pivoting  at  S, 
so  that  objects  will 
remain  in  the  optic 
axis  though  the  stage 
be  moved  in  arc. 
There  are  also  clips 
provided  for  the 
stage.  There  is  a 
condenser  at  U, 
which  slides  on  the 
stem  by  the  socket  u. 
The  mirror  also  slides 
on  the  stem.  There 
is  provided  a  rotating 
multiple  disc,  P,  of 
object-lenses,  and  a 
brass  cell  contains  a 
high  power,  of  .,',,  or 
^o  inch  focus,  which 
on  the  removal  of  the 
lens- disc  can  be 
screwed  into  the 
nose-piece. 

There  were  also 
designed  some  inte- 
resting forms  of  re- 
flecting microscopes, 
to  the  details  of  which 
we  can  afford  no 
space,  their  influence 
having  been  of  no 
value  in  the  develop- 
ment of  the  microscope  as  we  know  it.  There  was  a  reflecting 
microscope  suggested  by  Sir  Isaac  Newton  in  1672,  and  one  was 
devised  on  the  principle  of  the  Gregorian  telescope  by  Barker  in 
1  736  ;  another  of  the  Cassegrainian  form  was  made  in  1738  by  Smith, 
which  was,  perhaps,  the  most  perfect  of  the  Catoptric  forms. 

An  outline  of  its  construction  and  the  path  of  the  light-beams  is 

L 


JONES'S  MOST  IMPROVED  COMPOUND 

MICRO  Si-Ota    AND    AfPARATVX. 


FiGll2  (1798). 


146     THE   HISTOKY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 

given  in  fig.  113.  It  was  for  examining  transparent  objects  and  was 
similar  to  the  Cassegrainian  telescope,  but  with  an  extra  long  eye- 
piece tube  to  permit  the  focussing  by  movement  of  the  eye-lens. 
The  object  was  placed  at  M  1ST ;  the  image  was  taken  up  by  the 
concave,  reflected  on  the  convex,  and  again  reflected  to  the  eye-lens. 
He  advised  the  use  of  a  condensing  lens  for  the  illumination,  to  pre- 
vent *  the  mixture  of  foreign  rays  with  those  of  the  object/  otherwise 
the  instrument  gave  confused  images  of  distant  objects  when  it  was 
used  as  a  microscope. 

Even  without  a  condenser  there  are  good  images  attainable  with 
this  instrument,  but  with  the  condenser  they  would  be,  of  course, 
improved. 

We  have  not  followed  in  any  detail  the  forms  of  simple  micro- 
scopes as  they  presented  themselves,  but  in  1755  a  form  was  made 
by  Cuff  that  can  only  be  regarded  as  the  precursor  of  the  most  com- 


FIG.  118. — Smith's  reflecting  microscope  (173S). 

plete  and  perfect  of  our  simple  dissecting  microscopes  :  it  is  shown 
in  fig.  114.  A  disc  of  plane  glass,  C,  or  a  concave,  M,  was  applied, 
on  the  stage  of  which  dissections  &c.  could  be  made  ;  a  mirror,  1 , 
was  fitted  in  a  gimbal  with  a  stem  sliding  in  a  socket  in  the  pillar ; 
the  lens-carrier,  F,  alone,  or  with  Lieberkiihn,  F,  screwed  in  a  ring 
on  the  end  of  a  horizontal  arm,  E,  sliding  through  a  socket,  attached 
to  a  vertical  rod,  D,  sliding  and  rotating  in  a  socket  at  the  back  of 
the  pillar  for  focussing  etc.  This  motion  of  the  lens  over  the  object 
became  very  popular  and  was  employed  in  nearly  all  microscopes  up 
to  the  time  of  the  establishment  of  achromatism  ;  the  last  microscope 
so  fitted  was  that  designed  by  Mr.  W.  Valentine  and  made  by 
Andrew  Ross  1831.  The  movement  in  arc  lasted  much  longer,  and 
the  last  remnant  of  it  is  still  to  be  found  in  Powell's  No.  1. 
The  pillar  screwed  on  the  lid  of  the  box,  within  which  the  instru- 
ment was  packed  with  sundry  accessories. 

It  was  to  the  discovery  of  achromatism  as  applied  to  microscopic 


THE   KISE    OF  ACHROMATISM 


147 


object-glasses  that  we  must  attribute  the  strictly  scientific  value  and 
progress  in  development  of  this  now  extremely  valuable  and  beauti- 
ful instrument.  An  exhaustive  account  of  the  earliest  discovery 
and  progressive  application  to  our  own  day  of  achromatism,  so  far 
as  it  can  be  given  in  this  treatise,  will  be  found  in  the  chapter  on 
objectives.  We  can  here  only  attempt,  for  the  sake  of  completeness, 
a  very  broad  outline  of  the  facts. 

Martin  appears  to  have  constructed  an  achromatic  objective  in 
1759,  but  no  results  of  practical  value  were  obtained,  Martin  having 
formed  the  judgment  that  his  achromatic  microscope  was  not  equal 
to  a  reflecting  microscope  with  which  he  compared  it.  But  it  cer- 
tainly gives  him  a  place  of  interest  in  the  history  of  the  achromatism 
of  object-glasses  for  the  microscope. 


FIG.  114. — Ellis's  aquatic  microscope  (1755) 

In  1762  Euler  began  to  discuss  the  theory  of  achromatic 
microscopes,  and  in  1771,  in  his  *  Dioptrica,'  he  entered  upon  the 
subject  at  more  considerable  length.  A  pupil  of  his,  named  Nicholas 
Fuss,  published  in  St.  Petersburg,  in  1774,  a  volume  entitled  '  Detailed 
instruction  for  carrying  lenses  of  different  kinds  to  a  greater  degree 
of  perfection,  with  a  description  of  a  microscope  which  may  pass  for 
the  most  perfect  of  its  kind,  taken  from  the  dioptric  theory  of 
Leonard  Euler,  and  made  comprehensible  to  workmen  by  Nicholas 
Fuss.'  This  was  translated  into  German  by  Kliigel  in  1778,  but  no 
result  of  these  discussions  of  the  theory  of  achromatism  can  be 
discovered  earlier  than  1791,  when  Francois  Beeldsnyder  made  an 
achromatic  objective  which  was  presented  by  Harting  to  the  museum 
of  the  University  of  Utrecht ;  but  it  was  far  from  satisfactory.  It 

L2 


148      THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 


was  composed  of  two  biconvex  crown-glass  lenses,  and  a  biconcave 
flint  lens  placed  between  them. 

C.  Chevalier  tells  us  1  that  between  1800  and  1810  M.  Charles,  of 
the  '  Institnt,'  Paris,  made  small  achromatic  lenses;  but  they  were 
too  imperfect  to  be  of  real  service.  In  1811  Fraimhofer  made 
achromatic  doublets  with  no  great  success;  and  in  1823-4  an  achro- 
matic microscope  was  made  by  the  Messrs.  Chevalier,  with  four 
doublet  lenses  arranged  according  to  a  plan  devised  by  Selligue. 
Their  *  Microscope  d'Euler  '  followed,  and  in  1827  Amici  constructed  a 
horizontal  microscope  on  achromatic  principles,  which  was  spoken 

well  of.  But  while 
up  to  a  very  recent 
date  it  was  common 
to  assert  that  the  first 
to  suggest  the  plan 
of  combining  two, 
three,  or  four  plano- 
convex achromatic 
doublets  of  similar 
foci,  one  above  the 
other,  to  increase  the 
power  and  aperture, 
was  Selligue  in  1823, 
it  is  DOW  known  that 
this  had  been  antici- 
pated by  Marzoli  (ch. 
v.  353).  Selligue's 
plan  was  carried  into 
execution  by  the 
Messrs.  Chevalier. 
The  instrument  em- 
bodying tins  plan  is 
shown  in  fig.  115. 

In  a  report  to  the 
Academic  Royale  des 
Sciences,  the  well- 
known  mathema- 
tician Fresnel  says, 
concerning  this  mi- 


„ 


FIG.  115. — Selligue's  achromatic  microscope  (1823-4). 


croscope,  that  in  comparing  the  objectives  writh  those  of  one  of  Adams's 
best  non-achromatic  instruments — that  up  to  a  magnification  of 
two  hundred  times — Selligue's  was  decidedly  superior  ;  but  beyond 
that  magnification  there  was  no  superiority  in  the  achromatic  form, 
and  he  preferred  Adams's  form  for  prolonged  observations  because 
it  gave  a  larger  field  than  Selligue's. 

The  mechanism  of  this  microscope  was  similar  to  the  English 
model  of  Jones,  shown  at  fig.  112.  The  focussing  was  by  rack  and 
pinion  acting  on  the  stage,  the  pinion  travelling  with  the  stage  on 
the  rack.  Two  draw-tubes,  A  and  B,  were  applied  within  the 
body-tube,  C,  the  upper  one  having  a  biconcave  lens,  S,  at  the 
1  Des  Microscopes,  Paris,  1839,  p.  sc,. 


MODEL   STANDS   FOR  ACHROMATIC   OBJECTIVES 


149 


lower  end,  serving  as  an  amplifier,  which    was  probably  the  first 
application  of  a  *  Barlow  lens '  to  a  microscope. 

Illumination  for  opaque  objects  was  accomplished  by  a  lenticular 
prism,  P,  which  was  gimballed,  and  connected  with  a  ring  embracing 
the  body  tube. 

We  learn  from  Fresnel  that  the  range  of  magnification  was  from 
40  to  1,200  diameters. 
The  object-glasses  were 
composed  either  of  two 
doublet  systems  for  low- 
power  work  or  of  four 
doublet  systems  all 
screwed  together  for 
high-power  work,  and 
two  oculars  were  pro- 
vided of  different  power. 

It  is  interesting  to 
place  one  of  the  earliest 
known  English  models 
of  the  achromatic  micro- 
scope beside  that  of  Sel- 
ligue.  It  was  made  by 
Tully  the  optician,  of 
London,  who  at  Dr.  Gor- 
ing's  instance  had  been 
wrorking  at  the  achroma- 
tising  of  the  microscope. 
Selligue's  is  a  manifest 
modification  of  one  of 
the  best  forms  as  made 
by  Adams,  Jones,  or 
Dollond.  Tully  made  the 
microscope  figured  in 
116  from  the  working 
drawings  supplied  by 
Mr.  J.  J.  Lister,  who  saw 
that  great  accuracy  of 
workmanship  and  com- 
plete steadiness  in  the 
stand  were  needful  for 
achromatic  microscopes, 
and  to  this  end  they 
adopted  struts,  such  as 
were  used  in  telescopes, 
connecting  the  body-tube  with  the  base.  The  instrument  is  shown  in 
fig.  116.  He  also  provided  mechanical  movements  to  the  stage,  but 
no  fine  adjustment  was  applied.  There  was  a  sub-stage  provided  with 
a  rotating  disc  of  graduated  diaphragms.  This  microscope  was  made 
in  the  year  1826  by  Tully,  but  it  was  made  from  working  drawings 
supplied  by  Mr.  J.  J.  Lister,  who  therefore  is  responsible  for  the  entire 
The  sub-stage  held  a  combination  of  lenses  for  a  condenser. 

As  compared  with  single  lenses  of  equal  power,  from  which  so 


FIG.  116. — Lister's   achromatic  microscope  made  by 
Tully  (1826). 


150      THE   HISTOEY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 


much  light  was  inevitably  stopped  out  by  the  small  diaphragm  that 
it  was  needful  to  use  in  order  to  secure  a  fair  image,  the  objectives 
used  with  this  instrument  gave  a  vast  increase  of  light  by  permit- 
ting the  employment  of  the  full  aperture. 

An  extremely  interesting  instrument  by  C.  Chevalier,  made  very 
probably  not  long  after  1824,  and  bearing  much  resemblance  to  that 
of  Selligue,  is  shown  in  fig.  117.  It  is  provided  with  a  revolving 
disc  of  diaphragms  applied  below  the  dark  chamber  under  the  stage, 

and  this  is  a  plan  which  obtained 
a  permanent  place  in  the  micro- 
scopes of  the  future. 

The  report  of  Fresnel  con- 
cerning Selligue's  achromatic- 
microscope  determined  Professor 
Amici,  who  for  nine  years  had 
abandoned  his  experiments  on 
achromatic  object-glasses,  to  re- 
commence them  in  1826,  and  in 
1827  he  exhibited  in  Paris  and 
in  London  a  horizontal  micro- 
scope. The  real  novelty  shown 
in  it  was  the  application  of  a 
right-angled  prism  immediately 
above  the  objective  to  deflect 
the  rays  through  the  horizontal 
body-tube.  The  object-glasses 
were  composed  of  three  lenses 
superposed,  each  having  a  focus 
of  three  lines  and  a  greatly  in- 
creased aperture.  It  had  al^> 
extra  eye-pieces  by  means  of 
which  the  amplification  could  be 
increased. 

Meantime  the  subject  of 
achromatism  was  engaging  the 
attention  of  the  most  distin- 
guished English  mathematicians. 
Sir  John  Herschel,  Sir  George 
(then  Professor)  Airy,  Professor 
Barlow,  Mr.  Coddington,  and 

several  others,  worked  more  or  less  at  the  general  subject.  Cod- 
dington alone,  however,  confined  his  attention  to  the  microscope, 
and  his  work  w~as  limited  to  the  eye-piece.  Also,  for  some  years, 
Joseph  J.  Lister  had  been  earnestly  working  experimentally  and 
mathematically  on  the  same  subject,  and  he  discovered  certain  pro- 
perties in  an  achromatic  combination,  which  were  of  importance, 
although  they  had  not  been  before  observed.1  In  1829  a  p,-ij  :er 
from  Lister  was  received  and  published  by  the  Royal  Society,2 
and  putting  the  principles  it  laid  down  into  practice,  Lister  was 
enabled  to  obtain  a  combination  of  lenses  capable  of  transmitting  a 
1  Vide  Objectives,  ch.  v.  p.  355.  2  Trans.  Boy.  Sac.  for  1829. 


FIG.  117. — C.  Chevalier's  achromatic 
microscope  (circa  1824). 


FIG.  118. — One  of  Ross's  early  microscopes  designed  by  W.  Valentine  (1881) 


152      THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 


pencil   of  50°  with  a  large  corrected  field.     This   paper   and    its 

results  exerted  a  very  powerful  influence  on  the  immediate  improve- 
ment of  English  achro- 
matic object-glasses.  MI  id 
formed  a  per  ma  unit 
basis  of  advancement  for 
the  microscope,  not  only 
in  its  optical,  but  also 
indirectly  in  its  me- 
chanical construction 
and  refinements. 

For  convenience,  at 
this  point  we  may  ad- 
vance a  little  in  order 
to  complete  our  brief 
outline  of  the  mechani- 
cal application  of  achro- 
matism to  object-glasses. 
Mr.  A.  Ross  became 
practically  acquainted 
with  the  principles  of 
achromatism  as  applied 
to  combinations  of  lei  i  ><  «s 
in  working  with  Pro- 
t'es.xnr  Barlow  on  this 
subject,  and  having  ap- 
plied Lister's  principles 
with  great  success,  he 
discovered,  as  we  have 
already  pointed  out  in 
Ch.  I.,1  that  by  covering 
the  object  under  exami- 
nation by  a  thin  film  of 
glass  or  talc  the  correc- 
tions were  disturbed  if 
they  had  been  adapted 
to  an  uncovered  object ; 
and  we  have  seen  that, 
it  was  in  1837  that  Ross 
devised  a  simple  means 
of  correcting  this.  He 
was  an  indefatigable 
worker  in  the  interests 
of  the  advancement  of 
the  mechanical  as  well 

FIG.  119. — Pritchard's  microscope  with  '  Continental '   as  the  optical  side  of  the 
fine  adjustment  (1835).  microscope.        Fig.     1 1 8 

presents      a      form     of 

microscope,   from  an  extant   example  which  was  designed  by  W. 

Valentine  of  Nottingham  in  March  1831  and  made  by  Andrew  Ross. 

1  P.  20. 


A   BOSS'S    '  LISTER  '   MODEL 


153 


The  stage  is  actuated  in  diagonal  directions  on  either  side  of  the 
stem.  A  Pritchard  microscope  probably  made  by  Ross  is  shown  in 
fig.  119.  It  is  not  at  all  like  fig.  118.  The  stage  movement  is 
by  rack  and  pinion  and  not  by  screw  as  in  fig.  118,  but  it  will 
be  seen  that  it  has  also  a  curious  spiral  fine  adjustment,  which  is 
plainly  an  uncovered  '  Continental '  form,  either  adopted  in  England 
from  G.  Oberhauser,  or  it  may 
have  even  preceded  it.  It  is 
interesting  to  note,  however, 
that  the  sub-stage  arrange- 
ments in  both  these  instances 
are  the  same  as  those  employed 
by  Wollaston  in  connection 
with  his  celebrated  doublets, 
an  account  of  which  was  given 
in  the  Philosophical  Transac- 
tions of  that  date.1 

The  Ross  form  cannot  be 
inclined,  nor  can  the  Prit- 
chard; and  'the  fine  adjust- 
ment in  the  former  is  effected 
by  means  of  a  long  screw 
passing  up  the  pillar  and  act- 
ing on  a  triangular  sheath, 
within  which  the  stem  is 
applied,  to  move  with  rack  and 
pinion,  the  top  of  the  stem 
being  hollow  to  receive  either 
the  cross-arm  support  for  the 
single  lens  or  the  limb  of  the 
compound  body.  The  screw 
is  actuated  by  a  large,  gradu- 
ated, milled  head  below  the 
tripod.' 

The  stage  has  supports 
evidently  to  enable  dissection 
to  be  effected  without  flexure 
by  the  weight  or  pressure  of 
the  hands,  which  makes  it 
clear  that  it  is  the  Valentine 
microscope  that  is  referred  to, 
as  may  be  seen  by  reference 
to  fig.  118.  Rectangular  me- 
chanical movements  are  employed  acting  diagonally  on  either  side  of 
the  stein  by  rather  fine  screws,  so  that  the  motions  are  slow. 

But  A.  Ross  at  an  early  period  worked  out  a  *  Lister '  form  of 
microscope,  with  the  limb  supporting  the  body-tube.  He  applied  a 
fine  adjustment  in  this  to  act  upon  the  nose-piece  only,  which,  as 
we  shall  subsequently  see,  is  a  very  inferior  method.  This  instru- 
ment dates  from  1839,  and  is  shown  in  fig.  120.  In  1842  he 
1  Trans.  Boy.  Soc.  1829. 


FIG.  120. — A  Ross  microscope  (1839). 


^ 

FIG.  121. — H.  Powell's  microscope,  purchased  by  K.M.  Society  in  1841 


PKIXCIPAL  MODERN  STANDS  155 

changed  the  form  to  that  shown  in  fig.  123,  p.  158.  Ross  tried 
various  modifications  of  this  fine  adjustment  and  model,  but  from 
about  1843  he  worked  only  at  the  lever  method  as  applied  to  the 
nose-piece  through  the  *  cross  arm  '  and  brought  it  to  a  relatively 
high  state  of  perfection.  But  the  full  possibilities  of  this  method, 
as  concerned  its  sensitiveness,  were  never  utilised  by  Ross,  and  it 
was  Hugh  Powell  who  first  published  an  account  of  his  long  lever 
fine  adjustment  in  the  '  London  Physiological  Journal/  November 
1843.  The  published  account  of  Ross's  long  lever  fine  adjustment 
did  not  appear  until  a  month  later,  viz.  December  1843. 

In  1835  Powell  made  a  microscope  with  an  extremely  delicate 
fine  adjustment  applied  to  the  stage.  The  mechanism  and  the 
workmanship  were  excellent  (we  give  a  drawing  of  a  later  form  of 
the  instrument  at  fig.  121),  and  this  fine  adjustment  is  one  of  the 
slowest  and  steadiest  as  yet  made.  In  one  we  have  measured  the 
movement  only  amounts  to  ^-jir  °f  an  incn  f°r  one  revolution  of  the 
milled  head  ;  this  is  six  times  slower  than  the  fine  adjustment  applied 
to  the  best  Continental  microscopes.  The  disadvantage  of  this  fine 
adjustment  is  that  it  slightly  disturbs  the  focus  of  the  sub-stage  con- 
denser; therefore,  if  the  fine  adjustment  is  much  moved,  the  sub- 
stage  condenser  will  require  refocussing.  The  movement  usually 
required  is  so  slight  that  the  refocussing  of  the  condenser  is  seldom 
required. 

James  Smith  also  made  an  instrument  on  an  entirely  new 
plan.  It  is  illustrated  in  fig.  122,  being  the  first  model  made 
by  this  firm  in  this  form,  and  it  has  many  features  of  interest 
from  the  point  of  view  of  our  present  requirements.  But  after 
we  have  once  secured  steadiness,  the  crucial  points  in  a  microscope 
are  the  quality  of  the  fine  adjustment,  and  the  delicacy,  firmness, 
and  ease  with  which  we  can  centre,  focus,  and  otherwise  modify 
the  sub-stage  illumination.  To  the  former  certainly  this  model 
does  not  contribute. 

We  are  now  prepared  to  examine  and  endeavour  to  judge  im- 
partially from  a  practical  point  of  view  the  merits  of  the  principal 
English,  Continental,  and  American  models  which  are  offered  to 
the  microscopical  public.  It  is  impossible,  no  less  than  it  is  unde- 
sirable, to  attempt  to  describe  all  the  microscopes  of  every  maker, 
or  even  the  principal  forms  made  by  the  increasing  multitude  of 
opticians.  We  have  sought  no  opticians'  aid  ;  we  have  carefully 
examined  all  the  forms  that  lay  any  just  claim  to  presenting  an 
instrument  which  meets  the  full  requirements  of  modern  microscopy  ; 
and,  although  we  have  reason  to  know  that  the  judgments  we  express 
are  shared  by  the  leading  experts  of  this  country,  we  take  the  sole 
responsibility  for  these  judgments.  Having  sought  for  twrenty  years 
the  best  that  could  be  produced  in  microscopes  and  objectives  our 
judgment  is  given  with  deliberation  and  wholly  in  the  interests  of 
science. 

In  examining  the  principal  modern  microscopes  wre  shall  point 
out  whatever  is  of  absolute  importance  or  relative  value  ;  and  the 
absence  or  presence  of  this  in  any  form  provisionally  selected  is  all 
that  the  reader  will  need  to  enable  him  to  become  convinced  of  our 


156      THE   HISTOEY   AND   DEVELOPMENT   OF  THE   MICEO8COPE 

estimate  of  the  value  of  such  an  instrument,  whether  the  form  lu« 
illustrated  in  these  pages  or  found  in  the  catalogues  of  tin- 
makers. 


FIG.  122.— James  Smith's  microscope  (1839). 


STEADINESS  OF  THE  MICROSCOPE          157 

•  With  this  object  before  us  we  shall  facilitate  its  attainment  by 
at  once  considering  what  are  the  essentials  of  a  good  microscope. 
What  are  the  attributes  of  the  instrument  without  the  possession  of 
which  it  cannot  meet  modern  requirements  ? 

T.  Steadiness  is  absolutely  indispensable :  this  would,  in  fact, 
appear  to  be  obvious.  But  we  are  bound  to  admit  that  it  is,  in  what 
sometimes  claim  to  be  stands  of  the  first  class,  disregarded  ;  and  when 
the  height  of  the  centre  of  gravity  in  the  English  and  American 
stands  of  the  first  class  is  considered',  this  is  a  fatal  mistake. 

It  is  pointed  out  in  the  section  on  micrometry  J  and  drawing 
that  the  optic  axis  of  the  microscope  should  be  ten  inches  from  the 
table ;  therefore  a  first-class  microscope  whose  optic  axis  when 
placed  horizontally  is  either  more  or  less  than  this  is  found  wanting 
in  a  material  point.  But  to  possess  this  characteristic  it  must  have 
n  high  centre  of  gravity. 

Now  it  is  possible  to  secure  steadiness  by  (1)  weight  or  (2) 
design.  The  Continental  method  has  invariably  been  wreight.  The 
pillar  of  the  instrument  is  fixed  to  a  cumbrous  metal  foot  of  horse- 
shoe form,  which  bears  so  high  a  ratio  to  the  whole  remainder  of 
the  instrument  that  it  is  usually  steady.  This  secures  the  end 
certainly,  but  by  coarse  and  unwieldy  means.  It  promises  little 
for  the  instrument  as  a  whole. 

What  is  wanted  is  the  maximum  of  steadiness  writh  the  minimum 
of  weight.  An  old  plan  designed  by  Guff,  circa  1765,  of  rotating  the 
foot  below  the  pillar  has  been  frequently  reinvented.  It  was  used 
by  Adams  1771,  by  Ross  1842,  by  Sidle  and  Poalk  in  America  1880, 
by  A.  McLaren  1884,  and  recently  again  by  Ross.  This  is  a  very 
simple  method  of  obtaining  great  stability  for  the  instrument  when 
in  either  the  vertical  or  horizontal  positions.  An  instance  of  this  form, 
made  by  Andrew  Ross  in  1 842,  is  given  in  fig.  123 :  the  foot  is  seen  to 
be  circular,  with  a  vertical  pillar  attached  eccentrically  to  it,  and  the 
1  >ase  rotates,  securing  stability  in  either  a  vertical  or  inclined  position. 

Palpably,  the  mechanical  compensation  for  the  difficulty  of  an 
elevated  centre  of  gravity  is  an  extended  base.  The  leading  fault 
of  many  stands  claiming  the  first  rank  is  their  narrowed  bases.  A 
broad  base,  resting  on  three  points  only,  and  these  plugged  with 
cork,  is  the  ideal  for  a  perfect  instrument. 

II.  Next  in  or dei-  to  the  stand  of  the  microscope  comes  what  is 
known  as  the  body  of  the  instrument — the  tube  or  tubes  for  receiv- 
ing the  objective  at  one  end  and  the  eye-pieces  at  the  other.  The 
tube  of  the  monocular  is  always  provided  with  an  inner  tube  called 
the  draw-tube.  In  a  first-class  instrument  this  latter  should  always 
be  provided  with  a  rack-and-pinion  motion,  and  should  have  a  scale 
of  from  two  to  three  inches,  divided  into  tenths  or  millimetres.  This 
enables  the  operator  the  more  accurately  to  adjust  apochromatic  ob- 
jectives so  sensitive,  for  their  best  action,  to  accurate  adjustment  of 
tube-length.  In  fact,  it  is  always  important  to  remember  that  ob- 
jectives are  corrected  for  a  special  tube-length  ;  that  is  to  say,  for 
the  formation  of  the  image  at  a  certain  definite  distance. 

1  Chapter  IV. 


158      THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

There  are,  however,  two  kinds  of  tube-length :  (1)  an  optical  and 
(2)  a  mechanical. 

The  optical  tube-length  is  measured  from  the  posterior  principal 
point  of  the  objective  to  the  anterior  principal  point  of  the  eye-piece. 

Hie  mechanical  tube-length  should  be  measured  from  the  top  of 
the  tube  into  which  the  eye-piece  fits,  and  upon  which  the  bearings 


FIG.  123. — Old  Ross  stand  (1842),  rotating  foot  below  the  pillar.     From  the  cabinet 
of  the  Royal  Microscopical  Society. 

of  the  eye-piece  rest  to  the  end  of  the  nose-piece  into  which  the 
objective  is  screwed. 

Unfortunately  different  makers  estimate  tube-length  differently 
and  take  different  points  from  which  to  make  their  measurements. 
Looking  at  the  matter  broadly,  there  are  two  estimates  for  tube- 
length  in  practical  use  :  these  are  the  English  and  the  Continental. 


THE    'BODY'   OF   THE   MICROSCOPE  159 

What  was  formerly  known  as  the  English  standard  tube  had  an 
optical  length  for  high  and  moderate  power  objectives  of  ten  inches  ; 
with  low  powers,  however,  it  was  less.  The  mechanical  tube-length 
was  8 1  inches. 

Professor  Abbe,  in  constructing  his  apochromatic  objectives  for 
the  English  body,  has  taken  the  mechanical  tube -length  at  9*8 
inches  =  250  mm.  ;  and  the  optical  tube-length  at  10' 6  inches 
=  270  mm.  This  has  caused  an  increase  in  the  length  of  the  English 
standard  tube,  since  all  good  microscopes  are  made  to  work  with 
these  objectives  ;  and  the  addition  of  a  rack  and  pinion  to  the  l  draw- 
tube  '  becomes  of  great  practical  value. 

The  tube-length  of  the  Continental  mechanical  tube  is  6'3  inches 
=  160  mm.,  and  the  optical  tube-length  is  7'08  inches  =180  mm.,  and 
some  Continental  objectives  can  only  be  accurately  adjusted  on  an 
absurdly  short  tube  of  4|  or  5  inches. 

The  question  has  been  asked,  '  Which  is  the  better  of  these  two 
differing  tube-lengths  ? '  So  far  as  the  image  in  the  instrument  is 
concerned,  there  is  not  much  difference.  It  is  of  little  importance 
whether  the  initial  magnifying  power  of  an  objective  be  increased 
by  a  slightly  lower  eye-piece  used  at  a  longer  distance  or  a  slightly 
deeper  (higher)  eye-piece  at  a  shorter  distance.  But  it  is  of  practical 
importance  to  note  that  a  small  difference  of  tube-length  produces  a 
greater  effect  on  adjustment  with  a  short  body  than  with  a  long  one. 
Critical  work  is  carried  on  in  this  country  to  2^  mm.  adjustment 
on  the  long  tube  ;  with  a  short  tube  the  delicacy  would  be  greater. 
A  difference  of  5  mm.  on  a  short  tube  is  equivalent  to  the  difference 
between  a  good  and  a  bad  objective.  When  small  cones  of  illumina- 
tion are  used  lenses  are  far  less  sensitive,  but,  on  the  other  hand, 
they  are  not  doing  their  work.  Biologists  in  a  vast  majority  of  cases 
use  a  high  power  insufficiently  worked  ;  thus  a  J-inch  objective  with 
a  small  cone  is  used  in  place  of  a  1-inch  objective,  and  an  oil  im- 
mersion jL-inch  objective  with  small  cone  is  vised  to  do  what  a  J-inch 
would  have  done.  The  oil  rVinch  objective  is  never  fully  utilised, 
and  the  objects  that  it  will  show  if  properly  used  are  never  seen. 
The  principal  difference,  however,  between  the  long  and  the  short 
body  as  affording  a  datum  for  their  respective  values  is  that  when 
a  short  body  is  used  by  a  person  having  normal  accommodation  of 
sight,  the  stage  of  the  microscope  cannot  be  seen  unless  the  head  is 
removed  from  the  eye-piece,  whereas  with  the  long  body  the  eye 
need  not  be  taken  from  the  eye-piece  at  all,  as  the  stage  can  be  seen 
with  the  unused  eye.  We  are  informed  by  a  highly  competent 
German  optician  that  short  sight  is  the  most  common  form  of  vision 
amongst  German  microscopists.  This,  of  course,  for  Germans  so  far 
alters  the  case,  but  it  does  not  apply  in  this  country.  The  diameter 
of  the  body  tube  is  also  a  matter  of  importance,  because  when  a 
microscope  is  used  for  photomicrography  it  is  essential  that  it 
should  have  a  body  with  a  large  diameter. 

III.  Arrangements  for  focussing  stand  next  in  order  of  import- 
ance. Every  microscope  of  the  first  class  is  provided  with  two 
arrangements  for  focussing,  one  a  coarse  adjustment,  acting  rapidly, 
and  the  other  a  fine  adjustment,  which  should  act  with  great  delicacy 


l6o      THE   HISTORY  AND  DEVELOPMENT   OF  THE   MICROSCOPE 

and  precision.  A  good  'coarse  adjustment'  or  primary  movable 
part  of  the  instrument  is  of  great  importance.  The  first  requisite  is 
that  the  body  or  movable  part  should  move  easily,  smoothly,  but 
without  *  shake '  in  the  groove  or  slot  or  whatever  else  it  slides  in. 
We  have  found  in  practice  that  a  bar  shaped  like  a  truncated  prism 
sliding  in  a  suitable  groove  acts  best  and  longest.  But  a  bar  planed 
true  and  placed  in  a  groove  ploughed  to  suit  it  is  not  enough.  The 
inevitable  friction  determines  wear,  and  this  brings  with  it  a  fatal 


PIG.  124. — Diagonal  rack  and  twisted  pinion  devised  in  1881. 


*  shake.'  All  such  grooves,  which  are  usually  Y-shaped,  should  be 
cut  and  sprung  on  one  side,  so  that  by  '  tightening  up '  the  v's  by 
means  of  screws  the  bar  or  limb  is  again  firmly  gripped.  Further,  the 
bar  should  not  '  bear  '  for  its  whole  length  along  the  groove,  but  only 
on  points  at  either  end  and  in  the  middle.  Powell  introduced  these 
prime  essentials  to  a  good  'coarse  adjustment '  more  than  60  years 
ago  ;  yet  what  thousands  of  instruments  in  which  these  principles 
have  not  been  applied  have  been,  by  sheer  friction  wear,  soon 
changed  into  useless  brass  since  then !  But  instruments  made  by 


FOCUSSING   ARRANGEMENTS 


16 


this  firm  are  as  good  after  thirty  years'  use  as  they  were  when 
newT. 

Frequently  bad  workmanship  is  concealed  by  the  free  employment 
of  what  is  known  as  *  optician's  grease '  and  an  over-tightening  of  the 
pinion,  driving  its  teeth  into  the  rack,  which,  of  course,  speedily 
ends  in  disaster. 

If  we  desire  to  practically  test  this  part  of  a  microscope,  we 
must  remove  the  pinion,  take  out  .the  bar,  clean  off  the  '  optician's 
grease '  writh  petroleum  from  both  bar  and  groove,  oil  with  watch- 
maker's oil,  and  replace  the  bar  in  the  groove,  and  before  refixing 
the  pinion  see  if  it  slides  smoothly  and  without  lateral  shake. 

What  has  been  said  about  the  '  springing '  of  the  bar  in  this  special 
instance  applies  equally  to  all  moving  parts,  in  stage  and  sub-stage 
movements,  and  wherever  constant  friction  is  incurred ;  equally 
applicable,  too,  is  the 
lubricant  we  suggest. 
An  instrument  left 
unused  in  its  native 
*  grease  '  for  twelve 
months  becomes  so  im- 
mobile in  most  of  its 
parts  by  the  hardening 
of  its  *  normal '  lubri- 
cant that  motion  be- 
comes a  peril  to  its  future 
if  persisted  in  in  that 
condition. 

If  a  '  coarse  adjust- 

ment  'be  what  it  should  FlG  124A._Nelson,s ,  stepped ,  rack>  invented  in  1899. 
be,  all  lower  powers 

should  be  exclusively  and  perfectly  focussed  by  it,  and  with  the 
highest  powers  objects  should  be  found  and  focussed  up  to  the  point 
of  clear  visibility. 

The  exceedingly  useful  method  of  '  diagonal  rack  and  twisted 
pinion'  was  introduced  by  Messrs.  Swift  and  Son  about  1880  and 
has  since  been  universally  adopted.  Its  mode  of  operation  is  seen 
in  fig.  124,  a  sectional  drawing  of  this  part  of  one  of  Swift's  micro- 
scopes. The  advantages  gained  by  this  method  are  due  to  the  twist 
in  the  pinion  being  a  shade  steeper  than  the  diagonal  of  the  rack,  by 
which  expedient  there  is  more  gearing  contact  between  rack  and 
pinion,  which  prevents  '  loss  of  time '  and  obviates  the  necessity  for 
unduly  forcing  the  teeth  of  this  pinion  into  those  of  the  rack. 

Mr.  Nelson  has  had  made  by  Messrs.  Watson  and  Sons  a  still 
better  form  of  rackwork.  It  is  what  is  called  a  'stepped'  rack  (not 
of  the  diagonal,  but  of  the  straight  type).  In  this  very  admirable 
form  two  parallel  racks  engage  in  the  same  pinion  ;  one  rack,  how- 
ever, is  placed  so  that  its  teeth  are  stepped  an  amount  equal  to  the 
'  back-lash '  behind  those  of  the  other,  e.g.  rlr  of  the  pitch. 

These  racks  have  to  be  cut  together  and  fixed  in  the  position 
they  were  cut ;  the  object  of  this  plan  is  that  one  of  the  racks  shall 
be  in  action  when  the  bar  is  racked  up,  and  the  other  when  it  is 

M 


1 62     THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

racked  down ;  so  that  if  the  racks  are  properly  placed  relatively  to 
one  another  '  loss  of  time  '  is  impossible  ;  and  the  result  is  obtained 
without  forcing  the  teeth  of  the  pinion  into  the  rack.  If  the  teeth 
are  true,  the  friction  is  of  the  least,  and  the  smoothness  and  firm- 
ness all  that  can  be  desired.  But  what  gives  great  value  to  this 
form  of  rack  is  that  any  loss  of  time  as  the  result  of  wear  can  be 
taken  up  by  a  slight  alteration  of  the  position  of  the  second  rack. 
The  arrangement  is  shown  in  fig.  124  A,  and  it  will  be  seen  ,that  at 
the  top  of  the  right-hand  rack  as  we  look  at  the  illustration  there  is 
a  small  screw.  Now  the  racks  are  set  side  by  side,  one  being  fixed 
finally.  The  pinion  is  then  made  to  work  freely  and  smoothly  with 
this  one  rack  ;  the  second  rack  is  then  introduced,  and  is  provided 
with  slots  and  clamping  screws,  and  its  position  is  gradually  altered 
in  the  slots  in  a  vertical  direction  by  means  of  this  small  screw  over 
the  right-hand  rack  until  the  smoothest  position  of  action  is  secured. 
The  clamping  screws  are  then  tightened  and  the  rackwork  becomes 
fixed ;  and  subsequent  irregularity  in  it  is  at  once  corrected  by  the 
small  screw  to  which  we  have  referred. 

When  the  best  position  is  found  the  teeth  of  the  two  racks,  MS 
we  have  stated,  will  not  be  in  a  line,  but  those  of  the  loose  rack  will  be 
found  to  occupy  a  position  slightly  below  the  teeth  of  the  fixed 
one. 

There  is  a  defect  in  either  microscope  or  microscopist  if  the 
'  fine  adjustment '  is  resorted  to  before  the  object  is  focussed  into 
clear  view,  even  with  the  highest  powers. 

The  Fine  Adjustment. — This  part  of  the  modern  microscope 
possesses  an  importance  not  easily  exaggerated,  and  deficiency  or 
bad  principle  in  the  construction  of  this  makes  not  only  inferior, 
but  for  critical  purposes  absolutely  useless,  what  are  otherwise 
instruments  of  excellent  workmanship  and  real  value. 

There  are  two  kinds  of  fine  adjustment  usually  employed  : — 

i.  Those  which  simply  move  the  nose-piece  which  receives  the 
objective. 

ii.  Those  which  move  the  whole  body,  or  the  whole  body  including 
the  coarse  adjustment. 

All  constructions  of  the  second  class  formerly  proved  impracti- 
cable, and  even  pernicious.  They  inevitably  broke  down  just  as  the 
purchaser,  by  practice,  began  to  realise  the  value  of  perfect  action. 
With  a  large  experience  of  stands  of  every  class,  we  are  obliged  to 
say  that  generally  with  one  or  two  years  of  work  they  lost  whatever 
value  they  at  first  possessed. 

To  this  broad  statement  there  are  possibly  two  or  three  excep- 
tions, viz.  Swift's  side  lever  and  Campbell's  differential l  screw  and 
Watson's  long  lever,  to  which  we  shall  subsequently  refer. 

It  is,  however,  upon  the  model  above  referred  to,  with  all  its 
radical  and  glaring  imperfections,  that  the  majority  of  Continental 
microscopes  have  been  built. 

A  screw  with  an  extremely  fine  thread,  and  therefore  of  extremely 
shallow  incision — a  micrometer  screw  in  fact — has  to  bear  the  strain  of 

1  The  differential  screw  fine  adjustment  was  first  suggested  by  Dr.  Goring  in 
1830.  It  was  subsequently  made  by  Nobert  about  1865. 


IMPERFECT  MODERN  MODELS 


163 


lifting  ami  lowering  the  entire  weight  of  the  body,  with  its  coarse  ad- 
justment, lenses,  and  so  forth ;  while  the  sole  object  of  the  adjustment 
should  be  to  give  a  delicate,  almost  imperceptible,  motion  to  the 
object-glass  alone.  It  needs  no  great  experience  to  foresee  the  inevi- 
table result ;  the  screw  loses  its  power  to  act,  and  something  incom- 
parably worse  than  a  tolerable  coarse  adjustment  is  left  in  its  place. 

Yet  it  is  the  Con- 
tinental model  that 
has  become  the  dar- 
ling of  English  labo- 
ratories, and  that  still 
receives  the  appreci- 
ation of  professors 
and  their  students. 
True  they  answer  in 
the  main  the  purposes 
sought  —  the  exi- 
gencies of  a  limited 
course  of  practical  in- 
struction. But  how 
many  of  those  who 
receive  it  are  the 
medical  men  of  the 
future,  and  to  whom 
a  microscope — not  of 
necessity  a  costly  one 
— of  the  right  con- 
struction would  be 
of  increasing  value 
through  a  lifetime  ? 

Almost  any  in- 
strument, however 
inferior,  could  be  em- 
ployed successfully 
with  a  ^-inch  object- 
ive of  '  low  angle '  (to 
give  it  what  has  been 
called  '  the  needful 
penetration'  for  his- 
tological  subjects !)  to 
obtain  an  image  corresponding  to  a  figure  in  a  text-book  of,  say, 
a  Malpighian  corpuscle,  or  a  section  of  kidney,  brain,  or  spinal  cord. 
The  quality  of  a  fine  adjustment  is  never  tested  by  these  means, 
for,  in  point  of  fact,  a  delicate  fine  adjustment  is  not  even  necessary. 
We  write  in  the  interests  of  microscopical  research.  It  certainly 
may  be  taken  for  granted  that  the  end  sought  is  not  simply  to  use 
the  microscope  to  verify  the  illustrations  of  a  text-book,  a  treatise, 
or  a  course  of  lectures  ;  without  doubt  it  is  a  subsidiary  purpose  ;  but 
the  larger  aim  is  to  inspire  in  the  young  student  confidence, 
enthusiasm,  and  anticipation  in  the  methods  and  promise  of  histology 
and  all  that  it  touches.  But  for  this  there  must  be  potentiality  (with- 
it  2 


FIG.  125.— Ross-Zentmayer  model  (1878). 


164     THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

out  costliness)  in  the  mechanical  and  optical  character  of  the  micro- 
scopes commended  and  approved. 

A  low-priced  student's  microscope  of  good  workmanship  and 
perfect  design  could  easily  be  devised  if  the  demand  for  it  arose. 
Indeed,  quite  recently  a  certain  class  of  students'  microscopes  have 
been  improved  greatly  ;  this  has  been  a  concomitant  of  the  science  of 
bacteriology,  which  has  compelled  the  use  of  the  sub-stage  condenser. 
We  have  said  enough  of  the  value  of  this  instrument  in  a  succeeding 
chapter,  but  until  recent  years  histologists  did  not  use  it  because  it 
was  not  used  in  Germany  or  with  German  instruments  !  Its  present 
use,  nevertheless,  has  had  the  effect  of  improving  the  definition 
obtained  by  the  objectives  used  by  students  generally.  Some  who 
perceive  this,  endeavour  to  attribute  it  to  the  improvement  effected 
in  modern  objectives,  but  this  is  not  the  case  ;  the  objectives  in 
many  cases  are  not  even  new,  and  until  the  introduction  of  the  Jena 
glass  l  the  ordinary  students'  objectives  were  not  really  so  good  as 
the  English  objectives  of  forty-five  years  ago.  But  it  could  easily 
be  shown  that  one  of  these  early  objectives,  used  as  it  always  WM> 
with  a  condenser,  would  surpass  in  the  sharpness  of  its  definition  tilt- 
majority  of  those  now  supplied  to  'students '  with  Continental  models. 

But  it  must  not  be  supposed  that  it  is  only  the  Continental 
model  that  is  deformed  by  the  adoption  of  this  radical  error  in  the 
'  fine  adjustment '  with  which  we  are  dealing.  Even  during  the  last 
twenty  years  it  has  been  applied  to  some  of  the  most  imposing  and 
expensive  instruments  made  in  England  and  America  on  what  i> 
known  as  the  '  Lister '  model.  This  model  has  one  supreme  virtue, 
in  the  possession  of  a  solid  limb.  This  may  take  many  distinct 
forms,  but  it  is  sufficiently  represented  in  fig.  125,  where  it  will  be 
seen  that  the  '  limb,'  which  is  swung  between  the  pillars,  and  which 
carries  the  body-tubes  and  the  fine  adjustment,  is  in  one  solid  piece. 
If  nothing  were  sacrificed  this  would  be  a  boon.  Formerly,  this 
in  >dt'l  was  supplied  with  a  fine  adjustment  which  only  moved  the 
nose-piece,  but  on  a  principle  which  we  shall  see  was  wrrong,  and 
from  its  imperfections  it  was  abandoned,  and  the  solid  Lister  arm  was 
rtif.  and  the  whole  body  and  its  coarse  adjustment  was  pivoted  on  the 
lever  of  the  fine  adjustment.  Thus  its  normal  virtue  (a  solid  limb)  was 
sacrificed,  and  a  '  fine  adjustment,'  doomed  to  failure,  was  given  to  it. 

A  complex  roller,  a  wedge,  and  a  differential  screw  have  in  turn 
been  since  employed  to  redeem  this  instrument  from  the  failure  that 
had  overtaken  it.  Partially,  or  completely,  each  has  failed.  The 
differential  screw  certainly  conies  theoretically  nearest  to  success 
with  this  form  of  instrument.  But  at  the  outset  this  is  the  case 
only  where  it  wholly  abandons  the  lifting  and  lowering  of  the  body- 
tube  <fec.  by  the  action  of  a  *  fine  adjustment,'  and  its  motion  is  only 
brought  into  operation  upon  the  equivalent  of  a  nose-piece. 

The  form  of  differential  sci'ew  brought  into  practical  operation 
by  the  Rev.  J.  Campbell,  of  Fetlar,  Shetland,  was  adopted  by  Swift 
and  Son  in  1891,  but  had  been  exhibited  in  a  stand  made  by  Baker 
in  the  year  1886  at  the  Quekett  Micro.  Club.2  Its  object  is  to  sup- 

1    Vide  Chapter  I. 

'  Journ.  Q.M.C.  ser.  2,  vol.  ii.  pp.  283  and  287  (1886). 


THE    FINE   ADJUSTMENT 


plant  the  direct-action  screw,  where  the  form  of  the  microscope  may 
appear  to  make  that  a  necessity.  This  has  been  the  case  with  the 
Continental  model.  It  was  applied  by  its  inventor  to  a  microscope 
made  by  himself,  and  was  brought  before  the  Quekett  Club  by  Mr. 
E.  M.  Xelson. 

It  is  very  simple,  and  is  made  by  cutting  two  threads  in  the 
micrometer  screw.  Fig.  126  will  illustrate  the  exact  method.  D  is 
the  milled  head  of  the  direct-acting  screw.  The  upper  part,  S,  of 
the  screw  has  (say)  twenty  threads  to  the  inch,  and  the  lower  part,  T, 
twenty-fix •<•  threads  to  the  inch.  B  is  the  fixed  socket  forming  part 
of  the  limb  of  the  microscope,  and  H  is  the  travelling  socket  con- 
nected with  the  support  of  the  body-tube.  The  revolution  of  I) 
s  the  screw  thread  S  to  move  up  and  down  in  B  at  the  rate  of 


FIG.  126.— Campbell's 
differential  screw 
fine  adjustment 
(1886). 


PIG.  127. — Zeiss's  usual '  new  '  fine  adjustment  (1886) 


twenty  turns  to  the  inch,  whilst  the  screw  thread  T  causes  the 
travelling  socket  H  to  move  in  the  reverse  direction  at  the  rate  of 
twenty-five  turns  to  the  inch.  The  combined  effect,  therefore,  of 
turning  D  twenty  revolutions  is  to  raise  or  lower  T,  and  with  it  the 
body-tube  |th  of  an  inch,  or  ^th  of  an  inch  for  each  revolution. 
The  spiral  spring  below  H  keeps  the  bearings  in  close  contact. 

Of  course  any  desired  speed  can  be  attained  by  proper  combina- 
tion of  the  threads :  thus  32  and  30  would  give  ^th  of  an  inch 
for  each  revolution,  and  31  and  30  would  give  ^-j^th  of  an  inch. 

This  screw  has  provided  for  the  Continental  model  what  Swift's 
vertical  lever  has  done  for  the  Jackson  model ;  Mr.  Baker,  of 
Holborn,  has  adopted  it  and  with  very  satisfactory  results  ;  for  it 
has  passed  through  that  most  crucial  of  tests  for  a  fine  adjustment, 
its  employment  in  photo-micrography,  with  excellent  results  ;  and 


1 66     THE   HLSTOKY   AND   DEVELOPMENT   OF   THE   MICKOSCOPE 

we  hope  that  it  may  become  the  general  fine  adjustment  for  this 
form  of  microscope  in  place  of  the  old  form  of  direct-acting  screw. 

In  contrast  and  comparison  with  Campbell's  differential  screu 
we  may  put  the  principle  on  which  the  usual  simplified  construction 
of  the  fine  adjustment  of  the  Zeiss  stands  rests.1  In  fig.  127  the 
triangular  bar  C  is  screwed  firmly  to  the  stage  ;  on  it  moves  a  hollow 
piece  B,  which  is  connected  inseparably  with  the  arm  A  carrying  the 
tube.  At  its  upper  end  C  is  cut  away  for  about  15  mm.  and  B 
hollowed  out  at  a  corresponding  place  so  that  space  is  obtained  for  :\ 
spiral  spring.  This  spring  bears  below  against  the  hollowed-out 
part  of  B,  its  upper  end  being  connected  with  the  projections  of  the 
piece  E  screwed  into  C.  The  piece  B  is  closed  above  by  the  cap  F,  in 
which  is  the  female  screw.  On  the  top  of  the  micrometer  screw  i> 
fitted  a  bell-shaped  head,  and  at  its  lower  end  is  a  small  nut  for 
preventing  over-screwing.  The  lower  end  of  the  screw  is  rounded  off 
and  bears  against  the  flat  surface  of  a  hard  steel  cylinder  let  into  E.  . 

Clearly,  when  worked,  the  screw  remains  in  the  same  place, 
bearing  against  C.  The  female  screw,  on  the  other  hand.  UK  ACS  over 
it,  raising  and  lowering  the  tube  carrier  B  A  connected  with  it.  By 
its  own  weight  A  B  counteracts  the  rise  and  thus  supplies  the  place 
of  the  strong  spiral  spring  formerly  employed.  The  weak  spring 
here  adopted  acts  in  the  same  direction  as  the  weight  of  A  B,  and 
serves  to  assist  the  latter  when  the  upper  part  of  the  microscope  is 
placed  horizontally. 

Our  appreciation  of  all  that  is  done  by  the  great  firm  of  Zeiss  we 
need  not  reiterate  ;  it  is  well  known  ;  but  our  opinion  of  the  form  of 
stand  adopted  by  these  opticians  we  freely  expressed,  and  we  believe 
justified  in  the  last  edition  of  this  book  ;  but  it  is  well  to  get  the 
opinion  of  one  who  with  practical  knowledge  would  certainly  not  be 
prejudiced  against  the  Continental  stand.  Dr.  H.  E.  Hildebrand 
says2  that  in  teaching  establishments,  where  as  many  as  two 
hundred  microscopes  may  he  used,  the  weak  points  of  the  Continental 
stand  are  soon  brought  to  light.  The  fine  adjustment  screw  soon 
becomes  unsteady  (an  inevitable  consequence  of  the  weight  so  fine  a 
screw  has  to  carry),  the  prism  suffers  bending  or  rotation,  the  prism 
flange  or  the  hinge-block  under  the  object  stage  loosens  its  connec- 
tion with  the  stage  plate,  tfcc.  &c.,  all  of  which  and  much  more,  as  we 
believe,  is  the  result  of  the  adaptation  of  a  simple  and  primitive  form 
to  complex  appliances  for  which  it  was  never  designed  or  intended. 

It  is,  however,  an  admirable  characteristic  of  the  firm  of  Zeiss, 
that  while  they  adhere  doggedly  to  the  old  Continental  model,  they 
are  continuously  putting  forth  their  ingenuity  and  skill  to  counter- 
act what  are  shown  to  be  its  defects.  In  their  best  usual  form  the 
speed  of  the  fine  adjustment  is  T^T  inch  for  each  revolution  of  the 
milled  head.  This  is  undoubtedly  too  rapid,  but  it  could  scarcely  be 
made  a  finer  screw,  because,  as  we  have  seen,  it  had  the  coarse 
adjustment  and  tube  to  lift,  and  the  wear  and  tear  on  so  fine 
a  thread  in  constant  use  led  to  rapid  failure.  But  the  firm  has 

1  This  form  was  introduced  in  1886,  and  was  a  great  improvement  on  its  pre- 
decessor, which  was  mechanically  bad.     Vide  E.M.S.J.  1886,  p.  1051. 

2  Zeitschr.  f.  wiss.  Mikr.  xii.  (1895)  pp.  145-54. 


FIG.  128  (1898). 


1 68     THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 

introduced  a  very  complex  but  very  remarkable  modification  of  their 
fine  adjustment  which  is  intended  to  obviate  both  the  above  defects. 
It  is  a  model  ostensibly  constructed  for  photo-micrographic 
purposes,  but  if  successful  will  speedily  be  applied  to  all  their  stands, 
The  entire  microscope  is  shown  in  fig.  1 28,  while  a  vertical  section  of 
the  fine  adjustment  is  presented  in  fig.  129,  and  a  ground  plan  of 
the  same  in  fig.  130.  A  point  which  seems  to  be  considered  of 
importance  to  some  German  microscopists  is  the  provision  of  a 
handle  by  means  of  which  the  instrument  may  be  readily  moved, 
and  with  the  provision  of  this  the  usual  large  milled  head  controlling 
the  fine  adjustment  has  been  displaced.  This  is  shown  at  H  in  fig. 


FIG.  129  (1898). 

128.  But  with  the  accomplishment  of  this  there  was  a  great  desire 
to  bring  about  what  we  have  so  often  endeavoured  to  show  was  an 
indispensable  necessity  in  the  beautiful  productions  of  Jena,  viz.  that 
the  fine  adjustment  should  not  have  the  burden  of  carrying  the 
coarse  adjustment  and  the  tube.  They  have  not  succeeded  in  doing 
this  ;  the  weight  of  the  coarse  adjustment  and  tube  is  still  on 
the  fine  micrometer  screw.  They  have  diminished  the  weight 
that  'the  fine  adjustment  has  to  support  by  making  the  body  and 
draw- tube  of  aluminium.  The  fine  adjustment  is  placed  close  behind 
the  coarse  one,  both  being  fastened  quite  independently,  so  that  in 


THE   FINE   ADJUSTMENT 


169 


fact  the  object  holder  can  be  made  to  receive,  and  the  optical  appa- 
ratus arranged  to  examine,  preparations  of  almost  any  required  size. 
To  accomplish  this  H  (fig.  128)  is  made  hollow,  and  in  place  of 
the  usual  triangular  '  conductor'  of  the  fine  adjustment,  a  swallow- 
tail-shaped slide  F  (figs.  129, 130)  is  placed,  the  upper  part  of  which 
is  hollowed  out  to  receive  the  spiral  spring  U  (fig.  129).  The  lower 
part  of  this  is  also  hollowed  and  conceals  the  long  box  which  receives 


FIG.  130  (1898). 

the  micrometer  screw  M  (fig.  129).  The  pressure  of  the  spiral 
spring  is  in  the  direction  of  the  axis  of  the  micrometer  screw,  which 
works  against  a  hardened  point  shown  at  D2  fixed  on  the  dust-tight 
under-cover  of  H  (fig.  128).  This  <  conducting  slide '  F  (fig.  129) 
is  firmly  screwed  to  the  part  carrying  the  coarse  adjustment,  and  the 
aluminium  tube  T  is  connected  in  the  usual  manner  with  therackwork. 
To  avoid  what  appears  to  have  been  considered  a  peril  in  the 
exposure  of  the  milled  head  carrying  the  fine  adjustment  screw  in  the 
usual  form  of  the 
Zeiss  stand,  Dr. 
Czapski  caused  the 
fine  adjustment  to 
be  placed  in  the  . 
hollow  of  the  up- 
right H  (fig.  1 28),  I  ---'  \...  | 
so  that  the  screw 
itself  is  complete- 
ly removed  from 
direct  contact  with 
the  hand ;  the 
turning  of  the 
'  micrometer  '  or 


FIG.  131. — Eeichert's  new  patent  lever  fine  adjustment  (1899). 


fine  adjustment  screw  only  takes  place  by  means  of  the  motion  of 
the  small  milled  heads  W  W  (figs.  128  and  130)  which  work  the 
endless  screw  E  (fig.  130).  This  engages  the  wheel  S,  which  being 
fastened  on  to  the  flange  of  the  fine  adjustment  screw,  replaces  or 


I/O     THE    HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 


rather  supplants  the  usual  milled  head  ordinarily  placed  at  the  top  of 
H  ( fig.  128).  One  consequence  of  this  is  that  the  speed  of  the  fine 
adjustment  is  slowed  down  so  much  that  while  Zeiss  stands  of  the 


iiiiiiiiimiiiiiiii 

FIG.  132. — Watson's  lever  fine  adjustment  (1889J. 

usual  form  give  only  r^rth  inch  for  a  revolution  of  the  milled  head 
of  the  ordinary  micrometer  head,  this  form  of  fine  adjustment  gives 
•eis-th  inch  for  a  revolution  of  the  small  milled  heads  W  W  (figs.  128, 


THE   FINE   ADJUSTMENT 


I/I 


130).  That  this  is  an  advantage  of  a  very  high  order — if  experience 
proves  it  to  be  a  practical  method — there  can  be  no  doubt.  More- 
over, the  weight  which  this  newly  arranged  micrometer  screw  has 
to  lift  is,  as  the  firm  informs  us,  only  one-fifth  of  that  which  was  borne 
by  the  older  form,  and  there  are  special  arrangements  made  to  pre- 
vent this  delicate  construction  from  being  overscrewed  either  way. 

The  mechanical  stage  of  this  microscope  has  some  features  worthy 
of  note.  It  will  be  seen  that  the >milled  heads  which  wrork  the  stage 
are  on  Turrell's  plan,  but  the  outer"  head  gives  transverse  movement 
to  the  stage  plate  instead  of  verti- 
cal movement.  The  pitch  of  the 
sriwv  on  this  pinion  is  fine,  so  that 
the  motion  is  slow.  The  vertical 
movement  which  is  actuated  by  the 
inner  pinion  head  is  on  altogether 
a  novel  plan.  The  motion  is  one  in 
arc,  this  stage  plate  being  pivoted 
on  the  left-hand  side  ;  the  circular 
portion  on  the  right-hand  side 
has  rack  teeth  cut  in  it  into  which 
a  pinion  is  geared.  This  pinion 
has  a  toothed  wheel  fixed  to  it, 
which  engages  an  endless  screw 
attached  to  the  pinion  that  c;uri<-> 
the  inner  pinion  head. 

The  speed  of  the  object  at  the 
centre  of  the  stage  is  about  half 
that  of  the  rack,  because  the 
object  is  placed  about  halfway 
between  the  rack  on  the  right  and 
the  pivot  on  the  left  hand  side  of 
the  stage. 

The  stage  is  concentric  with 
simple  non-mechanical  rotation ;  it 
can  be  clamped  in  any  desired  po- 
sition by  a  small  screw  at  the  side 
of  the  stage  (not  shown  in  the 
figure) . 

We  may  now  describe  the  ex- 
ceedingly simple,  and  as  we  think 
beaut iful  because  essentially  prac- 
tical, fine  adjustment  invented  by 
Reichert,  which  we  believe  will 

prove   itself  the   most    useful   and    FlG'  138—Swiffs  patent  fine  adjustment 

conservative  adjunct  ever  devised 

to  make  the  Continental  stand  of  service  for  high -class  work  with- 
out increasing  its  expense  or  reducing  its  value  in  ordinary  work. 
It  consists  in  adapting  in  a  very  ingenious  manner  a  lever  of  the 
second  order  to  the  usual  direct  acting  screw.  It  will  be  seen  by 
fig.  131,  which  represents  this  part  of  the  microscope  open  at  B 
and  closed  as  in  use  at  A.  The  micrometer  screw  presses  on  two 


172     THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 


levers,  h,  h,  which  in  turn  press  the  arched  piece  with  its  appendix  f 
on  to  the  prism  support.  The  principal  screw  has  three  threads 
to  the  millemeter,  which  by  the  levers  is  reduced  by  about  one 
third.  The  pointer  for  reading  the  micrometer  scale  on  the 
milled  head  is  conveniently  arranged  so  that  it  can  be  changed  to  any 
figure  on  the  scale.  The  speed  of  the  adjustment  is  ^y^th  inch  to 
one  revolution  of  the  milled  head. 

We  may  now  profitably  consider  the  best  forms  of  fine  adjust- 
ment that  apply  to  the  Lister  model,  and  one  of  the  steadiest  and 


A 


FIG.  184. — Nelson's  model  with  Swift's  fine- 
adjustment  screw  to  the  left  hand  (1882). 


FIG.  185  (1885). 


most  delicate  of  these  is  that  devised  by  Messrs.  Watson  and  Sons. 
The  entire  body  is  raised  or  lowered  by  means  of  a  milled  head  fixed 
to  a  screw  having  a  hardened  steel  point,  acting  on  a  lever  with 
hardened  and  highly  polished  contact  surfaces,  against  a  point 
attached  to  the  body-slide,  in  a  perfect  dovetailed  fitting,  about 
2^  inches  long.  This  is  seen  in  the  section  shown  in  fig.  132.  By 
turning  the  milled  head  the  hard  steel  lever  B,  which  has  its  fulcrum 


THE   FINE    ADJUSTMENT  173 

at  C,  raises  or  lowers  the  body  with  great  smoothness  and  with  the 
great  delicacy  of  ^.^th  inch  for  every  revolution  of  the  milled  head, 
and  therefore  capable  of  yielding  good  service  with  the  highest  power 
objectives. 

We  may  now  direct  our  attention  to  the  former  of  the  two  divisions 
into  which  we  have  separated  the  various  kinds  of  fine  adjustment, 
viz.  that  in  which  the  nose-piece  only  is  controlled  by  the  adjustment 
screw. 

Swift's  vertical  side  lever  is  oije~of  the  new  forms  of  fine  adjust- 
ment worthy  of  careful  trial ;  it  has  in  it  elements  of  great  merit. 
It  can.  however,  only  be  applied  to  the  Lister  model,  and  with  the 
adjustment  described  above  certainly  places  this  form,  of  microscope 
beyond  the  danger  that  some  years  ago  promised  to  have  proved  its 
extinction  as  a  first-class  microscope.1 

The  first  form  of  this  adjustment  (1881)  was  sound  in  principle  and 
ingenious  in  construction,  and  although  the  patentee  introduced  a 
modification  2  of  it  (1885),  we  believe  the  original  form,  which  he  still 
makes,  to  be  the  best,  because  it  only  acts  on  the  nose-piece  while  the 
modification  acts  on  the  body-tube. 

The  early  form  employed  by  Swift  avoided  what  had  been  a  sheer 
necessity  of  all  successful  fine  adjustments  of  this  type,  viz.  the 
accuracy  and  perfection  of  the  fitting  of  the  nose-piece  tube.  This 
was  done,  as  shown  in  fig.  133,  by  attaching  a  vertical  prism-shaped 
bar,  A,  to  the  nose-piece,  and  sliding  this  in  V-grooves  in  a  box 
at  the  back  of  the  body.  A  horizontal  micrometer  screw  with  a 
milled  head,  F,  acts  on  a  vertical  bent  lever,  D,  on  which  a  stud,  E, 
fixed  to  the  prism  bar  bears. 

There  is  also  an  adjustment  for  tightening  up  the  prism  bar  in 
the  V-grooves,  B  B.  Side-shake  and  '  loss  of  time '  are  impossible 
with  this  form  of  adjustment ;  while  the  power  to  '  tighten  up  '  by 
means  of  the  capstan-headed  screws  enables  wear  and  tear  to  be 
compensated.  It  is  obvious  that  the  slowness  of  the  motion  is  here 
controlled  by  three  factors  :  (1)  the  length  of  the  lever,  D ;  (2)  the 
distance  of  the  lifting-stud,  E,  from  the  pivot  or  fulcrum  ;  and 
the  pitch  of  the  screw-thread  on  F. 

Manifestly,  where  a  side-lever  fine  adjustment  such  as  this  is 
employed  it  should  be,  as  it  now  always  is,  placed  on  the  left-hand 
side  of  the  operator :  we  can  readily  focus  with  the  left  hand,  and 
leave  the  right  hand  free  for  moving  the  slip  and  effecting  other 
adjustments.  Ambi-dexterity  is  not  at  present  a  common  gift,  and 
to  have  the  right  hand  free  is  important.  This  was  pointed  out  by 
Mr.  Nelson  when  this  fine  adjustment  was  first  introduced,  and  he 
had  a  student's  microscope  constructed  with  the  micrometer  milled 
nead  on  the  left  side,  as  in  fig.  134.  It  is  manifest,  however,  that 
it  wTould  greatly  improve  this  adjustment  if  the  screw-pinion 
were  carried  right  through  and  a  milled  head  placed  on  both  the 
right  and  the  left  sides  of  the  body. 

An  early  form  of  a  nose-piece-controlled  Jine  adjustment  was  em- 
ployed by  Andrew  Ross.  It  was  applied  to  a  microscope  having  a 

1  Journ.  B.M.S.  (1881)  p.  297,  fig.  43. 

2  Journ.  E.M.S.  (1885)  p.  120  and  (1886)  p.  1043,  fig.  207. 


174     THE   HISTOKY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

bar  movement.  It  consisted  of  a  lever  of  the  second  order  inserted 
within  the  bar,  and  actuated  by  a  micrometer  screw  with  a  milled 
head  at  one  end,  the  fulcrum  being  at  the  other,  and  the  nose- 
piece  between  them.  This  served  admirably  in  the  days  of  low- 
angled  objectives  ;  but  there  were  two  faults  belonging  to  it :  one 
was  that  the  tube  of  the  nose-piece  had  not  a  sufficient  length  of 
bearing  and  was  liable  to  a  lateral  shake ;  the  other  was  that  the 
adjustment  screw,  being  near  the  middle  of  the  bar,  involved  tremor. 

The  application  of  this  principle  in  its  very  highest  and  most 
perfectly  practical  form  was  invented  by  Powell.  His  instrument 
also  had  a  bar  movement :  but  the  bar  being  of  relatively  great 
length,  he  employed  a  lever  of  the  first  order,  the  micrometer-screw 
being  at  one  end,  the  nose-piece  at  the  other,  and  the  fulcrum  between 
them.  The  ratio  of  the  arms  of  the  lever  was  4:1;  and  the  screw 
is  so  arranged  that  a  complete  revolution  of  the  milled  head  is  equal 
to  the  -g-J-^th  of  an  inch.  The  position  of  the  screw  is  immediately 
behind  the  pivot  on  which  the  bar  turns,  and  this  precludes  the  possi- 
bility of  the  importation  of  vibration  to  the  body  ;  and.  as  tin '  nose- 
piece  tube  is  very  long,  and  only  bears  on  three  points  at  either  end, 
this  adjustment  is  the  steadiest,  the  smoothest,  and  the  most  reliable 
for  all  objectives  of  any  of  the  several  devices  which  have  come  before 
us  during  the  last  twenty  years.  In  fact,  this  fine  adjustment  has 
held  an  unrivalled  position  for  the  past  fifty  years  (fig.  157). 

The  fine  adjustment  that  was  employed  as  its  rival  on  the  earlier 
forms  of  the  Lister  model  was  known  as  the  short-side  lever,  and 
it  was  sometimes  employed  in  the  commoner  bar-movement  micro- 
scopes. Its  position  and  character  will  be  seen  on  the  right-hand 
side  of  the  body  of  the  Smith  model,  fig.  122.  In  the  light  of 
what  we  now  need,  we  are  bound  to  say  to  the  intending  pur- 
chaser of  a  microscope,  '  Avoid  it ; '  it  is  bad  alike  in  design  and 
construction.  The  screw  is  so  placed  that  tremor  is  inevitable  in 
the  body  when  it  is  touched,  while  the  nose-piece  tube  is  so  short 
that  steadiness  of  movement  does  not  belong  to  it.  It  is  only  that  it 
was  concurrent  with  the  belief  in  *  low  angles,'  and  consequent  *  pene- 
tration '  in  objectives  (with  which  no  critical  work  could  be  done), 
that  it  is  possible  to  account  for  the  toleration  for  so  long  in  num- 
bers of  English  microscopes  of  this  wholly  inefficient  adjustment. 

From  the  foregoing  we  learn  that  there  are  three  types  of  micro- 
scope models  for  which  a  suitable  fine  adjustment  has  been  found. 

i.  The  bar  movement  model,  for  which  Powell's  first  order  of 
lever  is  the  perfect  method. 

ii.  The  Lister  model,  for  which  Swift's  vertical  lever  and 
Watson's  long  horizontal  lever  are  the  best  forms  known. 

iii.  The  Continental  model,  for  which  Campbell's  differential 
screw  is  the  most  smooth  and  delicate  device  yet  suggested,  unless 
we  take  into  consideration  the  beautiful  lever  fine  adjustment  of 
Reichert. 

The  full  value  of  delicacy  in  the  fine  adjustment  can  of  course 
only  be  fully  appreciated  by  the  expert.  A  tolerable  speed  may  be 
permitted  in  this  adjustment  when  uncritical  images  with  small 
illuminating  cones  are  used,  because  objectives  so  used  are  far  less 


THE   MECHANICAL   STAGE 


175 


sensitive  to  focal  adjustment.  When,  however,  a  critical  image  is 
obtained  with  a  f  cone  the  conditions  are  changed  and  an  objective 
with  a  wide  aperture  becomes  excessiA~ely  sensitive  to  minute  focal 
alterations.  Hence  the  need  with  the  highest  class  of  microscopic 
investigation  of  at  least  as  slow  an  action  as  can  with  safety  to  the 
mechanism  be  secured,  and  therefore  comes  out  the  danger  of 
burdening  the  screw  of  the  fine  adjustment  with  a  fraction  of  an 
ounce  of  lifting  more  than  can  bo  avoided. 


FIG.  136. — Watson's  new  stage  (1898). 

So  fur  as  we  can  ascertain  the  speeds  of  the  several  fine  adjust- 
ments now  within  the  reach  of  the  worker,  they  are  as  follows,  viz. : 


Speed  for  one  revolution  of  the 

milled  head  in  fraction 

of  an  inch 

~st  =  two  threads  to  1  mm. 
£th 

=  four  threads  to  1  mm. 


nrr; 


th 


300 


Model 

Bausch  and  Lomb        .... 
Eeichert  (old  form)       .... 

Zeiss  (ordinary) 

Powell 

Baker  and  Swift  (Campbell  differential 

Eeichert  new  patent 

Swift  vertical  lever       ..... 

Watson's  long  lever 

Zeiss's  new  endless  screw  arrangement  for 

photo-micrographic  stand         .         .         .     j^-th 

IV.  The  stage  of  the  microscope  will  next  call  for  considera- 
tion. What  is  known  as  a  mechanical  stage  must  be  a  part  of  every 
first-class  microscope  ;  but  by  this  we  mean  one  of  perfect  work- 
manship and  construction,  otherwise  it  is  an  impediment  and  not  a 
help. 

To  this  end  we  would  say  at  the  outset  there  must  be  thoroughly 
well-made  movements.  The  employment  of  levers,  cams,  and  that 
class  of  stage-gear  is  in  practice,  for  critical  purposes,  a  mere 


176    THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

mechanical  mockery.  Better  trust  to  and  educate  the  fingers  to 
move  the  object  than  be  beguiled  by  any  such  practically  tormenting 
delusions.  They  are  simply  impossible  as  accompaniments  of  a 
first-class  microscope. 

The  principle  upon  which  alone  a  perfect  mechanical  stage  can 
be  constructed,  so  as  to  work  smoothly  without  '  loss  of  time,'  and  en- 
dure constant  use  without  failure,  must 
be  the  employment  of  prism-shaped 
plates  sliding  in  sprung  V-shaped 
grooves,  and  bearing  only  on  four  points. 
We  may  test  the  mechanical  quality 
of  the  movements  of  a  stage,  as  in  the 
case  of  the  coarse  adjustment,  by  re- 
moving the  parts,  cleaning  them,  and 
replacing  them,  when  they  should  \\ork 
smoothly  and  without  shake.  Where 
the  sliding  parts  are  tightened  into 
easily  fitting  and  merely  ploughed 
grooves  by  pressing  the  pinion  into  the 
rack,  the  desirable  result  of  smooth 
working  and  instant  responsiveno-  of 
sliding  plates  to  milled  heads  will  not 
present  itself. 

But  besides  the  perfect  action  of  the  sliding  parts,  a  perfect 
mechanical  stage  should  have  equal  speed  of  motion  vertically  and 
horizontally.  A  common  fault  is  that  the  speed  ot  the  rackwork 
giving  vertical  motion  is  greatly  in  excess  of  that  of  the  screw  giving 
lateral  or  horizontal  motion.  If,  for  example,  a  pinion  has  eight 
leaves,  and  the  rack  it  works  has  twenty-four  teeth  to  the  inch,  then 
three  turns  of  the  milled  head  (and  pinion)  would  cause  one  inch  of 
movement  to  the  stage.  In  order,  therefore,  to  get  the  same  rate  of 
movement  in  the  lateral  motion,  the  screw  should  be  so  pitched  as 
only  to  move  the  stage  through  an  inch  with  three  revolutions  of  the 
milled  head. 


FIG.  137  (1898). 


C  C  F  F 

FIG.  138  (1898). 

It  is  most  desirable  that  the  pinions  should  be  fixed,  not  movable 
with  the  movements  of  the  stage,  and  the  milled  heads  carrying  the 
respective  parts  should  be  as  near  to  each  other  as  possible.  The  best 
form  is  that  of  Turrell's,  devised  in  1832,  where  one  (a  screw)  is 
hollow,  and  the  other  (a  pinion)  passes  through  it ;  this  permits  both 
to  be  turned  at  the  same  time  with  one  hand,  giving  a  diagonal 


THE   MECHANICAL   STAGE.      HOW   TO   FOCUS  177 

motion,  as  well  as  the  separate  rectangular  ones,  and  gives  great 
facility  for  instantly  producing  any  motion  required  without  remov- 
ing the  hand  from  its  position  ;  a  most  desirable  attribute  of  a  stage 
when  the  rapid  movements  of  a  living  and  minute  organism  are 
being  followed. 

It  still  further  enhances  such  a  stage  if  a  pinion  is  carried  right 
through  the  stage  with  a  milled  head  at  each  end. 

A  new  stage  devised  by  WatsoiJ  has  in  it  some  features  of  interest, 
a  principal  one  being  that  the  milled  head  controlling  the  horizontal 
movements  is  in  a  fixed  position ;  in  other  words,  does  not  travel 
with  the  plate.  This  is  shown  in  fig.  136.  A  is  a  ball  upon  which  the 
turning  of  the  screw  takes  place  ;  it  will  be  noticed  that  this  ball  has 
a  groove  in  it  into  which  grease  or  dust  can  drift  without  affecting 
the  motion.  The  cap  B  covers  the  ball  when  fitted  together.  The 
manner  in  which  verniers  are  fitted  is  shown  at  D,  D,  and  the  screw 
for  adjusting  the  vertical  rack  movement  of  the  stage  is  shown  at  C. 
Fig.  137  shows  the  manner  in  which  the  plate  E  is  attached  to  the 
stationary  screw;  while  fig.  138  indicates  the  careful  manner  in 
which  this  stage  is  sprung  to  counteract  continuous  wear.  The  saw- 
cuts  shown  are  compressed  by  means  of  screws  which  are  situated  at 
the  points  F  F,  G  G,  and  any  amount  of  wear  can  be  corrected  by  the 
use  of  these  screws  in  these  slots. 

The  aperture  in  the  stage  should  always  he  large,  not  less  than  1 J 
inches  in  diameter.  There  ought  always  to  be  space  enough  above 
the  ordinary  slip  when  it  is  in  position  to  permit  of  the  easy  inser- 
tion of  the  index  finger,  for  by  its  proper  use,  focussing  with  the 
highest  powers  may  be  greatly  facilitated.  The  object  is  to  raise  or 
lower  the  slip,  as  the  objective  approaches  the  object,  so  as  to  dis- 
cover how  nearly  it  may  be  to  contact  with  the  front  lens  of  a  high 
power  in  approaching  focus.  The  focal  distance  should  always  be 
felt,  and  not  sought  with  the  eye. 

Let  it  be  supposed  that  we  are  using  a  dry  object-glass  with  a  full 
aperture,  and  consequently  short  working  distance.  With  the  right 
hand  the  coarse  adjustment  is  worked ;  with  the  elbow  of  the  left 
arm  on  the  table,  the  second  finger  of  the  left  hand  resting  on  an 
immovable  part  of  the  stage,  which  steadies  the  whole  hand,  the 
index  finger  should  rest  lightly  on  the  edge  of  the  slip,  and  the 
thumb  be  so  placed  as  to  graze  the  objective  as  it  advances  towards 
the  slip.  The  touch  of  the  thumb  indicates  whether  the  objective 
is  an  inch  off  or  only  a  quarter  of  an  inch  away  from  the  cover  of 
the  slip.  The  movement  of  the  coarse  adjustment  may  be  rapid  up 
to  Jth  or  Jth  of  an  inch,  but  after  this  there  must  be  a  cautious  but 
steady  advance.  The  body  may  be  racked  down  until  by  gentle 
upward  movement  the  slip  is  found  to  touch  the  front  of  the  objective  ; 
then  proceed  cautiously  by  delicately  lifting  the  slip  from  time  to 
time,  by  doing  which  we  can  proceed  in  perfect  safety  until  the  focus 
of  the  object  is  obtained.  In  this  way  focussing  becomes  easy  and 
rapid,  a  matter  of  touch,  and  not  of  discontinuous  procedure  to 
4  discover  where  the  front  of  the  lens  is ' — a  search  requiring  a  hand 
glass  and  often,  with  its  cumbrousness,  considerable  loss  of  time. 
The  above  simple  plan  with  brief  practice  will  enable  the  operator  to 

ff 


FIG.  139. — Zeiss  photo-micrographic  stand  (1895). 


THE   MECHANICAL  STAGE  179 

focus  an  object  in  the  field,  with  a  ^--inch  objective  in  ten  or 
twelve  seconds. 

If  a  perfect  mechanical  stage  cannot  be  obtained,  take  no  middle 
course,  have  ajifm,  'well-made  plain  one  with  a  smoothly  sliding  ledge. 
The  stage  should  be  large,  and  the  ledge  should  glide  with  perfect 
ease  and  without  catching  when  gently  pushed  from  one  corner.  For 
this  purpose  the  side-guides  should  be  long,  and  only  the  ends  of  the 
bar  should  bear  on  the  stage.  The  aperture  should  be  as  in  the 
mechanical  stage,  and  for  the  same  reason. 

Mr.  Nelson  suggested  a  stage  of  large  size,  which  should  have  a 
1^  or  1 J  inch  aperture  bored  in  it,  and  then  have  the  intervening 
brass  between  it  and  the  front  taken  away,  so  that  the  stage 
assumes  a  horse-shoe  form.  This  is  thoroughly  efficient,  and  the 
principle  is  seen  in  fig.  134. 

It  is  a  matter  of  great  interest  to  English  microscopists  to  note 
that  their  German  collaborateurs  in  Germany  and  the  leading 
German  makers  have  not  only  surrendered  to  the  sub-stage  condenser, 
and  even  in  its  achromatic  form,  but  that  at  length  they  have  also 
adopted  the  mechanical  stage ;  the  latest  form  adopted  by  Zeiss  is 
figured  in  the  accompanying  illustration  which  shows  the  complete 
instrument  (fig.  139).  We  specially  call  attention  to  it  here,  as  it  has 
Tun-ell  heads,  marked  H  Y,  and  a  rotating  stage  of  4  inches  diameter. 

It  must,  however,  be  noted  that  the  usual  Continental  model 
adopts  a  small  stage  with  a  %-inch  aperture  and  two  fixed  spring 
clips  with  no  sliding  ledge ;  that  is,  wanting  almost  everything 
required  to  do  good  modern  work. 

One  of  the  most  practical  rules  for  the  young  microscopist  in 
this  relation  is,  i  Have  your  mounted  slide  in  a  fixed  position,  but 
never  clip  it  if  it  can  possibly  be  avoided.' 

In  addition  to  perfect  rectangular  movements  a  first-class 
microscope  should  have  concentric  rotary  motion  to  the  stage.  This 
is  usually  effected  by  rack  and  pinion,  but  it  is  at  times  desirable 
to  move  it  with  greater  rapidity  than  this  admits  of.  In  very  well 
made  instruments  the  pinion  engages  the  rack  so  lightly  that  this 
rapid  motion  may  easily  be  given  to  it.  In  others  the  pinion  can  be 
disengaged  and  rapid  movement  effected. 

The  centre  of  rotation  of  the  stage  should  be  closely  approximate 
to  coincidence  with  the  optic  axis,  so  that  in  rotation  the  object 
should  never  be  out  of  the  field  when  a  fairly  high  power  is  used. 
Elaborate  rectangular  centring  gear  has  been  used  by  some  makers, 
and  is  found  in  some  high-class  instruments ;  but  this  is  not  needful, 
for  all  that  is  really  required  is  to  rotate  an  object  without  losing  it. 
In  fact  exact  centring  wrould  have  to  be  readjusted  for  every  separate 
objective  if  it  were  needed.  But  any  slight  departure  from  the 
axial  centre  can  be  much  more  readily  met  by  bringing  the  object 
into  centre  by  the  mechanical  stage. 

There  are  four  movements  in  every  microscope  ivhich  should  be 
graduated :  these  are  (1)  the  milled  head  of  the  fine-adjustment 
screw  ;  (2)  the  stage  movements  for  finders  ;  (3)  the  extension  draw- 
tube  carrying  the  eye-piece  ;  and  (4)  the  rotation  of  the  stage. 
Divided  arcs  are  imposing,  and  to  the  multitude  look  '  scientific  ; ' 

N  2 


ISO     THE   HISTOKY  AND   DEVELOPMENT  OF  THE   MICEOSCOPE 

but  in  practice  they  are  superfluous  in  the  most  complete  instrument 
beyond  those  indicated. 

There  is  a  simple  form  of  attachable  mechanical  stage  now  em- 
ployed by  many,  and  we  think  with  advantage,  when  the  cost  of  a 
complete  mechanical  stage  must  be  forgone.  This  consists  of  a  clip 
to  receive  the  object,  made  of  glass  or  .brass,  so  arranged  that  the 
friction  shall  be  reduced  to  a  minimum. 

Such  an  attachable  stage  can  be  made  to  work  with  remarkable 
smoothness ;  and  since  some  persons  have  not  sufficient  delicacy  of 
touch  to  move  so  small  and  thin  an  object  as  a  3  X  1-inch  slide  upon 
the  stage  with  steadiness  and  precision,  it  is  in  favour  of  the  super- 
stage  that  it  is  larger,  moves  easily,  and  can  be  furnished  with 
convenient  points  of  hold-fast  for  the  hands,  and  consequently  is 
more  manageable.  Against  its  employment  is  the  fact :  1st,  that 
the  slide  is  clipped  into  a  rigid  position  ;  and  2ndly,  that  the  aper- 
ture is  often  too  small  to  admit  of  the  employment  of  the  finger  in 


FIG.  140.— Swift's  attachable  mechanical  stage  (1894). 

moving  the  slide  to  assist  in  rapid  focussing.  But  these  are  defects 
which  are  rapidly  disappearing. 

Amongst  those  that  claim  the  attention  of  the  microscopist  is 
that  of  Messrs.  Swift  and  Son,  shown  in  fig.  140.  It  can  be  adapted 
to  most  microscopes ;  it  is  easily  applied  and  removed,  leaving  the 
stage,  if  required,  free.  The  up  and  down  motion  is  effected  by  a 
milled  head  below  the  stage.  The  lateral  movement  is  produced  by 
two  endless  screws  engaging  in  worm-wheels  fixed  to  smooth  rollers. 
The  lower  edge  of  the  slide  rests  on  these,  and  is  kept  in  gentle 
apposition  with  them  during  traverse  by  a  third  smooth  roller  at  the 
free  end  of  a  curved  spring  as  shown  in  the  figure.  This  is  readily 
turned  aside  when  changing  the  object.  In  its  most  recent  form  we 
have  used  this  stage  with  comfort  and  pleasure. 

Another  of  these  stages,  made  by  Baker  from  designs  by  Mr. 
Allen,  is  shown  in  fig.  141,  which  in  its  latest  form  is  so  arranged 
that  the  width  of  space  between  the  rest  and  the  spring  clip  can  be 


THE   MECHANICAL   STAGE 


181 


enlarged  so  that  a  much  wider  preparation  than  the  usual  one  inch 
may  be  worked  with  great  facility  on  this  stage.  The  method  of 
attachment  practically  makes  the  mechanical  stage  one  with  the  stage 
of  the  microscope,  as  it  is  in  contact  with  the  fixed  stage  throughout 
its  entire  length,  and  is  clamped  at  the  lower  end  to  the  top,  and  at 
the  upper  end  to  the  bottom  of  the  stage. 

Both  the  rectangular  movements  are  effected  by  rack  and  pinion, 
the  vertical  one  of  which  carries  a  bar  (fixed  as  to  horizontal  movement) 
against  which  the  slide  is  pressed  by  a  spring  clip,  and  upon  which 
is  mounted  the  rack  and  pinion  for  the  horizontal  movement ;  the 
end  which  presses  upon  the  slip  is  tipped  with  cork  in  order  to  grip 
the  slide,  and  move  it  along  the  fixed  bar  ;  when  the  milled  head  is 
rotated,  the  slide  actually  rests  on  two  small  raised  surfaces  at  either 
end  of  the  bar  to  minimise  friction.  This  is  without  question  a  well- 
made  practical  and  use- 
ful stage.  Amongst 
stages  of  this  kind,  how- 
ever, the  most  original 
and  useful  has  been  de- 
vised by  Mr.  Nelson. 
As  seen  in  fig.  142,  the 
sliding  bar  has  been 
slotted  and  a  movable 
piece,  which  may  be 
called  the  shuttle,  has 
been  fitted  in  the  slot ; 
this  shuttle  has  a  dia- 
gonal rackwork  at  the 
back,  and  a  vertical 
spiral  pinion  gears  in  it, 
as  is  shown  in  fig.  143. 
Above  this  pinion  there 
is  a  horizontal  bevel 
wheel  which  is  geared 
by  friction  to  a  vertical 
wheel  fixed  on  the  usual 


FIG.  141.— Baker's  attachable  stage  (1898). 


horizontal  pinion.  The  cock  which  holds,  and  is  close  to,  the  vertical 
bevel  wheel  in  fig.  143  is  slotted  underneath,  a  capstan-headed  screw 
(not  shown  in  the  figure)  is  fitted  for  the  purpose  of  compressing  this 
spring  part ;  the  amount  of  friction  between  the  copper  bevel  wheels 
can  therefore  be  regulated  at  will.  This  capstan-headed  screw  is 
placed  some  distance  from  the  bearing,  so  that  the  length  of  the  bar 
between  it  and  the  bearing  may  form  a  stiff  spring ;  this  renders  the 
motion  equable.  It  will  be  noticed,  therefore,  that  the  transverse 
movement  is  confined  to  the  sliding  bar.  This  sliding  bar  can  be 
removed  so  as  to  leave  the  stage  perfectly  plain.  The  heads  of  the 
pinions  which  control  the  vertical  movement  have  been  kept  below 
the  level  of  the  stage  so  as  to  be  out  of  the  way  of  culture  plates. 
Three  and  a  half  inches  of  transverse  movement  is  given  to  this 
stage,  and  the  manner  of  the  holding  the  clip  is  quite  new  and 
eminently  serviceable.  On  the  shuttle  there  are  two  sliding  pieces, 


1 82     THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICBOSCOPE 


FIG.  142. — Nelson's  new  mechanical  stage  (1897). 


FIG.  143. — Nelson's  new  mechanical  stage  (1897). 


J 


FIG    144. — Nelson's  new  mechanical  stage  (1888). 


THE  MECHANICAL  STAGE 


183 


and  these  hold  the  slip  by  the  two  lower  corners,  as  seen  in  fig.  142  ; 
and  this  mode  of  gripping  allows  for  the  employment  of  the  in- 
valuable method  of  touch  on  the  edge  of  the  slide  for  discovering 
working  distance  and  focus.  A  plain  sliding  bar  may  be  substituted 
for  the  mechanical  bar  ;  this  forms  a  semi-mechanical  stage  as  shown 
in  fig.  144.  The  mechanical  movement  being  only  imparted  to  the 
lugs  at  the  side  of  the  stage,  the  bar  may  be  moved  by  the  hand  by 
sliding  as  in  an  ordinary  plain  stage  without  the  employment  of  the 
mechanical  movement. 

The  stage  is  of  aluminium,  and  its  size  is  4^  x  7  inches. 

Another  attachable  stage  having  many  advantages  is  made  by 
Reichert  and  shown  by  fig.  145.  It  can  be  used  with  any  instrument 
of  the  Continental  type,  is  very  carefully  made,  and  the  scales 


FIG.  145. — Beichert's  attachable  stage.     (About  half  natural  size.)     (1892.) 

attached  are  divided  to  read  by  means  of  a  vernier  to  O10  mm.,  and 
the  range  of  movement  is  an  inch  in  both  directions. 

An  attachable  mechanical  stage  is  also  made  by  the  Bausch  and 
Lomb  Optical  Company  of  Rochester,  New  York,  having  great 
merit  and  some  special  points ;  and  this  firm  is  in  advance  of  all 
other  makers  that  we  know  of  in  making  an  attachable  revolving 
mechanical  stage. 

There  is  much  similarity  to  the  American  mechanical  stage  in  one 
made  by  Carl  Zeiss  and  illustrated  in  fig.  1 46 .  Of  course  the  principle, 
as  primarily  in  all  the  others,  is  that  suggested  by  the  late  Mr.  Mayall, 
and  afterwards  by  Reichert.  Two  sliding  pieces,  mounted  at  right 
angles  to  one  another,  are  moved  by  means  of  two  milled  heads,  S,  T. 
They  pass  along  millimetre  scales  which  serve  to  record  any  particular 
position. 

The  demand  for  these  attachable  stages  is,  we  presume,  consider- 


1 84      THE   HISTOKY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

able,  for  they  are  made  by  most  leading  opticians.  The  last  mechanical 
stage  we  illustrated  is  by  Messrs.  E,^  &  J.  Beck,  which  is  illustrated 
in  fig.  147.  It  has  vertical  rack  and  pinion  and  horizontal  screw 
motions  with  graduated  finer  divisions. 

To  Messrs.  Bausch  and  Lomb,  however,  we  are  indebted  for  the 
introduction  of  an  attachable  stage  in  which  the  iris  diaphragm  is  on 
the  plane  of  the  stage.  We  illustrate  this  in  fig.  147A.  Its  use  with 
a  condenser  we  do  not  commend.  But  especially  when  the  illumina- 


FIG.  146. — Zeiss's  attachable  mechanical  stage.     (§  full  size.)     (1895.) 


tion  is  daylight,  and  very  critical  results  are  not  sought,  it  will  be 
useful,  and  is  admirably  made. 

V.  The  sub-stage  is  scarcely  second  in  importance  in  a  first- 
class  microscope  to  the  stage  itself.  It  is  intended  to  receive  and 
enable  us  to  use  in  the  most  efficient  manner  the  optical  and  other 
apparatus  employed  to  illuminate  the  objects  suitably  with  the 
various  powers  found  needful.  Upon  this  much  of  the  finest 
critical  work  with  the  modern  microscope  depends. 

To  accomplish  this  a  good  sub-stage  must  have  rectangular 
movements,  and  a  rack-and-pinion  focussing  adjustment. 


THE   SUB-STAGE 


I85 


The  vertical  and  lateral  movements  need  not  be  as  elaborate  as 
those  of  the  stage,  since  only  a  small  movement  in  each  direction  is 
required.  The  object  is  to  secure  a  centring  motion,  a  motion  that 
will  make  the  optical  axis  of  the  sub-stage  combinations  continuous 
with  the  optical  axis  of  the  objective.  It  must  therefore  be  a  steady 
motion ;  the  sub-stage  must  move  decisively,  and  must  rigidly  re- 
main in  the  position  in  which  it  is  left. 

A  bad  sub-stage  moves  in  jerks,  and  is  liable  to  spring  from  the 
position  intended  to  be  final. 

It  is  not  needful  that  the  motion  should  be  in  right  lines  ; 
motion  in  arcs  whose  tangents  intersect  at  right  angles  are  quite  as 
efficient.  A  steady,  even,  reliable  motion  that  will  enable  a  centre 
to  befoimd  is  all  that  is  required. 


FIG.  147.— Beck's  mechanical  attachable  stage  (1896). 

The  focussing  adjustment  must  be  smooth,  steady,  and  firm,  acting 
readily  and  remaining  rigid.  The  recent  employment  of  achromatic 
condensers  of  wide  apertures  has  led  such  critical  workers  as 
Mr.  E.  M.  Nelson  to  suggest  a  fine  adjustment  to  the  sub-stage. 
There  are  times  when  it  is  a  great  luxury  and  a  facile  path  to 
delicate  and  desirable  results  ;  but  it  may  be  quite  simple,  a  direct- 
action  screw  of  fine  thread,  or  a  cone  which  the  revolution  of 
a  screw  pushes  horizontally  forward  upon  the  bottom  of  a  sliding 
bar  to  which  the  sub-stage  is  fixed,  or  an  inclined  plane  acting 
in  a  slot  in  the  same  way.  In  fact,  any  simple  device  for  focussing 
the  condenser  more  slowly  than  the  rackwork  will  do,  pushing  the 
condenser  up  to,  or  causing  it  to  recede  from,  the  under  surface  of 
the  slide  with  sufficient  delicacy.  But  no  means  should  be  employed 


1 86      THE   HISTOKY  AND   DEVELOPMENT  OF  THE   MICROSCOPE 

for  this  end  which  will  imperil  the  absolute  firmness  of  the  sub-stage, 
or  else  more  will  be  lost  than  can  be  gained.  The  need  of  such  a 
device  for  the  most  delicate  and  critical  microscopical  work  is  shown 
plainly  by  the  fact  that  during  the  past  few  years  several  ingenious 
and  practical  devices  have  been  used,  nearly  every  principal  Eng- 
lish maker  employing  a  method  of  his  own.  The  first  arrangement 
was  made  in  Powell  and  Lealand's  sub-stage  and  is  shown  in  fig.  148. 
The  nature  of  this  device,  which  was  suggested  by  Mr.  Nelson,  will 
be  readily  understood.  It  does  not  interfere  with  the  general 


FIG.  147A. — Attachable  stage  with  diaphragm  in  the  plane  of  the  stage.  Top 
view  and  cross  section  showing  construction  of  stage  and  attachment  of  iris  dia- 
phragm. 

mechanical  arrangements  of  the  sub-stage ;  it  will  be  seen  that  the 
milled  head  A  controls  a  screw  spindle  terminating  in  a  steel  cone  B. 
On  rotating  A,  B  turns,  and  with  a  very  slow  motion  forces  up  (or 
releases  as  the  case  may  be)  a  pin  C,  inserted  in  the  base  plate  E  of 
the  sub -stage.  The  motion  of  C  carries  with  it  the  condenser.  At 
right  angles  to  and  forming  part  of  E  at  the  back  an  inner  sliding 
plate  works  against  a  spring  at  the  upper  end  between  bearings  F  at 
each  side,  which  are  fixed  upon  the  usual  racked  slide  D  of  the  sub- 


THE   SUB-STAGE 


187 


stage  ;  the  inner  sliding  plate  is  the  essential  addition  to  the  usual 
racked  slide,  in  the  application  of  the  new  fine  adjustment  to  the 
sub-stage.  The  range  of  motion  is  about  ^th  in. — the  difference  in 
radius  between  the  smaller  and  larger  ends  of  the  steel  cone. 

A  very  simple  and  practical  device  for  the  same  purpose  was 
suggested  by  Mr.  G.  C.  Karop,  who  knew  that  if  the  best  possible 
resolutions  are  required,  the  image  of  the  flame  given  by  the  con- 
denser should  be  as  accurately  adjusted  in  the  focal  plane  as  the 
object  itself.  This  arrangement  of  Mr.  Karop's,  admirably  suited 
to  the  stands  of  Messrs.  Swift  and  Son,  was  patented  by  that  firm. 
It  consists  in  the  adaptation  of  their  well-known  'climax'  or 
;  challenge  '  fine  adjustment  to  the  slide  carrying  the  sub-stage  ;  but 
it  is  actuated  by  a  milled  head  borne  on  the  spindle  to  which  is  con- 
nected the  coarse  rack  motion.  As  will  be  seen  in  fig.  149,  it  is  a 
lever  actuating  a  stud  fixed  to  the  Dovetailed  slide  which  carries  the 


FIG.  148. — Fine  adjustment  to  sub-stage. 
Powell  (1882). 


FIG.  149.— Karop's  fine   adjustment  for    sub- 
stage,  made  by  Swift  (1892). 


sub-stage.  The  extreme  end  of  the  lever  is  not  acted  upon  by  a  fine 
screw,  but  there  is  a  cylindrical  pin  one  end  of  which  engages  the 
point  of  the  lever,  the  other  the  face  of  the  inner  milled  head  ;  the 
milled  heads  resemble  the  Turrell  stage  arrangement,  but  the  inner 
milled  head  works  on  a  screw  on  the  stem  of  the  outer  milled  head  ; 
when  the  inner  milled  head  is  turned  it  traverses  the  stem  of  the 
outer  one,  and  pressure  by  the  S-shaped  spring  in  the  fig.  causes  the 
stud  to  slowly  raise  or  lower,  as  may  be  desired,  the  sub-stage  which 
carries  it.  One  complete  turn  of  the  inner  head  presses  the  sub-stage 
the  T^th  in.  So  that  small  fractions  of  this  may  be  easily  obtained, 
and  it  is  an  advantage  that  the  milled  heads  of  both  movements 
are  so  close  to  each  other. 

Messrs.  W.  Watson  and  Sons  have  also  devised  a  useful  arrange- 
ment to  serve  the  same  end.  As  applied  to  their  Yan  Heurck 
microscope  it  is  shown  in  figs.  150  and  151.  A  is  a  controlling 
milled  head,  B  the  lever  which  is  seen  from  the  side  in  fig.  150 


1 88      THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 


and  from  the  front  in  fig.  151.  This  is  brought  round  at  one  end 
at  right  angles  to  the  front.  The  fulcrum  of  this  lever  is  at  C,  and 
it  fits  under  the  pin  D  which  is  attached  to  a  dovetailed  piece,  having 
at  the  back  of  it  enclosed  in  a  metal  casing  the  counteracting  spring 


FIG.  150.  FIG.  151. 

Watson's  sub-stage  fine  adjustment  (1899). 

shown  in  fig.  151  ;  when,  therefore,  the  lever  is  depressed  at  B,  the 
sub-stage  is  raised  at  D  and  vice  versa.  The  milled  head  A  is  placed 
at  the  side  of  the  stage  of  the  microscope  towards  the  back  slightly 

higher   than  the  surface  of  the 
stage. 

The  fine  sub-stage  adjust- 
ment of  these  makers  as  applied 
to  their  '  Royal '  microscope  is 
shown  as  it  is  in  its  complete 
form  in  fig.  152. 

Another  sub-stage  fine  ad- 
justment has  been  devised  by 
Baker,  which,  we  are  of  opinion, 
it  will  be  of  advantage  to  the 
student  to  understand.  It  em- 
ploys the  differential  screw,  and 
by  this  means  obtains  a  very 

FIG.  152.— Sub-stage  fine  adjustment  com-  slow  movement.    The  student  has 
plete  in  '  Royal '  microscope  (1899).        already  understood  that  the  prin- 
ciple of  this  screw  is  the  cutting 

of  two  threads  of  a  different  '  pitch,'  one  at  either  end  of  the  screw, 
the  proportion  of  one  to  the  other  determining  the  amount  of  move- 
ment. The  threads  found  most  suitable  for  their  sub-stage  fine  ad- 
justment were  40  and  50  to  the  inch.  In  fig.  153  the  screw  A  C 


THE   SUB-STAGE 


189 


has  40  threads  to  the  inch,  and  works  through  an  immovable  fitting, 
the  thread  is  discontinued  at  C,  and  from  C  to  D  a  screw  having  50 
threads  to  the  inch  is  cut,  working  through  a  fitting  E.  If  now 
the  milled  head  F  be  rotated  40  times,  the  screw  A  C  will  have 
travelled  one  inch.  So  will  the  screw  C  D  as  it  is  cut  on  the  same 
stem,  but  it  would  take  50  revolutions  of  screw  C  D  to  travel  one 
inch  through  the  fitting  E,  hence  the  fitting  E  must  have  been 
carried  up  bodily  the  remaining  10  revolutions — that  is  to  say,  ith 


FIG.  153.  —  Baker's  fine  adjustment  to  sub-stage  (1888). 


of  an 


of  an  inch  —  therefore  one  revolution  raises  the  fitting  E 
inch. 

The  fitting  E  is  attached  to  the  sub-stage  G  through  a  slot  cut  in 
the  cover  of  the  adjustment  ;  the  cover  is  also  grooved  on  either  side 
to  receive  that  part  of  the  sub-stage  H  which  insures  the  true 
vertical  movement  so  essential  with  this  screw. 

It  is  almost  a  matter  of  compulsion  to  refer  here  to  a  com- 
paratively recent  arrangement  known  as  a  swinging  sub-stage,  which 
is,  as  its  name  implies,  a  sub-stage  so  arranged  as  to  be  capable  of 


I QO      THE   HISTOEY   AND   DEVELOPMENT  OF  THE   MICEOSCOPE 

being  moved  laterally  out  of  the  axis  in  an  arc  which  has  the  object  on 
the  stage  for  its  centre. 

The  sole  purpose  of  this  is  to  secure  oblique  illumination,  which 
practically,  at  the  time  the  swinging  sub-stage  was  devised,  meant 
obtaining  a  more  oblique  pencil  than  the  condensers  then  provided 
could  command  ;  and  since  this  also  meant  sending  into  the  object  a 
small  portion  of  a  cone  of  light  in  one  azimuth,  many  tacitly  assumed 
that  this  alone  was  taken  to  be  '  oblique  illumination.'  But  whatever 
sends  oblique  light  through  an  object  into  the  objective  is  an  oblique 
illuminator.  Two  condensers  may  have  numerical  apertures  of  T4 
and  1'5  respectively;  a  stop  behind  the  back  lens  in  each  has  a 
narrow  sector  cut  out,  representing  the  conditions  of  the  so-called 
'  oblique  illuminators ; '  by  the  former  we  get  an  oil  angle  of  134°  10', 
by  the  latter  a  similar  angle  of  161°  23'.  These  sectors  of  the  cone 
of  light  of  67°  5'  and  80°  41'  respectively  are  in  every  sense 
'  oblique  illuminators,'  and  the  one  more  oblique  than  the  other. 

Whether  or  not  it  is  needful  or  best  to  use  such  a  sector  is 
scarcely  an  open  question  ;  it  is  manifest  that  by  taking  the  stop 
with  its  sector  away  from  each  condenser  and  sending  in  the  complete 
cone  of  light  formed  by  the  condenser,  we  are  still  using  oblique 
illuminators,  but  the  obliquity  is  in  all  azimuths. 

There  can  be  no  doubt  that  a  large  aperture  in  a  condenser 
provides  the  microscopist  with  far  greater  wealth  of  resource  than  an 
oblique  illuminator  in  one  azimuth  can  ever  give  him.  A  condenser 
with  an  oil  angle  of  161°  23'  is  much  more  valuable  than  even  the 
semi-angle  obtained  by  a  mere  section  of  a  luminous  cone.  The 
power  to  utilise  the  entire  cone  is  a  gain  of  the  highest  order. 

It  will  be  manifest  to  all  that  we  want  concentration  as  well  as 
obliquity. 

Ordinary  concentration  depends  upon  the  power  of  the  condenser. 
If  it  is  required  to  concentrate  the  light  from  the  edge  of  the  flame 
of  a  paraffin  lamp  upon  an  Amphipleura  pellucida,  the  condenser 
must  be  at  least  a  £th  inch  or  1th  inch  in  power,  which  will  give  an 
image  of  the  flame  nearly  the  same  size  as  the  object.  The  amount 
of  light  which  is  concentrated  upon  that  object  will  of  course  depend 
upon  the  aperture  of  the  condenser.  An  oblique  cone  of  great  in- 
tensity is  here  what  is  needed  ;  the  illuminating  cone  should  be 
equal  and  conjugate  to  that  which  exists  between  the  object  and  the 
objective. 

Now  it  is  certain  that  this  condition  cannot  be  met  by  an  '  oblique 
illuminator '  of  the  kind  commonly  undersood  by  that  name  ;  to  get 
immersion  contact,  which  is  of  course  a  sine  qua  non,  we  must  employ 
a  hemispherical  button — or  one  greater  than  a  hemisphere — placed 
in  immersion  contact  with  the  under  surface  of  the  slide.  This  may 
be  illuminated  by  a  beam  from  a  dry  combination,  made  oblique  by 
the  sub-stage  being  swung  out  of  the  axis.  Granted  that  the  angle 
is  attained  which  can  be  got  with  a  condenser  of  great  aperture,  we 
manifestly  obtain  only  a  portion,  and  an  attenuated  and  small  por- 
tion, of  the  light  given  in  every,  or  at  will  any,  azimuth  by  the  con- 
denser. 

Theoretically  perfect  illumination  of  an  objective,  for  example, 


THE   MIRROR 


191 


a  ^th  of  N.A.  1'4  or  1'5,  would  be  obtained  by  using  a  precisely 
similar  objective  as  a  condenser,  with  its  back  lens  stopped  down  by 
a  slotted  stop,  the  slot  being  of  the  size  of  the  peripheral  sector  re- 
quired to  be  illuminated.  The  cone  of  illumination  would  precisely 
equal  that  taken  up  by  the  objective,  and  would  be  of  maximum 
intensity. 

Xow  these  conditions  are  more  nearly  approached  by  a  high  -class 
achromatic  condenser  of  great  aperture  and  of  homogeneous  construc- 
tion than  by  any  other  means. 

The  value  of  oblique  illumination  is  not  here  in  question ;  what 
we  believe  clearly  shown  is  that,  however  much  may  have  been  done 
by  oblique  illuminators  dependent  on  swinging  sub-stages,  and  the 
like,  the  same  things  can  be  better  done  with  immersion  condensers  of 
great  apertures  and  perfect  corrections. 

The  swinging  sub-stage,  with  these  considerations — as  well  as  all 
other  *  oblique  illuminators '  of  its  order — is  a  useless  and  defective, 
not  to  say  deceptive,  adjunct  to  the  microscope ;  and  this  judgment 
has  so  far  obtained  amongst  practical  microscopists  as  to  cause  the 
virtual  disappearance  of  the  swinging  sub-stage.  It  has  no  valid 
function — is  unfruitful  specialisation  in  fact — which  does  not  pro- 
mote the  progress  of  either  the  instrument  or  the  worker. 

And  this  will  apply  to  those  complex  forms  of  microscope  known 
as  *  radial,'  '  concentric/  and  those  provided  with  stages  that  revolve 
or  '  turn  over '  in  an  axis  at  right  angles  to  the  optical  axis  of  the 
microscope. 

In  addition  to  the  features  enumerated  hitherto,  a  complete  sub- 
stage  should  also  be  provided  with  a  rack-and-pinion  rotary  motion  ; 
that  is  only  really  needed  in  order  to  use  the  polar iscope.  For  the 
purposes  of  its  successful  employment  this  is  important,  but  other- 
wise its  use  is  very  limited. 

VI.  The  mirror  is  also  an  indispensable  part  of  a  complete 
microscope.  In  a  first-class  stand  it  should  be  plane  and  concave 
and  from  2^  to  3  inches  in  diameter.  It  may  be  mounted  on  either 
a  single  or  a  double  crank  arm.  In  any  microscope,  if  there  be 
only  one  mirror,  it  should  be  concave.  This  mirror,  from  its  curve, 
has  a  focus,  a  point  in  which  the  reflected  rays  all  meet ;  and  the 
mirror  should  not  be  fixed,  but  so  mounted  that  it  may  be  focussed 
on  the  object. 

The  plane  mirror  is  sometimes  found  to  give  several  reflexions  of 
a  lamp  flame  at  one  time ;  we  find  a  very  efficient  explanation  of 
them  in  a  paper  by  Mr.  W.  B.  Stokes  in  Vol.  VI.  of  the  second  series  of 
the  Journal  of  the  Quekett  Micro.  Club,  p.  322  (1896).  His  idea  of  their 
origin  is  explained  in  fig.  154.  A  is  the  glass  surface,  B  the  silver 
surface,  O  the  object,  and  E  the  eye.  In  the  direction  1,  2,  3  appear 
the  first  three  images.  No.  1  is  from  the  glass  surface,  No.  2  from 
the  silver,  and  No.  3  is  from  the  silver  and  air  surfaces. 

Move  a  card  along  A  towards  1 ,  and  No.  3  disappears  first,  No.  2 
immediately  after,  and  No.  1  when  the  card  reaches  that  point. 
This  being  their  origin  it  may  be  asked  how  the  images  can  alter 
their  position  when  the  mirror  is  revolved  in  the  plane  of  A.  They 
cannot ;  the  mirror  A  B  has  parallel  surfaces,  but  microscope  mirrors 


192      THE   HISTOKY    AND   DEVELOPMENT   OF   THE    MICROSCOPE 


FIG.  154. 


are  not  completely  parallelised  ;  they  may  be  regarded  as  wedges. 
With  that  fact  before  us  we  can  see  how  images  approximate  and 
retire  when  the  mirror  is  revolved.  Let  the  surfaces  A  and  B, 
fig.  155,  have  an  inclination  of  1°  ;  then,  viewing  a  small  object  at  E 
(close  to  the  eye),  one  image  appears  towards  1  —  i.e.  at  right  angles 
to  A  —  and  another  in  the  direction  E  2,  1^°  from  E  1,  which,  after 

being  refracted  to  1  °  in  the 
glass,  is  reflected  at  right 
angles  from  surface  B. 

If  this  mirror  is  re- 
volved in  the  plane  of  A, 
of  course  No.  1  image  will 
remain  still,  and  No.  2  and 
subsequent  images  will  re- 
volve with  the  mirror 
round  No.  1. 

If  we  exaggerate  the 
wedge  shape  of  our  mirror, 
we  can  see  that  at  a  par- 
ticular angle  these  images 
can  be  made  to  superim- 
pose. In  fig.  156  let  the 
signs  be  as  before,  and  the 
images  whose  rays  pass  re- 
spectively from  0  to  1  and 
2  l  will  be  reflected  to  E  as 
one  image.  The  images 
vary  in  size  owing  to  the 
various  distances.  No.  2 
is  the  brightest  except  at 
great  obliquity. 

In  practice  we  find  that 
these  images  may  be  obvi- 
ated by  rotating  the  mirror 
in  its  cell  until  a  certain 
point  is  reached  where  all 
the  images  will  be  super- 
imposed. All  mirrors  should 
be  so  mounted  as  to  admit 
of  this  rotation. 

The  present  Editor  is 
greatly  in  favour  of  the  em- 
ployment of  a  rectangular 
prism  cut  with  care  and  precision.  We  get  by  this  means  total 
reflexion  and  no  double  reflexions  ;  and  he  believes  that  finer  images 
can  be  obtained  by  its  means  than  with  the  plane  mirror.  It  may 
be  mounted  in  the  place  of  the  plane  mirror—  that  is  to  say,  the 
concave  mirror  may  be  as  usual  in  its  cell  —  and  in  the  other  cell, 
which  would  have  received  the  plane  mirror,  the  rectangular  prism 
may  be  mounted  and  be  capable  of  rotation  as  the  plane  mirror 
would  have  been. 

It  should,  however,  be  noted  that  this  applies  only  when   the 


FIG.  155. 


FIG  156 


A   TYPICAL  MODERN    STAND 


193 


PIG.  157.— Powell  and  Lealand's  No.  1  stand  (1872) 


194      THE   HISTORY  AND   DEVELOPMENT   OF  THE   MICROSCOPE 

light  is  required  to  be  reflected  at  an  exact  right  angle.  It  is  of  the 
greatest  service  when  the  microscope  is  of  necessity  used  in  a  rigidly 
upright  position. 

If  it  be  used  for  angles  other  than  right  angles,  there  will  be 
refraction  as  well  as  reflexion  ;  and  as  the  necessary  decomposition 
of  the  light  into  a  spectrum  will  accompany  the  refraction,  care  must 
be  exercised  to  see  that  the  rays  emerging  from  the  prism  are  at 
right  angles  to  those  incident  to  it,  and  that  the  areas  of  the  square 
faces  of  the  prism  are  sufficiently  large  to  have  inscribed  within 
them  a  circle  equal  to  the  back  lens  of  any  condenser  used. 

Some  employ  what  has  been  known  as  a  '  white  cloud  illuminator ',' 
that  is,  a  disc  of  plaster  of  Paris,  or  opal  glass  with  a  polished 
surface .  But  a  disc  of  finely  ground  glass  dropped  into  the  diaphragm- 
holder  of  the  condenser  will  give  a  precisely  similar  result. 

Mr.  A.  Michael  has,  however,  pointed  out  the  curious  fact  that  an 
opalescent  mirror  becomes  an  inexpensive  and  excellent  substitute 
for  a  polarising  prism. 

Typical  Modern  Microscopes.— We  are  now  in  a  position  to  care- 
fully inspect  the  characteristics  of  the  chief  forms  of  microscope 
which  the  modern  manufacturers  of  England,  the  Continent,  and 
America  offer  to  the  microscopist. 

We  confine  ourselves  to  the  chief  models,  indicating  more  or  less 
suggestively  their  merits  or  defects.  We  neither  discuss  all  the 
instruments  of  any  maker  nor  in  every  case  even  one  instrument  of 
some  makers.  This  would  involve  simple  repetition  in  the  main 
features.  The  reader  can  compare  for  himself  the  microscope  of  any 
given  maker  from  whose  catalogue  he  proposes  to  select,  and  can 
discover  by  comparison  its  incidence  or  otherwise  with  the  type 
given  here  to  which  it  corresponds. 

Beginning  with  the  highest  types  we  place  first  on  the  list  Powell 
and  Lealand's  No.  I.  This  instrument  may  claim  a  seniority  over 
all  the  foremost  instruments,  because  for  nearly  fifty  years  it  has 
practically  remained  the  same.  All  its  principal  features  were 
brought  to  their  present  perfection  nearly  fifty  years  ago,  while  all 
other  microscopes  during  this  period  have  been  redesigned  and 
materially  altered  over  and  over  again.  This  is  no  small  commenda- 
tion, for  during  that  period,  as  the  reader  so  well  knows,  the  aper- 
tures of  objectives  have  been  enormously  enlarged,  and  with  this 
has  come  a  great  increase  of  focal  sensibility.  As  a  result  the 
majority  of  the  microscopes  of  forty  years  ago  are  absolutely  useless 
for  the  objectives  of  to-day,  but  the  focussing  and  stage  movements 
of  Powell  and  Lealand's  microscope  still  hold  the  first  place. 

Fig.  157  represents  the  instrument  in  its  monocular  form.  The 
foot  of  the  stand  is  a  tripod  in  one  casting ;  it  has  an  extended  base 
of  7  X  9  inches,  forming  at  once  the  steadiest  and  the  lightest  foot 
of  any  existing  microscope.  The  feet  are  plugged  with  cork,  and 
when  the  body  is  in  a  horizontal  position  the  optic  axis  is  (as  it 
should  be)  10  inches  from  the  table. 

The  coarse  adjustment  is  effected  by  a  bar,  consisting  of  a  mas- 
sive gun-metal  truncated  prism  in  form,  which  bears  only  on  a 
narrow  part  at  the  angles.  It  extends  sufficiently  to  focus  a 


POWELL  AND   LEALAND'S   BEST   STAND  195 

4-inch  objective.  The  arm  which  carries  the  body  is  of  unusual  length 
for  the  type  it  represents  ;  but  this  gives  a  large  radius  from  the 
optic  centre  of  the  instrument,  aii<l  makes  the  complete  rotation  of 
the  stage  easy.  Great  efforts  have  been  made  to  accomplish  this  in 
other  instruments.  The  older  Ross  form  from  the  shortness  of  the 
arm  only  allowed  of  a  two-thirds  rotation,  and  in  the  Lister  model 
many  different  devices  have  been  tried,  the  latest  being  the  placing 
of  the  stage  pinions  in  a  vertical  position  above  the  stage,  which  is 
an  unquestionable  error. 

The  rotation  of  the  stage  in  the  Powell  and  Lealand  model  is  by 
means  of  a  milled  head  most  conveniently  placed,  and  the  divided 
circle  is  on  a  plate  of  silver.1  It  will  also  rapidly  rotate  by  hand. 

The  arm  is  on  a  pivot,  which  allows  it  to  be  turned  away  from 
the  stage  altogether,  and,  as  we  have  already  indicated,  the  length 
of  the  arm  lent  itself  to  the  use  of, a  longer  lever  for  the  fine  adjust- 
ment (p.  174).  The  milled  head  is" placed  behind  the  strong  pivot 
of  the  arm,  where  vibration  is  impossible,  and  it  is  in  an  easy  and 
natural  position  for  the  access  of  either  hand. 

The  body  may  be,  with  great  ease,  entirely  removed  from  the  arm  ; 
this  makes  the  use  of  the  binocular  or  monocular  body  or  of  a  short 
or  long  body  a  matter  of  choice,  while  it  gives  access  for  cleaning  and 
other  purposes  to  the  nose-piece  tube,  as  well  as  for  the  insertion 
and  focussing  of  the  lens  used  with  an  apertometer,2  or  an  analysing 
prism.  So  also  it  is  of  service  in  low-power  photo-micrography. 

We  have  already  referred  to  the  stage  of  this  instrument ; 
but  it  may  be  briefly  stated  that  it  is  large,  has  complete  rotation, 
it  has  one  inch  of  rectangular  motion,  being  graduated  to  the  rtn>th 
inch  for  a  finder.  There  is  the  same  speed  in  the  vertical  and  the 
lateral  movements,  and  the  pinions  do  not  alter  their  positions.  The 
aperture  of  the  stage  is  amply  large. 

The  ledge  of  the  stage  has  a  stop  placed  on  its  left-hand 
side ;  this  is  held  by  a  screw,  but  is  removable  at  pleasure. 
Two  massive  brackets  under  the  stage  remove  all  possibility  of 
flexure. 

The  sub-stage  has  rectangular  movements  by  screw  in  either  direc- 
tion, as  well  as  a  rotary  movement  by  pinion.  The  coarse  adjust- 
ment is  by  rackwork,  and  a  fine  adjustment  is  added  when  desired. 
Fig.  158  illustrates  this  stage,  showing  its  under  side  in  order  to 
enable  the  fine  adjustment  to  be  seen. 

The  vertical  and  upper  horizontal  milled  heads  are  centring 
screws,  acting  at  right  angles  to  each  other,  while  the  diagonal  screw 
to  the  left  is  the  milled  head,  which  causes  the  stage  to  rotate,  the 
whole  acting  with  great  smoothness  and  accuracy,  also  enabling  the 
operator  to  centre  with  complete  precision,  while,  as  we  have 
already  seen  (pp.  187  and  196),  the  milled  head  A  works  by  an 
advancing  cone  the  fine  adjustment  to  this  stage. 

The  mirror  is  plane  and  concave,  with  double-jointed  arm. 

The  finish  and  workmanship  of  this  instrument  are  of  the  highest 
order.  The  seen  and  the  unseen  receive  equally  scrupulous  care. 

1  This  is  now  made  of  platinum  if  desired,  and  thus  tarnish  is  obviated. 

2  Chapter  V.  p.  337. 

02 


196      THE   HISTORY  AND  DEVELOPMENT   OF  THE   MICROSCOPE 


The  present  Editor  has  had  one  of  these  microscopes  in  constant,  and 
often  prolonged  and  continuous,  use  for  over  twenty  years,  and  the 
most  delicate  work  can  be  done  with  it  to-day.  It  is  nowhere 
defective,  and  the  instrument  has  only  once  been  '  tightened  up '  in 
some  parts.  Even  in  such  small  details  as  the  springing  of  the 
sliding  clip — the  very  best  clip  that  can  be  used — the  pivots  of  the 
mirror,  and  the  carefully  sprung  conditions  of  all  cylinders  intended 
to  receive  apparatus,  all  are  done  with  care  and  conscientiousness. 

An  instrument  of  this  kind  may  be  made  to  appear  perfect  to 
the  eye,  but  at  the  same  time  may  lack  some  most  important  elements 
as  a  finished  instrument.  But  this  is  an  instrument  of  the  highest 
order  as  such,  and  at  the  same  time  a  very  fine  specimen  of  highly 
finished  brass  work. 

A  note  must  be  made  before 
leaving  this  microscope  upon  the 
size  of  the  tubes  in  the  body  and 
the  sub-stage. 

Powell  and  Lealand  were  the 
only  makers  whose  gauge  of 
tubing  had  a  raison  d'etre ;  the 
size  of  the  tube  was  such  that  it 
would  take  in  a  binocular  body  a 
Huyghenian  2 -inch  eye-piece, 
having  the  largest  field-glass  pos- 
sible. The  size  of  this  field-glass 
depends  on  two  factors. 

1.  The  distance  between  the 
centres  of  the  eyes. 

2.  The       mechanical       tube- 
length. 

FIG.  158.-Powell  and  Lealand's  sub-stage  In  order  that  the  binocular 
with  fine  adjustment  (1882).  may  suit  persons  with  '  narrow 

centres'  to  their  eyes,  the  dis- 
tance between  them  should  not  be  greater  than  2^  inches.  The 
mechanical  tube -length  is  8|  inches  for  the  standard  tube.  When 
the  eye-pieces  were  '  home '  in  their  places  in  the  tubes  they  j  ust 
touched  each  other,  the  inner  sides  of  the  binocular  tubes  being  cut 
away ;  so  under  the  above  conditions  a  larger  field  than  is  thus 
obtained  is  simply  impossible.  The  size  of  the  field-glass  deter- 
mines the  size  of  the  eye-piece,  and  that  was  made  to  fix  the 
diameter  of  the  body-tube. 

Very  wisely  these  makers  made  the  tube  of  the  sub-stage  the 
same  size,  so  as  to  have  one  gauge  of  tubing  throughout.  This 
allows  a  Kellner  or  other  eye-piece  to  be  used  as  a  condenser,  thus 
reducing  the  number  of  adapters. 

Lately  this  firm  have  altered  their  sub-stage  tube  to  a  gauge 
recommended  by  the  Royal  Microscopical  Society.  This  involves 
an  adapter  where  the  sub-stage  apparatus  was  adapted  to  the  old 
gauge,  or  when  an  eye- piece  is  used  as  a  condenser  ;  as  the  size  is 
too  large  for  a  binocular. 

The  Ross  model,  in  its  completest  form  as  left  by  Andrew  Ross, 


T.   ROSS'S  MICROSCOPE 


197 


except  specially  ordered  is  never  made  by   this  firm,  but  for  its 
qualities  and  historical  relations  it  is   of  -tmuch  interest.      It  was 


FIG.  159.— The  model  by  T.  Koss  (1862). 

very  similar  to  the  model  by  T.  Ross  shown  in  fig.  159.  A.  Ross's 
first  model  had  a  triangular  bar,  was  monocular,  possessed  no 
proper  sub-stage,  the  condenser  was  attached  to  the  main  stage, 


198      THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 


which  was  without  arrangement  for  rotation ;  and  the  mirror  was 
not  jointed.  The  model  of  T.  Ross  had,  as  will  be  seen,  a  bar  move- 
ment, with  a  foot  formed  of  a  triangular  plate  to  which  were  bolted 
two  parallel  upright  plates  to  carry  the  trunnions  of  the  microscope. 
The  fine  adjustment  is  a  lever  of  the  second  order,  with  the  milled 
head  in  the  middle  of  the  bar,  which  involves  tremor,  and  the  tube  of 
the  nose-piece  is  short,  making  shake  possible. 

The  stage  movements  are  of  unequal  speed,  the  lateral  move- 
ment being  slower  than  the  vertical.       There  is  no  finder,  and  the 

rotation  of  the  stage  is 
but  partial.  The  sub- 
stage  and  mirror  are 
good.  It  was  a  com- 
manding instrument  in 
its  day,  and  was  of  ex- 
cellent workmanship 
and  finish ;  but  it  was 
not  equal  to  the  strain 
of  critical  work  with  im- 
mersion objectives  of 
great  aperture.  Never- 
theless the  defects  of  this 
stand  could  have  been 
readily  corrected.  With 
a  more  extended  base,  a 
better  arrangement  of 
the  fine  adjustment,  a 
mechanical  stage  con- 
structed on  better  prin- 
ciples, and  the  rotation 
made  complete  and  con- 
centric— which  it  was 
not — this  would  have 
been,  even  for  our  pre- 
sent requirements,  an 
admirable  instrument. 

This  important  firm 
were  otherwise  advised, 
however;  and,  instead 
of  correcting  the  errors 
of  the  instrument  whose 

history  they  had  made,  they  designed  an  entirely  view  model  in  which 
a  Lister  limb  was  substituted  for  the  bar  movement.  Fig.  160  illus- 
trates this  form  of  the  instrument,  from  which  it  will  be  seen  that  the 
foot  also  was  changed  for  the  worse  ;  the  base  was  not  sufficiently 
extended,  and  the  hinder  part  of  the  foot  was  too  large,  so  that  it 
sometimes  rocked  on  four  points,  because  the  hinder  part  was  too 
wide — a  flat  surface,  in  fact.  A  true  tripod  will  stand  firm  on  an 
uneven  table,  but  this  form  will  not.  It  is  a  form  frequently  used  by 
various  makers  now,  and  is  known  as  the  *  bent  claw.'  It  is  a  bad 
design,  and  may  be,  as  it  has  been,  easily  thrown  over  laterally.  It 


FIG.  160.— Ross-Zentmayer  model  (1878). 


THREE  GREAT  TYPES  OF  MICROSCOPE 


199 


was,  however,  eventually  cast  in  one  piece,  which  gave  it  a  solidity 
which  the  former  did  not  possess. 

The  introduction  of  the  Lister  limb  brought  its  inevitable 
troubles — notably,  with  the  fine  adjustment — to  which  we  have  fully 
referred  under  that  head.  But  in  the  Ross-Zentmayer  model,  a 
later  form,  the  body  and  the  coarse  adjustment  were  both  carried  by 
the  fine-adjustment  lever  and  screw. 

This  form  could  not — as  it  did  not — long  prevail.  Its  existence 
\vas  ephemeral,  and  in  its  place  was  put  a  modification  of  the  form 
devised  by  Zeiitmayer,  known  subsequently  as  the  Ross-Zentmayer 
model.  This  was  the  Ross- Jackson  instrument  with  a  '  swinging 
sub-stage.'  This  instrument  is  illustrated  in  fig.  161.  It  will  be 
seen  that  the  foot  is  a  true  tripod,  consisting  of  a  triangular  base 
with  two  pillars  rising  from  a  cross-piece,  which  carried  the  trun- 
nions. 

Here  it  may  be  as  well  to  point  out  the  differences  which  exist 
between  the  three  great  types  of  microscope,  viz.  the  bar  move- 
ment, the  Lister  limb,  and  the  Jackson  limb.  In  the  bar  movement 
we  find  a  transverse  bar  uniting  the  lower  end  of  the  body  to  the 
coarse  adjustment  bar  (figs.  157,  159).  In  the  Lister  the  body  is 
supported  through  a  greater  or  less  portion  of  its  entire  length, 
the  limb  being  formed  of  one  solid  casting  (figs.  160,  161,  162,  167). 
In  the  Jackson  the  dovetailed  groove  which  carries  the  sub-stage 
slide  is  included  in  the  casting,  and  the  groove  for  the  coarse  ad- 
justment of  the  body,  as  well  as  that  for  the  sub-stage,  is  ploughed  in 
one  cut  (fig.  165).  Jackson  also  designed  the  double  pillar  foot 
(fig.  161). 

We  have  already  assessed  the  value  of  a  swinging  sub-stage  and 
found  that  in  our  judgment  it  is  at  best  redundant  and  really 
adverse  to  the  accomplishment  of  the  best  scientific  work.1  No 
microscope  is  complete  without  a  good  condenser  ;  all  and  much  more 
than  all  that  can  be  done  by  a  swinging  sub-stage  can  be  done  with 
a  slotted  stop  at  the  back  of  the  condenser.  This  elaborate  appen- 
dage is  therefore  without  justification.  Yet  in  the  impatience  for 
large  illuminating  apertures,  which  were  not  at  that  time  provided  by 
condensers,  this  phase  of  pseudo-illumination  was  carried  to  a  still 
greater  and  more  elaborate  development  in  the  production  of  a  con- 
centric microscope.  This  was  a  Ross-Wenham,  known  as  the  radial 
microscope.  But  elaborate  and  costly  as  it  was  it  never  justified  its 
existence,  and  like  the  whole  group  of  *  concentric '  and  *  radial ' 
microscopes,  it  has  passed  away  simultaneously  with  the  abolition  of 
'  oblique  illumination,'  and  is  to-day  a  not  very  interesting  curiosity 
in  the  history  of  the  modern  microscope. 

A  large  and  extremely  well-finished  stand  is  made  by  Messrs. 
Watson,  known  as  the  Yan  Heurck  microscope  in  its  best  form  :  it  is 
illustrated  in  fig.  162.  The  body  has  two  draw  tubes,  one  of  which 
is  actuated  by  rack  and  pinion,  and  the  other  sliding  inside  it  so 
that  a  range  of  body  length  varying  from  142  mm.  to  300  mm.  can 
be  obtained.  The  coarse  and  fine  adjustments  have  very  wide 
bearings,  and  the  exact  relationship  of  the  pinion  to  the  rackwork 

1  P.  188  et  seq. 


FIG.  161  (1878). 


FIG.  163.— Section  of  bearings 
and  fittings  of  pinion. 


FIG.  162.— The  grand  model  Van  Ileurck.     Watson  and  Sons  (1895). 


202      THE   HISTOKY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

is  established  by  means  of  a  block  of  metal  which  fits  upon  the 
pinion  shaft  and  is  pressed  or  released  by  means  of  the  two  screws 
provided  for  the  purpose.  This  is  shown  in  section  in  fig.  163, 
where  the  pinion  is  P,  the  anti-friction  block  N,  and  one  of  the 
adjusting  screws  M.  The  perspective  view  of  the  coarse  adjustment 
showing  the  adjusting  screws  is  given  in  fig.  164. 

The  stage  can  be  completely  rotated  and  has  mechanical  move- 
ments on  the  Ttirrell  principle,  both  milled  heads  being  on  one  axis. 
The  sub-stage  has  a  fine  adjustment,  and  the  plane  mirror  is  care- 
fully worked  by  hand,  \vhile  exceptional  rigidity  for  the  whole  stand 
is  obtained  by  a  special  system  of  construction,  and  the  tripod,  which 
is  shod  with  cork,  has  a  spread  of  ten  inches. 

A  high-class  stand  of  distinguished  merit  is  made  by  the  firm  of 
Baker  of  Holborn.  It  is  illustrated  in  fig.  165,  is  made  with  great 


FIG.  164. — Complete  view  of  Watson's  coarse  adjustment  (1895). 

care  and  is  a.n  instrument  of  precision.  It  is  mounted  on  a  solid  tripod 
with  slotted  toes  so  that  it  can  be  firmly  clamped  to  the  baseboard 
of  a  photo-micrographic  apparatus.  The  body  is  mounted  on  a  mas- 
sive limb  in  one  piece  throughout,  and  on  to  this  the  stage  and 
sub-stage  are  mounted ;  in  this  way  the  chance  of  derangement  of 
the  optic  axis  is  reduced  to  a  minimum.  The  body  has  diagonal 
rack-and-pinion  coarse  adjustment  actuated  by  very  large  milled 
heads,  making  a  slow  movement  easy.  The  fine  adjustment  carries 
the  body  tube  only  each  revolution  of  the  graduated  milled  head, 
being  equal  to  the  ^^ih  of  an  inch ;  the  Campbell  differential  screw 
being  employed,  and  the  milled  head  being  placed  at  the  lower  end 
of  the  body.  The  body  can  be  extended  to  300  mm.  and  closed  to 
150  mm.  The  mechanical  stage  is  worked  on  the  Turrell  method 
by  stationary  milled  heads  working  on  a  common  centre  commanding 
oblique  as  well  as  rectangular  movements ;  the  rectangular  movements 
on  divided  silver  plates  for  recording  positions  ;  and  complete  rota- 
tion can  be  secured,  either  by  hand  or  by  rack  and  pinion,  and  can 
be  at  any  point  clamped. 

The  sub-stage  has  rectangular  mechanical  movements  controlled 


C.  BAKER'S   MODEL 


2O3 


by  fixed  milled  heads,  and  all  fittings  are  sprung  and  have  adjusting 
screws  to  compensate  for  wear,  and  fine  adjustment  by  Campbell  diffe- 
rential screw  which  Mr.  Baker  adopted  for  these  microscopes  in  1888. 


FIG.  165.— C.  Baker's  Model  (1895J. 

Swift  and  Son  formerly  made  two  instruments  of  the  first  class, 
one  having  a  bar  movement  similar  to  that  of  Andrew  Ross,  the 
other  a  Lister  similar  to  Beck's.  The  principal  difference  was  that 
the  foot  was  of  the  '  bent  claw '  form.  We  have  already  seen  that 


204      THE   HISTORY   AND   DEVELOPMENT  OF  THE   MICROSCOPE 

by  their  invention  of  the  vertical  lever  fine  adjustment  (figs.  133  and 
135)  Swift  and  Son  have  made  possible  a  useful  future  for  the  Lister 
limb  ;  and  their  model  of  this  form  is  shown  in  fig.  166,  known  as 
the  '  Best  "  Challenge  "  Microscope.'  It  has  a  beautifully  made  coarse 
adjustment,  the  special  fine  adjustment  invented  by  this  firm,  a  cir- 
cular rotating  stage  moved  by  rack  and  pinion  or  by  hand,  and  is 
provided  with  divided  silver  plates  to  the  rectangular  movement. 
The  sub-stage  is  complete  for  centring  as  well  as  focussing,  and  has 
rotary  movement  for  use  with  polar i scope.  The  stand  is  a  firm 
form  of  tripod,  and  the  mirrors  are  well  worked  and  mounted  on  a 
double  crank. 

All  the  movable  parts  of  Swift's  instruments  are  sprung  on  Powell 
and  Lealand's  method,  and  the  movements  are  smooth  and  sound. 
Many  stands  had  been  devised  by  American  opticians  up  to  the  time 
of  the  publication  of  our  last  edition  of  this  work,  but  they  were 
based  upon  one  or  other  of  the  great  English  models,  and  the  modi- 
fications, whether  for  good  or  evil,  were  adopted  into  the  then  modi- 
fications of  the  older  English  types,  and  were  incidentally  described. 
It  should  be  remembered  that  Zentmayer,  of  Philadelphia,  devised 
the  model  from  which  the  Ross-Zentmayer  was  finally  formed.  Its 
principal  feature  was  to  obtain  oblique  illumination  in  one  azimuth 
by  the  swinging  stage  which  we  have  emphatically  shown  in  this,  as 
we  did  in  the  last  edition,  to  be  a  pernicious  adjunct  for  practical 
purposes.  The  fine  adjustment  of  this  instrument  was  most  defec- 
tive. Tolles,  again,  who  wholly  deserves  the  very  high  reputation  he 
attained,  made  an  instrument  in  which  he  mounted  the  stage  on  a 
disc  ;  near  the  edge  of  this  disc  the  sub-stage  is  made  to  travel  in  a 
groove  carrying  the  condenser,  or  dry  combination,  in  an  arc  round 
the  object  as  a  centre.  This  was  only  another  elaboration  of  the 
same  swinging  sub-stage. 

In  later  constructions  of  this  form,  Tolles  first  used  the  mechanical 
stage  actuated  by  two  pinions  vertical  to  the  surface  of  the  stage, 
and  subsequently  adapted  by  Ross.  The  fine  adjustment  in  this 
instrument  had  the  fatal  defects  characteristic  of  its  form. 

Bulloch,  another  American  maker  of  note,  made  some  modifica- 
tions in  the  Zentmayer  model,  but  they  were  in  the  interests  of  the 
swinging  sub-stage,  and,  although  no  doubt  ingenious,  must  pass 
with  this  transient  form  of  the  microscope. 

A  modification  of  this  stand  was  devised  by  Bulloch  ;  it  presents 
no  special  point,  save  the  employment  of  a  Gillett  condenser  with 
the  diaphragm  drum  above  the  lenses ! 

A  later  development  of  this  form  of  instrument  by  the  same 
maker  was  made  some  years  after,  but  the  chief  difference  consists 
in  the  adoption  of  a  stage  in  which  the  milled  heads  stand  upon 
the  stage,  which  is  the  reverse  of  an  advance.  Since,  however,  the 
swinging  sub-stage  form  of  instrument  has  been  entirely  superseded, 
American  makers  have  adopted,  with  very  slight  modifications  in 
form,  none  in  principle,  the  Continental  stand,  which  is  made  with 
admirable  precision  and  conscientious  care,  but  still  retains  its  chief 
features.  It  may  therefore  be  of  service  to  consider  the  principal 
recent  modifications  of  the  Continental  stand,  so  that  they  may  be 


SWIFT'S    BEST    CHALLENGE    MICROSCOPE  205 


FIG.  166.— Swift's  best  challenge  microscope  (1881). 


206      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

fairly  compared  with  equally  recent  American  adaptations  of  the  same 
microscope ;  and  then  endeavour,  after  examining  instruments  of 
a  lower  class,  to  give  a  dispassionate  estimate  of  this  model  as  com- 
pared with  that  of  the  highest-class  English  type. 

Amongst  Continental  makers  the  firm  of  Zeiss  has  taken  a  fare- 
most  position  and  has  secured  a  well-deserved  world-wide  fame. 
Their  largest  microscope  is  shown  in  fig.  167.  It  is  a  model  of  fine 
workmanship  and  has  been  adapted  with  singular  ingenuity  to  the 
reception  of  all  their  accessory  apparatus.  The  upper  body  is 
inclinable  from  the  vertical  to  the  horizontal  position.  It  is 
provided  with  coarse  rack-and- pinion  adjustment,  and  fine  adjust- 
ment by  means  of  a  direct  acting  micrometer  screw  with  divided 
head.  The  sub -stage  takes  all  the  apparatus  provided  by  this  firm, 
and  in  addition  it  may,  by  means  of  a  small  lever,  be  swung  out  of 
its  central  position,  *  so  as  to  facilitate  rapid  transition  to  illumina- 
tion with  the  cylinder-diaphragm,'  wrhile  below  the  condenser  is  a 
movable  iris  diaphragm  fitted  with  a  rack-and-pinion  movement  to 
throw  it  out  of  the  centre,  and  which  can  be  rotated  about  the  axis 
or  entirely  swung  out. 

The  circular  object  stage  rotates  (not  by  rack  and  pinion),  but 
has  centring  screws.  The  aperture  in  the  stage  has  received  a 
more  oval  form.  The  rack-and-pinion  rectangular  movements  are 
1^-in.  vertical  and  2-iii.  lateral ;  the  milled  heads  are  small  but 
efficient  and  work  smoothly.  That  for  transverse  movement  being 
placed  upon  the  top  of  the  stage. 

Reichert,  of  Vienna,  makes  a  stand  which  in  the  main  cor- 
responds with  that  of  Zeiss,  and  we  are  enabled  to  speak  with 
confidence  of  the  high  quality  of  the  workmanship  ;  but  in  illustra- 
tion we  choose  not  the  IA  stand  but  the  large  stand  known  as  I  IB, 
an  illustration  of  wThich  is  given  at  fig.  168.  Our  object  in  choosing 
this  instrument  is  that  it  combines  every  essential  of  the  IA  stand, 
and  in  addition  is  furnished  with  the  new  lever  fine  adjustment, 
invented  so  recently  by  Reichert,  and  of  whose  value  we  have 
already  given  our  judgment.  It  will  be  seen  that  on  the  part  of  the 
body  which  the  fine  adjustment  milled  head  crowns  there  is  a 
protrusion  on  the  right  and  left  hand  side  of  the  pillar.  This  is  the 
only  addition  outwardly  that  the  new  fine  adjustment  makes  needful. 

A  very  high-class  microscope  is  made  by  Leitz  of  Wetzlar,  which, 
while  it  retains  the  principal  features  common  to  all  microscopes 
based  on  the  Continental  model,  has  yet  qualities  peculiar  to  itself, 
and  obtains  by  means  of  workmanship  and  ingenuity  the  most  ad- 
mirable results  attainable  from  the  model  on  which  it  is  based.  It 
'is  inclinable  with  a  hinged  joint  and  clamping  lever  ;  and  the  stage 
is  provided  with  a  revolving  centring  table.  The  mechanical  stai^c 
is  the  *  attachable '  one  already  described,  and  the  adjustment  of  the 
objective  is  by  rack-and-pinion  coarse  adjustment,  and  by  a  fine 
adjustment  depending  on  a  micrometer  screw  provided  with  a 
divided  screw  head.  The  draw-tube  is  furnished  with  a  millimetre 
scale.  The  sub-stage  is  planned  on  the  principle  of  the  Zeiss 
instrument  and  will  receive  the  illuminating  apparatus  as  devised 
by  Abbe,  which  is  worked  by  rack-and-pinion  adjustments,  which 


FIG.  1(57— Zeiss's  largest  and  complete  stand  (1895). 


208      THE   HISTOEY   AND   DEVELOPMENT  OF  THE   MICROSCOPE 

also  raise  and  lower  the  iris  diaphragm  and  provide  it  with  possible 
oblique  or  eccentric  movements ;  and  it  is  furnished  with  objectives 


FIG.  168. — One  of  Eeichert's  large  stands  (lib)  with  new  lever  fine  adjustment 
fitted  (1899). 


FIG.  169. — Leitz's  most  complete  stand  (1893). 


210     THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

ami  eye-pieces  that  give  it  magnifying  powers  ranging  from  15  to 
1,500  times.  This  instrument  is  shown  in  fig.  169,  and  with  the  two 
stands  immediately  preceding  it  furnishes  us  with  a  fair  view  of 
the  principal  and  latest  types  of  the  Continental  microscope  fitted 
with  the  apparatus  essential  to  the  production  of  good  work. 

But  another  most  interesting  model  of  Reichert's  has  just  been 
finished  which,  from  its  size  and  approximation  to  the  English 
stand  in  some  important  points,  we  are  constrained  to  notice  as 
these  sheets  are  passing  through  the  press.  The  instrument  is 
illustrated  in  fig.  169 A.  The  height  of  the  stand  in  the  position 
illustrated  is  16^  in.  The  distance  between  the  foot  and  the  stage  is 
3  J  in.  The  sub-stage  is  provided  with  centring  screws,  and  is  raised 
and  lowered  by  rack  and  pinion.  The  mirror  can  be  readily  moved 
towards  or  away  from  the  sub-stage  or  can  be  entirely  removed. 
The  tube  length  with  both  tubes  (A  A')  extended,  including  the  nose- 
piece,  is  10^  in.  The  stage  is  mechanical,  and  the  circle  is  divided  into 
360  degrees  ;  both  the  horizontal  and  vertical  motions  of  the  stage  have 
scales  read  by  verniers.  The  object  is  fixed  on  the  stage  by  spring 
fittings.  The  fine  adjustment  has  two  speeds  of  motion  by  two  screws, 
the  one  0'3  mm.,  the  other  O'l  mm.  per  revolution,  shown  at  M  M7. 
The  draw-tube  has  a  divided  scale  and  is  moved  by  rack  and  pinion. 

We  may  now  with  advantage  consider  the  different  classes  of 
microscopes  manufactured  by  the  opticians  of  Europe  and  America. 
To  do  this  without  prejudice  and  with  efficiency  it  is  necessary  to 
designate  the  characters  which  should  distinguish  each  class. 

Microscopes  placed  in  Class  I.  possess — 

1.  Coarse  and  fine  adjustments. 

2.  Concentric  rotation  of  the  stage 

3.  Mechanical  stage. 

4.  Mechanical  sub-stage. 
Class  II. 

1.  Coarse  and  fine  adjustments. 

2.  Mechanical  stage. 

3.  Mechanical  sub-stage. 
Class  III. 

1.  Coarse  and  fine  adjustments. 

2.  Plain  stage. 

3.  Mechanical  sub-stage. 
Class  IV. 

1.  Coarse  and  fine  adjustments. 

2.  Plain  stage. 

3.  Sub-stage  fitting  (no  sub -stage). 
Class  V. 

1.  Single  adjustment  (coarse  or  fine). 

2.  Plain  stage. 

3.  With  or  without  sub -stage  fitting  (no  sub-stage). 
This  classification  applies  also  to  portable  microscopes. 

The  recent  microscopes  of  the  best  American  makers  are 
characterised  by  the  highest  quality  of  workmanship  and  abundant 
ingenuity,  but  the  Continental  model  is  confessedly  made  their  fotmda- 


RECENT  AMERICAN  MICROSCOPES 


211 


tioii.  In  the  last  edition  of  this  work  it  was  shown  that  American 
opticians  made  their  first-class  microscopes  with  swinging  sub-stages, 
and  we  then  pointed  out  that  these  were  not  onlv  without  value, 


M 


FIG.  169A  (1900), 


p  2 


212      THE   HISTOKY    AND   DEVELOPMENT   OF  THE   MICROSCOPE 

but  injurious  to  the  best  work  possible  to  a  good  instrument.  In 
the  interval  the  swinging  sub-stage  has  been  given  up,  even  by  its 
most  ardent  advocates ;  but  at  the  same  time  in  the  majority  of 
cases  they  have  abandoned  the  sub-stage  proper  and  adopted  the 
Continental  condenser  fitting  instead.  In  fact,  the  American 
opticians  have  chosen  almost  exclusively,  as  the  basis  of  their  stands 
of  every  class,  the  microscope  that  has  been  so  long  in  vogue  on 
the  Continent  of  Europe. 

It  will  suffice  to  take  examples  of  the  unexceptionally  beautiful 
work  of  the  two  leading  opticians  of  America — The  Bausch  and 
Lomb  Optical  Company  and  The  Spencer  Lens  Company.  An 
illustration  of  the  best  instrument,  known  as  the  *  Grand  Model,'  of 
the  former  of  these  opticians  is  given  in  fig.  170.  It  is  designated 
a  '  Continental  Microscope,'  but  is  not  a  mere  copy  of  the  best  work 
of  Germany  or  France.  The  body-tube  is  large,  and  the  horseshoe 
base,  of  Continental  fame,  is  said  by  the  makers  to  be  improved  by 
the  '  back  claw '  being  prolonged  '  so  as  to  virtually  form  a  tripod 
base,'  and  it  is  commended  as  '  extra  heavy.'  From  the  figure,  how- 
ever, it  would  appear  to  be  the  extra  weight  rather  than  a  pro- 
longed claw  that  imparts  the  steadiness.  The  body  is  supported 
on  a  pillar  of  two  massive  columns.  The  stage  is  large,  and  rotal<>> 
with  centring  screws.  'The  heads  of  the  centring  screws  are 
provided  with  graduations  and  index,  and  with  a  series  of  line.s 
recording  the  number  of  revolutions  of  the  screw,'  so  that  the 
position  of  any  given  object  may  be  recorded  and  thus  be  referred  t<  > 
again  if  the  microscope  should  have  been  used  for  other  work  in  the 
interval.  The  mechanical  stage  is  worked  by  one  milled  head  at 
the  side  and  the  other  at  the  top  of  the  stage,  the  latter  position  (a^ 
we  pointed  out  in  the  last  edition  of  this  book  when  referring  to  the 
Tolles  mechanical  stage)  being  one  in  which  the  efficiency  of  the 
mechanism  is  reduced  to  its  lowest  value.  We  have  long  advocated 
the  adoption  of  Turrell  milled  heads  as  employed  in  Powell's  No.  1 
stand ;  they  give  the  worker  power  to  effect  not  only  rectangular 
but  diagonal  movements,  and,  without  displacing  the  fingers,  to 
work  the  stage  in  all  directions.  We  are  pleased,  as  we  have 
pointed  out,  to  note  that  the  eminent  firm  of  Zeiss  have  adopted 
these  in  their  best  stand  (fig.  139). 

The  sub-stage  is  composed  of  three  parts,  arranged  one  above  the 
other.  This  sub-stage,  with  the  parts  separated  to  show  their 
construction,  is  presented  in  fig.  171.  The  upper  part  is  a  ring 
carrying  a  removable  iris  diaphragm,-  so  arranged  as  to  come 
directly  into  contact  with  the  under  part  of  the  object  slide.  The 
middle  section  of  the  sub-stage  is  movable  vertically  on  the  main 
sub-stage  axis,  and  carries  an  Abbe  condenser  of  1-20  N.A.,  which 
can  be  swung  laterally  to  the  left  of  the  instrument  so  as  to  put  it 
out  of  optical  use  ;  but  on  the  other  hand  it  can  at  will  be  thrown 
back  into  position  and  placed  in  oil  contact  with  the  object  slide  with- 
out altering  the  position  of  the  upper  iris  diaphragm.  The  third  and 
lowest  section  of  the  sub-stage  carries  the  large  iris  diaphragm  used 
below  the  condenser.  Thus  it  is  clear  that  the  whole  can  be  used 
together,  or  any  one  of  the  three  sections  can  be  worked  separately. 


FIG.  170.— Bausch  and  Lornb's  'Grand  Model'  (1897). 


214     THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

We  note  one  admirable  feature  of  the  mechanical  finish  of  the 
microscopes  of  this  firm,  which  is,  that  they  avoid  sharp  angles  and 
knifelike  edges  to  all  their  instruments.  This  looks  a  trine,  but  the 
use  of  the  microscope  with  saprophytic,  pathogenic,  or  other  infective 
material  requires  the  utmost  caution  that  the  skin  of  the  hands 
should  be  unbroken,  and  there  can  be  but  little  doubt  that  all 
unconsciously  the  edges  and  corners  of  microscopes  finished  to  the 
just  pride  of  the  mechanic  do  often  break  the  skin,  and  are  wisely 


m 


FIG.  171.— Bausch  and  Lomb's  sub-stage, 


to  show  construction. 


and  happily  worked  into  rounded  edges  in  the  instruments  of  these 
distinguished  makers,  and,  we  may  add,  without  the  slightest  loss 
of  that  appearance  of  high  finish  which  has  always  been  correlative 
with  the  manufacture  of  microscopes. 

If  we  now  look  at  the  No.  1  stand  of  the  Spencer  Lens  Company, 
of  Buffalo,  N.Y.,  we  shall  find  again  that  the  model  of  Oberhauser 
is  adhered  to  and  the  instrument  is  of  the  third  class.  This 
microscope  is  illustrated  in  fig.  172.  It  is  beautifully  made,  and 
the  horseshoe  base  has  a  still  longer  '  claw '  than  those  of  Bausch, 


FIG.  172. — The  Spencer  Lens  Company's  Continental  form  No  1  (1896, 


FIG.  173. 


Watson's  Edinburgh  Student's ;  stand  ;  H  '  (with  horseshoe  foot  1889, 
with  tripod  foot  1893). 


RECENT   AMERICAN   MICROSCOPES 


217 


to  /give  the  stability  required  in  utilising  the  hinged  joint  for 
inclination  of  the  body,  which  stands  on  a  strong  uiiial  pillar. 
The  sub-stage  is  movable  by  a  quick  screw  ;  in  other  features  it 
resembles  the  majority  of  the  microscopes  of  the  type  to  which  it 
belongs  •  it  is,  however,  distinguished  by  rounded  in  contrast 
to  sharp  and  pointed  corners  and  edges  ;  and,  although  the  form 
presented  has  a  plain  stage  with  clips,  it  can  be  furnished  with 
a  circular  revolving  centring  stage,  or  with  an  '  attachable  '  stage 
made  by  the  Spencer  Lens  Company,  having  all  the  advantages  of 
the  several  forms  of  these  pieces  of  apparatus  already  described. 


FIG.  174.— Baker's  Model,  No.  2  (1898). 

We  note  with  some  surprise  that  such  accomplished  manu- 
facturers and  opticians  have  indicated,  so  far  as  we  can  discover,  110 
advance  in  their  sub-stage  condenser  beyond  that  of  the  now  old 
achromatic  of  Abbe,  and  that  there  is  no  evidence  before  us  of  their 
employment  of  a  sub-stage  fine  adjustment,  both  of  which  have  been 
found  of  such  great  practical  value  in  England,  and  which  have  been, 
as  we  shall  shortly  show,  adopted  for  the  more  critical  microscopical 
work  by  the  Messrs.  Zeiss,  the  leading  optical  firm  of  the  Continent. 


2l8     THE   HISTOKY   AND   DEVELOPMENT   OF   THE   MICEOSCOPE 

Second-class  microscopes  are  made  in  great  variety  by  English 
makers. 

One  of  the  finest  examples  of  this  class  of  microscope  at  present 
brought  within  the  reach  of  the  average  student's  means  is  that 
known  as  the  *  Edinburgh  Student's  Microscope  "  H,"  '  by  the  firm 
of  Watson  and  Sons.  It  is  the  most  complete  of  a  series  of  similar 
stands  varying  in  cost  and  completeness.  It  is  illustrated  in 
fig.  173,  where  it  will  be  seen  that  it  has  the  first  prime  requisite,  a 
rigid  foundation  combined  with  lightness — a  tripod  having  a  spread 
of  7  inches — and  it  is  also  possessed  of  a  well-constructed  mechanical 
stage  which  is  built  with  the  instrument,  an  advantage  over  the 
best  'attachable'  stage. 

It  is  essentially  a  student's  microscope,  and  although  of  so  low  a 
price  is  not  only  a  specimen  of  the  best  workmanship,  but  is  also 
extremely  complete  and  represents  an  advanced  type  of  construction 
capable  of  doing  all  ordinary  and  much  experimental  work. 

Belonging  to  this  class  is  an  instrument  by  Baker  known  as 
his  Model,  No.  2.  It  is  smaller  than  the  'A'  stand  of  the  same 
type  and  is  simplified,  but  is  capable  of  doing  the  most  refined  and 
critical  work.  It  is  illustrated  in  fig.  174.  The  coarse  and  fine 
adjustments  are  the  same.  The  mechanical  stage  has  rectangular 
movements  of  one  inch ;  the  Turrell  arrangement  is  not  adopted  ; 
but  the  whole  stage  can  be  rotated  through  an  arc  of  300°.  The 
sub-stage  has  diagonal  rack  and  pinion  focussing  movements  with 
centring  screws,  and  can  be  supplied  with  every  improvement 
applying  to  the  adjustment  of  the  sub-stage.  Taking  this  instru- 
ment as  a  whole — the  thoroughly  practical  character  of  the  model, 
the  high  quality  of  the  workmanship,  the  fact  that  it  will  take  all 
the  optical  apparatus  of  the  best  model,  and  that  all  fittings  are 
sprung  and  possessed  of  adjusting  screws  to  compensate  for  wear — 
we  have  in  this  microscope  one  of  the  very  best  of  its  class. 

Powell  and  Lealand  make  an  instrument  of  this  class,  having 
a  quality  of  work  not  second  even  to  their  large  stand.  It  is 
illustrated  in  fig.  175.  The  tube  length  is  the  same,  but  the  stage 
and  the  foot  are  smaller  than  in  the  large  instrument.  There  is 
no  rotary  movement  to  the  sub- stage,  and  its  centring  is  done  by 
the  crossing  of  sectors  and  not  lines  at  right  angles ;  but  this  is  in 
no  way  a  defect.  All  the  movements  and  adjustments  are  other- 
wise as  in  No.  1. 

Baker,  of  Holborn,  makes  a  very  admirable  and  useful  instru- 
ment of  this  class  known  as  his  D.P.H.  microscope,  No.  1.  It 
has  a  diagonal  rack  and  pinion  coarse  movement,  a  micrometer  screw 
and  lever  fine  adjustment,  giving  a  movement  of  3-^5-  of  an  inch  for 
each  revolution  of  the  milled  head  ;  a  draw- tube,  every  10  mm.  of 
which  is  engraved  with  a  ring,  extending  to  250  mm.  and  closing 
to  150  mm.,  thus  allowing  the  use  of  either  English  or  Continental 
objectives ;  it  possesses  a  mechanical  stage  giving  a  movement  of  25  mm. 
in  either  direction,  graduated  to  ^  mm. ;  the  milled  head  of  the  trans- 
verse motion  is  below  the  level  of  the  top  plate,  and  as  the  other  is 
removable  large  culture  plates  can  be  examined,  the  distance  from 
optic  axis  to  limb  (2^  in.)  allowing  of  their  easy  manipulation  ;  the 


BAKER'S   NEW   MICROSCOPES 


2I9 


top  plate  is  provided  with  three  adjustable  stops,  so  that  the  centre  of 
a  3  x  1  or  3  x  H  slip  is  identical  with  the  optic  axis  when  both 
the  rectangular  movements  are  at  the  centre  of  their  travel,  thus 
enabling  any  desired  field  to  be  recorded ;  the  stage  clips  are 


FIG.  175  (1852). 

mounted  on  two  of  these  stops,  all  of  which  are  removable ; 
a  centring  sub-stage  of  universal  size  (1'527  in.)  with  diagonal  rack 
and  pinion  focussing  adjustment,  plane  and  concave  mirrors  ;  the 
whole  mounted  on  a  solid  tripod  stand,  with  a  bracket  to  support  the 


220     THE    HISTORY  AND  DEVELOPMENT   OF  THE   MICROSCOPE 

instrument  in  a  horizontal  position  for  photo-micrographic  work. 
The  microscope  is  illustrated  in  fig.  176. 

A  modification  of  this  instrument  was  brought  out  as  these 
pages  are  passing  through  the  press,  which  is  entitled  to  rank  as  a 
first-class  instrument.  It  is  known  as  the  B.M.S.  1'27  gauge 
microscope,  and  is  illustrated  in  fig.  177.  It  has  a  diagonal  rack 
and  pinion  coarse  movement,  and  a  micrometer  screw  and  lever 
fine  adjustment  giving  a  movement  of  Oil  mm.  (-^5  in.)  for  each 
revolution  of  the  screw,  the  milled  head  of  which  is  divided  into 


FIG.  176.— Baker's  D.P.H.  stand  No.  1  (1899), 

ten  parts,  each  division  being  numbered.  It  also  possesses  two  draw- 
tubes  engraved  in  mm.,  every  tenth  numbered,  one  of  which  is 
provided  with  rack  and  pinion  adjustment,  so  that  objectives  may 
be  corrected  for  the  thickness  of  the  cover  glass,  &c.,  by  the  alteration 
of  the  tube  length  ;  these  draw-tubes  extend  to  250  mm.,  and  close  to 
120  mm.,  either  English  or  Continental  objectives  can  be  used  ;  this 
microscope  has  a  rotating  mechanical  stage  giving  a  movement  of 
25  mm.  (1  in.)  in  either  direction  graduated  to  J-  mm.  (?L  in.)  • 
the  milled  head  of  the  transverse  motion  is  below  the  level  of  the 


BAKER'S  LATEST  MICROSCOPE 


221 


top  plate,  and  the  other  being  removable  a  large  flat  stage  becomes 
available  if  required  ;  the  top  plate  is  provided  with  three  stops, 
adjustable,  so  that  the  centre  of  a  76  mm.  x  25  mm.  (3  in.  x  1  in.) 
or  76  mm.  x  38  mm.  (3  in.  x  H  in.)  slip  is  identical  with  the  optic 
axis  when  both  the  rectangular  movements  are  at  the  centre  of  their 
travel,  thus  enabling  any  desired  field  to  be  recorded  ;  the  stage 


FIG.  177.— Baker's  R.M.S.  1'27  gauge  microscope  (1900). 

clips  are  mounted  on  two  of  these  stops,  all  of  which  are  removable. 
It  has  a  centring  sub-stage  provided  with  diagonal  rack  and  pinion 
focussing  movement,  and  a  fine  adjustment,  the  milled  head  of  which 
is  so  placed  that  both  adjustments  can  be  conveniently  controlled 
without  shifting  the  hand,  and  it  is  provided  with  plane  and  con- 
cave mirrors,  and  the  microscope  is  mounted  upon  a  solid  tripod 
stand,  with  a  bracket  to  support  the  instrument  in  a  horizontal 
position  for  photo-micrographic  work. 


222      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

All  the  fittings  are  sprung  and  have  adjusting  screws  to  compen- 
sate for  wear. 

Coming  now  to  Third-class  microscopes,  we  note  that  the  dis- 
tinguished American  firm,  Bausch  and  Lomb,  make  a  very  useful 
instrument  which  must  be  placed  in  this  class.  It  is  intended  as 


FIG.  178.— Bausch  and  Lomb's  C.A.8.  microscope  (1897). 

a  high-class  laboratory  instrument  for  advanced  work  and  for  use 
in  independent  researches.  It  is  designated  by  the  firm  as  the  C.A.S. 
It  has  a  large  stage,  but  in  our  judgment  this  would  be  greatly  im- 
proved by  being  furnished  with  the  horseshoe  opening  so  valu- 
able for  hand  focussing  as  a  preliminary  in  the  use  of  high  powers 
and  immersion  lenses.  Of  course  the  mechanical  stage  of  the 


THIRD-CLASS   AMERICAN   MICROSCOPES 


223 


firm  can  be  added.     The  sub-stage  is  the  new  and  complete  one   of 
the  makers,  arranged  for  doing  critical  work  ;  the  fine  adjustment 


I 


FIG.  178A.— Reichert's  'Austrian  '  Baugh  stand  (1899). 


is  by  micrometer  screw  ;  the  weight  of  the  body  is  balanced,  the 
makers  tell  us,  by  a  spiral  spring  which,  they  believe,  subjects  the  fine 


224      THE   HISTOEY   AND   DEVELOPMENT   OF   THE   MICEOSCOPE 

micrometer  screw  only  to  the  friction  of  the  adjustment — and,  of 
course,  it  is  to  be  noted  that  the  screw  is  not  an  extremely  fine 
one  ;  and  the  makers  have  evidence  of  the  durability  of  the  adjust- 
ment, as  after  five  years  of  use  they  have  had  no  single  instance  of 
its  breakdown.  The  coarse  adjustment  is  by  diagonal  rack  and 
pinion  ;  the  draw-tube  is  graduated.  It  is  beautifully  made,  and  is 
by  no  means  an  expensive  instrument.  We  illustrate  it  in  fig.  178. 

A  well-made  and  remarkable  little  instrument  of  the  class  \\c 
are  considering  is  manufactured  by  Reichert,  of  Vienna,  known  as  the 
Austrian  stand.  It  is  illustrated  in  fig.  178A.  It  is  the  most 
modified  of  all  the  microscopes  we  know  based  on  the  Continental 
model ;  it  certainly  approximates  in  several  points  to  the  English 
type.  It  has  a  specially  extended  and  steady  horseshoe  foot,  and  is 
the  only  strict  Continental  form  with  the  axis  so  high  up.  The  re- 
sult is  that  the  body  is  balanced  when  in  a  horizontal  position.  The 
coarse  adjustment  is  by  spiral  rack  and  pinion  with  milled  heads. 
The  fine  adjustment  is  Reichert' s  recent  patent,  giving  extreme 
delicacy  to  the  movement,  and  having  a  movable  pointer,  i,  for 
reading  divisions  on  the  micrometer  screw.  It  is  provided  with  a 
double  rack  draw-tube  shown  at  B,  it  carries  the  Abbe  condenser  in  a 
sub-stage  that  focusses  by  a  screw  at  the  side,  and  centres  by  the 
screw-heads,  a,  a!.  In  its  most  complete  form  it  is  remark; il»ly 
low-priced,  and  certainly  will  meet  a  demand,  especially  as  the 
English  method  of  compensation  for  wear  and  tear  is  adopted. 
This,  indeed,  is  the  case  with  all  but  the  lowest-priced  instriim<>nt.s 
of  this  maker,  and  we  believe  him  to  be  the  only  Continental 
manufacturer  who  has  adopted  the  sprung  slots  and  screws  so  long 
used  with  success  by  English  makers  for  compensating  wear.  We 
should  have  suggested  slotting  the  edges  of  the  stage  for  sliding 
the  object-holder  or  ledge,  but  we  learn  from  the  maker  that  this 
is  to  be  done  in  all  future  instruments;  all  but  the  smallest  stands 
Reichert  is  willing  to  provide  with  English  pattern  sub-stages 
fitted  with  centring  screws  of  the  standard  size,  and  condensers  \\\ •<• 
mounted  to  suit  these. 

Another  instrument  of  the  same  class  and  general  designation, 
made  by  Messrs.  Watson  and  Sons,  and  distinguished  as  '  G,'  is  shown 
in  fig.  179.  It  is  identical  in  build  with  the  C  model,  but  the 
stage  is  plain,  and  it  has  only  a  tube  fitting  for  a  sub-stage  appa- 
ratus; the  workmanship  is  of  the  same  order,  the  movements  as 
delicate  and  true,  the  adjustments  as  reliable,  but  the  price  is  only 
one-half  that  of  the  more  complicated  form. 

Amongst  the  same  class  of  instruments  must  be  placed  another 
by  Messrs.  Swift  and  Son.  It  is  known  as  an  '  Improved  "  Wale's  " 
Microscope.' 

Mr.  George  Wale,  of  America,  devised  in  1879  a  plan  of  great 
merit  for  the  stands  of  microscopes.  The  '  limb  '  which  carries  the  body 
and  the  stage,  instead  of  being  swung  by  pivots — as  ordinarily — on 
the  two  lateral  supports  (so  that  the  balance  of  the  microscope  is 
greatly  altered  when  it  is  much  inclined),  has  a  circular  groove  cut 
on  either  side,  into  which  fits  a  circular  ridge  cast  on  the  inner  side 
of  each  support,  as  shown  in  fig.  180.  The  two  supports,  each 


WATSON'S 


MICEOSCOPE 


225 


having  its  own  fore-foot,  are  cast  separately  (in  iron),  so  as  to  meet 
to  form  the  hinder  '  toe,'  where  they  are  held  together  by  a  strong 
pin  ;  while  by  turning  the  milled  head  on  the  right  support  the  two 


FIG.  179.— Watson's  Edinburgh  Student's ;  stand  '  G  '  (1898). 


226      THE   HISTOKY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

are  drawn  together  by  a  screw,  which  thus  regulates  the  pressure 
made  by  the  two  ridges  that  work  into  the  two  grooves  on  the  limb. 
When  this  pressure  is  moderate,  nothing  can  be  more  satisfactory 
than  either  the  smoothness  of  the  inclining  movement  or  the 
balancing  of  the  instrument  in  all  positions;  while,  by  a  slight 


FIG.  180.— Swift's  improved  '  Wale's '  microscope  (1881  and  1883). 


tightening  of  the  screw,  it  can  be  firmly  fixed  either  horizontally, 
vertically,  or  at  any  inclination.  The  '  coarse '  adjustment  is  made 
by  a  smooth-working  rack  ;  the  fine  adjustment  is  Swift's  patent 
described  on  p.  172  (fig.  135),  and  the  attachable  mechanical  stage  of 
this  firm  can  be  readily  added  (as  in  fig.  180),  but  in  the  best  and 


LEITZ'S   ENGLISH  FORM  OF  MICROSCOPE 


227 


most  complete  form  of  the  instrument  a  large  mechanical  stage   is 
fitted,  and  sub-stage  apparatus  supplied. 

Leitz,  of  Wetzlar,  provides  a  very  useful  instrument  of  the  same 


FIG.  181.— Leitz's  IA  stand  (1898). 


228      THE  HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

class.  It  has  a  tripod  base  on  the  English  model,  and  is  a  thoroughly 
steady  instrument ;  it  has  rack  and  pinion  movement  to  the  coarse 
adjustment,  and  sub-stage  ;  the  draw -tube  has  a  mm.  scale,  and  a  fine 
adjustment  of  the  usual  Continental  type,  and  all  the  latest  adaptations 
for  sub- stage  illumination.  The  instrument  in  its  simplest  form  is 
remarkably  low-priced,  and  the  more  important  apparatus  can  be 
added  to  it  as  required.  It  is  illustrated  in  fig.  181. 

Beck's  third-class  microscope  is  shown  in  fig.  182.  It  has  a  good 
tripod  foot  with  a  single  pillar.  The  Jackson  model  is  used,  but 
a  peculiar  fine  adjustment  is  employed,  tire  lever  being  placed  below 
the  stage,  the  position  of  the  screw  being  immediately  behind  the 
pillar  which  supports  the  limb,  and  where  it  is  easy  of  access.  The 
body  is  not  affected  by  vibration  when  it  is  touched.  The  lever  is 
of  the  second  order,  and  it  draws  down  the  body  limb  and  coarse 
adjustment.  In  fact,  save  in  its  'fine  adjustment,  this  form  ap- 
proximates somewhat  to  the  Continental  model.  The  fine-adjust- 
ment lever  is  rather  short,  but  it  will  be  found  to  be  much  steadier 
and  slower  than  the  direct-acting  screw. 

The  stage  is  plain,  without  mechanical  movements  ;  but  it  has  a 
movable  glass  stage  over  the  principal  stage ;  to  this  the  slip  is 
clipped,  and  the  whole  super-stage  of  glass  is  moved  with  ease 
over  a  fair  area.  The  aperture  in  the  glass  stage  is  not  large 
enough  ;  it  should  be  cut  right  through  to  the  front,  which  would 
much  increase  its  usefulness. 

This  instrument  also  has  a  sub-stage  with  rack  and  centring 
movements. 

Swift  and  Son's  earlier  third-class  microscope  in  its  most 
suitable  form  dates  from  about  the  time  of  the  vertical  lever  fine 
adjustment  patented  by  that  firm  (q.v.)  It  was  first  made  from  the 
designs  of  Mr.  E.  M.  Nelson,  and  it  presented  three  distinctive 
features  : — 

(1)  The  milled  head  of  the  fine  adjustment  was  placed  on  the 
left-hanc1  side  of  the  limb. 

(2)  The  stage   was  of   a  horseshoe   form,    the   aperture   being 
entirely  cut  out  to  the  front  of  the  stage  ;  and 

(3)  The  body-tube,  which  was  of  standard  size,  viz.  8J  inches, 
was  made  in  two  pieces,  which  not  only  secured  portability,  but  also 
permitted  the  use  of  both  long  and  short  tubes. 

This  instrument  is  illustrated  in  fig.  135.  It  was  also  possessed 
of  a  cheaply  made  and  fairly  good  centring  sub-stage,  to  carry 
Powell  and  Lealand's  dry  achromatic  combination  fitted  with  a  turn- 
out rotary  arm  to  carry  stops.  The  sub-stage  was  made  by  adapting 
Swift's  centring  nose-piece,  and  providing  it  with  a  rack  and  pinion 
focussing  arrangement,  as  illustrated  in  fig.  183.  There  was  also  a 
graduated  stage-plate  and  sliding  bar,  a  plan  devised  by  Mr. 
Wright  for  a  finder.  The  eye-pieces  were  provided  with  rings,  like 
Powell  and  Lealand's,  outside  the  tube  to  govern  the  depth  which 
each  should  slide  into  the  draw-tube,  by  which  means  the  diaphragm 
is  in  the  same  place  whatever  the  depth  of  the  eye-piece  employed, 
and  it  was  constructed  to  do  critical  work  with  the  highest 
powers. 


FIG.  182.— Messrs.  R.  and  J.  Beck's  third-class  microscope  (1888). 


230     THE   HISTOKY  AND    DEVELOPMENT   OF   THE   MICROSCOPE 

Another  form  of  tJds  instrument  has  more  recently  been  intro- 
duced by  the  firm  of  Chas.  Baker,  of  Holborn,  London.  It  arose  in 
a  suggestion  by  Mr.  Nelson  that  this  form  should  be  adapted  to  the 
Campbell  differential  screw  fine  adjustment,  making  a  good  quality 
third-class  microscope.  It  should  be  noted  that  the  differential 
screw  permits  of  slow  action  being  obtained  by  means  of  coarse 
threads ;  it  is  therefore  very  strong.  In  the  ordinary  Continental 
form  of  direct-acting  fine-adjustment  screw,  if  the  motion  is  slow, 
the  thread  must  be  fine.  Hence  in  forms  where  the  fine  adjustment 
is  made  to  lift  the  body,  the  differential  screw  is  of  great  value. 

Further,  it  proved  on  testing  that  the  Campbell  differential  screw 
was  equal  to  the  most  critical  work,  and  could  be  used  in  photo- 
micrography. As  a  result  several  additions  were  made,  such  as 
rack  and  pinion  focussing  and  rectangular  movements  to  the  sub- 
stage  and  a  rack-work  arrangement  to  the  draw-tube.  Subse- 
quently a  larger  and  heavier  instrument  was  made,  having  a  J  inch 
more  of  horizontal  height.  In  this  model  the  milled  head  of  the 
differential  screw  is  placed  below  the  arm,  instead  of  above  it,  which 

is  an  improvement  for 
photo-micrographic  pur- 
poses, and  no  special 
detriment  in  ordinary 
work ;  and,  if  required, 
a  differential-screw  fine 
adjustment  can  be  fitted 
to  the  sub-stage.  A 
rotary  stage  is  also  some- 
times put  to  this  instru- 
ment, but  those  which 
we  have  seen  have  not 
FIG.  183.— Centring  nose-piece  used  as  sub-stage  given  the  aperture  suffi- 
cient dimensions  for 
modern  focussing. 

This  instrument  in  its  complete  form,  as  suggested  by  Mr.  Nelson 
and  devised  by  Baker,  gave  origin  to  an  entirely  new  group  of 
microscopes,  which  aimed  chiefly  at  supplying  the  student  with 
relatively  inexpensive  instruments,  but  which  at  the  same  time 
should  possess  all  the  qualities  and  be  capable  of  receiving  all  the 
apparatus  needful  for  an  efficient  use  of  the  microscope.  One  of  the 
higher  forms  arising  in  this  new  departure  is  the  instrument  shown 
at  fig.  177,  and,  with  the  Campbell  screw  fitted  behind  the  mirror 
for  the  fine  adjustment  of  the  condenser,  is  a  very  attractive  and 
useful  microscope,  and  may  be  safely  recommended  to  the  amateur 
and  the  student. 

Two  microscopes  by  Ross  certainly  deserve  the  attention  of  the 
student  seeking  a  reliable  instrument  belonging  to  the  class  we  are 
considering.  They  are  both  known  as  '  Ross's  New  Bacteriological 
Microscope.'  The  work  of  this  long  -established  firm,  it  is  needless 
to  say,  is  of  the  very  finest  quality  ;  and  these  microscopes  are  pro- 
vided with  all  the  required  adjuncts  for  the  work  they  specify.  The 
stage  is  of  horseshoe  form  ;  the  fine  adjustment  is  sensitive  and  firm. 


ROSS'S   RECENT  MICROSCOPES 


231 


The  principal  difference  between  the  two  instruments  is  in  their 
respective  stands.  The  one  shown  in  fig.  184  gives  a  wider  spread 
to  the  tripod  base  than  usual,  securing  greater  stability  .;  but  this  does 
not  involve  great  space  in  packing,  because  the  hind  '  toe '  of  the 


FIG.  184  — Ross's  new  (tripod)  bacteriological  microscope  (1898). 

tripod  is  made  to  fold  forward  between  the  two  fixed  front  toes  when 
not  in  use. 

The  other  similar  instrument  is  on  a  circular  foot,  to  which  is 
screwed  a  stout  supporting  pillar ;  the  upper  part  is  attached  to 
this  by  a  substantial  compass-joint ;  but  the  pillar  is  fixed  on  the  mar- 
gin of  the  ring,  thus  bringing  the  whole  weight  centrally  upon  the 


232     THE  HISTORY  AND   DEVELOPMENT  OF  THE   MICROSCOPE 

foot  when  the  instrument  is  in  an  upright  position.  When  inclined, 
the  centre  of  gravity  is  again  brought  directly  over  the  foot,  as 
shown  in  fig.  185,  by  rotating  the  pillar  upon  a  reliable  fitting  at 
its  base,  so  that  absolute  steadiness  is  secured.  This  is  a  revival 


FIG.  185. — Boss's  new  bacteriological  microscope  (1894). 

of  an  old  form  made  in   1760  by  J.  Cuff,  adapted  by  A.   Ross  in 
1842,  and  now  again  used  by  the  same  firm  (vide  fig.  128). 

Ross   also   manufactures   an    '  Educational '    microscope  having 
considerable  merit,  which  may  fairly  be  placed  in  this  class.     It 


MICROSCOPES   OF  THE   FOURTH   CLASS 


233 


is  presented,  on  a  small  scale,  in  fig.  186.  It  is  admirably  made, 
and  provides  all  that  is  required  in  coarse  and  fine  adjustments  ; 
it  is  also  provided  with  admirable  sub-stage  arrangements,  and  is 
placed  on  a  stand  that,  while  it  is  of  horseshoe  pattern,  has  the 
hind  'toe'  lengthened  considerably,  and  is  made  so  that  the  foot 
can  reverse  as  in  the  illustration,  and  lock,  thus  making  a  perfect 
balance  for  the  body,  however  it  may  be  inclined.  This  admirably 
made  instrument  is  considerably  under  51.  in  cost. 

Beck's  '  British  Student's  '  microscope  is  of  this  class,  as  is  also  the 
*  Star  '  microscope  by  the  same  makers.  The  former  has  a  firmly  made 
tripod,  as  fig.  187,  representing  this  instrument,  shows.  It  has  a 
spiral  rack  and  pinion  coarse  adjustment,  a  fine-adjustment,  a  draw- 
tube  with  mm.  scale,  and  a  focussing  sub-stage  which  swings  out  when 
not  in  use.  The  present 
Editor  can  speak  highly  of 
this  instrument  for  elementary 
class  work,  and  with  good 
workmanship  its  price  is  ex- 
ceedingly low.  The  *  Star  ' 
microscope  is  also  a  very  re- 
markable instrument,  suffi- 
ciently so  to  justify  us  in 
departing  from  a  rule  to 
point  out  that  with  two  eye- 
pieces, two  objectives  —  a 
J-inch  and  a  ^-inch  —  and  an 
iris  diaphragm,  the  whole, 
placed  in  a  cabinet,  is  sold  for 
U.  15s. 

We  come  now  to   micro- 
scopes of  the  fourth  class. 

A  small,  compact,  and 
thoroughly  useful  microscope, 
specially  adapted  for  medical 
students  and  Biological 
Schools,  is  made  by  Swift  and 
Son,  and  known  as  their 
'  New  Histological  and  Physio- 
logical Microscope.'  In  its  simplest  form  it  is  shown  in  fig.  188. 
The  stand  is  a  firm  tripod,  the  optical  tube  slides  in  a  cloth-lined 
fitting,  the  fine  adjustment  may  be  the  differential  screw  actuated 
by  a  large  milled  head,  and  capable  of  work  with  at  least  a  ^th-inch 
objective.  It  is  beautifully  swung,  and  is  firm  in  any  position. 
The  stage  is  large,  and  has  the  horseshoe  opening.  There  are 
several  grades  of  this  instrument,  involving  more  or  less  complexity 
and  apparatus  ;  but  it  was  designed  to  meet,  and  we  believe  does 
meet,  the  needs  of  students  who  want  a  strong,  practical,  and  well- 
equipped  instrument  at  a  very  moderate  price. 

Another  instrument  of  this  class  deserving  the  highest  commen- 
dation, and  offering  the  student  much  more  for  the  outlay  involved 
than  we  could  have  thought  possible  twenty  years  ago,  is  'The 


FIG. 


.  —  Eoss's  educational  microscope 


234      THE   HISTOKY  AND   DEVELOPMENT   OF   THE   MIOROSCOPE 

Fram'  microscope  of  Messrs.  Watson  and  Sons.  We  illustrate  it  in 
fig.  189.  It  is  strong  and  rigid,  and  its  workmanship  is  of  the 
highest  order.  It  has  a  completely  steady  tripod  foot  with  a  spread 


FIG.  187. — Beck's  British  student's  microscope  (1898). 


A    STAND   BY   SWIFT 


235 


of  7  inches,  and  its  steadiness  is  unaffected  in  whatever  position  the 
body  may  have  to  be  inclined.     The  coarse  adjustment  is  a  diagonal 


FIG,  188. — Swift's  histological  and  physiological  microscope  (1894). 


236      THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

rack  and  pinion,  while  the  fine  adjustment  is  the  now  celebrated 
lever   employed  by  this  firm.     One  revolution  of  the  milled  head 


FIG.  189.  —  Watson's  '  Fram  '  microscope  (1898). 


moves  the  body  the  ^^th  of  an  inch.     As  we  have  seen  (p.  170, 
fig.  132),  this  adjustment  is  sound  in  principle,  and  in  practice  all 


FIFTH   AND   SIXTH   CLASSES    OF  MICROSCOPES  237 

that  need  be  desired.  The  stage  has  the  horseshoe -shaped  aperture. 
The  sub-stage  fitting,  as  shown  in  the  illustration,  may  be  turned 
aside  out  of  the  optical  axis,  and  a  compound  sub-stage  may  be  made 
with  the  instrument  if  desired.  Throughout,  the  working  parts  are 
sprung,  and  wear  may  be  compensated  by  adjusting  screws.  We 
cannot  speak  too  highly  of  the  enterprise  and  skill  shown  in  the  design 
and  manufacture  of  this  instrument ;  and  yet  the  student  will  find 
that,  good  as  it  is,  it  is  one  of  the  least  costly  instruments  of  its  class. 

There  is  a  microscope  manufactured  by  Messrs.  Zeiss,  known  as 
'  Stand  VI.  A,'  which  comes  to  about  the  same  cost  as  the  above,  and 
which  we  illustrate  in  fig.  190.  It  is  of  course  a  strictly  Continental 
form,  having  a  fixed  stage  3J  ins.  square.  The  coarse  adjustment  is 
by  rack  and  pinion,  and  the  fine  adjustment  is  the  usual  micro- 
meter screw  of  these  makers.  The  stand  is  inclinable,  and  it  is 
provided  with  mirrors  and  a  cylinder  diaphragm  which  slides  in  a 
sleeve  fixed  belowT  the  stage  capable  of  receiving  the  illuminating 
apparatus.  It  is,  of  course,  made  with  the  accuracy  and  good 
quality  of  workmanship  for  which  this  firm  is  noted. 

Fifth  and  sixth  classes  of  microscopes  arc  made  by  the  best 
makers,  and  it  is  a  little  notable  that  the  best  of  these  classes  was  made 
by  the  late  Hugh  Powell,  whose  maxim  was  that  a  microscope  with 
only  a  good  coarse  adjustment  was  to  be  preferred  to  one  having  an 
indifferent  fine  adjustment  with  a  sliding  tube  for  the  coarse  adjust- 
ment. 

This  stand  is  of  cast  iron,  with  a  flat  tripod,  having  a  single 
pillar  to  which  is  jointed  the  Jackson  body.  The  focussing  is 
admirable  ;  the  stage  is  of  an  excellent  form,  being  4^  x  3^  inches, 
and  is  supplied  with  a  beautifully  made  sliding  ledge,  which  will 
move  easily  and  firmly  with  pressure  from  one  side  only. 

The  stage  is  fastened  to  the  upper  side  of  two  brackets  which 
are  cast  in  one  piece  with  the  limb  ;  on  the  under  side  of  these 
brackets  there  is  another  plate  which  holds  the  sub-stage  tube. 

This  instrument  is  supplied  with  large  plane  and  concave 
mirrors ;  and,  considering  that  it  constitutes  a  sixth  class  of 
microscope,  has  very  much  in  its  favour  as  a  secondary  instrument 
for  the  work-table.  Like  all  these  makers'  instruments,  the  feet  are 
plugged  with  cork  ;  and  we  know  of  some  of  these  microscopes  that 
have  been  in  use  for  forty  years,  and  are  still  the  trusted  'journey- 
men '  instruments  of  mounters  and  other  workers  of  various  orders 
in  many  departments  of  microscopy. 

Some  of  the  modern  forms  of  these  two  classes  of  microscope 
deserve,  on  behalf  of  beginners  with  limited  means,  some  considera- 
tion. A  thoroughly  good  but  extremely  simple  microscope  of  the 
fifth  class  is  made  by  Watson  and  Sons ;  it  is  illustrated  in  fig.  191. 
It  was  designed  for  educational  purposes  ;  the  workmanship  is  of  the 
finest  quality,  but  the  instrument  is  not  provided  with  a  fine 
adjustment ;  it  relies  on  a  very  perfectly  made  diagonal  rack  and 
pinion  coarse  movement.  From  practical  use  we  can  speak  in  the 
highest  terms  of  the  delicacy  of  this  focussing  arrangement,  with 
which  we  have  with  ease  used  powers  up  to  ^  inch,  and  often  have 
used  it  with  a  -fa-in.  objective.  The  stage  is  large,  the  body  has  a 


238      THE  HISTORY   AND   DEVELOPMENT   OF  THE  MICROSCOPE 

draw- tube,  can  be  inclined,  and  it  is  a  steady  useful  microscope.  It 
can  be  obtained  complete  in  a  case  with  one  eye-piece  for  the  sum 
of  21.  7s.  Qd. 


FIG.  190.— Zeiss's  stand  VI.A  (1898). 


LOW-PRICED   STANDS 


239 


Bausch  and  Lomb  manufacture  an  instrument  which  abandons 
the   coarse  adjustment,    but    provides   a    fine   adjustment    of  good 


FIG.  191. — Watson's  school  microscope  (1899). 


240      THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

quality,  and  is  thoroughly  well  made,  its  object  being  to  meet  the 
wants  of  schools  and  elementary  workers.     We   believe,  however, 


FIG.  192.— Reicliert's  stand  No.  15  (1890). 

for  many  reasons,  that  it  is  better  to  rely  on  an  excellent  rack  and 
pinion  coarse  adjustment  for  such  a  purpose.  This  instrument  is 
remarkable  as  meeting  a  distinct  demand,  for  though  of  excellent 


EECENT   AMERICAN   MICROSCOPES 


241 


workmanship  it  is  sold  for  twenty  shillings.     We  illustrate  it   in 
fig.  193. 


Fiu.  193. — Bausch  and  Lomb's  lowest-priced  microscope  (1897). 

Reichert,  of  Vienna,  manufactures  an  instrument  of  the  same 
class  with  a  good  coarse  adjustment  only,  built  on  a  tripod,  and  of 
almost  equally  low  price.  But  amongst  the  sixth  class  of  micro- 

E 


242      THE   HISTORY  AND   DEVELOPMENT  OF  THE  MICROSCOPE 

scopes  none  is  more  remarkable  for  its  strength,  good  form,  and 
excellent  finish  than  the  one  we  show  in  fig.  194,  made  by  Leitz.  Its 
coarse  adjustment  is  capable  of  doing  very  delicate  work,  and  it  is  a 
thoroughly  steady  instrument,  and  is  admirably  adapted  to  elemen- 


FIG.  194. — Leitz's  school  microscope. 

tary  work  and  school  use,  and,  whilst  its  finish  and  work  are  admirable, 
it  is  sold  for  II. 

A  really  beautiful  instrument  of  the  same  class  is  made  by 
Reichert,  designated  '  Stand  No.  15,'  which  is  illustrated  in  fig.  192. 
It  is  admirably  made,  and  the  maker,  as  we  think,  wisely,  has  thrown 


A   GOOD   LOW-PKICED   MICKOSCOPE   BY   LEITZ 


243 


the  best  possible  work  into  a  spiral  rack-and-pinion  coarse  adjust- 
ment which  works  with  great  accuracy  and  smoothness,  and  has 
dispensed  with  a  fine  adjustment.  Its  construction  is  neat,  but  it  is 


FIG.  195.— Powell  and  Lealand's  portable  microscope  (1848). 

one  of  the  most  rigid  of  this  class  of  microscope  which  we  have  seen 
or  used  ;  this  instrument  is  sold  for  twenty-five  shillings.  But  the 
maker  has  adopted  Mr.  Xelson's  plan,  using  a  Steinheil  magnifier  to 
be  mounted  as  a  sub-stage  condenser,  and  if  a  simple  iris  diaphragm 


FIG.  196. — Swift's  portable  histological  microscope  (1894). 


POETABLE   MICEOSCOPES 


245 


be  used  with  this,  there  are  very  few  but  will  be  astonished  at  the 
beautiful  results  attainable.  Certainly,  since  the  last  edition  of  this 
book  was  published,  large  and  successful  efforts  have  been  made  to 
supply  to  those  who  need  them  cheap  but  thoroughly  good  micro- 
scopes. 

Portable  Microscopes. — Microscopes  that  may  be  readily  taken 
from  place  to  place,  and  which  are  yet  provided  with  the  arrange- 
ments required  for  using 
the  principal  apparatus,  are 
of  importance  in  some  in- 
vestigations, and  are  de- 
sirable by  the  majority  of 
those  who  have  a  living 
interest  in  microscopic 
work. 

The  earliest  and  still 
the  best  form  of  this  kind 
of  microscope  was  made  by 
Powell  and  Lealand.  As 
opened  for  use  it  is  illus- 
trated in  fig.  195  ;  but  the 
tripod  foot  folds  into  what 
becomes  practically  a  single 
bar,  and  is  bent  by  means 
of  a  joint  to  occupy  the 
least  space.  The  body  un- 
screws, and  the  whole  lies 
in  a  very  small  space,  giving 
at  the  same  time  fittings  in 
the  cabinet  for  lenses,  con- 
densers, and  all  needful 
apparatus.  The  coarse  and 
fine  adjustments  to  the 
body  are  as  in  the  No.  1 
stand,  so  are  the  stage 

movements ;  and  the  sub-stage  has  rack-and-pinion  movements  and 
rectangular  sector  centring,  while  all  the  apparatus  provided  with 
the  largest  instrument  can  be  employed  with  it.  We  have  used 
this  instrument  for  delicate  and  critical  work  for  twenty  years,  and 
there  is  no  falling  off  in  its  quality ;  and,  when  packed  with  the 
additional  apparatus  required,  the  case  is  12  x  7  X  3  inches. 

Swift  and  Son  have  arranged  their  Histological  microscope 
(fig.  196)  as  a  portable  instrument,  to  which  from  its  peculiar  con- 
struction it  readily  lends  itself,  and  must  be  placed  in  the  third 
class  of  portable  microscopes. 

Mr.  Rousselet  has  designed  an  admirable  little  instrument  of 
portable  form  but  of  the  sixth  class.  It  is  binocular.  The  tripod 
folds ;  the  stage  is  plain,  with  a  sliding  ledge.  The  condenser 
focusses  by  means  of  a  spiral  tube,  within  which  an  inner  tube 
slides,  carrying  stops,  diaphragms,  &c.  The  mirror  is  jointed  so  as  to 
be  used  above  the  stage,  and,  as  its  focus  is  only  1^  inch,  can  be 


FIG.  197. — Baker's  diagnostic  travelling 
microscope  (1896). 


246      THE   HISTOKY  AND   DEVELOPMENT   01    THE    MICEOSCOPE 

used  as  a  side  reflector.  It  is  also  arranged  so  that  eye-pieces 
with  large  field-glasses  may  be  employed.  It  packs  in  a  box 
10^  X  5^  x  3^  inches,  and  weighs  6  pounds  complete. 


FIG.  198.— Bausch  and  Lomb's  portable  microscope  (1898). 

Baker  now  makes  a  small  useful  instrument  for  travelling  called 
'the  Diagnostic'  microscope,  designed  by  Surgeon-Major  Ross, 
medical  superintendent,  Indian  Army  Medical  Department. 
Fig.  197  illustrates  it.  The  tripod  stand  is  firm,  but  readily 


BAUSCH  AND  LOME'S  PORTABLE  MICROSCOPE     24? 

folds.  It  is  provided  with  sliding  tube,  coarse,  and  micrometer 
screw  fine  adjustments,  a  good  draw-tube  and  thoroughly  useful 
stage,  a  tubular  sub-stage  with  plane  and  concave  mirrors.  It  is 
packed  in  a  leather  case  with  shoulder  strap  and  loops  for  a 
.military  belt,  or  a  handle,  and  this  case,  with  three  objectives 
and  extra  eye-piece,  occupies  11  x  3^  x  3  inches.  It  can  also 
be  arranged  for  a  sub-stage  carrying  a  condenser  and  iris  dia- 
phragm, and  is  exceedingly  compact  and  wrell  made. 

A  very  old  device  has  been  utilised  by  Messrs.  Bausch  and  Lomb 
for  a  new  portable  stand,  that,  namely,  of  making  the  case  or  box  the 
foot  of  the  instrument.  The  microscope  itself  is,  in  every  other  respect 
save  size,  the  same  as  their  '  New'  stand  shown  in  fig.  193  ;  but  the 
addition  is  made  of  a  clamping  screw,  to  prevent  the  main  tube  from 


FIG.  199. — Bausch  and  Lomb's  portable  microscope  packed  (1898). 

dropping  or  turning.  An  illustration  of  this  microscope  is  given,  as 
set  up  for  use,  in  fig.  198.  It  will  be  seen  that  a  double  nose-piece 
may  be  used,  and  it  is  provided  with  a  useful  condenser,  the  sub- 
stage  having  a  screw  focussing  adjustment,  and  an  arrangement  for 
swinging  this  out  of  the  optic  axis.  The  microscope  is  rigid,  but 
can  be  inclined  at  any  angle  by  raising  the  cover  of  the  case  as  in 
the  figure.  It  can  be  closed  into  the  box  with  its  double  nose- 
pieces  in  position,  and  its  sub-stage  and  condenser  ready  for  use. 
The  size  of  the  case  complete  is  8f  x  5 J  x  2  J  inches,  and  its  weight 
is  3f  pounds. 

Microscopes  employed  for  the  purpose  of  minute  dissection  are 
of  considerable  importance  in  certain  kinds  of  work.  Many  instru- 
ments specially  adapted  are  made,  although  the  majority  are 
arranged  for  simple  lenses.  But  an  instrument  of  great  value, 


248      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

arranged  for  use  with  compound  lenses,  has  been  devised  by  employing 
the  binocular  of  Mr.  Stephenson.  This  instrument  is  illustrated  in 
fig.  200.  It  is  made  by  Swift  and  Son.  The  stage  may  be  enlarged 
as  a  dissecting  table,  with  special  rests  for  the  arms.  The  objective 
and  binocular  part  of  the  body  remain  vertical  and  focus  vertically 
by  a  rack-and-pinion  coarse  adjustment,  there  being  no  fine  adjust- 
ment. The  bodies  above  the  binocular  prisms  are  suitably  inclined, 
mirrors  being  placed  inside  them  to  reflect  the  image.  This  reflec- 
tion also  causes  the  erection  of  the  image,  which  is  valuable  to  the 
majority  engaged  in  insect  dissection  or  the  dissection  of  very 
delicate  and  minute  organisms  or  organs. 

Another  type  of  dissecting  microscope  has  been  introduced  (as 
we  have  seen  on  pp.  102-4)  by  the  firm  of  Zeiss ;  it  is  known 
as  Greenough's  Binocular  Microscope,  and  possesses  valuable 
and  interesting  features,  and  has  been  prepared  to  facilitate  the 
examination,  dissection,  and  -preparation  of  eggs,  Iarva3,  and 
other  solid  objects  by  furnishing  a  true  stereoscopic  and  erect 
image.  Hence  it  is  most  useful  for  zoologists,  botanists,  and 
embryologists.  To  accomplish  this  purpose  a  combination  of  Porro 
prisms  with  a  compound  microscope  of  the  usual  optical  type  has 
been  effected.  We  have  said  enough  of  this  instrument  in  an 
earlier  page,  and  merely  recall  its  adaptation  to  dissecting  purposes 
by  the  illustration  furnished  in  fig.  201,  and  we  would  remark  that  it 
is  only  when  two  such  complete  microscopes,  each  having  its  own 
objective  and  eye-pieces,  are  simultaneously  directed  upon  an  object 
that  the  truest  stereoscopic  images  can  be  obtained. 

Only  comparatively  low  powers  can  be  used  with  this  instrument, 
but  this  is  no  defect,  for  with  such  powers  alone  would  the  work  it 
is  intended  to  do  be  accomplished ;  but  two  special  eye-pieces  of 
different  powers,  corresponding  to  Huyghenian  eye-pieces  2  and  4, 
are  prepared  for  this  microscope  ;  they  are  known  as  orthomorphic. 
The  magnifications  resulting  from  the  combination  of  these  eye- 
pieces with  the  objective  are  respectively  25  and  40. 

We  have  now  to  consider  the  most  primitive  stands  adopted  for 
simple  microscopes.  That  in  the  form  of  a  bull's-eye  stand  is  the 
least  complex  form  possible.  This  instrument  holds  an  intermediate 
place  between  the  hand  magnifier  and  the  complete  microscope, 
being,  in  fact,  nothing  more  than  a  lens  supported  in  such  a  manner 
as  to  be  capable  of  being  readily  fixed  in  a  variety  of  positions 
suitable  for  dissecting  and  for  other  manipulations.  It  consists  in 
its  best  form  of  a  circular  foot,  wherein  is  screwed  a  short  tubular 
pillar  (fig.  202),  provided  with  a  rack-and-pinion  movement,  and 
carrying  a  jointed  arm  movable  in  many  directions  by  ball-and- 
socket  and  other  joints,  b,  c,  e,  but  capable  of  being  clamped  by 
thumb-screws  or  milled  heads,  a,  b,  e  ;  one  end  of  this  arm  carries  a 
joint,  to  which  is  attached  a  ring  for  holding  the  lenses.  By 
lengthening  or  shortening  the  pillar,  by  varying  the  angle  which 
the  arm  makes  with  its  summit,  and  by  using  the  various  joints, 
almost  any  position  and  elevation  may  be  given  to  the  lens  that  can 
be  required  for  the  purposes  to  which  it  may  be  most  usefully 
applied,  care  being  taken  in  all  instances  that  the  ring  which  carries 


LENS-HOLDERS 


249 


the  lens  should  (by  means  of  its  joint)  be  placed  horizontally.     The 
lenses  now  most  suitable   for  such  a  holder  are  those  constructed 


FIG.  200.— Stephenson's  binocular  by  Swift  (1887). 


upon  the  Steinheil  formula,  composed  of  three  cemented  lenses 
forming  a  system  which  gives  relatively  long  working  distances 
with  large  flat  field.  As  made  by  Zeiss  they  magnify  6,  12,  20,  and 


250      THE   HISTORY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

30  times,  and,  employed  in  such  a  stand  as  fig.  202,  they  are  ad- 
mirably adapted  for  picking  out  minute  shells  or  for  other  similar 
manipulations,  the  sand  or  dredgings  to  be  examined  being  spread 
upon  a  piece  of  black  paper,  and  raised  upon  a  book,  a  box,  or  some 
other  support  to  such  a  height  that  when  the  lens  is  adjusted 
thereto,  the  eye  may  be  applied  to  it  continuously  without  unneces- 
sary fatigue.  It  will  be  found  advantageous  that  the  foot  of  the 
microscope  should  not  stand  upon  the  paper  over  which  the  objects 
are  spread,  as  it  is  desirable  to  shake  this  from  time  to  time  in  order 
to  bring  a  fresh  portion  of  the  matters  to  be  examined  into  view  ; 


P2 


FIG.  201.— Greenougb's  binocular,  arranged  as  a  dissecting  microscope  (1897). 

and,  generally  speaking,  it  will  be  found  convenient  to  place  it  on 
the  opposite  side  of  the  object,  rather  than  on  the  same  side  with 
the  observer.  In  a  suitable  position  these  lenses  with  their  holder 
may  be  most  conveniently  set  for  the  dissection  of  objects  contained 
in  a  plate  or  trough,  the  sides  of  which,  being  higher  than  the  lens, 
would  prevent  the  use  of  any  magnifier  mounted  on  a  horizontal 
arm.  Although  the  uses  of  this  little  instrument  are  greatly 
limited  by  its  want  of  stage,  mirror,  &c.,  yet,  for  the  class  of  pur- 
poses to  which  it  is  suited,  it  has  advantages  over  perhaps  every 
other  form  that  has  been  devised.  Where,  on  the  other  hand, 


LENS-HOLDERS  25 1 

portability  may  be  altogether  sacrificed,  and  the  instrument  is  to  be 
adapted  to  the  making  of  large  dissections  under  a  low  magnifying 
power,  some  such  form  as  is  represented  in  fig.  203  constructed  by 
Messrs.  Baker,  on  the  basis  of  that  devised  by  Professor  Huxley  for 
the  use  of  his  Practical  Class  at  South  Kensington,  will  be  found 
decidedly  preferable.  The  framework  of  the  instrument  is  solidly 
constructed  in  mahogany,  all  its  surfaces  being  blackened,  and  is  so 
arranged  as  to  give  two  uprights  for  the  support  of  the  stage  and 
two  oblique  rests  for  the  hands.  Close  to  the  summit  of  each  of  these 
uprights  is  a  groove  into  which  the  stage-plate  slides  ;  and  this  may 
be  either  a  square  of  moderately  thick  glass  or  a  plate  of  ebonite, 
having  a  central  perforation  into  which  a  disc  of  the  same  material 
may  be  fitted,  so  as  to  lie  flush  with  its  surface,  one  of  these  being 
readily  substituted  for  the  other,  as  may  best  suit  the  use  to  be 


FIG.  202. — Zeiss's  lens-holder. 

made  of  it.  The  lens  is  carried  on  an  arm  working  on  a  racked 
stem,  which  is  raised  or  lowered  by  a  milled-head  pinion  attached  to 
a  pillar  at  the  further  right-hand  corner  of  the  stage.  The  length 
of  the  rack  is  sufficient  to  allow  the  arm  to  be  adjusted  to  any 
focal  distance  between  2  inches  and  J  inch.  But  as  the  height  of 
the  pillar  is  not  sufficient  to  allow  the  use  of  a  lens  of  3  inches 
focus  (which  is  very  useful  for  large  dissections),  the  arm  carrying 
the  lenses  is  made  with  a  double  bend,  which,  when  its  position  is 
reversed,  as  in  the  dotted  outline  (which  is  readily  done  by  unscrew- 
ing the  milled  head  that  attaches  it  to  the  top  of  the  racked  stem), 
gives  the  additional  inch  required.  As  in  the  Quekett  micro- 
scope, a  compound  body  may  be  easily  fitted,  if  desired,  to  a  separate 
arm  capable  of  being  pivoted  on  the  same  stem.  The  mirror  frame 


252      THE   HISTORY   AND   DEVELOPMENT  OF  THE  MICROSCOPE 

is  fixed  to  the  wooden  basis  of  the  instrument,  and  places  for  the 
lenses  are  made  in  grooves  beneath  the  hand- supports.  The  ad- 
vantages of  this  general  design  have  now  been  satisfactorily  de- 
monstrated by  the  large  use  that  has  been  made  of  it ;  but  the 
details  of  its  construction  (such  as  the  height  arid  slope  to  be 


FIG.  203. — Laboratory  dissecting  microscope  (1870), 

given  to  the  hand-rests)  may  be  easily  adapted  to  individual  require- 
ments. 

A  very  simple  and  well-known  form  of  dissecting  microscope  is 
made  by  Messrs.  Bausch  and  Lomb.  It  is  shown  in  fig.  204.  Its 
form  is  self-explanatory  :  a  plain  glass  stage,  and  a  mirror  at  a  suit- 
able angle  giving  abundant  light,  capable  of  being  replaced  by 


FIG.  204. — Bausch  and  Lomb's  (Barnes)  dissecting  microscope  (1896). 

a  white  or  black  enamelled  background,  suitable  rests  for  the 
arm,  and  a  sliding  holder  for  the  lenses.  It  is  these  latter  that 
are  special :  they  are  designed  for  the  instrument.  They  are 
doublets,  which  undoubtedly  give  a  large  aplanatic  field  and  fine 
definition. 

But  the  very  best  form  of  dissecting  microscope  for  simple  lenses 


A   GOOD   DISSECTING   STAND    BY   ZEISS 


253 


which  we  believe  to  be  at  present  constructed  is  made  by'Zeiss. 
We  illustrate  this  form,  fig.  205.  It  has  a  large  firm  stage  4  inches 
square  and  4^>  inches  from  the  table,  to  which  wooden  arm-rests  can 
be  attached  or  not,  as  may  be  desired.  Only  one  is  attached  in  the 


illustration,  and  the  points  of  attachment  of  the  other  are  seen. 
The  stage  has  a  large  opening,  3  x  3|  inches,  into  which  can  be 
placed  either  a  flat  brass  plate  or  a  glass  substitute,  or  a  metal  plate 
with  a  half- inch  hole  in  it.  Underneath  the  stage  are  black  and 
white  screens,  which  can  readily  be  turned  aside  by  the  use  of  the 


254      THE   HISTORY   AND   DE\7ELOPMENT   OF   THE    MICROSCOPE 

milled  heads,  A.  The  arm,  which  is  focussed  by  an  excellent  spiral 
rack- work  adjustment,  carries  either  a  Zeiss  dissecting  microscope, 
which,  with  and  without  its  concave  eye-lens,  yields  six  different 
powers,  varying  from  15  to  100  diameters,  or  the  arm  will  receive 
the  very  fine  Zeiss- Steinheil  simple  magnifiers. 

The  instrument  is  provided  with  a  large  plane  and  concave 
mirror  on  a  jointed  arm.  The  utility  of  this  simple  microscope  is 
very  great,  and  we  do  not  hesitate  to  pronounce  it  the  best  thing  of 
its  class  we  have  ever  seen. 

The  Continental  Model, — Our  one  purpose  in  this  treatise  is  to 
endeavour  to  promote  what  we  believe  to  be  the  highest  interests  of 
the  microscope  as  a  mechanical  and  optical  instrument,  as  well  as 
to  further  its  application  to  the  ever-widening  area  of  physical 
investigation  to  which,  in  research,  it  may  be  directed.  To  this  end 
throughout  the  volume,  and  especially  on  the  subject  of  the  value 
and  efficiency  of  apparatus  and  instruments,  we  have  not  hesitated 
to  state  definitely  our  judgment,  and,  where  needed,  the  basis  on 
which  it  rests.  Incidentally  we  have  expressed  perhaps  more  than 
once  our  disapproval,  and,  with  ourselves,  that  of  many  of  the  leading 
English  and  American  microscopists,  of  the  form  of  microscope  known 
as  the  Contiiwntal  model ;  we  believe  it  is  not  needful  to  say  that  we 
have  done  this  after  many  years  of  careful  thought  and  varied 
practice  and  experience,  and,  so  far  as  the  human  mind  can  analyse, 
without  bias.  It  is  not  where  a  microscope  is  made  that  the 
scientific  microscopist  inquires  first,  but  where  it  is  made  most 
perfectly,  and  we  cherish  strong  hopes,  in  the  interests  of  the  science 
of  microscopy,  that  so  enterprising  and  eminent  a  firm  as  that  of 
Zeiss,  of  Jena,  will  bring  out  a  model  that  will  comport  more  com- 
pletely with  the  needs  of  modern  microscopical  research  than  even 
the  best  of  the  models  that  they  now  produce.  It  is  to  this  house, 
under  the  cultivated  guidance  of  Dr.  Abbe  and  Dr.  Czapski,  that 
we  are  indebted  for  the  splendid  perfection  to  which  the  optical  side 
of  the  microscope  has  been  recently  brought ;  and  when  we  know 
that  the  '  Continental  model '  has,  in  the  hands  of  the  firm  of  Zeiss, 
passed  from  an  instrument  without  inclination  of  the  body  into  an 
instrument  that  does  so  incline,  and  from  an  instrument  without 
sub-stage  or  condenser  into  one  provided  with  the  latter  of  these 
absolutely  indispensable  appendages,  and  finally  from  an  instrument 
with  a  perfectly  plain  stage  with  '  clips '  into  what  is  now  a  stage 
with  mechanical  movements — we  can  but  hope  that  these  concessions 
to  what  has  belonged  to  the  best  English  models  for  over  forty  years 
may  lead  to  an  entire  resoiistruction  of  the  stand — a  wholly"  new 
model — intended  to  meet  all  the  requirements  of  modern  high-class 
work  in  all  departments,  and  with  a  fine  adjustment  of  the  most 
refined  class.  We  cannot  doubt,  if  this  were  so,  that  the  same 
genius  which  has  so  nobly  elevated  the  optical  requirements  of  the 
instrument  would  act  with  equal  success  on  its  construction  and 
mechanism.  We  have  been  told  in  the  friendliest  spirit,  by  one 
deeply  interested  in  the  Continental  stand,  and  a  master  in  optical 
knowledge,  that  on  the  Continent  the  microscope  is  '  actually  almost 
exclusively  used'  in  a  vertical  position.  Nevertheless  we  know 


CONTINENTAL  V.  ENGLISH  MODEL          255 

what  elaborate  arrangements  have  been  made  to  enable  the  body  to 
be  inclined  in  all  the  better  models,  and  surely  the  English  stand  is 
as  capable  of  being  vised  in  this  position  as  the  most  primitive  Con- 
tinental instrument ;  but  the  doubt  we  have  is  as  to  whether  the 
most  primitive  Continental  stand  possesses  the  same  primal  adapta- 
bility to  all  the  modern  optical  and  mechanical  improvements  of 
the  microscope  as  is  possessed  by  the  English  stand.  It  is  said 
that  '  the  Continental  microscope  has  closely  followed  the  wants  of 
the  microscopist,  and  that  in  its  mechanical  arrangements  it  has  kept 
pace  with  the  increasing  improvement  of  the  optical  parts,  without 
outrunning  them,  as  has  been  the  case  with  many  English  forms  of 
construction.'  "With  the  deference  and  good  feeling  with  which  we 
receive  this  statement  we  are  bound  to  say  that  it  does  not  present 
itself  as  historical.  The  mechanical  parts  have  not  in  reality  kept  pace 
with  the  optical  improvements,  fb£  when  apochromatic  lenses  of  0'95 
N.A.  to  1'4  IS". A.  are  used  with  large  illuminating  cones  they  become 
so  sensitive  to  focal  adjustment  that  the  Continental  fine  adjustment 
(the  best  form  of  which  has  hitherto  been  used  by  Zeiss)  is  not 
sufficiently  slow  to  permit  of  accurate  focussing  in  highly  critical 
work.  Applications  have,  for  instance,  been  made  to  Powell,  asking 
him  to  increase  the  slowness  of  his  fine  adjustment,  which  is  now 
twice  as  slow  as  the  best  Continental  form.  But  perhaps  the 
clearest  evidence  is  found  in  the  fact  that,  while  we  are  passing  this 
book  through  the  press,  two  striking  proofs  of  Continental  conviction 
that  their  fine  adjustment  should  be  rendered  slower  and  more  sen- 
sitive are  given,  first,  by  the  beautifully  simple  and,  as  we  believe^ 
most  admirable  invention  of  Reichert,  adapting  a  lever  movement 
to  his  stands  (vide  p.  169,  fig.  131),  by  which  he  makes  the  fine 
adjustment  more  than  three  times  as  slow  as  the  best  hitherto  used 
on  the  Continent ;  while  the  firm  of  Zeiss  themselves,  in  their 
newest  model  (p.  167,  fig.  128),  have  by  another  method  sur- 
passed all  other  makers ;  and,  as  I  learn  by  the  courtesy  of  the 
firm,  'the  micrometer  screw  of  this  new  stand  is  adjusted  for 
fii-th  of  an  inch  for  each  revolution  of  the  milled  head '  (figs.  129, 
130). 

We  cannot  but  believe  that  this  is  the  best  evidence  we  can 
have  of  the  validity  of  our  contention  in  the  last  edition  of  this 
book  that  the  Continental  fine  adjustment  was  too  coarse  or  quick 
for  the  almost  perfect  objectives  and  eye -pieces  they  themselves  had 
given  to  the  world. 

We  have  written  throughout  this  book  too  frankly  of  the  eminent 
services  of  Messrs.  Zeiss,  to  the  furtherance  of  the  interests  and  pro- 
gression of  the  microscope  as  a  scientific  instrument,  to  be  misunder- 
stood in  making  a  plain  estimate  of  the  quality  of  the  model  on  which 
their  elaborate  and  in  some  senses  beautiful  stands  are  built.  It 
will  be  seen  that  we  everywhere  justify  our  judgments  by  plain  and 
easily  comprehended  reasons,  and  the  very  eminence  of  the  makers 
renders  it  incumbent  that  practical  microscopists  should,  without  a 
shade  of  bias,  assess  the  value  of  a  stand  which  is  certainly  not 
built  on  lines  that  contribute  to  a  higher  and  still  more  efficient 
microscopy 


256      THE   HISTOEY  AND   DEVELOPMENT   OF   THE   MICEOSCOPE 

At  the  same  time  we  do  not  blind  ourselves  to  the  fact  that 
an  English  market  for  the  '  Hartnack '  model  has  had  very 
much  to  do  with  the  perpetuation  of  the  errors  which  that  form 
contains. 

The  reason  of  this  it  is  not  difficult  to  trace.  The  inductive 
method  advanced  but  slowly,  in  practice,  upon  the  professional 
activities,  and  even  the  professional  training,  of  medical  men.  The 
country  which  was  the  home  of  Bacon  and  Newton  and  Harvey  and 
Hunter  theoretically  accepted,  but  was  not  quick  to  apply,  the 
methods  of  induction  to  the  work  of  its  medical  schools.  Theory  and 
empiricism  held  a  powerful  place  in  both  the  teaching  and  practice 
of  medicine  in  England  until  the  earlier  years  of  the  present  century. 
Medicine  was  absolutely  unaffected  by  Bacon  until  the  latter  half  of 
the  seventeenth  century.  It  was  not  until  the  early  years  of  this  cen- 
tury that  the  modern  school  of  medicine  began  its  beneficent  career. 
But  at  that  time  the  microscope — one  of  the  most  powerful  instru- 
ments which  can  be  thought  of  in  the  application  of  experimental 
and  deductive  methods  to  the  science  of  medicine — was  looked  upon 
and  treated  by  the  faculty  as  a  philosophical  toy,  a  mere  plaything 
for  the  rich  dilettante.  But  in  spite  of  this  the  microscope  was 
brought  gradually  to  a  high  state  of  perfection,  and  by  the  end  of 
the  first  third  of  the  century  was  remarkably  advanced  as  a  practical 
instrument,  all  its  essentials  being  more  or  less  completely  developed. 
Meanwhile,  on  the  Continent  the  microscope  was  regarded  by  the 
Faculty  as  a  scientific  instrument  of  great  and  increasing  value, 
being  used  to  good  purpose  in  making  important  discoveries  in 
anatomy,  histology,  and  biology  generally. 

This  was  gradually  realised  in  this  country,  and  there  arose 
slowly  a  desire  to  employ  the  same  instrument  in  England.  But, 
although  English  instruments  of  the  most  practical  and  relatively 
perfect  kind,  representing  the  large  experience  of  many  careful 
amateurs,  were  easily  accessible  to  our  medical  men  in  their  own 
country — because  it  was  on  the  Continent  that  the  investigations 
referred  to  had  been  made — it  was  nothing  less  than  the  Continental 
microscope  that  was  sought  after  and  obtained.  We  have  been  told, 
indeed,  that  '  the  development  of  the  English  stands  has  not 
depended  on  the  wants  of  the  microscopist,'  but  has  been  the  result 
of  ingenuity  and  invention.  To  this  we  simply  say  that  it  may  be 
true  that  their  development  has  not  depended  on  the  immediate 
wants  of  the  microscopist,  but  was  in  many  cases  the  result  not  of 
ingenuity  so  much  as  of  powerful  insight  and  foresight.  And  how 
often  have  these  anticipations  been  realised  !  Because  early  obser- 
vations of  a  histological  character  (and  therefore  of  a  nature  to  lie 
beyond  the  sphere  of  the  lay  amateur)  had  been  successfully  made 
with  a  certain  form  of  microscope  on  the  Continent,  it  was  practi- 
cally argued  that  this  must  be  the  most  suitable  instrument  for  such 
a  purpose  ;  but  this  was  an  inference  made  without  knowledge  of  or 
reference  to  the  well-known  English  models. 

Let  us  carefully  examine  this  instrument.  The  typical  form 
was  that  made  by  Hartnack.  Seen  in  its  primitive  state,  we  have 
it  in  the  catalogues  of  all  the  Continental  makers — Zeiss,  Leitz, 


COMPARISON  OF  CONTINENTAL  AND  ENGLISH  MODELS      257 

Reichert,  and  the  rest.  It  is  a  non-inclining  instrument,  with  a 
short  tube  on  a  narrow  horseshoe  foot,  in  which  steadiness  is 
obtained  by  sheer  weight.  It  has  a  sliding-tube  as  a  coarse  adjust- 
ment, and  a  direct-acting  screw  for  the  fine  adjustment.  The  stage 
is  small,  and  the  aperture  in  it  is  relatively  still  smaller,  of  no  service 
in  reaching  the  focus  of  an  object  by  touch  with  a  high  power.  It 
is  provided  with  spring  clips,  and  a  diaphragm  immediately  below 
the  stage,  and  a  concave  mirror.  .  Now  it  has  been  said  that  the  fact 
that  the  Powell  stand,  e.g.  of  forty-five  years  ago,  adapts  itself  without 
material  change  to  the  most  modern  appliances  would  be  looked 
upon  by  the  German  student  as  being  *  no  commendation,'  because 
it  would  mean  that  they  were  more  elaborate  sthan  was  necessary, 
but  what  are  the  facts?  Let  us  take  an  Oberhauser  of  1837,  and 
compare  it  in  one  essential  particular  only  with  a  very  early  Powell, 
designed  in  1834.  It  was  a  stage-focussing  instrument.  As  a  fact 
the  Oberhauser  will  not  focus  a  low-angled  J-inch  objective  properly  ; 
the  fine  adjustment  works  in  jerks,  and  the  la-teral  movement  causes 
the  object  to  go  out  of  the  field.  The  Powell  will  now  work  an 
apochromatic  of  1*4  N.A.  oil  immersion  with  accuracy  and  precision  ; 
but  if  an  apochromatic  oil  immersion  of  1*4  were  placed  on  the  Ober- 
hauser it  would  be  at  great  risk  to  the  objective.  Now  even  in  early 
days  accurate  focussing  was  surely  a  vital  matter,  and  the  foresight 
that  could  anticipate  what  might  require  more  delicate  focussing  than 
the  objectives  then  in  use  was  wise,  and  to  the  student  profitable. 
The  Powell  No.  1  stand,  as  it  is  now,  was  in  the  main  constructed  in 
1849,  so  far  as  regards  tripod  foot,  limb,  coarse  adjustment,  and  fine 
adjustment  with  Turrell  stage.  The  alterations  that  have  been 
introduced  have  been  the  concentric  rotary  stage  (1861),  and  the 
present  form  was  manufactured  in  1869. 

A  sub-stage  condenser  was  rarely  used,  because  up  to  a  compara- 
tively late  date  (1874)  it  was  regarded  by  many  on  the  Continent 
as  a  mere  elegant  plaything  ;  its  true  value  was  not  perceived. 

On  this  model  all  the  microscopes  of  the  firm  of  Zeiss,  of  Jenar 
are  constructed,  as  they  are  used  almost  exclusively  on  the  Conti- 
nent, and  are  regarded  in  many  of  the  universities  and  medical 
schools,  both  here  and  in  America,  as  possessing  all  the  qualities 
required  for  the  best  biological  research. 

If  we  examine  the  finest  of  these  instruments  made  up  to  1885, 
we  are  impressed,  as  we  always  are,  with  the  beauty  and  care  of  the 
workmanship  and  finish  of  this  firm  ;  but  there  is  the  same  heavy 
horseshoe  foot,  steady  enough  while  the  instrument  is  non-inclining, 
only  needlessly  heavy,  requiring  common  ingenuity  alone  to  get 
equal  steadiness  with  one-fourth  the  weight.  But  since  this  instru- 
ment has  been  adapted  to  the  English  form  by  being  made  to  incline 
to  any  angle  up  to  the  horizontal,  the  foot  but  insecurely  balances 
the  instrument,  and  it  is  not  difficult,  as  it  is  not  uncommon,  to  topple 
it  over.  Indeed  in  their  photo-micrographic  outfit  the  Messrs. 
Zeiss  practically  see  this,  for  they  supply  another  foot  to  ivhich  the 
microscope  is  clamped.  Messrs.  Bausch  and  Lomb  tell  us  that  the 
foot  of  their  '  B  B '  Continental  microscope  is  '  heavily  leaded  to  ensure 
greater  stability.'  Sidle  and  Poalk  (1880)  and  McLaren  (1884),  and 


258      THE   HISTOEY    AND   DEVELOPMENT   OF  THE   MICROSCOPE 

now  Ross,  adopting  this  foot,  employ  the  added  mechanism  of  the 
revolution  of  the  pillar  on  the  foot  (an  old  device)  to  secure  stability 
at  all  inclinations  (vide  fig.  185,  p.  232).  Surely  if  the  horseshoe 
foot  were  satisfactory  for  the  inclining  microscope  these  modifications 
would  not  have  been  deemed  needful.  Besides  which  we  note  that 
for  the  same  purpose  the  C  mtiiiental  maker,  whom  we  venture  to 
think  very  alert  to  the  true  needs  of  modern  microscopy,  Reichert, 
prolongs  the  projecting  'toe'  of  the  horseshoe,  giving  it  almost  a 
tripod  form. 

It  must  not  be  forgotten  that  this  want  of  balance  is  with  the 
short,  not  the  long  body. 

The  diameter  of  the  tube  is  small,  being  slightly  over  seven- 
eighths  of  an  inch.  No  doubt  a  low-power  eye-piece  with  a  large 
field  is  extremely  useful  as  a  finder,  but  this  advantage  is  completely 
lost  with  the  original  small  Continental  tube.  That  this  is  seen  to 
be  a  disadvantage  would  appear  certain,  because  the  photographic 
microscope  model  of  Zeiss  has  a  larger  body -tube  ;  and  in  their  recent 
'  Appendix '  to  their  latest  catalogue  they  admit  that  for  certain  pur- 
poses other  stands  made  by  them,  '  owing  to  the  limited  diameter 
of  their  tubes,  cut  off  the  field ; '  a  significant  fact  for  those  who 
would  narrow  the  English  body,  when  it  is  remembered  that  Powell's 
is,  and  has  been,  suitable  for  all  purposes  without  alteration,  and 
long,  short,  and  binocular  bodies  are  interchangeable. 

At  the  date  of  the  publication  of  our  last  edition,  out  of  eighteen 
models  ten  were  made  with  inclining  bodies,  and  three  had  sliding 
coarse  adjustment.  But  in  the  twelve  models  for  1889  ten  incline, 
while  only  two  are  rigid,  and  eight  have  rack-work,  against  four 
having  sliding  tubes  for  coarse  adjustment ;  but  in  the  current 
<?atalogue  of  Messrs.  Zeiss  six  out  of  eight  models  have  inclining 
bodies,  two  are  rigid,  and  one  has  sliding  coarse  adjustment.  This 
is  a  manifest,  if  slow,  conformity  of  the  primitive  model  to  the 
English  type,  and  hardly  supports  the  affirmation  '  that  (during  the 
last  forty  years)  the  Continental  microscope  has  closely  followed  the 
wants  of  the  microscopist.' 

The  direct-acting  screw,  only  slightly  modified,  obtains  universally 
in  these  models.  We  have  already  plainly  said  that  this  is  not  suf- 
ficiently delicate  in  its  action  for  critical  work  with  an  apochromatic 
objective  of  1-4  or  1 '5  numerical  aperture,  especially  as  a  micrometer 
screw  with  a  necessarily  delicate  thread  is  bound  to  carry  the  com- 
bined weight  of  the  body,  limb,  coarse  adjustment,  and  the 
opposing  spring ;  that  it  will  wear  loose  under  the  stress  of 
constant  work  is  inevitable,  and  thus  its  utility  must  be  wholly 
gone. 

The  1889  model  has  a  new  form  of  fine  adjustment,  the  alteration 
being  that  the  micrometer  screw  acts  on  a  hardened  steel  point.  This 
may  cause  it  to  work  smoother ;  but  as  no  weight  is  taken  off,  there 
is  difficulty  in  discovering  any  reason  for  its  admitting  of  more 
prolonged  use  without  injurious  wear.  In  support  of  this  is  the 
fact  that  in  the  new  photographic  stand  made  by  this  celebrated 
firm,  with  so  extremely  delicate  a  fine  adjustment  (fig.  129),  we 
have  learned  through  their  English  representatives  that  only  one- 


CRITICISM   OF  MECHANICAL   PARTS 


259 


fifth  of  the  amount  lifted  by  the  micrometer  screw  of  the  1889 
model  is  lifted  by  the  same  screw  in  the  new  model.  It  should  be 
remembered  that  few  makers  of  microscopes  in  England,  though 
they  may  be  for  class  and  school  purposes,  if  they  use  a  fine  adjust- 
ment at  all,  use  anything  less  delicate  than  the  Campbell  differential 
screw  ;  although  it  seems  on  the  Continent  to  be  believed  that  the 
direct-acting  micrometer  screw  of  the  Continental  form  is  still  in 
vogue. 

It  must  be  plain  that  a  screw  of  T^th  inch  to  a  revolution 
cannot  bear  for  long  the  heavy  strain  of  the  body  of  a  microscope. 
The  remodelling  of  Zeiss  fine  adjustments  in  1886  undoubtedly 
improved  their  construction  and  quality  of  work  ;  but  so  fine  a  steel 
thread  is  not  meant  to  carry  weight  and  strain.  This  applies  to  all 
delicate  instruments  of  precision. 

The  stage  of  this  instrument, -in  common  with  all  built  on  the 
same  model,  has  three  fundamental  errors  of  design  : — 

i.  The  stage  is  so  narrow  that  the  edges  of  the  3x1  slips  are,  in 
some  Continental  stands,  allowed  to  project  over  the  edges.  Messrs. 
Zeiss  have  profitably  departed  from  this  fault  by  giving  to  their 
larger  stands  a  stage  in  size  more  like  the  English  type. 

ii.  The  stages  have  an  aperture  so  small  as  to  limit  their  useful- 
ness in  focussing  with  high  powers. 

iii.  Instead  of  a  sliding  ledge  they  provide  what  still  more 
efficiently  militates  against  easy  and  rapid  focussing,  viz.  spring 
clips.  It  is  unfortunate  that  no  stage  on  this  model  admits  of  the 
use  of  the  finger  to  aid  in  reaching  the  focus.  This  gentle  tilting 
up  of  the  object,  as  we  approach  the  focal  point,  would  save  hundreds 
of  cover-glasses  and  objective  fronts — and  we  have  reason  to  know 
that  not  a  few  are  broken  with  this  form  of  stage  ;  but  we  have  never- 
seen  put  forward,  and  do  not  know,  a  single  reason  in  justification 
of  a  small  aperture  in  the  stage. 

Another  important  point  is  the  absence  of  rotation  in  the 
ordinary  Continental  stand.  True  rotation  is  a  strictly  English 
feature,  which  has  been  in  use  and  carefully  constructed  for  many 
years.  And  its  value  is  great ;  it  is  an  indispensable  adjunct  to 
practical  work. 

Messrs.  Zeiss,  some  twenty  years  since,  copied  the  Oberhauser 
form  of  rotation  for  the  stage  ;  they  did  this  by  making  the  body 
and  limb  solid  with  the  stage,  so  that  the  whole  rotates  to- 
gether. 

Practically  there  is  only  one  point  in  favour  of  such  a  move- 
ment, and  that  is,  that  the  object  remains  exactly  in  the  same 
position  in  regard  to  the  field.  But  against  this  arrangement  there 
is — 

1 .  The  liability  of  throwing  the  optic  axis  above  the  stage  out  of 
centre  with  that  below  the  stage,  and  this  though  the  workmanship 
be,  as  it  is,  of  the  highest  order. 

2.  The  rotation  of  a  microscope  object  for  ordinary  examination 
is  really  unimportant,  as  there  can  be  no  top  or  bottom  to  it.     Even 
for  oblique  illumination  it  is  not  required,  as  it  is  always  easier   to 
rotate  the  illuminating  pencil.     The  only  instances  in  which  rotation 


260      THE    HISTOEY  AND   DEVELOPMENT   OF   THE   MICROSCOPE 

of  the  object  is  important  are  :  (a)  When  the  object  is  polarised,  and 
then  it  is  a  distinct  disadvantage  not  to  be  able  to  rotate  the  object 
independently  of  the  body  which  carries  the  analyser.  In  short,  the 
stage  rotating  independently  of  the  body  would  be  preferable  because, 
if  it  is  required  to  rotate  the  object  on  a  dark  polarised  field,  the 
polarising  and  analysing  prisms  can  be  set  at  the  proper  angles,  and 
then  the  object  rotated  without  disturbing  the  relative  positions  of 
the  prisms. 

But  this  cannot  be  done  with  the  arrangement  of  the  Zeiss 
model,  which  rotates  body  and  stage.  The  firm  have,  however, 
more  recently  introduced  a  rotating  stage  based  on  the  English 
model,  and  we  are  glad  to  give  our  testimony  to  its  admirable 
workmanship  and  perfection  of  centring.  The  contention,  however, 
that  we  think  in  all  friendliness  is  sustained,  is  that  the  charac- 
teristics of  the  English  model  were  not  superfluous,  and  that  the 
Continental  model  has  only  too  slowly  followed  the  requirements 
discovered  and  used  by  the  makers  of  the  best  English  models  so  long 
ago. 

((3)  For  photo -miwographic  purposes. — In  this  case,  in  the  Zeiss 
stand,  the  head  of  the  fine-adjustment  screw  is  geared  to  the  focussing 
rod  ;  so,  manifestly,  rotation  of  the  body  becomes  impossible. 

Thus,  by  adopting  rotation  in  the  form  chosen,  the  highest  ends 
for  which  the  microscope  stage  should  revolve  cannot  be  accomplished, 
and  the  newer  form  of  stand  must  be  adopted. 

The  sub-stage  is  often  quite  wanting  in  the  common  Continental 
forms.  This  was  true  of  the  Hartnack  stands,  with  rare  excep- 
tions ;  the  Nachet  instruments  were  provided  with  an  elementary 
form. 

As  we  have  seen,  until  quite  recent  times,  the  condenser  was 
regarded  on  the  Continent  as  a  superfluous,  if  not  a  foolish,  appliance ; 
but  that  prejudice  has  been  killed  by  the  light  thrown  on  the  whole 
question  by  (1)  the  chromatic  (1873),  and  now  (2)  the  achromatic 
condenser  of  Abbe,  and  finally  (3)  by  the  '  centring  achromatic 
condenser,'  only  just  made  accessible  by  this  firm.  This  condenser  is 
not  only  focussed  by  the  rack-and-pinion  movement,  but  also  by 
means  of  a  special  fine  adjustment  for  bringing  out  its  most  delicate 
results.  But  even  a  condenser  was  in  use  in  England  in  the  year 
1691  (vide  fig.  101,  p.  133),  and  the  best  work  in  England  since  the 
invention  of  achromatism  has  never  been  done  without  one. 

In  the  mounting  of  the  Abbe  condenser  every  possible  ingenuity 
has  been  displayed  to  make  it  do  its  work  without  a  sub-stage  ;  but 
a  permanent  centring  and  focussing  sub-stage,  into  which  this  optical 
arrangement  could,  amongst  others,  fit,  might  be  made  with  half  the 
labour,  ingenuity,  and  cost.  But  rather  than  this,  we  have  in  the 
less  recent  forms  the  condenser  made  to  slide  on  the  tail-piece,  and 
to  be  jammed  with  a  screw. 

It  has  therefore  neither  centring  nor  focussing  gear ;  but,  striking 
as  it  may  appear,  a  diaphragm,  which  cannot  be  used  with,  and  is  no 
part  of,  the  condenser,  is  supplied  in  a  stand  not  of  the  most  recent, 
but  of  comparatively  recent  make,  with  mechanical  centring  and 
rack-work  focussing  movements  !  That  is  to  say,  the  delicate  centre 


THE   PURCHASE   OF  A   MICROSCOPE  261 

of  an  optical  combination  might  in  that  instrument  take  care  of  itself, 
but  a  diaphragm  aperture  must  be  centred  by  mechanism  and 
focussed  by  rack. 

We  know  that  the  idea  involved  in  a  rack- work  diaphragm  is 
the  graduation  in  the  angle  of  the  cone-  of  illumination  from  the 
plane  mirror  by  racking  a  certain-sized  diaphragm  up  or  down. 
But  this  can  be  better  done  by  an  iris  diaphragm,  or  perhaps  more 
perfectly  still  by  a  wheel  of  diaphragms. 

Now,  in  reality  nothing  is  so  important  as  the  centring  and 
focussing  of  the  condenser,  after  we  are  once  provided  with  perfect 
objectives ;  and  any  mechanical  arrangement  that  would  enable  us 
to  perfectly  centre  an  iris  diaphragm  or  a  wheel  of  diaphragms 
would  enable  us  to  centre  the  condenser.  For  the  racking  and 
centring  of  condensers  there  was,  until  very  recent  times,  nothing 
in  the  best  stands,  of  what  is,  doubtless  the  largest  and  most 
enlightened  house  for  the  manufacture  of  microscopes  in  the 
world,  to  supply  this  indispensable  need  which  the  modern  con- 
denser involves. 

We  observe  with  pleasure  advances  in  every  direction  in  which 
we  have  called  attention  to  defects.  The  more  recent  instruments 
are  marvels  of  ingenuity;  we  present,  in  fig.  167,  the  latest  and 
finest  form  of  Zeiss's  best  microscope. 

There  is  no  fault  in  the  workmanship ;  it  is  the  best  possible. 
The  design  only  is  faulty,  there  is  nothing  to  command  commenda- 
tion in  any  part  of  the  model ;  and,  seeing  that  the  Messrs.  Zeiss 
have  now  progressed  so  far  as  to  furnish  their  first-class  stand  with 
the  English  mechanical  movement,  and  even  stage  rotation,  and  fine 
adjustment  to  their  newest  and  best  sub-stage  condenser,  we  can 
but  believe  that  the  advantages  of  these  improvements  will  make 
plain  the  greater  advantage  that  would  accrue  from  an  entirely  new 
model.  To  all  who  study  carefully  the  history  of  the  microscope 
arid  have  used  for  many  years  every  principal  form,  it  will,  we 
believe,  be  manifest  that  the  present  best  stand  of  the  best  makers 
of  the  Continent  is  an  over-burdened  instrument.  Its  multiplex 
modern  appliances  were  never  meant  to  be  carried  by  it.  The 
attempt  to  combine  a  dissecting  microscope  with  an  observing 
microscope  required  to  do  the  most  critical  work  is  not,  we  submit 
with  all  friendliness,  compatible. 

The  Purchase  of  a  Microscope. — A  desire  to  possess  a  good  but 
not  costly  microscope  is  extremely  common,  but  as  a  rule  the 
intending  purchaser  has  little  knowledge  of  the  instrument,  and 
does  not  profess  to  know  what  are  the  indispensable  parts  of  such  an 
apparatus,  or  what  parts  may,  in  the  interests  of  economy  and  his 
special  object,  be  dispensed  with,  leaving  him  still  possessed  of 
a  sound  and  well-made  instrument.  We  may  briefly  consider  this 
matter. 

The  first  question  to  be  asked  when  a  microscope  is  to  be  pur- 
chased is,  *  What  is  the  order  of  importance  of  the  various  parts  of 
a  microscope  ? '  In  answering  this  query  it  will  be  to  some  extent 
true  that  subjectivity  of  judgment  will  appear.  But  we  believe 
that  the  following  table  of  the  relative  order  of  importance  of  the 


262      THE   HISTOEY   AJND   DEVELOPMENT   OF  THE   MICEOSCOPE 

parts  of  a  microscope  will  commend  itself  to  all  workers  of  large  and 
broad  experience  : — 

1.  A  coarse  adjustment  by  rack  and  pinion. 

2.  A  sub-stage. 

3.  A  fine  adjustment. 

4.  Mechanical  movements  to  sub-stage,  i.e.  focussing  and  centring. 

5.  Mechanical  stage. 

6.  Rack- work  to  draw- tube. 

7.  Finder  to  stage, 

8.  Plain  rotary  stage. 

9.  Graduation  and  rack- work  to  rotary  stage. 

10.  Fine  adjustment  to  sub-stage. 

1 1 .  Rotary  sub-stage. 

12.  Centring  to  rotary  stage. 

This  table  gives  in  order  the  relative  values  of  the  several  parts  ; 
thus  a  microscope  with  a  rack-and-pinion  coarse  adjustment  and  a 
sub-stage  is  to  be  preferred  before  a  microscope  with  a  rack-and- 
pinion  coarse  adjustment,  a,  fine  adjustment,  but  no  sub-stage.  Or  a 
microscope  with  a  coarse  adjustment  by  rack  and  pinion,  a  sub-stage, 
and  a  fine  adjustment,  is  to  be  preferred  before  one  with  the  same 
coarse  adjustment  and  a  mechanical  stage  movement,  but  no  sub- 
stage  or  fine  adjustment ;  and  so  on.  The  last  item  is  of  least 
importance,  and  the  importance  of  all  the  others  is  in  the  order  of 
their  numeration. 

Another  matter  of  some  significance  to  the  tyro  is  the  relative 
value,  from  the  point  of  view  of  time  consumed,  and  therefore  of 
prime  cost,  in  producing  the  several  kinds  of  microscopes.  The 
No.  1  stands  of  half  a  dozen  makers  may  be  near  the  same  cost,  but 
may  nevertheless  have  involved  the  consumption  of  very  different 
quantities  of  the  highest  class  of  skilled  labour  in  their  production. 

Manifestly  the  first  thing  to  be  looked  at  in  a  microscope  making 
any  pretensions  to  quality  is  the  character  of  the  ivorkmanship  ;  and 
this  should  carry  with  it  the  question  how  much  machine,  and  how 
much  hand  work  and  fitting  there  is  in  it.  Arcs  graduated  on 
silver,  for  example,  are  very  attractive,  and  with  many  are  most 
impressive ;  but  they  are  simply  machine  work,  and  quite  inex- 
pensive. 

In  the  two  great  types  of  models,  the  bar  movement  and  the 
Jackson  limb,  the  bar  movement  involves  more  than  double  the 
actual  hand-fitting ;  while  a  fine  adjustment  with  a  movable  nose- 
piece  takes  twice  the  fitting  of  one  in  which  the  wrhole  body  is  moved 
by  the  fine -adjustment  screw.  In  the  same  way  a  mechanical  stage 
which  is  made  of  machine-planed  plates,  sliding  in  a  machine-ploughed 
groove,  is  much  less  costly  in  time  and  quality  of  labour  than  a  hand- 
made sprung  stage.  So  a  sub-stage  having  a  movable  ring  pressed 
by  two  screws  against  a  spring  has  very  far  less  work,  and  work  of 
a  lower  class,  than  one  with  a  true  rectangular  centring  movement. 

It  will  follow,  then,  that  a  Jackson-limbed  microscope  with  no 
movable  nose-piece,  with  a  machine-made  mechanical  stage  and  a 
movable  ring  for  sub-stage,  will  not  have  involved  more,  perhaps, 
than  a  third  of  the  skilled  work  which  must  be  expended  on  a  well- 


SPECIAL  MICROSCOPES 


263 


made  instrument  of  the  same  size  with  a  bar  movement.  But  if  we 
compare  the  range  of  prices  as  presented  by  English  and  American 
makers,  we  rarely  find  an  equivalent  difference  in  cost. 

Then  the  tyro  will  be  warned  by  this  not  to  purchase  a  pretentious 
instrument  with  a  bar  movement  and  mechanical  stage  for,  say,  51. 

But  if  a  loiv-priced 
instrument  is  to  be 
purchased,  if,  as  is 
almost  certain,  it  be  a 
Jackson  model,  see 
that  it  has  a  rack- 
work  coarse  adjust- 
ment, eschew  the  short- 
lever  nose-piece,  and 
have  a  differential 
screw  fine  adjustment, 
a  large  plain  stage, 
and  an  elementary 
centring  sub-stage. 
Such  an  instrument 
should  be  obtained  for 
51.  10s. 

Although  not  fre- 
quently used,  it  would 
be  doing  our  work  im- 
perfectly not  to  refer 
to  a  form  of  micro- 
scope devised  for 
chemical  purposes  by 
Messrs.  Bausch  and 
Lomb.  The  object  of 
Prof.  E.  Chamot,  of 
the  Cornell  University, 
in  inducing  these  op- 
ticians to  make  this 
microscope  was,  he 
says,  to  enable  the 
chemist  who  had 
mastered  the  use  of  the 
microscope  '  to  employ 
the  elegant  and  time- 
saving  methods  of 
micro -analysis/  thus 
giving  him  ability  '  to 
examine  qualitatively 
the  most  minute  amounts  of  material  with  a  rapidity  and  accuracy 
which  are  truly  marvellous,  not  to  speak  of  the  many  substances 
for  which  no  other  method  of  identification  is  known.' 

An  illustration  of  this  instrument  is  given  in  fig.  206.  It  will 
be  observed  that  it  follows  the  Continental  model ;  ;  since  in  all  the 
work  for  which  it  is  intended  the  stand  is  always  used  in  an  upright 





FIG.  206. — Microscope  for  chemical  purposes  (1897). 


264      THE   HISTORY   AND   DEVELOPMENT   OF   THE   MICROSCOPE 

position/  it  is  not  provided  with  a  jointed  pillar  to  secure  inclination. 
The  coarse  adjustment  is  by  rack  and  pinion  ;  the  fine,  by  the  usual 
micrometer  screw  of  this  firm.  The  stage  is  circular  and  rotates, 
being  provided  with  centring  screws,  and  its  margin  is  graduated 
into  degrees  for  measuring  crystal  angles.  Except  for  this  graduated 
circle  the  stage  is  faced  with  hard  rubber.  The  sub-stage  is  adjust- 
able by  means  of  a  quick-acting  screw.  This  is  fitted  with  polarising 
apparatus,  consisting  of  a  large  Nicol  prism  so  mounted  that  by 
means  of  a  pin  fitting  into  a  slot  in  the  sub-stage  the  prism  can 
always  be  replaced  in  exactly  the  same  position,  and  rotated  with  a 
circle  graduated  in  degrees  ;  or  it  can  be  swung  aside  when  polarised 
light  is  not  needed.  The  analysing  Nicol  prism  is  also  provided 
with  a  graduated  circle,  and  is  so  mounted  that  it  fits  over  and  above 
any  eye-piece.  The  draw-tube  of  the  microscope  is  furnished  with 
a  small  projecting  pin,  which  fits  into  a  slot  cut  in  the  bottom  of 
the  tube-mounting  of  the  analyser.  This  slot  lies  in  the  same 
vertical  plane  as  the  zero  points  of  the  analyser,  the  polariser,  and 
the  stage.  The  zero  points  of  the  two  former  are  arranged  as  usual 
for  the  position  of  crossed  Nicols ;  hence,  when  the  polariser  is  in 
position  and  at  zero,  and  the  analyser  is  at  zero  and  is  in  position 
by  its  pin  and  slot,  the  Nicols  are  crossed  without  further  adjust- 
ment ;  this,  of  course,  saves  much  time.  But  it  is  clearly  a  simplified 
petrological  microscope ;  it  is  not  intended  for  petrological  or 
mineralogical  work,  it  is  simply  an  instrument  made  at  a  very 
low  price,  but  stated  by  Prof.  Chamot  to  be  competent  for  all 
chemical  work  or  food  examinations. 

An  equally  important  special  form  of  microscope  has  been  made 
by  Reichert  for  the  examination  of  metals.1  Fig.  207  shows  this 
instrument  made  according  to  the  instructions  of  Dr.  A.  Rejto,  of 
Budapest.  In  general  appearance  it  resembles  the  ordinary  horse- 
shoe stand,  but  it  has  no  mirror,  and  the  stage,  which  is  made 
adjustable  in  height,  may  also  be  removed  altogether. 

With  very  low  powers  the  specimen  may  be  illuminated  by 
diffused  daylight  or  artificial  light  falling  freely  upon  its  surface. 
With  higher  powers  an  illuminator  is  used  which  fits  the  tube  of 
the  microscope,  and  is  provided  with  an  extension  to  receive  the 
eye-piece.  The  illuminator  consists  of  a  thin  plate  of  glass  placed 
at  an  angle  of  45°  with  regard  to  the  axis  of  the  tube,  and  of  a  con- 
densing lens  whose  focal  length  is  equal  to  the  sum  of  distances 
between  the  lens  and  the  plate  of  glass,  and  between  the  latter  and 
the  object. 

The  question  of  illumination  is  a  very  important  one,  to  whic 
great  attention  is  to  be  devoted. 

As  source  of  light  the  '  Auer,'  a  triplex  burner,  adjustable  in 
height,  may  be  recommended  ; 2  it  is  placed  at  a  distance  of  one 
metre  from  the  illuminator.  The  flame  is  surrounded  by  an  iron 
or  asbestos  cylinder,  with  only  the  necessary  aperture  for  illumination 
of  the  object.  The  source  of  light  should  be  at  exactly  the  same 
level  with  the  lens,  6,  of  the  illuminator.  On  removing  the  eye- 

1  Central-ZeMung  fur  Optik  und  Mechanik,  No.  17,  1897. 

2  Supplied  by  Reichert. 


SPECIAL  MICROSCOPES 


265 


piece  and  looking  through  O  c,  it  will  generally  be  found  that  the 
microscopical  field  is  not  evenly  illuminated  ;  the  light  should  then 
be  lowered  or  raised  until  perfectly  uniform  illumination  is 
obtained. 

The  beam  of  light  received  by  the  lens,  b,  is  made  to  converge,  and 


Oc 


FIG.  207. — Reichert's  microscope  for  the  examination  of  metals  (1897). 

is  reflected  downwards,  in  the  direction  of  the  axis  of  the  instrument, 
by  the  glass-plate,  a.  It  is  then  condensed  upon  the  object  by  the 
lenses  of  the  objective  itself.  The  illuminated  object  sends  back  a 
portion  of  the  light,  which  passes  through  the  objective  and  the  plate 
^,  reaching  the  eye  at  O  c. 

The  object  to  be  examined  should  have  two  parallel  surfaces,  so 


266      THE   HISTORY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 

that  it  may  be  placed  on  the  stage  of  the  microscope  in  a  perfectly 
horizontal  position.  With  a  view  of  compensating  for  small  de- 
ficiencies in  the  parallelism  of  the  two  surfaces,  the  stage  is  provided 
with  the  screws,  S  S,  by  which  means  it  may  be  tilted,  and  the  upper 
surface  of  the  object  made  to  lie  in  a  truly  horizontal  plane,  which 
of  course  is  necessary  in  order  to  place  the  entire  field  in  the  focus 
of  the  instrument.  The  stage  is  a  mechanical  one,  the  milled  heads, 
T'"  and  T"",  imparting  to  it  a  forward  and  backward  movement  and 
a  lateral  movement  respectively. 

After  the  source  of  light  has  been  placed  in  the  most  desirable 
position  for  the  examination  of  a  certain  specimen,  if  a  sample  of 
different  thickness  be  placed  on  the  stage,  the  microscope  must  be 
lowered  or  raised,  with  the  result  that  the  light  is  no  longer  in  the 
proper  position  and  must  again  be  adjusted.  To  avoid  this  trouble- 
some manipulation,  the  stage  of  the  microscope  is  made  adjustable 
in  height  by  turning  the  milled  head  T".  When  the  object  is  too 
thick  to  be  placed  on  the  stage,  the  latter  may  be  turned  to  one  side 
and  the  preparation  laid  on  the  foot  of  the  microscope.  For  still 
larger  pieces  of  metal,  the  stage  may  be  removed  altogether,  the 
body  of  the  instrument  turned  around  180°,  and  the  metal  placed  on 
the  table  by  the  side  of  the  stand ;  pr  the  body  of  the  microscope  is 
connected  directly  with  its  foot,  for  which  purpose  the  intermediate 
piece  bearing  the  stage  must  be  removed. 

Prof.  Rejto's  method  for  the  preparation  of  the  sample  is  as 
follows  : — 

The  piece  of  metal  to  be  examined  has  two  of  its  sides  planed  oft" 
and  made  parallel.  The  upper  surface  is  polished  until  it  is  free 
from  scratches.  It  is  then  washed  with  absolute  alcohol,  and  wiped 
with  a  soft  clean  cloth  in  order  to  remove  all  fatty  substances.  The 
polished  surface  is  next  surrounded  with  a  layer  of  wax  so  as  to  form 
a  rim  projecting  a  little  above  the  surface.  Being  placed  horizon- 
tally, pure  concentrated  hydrochloric  acid  is  poured  over  it  to  a 
depth  of  about  three  millimetres,  and  allowed  to  act  for  five  minutes. 
It  is  then  poured  off,  and  the  surface  covered  with  concentrated 
ammonia.  The  wax  is  removed,  and  the  surface  wiped  dry  with  a 
soft  cloth.  A  little  oil  is  next  poured  over  it  and  allowed  to  remain 
for  fifteen  minutes. 

It  is  then  dried  again  and  rubbed  on  a  piece  of  chamois  leather 
until  it  assumes  a  shiny  appearance. 

When  large  pieces  of  metal  are  to  be  examined,  small  portions 
must  be  polished  by  hand  and  etched  as  described  above. 

Figs.  208  and  209  are  photomicrographs  taken  with  this  instru- 
ment, which  are  self-explanatory  of  the  nature  of  the  work  it  does. 

Tank  microscopes  (also  called  aquarium  microscopes)  have,  for 
certain  kinds  of  work,  a  value  of  their  own.  They  may  be  used 
with  low  powers  outside  the  glass  or  above  the  water  ;  or  the 
object-glass  may  be  protected  by  a  water-tight  tube  outside  it,  and 
with  a  disc  of  glass  fixed  (also  water-tight)  into  that  end  of  the  tube 
which  stands  below  the  front  lens  of  the  objective,  at  a  proper- 
distance  for  the  focus,  may  then  be  plunged  into  the  aquarium, 
Indeed,  the  tube  of  the  instrument  may  be  so  protected  as  to  work 


TANK  AND    AQUARIUM   MICROSCOPES  267 

for  some  depth,  and  have  some  range  in  the   water  of  a  good-sized 
tank. 

A  beautiful  instrument  of  this  class  has  been  devised  by  Mr.  J. 
W.  Stephenson  for  the  examination  of  living  objects  in  an  aquarium. 
A  brass  bar  is  laid  across  the  aquarium  as  shown  in  the  woodcut  i 


FIG.  208. — Wrought  iron  magnified  250  diameters. 


FIG.  209. — Ordinary  steel  magnified  250  diameters. 

(fig.  210).  To  adjust  it  to  aquaria  of  different  widths  the  support 
on  the  left  is  made  to  slide  along  the  bar,  and  it  can  be  clamped  at 
any  given  point  by  the  upper  milled  head.  The  milled  head  at  the 
side,  by  pressing  on  a  loose  plate,  fastens  the  bar  securely  to  the 
aquarium. 


268       THE   HISTOEY   AND   DEVELOPMENT   OF  THE   MICROSCOPE 


Between  the  ends  of  the  bar  slides  an  arm  carrying  a  sprung 
socket,  and  the  arm  can  be  clamped  at  any  given  point  of  the  bar. 
Through  the  socket  is  passed  a  glass  cylinder,  cemented  to  a  brass 
collar  at  the  upper  end,  and  closed  at  the  lower  by  a  piece  of  cover- 
glass.  Into  this  cylinder  is  screwed  the  body-tube  of  the  microscope 
with  eye-piece  and  objective,  which  are  thus  protected  from  the 
water  of  the  aquarium.  The  microscope  is  focussed  by  rack  and 
pinion  (milled  head  just  below  the  eye-piece),  and  in  addition  the 
objective  is  screwed  to  a  draw-tube,  so  that  its  position  in  the  cylinder 
may  be  approximately  regulated. 
is  The  arm  of  the  socket  is  hinged  to  allow  of  the  microscope  being 


FIG.  210 


inclined  in  a  plane  parallel  to  the  sides  of  the  aquarium.  The  lower 
milled  head  clamps  the  hinge  at  any  desired  inclination. 

The  socket  also  rotates  on  the  arm,  so  that  the  microscope  can  be 
inclined  in  a  plane  parallel  to  the  front  of  the  aquarium.  Thus  any 
point  of  the  aquarium  can  be  reached. 

As  an  adjunct,  and  admirable  aid  to  the  student  of  the  tank  and 
pond,  as  well  as  a  simple  and  easy  means  by  which  specific  forms  of 
microscopic  life  may  be  found  and  readily  taken,  we  call  attention 
to  the  tank  microscope  of  Mr.  C.  Rousselet.  It  is  illustrated  in  fig. 
211  and  scarcely  needs  further  description. 

One  of  Zeiss's    Steinheil   aplanatic   lenses,   to   which .  we   have 


MR.   ROCTSSELET'S   TANK  MICROSCOPE 


referred,  is  carried  on  a  jointed  arm,  which  is  clamped  to  the  tank,1 
the  tank  being  nowhere  deeper  than  the  range  of  focus  of  the  lens 
employed.  The  arm  moves  on  a  plane  parallel  to  the  side  of  the 
tank,  and  the  lens  is  foctissed  by  means  of  a  rack  and  pinion, 

arranged  upon  the  body        

of  the  clamp,  as  seen 
upon  the  left-hand 
corner  of  the  figure. 
The  following  points 
will  recommend  them- 
selves to  those  who  are 
in  the  habit  of  looking 
at  their  captures  with 
the  pocket  lens  in  the 
ordinary  way  : — 

When  an  object  of 
interest  is  found,  it  can 
be  followed  with  the 
greatest  ease  and  taken 
up  with  a  pipette,  both 
hands  being  free  for  this 
operation. 

It  so  frequently 
happens  that  a  minute 
object  is  lost  simply  by  removing  the  pocket  lens  for  an  instant  to 
take  up  the  pipette  ;  in  the  above  apparatus  the  lens  remains  in  the 
position  in  which  it  has  been  placed.  By  a  new  process  glass  tanks 
are  made  with  melted  seams  ;  these  cannot  possibly  leak,  and  are  to 
be  preferred  to  those  with  the  ordinary  cemented  joints. 

1  We  prefer  to  have  a  stand  or  c  rest '  for  the  tank,  and  on  one  side  of  this  a  firm 
pillar  to  which  (and  not  to  the  side  of  the  aquarium)  the  jointed  arm  is  clamped. 
This  enables  shallower  and  deeper  tanks  to  be  employed  without  shifting  the  rack 
carrying  the  lens. 


FIG.  211. — Rousselet's  aquarium  microscope. 


270 


CHAPTER   IV 

ACCESSORY    APPARATUS 

THIS  chapter  on  apparatus  accessory  to  the  microscope  might  be 
easily  made  to  occupy  the  whole  of  the  space  we  propose  to  devote 
to  the  entire  remainder  of  the  book  ;  the  ingenuity  of  successive 
microscopists,  and  the  variety  of  conditions  presented  by  successive 
improvements  in  the  microscope  itself,  have  given  origin  to  a 
variety  of  appliances  and  accessory  apparatus  that  it  would  be  futile 
in  a  practical  handbook  to  attempt  to  figure  and  describe.  We  pro- 
pose, therefore,  only  to  describe,  and  to  explain  the  mode  of  success- 
fully employing,  the  essential  and  the  best  accessories  now  in  use, 
neglecting,  or  only  incidentally  referring  to,  those  which  are  either 
supplanted,  or  which  present  modifications  either  not  important  in 
themselves  or  accounted  for  by  the  fact  of  their  production  by 
different  opticians. 

I.  Micrometers  and  Methods  of  Measuring  Minute  Objects. — It 
is  of  the  utmost  importance  to  be  able  with  accuracy,  and  as  much 
simplicity  as  possible,  to  measure  the  objects  or  parts  of  objects  that 
are  visible  to  us  through  the  microscope. 

The  simplest  mode  of  doing  this  is  to  project  the  magnified 
image  of  the  object  by  any  of  the  methods  described  under 
'  Camera  Lucida  and  Drawing.'  We  carefully  trace  an  outline  of 
the  image,  and  then,  without  disturbing  any  of  the  arrangements, 
remove  the  object  from  the  stage,  and  replace  it  with  a '  stage  micro- 
meter,' which  is  simply  a  slip  of  thin  glass  ruled  to  any  desired  scale, 
such  as  tenths,  hundredths,  thousandths  of  an  inch  and  even  less. 
Trace  now  the  projected  image  of  this  upon  the  same  paper,  and  the 
means  are  at  once  before  us  for  making  a  comparison  between  the 
object  and  a  known  scale,  both  being  magnified  to  the  same  extent. 
The  amount  of  magnification  in  no  way  affects  the  problem.  Thus, 
if  the  drawn  picture  of  a  certain  object  exactly  fills  the  interval 
between  the  drawing  representing  the  '01  inch,  the  object  measures 
the  '01  inch,  and  whether  we  are  employing  a  magnifying  power  of 
a  hundred  or  a  thousand  diameters  is  not  a  factor  that  enters  into 
our  determination  of  the  size  of  the  object.  In  fact,  all  drawings  of 
microscopic  objects  are  rendered  much  more  practically  valuable  by 
having  the  magnified  scale  placed  beneath  them,  so  that  measure- 
ments may  at  any  time  be  made. 

In  favour  of  the  above  method  of  micro-measurement,  it  will  be 
noted  (1)  that  no  extra  apparatus  is  required,  (2)  that  it  is  extremely 
simple,  and  (3)  that  it  is  accurate. 


MICROMETER   EYE-PIECES 


271 


The  most  efficient  piece  of  apparatus  for  micro-measurement  is 
without  doubt  the  SCREW-MICROMETER  EYE-PIECE  ;  it  was  invented 
by  William  Gascoigne  in  1639  for  telescopes,  and  if  well  constructed 
is  a  most  valuable  adjunct  to  the  microscope.  It  is  made  by 
stretching  across  the  field  of  an  eye-piece  two  extremely  fine  parallel 
wires,  one  or  both  of  which  can  be  separated  by  the  action  of  a 
micrometer  screw,  the  circumference  of  the  brass  head  of  which  is 
divided  into  a  convenient  number  of  parts,  which  successively  pass 
by  an  index  as  the  milled  head  is  turned ;  it  is  seen  in  fig.  212,  B. 
A  portion  of  the  field  of  view  on  one  side  is  cut  off  at  right  angles 
to  the  filaments  by  a  scale  formed  of  a  thin  plate  of  brass  having 
notches  at  its  edge,  whose  distance  corresponds  to  that  of  the  threads 
of  the  screw,  every  fifth  notch  being  made  deeper  than  the  rest  to 
make  the  work  of  enumeration  easier.  Formerly  one  filament  was 
stationary,  the  object  being  brought  into  such  a  position  that  one 
of  its  edges  appeared  to  touch  tKe  '"fixed  wire,  the  other  wire  being 
moved  by  the  micrometer  screw  until  it  appeared  to  lie  in  contact 


m 


FIG.  212. — The  micrometer  eye-piece. 

with  the  other  edge  of  the  object ;  the  number  of  entire  divisions 
on  the  scale  then  showed  how  many  complete  turns  of  the  screw  had 
been  made  in  the  separation  of  the  wires,  while  the  number  of 
index  -points  on  the  edge  of  the  milled  head  showed  the  value  of  the 
fraction  of  a  turn  that  might  have  been  made  in  addition.  Usually 
a  screw  with  100  threads  to  the  inch  is  employed,  which  gives  to 
each  division  in  the  scale  in  the  eye-piece  the  value  of  x^yth  of  an 
inch,  whilst  the  edge  of  the  milled  head  is  usually  divided  into  100 
parts. 

Both  wires  or  filaments  have  since  been  made  to  move,  a  screw 
and  divided  head  being  fixed  to  the  stationary  wire.  There  is  no 
advantage  in  this  plan,  and  it  involves  needless  complexity  in  calcu- 
lation. The  best  method,  there  can  be  no  doubt,  is  the  one  employed 
by  Mr.  Kelson,  which  is  to  have  one1  thread  fixed,  but  not  in  the 
centre  of  the  eye-piece,  but  five  notches  in  the  scale  from  the  centre 
on  the  side  furthest  from  the  screw-head.  This  not  only  permits  of 
a  much  larger  object  being  spanned,  but  also  keeps  the  average  of 
measurements  in  the  middle  of  the  '  field.'  This  is  not  only 


2/2 


ACCESSORY  APPARATUS 


convenient  but  important,  because  the  magnification  is  not  uniform 
throughout  the  field.  If  the  power  employed  is  high,  in  order  to 
effect  the  span  of  the  great  magnification,  one  wire  (the  fixed  central 
one)  will  be  in  the  middle  of  the  field,  the  other  at  the  margin,  and 
the  comparison  will  not  be  true  on  account  of  the  unequal  magnifi- 
cation of  the  eye-piece  throughout  the  field,  whereas  if  the  wire  be 
placed  five  notches  on  one  side,  both  measurements  are  brought  more 
within  the  centre  of  the  field. 

Messrs.  Zeiss  now  make  a  Ramsden  micrometer  eye-piece.  It  is 
provided  with  a  glass  plate  with  crossed  lines,  which  together  with 
the  eye-piece  are  carried  across  the  image  formed  by  the  objective 
by  means  of  the  measuring  screw,  so  that  the  adjustment  always 
remains  in  the  centre  of  the  field  of  view. 


FIG.  213. 

Fig.  213  illustrates  this  instrument,  complete  and  in  longitudinal 
section. 

Each  division  on  the  edge  of  the  drum  corresponds  to  0*002  mm. 
Whole  turns  are  counted  on  a  numbered  scale  seen  in  the  visual 
field,  and  the  image  may  be  measured  up  to  8  mm. 

A  modification  of  this  instrument,  facilitating  both  accuracy  and 
simplicity,  was  in  1890  devised  by  Mr.  Nelson,1  of  which  we  think 
highly,  and  of  which  we  give  an  illustration  in  fig.  214. 

This  screw  micrometer  eye-piece  differs  from  those  of  the  old 
form  mainly  in  two  respeets  :  first,  the  optical  part  is  compensated  ; 
secondly,  the  micrometer  part  with  both  webs  can  be  made  to 
traverse  en  bloc  the  field  of  the  eye-piece  by  screw  motion. 

More  particularly  speaking,  the  instrument  consists  of  two  parts  : 
1  Journ.  B.  M.  S.  1890,  p:  508. 


THE   BEST   FORM   OF  MICROMETER   EYE-PIECE  273 

one,  a  flat  rectangular  box  containing  the  fixed  and  movable  webs, 
the  micrometer  screw,  and  divided  head  complete ;  the  other  part 
may  be  called  an  '  eye-piece  adapter,'  with  an  outer  case  to  hold  the 
above-mentioned  rectangular  box. 

The  flat  inner  box  has  a  screw  attached  to  it  which  engages  with 
a  head  on  the  exterior  of  the  outer  box.  This  gives  about  one  inch 
of  screw  movement  to  the  inner  box,  which  causes  the  webs  to 
traverse  the  field  of  the  microscope.  It  must  be  remembered  that 
this  in  no  way  affects  the  movement  of  the  movable  web  from  the 
fixed,  which  can  alone  be  accomplished  by  turning  the  graduated 
micrometer  head  as  in  the  old  form. 

The  '  eye-piece  adapter '  portion  of  the  instrument  is,  as  its  name 
implies,  merely  an  adapter  to  take  the  optical  part  of  positive  com- 
pensating eye-pieces  of  various  powers. 

Immediately  below  the  web  itfaji  iris  diaphragm.  This  permits 
a  diaphragm  to  be  used  suitable  to  the  power  of  the  eye-piece 
employed.  A  guiding  line  at  right  angles  to  the  webs  has  been 
added.  Care  must  be  taken  to  observe  that  when  the  movable  web 
coincides  precisely  with  the 
fixed  web,  the  indicator  on  the 
graduated  head  stands  at  zero. 
If  this  is  not  the  case,  the 
finger  screw  must  be  loosed, 
which  will  liberate  the  gradu- 
ated head,  and  then  it  can  be 
placed  in  its  proper  position 
and  fixed.  This  is  of  universal 
application  to  all  screw  micro- 
meters. 

Four  points  are  gained  by  _     __ 

this  arrangement :-  FlG  214._Nelson.s  new  form  of  8Crew 

(1)  The  compensating  eye-  micrometer  eye-piece, 
piece  yields  far  better  defini- 
tion when  measuring  with  apochromatic  objectives  than  either  the 
Huyghenian  or  Ramsden  forms. 

(2)  Different-powered  eye-pieces  can  be  employed. 

(3)  By  means  of  the  screw  which  moves  the  micrometer  webs 
across  the  field  it  is  possible  to  perform  measurements  with  the  webs 
equidistant  from  the  centre  of  the  field,  and  thus  eliminate  errors 
due  to  distortion. 

(4)  The  preceding  advantage  is  secured  without  sacrificing  the 
benefit  of  a  fixed  zero  web. 

Messrs.  Zeiss  have  since  .adapted  the  compensating  eye-piece  to 
their  best  screw  micrometer. 

To  use  the  screw  micrometer  vnik  success  it  should  not  be  inserted, 
as  the  custom  has  been,  like  an  ordinary  eye-piece  into  the  tube  of 
the  microscope,  but  it  should  have  a  form  stand  quite  independently, 
preventing  actual  contact  with  the  body- tube. 

Plate  II.  gives  the  mode  of  its  employment,  the  illustration  being 
made  from  a  photograph  by  Mr.  Nelson.  The  micrometer  eye-piece, 
it  will  be  seen,  is  fitted  into  a  stand  wholly  independent  of  the 

T 


2/4  ACCESSORY   APPARATUS 

microscope.  This  consists  of  a  strong  upright,  fitted  into  a  massive 
tripod  or  circular  foot.  The  foot  in  either  case  only  rests  on  three 
points  ;  the  upright  is  capable  of  telescopic  extension  by  a  clamping 
tube  ;  a  short  tube  which  takes  the  eye-piece  is  fixed  to  this  upright 
by  a  compass  joint. 

To  use  it,  the  object  to  be  measured  is  placed  in  position,  and 
the  microscope  inclined  in  the  usual  way.  The  ordinary  eye-piece 
is  removed,  and  the  separate  stand  with  the  micrometer  in  its  place 
is  put  in  front  of  the  microscope,  the  extension  tube  being  raised  or 
lowered  until  the  tube  at  the  top  of  it,  carrying  the  micrometer  v  is 
made  continuous  with  the  tube  of  the  microscope,  as  seen  in  the 
drawing.  It  is  well  to  leave  from  j-th  to  f^ths  of  an  inch  of  space 
between  the  body-tube  and  the  micrometer  tube.  It  will  be  now- 
needful  to  employ  corrections  to  compensate  for  the  increased  length 
of  tube.  If  the  objective  be  provided  with  a  '  correction  collar  '  the 
adjustment  must  be  re-corrected  ;  but  if  it  is  not  so  provided  the  tube 
of  the  microscope  must  be  shortened  exactly  as  much  as  the  tube 
carrying  the  micrometer  will  have  lengthened  it. 

By  this  arrangement  it  will  be  found  that  manipulation  can  be 
effected  without  the  vibration  of  the  microscopical  image  which  is  in- 
evitably the  result  of  the  revolving  of  the  micrometer  screw  head  when 
the  micrometer  eye-piece  is  placed,  as  it  usually  has  been,  in  the  body- 
tube  of  the  microscope.  The  consequence  is  that  much  more  minute 
spaces  can  be  measured,  and  with  much  greater  accuracy.  Mr. 
Nelson  has  repeatedly  spanned  the  y^^th  of  an  inch  by  means  of 
a  stage  micrometer  in  the  focus  of  the  objective  ;  this  was  replaced 
by  a  mounted  specimen  of  Amphiplvura  pellucida,  and  he  has  counted 
ninety-six  lines  in  the  y  oVo^n  °f  an  incn  by  making  the  movable  wire 
pass  successively  over  them  until  the  fixed  wire  was  reached.  By 
similar  means  the  Editor  has  measured  single  objects  less  than  the 

iooWtn  of  an  inch- 

It  will  have  been  premised  by  the  careful  reader  that  the  stage 

micrometer  must  be  used  in  every  set  of  measurements ;  at  least  we 
would  strictly  emphasise  this  as  the  only  accurate  and  scientific 
method.  It  has  been  advised  that  a  record  of  comparisons  with  the 
various  lenses  in  the  possession  of  the  microscopist  should  be  made 
once  for  all.  We  decidedly  deprecate  this  method,  unless  it  be  in 
such  utterly  valueless  work,  as  is  sometimes  done,  where  lenses  are 
uncorrected  and  accuracy  of  tube-length  forgotten  or  ignored.  The 
correction  of  an  objective  and  the  tube-length  ought  to  vary  with 
every  object,  and  therefore  a  comparison  of  the  stage-micrometer 
and  the  screw-micrometer  should  be  made  with  every  set  of  measure- 
ments. 

Moreover,  the  majority  of  stage  micrometers  exhibit  very  con- 
siderable discrepancies  in  the  several  intervals  between  the  lines ; 
it  is  well  in  the  interests  of  accuracy  to  take  the  screw  value  of  each 
under  a  high  power,  find  the  value  of  the  average,  and  then  note  the 
particular  space  or  spaces  that  may  be  in  agreement  with  the  average 
and  always  use  it.  An  illustration  will  make  this  clear. 

Zeiss  provides  a  stage  micrometer  of  1  mm.  divided  into  '1  and 


rt  - 
£* 


: 


TO    OBTAIN   THE    VALUE    OF   A   MICROMETER   INTERVAL  275 

•01.     The  following  are  the  actual  values  obtained  for  each  of  the  -05 
divisions,  viz.  : —  ft-4.0 

8-37 
8-38 
8-38 
8-36 
8-36 
8-58 
8-33 
8-31 
8-47 
8-33 
8-33 
•1B.-.38 
8-44 
8-38 
8-40 
8-37 
8-40 
8-25 
8-38 
20)16-760 

8' 38  mean  value. 

In  this  instance  it  will  be  seen  that  the  last  division,  8'38,  agrees 
with  the  mean,  and  is  the  best  for  all  future  use.1 

Having  thus  obtained  a  screw-micrometer  value  for  a  certain 
known  interval,  the  screw-micrometer  value  for  any  other  object 
being  known,  the  size  of  the  object  may  be  found  by  simple  propor- 
tion ;  thus,  viz.  if  8*38  is  the  screw-micrometer  value  for  '05  mm. 
and  6'45  that  for  a  certain  object,  the  size  of  the  object  is 

(i)  8-38  :  6-45:: -05 


6-45  x  -05      AOOK 
*=-83g-=-0386mm- 

If  the  answer  is  required  in  fractions  of  an  English  inch,  all  that 
we  need  remember  is  that  1  inch=25"4  mm. ;  then 

(ii)  8-38  :  6-45::^|  :  x  inch ; 

6-45  x -00197     -0127 
*=      ~¥38— =8^- ='001515  inch. 

If  the  stage-micrometer  is  ruled  in  fractions  of  English  inches, 
then  suppose  the  screw-micrometer  value  for  -nnro^h  inch =4- 257, 
and  that  for  the  object=6'45  as  before. 

(iii)  4-257  :  6'45  : :  '001  :  x  inch  ; 

6-45  x  -001 


4-257 


-=•001515  inch. 


1  In  the  number  given  for  screw  value  the  whole  number  stands  for  a  complete 
revolution  or  number  of  revolutions  of  the  screw  head,  and  the  decimal,  the  portion 
of  a  revolution  read  off  beyond  this. 

T2 


2/6  ACCESSOEY  APPARATUS 

If  the  answer  is  required  in  metrical  measurement,  then  as   1 
inch  =  25*4  mm., 

(iv)  4-257  :  6'45  ::  (-001  x25'4)  :  xmm.  ; 

6-45  x  '0254     -1638 


In  this  connection  it  will  be  as  well  to  give  two  examples  of 
scale  comparison  which  are  sometimes  required.  Thus  you  have  a 
certain  interval  on  a  metrical  stage  micrometer  which  you  know  to 
be  accurate,  and  you  wish  to  compare  an  English  stage  micrometer 
with  this  scale  in  order  to  find  out  which  particular  interval  of  T^OTT 
inch  agrees  with  it.  Suppose  '05  mm.  =  8'38  screw  value  as  above, 
then  all  that  is  necessary  is  to  find  the  point  to  which  the  screw 
micrometer  must  be  set  in  order  that  it  may  accurately  span  the 
y^Vo  incn-  Take  1  inch  =25  -4  mm.  as  before  ;  then  '001  inch= 
•0254. 

(v)         -05mm.  :  '0254mm.  ::  8'38  :  x  screw  value; 


•05 

Conversely,  if  a  metrical  scale  is  to  be  compared  with  an  accurate 
English  one  where  '001   inch=4*257  screw  value,  then  the  screw 
value  for  '05  mm.  may  be  found  thus  :  '001  iiich='0254  mm. 
(vi)         -0254mm.  :  '05mm.  ::  4'257  :  x  screw  value; 
-05  x  4'257  vaiue  for  -05  mm. 


•0254 

A  cheap  substitute  for  the  screw  micrometer  has  been  devised  by 
Mr.  G.  Jackson.     It  consists  in  having  a  transparent  arbitrary  scale 

inserted  into  an  or- 
dinary Huyghenian 
eye-piece  in  the  focus 
of  the  eye-lens,  so 
that  it  will  be  in  the 
same  plane  as  the 
magnified  image  of 
the  object  to  be 
measured.  It  is  seen 
in  fig.  215.  The 
method  of  using  it  is 
precisely  similar  to 
that  of  the  screw 
micrometer ;  the 

value  of  T^Q-Q  incn  01' 
-jL  mm.,  as  the  case 
may  be,  is  found  in 


J 


FIG.  215.— Jackson's  eye-piece  micrometer.  terms  of  the  arbitrary 

scale.     The  value  of 

the  object  in  terms  of  the  same  scale  is  also  found,  and  comparison 
made  accordingly.  All  that  need  be  done  is  to  substitute  the  terms 
of  the  arbitrary  scale  for  screw  values  in  the  preceding  examples,  and 
they  will  meet  the  case. 


ESTIMATING   THE   EDGES   OF  MINUTE   OBJECTS          277 

The  arbitrary  scale  should  be  capable  of  movement  by  a  screw, 
otherwise  the  appliance  is  hardly  as  accurate  as  the  first  method  of 
micrometry  by  simple  drawing  described  above. 

Of  all  the  methods  of  micrometry  the  most  accurate  is  that 
performed  by  photo-micrography.  A  negative  of  the  object  to  be 
measured  is  taken,  and  then,  without  any  alteration  in  tube-  or 
camera-length,  the  magnified  image  of  the  stage  micrometer  is  pro- 
jected on  the  ground  glass ;  this  is  spanned  by  means  of  a  pair  of 
spring  dividers.  The  negative  film  is  then  scratched  by  these 
dividers.  Then  you  are  in  a  position  to  make  the  most  accurate 
measurement  the  microscope  is  capable  of  yielding. 

It  is  exceedingly  important,  when  performing  micrometric 
measurements,  to  remember  that  the  precise  edges  of  all  objects  in 
the  microscope  are  never  seen.  Consequently  it  is  impossible  to 
ascertain  from  what  point  to  wh^it  point  the  measurement  is  to  be 
made. 

This,  while  hardly  affecting  large  and  coarse  objects,  becomes 
supremely  important  with  small  objects. 

Instead  of  a  real  edge  to  an  object  you  get  diffraction  bands. 
These  bands  alter  with  focus,  and  also  to  a  greater  extent  with  the 
angle  of  the  illuminating  cone  as  well  as  with  the  aperture  of  the 
objective.  Hence  it  ensues  that  the  accurate  micrometry  of  delicate 
objects  presents  one  of  the  most  difficult  matters  encountered  in 
practical  microscopy.  At  the  present  time  opinions  differ  greatly 
as  to  the  treatment  of  particular  cases. 

The  following  plan  of  Mr.  Nelson's  is  the  outcome  of  a  long  series 
of  experiments  : — 

1.  The  focus  and  adjustment  to  be  chosen  may  be  termed  that  of 
the    *  black   dot '    (see  Elimination  of  Errors   of   Interpretation)  ; 
in  other  words,  if  the  object  were  a  slender  filament  it  would  be 
represented  white  with  black  edges.     These  black  edges  are  due  to 
diffraction.     If  the  filament  is  very  slender  and  the  illuminating  cone 
small,  there  may  be  seen  a  white  diffraction  edge  outside  the  black 
one,  and  perhaps  another  faint  black  one  outside  that  again. 

2.  Reduce  as  far  as  possible  the  extent  of  these  diffraction  bands 
by  (a)  using  an  objective  with  as  large  an  aperture  as  possible  ;  (b) 
by  using  as  large  an  illuminating  cone  as  possible. 

3.  Measure  from  the  inner  edge  of  the  inner  diffraction  band  to 
the  inner  edge  of  the  inner  diffraction  band  on  the  opposite  side. 

4.  But  if  the  diameter  of  a  hole  be  required,  then  the  measure- 
ment must  be  made  from  the  outer  edge  of  the  outer  black  diffraction 
band  to  the  outer  edge  of  outer  diffraction  band  on  the  opposite  side. 
It  must  not  be  forgotten,  however,  that  these  rules  only  apply  for  a 
particular  focus  and  a  particular  adjustment. 

II.  The  Camera  Lucida  and  its  Uses. — There  are  a  large  number 
of  contrivances  devised  for  the  purpose  of  enabling  the  observer 
to  see  the  image  of  an  object  projected  on  a  surface  upon  which  he 
may  trace  its  outlines,  but  they  resolve  themselves  practically  into 
two  kinds,  viz.  : — 

1 .  Those  intended  for  use  when  the  microscope  is  in  a  horizontal 
position. 


2/8  ACCESSORY   APPARATUS 

2.  Those  provided  for  it  when  used  in  a  vertical  position. 
We  shall  describe  what  we  consider  the  most  practical  forms  of 
each. 

In  point  of  antiquity  Wollastoris  camera  lucida  claims  the  post 
of  honour ;  but  to  use  it  the  microscope  must  be  placed  in  a  hori- 
zontal position.  Its  general  form  is  shown  in  fig.  216.  The  rays 
on  leaving  the  eye-piece,  above  which  it  is  fixed  by  a  collar,  enter  a 
prism,  and  after  two  internal  reflections  pass  upwards  to  the  eye  of 
the  observer.  It  is  easy  to  see  a  projection  of  the  microscopic  image 
with  this  instrument,  but  it  is  when  we  desire  at  the  same  time  to 
see  the  paper  and  the  fingers  holding  the  pencil  that  the  difficulty 
begins.  The  eye  has  to  be  held  in  such  a  position  that  the  edge  of 
the  prism  bisects  the  pupil,  so  that  one-half  of  the  pupil  receives 
the  microscopic  image  and  the  other  half  the  images  of  the  paper 
and  the  hand  employed  in  drawing.  If  this  bisection  is  not  equal, 
too  much  of  one  image  is  seen  at  the  expense  of  the  other.  This 
was  in  some  sense  supposed  to  be  compensated  by  the  use  of  lenses, 
as  seen  in  the  figure ;  but  the  difficulty  of  keeping  the  eye  precisely 
in  one  position  has  caused  this  instrument  to  fall  into  disuse,  several 

cameras  being  now  devised  free  from 
this  defect.  It  has  nevertheless  one 
special  point  in  its  favour — it  does 
not  invert  the  image,  causing  the 


FIG.  216.  FIG.  217.— Simple  camera. 

right  to  be  turned  to  the  left,  and  vice  versa.  This  is  an  advantage 
the  value  of  which  we  shall  subsequently  see. 

A  simple  camera  was  made  by  Soemmering  by  means  of  a  small 
circular  reflector,  usually  made  of  highly  polished  steel,  which  is 
placed  in  the  path  of  the  emergent  pencil  at  an  angle  of  45°  to  the 
optic  axis,  thus  reflecting  rays  from  the  image  upwards.  The 
instrument,  though  rarely  used  now,  is  shown  in  fig.  217,  and  slides 
on  to  the  eye-piece.  The  reflector  must  be  smaller  than  the  pupil 
of  the  eye,  because  it  is  through  the  peripheral  portion  of  the  pupil 
that  the  rays,  not  stopped  out  by  the  mirror,  come  from  the  paper 
and  pencil.  Hence,  as  in  the  case  of  Wollaston's  camera,  the  pupil 
of  the  eye  must  be  kept  perfectly  centred  to  the  small  reflector. 
As  there  is  but  one  reflection,  the  image  is  inverted,  but  not  trans- 
posed. To  see  the  outline  of  the  image  as  it  is  in  the  microscope, 
the  drawing  must  be  made  upon  tracing  paper,  and  inverted,  looking 
at  it  as  a  transparency  from  the  wrong  side. 

There  is  considerable  variety  in  the  experience  of  different 
microscopists  as  to  the  facilitv  with  which  these  two  instruments 
can  be  used.  The  difference  in  all  probability  depends  on  the 


CAMERA   LUCID^E 


279 


FIG.  218. 
Beale's  camera. 


greater  normal  diameter  of  the  pupils  of  the  eyes  of  some  observers 
in  comparison  with  that  of  others'. 

Dr.  Lionel  Scale  devised  one  of  the  simplest  cameras,  which  has 
the  advantage  of  being  thoroughly  efficient.  It  consists  of  a  piece 
of  tinted  glass  placed  at  an  angle  of  45°  to  the  optic 
axis,  in  the  path  of  the  emergent  pencil.  The  idea 
was  first  suggested  by  Amici,  but  he  employed  un- 
colourecl  glass  ;  Dr.  Beale  made  it  practical  by  the 
employment  of  tinted  glass.  The  first  surface  of  the 
glass  reflects  the  magnified  image  upwards  to  the  eye, 
the  paper  and  pencil  being  seen  through  the  glass. 
In  its  simplest  form  it  is  seen  in  fig.  218.  The  glass 
is  tinted  to  render  the  second  reflection  from  the  internal  surface 
of  the  glass  inoperative.  The  reflection  of  the  image  is  identical 
with  that  of  Soemmer ing's. 

Another  camera  lucida  of  soihje.  merit  is  that  devised  by  Amici, 
and  adapted  to  the  horizontal  microscope  ^by  Chevalier.  The  eye 
looks  through  the  microscope  at  the  object  (as  in  the  ordinary  view 
of  it),  instead  of  looking  at  its  projection  upon  the  paper,  the  image 
of  the  tracing  point  being  projected  upon  the  field — an  arrangement 
which  is  in  many  respects  more  advantageous.  This  is  effected  by 
combining  a  perforated  silver-on-glass  mirror  with  a  reflecting 
prism  ;  and  its  action  will  be  understood  by  the  accompanying 
diagram  (fig.  219).  The  ray  a  b  proceeding  from  the  object,  after 
emerging  from  the  eye-piece  of 
the  microscope,  passes  through 
the  central  perforation  in  the 
oblique  mirror  M,  which  is  placed 
in  front  of  it,  and  so  directly 
onwards  to  the  eye.  On  the  other 
hand,  the  ray  a!,  proceeding  up- 
wards from  the  tracing  point, 
enters  the  prism  P,  is  reflected 
from  its  inclined  surface  to  the 
inclined  surface  of  the  mirror  M, 
and  is  by  it  reflected  to  the  eye 
at  b',  in  such  parallelism  to  the 
ray  b  proceeding  from  the  object 
that  the  two  blend  into  one 
image. 

A  valuable  and  simple  little 
camera  was  devised  by  Mr.  E.  M. 
Nelson  in  1894.1  It  takes  into  FIG.  219. 

account  the  fact  that  while  that 

form  known  as  Beale's  neutral  tint  (fig.  218)  has  been  of  great 
value  and  persistence,  it  is  yet  a  defective  form ;  the  microscopic 
image  as  received  at  the  eye-piece  is  inverted  and  transposed. 
Beale's  camera  corrects  the  inversion,  while  it  leaves  the  transposi- 
tion unaltered ;  therefore  all  the  objects  drawn  with  this  camera 
are  unlike  the  originals.  In  illustration  place  the  letter  p  on 
1  Journ.  B,  M.  S,  1895,  p,  21  et  seq. 


280 


ACCESSOKY   APPARATUS 


the  stage  in  the  position  as  here  printed  ;  when  examined  by  the 
microscope  it  will  appear  thus  -J .  In  order  to  look  at  this  letter  as 
the  original,  all  that  we  have  to  do  is  to  turn  this  paper  round. 
But  this  object,  as  drawn  by  a  Beale's  camera,  will  appear  "^,  and  no 
turning  of  the  paper  can  cause  it  to  appear  as  the  original ;  it  will 
only  become  so  when  it  is  viewed  as  a  transparency  from  the  other 
side  of  the  paper. 

This  is,  of  course,  important  in  many  matters  with  which  the 
microscopic  biologist  is  concerned. 

In  many  forms  of  camera  this  difficulty  has  been  overcome  by 
reflecting  the  image  of  the  paper  and  pencil  down  the  tube  of  the 
microscope.  The  drawing  there  made  will  be  inverted  and  trans- 
posed, but  by  turning  the  picture  round  we  at  once  get  a  correct 
representation  of  the  object  itself. 

The  new  camera  devised  by  Mr.  Nelson  consists  of  a  right-angled 
prism  or  small  glass  mirror  fixed  at  an  angle  of  45°  to  an  eye-piece 
cap.  This,  when  the  microscope  is  placed  in  a  horizontal  position, 
reflects  the  rays  horizontally  arid  at  right  angles  to  the  optic  axis  ; 
these  rays  then  fall  on  a  piece  of  neutral-tint  glass  placed  at  an 
angle  of  45°  to  those  rays  so  as  to  reflect  them  upwards  to  the  eye. 

The  mirror  corrects  the  transposition,  and  the  neutral-tint  the 
inversion  ;  an  erect  image  is  therefore  seen  on  the  table.  The  neutral- 
tint  glass  is  mounted  on  a  pivot  so  that  it  may  be  turned  round  at 
a  right  angle  ;  this  adapts  the  instrument  for  use  with  either  the 
right  or  left  eye.  Should  the  light  be  too  strong,  it  must  be 
modified  by  screens,  not  by  change  of  focus  in  the  condenser,  assum- 
ing that  the  perfect  image  has  been  obtained. 

On  the  important  subject  of  the  inversion  and  transposition  of 
microscopic  images  brief  but  valuable  data  are  given  and  put  in  the 
clearest  light,  thus  : — 


Object  on  the 
stage. 

F 


Image  seen  through 
the  eye-piece. 


Image  projected  on  screen 
or  on  sensitive  plate. 


Image  seen  through  Woll- 
aston's  camera. 


linage  seen  through 
ground  glass. 

F 
6 

Image  seen  through 
Beale's  neutral  tint  or 
Soenmieriug's  reflector. 


Image  seen  through  Nel- 
son's camera. 


Image  projected  on.  table 
by  45°  mirror  or  right- 
angled  prism,  as  devised 
by  C.  W.  Cooke. 

F 

The  instrument  referred  to  in  (7)  of  the  above  table  of  inversion 
and  transposition  in  microscopic  images  is  a  somewhat  distinct  form 
of  camera  called  by  Mr.  Conrad  W.  Cooke,  who  devised  it  in  1865,  a 
'  Micrographic  Camera.'  The  projection  of  the  image  is  dependent 
on  a  silvered  mirror  fixed  at  45°,  or  a  right-angled  prism.  By  the 
arrangement  of  this  instrument  an  image  can  be  thrown  on  a  sheet 
of  paper  placed  in  a  horizontal  position,  so  that  one  can  readily  trace 


ABBE'S    CAMERA   LUC1DA 


28l 


on  the  paper  the  outlines  and  details  of  the  image  with  ease  and 
accuracy;  only  it  must  be  remembered  that  the  mirror  or  prism 
erects  the  inverted  image  (No.  2  in  the  above  table),  but  its  trans- 
position is  due  to  the  fact  of  its  not  being  viewed  as  a  transparency. 

This  instrument  is  also  useful  for  the  purpose  of  demonstrating 
where  two  or  three  persons  may  at  the  same  time  examine  the 
image,  and  it  can  be  used  on  many  opaque  objects,  and  objects  pre- 
sented by  dark  ground  illumination  ;  but  to  use  it  the  external  light 
must  be  carefully  screened  from  the  observer. 

Coming  now  to  the  second  group  of  cameras,  there  stands  first  on 
the  list  an  instrument  devised  by  Professor  Abbe  ;  although,  like 
many  '  new  '  apparatus  for  the  microscope,  the  idea  it  embodies  is 
not  a  new  one,  but  was  suggested  for  micrometric  purposes  by  Mr. 
G.  Burch  in  1878  (Journ.  Qvek.  Micro.  Club,  v.  p.  47).  We  have 
used  this  admirable  instrument  With  complete  success. 

The  accompanying  drawing  (fig.  220)  will  at  once  show  the 
simplicity  of  its  action.  The  image  of  the  paper  and  pencil  coming, 
say,  in  a  vertical  direction  (S2  fig.  220),  is  reflected  by  a  large  mirror 


-sp 


FIG.  220. — Abbe's  camera  lucida. 


in  a  horizontal  direction,  W,  to  a  cube  of  glass  which  has  a  silvered 
diagonal  plane  with  a  small  circular  hole  in  it  in  the  visual  point  of 
the  eye-piece.  The  microscopic  image  is  seen  directly  through  this 
aperture  in  the  silvering  of  the  prism,  while  the  silvered  plane  of 
the  prism  transmits  the  image  of  the  paper  and  the  operator's  fingers 
and  pencil.  By  the  concentricity  thus  obtained  of  the  bundle  of 
rays  reaching  the  eye  from  both  the  microscope  and  the  paper,  the 
image  and  the  pencil  with  which  it  is  to  be  drawn  are  seen  coinci- 
dentally  without  any  straining  of  the  eyes. 

This  instrument  requires  the  paper  to  be  placed  in  a  plane 
parallel  to  that  of  the  object ;  thus,  if  the  microscope  is  vertical  the 
paper  must  be  horizontal,  and  vice  versa,  and  it  presents  the  image 
precisely  as  it  is  seen  in  the  microscope.  For  the  purpose  of  drawing 
simply,  and  where  the  observer  has  had  no  experience  in  the  use  of 
a  camera  lucida,  we  should  be  inclined  to  recommend  this  one  as  the 
instrument  presenting  to  the  tyro  the  greatest  facility.  But  there 
is  a  use  to  be  made  of  the  camera  lucida  to  which  this  one  does  not  so 
readily  lend  itself,  which  is  none  the  less  of  great  importance  ;  that  is, 


282  ACCESSOEY   APPARATUS 

the  determining-  of  the  magnifying  power  of  objectives.  It  is  manifest 
that  the  distance  between  the  paper  and  the  eye  of  the  observer 
cannot  be  so  readily  determined  in  this  case  as  in  those  forms  of  the 
instrument  where  the  image  of  the  paper  and  pencil  is  seen  direct. 

The  same  apparatus  arranged  so  that  the  prism  casing  together 
with  the  mirror  may  be  swung  back  while  the  clamping  collar 
remains  on  the  tube  in  its  adjusted  position,  is  shown  in  fig.  221. 
The  mirror  has  a  surface  of  75x50  mm.  (3x2  in.),  and  may  be 
inclined  at  any  angle  between  the  horizontal  plane  and  45°,  the 
latter  position  being  marked  by  a  stop.  The  length  of  the  arm 
supporting  the  mirror  being  lO5cm.  (4  in.),  it  is  only  with  very 
large  drawings  necessary  to  incline  or  raise  the  drawing  surface. 

But  the  latest  modification  of  this  instrument  is  shown  in  figs. 
222  and  223,  where  it  will  be  observed  that  the  camera  is  attached 
to  the  tube  by  means  of  the  clamping-ring  K,  and  the  Abbe  double 


FIG.  221. — Abbe's  camera,  improved. 

prism  is  centred  by  means  of  the  screws  L  and  H.  The  brightness 
of  the  drawing  surface  and  the  microscopic  image  is  respectively 
regulated  by  a  cap  R  encasing  the  prisms,  which  is  provided  with  a 
clear  opening  and  five  moderating  glasses  of  varying  degrees  of 
density,  and  by  an  eccentric  disc  B  pivoted  below  the  prisms,  which 
is  also  provided  with  a  clear  opening  and  five  moderating  glasses. 

In  order  to  completely  utilise  the  increased  cone  of  emerging- 
rays  obtained  with  low  magnifications,  the  usual  prism,  having  in  its 
silvering  an  aperture  of  1  mm.,  can  quickly  and  conveniently  be 
exchanged  for  another  with  an  aperture  of  2  mm. 

The  prism,  together  with  the  moderating  glasses,  may  be  turned 
aside  about  the  vertical  pin  Z  into  the  position  indicated  by  the 
dotted  lines  shown  in  fig.  222.  When  the  prism  is  returned  to  its 
original  position  it  is  fixed  by  a  catch,  which  is  not  externally 
visible. 

In  the  use  of  a  good  drawing  apparatus  (1)   the  light  from  the 


LATEST   FORMS   OF   ABBE'S   CAMERA   LUCID  A  283 

image  must  not  to  any  serious  extent  be  weakened  by  the  light  from 
the  drawing  material.     (2)  The  image  of  the   drawing  paper  must 

11 


FIG.  222. — Latest  modification  of  Abbe's  camera 


reach  the  eye  with  the  least  possible  intensity  and  be  coaxial  with  the 
microscopic  image.     (3)  There  should  be  an  arrangement  by  which 


FIG.  223. 


the  relation  of  the  intensities  of  these  two  images  can  be  modified 
to  suit  each  other.     (4)  The  apparatus  must  be  adjustable  in  height 


284  ACCESSORY   APPARATUS 

and  capable  of  being  centred  in  its  horizontal  plane.  (5)  It  should 
be  possible  to  easily  separate  the  apparatus  from  the  eye-piece  and 
replace  it  again  in  its  former  position  at  will.  (6)  The  image  of  the 
plane  of  the  drawing,  and  the  image  of  the  microscopic  object  pro- 
jected on  it,  must  be  seen  with  the  apparatus  without  distortion. 
As  regards  the  first  two  conditions  the  arrangement  of  the  original 
Abbe  camera  is  adopted,  viz.  two  rectangular  prisms  with  the  hypo- 
tenuses cemented  together,  of  which  one  is  silvered,  with  a  small 
portion  of  the  silver  deposit  in  the  centre  taken  away,  and  with  these 
a  second  mirror  A,  fig.  222,  for  transmitting  the  image  of  the  plane 
of  the  drawing  to  this  prism.  But  since  one  and  the  same  prism, 
with  a  fixed  opening  in  its  silver  deposit,  cannot  suffice  for  all 
purposes  and  changes  of  magnification,  an  arrangement  is  added  by 
which  the  prism  P,  fig.  223,  with  its  fastening,  can  be  easily  taken 
out  of  the  apparatus  and  replaced  by  another  \vith  an  opening  of 
different  size. 

With  respect  to  the  third  condition  securing  a  due  relation 
between  the  intensities  of  the  two  images,  an  arrangement  of  two 
smoked-glass  wedges  was  made  to  move  over  each  other  so  as 
to  form  a  plate  of  continuously  varying  thickness.  This  was  most 
satisfactory  but  too  costly,  so  smoked-glass  plates  were  employed 
and  set  in  the  cylindrical  wall  of  a  small  cap,  R,  figs.  222,  223,  which 
was  simply  placed  over  the  prism.  Each  smoked  glass  in  turn  can 
be  interposed  in  the  path  of  the  rays  by  turning  the  cap  on  its 
upper  edge  until  a  small  pin  engages  in  a  corresponding  small  hole  on 
the  lower  edge  of  the  cylinder.  There  are  five  smoked  glasses  of 
different  densities  of  colour,  while  one  aperture  is  left  empty. 

The  adjustment  in  height  is  satisfied  by  the  apparatus  being 
attached  to  the  body-tube  by  means  of  a  clamping  screw,  while  the 
adjustment  from  side  to  side  is  effected  by  the  prism,  together  with 
the  cap  and  smoked-glass  disc,  being  centred  from  front  to  back  by 
means  of  a  screw,  H,  figs.  222,  223,  working  through  a  spring  socket, 
and  from  right  to  left  by  means  of  a  second  screw  L,  against  which 
works  a  counter-spring  not  shown  in  the  figures. 

In  order  to  pass  conveniently  from  observation  through  this  ap- 
paratus to  observation  through  the  free  eye-piece,  the  prism  with  its 
diaphragm  arrangement  can  be  rotated  to  one  side  about  a  vertical 
pin  Z  ;  the  return  of  the  prism  to  its  central  position  is  marked  by 
a  spring  catch.  To  obtain  drawings  free  from  distortion,  a  drawing 
table  similar  to  that  described  by  Dr.  Bernhard  ought  to  be 
employed.1 

This  useful  instrument  has,  however,  been  modified  and  made 
simpler  by  more  than  one  optical  firm.  Messrs.  Swift  have  con- 
structed a  very  handy  and  easily  applied  form,  which  is  so  arranged 
that  the  microscope  may  be  employed  with  it  not  only  in  the 
vertical  but  also  in  an  inclined  position.  It  is  illustrated  in  fig.  224. 

This  camera  lucida  is  precisely  on  the  same  principle  as  the  Abbe 
form  used  for  the  same  purpose,  but  being  manifestly  less  bulky  it  is 
far  more  convenient  and  easier  to  use,  although  less  efficient  for  very 
careful  work. 

1  Zeitschr.  f.  wiss.  Mikr.  xi.  (1894),  pp.  289-801. 


ENGLISH  AND   AMERICAN  MODIFICATIONS 


285 


When  this  form  of  camera  is  used,  the  paper  upon  which  the 
object  is  received  should  be  tilted  to  the  same  plane  as  the  stage  of 
microscope  upon  which  the  object  rests,  as  this  will  pi-event  any 
marginal  distortion. 

Another  extremely  good  and  easily  applied 
modification  of  the  Abbe  form  is  manufactured  by 
Bausch  and  Lomb.  and  is  illustrated  in  fig.  225. 
The  Abbe  prism  is  used  as  in  the  large  Abbe 
drawing  camera  ;  the  mirror  is  reduced  in  size 
and  is  fixed.  The  path  of  the  light  is  seen  to  be 
the  same  as  the  white  dotted  lines  and  arrows 
show,  as  in  the  complete  form  of  Abbe  ;  and  the 
camera  may  be  swung  back  when  not  in  use,  as 
shown  in  the  dotted  outline.  We  can  testify  that 
the  image  off  both  object  and  pencil-point  are  clear, 
and  this  instrument  can  be  used  with  most  eye-pieces  ;  but  cannot  for 
complete  results  be  counted  equal  to  the  drawing  camera  of  Abbe. 

The  Editor  has  used  with  great  facility  and  success  a  camera  devised 
by  Dr.  Hugo  Schroder,  and  produced  by  Messrs.  Ross.  It  is  figured 
at  226,  and  consists  of  a  combination  of  a  right-angled  prism  (fig. 
227)  ABC,  and  a  rhomboidal  prism  D  E  F  G,  so  arranged  that  when 


FIG.  224.  —  Swift's 
camera  lucida  on 
the  Abbe  principle. 


FIG.  225. — Bausch  and  Lomb's  modification  of  Abbe's  camera. 

adjusted  very  nearly  in  contact  (i.e.  separated  by  only  a  thin  stra- 
tum of  aii-)  the  faces  B  C  and  D  E  are  parallel,  and  consequently 
between  D  E  and  B  E'  they  act  together  as  a  thick  parallel  plate  of 
glass  through  which  the  drawing  paper  and  pencil  can  be  seen. 
The  rhomboidal  prism  is  so  constructed  that  when  the  face  G  F  is 
applied  at  right  angles  to  the  optic  axis  of  the  microscope,  the  axial 
ray  H  passes  without  refraction  to  I  on  the  internal  face  E  F ; 
whence  it  is  totally  reflected  to  J  in  the  face  D  G .  At  J  a  part  of 


286 


ACCESSORY   APPARATUS 


the  ray  is  reflected  to  the  eye  by  ordinary  reflection  in  the  direction 
of  J  K,  and  a  part  transmitted  to  3'  on  the  face  A  C  of  the  right- 
angled  prism.  Of  the  latter  a  portion  is  also  reflected  to  K  by 
ordinary  reflection  at  J'.  The  hypotenuse  face  A  C  is  cut  at  such 
an  angle  that  the  reflection  from  J'  coincides  with  that  from  J  at 
the  eye-point  K,  thus  utilising  the  secondary  reflection  to  strengthen 
the  luminosity  of  the  image.  The  angle  G  is  arranged  so  that  the 
extreme  marginal  ray  H'  from  the  field  of  the  B  eye-piece  strikes 
upon  D  G  at  a  point  just  beyond  the  angle  of  total  reflection,  the 
diffraction  bands  at  the  limiting  angle  being  faintly  discernible  at 
this  edge  of  the  field.  This  angle  gives  the  greatest  amount  of 
light  by  ordinary  reflection,  short  of  total  reflection. 

In  use,  the  microscope  should  be  inclined  at  an  angle  of  45°,  and 
the  image  focussed  through  the  eye-piece  as  usual ;  the  camera  is 
then  placed  in  position  on  the  eye-piece,  and  pushed  down  until  the 
image  of  the  object  is  fully  and  well  seen.  The  drawing  paper 
must  be  fixed  upon  a  table  on  a  level  with  the  stage  immediately 


FIG.  226.— Schroder's 
camera  lucida. 


FIG.  227. — Diagram  explaining  Schroder's  camera  lucida. 


under  the  camera.  The  observer  will  then  see  the  microscopical 
image  projected  on  the  paper,  and  the  fingers  carrying  the  pencil 
point  will  be  clearly  in  view,  the  whole  pupil  of  the  eye  being 
available  for  both  images,  the  diaphragm  on  the  instrument  being 
considerably  larger  than  the  pupil.  The  eye  may  be  removed  as 
often  as  required,  and,  if  all  is  allowed  to  remain  without  alteration, 
the  drawing  may  be  left  and  recommenced  without  the  slightest  shift- 
ing of  the  image. 

If  a  vertical  position  of  the  microscope  be  needful,  this  may  be 
done  by  inclining  the  table  and  drawing  paper  to  an  angle  of  45° 
either  in  front  or  at  the  side  of  the  microscope.  For  accurate 
drawing,  in  all  azimuths,  the  drawing  paper  should  of  course  coin- 
cide with  the  plane  of  the  optical  image.  When  the  paper  is  in  its 
proper  position,  the  limiting  circle  of  the  field  of  the  microscope 
will  be  projected  as  a  true  circle,  but  if  otherwise  it  will  appear 
elliptical.  It  is  recommended  that  a  circle  about  the  size  of  the 
field  be  drawn  upon  the  paper,  and  its  coincidence  with  the  projected 
field  compared. 


T.HE  USE  OF  THE  CAMEEA  LUCID  A       287 

This  camera  may  be  used  with  a  hand-magnifier,  or  with  simple 
lenses  used  for  dissection  and  other  purposes. 

With  one  or  other  of  the  foregoing  contrivances,  every  one  may 
learn  to  draw  an  outline  of  the  microscopic  image  ;  and  it  is  extremely 
desirable  for  the  sake  of  accuracy  that  every  representation  of  an 
object  should  be  based  on  such  a  delineation.  Some  persons  will  use 
one  instrument  more  readily,  some  another,  the  fact  being  that 
there  is  a  sort  of  '  knack '  in  the  use  of  each  which  is  commonly 
acquired  by  practice  alone,  so  that  a  person  accustomed  to  the  use 
of  any  one  of  them  does  not  at  first  work  well  with  another. 
Although  some  persons  at  once  acquire  the  power  of  seeing  the 
image  and  the  tracing  point  with  equal  distinctness,  the  case  is  more 
frequently  otherwise  ;  and  hence  no  one  should  allow  himself  to 
be  baffled  by  the  failure  of  his  f rst  attempt.  It  will  sometimes 
happen,  especially  when  the  Wolfaston  prism  is  employed,  that  the 
want  of  power  to  see  the  pencil  is  due  to  the  faulty  position  of  the 
eye,  too  large  a  part  of  it  being  over  the  prism  itself.  When  once 
a  good  position  has  been  obtained,  the  eye  should  be  held  there  as 
steadily  as  possible,  until  the  tracing  shall  have  been  completed.  It 
is  essential  to  keep  in  view  that  the  proportion  between  the  size  of 
the  tracing  and  that  of  the  object  is  affected  by  the  distance  of  the 
eye  from  the  paper  ;  and  hence  that  if  the  microscope  be  placed 
upon  a  support  of  different  height,  or  the  eye-piece  be  elevated  or 
depressed  by  a  slight  inclination  given  to  the  body,  the  scale  will  be 
altered.  This  it  is,  of  course,  peculiarly  important  to  bear  in  mind 
when  a  series  of  tracings  is  being  made  of  any  set  of  objects  which 
it  is  intended  to  delineate  on  a  uniform  scale. 

A  valuable  adjunct  to  a  camera  lucida  is  a  small  paraffin  lamp, 
seen  to  the  left  of  plate  III.,  which  illustrates  the  correct  method  01 
using  the  camera  lucida.  This  lamp  is  simple,  and  is  capable  of  being 
raised  or  lowered,  fitted  with  a  paper  shade,  for  a  great  deal  of  the 
success  attendant  on  the  use  of  the  camera  depends  on  the  relative 
illumination  of  the  microscopic  image  on  the  one  side,  and  of  the  paper 
and  fingers  and  pencil  of  the  executant  on  the  other.  It  is  not  a 
matter  to  be  determined  by  rules ;  personal  equation,  sometimes 
idiosyncrasy,  determines  how  the  light  shall  be  regulated.  Many 
finished  micro-draughtsmen  use  a  feeble  light  in  the  image  and  a 
strong  light  on  the  hand  and  paper,  and  others  equally  successful 
manipulate  in  the  precisely  reverse  way.  But  upon  the  adjustment 
of  the  respective  sources  of  light  to  the  personal  comfort  of  the 
draughtsman  will  depend  his  success. 

Care  must  be  exercised  in  this  work  in  the  case  of  witical  images. 
These  must  not  be  sacrificed  either  by  racking  the  condenser  into  or 
out  of  focus,  or  by  reducing  its  angle  by  a  diaphragm.  If  the  in- 
tensity of  the  light  has  to  be  reduced,  it  must  be  done  by  the  inter- 
position of  glass  screens,  and  this  is  beautifully  provided  in  Abbe's 
camera.  The  illustration  of  how  the  various  apparatus  for  the  use 
of  the  camera  lucida  should  be  disposed,  given  in  plate  III.,  may  be 
profitably  studied.  Both  mirror  and  bull's-eye  are  turned  aside, 
and  the  hand  and  pencil  are  illuminated  by  the  shaded  lamp. 

The  lamp  illuminating  the  image  is  seen,  with  such  a  screen  of 


288  ACCESSORY   APPARATUS 

coloured  glass  as  may  be  found  needful,  and  the  lamp  illuminating 
the  paper  and  pencil,  and  carefully  shaded  above,  is  also  seen  at  the 
eye-piece  end  of  the  body-tube.  Often,  if  the  image  is  too  bright, 
we  find  that  bringing  the  lamp  down  to  illuminate  the  paper  more 
intensely  suffices  If  not,  use  screens  ;  the  illuminating  cone  must 
not  be  tampered  with. 

III.  The  Determination  of  Magnifying  Power  is  an  important 
and  independent  branch  of  this  subject.  For  this  purpose,  and  for 
the  reason  given  above,  Beale's  neutral -tint  camera  l  is  eminently 
suitable — indeed,  is  the  best.  We  can  easily  and  accurately  measure 
the  path  of  the  ray  from  the  paper  to  the  eye.  What  is  necessary  is 
to  project  the  image  of  a  stage  micrometer  on  to  an  accurate  scale 
placed  ten  inches  from  the  eye-lens  of  the  eye-piece.  There  must  be 
complete  accuracy  in  this  matter. 

We  can  best  show  how  absolute  magnifying  power  is  thus  deter- 
mined by  an  example. 

Suppose  that  the  magnified  image  of  two  TTJL^ths  of  an  inch 
divisions  of  the  stage  micrometer  spans  T8^ths  of  an  inch  on  a  rule 
placed  as  required  ;  then 

(i)     '002  inch  :  '8  inch  ::  1  inch  :  x  power  ; 

x-='-~—  =400  diameters ; 
•002 

for  it  is  obvious  that  under  these  conditions  one  inch  bears  the  same 
proportion  to  the  magnifying  power  that  T^¥ths  of  an  inch  bears  to 
i%-ths  of  an  inch. 

Suppose,  now,  as  it  sometimes  happens,  that  the  operator  is  pro- 
vided with  a  metrical  stage  micrometer,  but  is  without  a  metrical 
scale  to  compare  it  with,  there  being  nothing  but  an  ordinary  foot- 
rule  at  hand. 

Let  it  be  assumed  that  the  magnified  image  of  two  T^  mm.  when 
projected  covers  T8F  inch ;  then,  as  there  are  25'4  mm.  in  one  inch, 

(ii)     '02  mm.  :  (-8  inch  x  25'4) ::  1  :  x  power  ; 

•8x25-4x1      1n1«   ,. 

x= — =1015  diameters. 

'02 

If  the  reverse  is  the  case,  viz.  that  you  have  an  English  stage 
micrometer  and  a  metrical  scale,  then,  if  the  magnified  image  of  two 
1TTi__ths  of  an  inch  spans  18  mm., 

1  R 

(iii)       -002  inch  :    "L  : :  1  :  x  ; 
.2o*4 

•7087x1     o^  o  A- 
x= .=  d54'o  diameters. 

The  above  results  indicate  the  combined  magnifying  power  of  the 
objective  and  eye-piece  taken  at  a  distance  of  ten  inches.  The  arbi- 
trary distance  of  ten  inches  is  selected  as  being  the  accommodation 
distance  for  normal  vision. 

The  magnifying  power,  however,  is  very  different  in  the  case  of 

1  Page  279. 


TO   FIND   THE   INITIAL   POWER   OF   A  LENS  289 

a  myopic  observer.  Let  us  investigate  the  case  of  one  whose  accom- 
modation distance  is  five  inches. 

Here  he  will  be  obliged,  in  order  to  see  the  object  distinctly,  to 
form  the  virtual  image  from  the  eye -piece  at  a  distance  of  five  inches. 
To  do  this  he  must  cause  the  objective  conjugate  focus  to  approach 
the  eye-lens ;  consequently  he  must  shorten  his  anterior  objective 
focus.  In  other  words,  he  must  focus  his  objective  nearer  the 
object.  This  will  have  the  effect  of  causing  the  posterior  conjugate 
focus  to  recede  from  the  objective  towards  the  eye -lens,  and  the  fact 
of  bringing  the  inverted  objective  image  nearer  the  eye-lens  brings 
also  the  virtual  image  of  the  eye-lens  nearer. 

Shortening  the  focus  of  the  objective  has  the  effect  of  increasing 
its  power ;  but  as  this  alteration  is  proportionately  very  little,  the 
increase  in  power  is  very  small ;  but  the  shortening  of  the  eye-piece 
virtual  from  ten  to  five  inches  has  the  effect  of  nearly  halving  its 
power.  Consequently  the  combined  result  of  the  eye-piece  and 
objective,  in  the  case  of  halving  the  eye-piece  virtual,  is  to  nearly 
halve  the  power  of  the  microscope.  The  increase  of  the  objective 
power  is  practically  so  small  that  it  may  be  neglected.1  In  practice 
it  is  found  by  us  that  if  the  image  is  projected  on  a  ground-glass 
screen  ten  inches  from  the  eye-piece,  the  image  is  nearly  the  same 
size  whether  focussed  by  ordinary  or  myopic  sight.  This  is  in 
harmony  with  Abbe's  demonstration  that  both  images  are  seen 
under  the  same  visual  angle.  But,  on  the  other  hand,  if  a  myopic 
sight  compares  the  image  with  a  scale,  the  magnification  will  be 
less  than  with  ordinary  vision,  because  the  observer  with  myopic 
sight  must  bring  the  scale  to  a  shorter  distance  than  ten  inches 
in  order  to  see  it. 

To  find  the  precise  initial  power  of  any  lens,  or  to  find  the  exact 
multiplying  power  of  any  eye-piece,  is  not  so  easy.  A  laborious 
calculation,  involving  the  knowledge  of  the  distances,  thickness,  and 
refractive  indices  of  the  lenses,  is  required.  But  a  very  approximate 
determination,  sufficiently  accurate  for  all  practical  purposes,  may 
be  easily  made,  especially  if  one  has  a  photo-micrographic  camera  at 
hand.  The  principle  is  as  follows  : — 

Select  a  lens  of  medium  power — a  J-inch  is  very  suitable.  Now, 
with  the  microscope  in  a  horizontal  position,  and  with  a  powerful 
illumination,  project  the  image  of  the  stage  micrometer  on  to  a  screen 
distant  five  feet,  measured  from  the  front  lens  of  the  objective.  If  no 
photo-micrographic  camera  is  at  hand,  it  will  be  necessary  to  perform 
the  experiment  in  a  darkened  room,  shading  the  illuminating  source. 
Divide  the  magnifying  power  thus  obtained  by  6  ;  the  quotient  will 
give  the  initial  powder  of  the  lens  at  ten  inches  to  a  very  near  approxi- 
mation. 

The  reason  why  the  result  is  not  perfectly  accurate  is  that  the 
ten  inches  must  be  measured  from  the  posterior  principal  focus  of 
the  lens,  and  that  is  a  point  which  is  not  given.  But  in  the  case  of 
a  power  such  as  a  J,  it  is,  in  practice,  found  to  be  very  near  the  front 
lens  of  the  objective.  So  by  taking  a  long  distance,  such  as  five  feet, 

1  English  Mechanic,  vol.  xlvi.  No.  1185.  Article  on  measurements  of  magnifying 
power  of  microscope  objectives,  by  E.  M.  Nelson. 


290 


ACCESSOKY  APPARATUS 


the  error  introduced  by  a  small  displacement  of  the  posterior  prin- 
cipal focus  does  not  materially  amount  to  much. 

There  is  a  further  error  introduced  by  the  approximation  of  the 
objective  to  the  stage  micrometer  in  order  to  focus  the  conjugate  at 
such  a  distance,  but  this  is  small.  We  can  see,  therefore,  that  this 
error  tends  to  slightly  increase  the  initial  magnifying  power. 

The  initial  power  of  the  J  being 
found,  and  its  combined  magnifying 
power,  with  a  given  eye-piece,  being 
known,  the  combined  power  divided 
by  the  initial  power  gives  the  multi- 
plying power  of  the  eye-piece.  Care 
must  be  of  course  taken  to  notice  the 
tube-length  l  when  the  combined  power 
is  measured.  The  initial  power  of  any 
other  lens  may  be  found  by  dividing 
the  combined  power  of  that  lens  with 
the  eye -piece,  whose  multiplying  power 
has  been  determined,  by  the  multiplying 
power  of  that  eye-piece.2 

Nose-pieces. — The  term  *  nose-piece' 

primarily  means  that  part  of  a  micros'cope  into  which  the  objective 
screws,  but  the  term  is  also  applied  to  various  pieces  of  apparatus 
which  can  be  fitted  between  the  nose-piece  of  the  microscope  and 
the  objective.  There  are,  for  instance,  rotating,  calotte,  centring, 
changing,  and  analysing  nose-pieces. 

Nose-pieces,  although  thought  to  be  so,  are  not  a  modern  idea ; 
our  predecessors  of  a  century  ago  employed  similar  means.  Mr. 
Crisp  has  recently  acquired  a  microscope  which  possesses  a  double 
arm,  at  the  end  of  which  is  a  cell  for  receiving  different  lenses. 
This  cell  fits  over  the  end  of  the  nose-piece,  and  so  keeps  the  several 
objectives  which  may  be  inserted  in  position.  It  dates,  in  all  proba- 
bility, from  the  end  of  the  seventeenth  or  the  early  part  of  the 
eighteenth  century. 

But  in  the  early  days  of  the  microscope  rotating  discs  of  objec- 
tives, as  shown  in  fig.  228  (or,  perhaps,  older  still,  a  long  dovetailed 


FIG.  228. — Kotating  disc  of 
objectives.  Benj.  Martin 
(circa  1776). 


FIG.  229.— .Sliding  plate  of  objectives.     Adams  (1771). 

slide    of    objectives,    such    as    fig.     229    shows),    were    frequently 
employed. 

It  is  continually  desirable  to  be  able  to  substitute  one  objective 
for  another  with  as  little  expenditure  of  time  and  trouble  as  possible, 
so  as  to  be  able  to  examine  under  a  higher  magnifying  power  the 
details  of  an  object  of  which  a  general  view  has  been  obtained  by 

1  English  Mechanic,  vol.  xxxviii.  No.  981,  '  Optical  Tube-length,   by  Frank  Crisp. 

2  Ibid.  vol.  xlvi.  No.  1178, '  Measurement  of  Power,'  by^E.  M.  Nelson. 


NOSE-PIECES 


291 


means  of  a  lower  ;  or  to  use  the  lower  for  the  purpose  of  finding  a 
minute  object  (such  as  a  particular  diatom  in  the  midst  of  a  slideful) 
which  we  wish  to  submit  to  higher  amplification.  This  was  con- 
veniently effected  by  the  nose-piece  of  Mr.  C.  Brooke,  which,  being 
screwed  into  the  object  end  of  the  body  of  the  microscope,  .carries 
two  objectives,  either  of  which  may  be  brought  into  position  by 
turning  the  arm  on  a  pivot.  This  is  shown  in  fig.  230. 

The  most  generally  useful  of 
all    nose-pieces   IIOWT  in  use   are 
the  rotating  forms,  which  enable 
one  to  carry  two,  three,  or  four 
objectives  on    the  microscope  at 
one  time,  and  by  mere  rotation 
each     is     successively      brought 
central  to  the  optic  axis,  seen  ih.4 
figs.   231,   232,   233,  as  supplied 
by  Messrs.  Beck.     It  is  almost 
unnecessary  now  to  point  out  the 
disadvantage  of  those  older  and 
straight     forms    which    involved 
the  danger  of  knocking  out  the 
front  lens  of  the   objectives  by 
bringing   it   into   contact  with  some  part   of  the  stage  while  the 
other  objective  was   being  focussed.     This   objection   was   entirely 
removed  by  the  introduction  of  the  bent  form  by  Messrs.  Powell  and 
Lealand,  and  adopted  in  the  forms  shown  in  figs.  231-233.    There  can 


FIG.  230.— Brooke's  nose- 
piece,  as  made  by  Swift. 


FIG.  232. 


FIG.  233. 


be  no  doubt  that  for  ordinary  dry  lens  \vork  some  such  device  is  im- 
perative. Some,  however,  who  do  a  very  large  amount  of  microscopical 
work  prefer  to  use  two  microscopes  ;  the  one  a  third-  or  fourth-class 
microscope,  with  only  a  coarse  adjustment  and  a  1-inch  objective  and 
mirror,  the  other  having  a  coarse  and  fine  adjustment  and  a  J-inch 
objective,  with  a  simple  form  of  condenser  and  plane  mirror,  all  fine 
and  higher-power  work  being  left  for  a  special  microscope. 

The  one  drawback  to  the  use  of  a  rotating  nose-piece  is  the  extra 
weight  it  throws  upon  the  fine  adjustment.     As  this  subject  is  fully 

u2 


292  ACCESSOEY  APPARATUS 

treated  under  the  heading  of  '  Microscope/  no  more  will  be  said  at 
present  than  that  a  double  nose-piece  is  to  be  preferred  to  a  triple, 
and  a  quadruple  need  not  be  entertained  for  a  delicate  instrument 
when  made  of  ordinary  metal,  unless  it  is  required  to  find  out  in  how 
short  a  time  a  fine  adjustment  may  be  ruined  ;  for  let  it  be  noted 
that  a  2-inch,  1-inch,  J-inch,  and  J-inch  objective  of  English  make 
weigh  together  8J  oz.  without  any  nose-piece.  But  Messrs.  Watson 
and  Son  have  devised  and  made  in  aluminium  a  dust-proof  triple 
nose-piece,  which,  where  it  is  required  to  be  used,  reduces  the  objec- 
tions to  its  employment  to  their  minimum,  and  not  only  in  greatly 
reduced  weight,  but  in  other  ways,  makes  its  use  more  feasible 
without  strain  upon  the  fine  adjustment  or  danger  of  injury  to  the 
objectives.  In  many  nose-pieces,  if  the  objectives  should  be  acci- 
dentally left  so  that  neither  of  them  is  in  the  optical  axis  of  the 
microscope,  there  is  nothing  to  guard  the  back  lenses  of  the  objec- 
tives from  dust  and  moisture.  Messrs.  Watson  devised  a  dust- 


FIG.  234. — Watson's  dust-proof  aluminium  nose-piece. 


FIG.  235. — Section  of  the  above. 

proof  arrangement,  consisting  of  an  upper  and  an  under  disc,  having 
a  spherical  curve  ;  to  the  lower  disc  are  fitted  three  small  screw 
tubes  which  receive  the  objectives.  This  plate  rotates  upon  a  centre 
pin,  and  as  each  objective  is  brought  into  the  optical  axis  of  the 
microscope  its  axial  coincidence  is  indicated  by  a  spring  catch.  The 
edge  is  covered  with  a  metal  rim,  making  it  dust-proof.  The  weight 
of  the  ordinary  brass  nose-piece  is  4|  oz. ;  the  weight  of  this  one  is 
lj  oz.  Similar  instruments  are  made  by  other  makers,  but  the 
dust-proof  arrangement  and  the  extreme  lightness  are,  so  far  as  we 
know,  characteristic  of  the  instrument  of  Messrs.  Watson.  We 
illustrate  this  nose-piece  complete  in  fig.  234,  and  in  an  enlarged 
section  in  fig.  235. 

For  the  proper  use  of  a  rotating  nose-piece  the  length  of  the 
objective  mounts  should  be  so  arranged  that  when  the  objective  is 
changed  little  focal  adjustment  will  be  necessary. 

An  excellent  calotte  nose-piece  for  four  objectives  is  made  by 


CHANGING   NOSE-PIECES  393 

Zeiss ;  this  is  so  arranged  that  only  the  optical  portion  of  the  objec- 
tive is  screwed  into  the  nose-piece.  This  plan  much  lightens  it,  so 
that  the  nose-piece  and  the  four  lenses  weigh  3|  oz.,  or  only  1  oz. 
more  than  an  English  J-inch  with  a  screw  collar,  and  ^  oz.  more  than 
an  English  ^-inch  of  wide  angle. 

A  centring  nose-piece  has  been  made  with  the  view  of  placing 
any  objective  central  to  the  axis  of  rotation  of  the  stage.  It  is,  of 
course,  much  cheaper  to  centre  an  objective  by  means  of  a  nose-piece 
to  the  axis  of  rotation  of  the  stage  than  to  centre  the  rotary  stage 
to  the  objective.  This,  like  all  other  adapters,  is  an  additional 
weight ;  but  here  there  is  very  little  to  be  gained  by  it,  for  if  the 
rotary  stage  is  well  made  any  objective  will  be  sufficiently  centred 
for  all  practical  purposes.  Mr.  Nelson,  as  we  have  seen,  pointed 
out,  at  a  time  when  the  sub-stage  was  costly,  that  such  a  nose- 
piece  turned  upside  down,  with  a,turn-out  rotating  ring  for  stops,  &c., 
fitted  below,  made  a  very  efficient  rectangular  centring  sub-stage 
at  a  small  cost.  Sub-stages  are  now  quite  common  and  cheap,  and 
centring  nose-pieces  are  seldom  used  for  any  purpose. 

Next  to  the  rotating,  probably  the  changing  nose-piece  is  the 
most  important.  We  do  not  know  from  whom,  and  when,  the  idea 
of  an  arrangement  by  which  an  objective  could  be  rapidly  attached 
or  detached  originated ;  but  certain  it  is  that  the  idea  is  admirable, 
and  one  which  is  scarcely  yet  as  fully  appreciated  as  it  should  be. 
It  will  be  quite  impossible  to  go  through  a  tithe  of  the  appliances 
which  have  been  invented  for  this  purpose ;  it  will  be  sufficient  to 
lay  down  some  principles,  and  mention  a  few  in  which  those  prin- 
ciples are  fulfilled. 

The  first  principle  is  that  the  objective  or  nose-piece,  adapter, 
or  whatever  else  is  used,  should  '  face  up.'  This  means  that  a  flange 
turned  true  in  the  lathe  should  '  face  up  '  to  the  flat  side  of  the  nose- 
piece,  which  has  also  been  turned  true.  This  '  facing  up  '  should  be 
made  tight  by  a  screw,  inclined  plane,  or  wedge,  itc.  Unless  this 
is  done  you  have  no  guarantee  that  the  axis  of  the  objective  is 
parallel  to  that  of  the  body.  Therefore  all  those  appliances  which 
merely  grip  the  objective,  or  an  adapter  screwed  on  to  the  objective, 
are  simply  of  no  value.  Secondly,  the  appliance,  whatever  it  is, 
should  be  light. 

Xachet's  changing  nose-piece,  which  fulfils  none  of  these  con- 
ditions, cannot  be  called  good.  The  nose-piece  is  large  and  heavy, 
even  for  the  small  objective  it  is  intended  to  take,  the  screws  of 
which  are  -/¥  only  in  diameter,  against  the  |-|  of  that  of  the  Society. 
The  objectives  are  held  by  a  spring  clip  on  a  small  flange.  Of  course, 
screw-collar  adjustment  with  such  a  device  would  be  simply  im- 
possible. Zeiss's  sliding-objective  changer  is  most  elaborate  and 
efficient,  although,  as  wTe  think,  much  heavier  than  it  need  be.  It 
consists  of  a  grooved  slide  which  screws  on  to  the  nose-piece.  On 
each  objective  is  screwed  an  adapter  to  slide  into  the  grooved  nose- 
piece.  These  adapters,  which  are  wedge-shaped  and  i  face  up/  have 
two  novel  features,  the  first  being  that  they  are  each  fitted  with 
rectangular  centring  adjustments,  which  permit  the  objectives 
to  be  centred  to  one  another  ;  and  the  second  is  that  they  have 


294 


ACCESSORY  APPARATUS 


adapters  to  equalise  the  length  of  the  objectives,  so  when  a  change 
of  objectives  is  made  little  change  of  focal  adjustment  is  required. 
Figs.  236,  237  show  the  nature  of  this  arrangement.  In  Nelson's 
changing  nose-piece  a  small  ring  with  three  studs  is  screwed  on  to 
the  objective  ;  a  nose-piece  is  screwed  on  the  microscope,  having 
three  slots  and  three  inclined  planes.  Therefore,  by  placing  the  studs 
into  the  slots  and  giving  the  objective  a  quarter  of  a  turn,  the 
studs  run  up  the  inclined  planes,  thus  causing  the  flanges  to  '  face 
up '  tightly. 

Mr.  Nelson  has  pointed  out  a  far  better  and  simpler  method 
which  dispenses  with  all  extra  apparatus. 

Three  portions  of  the  thread  in  the  nose-piece  of  the  microscope 
itself  are  cut  away,  and  also  three  portions  on  the  screw  of  the 


FIG.    236.  —  Zeiss's    sliding-objective 
changer,  with  objective  in  position. 


FIG.  237. — The  objective  detached  from 
the  body-slide. 


objective.  Those  portions  where  the  thread  is  left  on  the  objective 
pass  through  those  spaces  in  the  nose-piece  where  it  has  been  cut 
away.  The  screw  engages  just  as  if  the  whole  screw  were  there,  and 
the  objective  faces  up  in  the  usual  manner.  This  plan  in  no  way 
injures  either  the  microscope  or  the  objectives  for  use  in  the  ordinary 
way ;  thus  uncut  objectives  will  screw  into  the  nose-piece,  and  cut 
objectives  will  screw  into  an  uncut  nose-piece.  This  plan  is  similar 
to  that  employed  in  closing  the  breech  of  guns,  and  it  was  seeing  one 
of  them  in  1882  which  suggested  to  Mr.  Nelson  to  adapt  the  same 
principle  to  the  microscope.  Subsequently  it  has  been  found  that 
in  1869  Mr.  James  Yogan  had  proposed  much  the  same  plan,  only 
cutting  away  two  portions  instead  of  three  ;  it  is  curious  that  such 
an  excellent  idea  was  allowed  to  drop. 

An  analysing  nose-piece  is  that  which  carries  a  Nicol's  analysing 


FINDEES 


295 


prism  for  polariscope  purposes.  In  some  the  prism  is  fixed  in  the 
nose-piece,  whereas  it  ought  to  be  capable  of  rotation.  Lastly  we 
have  a  revolving  nose-piece  for  the  purpose  of  testing  objectives. 
Mr.  Nelson,  in  a  paper  read  before  the  Quekett  Microscopical  Club, 
February  1885,  stated  that  he  had  observed  that  certain  objectives 
performed  better  when  the  object  was  placed  in  a  definite  azimuth. 
With  a  view  to  eliminate  any  possible  alteration  which  might  arise 
from  the  revolution  of  the  object  with  regard  to  the  light,  he  had 
designed  a  revolving  nose-piece  which  enabled  the  objective  itself  to 
be  revolved  true  to  the  optic  axis  when  any  imperfection  in  its 
performance  in  a  particular  azimuth  could  be  immediately  noted. 
This  plan  had,  however,  been  previously  in  use  by  Professor  Abbe 
for  a  similar  purpose,  but  not,  as  wre  believe,  made  public. 

Finders. — A  finder  is  a  very  important  and  valuable  addition  to 
a  microscope.  By  its  means  the, position  of  any  particular  object  or 
part  of  an  object  in  a  mount  can  be  noted,  so  that  it  may  be  found 
again  on  any  subsequent  occasion.  In  working  on  a  microscope 
without  a  finder  it  frequently  happens  that  in  the  prosecution 
of  special  research,  or  in  the  examination  of  unknown  objects, 
something  is  seen  wrhich  it  would  be  of  the  utmost  value  to  recur  to 
again ;  but  the  amount  of  time  lost  in  transferring  the  object  to  a 
stand  with  a  finder  is  so  great  that  most  experienced  microscopists 
do  all  their  search  and  general  wrork  on  their  best  instruments  with 
finders. 

The  usefulness  of  the  finder  has  caused  a  large  number  to  be  de- 
vised ;  but,  as  in  all  cases,  we  consider  only  those  which  we  believe 
embody  the  best  practical  principles. 

The  first,  and  by  far  the  best,  is  the  graduation  of  the  stage 
plates  of  a  mechanical  stage  by  dividing  an  inch  into  100  parts,  both 
on  the  vertical  and  horizontal  plates.  The  vertical  stage-plate  will 
then  indicate  the  latitude,  and  the  horizontal  plate  the  longitude  of 
the  object,  the  slip  being  always  pressed  close  home  against  a  prepared 
stop.  For  many  years  Messrs.  Powell  and  Lealand  have  supplied 
their  No.  1  stand  with  this  kind  of  finder  ;  and  its  permanent  position 
and  ease  in  use  not  only  give  greater  facility  in  special  researches, 
but  in  reality  attach  a  new  value  to  every  slide  in  the  cabinet.  Such 
a  worker  at  critical  images  as  Mr.  Nelson  has  weeks  of  close  work 
'  logged  '  on  the  labels  of  his  slides.  A  still  better  plan  is  to  '  log  ' 
in  books  in  which  the  slides  are  numbered.  The  result  is  that  the 
labour  of  days  and  weeks  can  be  in  a  moment  recalled  for  demon- 
stration ;  and  so  accurate  is  this  method  that  an  object  so  small  as 
a  Bacterium  termo  or  a  specified  minute  diatom  in  a  thickly  scattered 
mounting  may  be  at  once,  and  as  often  as  we  please,  replaced  in  the 
field  with  even  high  powers. 

These  finders  of  course  are  only  suitable  for  the  microscope  on 
which  the  'log'  was  taken.  It  is  beneficial,  and  even  needful  at 
times,  to  interchange  specimens  or  refer  an  object  to  an  expert  at  a 
distance.  In  that  case  a  minute  dot  may  be  placed  on  the  cover,  or 
a  single  selected  diatom  or  other  object  may  be  fixed  upon  and  its 
latitude  and  longitude  as  read  on  the  microscope  of  the  sender  marked 
on  the  slide.  If  the  receiver  then  places  this  on  his  microscope  and 


296  ACCESSOKY  APPARATUS 

centres  it,  the  differences  in  latitude  and  longitude  may  be  noted,  and 
will  give  the  constants  for  the  correction  which  must  be  added  to 
or  subtracted  from  the  figures  given  by  the  sender. 

Mr.  Nelson  has  made  some  very  practical  suggestions  touching 
the  improvement  of  finders.  He  suggests,  what  we  heartily  accord 
with — 

1.  That  the  stage-stop  shall  be  always  on  the  left  hand  of  the 
stage. 

2.  That  the  zero  of  the  horizontal  graduation  shall  be  on  the  left 
hand  of  the  scale. 

3.  That  the  zero  of  the  vertical  graduation  shall  be  on  the  top  of 
the  scale. 

4.  That  when  the  finder  is  placed  to  0,  0,  a  spot  marked  on  the 
bottom  edge  of  a  3  x  1  inch  brass  template  two  inches  from  the  stop 
shall  be  in  the  optic  axis  of  the  instrument.     In  other  words,  the 
latitude  and  longitude  of  the  centre  of  a  3  x  1  inch  glass  slip  shall 
be  50,  50. 

5.  That  the  division  shall  be  in  T^ths  of  an  inch,  and  the  scales 
one  inch  long. 

If  these  very  simple  suggestions  were  adopted  generally,  an  object 
found  on  one  microscope  could  be  easily  found  on  any  other.  This, 
like  the  '  Society's  screw'  for  object-glasses  and  a  universal  sub-stage 
fitting,  deserves,  in  the  interests  of  international  microscopy,  the 
consideration  of  opticians. 

In  practical  '  logging '  the  use  of  a  hand  lens  will  enable  the  ob- 
server to  read  by  estimation  very  accurately ;  half  a  division  can  be 
very  approximately  judged  of,  and  this  is  as  close  as  will  be  required 
with  the  highest  powers.  We  have  found,  for  very  delicate  work, 
that  we  could  log  with  advantage  between  the  divisions,  thus :  say 
'  long.  41  ;'  but  if  slightly  over,  but  not  an  estimated  half,  *  41  +  ; '  if 
half,  ' 41^ ; '  if  more  than  this,  but  less  than  42,  it  is  logged  '  —  42.' 
For  logging  purposes  the  lens  we  recommend  is  one  of  Zeiss's  '  loups,' 
magnifying  six  diameters.  They  are  admirable  instruments,  and 
are  furnished  with  a  handle,  which  may  be  used  or  riot  at  the  wrill  of 
the  worker. 

The  other  finder  we  desire  to  consider  is  called  after  its  inventor, 
and  is  known,  as  '  Maltwood's  finder.' 1 

It  consists  of  a  micro-photograph,  one  square  inch  in  size,  divided 
into  2,500  little  squares,  so  that  each  is  ^th  inch  square.  Each 
square  contains  two  numbers,  one  indicating  the  latitude  and  one 
the  longitude.  To  log  any  object  the  slide  containing  the  object 
must  be  removed  and  the  slip  holding  the  micro- photograph  substi- 
tuted for  it ;  then  the  figure  in  the  square  which  most  nearly  agrees 
with  the  centre  of  the  field  is  noted.  Of  course,  both  the  object  and 
the  Maltwood  finder  must  be  carefully  made  to  abut  against  the 
stop. 

There  are  two  drawbacks  to  this  finder. 

1.  The  divisions  are  not  fine  enough,  so  that  it  is  only  suitable 
for  low  powers. 

2.  The  removal  of  the  slide,  and  its  substitution  by  the  Maltwood 

1  Trans,  of  the  Micro.  Soc.  new  series,  vol.  vi.  1858,  p.  59. 


DIAPHRAGMS  297 

finder,  render  it  extremely  unhandy  when  using  an  immersion  objec- 
tive, all  the  more  so  if  the  condenser  happens  to  be  immersed  as 
well. 

If  the  Maltwood  finders  are  made  alike,  they  are  then,  of  course, 
interchangeable. 

Diaphragms. — There  are  three  kinds  of  diaphragms  in  use. 
First,  the  commonest  form  is  that  of  a  rotating  disc  of  several  aper- 
tures graduated  as  to  size.  Secondly,  a  series  of  separate  small 
discs  of  metal,  with  a  single  central  aperture,  which  fits  in  a  suitable 
carrier.  Thirdly,  there  is  what  is  known  as  the  *  iris '  diaphragm, 
which  is  shown  in  fig.  238.  Upwards  of  30  years  ago  it  was  applied 
to  the  microscope  by  Beck  ;  it  has  since  been  brought  to  great  per- 
fection, some  being  made  with  as  many  as  sixteen  leaves ;  all  makers 
now  provide  them.  In  whatever  form  the  diaphragm  may  be  which 
is  for  use  with  the  mirror,  it  ite  important  that  it  should  not  be 
placed  too  near  the  object,  as  then" its  position  lies  so  near  the  apex 
of  the  cone  of  illumination 
that  it  will  not  cut  it  unless 
the  hole  be  exceedingly  small. 
A  very  small  diaphragm  aper- 
ture is  objectionable,  as  it  is 
liable  to  introduce  diflractional 
effects.  Therefore  it  is  better 
to  use  a  larger  aperture 
further  away  from  the  stage 
than  a  pin-hole-  near  the 
stage.  When  a  diaphragm  is 
used  in  connection  with  a 
condenser,  it  should  be  placed 
just  behind  the  back  lens,  and 
never  above  the  front  lens. 
Calotte  diaphragms  placed 
close  under  the  stage,  and  FIG.  238.— Zeiss's  iris  diaphragm, 
which  have  been  much  in  use 

lately,  both  here  and  on  the  Continent,  are  a  mistake  for  critical 
work.1 

A  very  good  wTay  of  cutting  down  a  cone  from  a  mirror  is  to 
have  the  diaphragm  fitted  in  the  sub-stage,  so  that  it  can  be  made 
to  advance  or  recede  from  the  object.  The  advantage  thus  gained 
is  that  one  aperture  is  made  to  do  the  duty  of  several.  It  also 
permits  of  careful  adjustment. 

The  iris  diaphragms  are  so  comparatively  inexpensive,  that  they 
have  superseded  for  general  work  and  ordinary  purposes  all  others  ; 
but  whatever  diaphragm  is  used  it  should  work  easily.  Iris  dia- 
phragms work  sometimes  so  stiffly  that  the  microscope  may  be  moved 
before  the  diaphragm.  So,  too,  with  the  diaphragm  wheels  ;  some 
require  a  pair  of  pliers  before  they  can  be  rotated.  This  is  easily 
accounted  for  when  we  examine  the  way  in  which  they  are  fixed.  The 
usual  method  is  to  screw  the  wheel  to  the  under  side  of  the  metal  stage. 
Now,  if  there  are  neither  washers  nor  a  shoulder  to  the  screw,  it  is 
1  Quekett,  Micro.  Journ.  vol.  iv.  p.  121  et  seq. 


298  ACCESSORY  APPARATUS 

more  than  probable  that  when  the  diaphragm  is  rotated  it  will 
screw  up  and  jam.  The  purchaser  may  easily  observe  a  matter  of 
this  kind.  Cylinder  diaphragms,  which  were  invented  in  1832  by 
C.  Yarley,  are  much  used  on  the  Continent ;  they  are  also  often  made 
into  iris  forms.  Also  diaphragms  with  a  very  minute  circular  hole 
in  the  line  of  the  optical  axis  are  largely  used  just  behind  the 
object -slip.  These  are  employed  with  the  mirror  only  (without 
condenser)  and  with  daylight  alone.  The  object  of  this  method 
of  illumination  being  to  render  very  translucent  objects  visible  by 
increasing  the  size  of  the  black  diffraction  bands  at  their  edges,  it 
is,  as  before  stated,  of  no  use  for  critical  work. 

Condensers  for  Sub-stage  Illumination.1 — This  condenser  is  an 
absolutely  indispensable  part  of  a  complete  microscope.  Its  value 
cannot  be  overrated,  for  the  ability  of  the  best  lenses  to  do  their 
best  work,  even  in  the  most  skilful  hands,  is  determined  by  it. 
Perfection  in  the  corrections  of  object-glasses  is  indispensable  ;  but 
those  who  suppose  and  affirm  that  this  is  all  that  we  need — that  the 
objective  is  the  microscope — cannot  understand  the  nature  of  modern 
critical  work.  The  importance  of  it  could  not  have  been  realised  in 
the  sense  in  which  we  know  it  in  the  earlier  dates  of  the  history  of 
the  instrument ;  but  at  as  early  a  period  as  1691  we  pointed  out 
(p.  134)  that  a  drawing  of  Bonanni's  horizontal  microscope  showed 
the  presence  of  a  condenser.  It  is,  in  fact,  of  some  interest  to  note 
how  our  modern  condensers  gradually  arose. 

The  microscope  that  amongst  the  older  forms  (1694)  appears 
most  efficient  and  suited  for  the  examination  of  objects  by  trans- 
mitted light  was  that  of  Hartsoeker  (p.  134,  fig.  102).  It  will  be 
remembered  that  it  was  furnished  not  only  with  a  condenser,  but  with 
a  focussing  arrangement  to  be  used  with  it,  which  was  not  in  any 
way  affected  by  a  change  of  focus  in  the  object.  This  is  a  feature 
which,  although  not  then  important,  is  of  the  utmost  importance  now. 

In  the  correction  of  dispersion  in  the  lenses  employed  in  the 
dioptric  form  of  microscope  so  much  difficulty  was  experienced  that 
several  efforts  were  made  to  produce  catoptric  forms  of  the  instru- 
ment ;  the  most  successful  of  these  was  that  of  Dr.  Smith,  of  Cam- 
bridge, in  1838  ;  but  this  and  all  other  forms  of  reflecting  microscope 
had  but  a  brief  existence,  and  passed  for  ever  away.  To  the  improve- 
ment of  simple  lenses  much  of  the  earlier  progress  of  microscopic 
investigation  is  attributable ;  and  that  known  as  '  Wollaston's 
doublet,'  devised  in  1829,  was  a  decided  improvement  in  all  respects. 
It  consisted  of  two  plano-convex  lenses  ;  but  this  was  again  improved 
by  Pritchard,  who  altered  the  lens  distances  and  placed  a  diaphragm 
between  the  lenses.  When  the  object  was  illuminated  with  a  con- 
denser this  formed  what  was  the  best  dioptric  microscope  of 
pre-achromatic  times. 

Good  results,  within  certain  limits,  may  be  obtained  by  means  of 
the  best  Pritchard  doublets.  With  a  -j^th  inch  the  surface  of  a 
strong  Podura  scale  may  be  seen  as  a  surface  symmetrically  scored 
or  engraved  ;  but  the  Editor  has  never  himself  been  able  to  reveal  the 

1  The  word  '  condenser  '  throughout  this  work  is  applied  to  optical  appliances  for 
the  sub-stage  ;  what  is  known  as  the  '  bull's-eye  '  is  not  called  a  '  condenser.' 


EAKLY   CONDENSERS  299 

'  exclamation '  marks,  and  as  this  is  the  experience  of  the  majority 
of  efficient  experts,  it  may  be  taken  that  no  resolution  of  these  was 
accomplished  in  pre-achromatic  days ;  these  lenses,  in  fact,  over- 
lapped the  discovery  of  Achromatism. 

But  the  practical  results  of  the  use  of  achromatic  lenses  soon  led 
experienced  men,  understanding  their  theory  and  practice,  to  perceive 
that  if  it  were  good  for  the  lenses  which  formed  the  image,  it  was 
also  good  for  the  condenser.  Thus  Sir  David  Brewster  in  1831  ad- 
vocated an  achromatic  condenser  in  these  remarkable  words,  viz.  :  '  I 
have  no  hesitation  in  saying  that  the  apparatus  for  illumination 
requires  to  be  as  perfect  as  the  apparatus  for  vision,  and  on  this  account 
I  would  recommend  that  the  illuminating  lens  sho^dd  be  perfectly 
free  from  chromatic  and  spherical  aberration,  and  that  the  greatest 
care  be  taken  to  exclude  all  extraneous  light,  both  from  the  object 
and  from  the  eye  of  the  observer. !>  ^This  is  a  judgment  which  every 
advance  in  the  construction  of  the  optical  part  of  the  microscope,  as 
used  by  the  most  accomplished  experts,  has  fully  confirmed. 

We  have  no  knowledge,  from  an  inspection  of  the  piece  of 
apparatus  itself,  of  the  construction  of  the  compound  sub-stage  con- 
denser of  Bonanni  (fig.  101);  it  does  not  appear  to  have  attracted 
much  attention,  and  of  course  it  was  quite  impossible  to  secure  a 
critical  image  by  its  means.  It  was  focussed  on  the  object  merely 
to  obtain  as  bright  an  illumination  as  possible,  in  order  that  the 
object  might  be  seen  at  all. 

In  the  condenser  used  by  Smith  in  his  catoptric  microscope 
(fig.  113)  we  have  the  earliest  (1738)  known  condenser,  by  means  of 
which  a  distinction  between  a  '  critical ;  image — that  is,  an  image  in 
which  a  sharp,  clear,  bright  definition  is  given  throughout,  free  from 
all  '  rottenness  '  of  outline  or  detail — and  an  '  uncritical '  or  imperfect 
image  could  be  made.  It  was  not,  apparently,  at  the  time  it  was 
first  used,  considered  to  be  so  important  as  we  now  know  it  to  be  ; 
and  it  is  probable  that  the  mode  of  focussing  the  light  upon  the 
object  by  its  means  was  to  direct  the  instrument  to  the  sky  with 
one  hand  and  to  use  the  biconvex  condenser  with  the  other.  In 
1837  Sir  D.  Brewster  writes  of  it  with  appreciation,  saying  that 
'  it  performs  wonderfully  well,  though  both  the  specula  have  their 
polish  considerably  injured.  It  shows  the  lines  on  some  of  the  test 
objects  with  very  considerable  sharpness.' 

No  advance  was  made  on  this  condenser  for  nearly  a  century. 
In  1829  Wollaston  recommends  the  focussing  of  the  image  of  the 
diaphragm  by  means  of  a  plano-convex  lens  of  J  of  an  inch  focus 
upon  the  object,  and  Goring  in  1832  says  concerning  it :  '  There  is  no 
modification  of  daylight  illumination  superior  to  that  invented  by 
Dr.  Wollaston.'  But  Sir  D.  Brewster  objected  to  this,  contending 
that  the  source  of  light  itself  should  be  focussed  upon  the  object.  He 
preferred  a  Herschelian  doublet  placed  in  the  optic  axis  of  the  micro- 
scope. But,  whilst  there  is  a  very  clear  difference  between  these 
authorities,  we  can  now  see  that  both  were  right. 

Goring,  who  was  also  a  leader  in  the  microscopy  of  his  day,  used 
diffused  daylight,  and  as  the  lens  he  employed  was  a  plano-convex 
of  |  of  an  inch  focus,  the  method  of  focussing  the  diaphragm  was  as 


300 


ACCESSORY  APPAEATUS 


good  sis  any  other,  because  the  diaphragm  was  placed  at  a  distance 
from  the  lens  of  at  least  five  times  its  focus,  so  that  the  difference 
between  diaphragm  focus  and  '  white  cloud '  focus,  or  the  focussing 
of  the  image  of  a  white  cloud  upon  the  object,  was  not  very  great. 
But  Brewster  was  writing  of  a  flame  from  a  saucer  of  burning  spirit 
and  salt  when  he  insisted  on  the  bringing  of  the  condenser  to  a 
focus  on  the  object,  and  in  this  he  was,  beyond  all  cavil,  right. 

In  1839  Andrew  Ross  gave  some  rules  for  the  illumination  of 
objects  in  the  '  Penny  Cyclopedia.'  These  were  : — 

1 .  That  the  illuminating  cone  should  equal  the  aperture  of  the 
objective,  and  no  more. 

2.  With  daylight,  a  white  cloud  being  in  focus,  the  object  was 
to  be  placed  nearly  at  the  apex  of  the  cone.     The  object  was  seen 
better  sometimes  above,  and  sometimes  below,  the  apex  of  the  cone. 

3.  With  lamplight  a  bull's-eye  is  to  be  used  to  parallelise  the 
rays,    so  that  they  may  be  similar  to  those  coming  from  a  white 
cloud. 

Of  the  old  forms  of  condenser,  that  devised  by  Mr.  Gillett  wras, 

there  can  be  no  doubt,  the 
best.  It  was  achromatic,  and 
had  an  aperture  of  80°.  Fig. 
239  illustrates  it.  It  was 
fitted  with  a  rotating  ring  of 
diaphragms  placed  close  be- 
hind the  lens  combination. 
This  was  formed,  as  the  figure 
shows,  by  a  conical  ring  with 
apertures  and  stops.  The 
large  number  of  apertures 
and  stops  it  would  admit, 
provided  they  are  care- 
fully 'centred,'  are  of  great 
value  in  practical  wrork  ; 
and  the  fact  that  they  are 
so  placed  as  not  to  inter- 
fere with  the  stage,  makes 
this  arrangement  of  dia- 
phragms and  stops  an  excellent  one,  and  it  is  not  clear  why  it  has 
fallen  into  disuse. 

It  had  been  the  custom  to  recommend  the  use  of  this  instrument 
racked  either  within  or  without  its  focus.-  Carpenter  employed  it 
without,  and  Quekett  within,  and  one  or  other  of  these  methods  was 
general.  But  in  the  use  of  good  achromatic  condensers  with  high- 
power  work  it  soon  became  manifest  to  practical  workers  that  it  is 
only  when,  as  Sir  David  Brewster  pointed  out,  the  source  of  light  is 
focussed  by  the  condenser  on  the  object  that  a  really  critical  image 
is  to  be  obtained.  And  Mr.  Nelson  readily  demonstrated  this  fact 
even  with  the  condenser  Gillett  had  devised. 

The  next  condenser  of  any  moment  is  a  most  valuable  one,  and 
constitutes  one  of  the  great  modern  improvements  of  the  microscope. 
It  was  an  achromatic  condenser  of  1 70°  devised  and  manufactured 


FIG.  239. — Grillett's  condenser,  from 
'  Hogg  on  the  Microscope.' 


POWELL   AND   LE ALAND'S   CONDENSER  301 

by  Messrs.  Powell  and  Lealand.  We  have  used  this  instrument  for 
thirty-five  years  on  every  variety  of  subject,  and  we  do  not  hesitate 
to  affirm  that  for  general  and  ordinary  critical  work  it  is  still  un- 
surpassed. Fig.  240  illustrates  this  apparatus.  The  optical  com- 
bination is  a  1th  of  an  inch  power,  and  it  is  therefore  more  suitable 
for  objectives  from  a  Jth  of  an  inch  and  upwards;  but  by  removing 
the  front  lens  it  may  be  used  with  objectives  as  low  as  one  inch. 

Having  given  to  this  condenser  so  high  a  place  amongst  even 
those  of  our  immediate  times,  it  may  be  well  to  specify  what  the 
requirements  arc  which  a  condenser  employed  in  critical  work  with 
high  powers  should  meet.  It  is  needful  that  we  should  be  able 
(1)  to  obtain  at  will  the  largest  '  solid '  cone  of  light  devoid  of 
spherical  aberration.1  Directly  spherical  aberration  makes  itself 
apparent  the  condenser  fails  ;  that  is,  when,  on  account  of  under- 
correction,  the  central  rays  are  brojught  to  a  longer  focus  than  the 
marginal  rays,  or  when,  because  of  over-correction,  the  marginal 
rays  have  a  longer  focus  than  the  central. 

But  (2)  it  is  also  an  absolute  essential  that  if  a  condenser  is  to 
be  of  practical  service  it  must  have  a 
working  distance  sufficiently  large  to 
enable  it  to  be  focussed  through 
ordinary  slips.  It  would  be  an 
advantage  if  all  objects  mounted 
for  critical  high-power  work  were 
mounted  on  slips  of  a  fixed  gauge,  say 
•06-  inch,  which  would  be  '  medium,' 
•05  inch  being  accounted  '  thin,'  and 
•07  inch  '  thick.' 

It     is     plain,     however,     that     to       FIG.  240.-Powell  and  Lealand's 
combine    a    large    aperture    with    a  condenser, 

great   working   distance  the  skill  of 

the  optician  is  fully  taxed,  for  this  can  only  be  accomplished  (a)  by 
keeping  the  diameter  of  the  lenses  just  large  enough  to  transmit 
rays  of  the  required  angle  and  no  more  ;  (b)  by  working  the  convex 
lenses  to  their  edge  ;  (c)  by  making  the  flint  lenses  as  thin  as 
possible. 

Xow  it  is  due  to  the  eminent  firm  whose  condenser  we  have 
been  considering  with  such  appreciation  to  say  that  the  condenser 
referred  to  (d)  did,  when  it  was  first  devised  and  for  many  years 
after,  transmit  the  largest  '  solid  '  cone  free  from  spherical  aberra- 
tion ;  (e)  that  it  has  the  greatest  working  distance ;  (/ )  that  its 
chromatic  aberrations  are  perfectly  balanced.  In  the  possession  of 
these  three  essential  qualities  it  stood  unrivalled  for  upwards  of 
thirty  years. 

The  removal  of  the  front  lens  of  this  condenser,  which  may  be 
readily  unscrewed,  reduces  it  in  power  and  angle,  and  therefore 
makes  it  suitable  for  objectives  of  lower  power.  This,  however,  is 
rather  an  adaptation  involving  compromise  than  an  ideal  condenser 

1  This  is  one  of  the  many  expressions  which  are  inevitable  to  the  practical  use  of 
apparatus;  it  is  simply  convenient,  and  means  a  full  cone  of  light — a  cone  with  none 
of  its  rays  stopped  out. 


302  ACCESSORY   APPARATUS 

for  low  powers.  When  the  highest  class  of  work  has  to  be  done  it 
is  -needful  to  have  condensers  suited  to  the  power  of  the  objective  used. 

A  dry  apochromatic  condenser  -of  merit  is  made  by  Swift  and 
Son ;  it  has  a  N.A.  of  0'95  and  an  aplanatic  cone  approximating 
O92.  and  works  with  ease  through  any  object- 
slide,  but  is  corrected  to  do  this  by  thinning 
the  front  lens  and  setting  the  front  and  back 
combinations  further  apart  than  would  be  the 
case  if  they  were  used  as  an  objective.  The 
lower  combination  has  a  large,  clear  aperture. 
The  optical  part  of  this  instrument  is  shown  in 
fig.  241  ;  we  have  used  it,  and  find  it  a  tho- 
roughly practical  and  serviceable  condenser. 
FIG  241  —Swift's  apo-  Before  the  introduction  of  the  homogeneous 

chromatic  (1899)  con-  system,  and  the  production  of  such  great  aper- 
denser,  N.A.  0-95.  tures  by  Powell  and  Lealand  as  a  1/5  in  a  £th, 

a  ^Vth,  and  a    .>Vfch  of  an  inch  focus,  the   cone 

transmitted  by  Powell's  dry  achromatic  condenser  was  as  large  as 
could  be  utilised.  But  with  apertures  such  as  these,  and  because 
of  the  subsequent  introduction  of  the  apochromatic  system  of  lenses, 
much  larger  cones  were  required.  To  meet  this  necessity  PowTell 
and  Lealand,  at  the  urgent  suggestion  of  English  experts,  made  first 
a  chromatic  condenser  on  the  homogeneous  system ;  but  this  was 
subsequently  succeeded  by  an  achromatic  instrument  of  great  value 
on  the  same  system.  This  combination  consisted  of  a  duplex  front 
with  two  doublet  backs  ;  it  is  nearly  of  the  same  power  as  their 
dry  achromatic  condenser,  but  is  of  much  greater  aperture.  It  was 
brought  afterwards  to  a  very  high  state  of  perfection,  Imving  an 
aperture  of  1'40,  and  will  work  through  a  mounting  slip  of  *07,  and 
for  aperture  and  working  distance  is,  like  its  dry  predecessor,  quite 
unappr  cached . 

Messrs.  Powell  and  Lealand  have  produced  an  entirely  new 
condenser,  strictly  apochromatic,  employing  a  fluorite  lens  in  the 
combination,  and  presenting  features  in  the  highest  degree  desirable. 
We  find  its  N.A.  to  be  0*95,  its  focal  length  long  enough  for  a  thick 
slip,  its  aplanatic  aperture  '9.  We  have  found  it  of  the  utmost 
practical  value  in  critical  work,  and  this  valuable  apparatus  has 
been  greatly  increased  in  efficiency  by  the  application  of  a  device 
by  Mr.  E.  M.  Nelson,  providing  it  with  a  correction  collar,  which 
can  be  used  with  the  utmost  ease,  no  matter  in  what  position  the 
microscope  may  be.  It  is  similar  in  practice  to  the  correction 
collar  of  an  ordinary  objective;  it  has  a  steeper  spiral  slot,  and 
only  half  a  revolution  of  movement ;  a  long  arm  is  fixed  to  the 
collar,  so  that  it  may  be  conveniently  reached  by  the  finger.  The 
whole  condenser  is  represented  in  fig.  242,  and  the  arm  for  moving 
the  correction  collar  is  seen  on  the  right  of  the  optical  tube  :  it 
turns  at  the  slightest  touch,  and  the  collar  moves  only  the  back 
lens  of  the  combination,  leaving  the  mount  rigid. 

The  object  of  this  correctional  movement  is  primarily  to  increase 
the  maximum  aplanatic  aperture  of  the  condenser ;  this  is  effected 
by  separating  the  lenses.  If  the  back  of  a  wide -angled  objective  be 


NELSON'S  CONDENSER  'CORRECTION  COLLAR 


303 


examined  when  an  object  is  illuminated  by  the  full  aperture  of  the 
condenser,  the  edge  of  the  flame  being  in  focus,  it  will  be  noticed 
that  the  illuminated 
portion  of  the  back  lens 
will  be  oval  and  pointed 
instead  of  circular. 
Also  that  when  the 
condenser  is  racked  up, 
although  the  exterior 
shape  of  the  illuminated 
portion  will  become 
more  circular,  two  dark 
patches  will  appear  on 
either  side  of  the  centre, 
showing  the  operation 
of  the  spherical  aber- 
ration of  the  condenser. 

If  under   these   circum-      FlG  u*.^l*on>*  correction  collar  to  Powell's 
Stances      me      lenses      be  apochromatic  condenser. 

separated    by   means    of 

the  collar  adjustment,  the  black  spots  will  be  closed  up,  and  a  circular 
and  evenly  illuminated  disc  will  appear.  This  is  a  distinct  optical 
gain,  and  will  enable  the  observer  to  see  more  than  he  could 
have  seen  before.  Mr.  Nelson  made  this  manifest  on  the  examina- 
tion of  a  well-known  diatom,  Navicula  major.  If  examined  in  its 
'  principal  view/  two  vertical  stripes  will  be  seen  running  down  the 
centre  of  the  hoop  (fig.  243,  a)  ;  these  can  easily  be  resolved  into 
striae  with  a  J-inch  objective,  but  the  probability  is  that  these  striae 
are  not  the  real  structure  but  rows  of  minute  perforations  incom- 


FIG.  243. 


FIG.  244. — Watson's  oil-immersion  condenser. 


pletely  resolved  (fig.  b) ;  by  using  the  condenser  with  the  collar 
correction  these  striae  were  resolved  by  means  of  the  enlarged 
aplanatic  cone  it  produced,  as  shown  in  c. 

Another  advantage  of  the  correction  collar  is  that  it  enables  the 
worker  to  determine  most  delicately  the  size  of  the  illuminating 
cone,  and  so  to  record  it  that  it  can  be  with  facility  exactly  resumed 
at  any  time  (Journ.  A3.  Micro.  Soc.  1895,  pt.  ii.  p.  231-2). 

One  of  the  most  valuable  condensers  introduced  by  any  maker 
lately  is  an  oil-immersion  one  by  Messrs.  Watson  and  Sons.  It  has 
special  claims  upon  the  attention  of  those  who  work  with  high 


304 


ACCESSORY   APPARATUS 


FIG.  245. — Watson's  parachromatic  condenser. 


powers,  for  we  know  of  no  similar  instrument  that  yields  so  large  a 
'  solid  cone '  of  illumination.  The  construction  is  an  unusual  one, 
the  corrections  for  both  spherical  and  chromatic  aberrations  being 
effected  by  means  of  a  cemented  triple  back  lens,  as  is  shown  in  the 
illustration  of  the  optical  system  in  fig.  244.  The  only  flint  glass 
used  in  it  is  the  middle  of  the  triple  back.  The  total  numerical 
aperture  is  1'33,  the  aplanatic  aperture  being  in  excess  of  1'25. 
The  magnifying  power  is  J  inch,  and  the  clear  aperture  at  the  back 
of  the  lens  is  y%ths  inch,  and  it  works  through  a  slip  of  '073 
thick. 

With  the  front  lens  removed  it  is  an  efficient  dry  condenser  for 
medium  powers,  magnifying  fths  inch,  with  a  total  N.A.  of  -56,  the 

aplanatic  aperture  being 
over  '5.  It  is  mounted 
like  their  '  Parachro- 
matic condenser '  shown 
in  fig.  245,  which  is  also 
a  very  useful  instru- 
ment, with  a  total  N.  A. 
of  I/O,  a  power  of  fths 
inch.  It  is  shown  here 
principally  for  the 
mounting,  which  is 
identical  with  that  used 
with  fig.  244.  The 

collar  into  which  the  optical  part  fits  carries  an  iris  diaphragm ;  on 
the  diametrical  edge  of  this  is  engraved  a  scale  showing  the  NJL  at 
which  the  condenser  is  working  when  the  iris  diaphragm  is  in  a 
given  position.  We  have  used  this  condenser  with  much  pleasure 
and  profit,  and  can  commend  it  as  a  truly  valuable  instrument  and 
yet  remarkably  low  in  price. 

A.  condenser  satisfying  modern  necessities  has  also  recently  been 

made  by  Messrs.  R.  and  J. 
Beck,  which  wre  illustrate  in  fig. 
246  in  its  complete  condition. 
The  optical  combination  consists 
of  four  systems  of  lenses,  the 
front  of  which  is  a  hemisphere, 
with  three  combinations  behind, 
and  the  whole  is  constructed  on 
the  principle  of  an  oil-immersion 
objective.  The  N".A.  varies  from 
1'35  to  1*4,  and  the  aplanatic 
cone  is  about  1*3  N.A.,  the 
working  distance  being  fully  "06. 
We  can  speak  highly  of  this 

instrument ;    it    is    in  our  judg- 
FiG.246.-Beck's  new  achromatic          ment    the    begt    condenser     eyer 
coiiaenbei.  .  . 

made  by  this  firm. 

Another  condenser  has  been  made  recently  by  the  same  firm, 
with    K.A.    of   1*0,   the    maximum    obtainable    without    immersion 


SUJKSTAGE    CONDENSEBS 


305 


contact.     Its  aplanatic  aperture  is  -9  X.A.  1-0.     We  illustrate  thi< 
form  in  fig.  247. 


FIG.  247.— Beck's  condenser  with  N.A.  1-0. 

It  is  with  great  pleasure  that  we  are  able  to  announce  the 
production,  by  the  firm  of  Zeiss,  of  a  '  centring  oil-immersion  achro- 
matic condenser'  of  N.A.  1-30.  This  is  what  we  have  long  desired 
to  see,  and  we  have  used  it  with  admirable  results.  It  gives  a  large 
illuminating  aplanatic  cone,  hence  very  oblique  illuminating  rays. 
The  centring  ar- 
rangement is  the 
same  as  that  of  the 
achromatic  conden- 
ser of  the  same  firm 
having  1-0  N.A.  It 
is  supplied  with  an 
iris  diaphragm  of 
the  most  perfect 
workmanship,  and 
the  condenser  is 
focussed  not  only  by 
rack  -  and  -  pinion 
movement,  but  also 
by  means  of  a  special 
fine  adjustment ;  this  FIG.  248.— Zeiss's  centring  oil-immersion  achromatic 
is  accomplished  by  condenser  (1899). 

the  aid  of  a  rotating 

ring  provided  with  a  differential  thread,  as  wTill  be  seen  by  examining 
the  illustration  we  give  in  fig.  248.  This  allows  the  condenser  to 
be  easily  focussed  *  at  intervals  of  about  O'Ol  mm.'  '  By  means  of 
this  fine  adjustment  the  condenser  may  be  focussed  up  to  about 
1  mm.' 

Messrs.  Swift  and  Son  make  a  panachromatic  dry  condenser 
having  a  N.A.  I'O,  an  aplanatic  cone  of  O93,  and  it  works  well  when 
a  critical  image  is  desired.  It  is  well  corrected  for  colour ;  they 
also  make  a  panaplanatic  oil-immersion  of  N.A.  T40,  with  an 
aplanatic  cone  of  1'25.  The  new  optical  glass  is  used  throughout 
the  system.  It  is  mounted  in  an  adjustable  cell,  if  desired,  for 
correcting  the  variations  in  the  thickness  of  the  glass  slide.  The 

x 


306  ACCESSOKY  APPARATUS 

iris  diaphragm  supplied  with  this  condenser  is  graduated  to  show 
the  NjL  when  greater  accuracy  is  required,  but  the  still  more 
accurate  method  of  employing  fittings  with  separate  discs  with  their 
N.A.  marked  on  them  is  also  supplied  by  the  makers. 

A  very  complete  achromatic  condenser  is  now  made  by  Baker  of 
Holborn.  This  condenser  is  a  modification  of  the  well-known  Abbe 
form,  in  that  the  diameter  of  the  component  lenses  is  considerably 
smaller  :  this  reduction  in  the  size  of  the  lenses,  allowing,  as  it  does,  of 

greater  freedom  of  move- 
ment of  the  mechanical 
stage,  has  been  effected 
without  in  any  way  de- 
creasing its  optical  effi- 
ciency ;  on  the  contrary  the 
aplanatic  aperture  has  been 
increased,  thus  rendering 
it  especially  suitable  for  use 
with  high  powers.  The 
total  aperture  is  N.A.  TO, 
of  which  KA.  0-90  is 

FIG.  '249.     Bakers  new  achromatic  condenser  ..  .  ,  ... 

N.A.  l-o.  aplanatic  :    the     diameter 

of  the  back  lens  is  22  mm. 

(-}-£-  in.)  and  the  power  of  the  condenser  as  a  whole  is  10  mm.  (  ^  in.) 
with  a  working  distance  of  2'5  mm.  (^in.):  with  the  front  lens 
removed  for  low-power  work  the  power  is  reduced  to  20  mm.  (-J^T  in.), 
and  the  working  distance,  which  is  calculated  with  the  lamp  name 
at  ten  inches,  is  increased  to  10'5  mm.  (^f  in.). 

The  above  is  mounted  in  the  usual  sub-stage  fitting  of  universal 
gauge  with  iris  diaphragm  and  carrier  with  dark-ground  stops,  as 
shown  in  the  illustration  of  it  in  fig.  249. 

It  is  essential  for  ideal  illumination  with  transmitted  light 
(1)  that  the  illuminating  axial  cone  should  be  approximately  equal 
to  the  aperture  of  the  objective  used  ;  (2)  that  the  object  should  be 
placed  at  the  apex  of  this  cone. 

If  an  objective  breaks  down  with  this  ideal  illumination,  wrhich 
is  very  probable,  we  must  be  content  to  sacrifice  the  ideal ;  or,  as 
is  also  exceedingly  probable,  if  the  object  under  examination  lacks 
contrast,  the  ideal  method  must  be  modified.  But  if  we  have  a 
suitable  object  and  a  perfect  objective,  it  is  the  strong  conviction 
of  some  leading  experts  that,  as  we  increase  the  cone  in  aperture,  we 
increase  the  perfect  rendering  of  the  image,  until  the  point  is  reached 
where  the  cone  from  the  condenser  is  equal  to  the  aperture  of  the 
objective.  This  ideal  can  be  realised  with  fine  apo-  and  semi- 
apochromatics  up  to  '3  to  -4  N.A.  With  the  most  perfect  objectives 
of  the  present  day  of  *5  N.A.  and  upwards  we  find  in  practice  that 
the  best  results  are  obtained  when  a  cone  of  light  is  used  which,  on 
the  removal  of  the  eye-piece,  is  found  to  occupy  three-quarters  of 
the  area  of  the  back  lens  of  the  objective. 

No  condenser  is  sufficiently  free  from  spherical  aberration  to 
transmit  a  cone  equal  to  its  own  aperture.  Condensers  are  all  more 
or  less  under-corrected,  and  consequently  focus  their  central  rays  at 


EFFECTIVE    APERTURE    OF   CONDENSER  307 

a  greater  distance  than  their  marginal  rays.  If  we  rack  up  the 
condenser  so  that  the  marginal  rays  are  focussed  on  the  object,  the 
focus  of  the  rays  which  pass  through  the  centre  will  be  beyond  the 
object. 

It  is  well  known  to  those  practised  in  microscopy  that,  in  the 
case  of  a  narrow  cone  from  a  well-stopped-down  condenser — that  is, 
a  condenser  used  with  diaphragms  of  relatively  small  diameter — the 
illumination  is  at  its  greatest  intensity  when  the  object  is  at  the 
apex  of  the  illuminating  cone,  and,  if  the  condenser  is  racked  either 
up  or  down,  the  intensity  of  the  illumination  is  rapidly  diminished. 
But  in  the  case  of  a  condenser  with  great  aperture,  if  it  be  racked 
up,  the  marginal  rays  will  have  their  full  intensity,  while  those  which 
pass  through  the  central  portion  of  the  condenser  will  have  a 
diminished  intensity. 

The  extent  to  which  this  wil>  take  place  will  be  wholly  dependent 
on  the  amount  of  under-correction  present  in  the  condenser.  In 
some  condensers  the  under-correction  is  so  serious  that  to  obtain  a 
wide  or  even  a  moderate  cone  we  so  enfeeble  the  central  cone  as  to 
reduce  it  almost  to  a  mere  annular  illumination,  which  is  not  a  desir- 
able quality. 

It  will  be  seen,  then,  that  the  aperture  of  the  cone  of  light  trans- 
mitted by  a  condenser  plays  a  very  important  part  in  giving  critical 
quality  to  an  image  with  different  objectives.  We  should  therefore, 
to  use  a  condenser  accurately,  be  able  to  determine  the  aperture  of 
the  cone  we  are  using. 

We  may  measure  the  total  aperture  of  a  condenser  just  as  we 
do  that  of  an  objective,  viz.  by  means  of  Abbe's  apertoineter.1  But 
the  effective  aperture  cannot  be  measured  in  that  way ;  that  is  to 
say,  the  aperture  of  the  largest  aplanatic  cone  (or  cone  free  from 
spherical  aberration)  the  condenser  is  capable  of  giving,  cannot  be  so 
discovered. 

To  do  this,  place  the  condenser  in  the  sub-stage  and  an  objective 
on  the  nose-piece  ;  focus  both  upon  an  object.  Let  the  edge  of  the 
lamp-name  be  used,  and  so  arrange  the  focus  of  both  optical  com- 
binations that  the  edge  of  the  clear  image  of  the  lamp-flame  falls 
centrally  upon  the  object.  Now  move  the  object  just  out  of  the 


FIG.  250.  FIG.  251.  FIG.  252.  FiG.253. 

field,  remove  the  eye-piece  and  examine  the  back  of  the  objective, 
and  if  the  aperture  of  the  aplanatic  illuminating  cone  is  greater  than 
that  of  the  objective  it  will  show  the  back  lens  to  be  full  of  light  (fig. 
250).  Therefore,  if  the  aperture  of  the  objective  is  '5,  we  know  that 
the  aplanatic  illuminating  cone  cannot  be  less  than  "5.  If  now  we 

1  Chapter  v. 

x2 


308  ACCESSORY  APPARATUS 

close  the  diaphragm  so  that  the  image  of  it  just  appears  at  the  back 
of  the  objective,  we  are  able  to  determine  the  aperture  of  the  illumi- 
nating cone  with  that  given  opening  in  the  diaphragm  ;  thus  in  fig. 
251  it  is  a  trifle  less  than  '5  N.A. 

In  a  similar  manner  the  apertures  of  the  other  diaphragm  open- 
ings can  be  determined. 

Now  let  the  diaphragm  be  opened  to  the  full  aperture,  and  an 
objective  with  a  wider  aperture,  say  "95,  be  used.  It  will  perhaps 
be  found  that  before  we  are  able  to  fill  the  back  of  the  objective  with 
light  by  racking  up  the  condenser,  two  black  spots  will  be  formed  on 
either  side  the  middle  of  the  disc.  When  we  reach  the  disc  of  light 
that  is  largest  (fig.  252),  any  further  racking  up  causes  the  appear- 
ance shown  in  fig.  253.  The  last  point  before  the  appearance  of  the 
black  spots  indicates  the  largest  aplanatic  aperture  of  the  condenser,  and 
is  the  limit  of  the  condenser  for  critical  work}- 

There  are  many  other  condensers  of  more  or  less  merit  and  use- 
fulness than  those  which  we  have  already  described  and  illustrated  ; 
but  for  most  recent  lenses,  and  for  the  finest  critical  results,  we  have 
given  them  as  full  a  representation  as  can  be  fairly  desired.  But 
there  are  still  some  forms  that  either  from  their  own  peculiar  value 
or  their  historic  importance  deserve  consideration. 

A  condenser  known  as  the  *  Webster '  was  first  made  in  1865,  and 
is  still  a  very  useful  one  for  low  powers.  It  is  the  same  as  that 
made  by  Swift,  but  without  the  middle  combination.  Its  angle  is 
less,  and  its  range  is  not  so  extensive  ;  but  its  chief  commendation  in 
possessing  these  qualities  is  that,  having  one  combination  less  than 
Swift's,  it  is  of  necessity  lower  in  price,  and  on  that  account  will  be 
welcome  to  some  workers. 

In  its  present  form  it  reverses  its  primary  construction.  It  is 
now  made  with  a  double  front  and  a  single  back,  instead  of  a  single 
front  and  a  double  back. 

A  chromatic  condenser  which  has  been  very  largely  used  in 
England  and  America,  and  which  has  secured  a  great  deal  of  com- 
mendation, is  that  of  Professor  Abbe.  The  optical  productions  of 
Abbe  are  too  well  known  and  too  valuable  as  a  rule  to  make  it 
needful  to  be  other  than  perfectly  frank  concerning  so  important  a 
piece  of  apparatus  as  this  ;  and  there  can  be  no  doubt  that  the  wide 
popularity  of  this  instrument  is  due,  not  so  much  to  intrinsic  merit 
as  to  the  fact  that  it  has  been  employed  much  by  those  who, 
previously  ignorant  of  the  value  of  any  .condenser,  have  at  once 
perceived  the  enhanced  value  of  the  results  yielded  by  its  means. 

To  those  who  have  made  the  scientific  use  of  the  microscope  a 
careful  study  in  England  it  has  been  a  persistent  source  of  regret 
that  it  was  so  long  and  pertinaciously  taught  that  the  *  correct ' 
histological  microscope  must  be  of  the  Hartnack  type,  and  that  it 
should  be  used  with  narrow-angled  dry  lenses,  perhaps  a  j-th-inch 
focus,  and  no  illumination  but  that  afforded  by  a  small  concave 
mirror,  the  focal  point  of  which  is  extremely  doubtful  or  unknown, 

1  '  The  Back  of  the  Objective  and  the  Condenser.'  E.  M.  Nelson,  Eng.  Mech. 
vol.  xlviii.  No.  1234,  1883. 


IMPORTANCE    OF   APLANATIC    CONDENSER  309 

Miid  in  practice  wholly  disregarded.  No  doubt  a  student  instructed 
011  these  lines  would  be  astonished  indeed  when  he  exchanged  such  a 
practice  for  the  illumination  and  improved  image  afforded  by  an 
Abbe  condenser. 

Usually  such  exchange  of  illuminating  method  presages  an  ex- 
change of  instrument,  for  the  scientifically  imperfect  and  wholly 
unsatisfactory  '  tool '  that  is  in  the  majority  of  cases  put  into  the 
hands  of  the  medical  student  will  not  lend  itself  even  to  an  Abbe 
condenser. 

The  fact  is  that  a  large  part  of  the  admiration  that  has  been  ex- 
pressed for  this  condenser  has  resulted,  not  from  a  comparison  of  its 
results   with  those    of  other   high-class 
achromatic  condensers,    but  of  images 
obtained  without  any  sub-stage  optical 
arrangements  at  all,  placed  in  contrast 
with  the  results  obtained  by  usingthis 
condenser  against  the  same  objective 
when  used  without  its  aid.     But  that 
even  these  images  are  entirely  inferior 
to  the  images  obtained  by  the  higher     PIG.  254. — Optical  arrangement  of 
order  of  achromatic  condensers  we  only         Abbe's  chromatic  condenser, 
require    the    practical     testimony    of 

Professor  Abbe  to  prove  ;  for  he.  has  since  produced  an  achromatic 
condenser  of  much  merit,  to  which  we  give  consideration  below. 

In  its  most  perfect  form  this  chromatic  condenser  of  Abbe's  con- 
sists of  three  single  lenses,  the  front  being  hemispherical,  and  the 
two  lower  lenses  form  a  Herschelan  doublet.  This  combination  is 
shown  in  fig.  254,  and  the  general  form  of  the  instrument,  as  applied 
to  Zeiss's  own  microscopes,  is  shown  in  fig.  255. 

The  power  of  this  condenser  is  low,  and  its  aperture  is  very  large 
(1*36);  hence,  beyond  the  fact  that  it  is  not  achromatised,  it  has 
enormous  spherical  aberration.  The  distance  between  the  foci  of 
the  central  portion  and  of  a  narrow  annular  zone  whose  internal 
diameter  is  f  th  inch  is  ?V*n  incn-  Its  aplanatic  aperture  is  therefore 
only  *5.  Now,  whilst  it  is  a  gain  of  no  inconsiderable  character  to 
have  an  achromatised  condenser,  yet  the  point  of  vital  importance  is 
that  it  should  be  aplanatic ;  the  best  condenser  is  always  that  which 
will  transmit  the  largest  aplanatic  cone.  At  the  close  of  this  section 
we  furnish  a  table  of  the  relative  qualities  of  the  condensers  of  the 
best  construction  now  accessible  to  the  microscopist,  and  a  reference 
to  this  will  show  that  Powell  and  Lealand's  dry  achromatic  (fig.  240), 
with  the  top  removed,  is  in  this  respect  as  efficient  as  this  form  of 
Abbe's. 

This  condenser  can  be  used  either  dry  or  homogeneously ;  but  of 
course  with  objectives  of  greater  aperture  than  1*0  the  base  of  the 
slide  should  always  be  in  oil  contact  with  the  condenser. 

It  gives  the  principal  modifications  from  direct  to  oblique  illu- 
mination with  transmitted  light  by  changing  and  moving  a  set  of 
diaphragms  placed  in  a  movable  fitting,  ancl  the  diaphragm  may  be 
moved  eccentrically  to  the  optical  axis  of  the  condenser  by  moving 
the  milled  head.  It  gives  dark-ground  illumination  with  objectives 


3io 


ACCESSORY   APPARATUS 


of  "5  N.A.  ;  for  such  illumination,  in  fact,  it  is  perhaps  the  best 
illuminator  extant,  and  shows  objects  on  a  dark  ground  with 
sparkling  brilliancy,  and  may  be  used  with  polarised  light. 

A  chromatic  condenser,  somewhat  similar  in  construction  to  this, 
and  of  low  price,  is  made  by  Messrs.  Powell  and  Lealand ;  but  it  is 
of  much  higher  power,  so  that  the  distance  between  the  foci  for 
the  central  and  peripheral  rays  is  not  so  great,  and  on  this  account 
it  yields  a  somewhat  larger  aplanatic  cone.  This  instrument  with 
its  diaphragms  is  shown  in  fig.  256.  It  is  more  convenient  in  form, 


FIG,  255. — Abbe's  chromatic  condenser  as  applied  to  the  Zeiss  microscopes. 

and  can  be  handled  and  adjusted  with  greater  facility,  than  that  oi 
Abbe.  The  size  of  their  respective  back  lenses  is  significant  in  this 
regard,  that  of  Powell's  being  fa  inch,  and  that  of  Abbe's  being 
1  fa  inch.  This  instrument  of  Powell's,  if  fitted  in  the  usual  way, 
would  be  now  a  very  efficient  instrument  of  its  kind  and  quality. 
The  particular  quality  of  oblique  illumination  was  in  fact  still 
further  advanced  by  a  modified  form  by  the  same  makers  known  as 
Powell's  truncated  condenser,  which  gives  great  obliquity  with 
abundance  of  light,  but  it  is  as  a  matter  of  course  very,  chromatic. 
The  diaphragms  (fig.  256,  A)  have  a  central  aperture  for  the 


POWELL'S   CHROMATIC,    ABBE'S   ACHROMATIC    CONDENSER 


1  I 


purpose  of  centring,  and  the  movement  is  made  by  means  of  an 
outer  sliding  tube  b:  with  a  slot  at  the  top  in  which  the  arm  A  fits, 
and  another  arm,  B,  is  placed  at  the  lower  end  so  as  to  give  ready 
command  of  the  rotation.  This  plan  allows  of  the  use  of  one  or  two 
oblique  pencils  incident  90°  apart  in  azimuth.  The  condenser  thus 
mounted  is  only  intended  as  an  oblique  illuminator.  It  forms  one 
of  the  best  of  the  very  cheap  condensers  when  it  is  mounted  in  a 
plain  tube  mount  with  a  ledge  to  hold  the  diaphragms.  I)  is  the 
optical  part  of  the  condenser  placed  immediately  above  the  dia- 
phragms and  in  oil-immersion  contact  with  the  base  of  the  slide. 
The  circular  diaphragm  is  fixed  into  the  inner  tube  attached  to  the 
sub  stage  tube  C,  just  below  the  position  of  the  arm  A  ;  the  other 
diaphragm  is  screwed  to  it  by  a  screw  in  the  eccentric  hole,  shown 
in  each.  It  will  be  seen  that  when  the  diaphragms  are  placed 
together  in  this  manner  the 
movement  of  the  arm  will 
I »rod uce  the  changes  in  the 
light  as  above  mentioned. 

As  we  intimated  above, 
Professor  Abbe  subsequently 
produced  an  achromatic  con- 
denser, ostensibly  for  use  in 
high-power  photographic 
work,  but  in  fact  of  much 
more  general  utility.  It 
consisted  of  a  single  front 
with  two  double  backs,  and 
it  projects  a  sharp  and  per- 
fectly achromatic  image  of 
the  source  of  light  in  the 
plane  of  the  object.  Its 
power  is  low,  being  \  inch 
focus,  and  it  has  a  total 
aperture  of  1*0.  Its  great 
superiority  over  the  chro- 
matic form  is  that  it  trans- 
mits a  much  larger  aplanatic  cone  than  that  ;  for  whereas  the  former 
gave  only  an  aplanatic  cone  of  '5,  this  instrument  yields  a  similar 
cone  of  "65.  But  we  have  already  expressed  our  pleasure  that  even 
this  form  has  been  surpassed  by  the  high  quality  condenser  illustrated 
in  fig.  257.  Like  its  predecessor,  it  is  large  and  heavy  ;  and,  with 
great  deference  and  respect  to  our  Continental  neighbours,  we  would 
suggest  that  this  is  a  too  general  characteristic  ;  the  back  lens  in  this 
case  is  more  than  an  inch  in  diameter,  while  barely  f  of  an  inch  is 
utilised  when  it  is  transmitting  its  largest  cone.  A  very  excellent 
modification  in  fitting  it  to  English  microscopes  has  been  made  by 
Mr.  Charles  Baker,  the  optician,  which  is  shown  in  fig.  258,  where 
it  will  be  seen  that  the  fitting  for  stops  is  conveniently  placed,  and 
an  iris  diaphragm  can  be  used  with  great  ease  below  this.  This 
1  turn-out ?  arm  carries  a  disc  of  metal  to  receive  the  diaphragms, 


FIG.  256.— Powell  and  Lealand's 
chromatic  oil  condenser  (1880). 


312 


ACCESSORY   APPARATUS 


stops,  &c.     Over  this  is  fitted  a  ring  into  which  screw  adapters,  which 
will  allow  other  condensers  to  be  used  on  the  one  mechanism. 

The  metal  disc  should  have  a  central  aperture  as  large  as  the 
largest  back  lens  of  any  of  the  combinations  to  be  used  with  the 
mount.  It  should  be  thick  enough  to  receive  two  stops  or  dia- 
phragms at  a  time.  This  power  to  alter  a  diaphragm  or  stop  so  as 
to  secure  any  required  arrangement  of  apertures  and  stops  without 


FIG.  257. — Abbe's  achromatic  condenser  (1888). 

in  the  least  disturbing  any  of  the  adjustments  of  the  condenser  is  a 
practical  gain  of  a  very  valuable  kind. 

Diaphragms  should  be  marked  with  the  numerical  aperture  they 
yield,  and  stops  should  be  marked  with  the  numerical  aperture  of 
the  cone  they  cut  out.  Empirical  numbers  are  misleading  and 

valueless.  This  special  mark- 
ing need  not  involve  two  sets 
of  diaphragms  with  two  con- 
denser combinations,  one  for 
high  and  the  other  for  low 
powers  ;  the  different  numeri- 
cal apertures  for  each  may  be 
marked  on  either  side  of  the 
diaphragm  or  stop.  Memory 
cannot  fail  if  we  make  the 
loiver  side  of  the  diaphragm 
indicate  the  apertures  for  the 
lower-power  condenser,  and 
vice  versa. 

We  may  note  that  for 
dark-ground  work,  stops 
should  be  placed  close  to  the  back  lens  of  the  condenser,  and  in  the 
case  of  a  diaphragm — which  is  less  important — an  inch  of  distance 
should  not  be  exceeded.  This  condenser  gives  dark-ground  illumi- 
nation with  objectives  of  '5  N.A.  ;  for  such  illumination  it  is  one  of 
the  best  illuminators  extant. 


FIG.  258. — Baker's  fitting  for  Abbe's  achro- 
matic condenser  used  in  English  micro- 
scopes. 


A   SIMPLE    CONDENSEK  313 

The  iris  diaphragm  is  for  general  pin-poses  more  convenient  than 
the  usual  circular  plate,  but  it  has  the  drawback  of  being  incapable 
of  setting  to  any  exact  size.  A  delicate  point  in  an  image,  caught 
with  a  certain-sized  diaphragm,  is  not  regained  with  ease  and  cer- 
tainty with  the  iris,1  and  may  involve  much  patience  and  labour; 
but  a  well-made  large  plate  of  graduated  diaphragms  will  wholly 
remove  this  difficulty.  Moreover,  for  testing  object-glasses  it  is 
supremely  important  that  a  metal  diaphragm  be  used,  so  that  the 
conditions  of  illumination  may  be  readily  and  accurately  reproduced. 

It  may  be  of  service  to  those  who  are  unable  or  indisposed  to 
spend  considerable  sums  upon  condensers  to  state  that  an  excellent 
achromatic  condenser  can  be  made  by  placing  a  Zeiss  '  aplanatische 
Lupen '  on  Steinheil's  formula  in  the  sub-stage.2  This  plan  has 
been  adopted  in  one  of  Reichert's  stands,  as  we  have  seen. 
These  are  made  in  two  differen^ppwers,  viz.  1  inch  and  1^  inch,  and 
we  can  fully  testify  to  their  being  the  most  useful  hand-lenses  for 
ordinary  work  that  can  be  employed.  Great  credit  is  due  to  Dr. 
Zeiss  for  bringing  out  such  excellent  achromatic  lenses  at  so  low  a 
price,  and  so  meeting  a  want  long  and  generally  felt.  Excellent 
forms  of  triplet  lenses  answering  a  similar  purpose  are  made  by 
Bausch  and  Lomb  after  the  calculations  of  Professor  Hastings,  and 
most  leading  makers.  Continental  arid  English,  make  similar  magni- 
fiers to  those  of  Zeiss.  An  achromatic  loup  of  this  kind  is  almost  an 
indispensable  accompaniment  of  a  microscopic  outfit,  and,  if  a  tube  to 
receive  it  be  arranged  in  the  sub-stage,  these  lenses  make  really  ex- 
cellent condensers  for  low  powrers.  It  need  not  have  a  centring  sub- 
stage,  but  only  a  central  fitting.  It  is  not  of  course  qualified  to 
supplant  the  condenser  of  larger  and  more  perfect  instruments,  but  it 
is  capable  of  raising  students'  and  other  simple  microscopes  to  a  much 
higher  level. 

Without  a  condenser  the  microscope  is  either  (by  construction) 
not  a  scientific  instrument,  or  it  is  an  instrument  unscientifically 
used.  It  becomes  a  mere  '  magnifying  glass.'  It  is  the  adaptation 
for  and  use  of  a  condenser — though  as  simple  as  a  hemispherical  lens 
fitted  into  a  stage  plate — that  raises  it  to  a  microscope. 

We  have  already  referred  to  the  nature  of  the  mechanical 
arrangements  needful  for  the  condenser  in  a  general  way  (Chapter 
III.,  pp.  185-190) ;  we  may  add  here  that  the  simplest  form  of  sub- 
stage  being  a  tube  fixed  centrally  in  the  optic  axis  of  the  microscope, 
the  simplest  form  of  condenser-mount  will  be  a  tube  sliding  into 
this.  It  must  not  screw,  it  must  push,  and  there  should  be  a  little 
below  the  back  lens  a  shoulder  to  hold  the  diaphragms,  stops,  glasses, 
Arc.  Centring  gear  is  not  necessary  with  students'  and  elementary 
microscopes.  The  slight  displacements  due  to  varying  centres  of 

1  It  will  be  urged  that  apertures  can  be  exactly  reproduced  with  the  iris  in 
photographic  lenses ;  why  cannot  they,  therefore,  in  the  case  of  the  microscope  ? 
The  answer  is  (1)  that  with  wide-angled  condensers  a  very  slight  difference  in  the 
aperture  makes  a  very  great  difference  in  the  angle ;  a  similar  difference  would  be 
inappreciable  in  the  case  of  a  photographic  lens.  i2)  It  is  in  small  apertures  such 
as  are  seldom  used  in  photographic  lenses  where  the  difficulty  arises  in  the  case  of 
the  microscope.  (3)  It  is  in  the  small  apertures  that  the  iris  fails  to  respond  to 
the  movement  of  the  lever. 

-  Journ.  QueJcettMic. Club, \o\.i\. ser.ii.p. 77  (1889), onZeiss'sloup.   E.M.Nelson. 


314  ACCESSORY  APPARATUS 

different  objectives  will  with  such  microscopes  prove  of  no  moment 
if  the  sub-stage  is  once  for  all  carefully  fixed  centrally  in  the  axis. 

What  we  require  to  do  is  to  centre  the  image  of  the  lamp  flame, 
as  seen  with  a  low-power  lens  through  the  condenser,  so  that  it 
stands  in  the  middle  of  the  field.  This  can  be  done  by  moving  the 
lamp  or  the  mirror,  and  until  this  is  satisfactory  the  best  results 
cannot  be  obtained.  To  obviate  the  inconvenience  of  having  to  re- 
move the  combination  in  order  to  alter  a  diaphragm  l  or  stop  in 
this  simple  mount  an  internal  sliding  tube  may  be  used.  It  will  be 
a  further  advantage  to  have  a  separate  cell  to  fit  into  the  bottom  of 
the  sliding  tube  to  receive  coloured  glasses ;  a  spiral  slot-focussing 
arrangement  may  be  added  with  advantage  to  this  kind  of  mount, 
acting  like  a  pocket  pencil.  For  students'  arid  elementary  micro- 
scopes— still  so  often  and  so  unwisely  without  condensers — this  is  a 
most  inexpensive  and  most  convenient  arrangement. 

An  epitome  of  its  principal  points  may  be  of  service. 

1.  A  sub-stage  tube  fixed  centrally  to  the  body  of  the  microscope. 

2.  A  spiral  slotted  tube  to  push  into  (1). 

3.  A  tube  carrying  the  optical  combination   of  the   condenser 
sliding  into  (2),  with  a  pin  moving  in  the  spiral  slot. 

4.  A  long  tube  carrying  the  diaphragm  and  slots  sliding  into  (3). 

5.  A  cell  carrying  coloured  glasses  sliding  into  the  bottom  of  (4). 
Condensers  require  special  mounting  for  use  with  the  polariscope. 

Then  at  least  two  '  turn-out '  rotating  rings  are  required  to  hold 
selenites.  Swift  makes  an  ingenious  multum  in  parvo  mount  for 
employing,  amongst  other  things,  the  condenser  with  the  polariscope, 
to  which  we  call  attention  in  describing  the  polariscope.  But  we 
know  of  no  plan  equal  to  that  found  in  the  best  stand  of  Powell  and 
Lealand.  The  sub-stage  has  a  double  ring,  one  placed  concentrically 
within  the  other.  The  inner  one  revolves  by  a  milled  head  and 
receives  the  usual  sub-stage  apparatus.  The  outer  one  receives  a 
mount  of  three  selenites  which  revolve,  and  are  placed  on  '  turn-out ' 
arms.  On  the  upper  part  of  this  mount  of  selenites  is  a  screw,  which 
receives  the  optical  combination  of  their  dry  achromatic  condenser. 
When  this  is  screwed  in  its  place  we  have  a  condenser  of  the  first 
order,  with  a  mount  of  three  plates  of  selenites  taking  the  place  of  a 
mount  of  diaphragms,  &c.  Now  from  the  under  part  of  the  sub- 
stage  into  the  inner  and  revolving  ring  is  fitted  the  polariser,  and 
this  leaves  little  to  be  desired  in  practice. 

We  would  advise  the  microscopist  to  avoid  condenser  mounts 
which  carry  their  own  centring  movements  apart  from  the  suit- 
stage.  It  is  with  regret  that  we  find  tha,t  this  plan  has  been  adopted 
in  Abbe's  new  achromatic  condenser.  It  is  manifestly  better  to  fit 
the  rectangular  movements  to  the  sub-stage,  and  then  they  become 
available  for  all  the  apparatus  employed  with  the  sub-stage.  A  plan 
which  requires  that  each  piece  of  sub-stage  apparatus  which  needs 
centring  should  be  provided  with  separate  fittings  for  this  purpose 
can  have  nothing  to  recommend  it. 

1  In  the  technical  language  or  usage  of  microscopists  a  diaphragm  means  a  hole 
or  aperture ;  thus  a  '  large  diaphragm '  means  that  the  opening  in  the  diaphragm 
plate,  disc,  or  iris  is  large.  A  '  stop  '  is  an  opaque  disc  stopping  out  central  rays. 


A   COMPABISON   OF   CONDENSERS 


315 


We  give  below  a  list  presenting  the  most  important  features  of 
the  most  important  condensers,  which  we  believe  will  be  of  service 
to  the  student  and  worker. 

The  aplanatic  aperture  given  in  the  table  means  the  N.A.  of  the 
greatest  solid  cone  a  condenser  is  capable  of  transmitting,  the 
source  of  light  being  the  edge  of  the  flame  placed  in  the  axis. 

The  cone  transmitted  by  any  condenser  is  assumed,  for  practical 
purposes,  to  be  a  solid  one.  so  long  as  the  image  seen  at  the  back  of 
the  object-glass  when  the  eye-piece  is  removed  (the  condenser  and 
flame  being  centred  to  the  optic  axis  of  the  objective,  and  the  source 
of  light  focussed  by  the  condenser  on  the  object)  presents  an  un- 
broken disc  of  light. 

The  moment,  however,  the  disc  breaks,  that  is,  black  spots 
appear  in  it,  or  its  periphery  breaks  away  from  its  centre,  then,  as 
we  have  shown  above,  spherical  .aberration  comes  into  play,  and  the 
limit  of  aperture  for  which  that  condenser  is  aplanatic  has  been  ex- 
ceeded. 


Condenser 

Total 
aperture 
N.A.. 

Aplauatic 
aperture 
N.A. 

Power 

1.  Powell  and  Lealand's  dry  achromatic  (1854) 

•99 

•8 

i 

;,         „           ,,           new  formula  (1859)    . 

•99 

•8 

| 

2.        ,,         ,,           ,,           top  lens  removed 

— 

•5 

3.        ,,         „           „           bottom  lens  only 

•24 

| 

4.  Swift's  achromatic  (1868)     .... 

•92 

•5 

^ 

5.         ,,             „            top  lens  removed     . 

— 

•22 

1 

6.  Abbe's  chromatic  (3  lenses)  (1873) 

1-36 

•5 

i 

7.         „             „          top  lens  removed 

— 

•3 

| 

8.  Powell   and   Lealand's    chromatic    (Abbe's 

formula)  (1880)  

1-3 

•7 

1 

9.  Powell  and  Lealand's  oil  achromatic  (1886) 

1-4 

1-1 

i 

10.         „         „             „          „          „         used  dry 

1-0 

•8 

•i 

11.         „         ,,             „          „     top  lens  removed 



•4 

A 

12.  Abbe's  achromatic  (1888)      .... 

•98 

•65 

* 

13.         „             „            top  lens  removed 
14.  Powell    and    Lealand's     low  -power    achro- 

— 

•28 

matic  (1889)       

•83 

•5 

15.  Powell  and  Lealand's  apochromatic  (1891)  . 

•95 

•9 

j. 

16.  Zeiss's     '  aplanatische    lupen,'    large    field 

(Steinheil  formula)     ..... 

— 

•32 

1 

17.  Beck's  achromatic,  dry  (1883) 

•0 

•9 

18.       „       oil  achromatic  (1900) 

•4 

1-3 

19.  Swift's  apochromatic,  dry  (1892)  . 

•95 

•92 

20.         „      panaplanatic,  dry  (1897)   . 

•0 

•93 

21.         „                 „             oil  (1898)     . 

•4 

1-30 

" 

22.  Watson's  panachromatic,  dry  (1898)     . 

•0 

•95 

23.         „                    „                 oil  (1899)     . 

•33 

1-25 

i 

24.  Zeiss's  oil  achromatic  (1899) 

•30 

— 

25.  Baker's  semi-apochromatic  dry  (1900) 

1-0 

•95 

7 

The  values  of  the  first  sixteen  and  of  Nos.  22,  23,  and  25  have 
been  obtained  from  actual  measurements ;  the  others  are  from  the 
estimates  of  the  makers. 

The  limit  given  in  the  table  is  for  the  edge  of  the  flame  as  a 


31,6  ACCESSORY   APPARATUS 

source  of  light.  When,  however,  a  single  point  of  light  in  the 
axis  is  the  source,  the  condenser  will  be  much  more  sensitive,  and 
a  lower  value  for  the  aplanatic  aperture  than  that  given  in  the  table 
will  be  obtained.  But  as  a  single  point  of  light  is  seldom,  if  ever, 
practically  used  in  microscopy,  it  was  deemed  better  to  place  in 
the  table  a  practical  rather  than  a  theoretical  and  probably  truer 
result. 

It  has  been  stated  that  the  best  dark  grounds  are  obtained  when 
a  stop  is  used  which  is  of  just  a  sufficient  size  to  give  a  suitable  dark 
field  and  no  more. 

When  such  a  stop  has  been  chosen,  and  excellent  results  are  ob- 
tained with,  say,  balsam-mounted  objects,  if,  in  the  place  of  this, 
living  animalcules  in  water  be  examined,  it  will  probably  be  found 
that  a  dark  field  can  no  longer  be  obtained. 

For  animalcules  in  water  and  ;  pond  life  '  generally  a  stop  larger 
than  that  employed  for  ordinary  objects  will  be  necessary. 

Other  Illuminators.— In  the  course  of  the  history  of  the  micro- 
scope a  large  number  of  special  pieces  of  apparatus  have  been  devised 
for  the  purpose  of  accomplishing  some  real  or  supposed  end  in  illumi- 
nation. Many  of  these  have  proved  wholly  impracticable  and  had  a 
mere  ephemeral  existence ;  many  more  never  accomplished  the  end 
for  which  they  were  supposed  to  be  constructed ;  and  a  still  larger 
number  have  been  superseded  by  high-class  condensers. 

The  great  majority  of  these  illuminators  were  devised  for  the 
production  of  oblique  light.  In  the  sense  in  which  it  was  employed 
a  few  years  ago,  it  is  rendered  needless  by  condensers  of  great  aper- 
ture. All  the  obliquity  at  present  needed  can  be  obtained  with  good 
condensers. 

To  give  completeness  to  this  part  of  our  subject  it  is  needful  to 
refer  to  the  SPOT-LENS  and  the  PARABOLOID,  although  they  are  only 
serviceable  for  very  low  powers,  such  as  3-inch  to  H-inch  objec- 
tives, and  for  use  with  higher  powers  they  are  superseded  by  the 
condenser. 

A  spot  lens  is  a  condenser  with  a  permanent  axial  stop  fixed  in 
it  to  cut  off  the  central  rays  for  the  purpose  of  obtaining  a  dark 
ground  upon  which  the  illuminated  object  lies.  Its  use  is  very 
beneficial  in  low-power  work.  Large  insect  preparations  are  pro- 
bably better  shown  with  this  device  than  with  any  condenser,  but 
when  the  moderate  powers  are  brought  into  operation  the  condenser 
at  once  makes  manifest  its  superior  qualities. 

The  paraboloid,  or  parabolic  illuminator,  as  devised  by  Mr. 
Wenham,  and  subsequently  improved  by  Mi'.  Shadbolt,  ingenious 
and  beautiful  instrument  as  it  is,  comes  under  the  same  category. 
It  consists  of  a  paraboloid  of  glass  that  reflects  to  its  focus  the  rays 
which  fall  upon  its  internal  surface.  A  diagrammatic  section  of 
this  instrument,  showing  the  course  of  the  rays  through  it,  is  given 
in  fig.  259,  the  shaded  portion  representing  the  paraboloid.1  The 

1  A  parabolic  illuminator  was  first  devised  by  Mr.  Weiiham,  who,  however, 
employed  a  silver  speculum  for  the  purpose.  About  the  same  time  Mr.  Shadbolt 
devised  an  annular  condenser  of  glass  for  the  same  purpose  (see  Trans.  Micro. 
Soc.  ser.  i.  vol.  iii.  1852,  pp.  85,  132).  The  two  principles  are  combined  in  the  glass 
paraboloid. 


PAEABOLIC   ILLUMINATOR 


317 


parallel  rays  r  r'  v"  (fig.  259),  entering  its  lower  surface  perpendicu- 
larly, pass  on  until  they  meet  its  parabolic  surface,  on  which  they  fall 
at  such  an  angle  as  to  be  totally  reflected  by  it,  and  are  all  directed 
towards  its  focus,  F.  The  top  of  the  paraboloid  being  ground  out  into 
a  spherical  curve  of  which  F  is  the  centre,  the  rays  in  emerging  from 
it  undergo  no  refraction,  since  each  foils  perpendicularly  upon  the 
part  of  the  surface  through  which  it  passes.  A  stop  placed  at  S 
prevents  any  of  the  rays  reflected  upwards  by  the  mirror  from 
passing  to  the  object,  which,  being  placed  at  F,  is  illuminated  by 
the  rays  reflected  into  it  from  all  sides  of  the  paraboloid.  Those 
rays  which  pass  through  it  diverge  again  at  various  angles  ;  and  if 
the  least  of  these,  G  F  H,  be  greater  than  the  angle  of  aperture  of 
the  object-glass,  none  of  them  can  enter  it.  The  stop  S  is  attached 
to  a  stem  of  wire,  which  passes  vertically  through  the  paraboloid 


PIG.  260.— Parabolic 
illuminator. 


FIG.  259. 


and  terminates  in  a  knob  beneath,  as  shown  in  fig.  260 ;  and  by 
means  of  this  it  may  be  pushed  upwards  so  as  to  cut  off  the  less 
divergent  rays  in  their  passage  towards  the  object.  It  is  claimed 
that  this  instrument  has  great  capabilities  of  giving  dark-ground 
illumination  with  lenses  of  '  wide  apertures  ; '  but  that  has  application 
to  the  lenses  contemporary  with  its  introduction,  and  not  to  wide 
apertures  as  applied  to  the  lenses  of  to-day.  In  comparison  with 
what  can  be  done  with  condensers  it  suffers  greatly  after  we  pass 
the  Vinch  objective,  although  it  does  give  excellent  results  with 
very  low  powers  such  as  1-inch,  1^-inch,  2-inch,  and  3-inch 
objectives  when  employed  to  illuminate  large  objects  such  as  whole 
insects,  because  this  instrument  gives  more  diffusion  of  light  over 
the  whole  of  a  large  object  than  a  condenser  does. 

Polarising  Apparatus. — In  order  to  examine  transparent  objects 
by  polarised  light,  it  is  necessary  to  employ  some  means  of  polarising 


3l8  ACCESSOKY  APPARATUS 

the  rays  before  they  pass  through  the  object,  and  to  apply  to  them, 
in  some  part  of  their  course  between  the  object  and  the  eye,  an 
analysing  medium.  These  two  requirements  may  be  provided  for 
in  different  modes.  The  polariser  may  be  either  a  bundle  of  plates 
of  thin  glass,  used  in  place  of  the  mirror,  and  polarising  the  rays  by 
reflexion  ;  or  it  may  be  a  '  single  image  '  or  '  Nicol '  prism  of  Iceland 
spar,  which  is  so  constructed  as  to  transmit  only  one  of  the  two 
rays  into  which  a  beam  of  ordinary  light  is  made  to  divaricate  by 
passing  through  this  substance.  Of  these  two  methods  the  *  Nicol ' 
prism  is  the  one  generally  preferred,  the  objection  to  the  reflecting 
polariser  being  that  it  cannot  be  made  to  rotate.  This  polarising 
prism  is  usually  fixed  in  a  tube,  and  is  shown  in  a  simple  form  in 
A,  fig.  261  ;  it  is  usually  employed  in  a  sub-stage  which  rotates  by  a 
rack-and-pinion  arrangement,  so  that  rotation  of  the  prism  is  easily 
effected.  For  the  analyser  a  second  '  Nicol '  prism  is  usually  em- 
ployed ;  and  this,  fixed  in  a  short  tube,  may  be  fitted  into  a  collar 
interposed  between  the  lower  end  of  the  body  and  the  objective, 
as  is  shown  in  B,  fig.  261.  The  prism  in  this  fitting  can  also 

be  rotated  by  the  fingers 
grasping  and  giving  circular 
motion  to  the  inner  fitting  of 
B,  arid  it  is  always  important 
that  the  polarising  prism 
should  be  large,  so  as  not  to 
act  as  a  diaphragm  to  the  con- 
denser, thus  cutting  off  the 
light  when  it  is  used  ;  for  the 
polarising  apparatus  may  be 
FIG.  261.— Polarising  apparatus.  worked  in  combination  either 

with  the  achromatic  con- 
denser, by  which  means  it  may  be  employed  with  high-power 
objectives,  or  as  a  '  dark-ground '  illuminator,  which  shows  many 
objects — such  as  the  horny  polyparies  of  zoophytes — gorgeously 
projected  in  colours  upon  a  dark  field. 

For  bringing  out  certain  effects  of  colour  by  the  use  of  polarised 
light  it  is,  as  already  stated,  desirable  to  interpose  a  plate  of  selenite 
between  the  polariser  and  the  object ;  and  it  is  advantageous  that 
this  should  be  made  to  revolve.  A  very  convenient  mode  of  effecting 
this  is  to  mount  the  selenite  plate  in  a  revolving  collar,  which  fits 
into  the  upper  end  of  the  tube  that  receives  the  polarising  prism. 
In  order  to  obtain  the  greatest  variety  of  coloration  with  different 
objects,  films  of  selenite  of  different  thicknesses  should  be  employed  ; 
and  this  may  be  accomplished  by  substituting  one  for  another  in  the 
revolving  collar.  A  still  greater  variety  may  be  obtained  by  mounting 
three  films,  which  separately  give  three  different  colours,  in  collars 
revolving  in  a  frame  resembling  that  in  which  hand-magnifiers  are 
usually  mounted,  this  frame  being  fitted  into  the  sub-stage  in  such 
'a  manner  that  either  a  single  selenite,  or  any  combination  of  two 
selenites,  or  all  three  together,  may  be  brought  into  the  optic  axis 
above  the  polarising  prism  (fig.  262).  As  many  as  thirteen  different 
tints  may  thus  be  obtained.  When  the  construction  of  the  micro- 


POLARISING  APPARATUS— RINGS  AND   BRUSHES 


319 


scope  does  not  readily  admit  of  the  connection  of  the  selenite  plate 
with  the  polarising  prism,  it  is  convenient  to  make  use  of  a  plate  of 
brass  (fig.  263)  somewhat  larger  than  the  glass  slides  in  which 
objects  are  ordinarily  mounted,  with  a  ledge  near  one  edge  for 
the  slide  to  rest  against  and  a  large  circular  aperture  into  which 
a  glass  is  fitted,  having  a  film  of  selenite  cemented  to  it ;  this 
'  selenite  stage '  or  object-carrier  being  laid  upon  the  stage  of  the 
microscope,  the  slide  containing  the  object  is  placed  upon  it,  and,  by 
an  ingenious  modification  contrived  by  Dr.  Leeson,  the  ring  into 
which  the  selenite  plate  is  fitted  being  made  movable,  one  plate  may 
be  substituted  for  another,  whilst  rotation  may  be  given  to  the  ring 
by  means  of  a  tangent-screw  fitted  into  the  brass  plate.  The 
variety  of  tints  given  by  a  selenite  film  under  polarised  light  is  so 
greatly  increased  by  the  interposition  of  a  rotating  film  of  mica  that 
two  selenites — red  and  blue — wilh  a  mica  film,  are  found  to  give  the 
entire  series  of  colours  obtainable  from  any  number  of  selenite 
films,  either  separately  or  in  combination  with  each  other. 

The  compact  apparatus  made  by  Swift  as  a  general  sub-stage 
illuminator  is  useful  and  commendable, 
and  is  capable  of  adaptation  to  most 
English  microscopes.  It  is  shown  in  fig. 
264.  The  special  a,  A  vantage  of  this  con- 
denser lies  in  its  having  the  polarising 


FIG.  263. 


prism,  the  selenite  and  mica  films,  the  black  ground  and  oblique- 
light  stops,  and  the  moderator  all  brought  close  under  the  back  lens 
of  the  achromatic  ;  whilst  it  combines  in  itself  all  the  most  important 
appliances  which  the  sub-stage  of  a  good  moderate  microscope  can 
require. 

Rings  and  Brushes.- — Mr.  Nelson  has  pointed  out  ('  Journ. 
II. M.S..'  1892)  that  it  is  remarkable  the  microscopical  text-books 
give  no  account  of  the  method  of  viewing  the  rings  and  brushes 
which  certain  minerals  show  under  polarised  light.  If  the  instru- 
ment be  set  up  as  if  for  viewing  ordinary  polariscope  objects,  not  a 
ring  or  a  brush  will  be  seen. 

The  whole  point  lies  in  the  fact  that  it  is  a  wide-angled  telescope 
that  is  required,  and  not  a  microscope.  Once  this  is  recognised  the 
whole  matter  is  simple.  As  the  microscope  has  to  be  turned  into  a 
wide-angled  polarising  telescope,  all  that  is  necessary  is  to  screw  a  low 
power  on  the  end  of  the  draw-tube,  as  in  fig.  265 .  As^the  light  requires 
to  be  passed  through  the  crystal  at  a  considerable  angle,  a  wide- 
aiigled  condenser  should  be  employed,  but  it  need  not  be  achromatic. 


320 


ACCESSOEY  APPARATUS 


The  objective  most  suitable  is  a  nyths  of  '65  N.A. ;  but  a  Jth  of  -71 
N.A.,  or  a  ^-rd  of  '65  N".A.  will  do  equally  well,  as  the  whole  of  the 
back  lens  of  the  objective  should  be  visible  through  the  analysing 
'  Nicol ; '  the  back  lens  of  the  objective  must  not  be  too  large,  thus  a 
^  inch  of  '65  N.A.  would  not  do  so  well.  The  analysing  prism  may 
be  placed  either  where  it  is  in  the  drawing  or  above  the  eye-piece. 
Practically  it  works  very  well  above  the 
objective,  which  is  the  position  it  occupies 
in  '  ordinary  microscopical  outfits.' 

For  the  draw-tube  a  2 -inch  objective 
and  a  B  or  C  eye-piece  will  answer 
admirably. 


FIG.  264. — Swift's  illuminating  and  polarising 
apparatus. 


FIG.  265. — In  this  diagram  P 
is  the  polarising  prism  in 
the  sub-stage,  C  sub-stage 
condenser.  On  the  stage 
M  mineral.  On  nose-piece 
Ol  objective  tfeths  '64  N.A. ; 
A  analysing  prism.  In  the 
draw-tube,  Oa  objective  2 
or  3  in.  H,  Huyghenian 
eye-piece. 


For  setting  up  the  instrument  it  is  better,  before  screwing  the 
objective  in  the  end  of  the  draw  tube,  to  centre  the  light  in  the 
usual  manner,  the  '  Mcols '  being  turned  so  as  to  give  a  light  field. 
Next  fix  the  objective  in  the  draw-tube,  open  the  sub-stage  con- 
denser to  full  aperture,  and  put  the  mineral  on  the  stage.  Rack 


MONOCHROMATIC   ILLUMINATION 


321 


down  the  body,  so  that  the  objective  on  the  nose-piece  nearly 
touches  the  crystal ;  then  focus  with  the  draw-tube  exclusively.  The 
sub-stage  condenser  should  be  racked  up  close  to  the  under  side  of 
the  crystal. 

The,  use  of  monochromatic  light  is  frequently  desirable  in  micro- 
scopic work,  especially  blue  light,  although  of  less  moment  than 
in  pre-achromatic  days.  The  usual  method  of  obtaining  coloured 
light  is  to  pass  sunlight  through  coloured  glass,  or  through  a 
coloured  solution,  such  as  the  ammonio-sulphate  of  copper  ;  but  this 
is  a  most  imperfect  and  unsatisfactory  method,  and  does  not  give 
//w achromatic  light.  This  most  valuable  mode  of  illumination  has 
been  made  possible  by  the  use  of  what  is  now  known  as  the  Gifford 

screen,  from  the  name  of  its 
inventor,  Mr.  J.  W.  Gifford  ; 
and  when  artificial  light  is 
used  one  of  these  screens 
should  be  interposed  between 
the  lamp  and  the  sub-stage 
condenser.  It  is  shown  in  fig. 
266,  and  consists  of  a  glass 
trough,  about  3  inches  long  by 
2  inches  broad  and  ^ths  deep, 
filled  with  a  solution  of  methyl 
green  and  glycerin  mixed 

E  6 


FIG.  266. — Gifford  screen  with  an  adjustable 
stand. 


FIG.  267. — Gifford' sF-line  mono- 
chromatic light  screen. 


warm.  Now  this  solution  passes  a  little  band  of  infra  red,  which 
must  be  cut  out.  To  do  this  a  piece  of  signal  green  glass  just  fitting 
the  trough  is  placed  in  it. 

A  piece  of  ordinary  commercial  signal  green  would  cut  out  too 
much  light,  and  render  the  screen  too  opaque ;  therefore  it  is 
requisite  to  have  this  signal  green  glass  worked  down  to  about  half 
its  thickness,  so  that  only  the  infra  red  passed  by  the  methyl 
green  is  cut  out,  and  nothing  more.  This  screen  is  called  an 
F-line  screen,  because  the  F  line  is  in  the  centre  of  the  band  passed 
by  it.  The  band  for  general  microscopical  purposes  may  usefully 
extend  from  E  to  G.  The  importance  of  this  screen  cannot  be  held 
too  high  by  the  modern  microscopist.  It  makes  semi-apochromatic 


322 


ACCESSOKY  APPARATUS 


objectives  equal  to  real  apochromatics,  and  it  sharpens  the  images 
yielded  even  by  the  latter,  whilst  it  increases  resolving  power  in  all 

lenses,  arid  amelior- 
ates the  strain  often 
felt  by  workers  who 
have  not  before  used 
it. 

The  cell  contain- 
ing the  solution  and 
worked  glass  may 
either  have  its  upper 
end  sealed  hermeti- 
cally with  paraffin,  or 
be  simply  carefully 
corked ;  the  latter 
plan,  if  the  cork  is 
carefully  made,  ad- 
mits of  the  easy 
opening  of  the  cell 
and  renewal  of  the 
fluid.  A  diagram- 
matic illustration  of 
the  effect  of  the  use 
of  the  screen  is  given 
in  fig.  267,  which 
represents  the  band 
of  colour  passed 
through  the  F-line 
screen.  The  green 
is  represented  by  the 
horizontal  lines,  and 
the  blue,  in  which 
the  F  line  is  situated, 
by  the  diagonal  lines. 
The  cell  itself  is 
prepared  by  the  Ley- 
bolds  process,  and  is 
fused  at  the  joints 
and  never  leaks  ;  a 
still  simpler  and  less 
expensive  means  of 
making  such  a  filter 
has  been  devised  by 
Dr.  A.  Meithe,  pro- 
fessor of  spectral 
analysis  at  Berlin. 
The  filter  consists  of 

_  a  trough  containing 

|  of  an  inch  in  thickness  of  saturated  solution  of  acetate  of  copper 
filtered ;   a  variation  in  the  thickness  of  the  troughs  or  tanks 
desirable,  but  the  results  are  excellent. 


MICRO-SPECTKOSCOPE  323 

Equally  perfect  monochromatic  illumination  can  be  obtained  by 
prismatic  dispersion. 

A  method  of  approximating  to  monochromatic  illumination  has 
been  devised  by  Mr.  Nelson  which  answers  admirably  with  an 
ordinary  ^-inch  wick  paraffin  lamp.  Briefly,  the  rays  proceeding 
from  the  radiant  are  passed  through  a  slit,  as  in  fig.  268,  and 
dispersed  by  a  prism  of  glass,  and  by  means  of  a  second  slit  any 
portion  we  wish  may  be  selected  from  the  spectrum  to  be  used  for 
the  purpose  required. 

First  an  image  of  the  edge  of  the  flame  is  focussed  upon  the  slit 
by  means  of  a  bull's-eye  consisting  of  three  lenses ;  next  the  slit  is 

placed  in  the  principal  focus  of  a  lens  known  as  a  Wray  5   x  4  R  R, 

£ 

working  at  -z—  (this  lens  is  not  shown  in  the  cut).      In  the  parallel 
5*6*  % 

beam  from  this  lens  and  close  to  it  is  placed  an  equilateral  prism  of 
dense  flint  set  at  minimum  deviation.  Close  to  the  prism  is  placed 

another  Wray  5  x  4  R  R,  working  at  J— .    If  a  cardboard  screen  be 

O'D 

held  at  the  principal  focus  of  this  lens,  there  will  be  seen  a  spectrum 
brilliantly  illuminated.  A  slit  nyth  inch  in  diameter  is  cut  in  the 
cardboard  screen,  through  which  the  required  colour  is  allowed  to 
pass  to  the  mirror  of  the  microscope,  thence  to  the  sub-stage  con- 
denser. For  visual  work  blue  green  is  the  best,  but  for  photo- 
graphic work  blue  would  be  chosen  unless  orthochromatic  work 
required  a  colour  lower  down  the  spectrum. 

Sorby-Browning  Micro-spectroscope.1 — When  the  solar  ray  is 
decomposed  into  a  coloured  spectrum  by  a  prism  of  sufficient  disper- 
sive power,  to  which  the  light  is  admitted  by  a  narrow  slit,  a 
multitude  of  dark  lines  make  their  appearance.  The  existence  of 
these  was  originally  noticed  by  Wollaston  ;  but  as  Fraunhofer  first 
subjected  them  to  a  thorough  investigation  and  mapped  them  out, 
they  are  known  as  Fraunhofer  lines.  The  greater  the  dispersion 
given  by  the  multiplication  of  prisms  in  the  spectroscope,  the  more 
of  these  lines  are  seen ;  and  they  bear  considerable  magnification. 
They  result  from  the  interruption  or  absorption  of  certain  rays  in 
the  solar  atmosphere,  according  to  the  law,  first  stated  by  Angstrom, 
that  '  rays  which  a  substance  absorbs  are  precisely  those  which  it 
emits  when  made  self-luminous.'  Kirchhoff  showed  that  while  the 
incandescent  vapours  of  sodium,  potassium,  lithium,  <fcc.  give  a 
spectrum  with  characteristic  bright  lines,  the  same  vapours  intercept 
portions  of  the  spectrum  so  as  to  give  dark  lines  at  the  points  where 
the  bright  ones  appeared,  absorbing  their  own  special  colour,  but 
allowing  rays  of  other  colours  to  pass  through.  Again,  when  ordinary 
light  is  made  to  pass  through  coloured  bodies  (solid,  liquid,  or 
gaseous),  or  is  reflected  from  their  surfaces  so  as  to  affect  the  eye 
with  the  sensation  of  colour,  its  spectrum  is  commonly  found  to 
exhibit  absorption  bamls,  which  differ  from  the  Fraunhofer  lines  not 
only  in  their  greater  breadth,  but  in  being  more  or  less  nebulous  or 

1  For  general  information  on  the  spectroscope  and  its  uses  the  student  is  referred  to 
Professor  Roscoe's  Lectures  on  Spectrum  An  a  lysis,  or  the  translation  of  Dr.  Schellen's 
Spectrum  Analysis,  and  How  to  use  the  Spectroscope,  by  Mr.  John  Browning. 

Y2 


324 


ACCESSORY  APPARATUS 


cloudy,  so  that  they  cannot  be  resolved  into  distinct  lines  by  magni- 
fication, while  too  much  dispersion  thins  them  out  to  indistinctness. 
Now,  it  is  by  the  character  of  these  bands,  and  by  their  position  in 
the  spectrum,  that  the  colours  of  different  substances  can  be  most  ac- 
curately and  scientifically  compared,  many  colours  whose  impressions 
on  the  eye  are  so  similar  that  they  cannot  be  distinguished  being 
readily  discriminated  by  their  spectra.  The  purpose  of  the  micro- 
spectroscope  l  is  to  apply  the  spectroscopic  test  to  very  minute 
quantities  of  coloured  substances  ;  and  it  fundamentally  consists  of 
an  ordinary  eye-piece  (which  can  be  fitted  into  any  microscope)  with 
certain  special  modifications.  As  originally  devised  by  Dr.  Sorby 
and  worked  out  by  Mr.  Browning,  the  micro-spectroscope  is  con- 
structed as  follows  (fig.  269) :  Above  its  eye-glass,  which  is  achro- 
matic, and  made  capable  of  focal  adjustment  by  the  milled  head,  B, 
there  is  placed  a  tube,  A,  containing  a  series  of  five  prisms,  two  of 
flint  glass  (fig.  270,  F  F)  interposed  between  three  of  crown 
(C  0  C)  in  such  a  manner  that  the  emergent  rays,  r  r,  which  have 
been  separated  by  dispersion,  leave  the  prisms  in  much  the  same 

direction  as  the  immergent  ray 
entered  it.  Below  the  eye-glass, 
in  the  place  of  the  ordinary  stop, 
is  a  diaphragm  with  a  narrow  slit 
which  limits  the  admission  of  light 
(fig.  269) ;  this  can  be  adjusted  in 
vertical  position  by  the  milled  head, 
H,  whilst  the  breadth  of  the  slit  is 


FIG.  269. — Micro-spectroscope. 


FIG.  270. 


regulated  by  C.  The  foregoing,  with  an  objective  of  suitable  power, 
would  be  all  that  is  needed  for  the  examination  of  the  spectra  of 
objects  placed  on  the  stage  of  the  microscope,  whether  opaque  or 
transparent,  solid  or  liquid,  provided  that  they  transmit  a  sufficient 
amount  of  light.  But  as  it  is  of  great  importance  to  make  exact 
comparisons  of  such  artificial  spectra,  alike  with  the  ordinary  or 
natural  spectrum  and  with  each  other,  provision  is  made  for  the 
formation  of  a  second  spectrum  by  the  insertion  of  a  right-angled 
prism  that  covers  one  half  of  this  slit,  and  reflects  upwards  the  light 
transmitted  through  an  aperture  seen  on  the  right  side  of  the  eye- 
piece. For  the  production  of  the  ordinary  spectrum,  it  is  only 
requisite  to  reflect  light  into  this  aperture  from  the  small  mirror,  I, 
carried  at  the  side  ;  whilst  for  the  production  of  the  spectrum  of  any 
substance  through  which  the  light  reflected  from  this  mirror  can  be 
transmitted,  it  is  only  necessary  to  place  the  slide  carrying  the 
section  or  crystalline  film,  or  the  tube  containing  the  solution,  in 

1  We  do  not  make  the  change,  lest  complications  should  arise;  but  we  think  it 
would  be  more  harmonious  with  analogy  to  call  this  instrument  the  spectro-micro- 
scope. 


USE    OF   THE   MICRO-SPECTROSCOPE 


325 


the  frame,  D  D,  adapted  to  receive  it.  In  either  case  this  second 
spectrum  is  seen  by  the  eye  of  the  observer  alongside  of  that  pro- 
duced by  the  object  viewed  through  the  body  of  the  microscope,  so 
that  the  two  can  be  exactly  compared. 

The  exact  position  of  the  absorption  bands  is  as  important  as 
that  of  the  Fraunhofer  lines ;  and  some  of  the  most  conspicuous  of 
the  latter  afford  fixed  points  of  reference,  provided  the  same  spectro- 
scope be  employed.  The  amount  of  dispersion  determines  whether 
the  Fraunhofer  lines  and  absorption  bands  are  seen  nearer  or 
farther  apart,  their  actual  positions  in  the  field  of  view  varying 
according  to  the  dispersion,  while  their  relative  positions  are  in 
constant  proportion.  The  best  contrivance  for  measuring  the 
spectra  of  absorption  bands  is 
Browning's  bright-line  micro- 
meter, shown  in  fig.  271.  At-H 
is  a  small  mirror  by  which  light 
from  the  lamp  employed  can  be 
reflected  through  E  D  to  the 
lens  C,  which,  by  means  of  a 
perforated  stop,  forms  a  bright 
pointed  image  on  the  surface  of 
the  upper  prism,  whence  it  is 
reflected  to  the  eye  of  the  ob- 
server. The  rotation  of  a  wheel 
worked  by  the  milled  head,  M, 
carries  this  bright  point  over  the 
spectrum,  and  the  exact  amount 
of  motion  may  be  read  off  to 
loooo^h  inch  on  the  graduated 
circle  of  the  wheel.  To  use  this 
apparatus,  the  Fraunhofer  lines 
must  be  viewed  by  sending  bright 
daylight  through  the  spectro- 
scope, and  the  positions  of  the 
principal  lines  carefully  measured, 
the  reading  on  the  micrometer- 
wheel  being  noted  down.  A 
spectrum  map  may  then  be  drawn  FIG-  271. 
on  cardboard,  on  a  scale  of  equal 
parts,  and  the  lines  marked  on  it,  as  shown  in  the  upper  half  of 
fig.  272.  The  lower  half  of  the  same  figure  shows  an  absorption 
spectrum,  with  its  bands  at  certain  distances  from  the  Fraunhofer 
lines.  The  cardboard  spectrum  map,  when  once  drawn,  should  be 
kept  for  reference.1 

A  beginner  with  the  micro-spectroscope  should  first  hold  it  up  to 
the  sky  on  a  clear  day,  without  the  intervention  of  the  microscope, 

1  Mr.  Swift  has  devised  an  improved  micro- spectroscope,  in  which  the  micro- 
metric  apparatus  is  combined  with  the  ordinary  spectroscopic  eye-piece,  and  two 
spectra  can  be  brought  into  the  field  at  once.  Other  improvements  devised  by 
Dr.  Sorby  and  a  new  form  devised  by  Mr.  "F.  H.  Ward  have  been  carried  into 
execution  by  Mr.  Hilger.  (See  Journ.  of  Roy.  Microsc.  Soc.  vol.  i.  1878,  p.  326, 
and  vol.  ii.  1879,  p.  81.) 


-Bright-line  spectro-micrometer. 


326 


ACCESSOKY  APPARATUS 


and  note  the  effects  of  opening  and  closing  the  slit  by  rotating  the 
screw,  C  (fig.  269) ;  the  lines  can  only  be  well  seen  when  the  slit  is 
reduced  to  a  narrow  opening.  The  screw  H  diminishes  the  length 
of  the  slit,  and  causes  the  spectrum  to  be  seen  as  a  broad  or  a  narrow 
ribbon.  The  screw  E  (or  in  some  patterns  two  small  sliding  knobs) 


FIG.  272. — Upper  half,  map  of  solar  spectrum,  showing  Frauiihofer  lines.     Lower 
half,  absorption  spectrum,  showing  position  of  bands  in  relation  to  lines. 

regulates  the  quantity  of  light  admitted  through  the  square  aperture 
seen  between  the  points  of  the  springs,  D  D.  Water  tinged  with 
port  wine,  madder,  and  blood  are  good  fluids  with  which  to  com- 
mence this  study  of  absorption  bands.1  As  each  colour  varies  in 
refrangibility,  the  focus  must  be  adjusted  by  the  screw  B,  fig.  269, 

according  to  the  part  of  the 
spectrum  that  is  examined. 
When  it  is  desired  to  see  the 
spectrum  of  an  exceedingly 
minute  object,  or  of  a  small 
portion  only  of  a  larger  one, 
the  prisms  are  to  be  re- 
moved by  withdrawing  the 
tube  containing  them  ;  the 
slit  should  then  be  opened 
wide,  and  the  object,  or  part 
of  it,  brought  into  the  centre 
of  the  field ;  the  vertical  and 
horizontal  slits  can  then  be 
partly  shut  so  as  to  enclose 
it ;  and  if  the  prisms  are 
then  replaced  and  a  suitable 
objective  employed,  the  re- 
quired spectrum  will  be  seen, 
unaffected  by  adjacent  ob- 
j ects.  For  ordinary  observa- 
tions objectives  of  from  two 

inches  to  ^-inch  focus  will  be  found  most  suitable  ;  but  for  very 
minute  quantities  of  material  a  higher  power  must  be  employed. 
Even  a  single  red  blood-corpuscle  may  be  made  to  show  the 

1  A  series  of  specimens,  in  small  tubes,  for  the  study  of  absorption-spectra,  is 
kept  on  sale  by  Mr.  Browning  :  and  the  directions  given  in  his  How  to  work  with 
the  Micro-spectroscope  should  be  carefully  attended  to. 


273. 


USE   OF   THE   MICKO-SPECTKOSCOPE 


327 


characteristic  absorption  bands  represented  (after  Professor  Stokes) 
in  fig.  273.1 

For  the  study  of  coloured  liquids  in  test-tubes  or  small  cells,  the 
binocular  spectrum  microscope,  described  by  Dr.  Sorby  in  the  *  Pro- 
ceedings of  the  Royal  Society,'  No.  92,  1867,  p.  33,  is  extremely 
convenient. 

Tlie  spectral  ocular  by  Zeiss  is  another  and  a  very  perfect  form  of 
the  micro-spectroscope.  This  is  an  opinion  expressed  by  Dr.  Sorby 
and  other  experts,  and  it  is  manifest  in  the  character  of  the  in- 
strument. Fig.  274  represents  a  sectional  view  of  the  instrument. 
It  will  be  seen  that  the  lower  part  is  an  ordinary  eye-piece  with  its 
two  lenses,  but  in  place  of  the  ordinary  diaphragm  there  is  a  slit 
adjustable  in  length  and  breadth,  shown  in  fig.  275.  By  studying 
this  figure  the  method  of  adjustment  with  two  screws,  F  and  H, 
and  the  projecting  lever,  whi'ch.  carries  a  reflecting  prism,  can  be 


FK;.  274. 


FIG.  275. 


readily  understood.  The  upper  part  of  the  instrument  swings 
about  the  pivot,  K,  so  that  by  opening  the  slit  the  eye-piece  can  be 
used  for  focussing  an  object,  the  slit  being  the  diaphragm.  The 
upper  portion  contains  the  prisms,  and  also  a  scale  in  the  tube,  N, 
which  is  illuminated  by  the  mirror,  0.  The  image  of  the  scale  is 
reflected  from  the  upper  surface  of  the  last  prism  to  the  eye,  and 
when  properly  adjusted  gives  the  wave-length  of  the  light  in  any 
part  of  the  spectrum.  There  is  also  a  supplementary  stage,  not 
shown  in  the  figure,  upon  which  a  specimen  can  be  placed,  and  its 
light  thrown  up  through  the  slit  by  reflection  from  the  prism  on  the 
lever  shown  in  fig.  274,  alongside  of  the  light  from  the  object  on  the 
stage  of  the  microscope,  thus  enabling  the  spectra  from  the  two 
sources  to  be  directly  compared. 

1   For  further  information  on  '  The  Spectrum  Method  of  Detecting  Blood,'  see  an 
important  paper  by  Dr.  Sorby  in  Monthly  Microsc.  Journ.  vol.  vi.  1871,  p.  9. 


328  ACCESSORY   APPARATUS 

The  Method  of  using  the  Micro-spectroscope, — The  objects  to  be 
investigated  are  of  two  sorts,  liquid  and  solid.  Colouring  substances, 
as  chlorophyll,  the  colouring  matter  of  hair,  blood,  &c.,  will  fre- 
quently come  under  inicro-spectroscopic  investigation  in  the  form  of 
a  solution.  In  general  we  need  scarcely  say  anything  concerning 
the  preparation  of  the  solution.  In  reference  to  the  chlorophyll  of 
the  phanerogams  especially,  the  particular  part  of  the  plant  from 
which  the  preparation  is  to  be  made,  as,  for  instance,  the  foliage 
leaves,  is  put  for  a  short  time  in  boiling  water,  then  quickly  dried 
by  means  of  bibulous  paper,  and  then  immersed  for  a  longer  time  in 
absolute  alcohol,  ether,  or  benzole  in  a  dark  place,  for  the  purpose 
of  extracting  the  chlorophyll  colouring  matter.  The  concentration 
of  the  solution  thus  produced,  which  influences  the  intensity  of  the 
absorption  spectrum  and  the  number  and  length  of  the  absorption 
bands,  depends  naturally  upon  the  time  during  which  the  material  is 
in  the  extracting  medium,  as  well  as  on  the  quantity  of  the  material. 

Commonly  also  a  solution  of  less  concentration  will  give  the  same 
intensity  of  spectrum  if  a  sufficiently  thick  layer  of  it  be  used.  The 
solution  can  generally  be  examined  in  an  ordinary  test-tube.  The 
test-tube  is  filled  and  carefully  corked,  and  then  laid  on  the  stage  of 
the  microscope  or  held  before  the  opening  of  the  comparison  prism, 
as  the  case  may  be.  For  the  latter  purpose  (bringing  liquids  before 


the  opening  of  the  comparison  prism)  a  small  open  trough  of  glass, 
with  two  parallel  glass  plates,  is  very  useful.  For  exact  investiga- 
tions, however,  the  trough-flask  is  preferable.  It  is  a  flask  whose 
two  sides,  back  and  front,  are  parallel,  furnished  with  a  carefully 
fitted  ground-glass  stopper.  It  should  be  filled  quite  full  of  the 
solution  and  then  laid  with  its  broad  side  on  the  stage.  It  is 
especially  indispensable  when  we  wish  to  study  the  combination 
spectrum  of  two  solutions.  In  that  case  two  flasks  are  filled  each 
with  a  different  solution,  and  both  laid  upon  the  stage,  one  upon 
the  other.  For  the  purpose  of  examining  small  quantities  of 
any  liquid,  a  sufficient  depth  being  obtained  with  very  little 
material,  vertical  glass  tubes  attached  to  horizontal  plates  are  used, 
as  proposed  by  Mr.  Sorby  and  shown  in  fig.  276.  The  narrow  tubes 
are  made  of  various  lengths  from  sections  of  barometer  tubing,  in 
order  to  present  different  thicknesses  of  the  contained  fluid,  the 
broad  tube  being  higher  on  one  side  than  the  other,  and  thus  con- 
stituting a  wedge-shaped  cell,  which,  when  filled  and  closed  by  a 
thin  cover-glass,  will  present  a  varying  thickness  of  fluid  for  study 
and  comparison.  If  the  object  to  be  investigated  is  not  a  solution, 
but  a  preparation  of  the  kind  which  we  commonly  employ  in  micro- 
scopic inquiries,  we  must  first  of  all  bring  it  into  the  focus  of  the 
objective  system.  To  do  this  we  must  first  remove  the  tube  bearing 


ILLUMINATION   BY    REFLECTION 


329 


the  prisms,  open  the  slit  somewhat,  and  use  the  apparatus  as  a 
simple  ocular.  If  one  has  to  deal  with  a  small  object  which  does 
not  entirely  fill  the  slit,  but  allows  rays  of  light  to  come  in  past 
it  and  disturb  the  spectrum,  he  should  turn  the  comparison  prism 
so  as  to  shut  up  some  of  the  slit,  without,  however,  letting  in  the 
light  upon  it,  and  then  bring  the  object  up  near  to  it,  and  from  the 
other  side  push  up  the  shortening  apparatus  as  close  as  is  necessary. 
On  the  other  hand,  should  the  object  consist  of  a  number  of  single 
minute  grains,  which  would  cause  to  be  drawn  across  the  spectrum, 
in  the  direction  of  its  length,  perpendicular  to  the  Fraunhofer  lines' 
a  like  number  of  dark  lines, 
one  must  adjust  the  micro- 
scope so  that  the  object  will 
be  a  little  out  of  focus,  some-  > 
what  above  or  below  the  true' 
focus.  In  this  way  we  shall 
get  a  uniform  spectrum.  The 
spectrum  can  also  be  improved 
in  some  other  cases  by  like- 
wise throwing  the  object 
somewhat  out  of  focus. 

Illumination  by  Reflec- 
tion.— Objects  of  almost  every 
description  \vill  require  at 
times  to  be  examined  and 
studied  by  what  is  called  re- 
flected light ;  the  light  in 
this  case  is  thrown  down  upon 
the  object  by  various  devices, 
and  is  reflected  upwards 
through  the  objective.  This 
has  been  called  '  opaque  illu- 
mination,' which,  however, 
is  not  a  comprehensive,  nor 
even  an  accurate  designation. 
Only  a  small  proportion  of 
the  objects  examined  in  this 
way  are  opaque ;  the  same 
diatom,  for  example,  may 
often  with  advantage  be  ex- 
amined with  transmitted 
light,  being  transparent,  and  again  by  means  of  an  illumination 
thrown  upon,  and  reflected  up  from,  its  surface ;  also  a  condenser 
with  a  central  stop,  when  used  for  a  dark  ground,  shows  objects  by 
reflected  light,  but  it  is  manifestly  not  ;  opaque  illumination.'  The 
designation  of  this  method  of  illumination  is  consequently  more 
accommodating  than  accurate. 

There  are  two  very  simple  means  of  obtaining  this  superficial 
illumination  when  low  powers  are  employed.  The  first  is  the 
'  bull's-eye '  (which  is  nowhere  in  this  work  called  a  '  condenser  ;  ' 
this  would,  as  it  often  has  done,  lead  to  confusion ;  it  is  enough  to 


FIG.  277.— The  English  form  of  bull's-eye. 


330 


ACCESSORY   APPARATUS 


designate  it  as  we  have  done).  It  is  a  plano-convex  lens  of  short 
focus,  two  or  three  inches  in  diameter,  mounted  upon  a  separate 
stand  in  such  a  manner  as  to  permit  of  its  being  placed  in  a  great 
variety  of  positions.  The  mounting  shown  in  fig.  277  is  the  usual 
adopted  in  England ;  the  frame  which  carries  the  lens  is  borne 
at  the  bottom  upon  a  swivel  joint,  which  allows  it  to  be  turned  in 
any  azimuth  ;  whilst  it  may  be  inclined  at  any  angle  to  the  horizon, 
by  the  revolution  of  the  horizontal  tube  to  which  it  is  attached, 
around  the  other  horizontal  tube  which  projects  from  the  stem.  By 
the  sliding  of  one  of  these  tubes  within  the  other,  again,  the  hori- 
zontal arm  may  be  lengthened  or  shortened;  the  lens  may  be 
secured  in  any  position  (as  its  weight  is  apt  to  drag  it  down  when 


•      FIG.  277A. 

it  is  inclined,  unless  the  tubes  be  made  to  work,  the  one  into  the 
other,  more  stiffly  than  is  convenient)  by  means  of  a  tightening 
collar  milled  at  its  edges ;  and  finally  the  horizontal  arm  is 
attached  to  a  spring  socket  which  slides  up  and  down  upon  a  vertical 
stem. 

A  good  form  of  the  bull's-eye  is  made  by  Leitz,  and  is  illustrated 
fig.  277A.  All  the  required  movements  are  provided  for,  but  in  a 
different  way  ;  the  clamping  screws  are  by  means  of  usual  milled 
heads. 

The  plane  side  of  the  bull's-eye  should  be  turned  towards  the 
object.  Some  microscopists  like  to  have  their  bull's-eye  attached'  to 
some  part  of  the  microscope  ;  but  if  this  is  done,  care  must  be  taken 
to  attach  it  to  a  fixed  part  of  the  microscope,  and  not  to  either  the 


THE   USE   OF   THE   BULL'S-EYE  331 

mechanical  stage  or  to  the  body,  as  is  so  often  done.  If  it  is  fixed 
to  the  mechanical  stage,  when  the  object  is  moved  the  light  will 
require  to  be  readjusted,  to  say  nothing  of  the  probable  injury  to 
the  stage  by  the  weight  of  the  bull's-eye.  If  it  is  fixed  to  the 
body  the  light  will  be  displaced  when  the  focus  of  the  objective  is 
altered.  Hence  the  bull's-eye  should  either  have  a  weighted  separate 
stand,  or  be  attached  to  the  stand  or  holder  of  the  lamp  or  other 
illuminant. 

The  optical  effect  of  such  a  bull's-eye  differs  according  to  the 
side  of  it  turned  towards  the  light  and  the  condition  of  the  rays 
which  fall  upon  it.  The  position  of  least  spherical  aberration  is  when 
its  convex  side  is  turned  towards  parallel  or  towards  the  least  diverging 
rays ;  consequently,  when  used  by  daylight,  its  plane  side  should  be 
turned  towards  the  object,  and  tjie  same  position  should  be  given  to  it 
when  it  is  used  for  procuring  converging  rays  from  a  lamp,  this  being 
placed  four  or  five  times  farther  off  on  one  side  than  the  object  is  on 
the  other.  But  it  may  also  be  employed  for  the  purpose  of  reducing 
the  diverging  rays  of  the  lamp  to  parallelism,  for  use  either  with  the 
paraboloid,  or  with  the  parabolic  speculum  to  be  presently  described  ; 
and  the  plane  side  is  then  to  be  turned  towards  the  lamp,  which  must 
be  placed  at  such  a  distance  from  the  bull's-eye  that  the  rays  which 
have  passed  through  the  latter  shall  form  an  inverted  image  of 
the  lamp  flame  on  the  wall  or  a  distant  screen.  For  viewing  minute 
objects  under  high  powers,  a  smaller  lens  may  be  used  to  obtain  a 
further  concentration  of  the  rays  already  brought  into  convergence 
by  the  bull's-eye.  An  ingenious  and  effective  mode  of  using  the 
bull's-eye  for  the  illumination  of  very  minute  objects  under  higher- 
power  objectives  has  been  devised  by  Mr.  James  Smith.  The  micro- 
scope being  in  position  for  observation,  the  lamp  should  be  placed 
either  in  the  front  or  at  the  side  (as  most  convenient),  so  that  its 
flame,  turned  edgeways  to  the  stage,  should  be  at  a  somewhat  lower 
level,  and  at  a  distance  of  about  three  inches.  The  bull's-eye  should 
be  placed  between  the  stage  and  the  lamp,  with  its  plane  surface 
uppermost,  and  with  its  convex  surface  a  little  above  the  stage. 
The  light  entering  its  convex  surface  near  the  margin  turned  towards 
the  lamp  falls  on  its  plane  surface  at  an  angle  so  oblique  as  to  be 
almost  totally  reflected  towards  the  opposite  margin  of  the  convex 
surface,  by  which  it  is  condensed  on  to  the  object  on  the  stage,  on 
which  it  should  cast  a  sharp  and  brilliant  wedge  of  light.  The  ad- 
justment is  best  made  by  first  placing  a  slip  of  white  card  on  the 
stage,  and,  when  this  is  well  illuminated,  substituting  the  object 
slide  for  it,  making  the  final  adjustment  while  the  object  is  being 
viewed  under  the  microscope.  No  difficulty  is  experienced  in 
getting  good  results  with  powers  of  from  200  to  300  diameters,  but 
higher  powers  require  careful  manipulation  and  yield  but  doubtful 
results. 

The  second  simple  method  of  securing  this  illumination  is  to  have 
the  concave  mirror  of  the  microscope  capable  of  being  used  above 
the  stage,1  so  that  the  source  of  light  may  by  its  means  be  focussed 
on  the  object.  Neither  of  these  plans  will  answer  for  other  than  low 

1  See  Jo  urn.  Boy.  Microsc.  Soc.  vol.  iii.  1880,  p.  398. 


33 2  ACCESSOEY   APPARATUS 

powers,  where  there  is  plenty  of  room  for  the  light  to  pass  between 
the  objective  and  the  object.  The  ingenious  use  of  the  bull's-eye 
employed  by  Mr.  James  Smith,  as  detailed  above,  increases  the  possi- 
bility of  magnification,  but  it  needs  practice  and  care.  With  the 
great  improvement  which  has  been  effected  in  objectives  and  con- 
densers the  need  of  a  bull's-eye  which  should  give  the  minimum  of 
aberration  has  become  a  desideratum  ;  and  Mr.  Nelson  has  calculated 
and  had  constructed  a  doublet  bull's-eye  which  gives  admirable 
results.  There  are  described  in  most  treatises  on  optics  doublets 
devised  by  Herschel  which  are  said  to  be  of  '  no  aberration.'  Mr. 
Nelson  has  shown  ('Journ.  Q.  M.  S.,'  vol.  vi.  ser.  ii.  p.  197,  1896) 
that  they  are  by  no  means  free  from  spherical  aberration,  and  that 
their  forms  are  such  as  will  not  even  yield  a  minimum  amount  of 
such  aberration ;  also  that  there  is  a  numerical  error  in  the  focal 
length  of  the  high-power  doublet.  He  has  computed  that  the  spheri- 
cal aberration  in  the  Herschel  doublets  amounts  to  — * 296^,  and 
he  gives  the  following  formula  for  a  combination,  the  spherical 
aberration  of  which  is  —  *207^  ;  or  30  per  cent,  less  than  in  either  of 

JF 

those  proposed  by  Sir  John  Herschel. 

Boro-silicate  glass,  Jena  catalogue  No.  5  ;  /z=T51 . 


1st  lens  crossed,  r=+   2*359)  ,. 

s=-15-078jdlameter2'1; 
2nd  lens  meniscus,  r=+    1;280  Jj.^^.  ^ 

Distance  between  the  lenses  *05,  equivalent  focus  2*0,  working 
distance  or  back  focus  1*55,  total  aberration  —  *1035,  clear  aperture 
2*0,  angle  62°.  The  second  Gauss  point  of  the  combination  is  close 
to  the  posterior  surface  of  the  crossed  lens. 

As  there  are  some  microscopists  who  might  require  a  combina- 
tion of  this  kind,  but  with  a  different  focal  length,  and  who  are 
unable  to  transpose  the  formula,  the  following  rule  may  be  of  use. 
Halve  all  the  radii  and  diameters  and  multiply  the  results  by  the  focal 
length  that  is  required.  Example. — Required  a  doublet  on  this 
formula  with  3^  inches  of  equivalent  focus.  Halving  the  data  for 
the  crossed  lens  in  the  given  formula,  we  have  r=  +  l*1795, 
s=  —  7*539,  diameter  1*05;  multiplying  these  results  by  3i  we 
obtain  r=  +4*128,  s=  —  26*386,  diameter  3*7.  Treat  the  meniscus 
in  the  same  way ;  the  lens  distance  may  with  advantage  be  kept 
•05. 

The  following  bull's-eye  is  not  so  expensive  to  manufacture,  and 
may  on  that  account  be  preferred  to  the  doublet  of  minimum  aber- 
ration just  described.  Its  form,  though  of  minimum  aberration  for 
two  plano-convex  lenses,  possesses  43  per  cent,  more  aberration  than 
the  former.  It  will  on  this  account  not  be  possible  to  obtain  such 
an  even  and  unbroken  disc  of  light  with  this  form  of  bull's-eye  as 
with  the  other.  The  data  are  as  follows. 


NELSON'S   COMPUTATION  333 

Glass,  boro-silicate,  the  same  as  before. 

Radii  r  = -f  272)  ,. 

diameter  2-1; 

diameter  1-9. 

Distance    of  lenses  apart    '05,    equivalent  focus  2'0,    working  dis- 
tance 1-50,  angle  60°. 

It  is  illustrated  in  a  mounted  form  in  fig.  278.  Combinations 
having  different  foci  may  be  constructed  in  the  same  manner  as  in 
the  example  above. 

An  illuminator  not  so  well  known,  or  at  least  so  much  used,  as 
its  merits  justified,  is  Powell  and  Lea*land's  small  bull's-eye  of  J  inch 
focus,  which  slides   into   an  adapter  fixed  into  the  sub-stage,  and 
susceptible  of  its  rack  motion  upland  down.     The  object  is  placed 
on  a  super-stage,  and  lies  considerably  above,  but  parallel   with,  the 
ordinary   stage.     The  bull's-eye,  capable  thus  of 
being  raised  or  lowered,  and  of  being  moved  by 
sliding  away  from  or  close  to  the  mounted  object, 
has  its  plane  side  placed  against  the  edge,  and  at 
right  angles  to  the  plane   of  the  slip.     By  this 
means    illumination    of    great    obliquity    can    be 
obtained,  and  very  surprising  effects  secured  even 
with  high  powers.     It  was  much  used  by  the  Editor 
and    Dr.   Drysdale  in  their  earlier  work   on   the 
saprophytic  organisms,    and,    in    the    days  before 
homogeneous  lenses,   helped    us  over   many  diffi- 
culties of  detail.     It  was   the  first  illuminator  to 
actually   resolve   the   Amphipleura  pellucida.      It 
could   be   very   easily   obtained  with  a  student's  FIG.  278.— Bull's-ey 
microscope  provided  with  Nelson's  open  stage,1  for     of  good  but  not  the 
on  this  the  bull's-eye  could  be  placed  against  the 

,  „      ,          ,.  ~  ,  L     .    . 

edge  of  the  slip  without  any  special  apparatus  or 
fitting. 

Another  and  popular  method  of  '  opaque  illumination '  is  by 
means  of  a  specialised  form  of  mirror,  generally  of  polished  silver, 
called  a  side  reflector,  and  fixed,  as  in  the  case  of  the  bull's-eye,  and 
for  the  same  reasons,  to  an  immovable  part  of  the  microscope. 

The  manner  of  employing  this  reflector,  as  provided  with  Powell 
and  Lealand's  best  stand,  is  seen  in  Plate  III.  The  arm  of  the 
side  reflector  is  fixed  to  an  immovable  part  of  the  stand,  and  is  thus 
unaffected  by  the  racking  up  or  down  of  the  body.  The  lamp  placed 
on  the  right  of  the  observer  is  set  at  such  a  height  that  its  beams 
fall  full  upon  the  reflector;  this,  by  means  of  a  ball-and-socket 
joint,  can  be  easily  manipulated  until  the  full  image  of  the  flame  is 
caused  to  fall  upon  the  object.  For  the  same  purpose  a  parabolic 
speculum  is  commonly  employed,  mounted  either  on  the  objective, 
as  in  Beck's  form,  fig.  279,  or  on  an  adapter,  as  in  Crouch's,  shown 
in  fig.  280,  where  a  collar  is  interposed  between  the  lower  end  of 
the  body  of  the  microscope  and  the  objective  seen  at  A.  This  is  not 

1  Fig.  134. 


334 


ACCESSORY  APPARATUS 


a  commendable  plan,  for  it  increases  the  distance  between  the  ob- 
jective and  the  Wenham  binocular  prism  ;  and  as  the  binocular  is 
specially  suited  for  the  kind  of  object  usually  examined  with  this 
speculum,  this  increased  distance,  acting  detrimentally  on  the  be- 
haviour of  the  binocular  prisms,  and  causing  the  available  racking 
distance  for  the  focus  of  objectives  of  very  low  power  to  be  shortened 
by  the  width  of  such  collar,  is  to  be  avoided. 

The  best  plan  without  doubt  is  to  attach  the  speculum  to  a  fixed 


FIG.  279. 


FIG.  280. 


part  of  the  stand,  as  is  done  in  the  Powell  and   Lealand,  the   Ross, 
and  the  Beck  stands. 

A  modification  oj  the  parabolic  reflector  was  devised  by  Dr.  Sorby, 
and  has  proved  to  be  very  useful  in  certain  investigations,  such  as 
the  microscopic  structure  of  metals.  It  consists  of  a  parabolic 
reflector,  in  the  centre  of  which,  in  a  semi-cylindrical  tube  open  in 
front,  is  placed  a  small  plane  reflector  which  covers  half  of  the 
objective,  and  throws  the  light  directly  down  upon  the  object  and 

back  through  the  other  half.  It  is 
shown  in  fig.  281  with  the  cylinder  in 
place,  and  in  the  dotted  lines  with  the 
same  turned  out.  This  arrangement 
™  ^  allows  of  two  kinds  of  illumination, 

oblique  and  direct,  being  readily  used, 

Fl°-  ^SliV^etctf0'1  °f  «  the  Plane  '^tor  is  attached  to  an 

arm  so  that  it  can  be  swung  out  of  the 
way  when  not  required,  as  shown  in  the  figure. 

Dr.  Sorby  was  able  to  get  results  in  the  examination  of  polished 
sections  of  steel  not  otherwise  attainable. 

No  opaque  illumination,  however,  has  yet  surpassed  the  venerable 
Lieberkiihn  ;  the  best  experts  freely  admit  that  the  finest  critical 
images  to  be  obtained  by  this  method  of  illumination  are  secured 
by  the  Lieberkiihn.  This  mode  of  illuminating  opaque  objects  is 
by  means  of  a  small  concave  speculum  reflecting  directly  down  upon 
them  to  a  focus  the  light  reflected  up  to  it  from  the  mirror ;  it  was 


LIEBEEKUHN — ITS   DRAWBACKS 


335 


formerly  much  in  use,  but  is  now  comparatively  seldom  employed. 
This  concave  speculum,  termed  a  '  Lieberkiihn,'  from  the  celebrated 
microscopist  who  invented  it,  is  made  to  fit  upon  the  end  of  the 
objective,  having  a  perforation  in  its  centre  for  the  passage  of  the 
rays  from  the  object  to  the  lens;  and  in  order  that  it  may  receive 
its  light  from  a  mirror  beneath  (fig.  282,  A),  the  object  must  be  so 
mounted  as  only  to  stop  out  the  central  portion  of  the  rays  that  are 
reflected  upwards.  The  curvature  of  the  speculum  is  so  adapted  to 
the  focus  of  the  objective  that,  when  the  latter  is  duly  adjusted,  the 
rays  reflected  up  to  it  from  the  mirror  shall  be  made  to  converge 
strongly  upon  the  part  of  the,  object  that  is  in  focus ;  a  separate 
speculum  is  consequently  required  for  every  objective. 

It  has  two  manifest  drawbacks :  the  first  one,  that  of  requiring  a 
separate  Lieberkuhn  for  each  objective,  is  a  difficulty  which  in  the 
nature  of  things  cannot  be  overcome.  The  radius  of  the  Lieberkiihn 


FIG.  282. 

must  alter  with  the  focus  of  the  objective  employed,  and  each  should 
have  a  certain  amount  of  play  on  the  objective  to  allow  for  slight 
alterations  of  focus  ;  for  if  we  employ  parallel  rays  it  is  obvious  that 
the  Lieberkiihn  will  focus  nearer  to  the  object  than  if  divergent 
rays  are  used.  This  is  met  by  an  allowance  being  made  to  com- 
pensate it  on  the  tube  which  slides  the  Lieberkiihn  on  to  the  nose 
of  the  objective. 

The  second  drawback  has  reference  to  the  special  way  in  which 
objects  have  to  be  mounted  in  order  to  be  suitable  for  the  Lieberkiihn. 
This  could  be  easily  avoided  if  professional  and  other  mounters 
would  attend  to  the  following  simple  suggestions : — 

1 .  Slides  should  never  be  covered  with  paper ;  it  is  without  use, 
and  fails  as  an  ornament ;  and  opaque  glass  slips  should  be  entirely 
avoided. 

2.  The  ring  of  cement  should  not  be  made  of  greater  width  than 
is  necessary  for  security. 


336  ACCESSORY  APPARATUS 

3.  A  stop  of  paper  or  varnish  should  never  be  placed  behind  an 
object. 

Let  every  opaque  mount  be  also  a  transparent  one,  since  it  is 
often  most  useful  to  examine  an  opaque  object  afterwards  by  trans- 
mitted light.  The  stop  should  always  be  a  separate  one ;  this  may 
be  a  disc  on  a  pin  held  in  the  sub-stage,  or,  what  is  still  simpler,  a 
piece  of  moderately  thick  '  cover '  glass,  cut  to  the  3x1  inch  size, 
or  rather  shorter,  should  have  a  small  disc  of  Brunswick  black  put 
on  it  centrally  on  the  '  turn-table,' J  and  this  may  be  placed  under 
the  slide  when  the  Lieberkiihn  is  to  be  used.  There  may  be  two  or 
three  such  slips  with  stops  of  different  sizes  ;  in  this  way  every 
mount  may  be  examined  either  with  the  Lieberkiihn  or  by  directly 
transmitted  light,  and  of  course  by  having  a  larger  stop  the  same 
object  may  be  examined  by  any  kind  of  reflected  light.  Many  a 
valuable  preparation  has  been  spoiled  by  placing  a  stop  on  it  which 
cannot  be  removed. 

4.  It  would  be  a  most  appreciable  benefit  to  the  cause  of  micro- 
scopy, as  we  have  already  hinted,  if  a  uniform  gauge  of  thickness  of 
slip  and  diameter  of  cover-glass  were  adopted.     For  the  thickness 
of  the  slip,  the  ^th  °f  an  ^ncn  would  prove  most  suitable,  and  for 
the  diameter  of  the  cover-glass  J  of  an  inch  would  be  most  con- 
venient, and  if  the  thickness  of  the  cover-glass  were  uniformly  from 
•006  to  '008  the  gain  would  be  still  greater.     Certainly  no   mount 
ought  to  be  finished  without  the  thickness  of  the  cover-glass  being 
marked  in  diamond  point  upon  it,  and   a  narrow  ring  of  shellac 
cement  should  be  put  round  every  cover-glass  where  there  is  even  a 
probability  that  a  homogeneous  lens  will  be  admissible  in  examining 
the  object  mounted. 

Very  minute  cover-glasses — such  as  those  ^ths  of  an  inch  in 
diameter — are  to  be  wholly  condemned.  They  do  not  allow  the 
conditions  required  by  modern  microscopy,  being  adverse  to  the 
employment  of  oil-immersion  lenses  in  anything  like  the  most 
efficient  way. 

Lieberkiihns  can  be  used  with  objectives  as  high  as  £  of  an  inch 
focus  of  '77  N.A.  For  higher  powers  than  this  a  perfectly  flat 
speculum  may  replace  the  conical  form,  being  illuminated  by  a 
condenser  with  a  stop,  and  racked  up  well  within  its  focus.  The 
oblique  annular  ring  of  light  falls  on  the  flat  speculum,  and  is  then 
reflected  on  the  object. 

The  light  suitable  for  illumination  by  Lieberkiihn  may  be  either 
the  flat  of  the  lamp  flame,  reflected  by  the  plane  mirror,  or  the  edge 
of  the  flame,  the  rays  being  rendered  parallel  by  a  bull's-eye,  and 
reflected  from  the  plane  mirror  to  the  Lieberkiihn. 

There  is  one  other  kind  of  reflected  illumination  em- 
ployed, produced  by  the  vertical  illuminator,  which,  although  it 
has  been  in  use  for  some  years,  has  received  an  accession  of  value 
from  the  employment  of  immersion  lenses.  The  earliest  device  for 
accomplishing  this  was  invented  by  Professor  H.  L.  Smith,  of 
Geneva,  U.S.A. 

The  principle  of  this  illuminator  is  to  employ  the  objective  as 
1  Chapter  vii. 


VERTICAL  ILLUMINATOR 


337 


its  own  illuminator  ;  which  Professor  Smith  did  by  means  of  a 
speculum.  A  pencil  of  light  was  admitted  from  a  lateral  aperture 
above  the  objective  and  then  reflected  downwards  upon  the  object 
through  the  lenses  by  means  of  a  small  silvered  speculum  placed  on 
one  side  of  its  axis. 

Messrs.  R.  and  J.  Beck,  in  place  of  a  speculum,  employ 
a  disc  of  cover-glass.  The  cover-glass  is  mounted  on  a  pin,  B, 
fig.  284,  in  order  that  it  may  be  rotated,  and  oblique  light  obtained 
by  the  milled  head,/,  A,  fig.  284. 

Powell  and  Lealand's  metkod  is  to  fix  a  piece  of  glass,  worked 
flat,  at  an  angle  of  45°  to  the  optic  axis,  with  a  rotating  diaphragm 
in  front  of  the  aperture  admitting  the  light. 


FIG.  283. 


FIG.  284. 


To  use  these  instruments  the  edge  of  the  lamp  flame  should  be 
placed  in  front  of  the  reflector,  so  that  the  rays  may  be  reflected  on 
to  the  back  lens  of  the  objective  in  a  line  parallel  to  the  optic  axis. 
The  distance  from  the  lamp  to  the  reflector  must  exactly  equal  the 
distance  from  the  reflector  to  the  diaphragm  of  the  eye-piece  in  a 
positive  eye-piece,  or  the  eye-lens  of  a  negative  eye-piece,  otherwise 
the  rays  will  not  be  focussed  on  the  object. 

This  illumination  is  only  suitable  for  objects  mounted  dry  on  the 
cover,  and  with  immersion  lenses.  No  good  result  was  ever  obtained 
until  the  immersion  lenses  were  brought  into  use,  but  it  is  now 
largely  used  in  the  examination  of  metals.  The  microscope  adapted 
to  its  employment  is  shown  in  fig.  207. 

Of  all  the  light  which  is  caused  to  pass  out  of  the  front  lens  of 
the  objective,  through  the  oil  and  into  the  cover-glass,  that  which 
has  an  obliquity  less  than  the  critical  angle  for  glass  (41°)  passes 
through  the  cover  and  object  and  is  lost ;  but  all  the  light  which  is 
of  greater  obliquity  than  the  critical  angle  for  glass  is  totally  reflected 


338  ACCESSORY  APPARATUS 

from  the  under  surface  of  the  cover-glass,  and  comes  back  through 
the  oil  and  the  objective  to  the  eye-piece  and  the  eye ;  they  are,  in 
fact,  all  optically  continuous,  so  that  the  upper  surface  of  the  cover- 
glass  has  ceased  to  exist  optically,  the  only  reflexion  being  from  its 
inner  surface.  It  is  here,  therefore,  that  the  oil-immersion  system 
gives  a  new  value  to  this  illuminator,  by  this  means  enabling  it  to 
utilise  a  larger  aperture  otherwise  unavailing. 

When  this  illumination  is  employed,  if  the  eye-piece  be  removed 
and  the  back  of  the  objective  be  examined,  it  will  be  seen  that  all 
that  portion  of  the  back  of  the  objective  whose  aperture  exceeds  1*0 
is  brilliantly  illuminated.  This  annulus  represents,  and  is  produced  by, 
the  excess  of  aperture  beyond  the  equivalent  air  angle  of  180°,  of 
which  it  is  also  a  measure.  The  internal  dark  space  is  of  the  exact 
diameter  of  that  of  a  dry  objective  of  the  same  focus,  and  is  the 
maximum  space  which  it  can  itself  utilise  on  a  dry  object  by  trans- 
mitted light. 

By  means  of  this  instrument  carefully  used,  some  difficult  tests 
and  lined  objects  have  been  resolved  ;  but  its  principal  use  at  the 
present  day  is  for  the  examination  of  metals,  and  it  is  eminently 
serviceable  in  determining  whether  any  dry-mounted  object  is  in 
optical  contact  with  the  cover-glass  or  not.  If  it  be  not  so  it  is  in- 
visible with  the  vertical  illuminator.  So  also  it  is  instructive  to 
examine  the  backs  of  objectives  of  various  apertures  with  this  mode 
of  illumination.  A  dry  objective  will  be  wholly  without  the  bright 
annulus,  while  an  immersion  of  1*1  N.A.  will  have  a  narrow  annulusr 
and  that  of  1*4  or  1'5  a  broad  and  still  broader  one.  In  this  way, 
by  practice,  a  fair  approximation  to  the  aperture  of  an  objective  may 
be  obtained. 

It  is  not  the  absolute  size  of  the  annulus,  but  the  relation  of  the 
size  of  the  annulus  to  that  of  the  whole  back,  that  must  be  estimated. 
Thus  ^-th  of  N.A.  1'2  will  have  as  broad  an  annulus  as  -j-^th  of 
1*4  N.A.,  but  the  diameter  of  the  back  of  the  Jth  is,  of  course,  much 
larger  than  that  of  the  iVth,  and  this  involves  the  necessity  for  a 
relative  comparison. 

Appliances  for  the  Practical  Study  of  Living  and  other  Objects 
with  the  Microscope. — Stage-forceps  and  Vice. — For  bringing  under 
the  object-glass  in  different  positions  such  small  opaque  objects  as 
can  be  conveniently  held  in  a  pair  of  forceps,  the  stage-forceps  (fig. 
285)  supplied  with  most  microscopes  provide  a  ready  means.  These 
are  mounted  by  means  of  a  joint  upon  a  pin  which  fits  into  a  hole 
either  in  the  corner  of  the  stage  iteelf  or  in  the  object-platform  ;  the 
object  is  inserted  by  pressing  the  pin  that  projects  from  one  of  the 
blades,  whereby  it  is  separated  from  the  other  ;  and  the  blades  close 
again  by  their  own  elasticity,  so  as  to  retain  the  object  when  the 
pressure  is  withdrawn.  By  sliding  the  wire  stem  which  bears  the 
forceps  through  its  socket,  and  by  moving  that  socket  vertically 
upon  its  joint,  and  the  joint  horizontally  upon  the  pin,  the  object 
may  be  brought  into  the  field  precisely  in  the  position  required  ; 
and  it  may  be  turned  round  and  round,  so  that  all  sides  of  it 
may  be  examined,  by  simply  giving  a  twisting  movement  to  the 
wire  stem.  The  other  extremity  of  the  stem  often  bears  a  small 


STAGE   APPLIANCES 


339 


FIG.  285.— Stage-forcep?. 


FIG.  286.— Stage-forceps. 


brass  box  filled  with  cork,  and  perforated  with  holes  in  its  side, 
seen  in  fig.  286  ;  this  affords  a  secure  hold  to  common  pins,  to  the 
heads  of  which  small  objects  can  be  attached  by  gum,  or  to  which 
discs  of  card,  etc.,  may  be  attached,  whereon  objects  are  mounted 
for  being  viewed  with  the  Lieberkiihn.  This  method  of  mounting- 
was  formerly  much  in  vogue,  but  has  been  less  employed  of  late, 
since  the  Lieberkiihn  has  unfortunately  fallen  into  comparative 
disuse.  The  forceps  in  fig.  287  are  also  often  of  great  practical  value, 
and  are  adjusted  for  holding  by  a  screw.  That  which  is  known  as 
the  stage-vice,  for  the 
purpose  of  holding  small 
hard  bodies,  such  as 
minerals,  apt  to  be  jerked 
out  by  the  angular  motion 
of  the  blades  of  the  for- 
ceps, or  very  delicate 
substances  that  will  not 
bear  rough  compression, 
is  very  useful,  and  is  seen 
in  fig.  288.  The  stage- 
vice  fits  into  a  plate,  as 
is  the  case  with  Beck's 
disc-holder,  fig.  289,  or 
it  may  simply  drop  into 
a  stage  fitting,  as  in  the 
figure. 

For  the  examination 
of  objects  which  cannot 
be  conveniently  held  in 
the  stage-forceps,  but 
which  can  be  temporarily 
or  permanently  attached 
to  discs,  no  means  is 
comparable  to  the  disc- 
holder  of  Mr.  R.  Beck 
(fig.  289)  in  regard  to 
the  facility  it  affords  for 
presenting  them  in  every 
variety  of  position.  The 
object  being  attached  by 
gum  (having  a  small 
quantity  of  glycerine 
mixed  with  it)  or  by 

gold  size  to  the  surface  of  a  small  blackened  metallic  disc,  this  is 
fitted  by  a  short  stem  projecting  from  its  under  surface  into  a 
cylindrical  holder  ;  and  the  holder  carrying  the  disc  can  be  made  to 
rotate  around  a  vertical  axis  by  turning  the  milled  head  on  the 
right,  which  acts  on  it  by  means  of  a  small  chain  that  works  through 
the  horizontal  tubular  stem  ;  whilst  it  can  be  made  to  incline  to  one 
side  or  to  the  other,  until  its  plane  becomes  vertical,  by  turning 
the  whole  movement  on  the  horizontal  axis  of  its  cylindrical 

z  2 


FIG.  287. 
Three-pronged  forceps,  screw  adjustment. 


FIG.  288. — The  stage-vice. 


FIG.  289. — Beck's  disc-holder. 


340 


ACCESSOKY   APPARATUS 


socket.1  The  supporting  plate  being  perforated  by  a  large  aperture, 
the  object  may  be  illuminated  by  the  Lieberkiihn  if  desired.  The  discs 
are  inserted  into  the  holder,  or  are  removed  from  it,  by  a  pair  of 
forceps  constructed  for  the  purpose ;  and  they  may  be  safely  put 
away  by  inserting  .their  stems  into  a  plate  perforated  with  holes. 
Several  such  plates,  with  intervening  guards  to  prevent  them  from 
coming  into  too  close  apposition,  may  be  packed  into  a  small  box. 
To  the  value  of  this  little  piece  of  apparatus  the  Author  can  bear 
the  strongest  testimony  from  his  own  experience,  having  found  his 
study  of  the  Foraminifera  greatly  facilitated  by  it. 

Glass  Stage-plate. — Every  microscope  should  be  furnished  with 
a  piece  of  plate  glass,  about  3^  in.  by  2  in.,  to  one  margin  of  which  a 
narrow  strip  of  glass  is  cemented,  so  as  to  form  a  ledge.  This  is 
extremely  useful,  both  for  laying  objects  upon  (the  ledge  preventing 
them — together  with  their  covers,  if  used — from  sliding  down  when 
the  microscope  is  inclined),  and  for  preserving  the  stage  from  injury 
by  the  spilling  of  sea-water  or  other  saline  or  corrosive  liquids  when 
such  are  in  use.  Such  a  plate  not  only  serves  for  the  examination 
of  transparent,  but  also  of  opaque  objects  ;  for  if  the  condensing 
lens  is  so  adjusted  as  to  throw  a  side  light  upon  an  object  laid  upon 
it,  either  the  diaphragm  plate  or  a  slip  of  black  paper  will  afford  a 
•dark  background  ;  whilst  objects  mounted  on  the  small  black  discs 
.suitable  to  the  Lieberkiihn  may  conveniently  rest  on  it,  instead  of 
being  held  in  the  stage-forceps. 

Growing  Slides  and  Stages. — A  number  of  contrivances  have  been 
devised  of  late  years  for  the  purpose  of  watching  the  life  histories  of 


FIG.  290. 

minute  aquatic  organisms,  and  of  '  cultivating  '  such  as  develop  and 
multiply  themselves  in  particular  fluids.  One  of  the  simplest  and 
most  effective,  that  of  Mr.  Botterill,  represented  in  fig.  290,  consists 
of  a  slip  of  ebonite,  three  inches  by  one,  with  a  central  aperture  of 
three-fourths  of  an  inch  at  its  under  side  ;  this  aperture  is  reduced 
by  a  projecting  shoulder,  whereon  is  cemented  a  disc  of  thin  glass, 
which  thus  forms  the  bottom  of  a  cell  hollowed  in  the  thickness  of 
the  ebonite  slide.  On  each  side  of  this  central  cell  a  small  lateral  cell 
communicating  with  it,  and  about  a  fourth  of  an  inch  in  diameter,  is 
drilled  out  to  the  same  depth  ;  this  serves  for  the  reception  of  a  supply 

1  A  small  pair  of  forceps  adapted  to  take  up  minute  objects  may  be  fitted  into 
the  cylindrical  holder  in  place  of  a  disc. 


GROWING-   SLIDES  341 

of  water  or  other  fluid,  which  is  imparted,  as  required,  to  the  central 
'  growing '  cell,  which  is  completed  by  placing  a  thin  glass  cover  over 
the  objects  introduced  into  it,  with  the  interposition  of  a  ring  of  thin 
paper,  or  (if  a  greater  thickness  be  required)  of  a  ring  of  cardboard 
or  vulcanite.  If  the  fluid  be  introduced  into  one  of  the  lateral  cells, 
and  be  drawn  off  from  the  others — either  by  the  use,  from  time  to 
time,  of  a  small  glass  syringe,  to  be  hereafter  described,  or  by 
threads  so  arranged  as  to  produce  a  continuous  drip  into  one  and 
from  the  other — a  constantly  renewed  supply  is  furnished  to  the 
central  cell,  which  it  enters  on  one  side  and  leaves  on  the  other, 
by  capillary  attraction.  •*  ^ 

Dr.  Lewis's  ami  Dr.  Maddox's  growing  slides  axe  shown  in  figs.  291 
and  292.     Two  semicircles  of  asphalte  varnish  are  brushed  on  the 
slide,  one  being  rather  larger  than  the  other,  so  that  the  ends  of  one 
half-circle    may  over- 
lap the  other,  but  not 
so   closely   as   not   to 
permit    the    entrance 
and  exit  of  air.    When 
nearly   dry  a  minute 
quantity    of   growing 
fluid  is  placed  in  the 
centre,     upon     which 
a      few      spores     are  FIG.  291. 

sown,     a     cover-glass 

being  placed  over  it,  which  adheres  to  the  semi-dried  varnish.  The 
slide  should  be  placed  under  a  bell-glass,  kept  damp  by  being  lined 
with  moist  blotting-paper. 

Dr.  Maddoxs  growing  slide  will  be  understood  from  the  annexed 
sketch,  fig.  292.  The  shaded  parts  are  pieces  of  tinfoil  fastened  with 
shellac  glue  to  a  glass  slide.  The 
minute  fungi  or  spores  to  be 
grown  are  placed  on  a  glass  cover 
large  enough  to  cover  the  tinfoil, 
with  a  droplet  of  the  fluid  re- 
quired. This,  after  examination 
to  see  that  no  extraneous  matter 
is  introduced,  is  placed  over  the 
tinfoil,  and  the  edges  fastened  ~"x  x" 

with  wax  softened  with  oil,  leav-         pIG.  292.— Maddox's  growing  stage. 

ing   free    the   spaces,    X  X,   for 

entrance  of  air.     Growing  slides  of  this  description  could  be  made 

cheaply  with  thin  glass  instead  of  tinfoil. 

Dallinger  and  Drysdak's  Moist  Stage  for  Continuous  Observa- 
tions.—It  is  needful  in  working  out  the  life  histories  of  minute 
forms  to  be  able  to  keep  the  organisms  in  a  normal  and  un- 
disturbed condition  for  sometimes  weeks  at  a  time;  only  a  small 
drop  of  fluid  containing  the  organism  can  be  under  observation,  and 
this,  without  proper  provision,  is  constantly  evaporating.  To 
prevent  this,  and  still  to  employ  very  high  powers  in  prolonged 
study  of  a  given  organism,  is  the  object  of  this  device.  It  consists  of 


342 


ACCESSORY  APPARATUS 


a  plain  glass  stage,  fig.  293,  a,  a,  so  fitted  as  to  slide  on  in  the  place 
of  the  ordinary  sliding  stage  of  a  Powell  and  Lealand  or  Ross  stand. 
It  is  thus  susceptible  of  the  mechanical  motions  common  to  those 
stages.  Its  foundation,  fig.  293,  a,  a,  is  plate  glass,  about  the  tenth 
of  an  inch  thick,  in  order  to  give  it  firmness.  But  this  is  too  thick 
to  work  through  with  a  condenser  and  high  powers,  and  therefore  a 


FIG.  293. — Dallinger  and  Drysdale's  moist  continuous  growing  stage. 

circular  aperture,  6,  is  cut  through  it,  and  a  thin  piece  of  good  glass, 
c,  d,  e,f,  is  fixed  over  the  under  surface  of  it  with  Canada  balsam  ; 
this  may  be  as  thin  as  the  condenser  may  require.  At  the  end  of 
the  arm  a,  which  extends  some  distance  beyond  the  stage  to  the 
right  of  the  reader,  but,  when  the  arrangement  is  set  up  on  the 
microscope,  to  the  left  of  the  operator,  a  brass  socket  with  a  ring 
attached  is  fixed  with  marine  glue.  It  is  marked  in  the  drawing 
g,  g,  g.  The  object  of  this  ring  is  to  hold  a  glass 
vessel,  fig.  294,  about  1 J  or  2  inches  deep.  It 
simply  drops  in,  and  the  top,  a,  being  slightly 
larger  than  the  ring,  g,  fig.  293,  it  is  prevented 
from  slipping  through. 

-Let  us  suppose  the  stage 
to  be  in  its  position  on  the 
microscope,  and  the  vessel, 
fig.  294,  inserted  in  this 
manner  into  </,  fig.  293.  A 
piece  of  good  new  linen  is 
now  cut  to  the  shape  drawn 
FIG.  294.  FIG.  295.  in  fig.  297,  the  part  a  being 

long  enough  to  reach  to  the 

end  of  the  glass  stage,  and  then  at  b  bent  over,  leaving  the  part  in 
the  vessel,  fig.  294,  which  is  inserted  into  g,  fig.  293.  Its  position 
is  indicated  in  fig.  293  by  the  clotted  lines,  h,  h,  h,  &c.  But  before 
it  is  laid  upon  the  stage  a  circular  aperture,  d,  fig.  297,  is  cut  out, 
which  must  be  much  larger  in  diameter  than  the  covering  glass 
which  it  is  intended  to  use.  We  therefore  employ  small  covers. 


GROWING-   STAGE   FOR   CONTINUOUS   WORK 


343 


The  glass  with  the  flap  of  linen  in  it  is  now  filled  with  water, 
and  the  linen  is  wetted  and  wrung  so  as  not  to  drip,  and  the  whole 
is  very  soon,  by  capillary  action,  constantly  and  evenly  wet.  A 
drop  of  the  fluid  to  be  examined  must  now  be  placed  at  k,  fig.  293, 
and  the  covering  glass,  i,  must  be  laid  on.  It  will  be  seen  that 
there  is  a  broad,  clear  space  between  the  covering  glass  and  the  linen. 
We  now  want  to  form  a  chamber  into  which  the  object-glass  can  be 
inserted,  and  which  shall  enclose  a  portion  of  the  constantly  wet 
linen,  and  be  to  a  very  large  extent  air-tight.  The  consequence 
will  be  that  the  evaporation  within  the  chamber  will  be  always 
greater  in  quantity  from  the  linen,  on  account  of  its  continual 
renewal,  than  it  can  be  from  the  film  of  fluid. 

Indeed,  the  moisture  in  the  chamber  is  so  great  under  favourable 
circumstances  that  it  rather  increases  than  allows  a  diminution  of 
the  film  of  fluid.  The  manner 
in  which  we  effect  this  is 
simple.  A  piece  of  glass 
tubing,  about  1^  inch  in  dia- 
meter, is  cut  to  about  J  of  an 
inch  in  length.  At  one  end 
of  this  a  piece  of  thin  sheet 
caoutchouc  is  firmly  stretched, 
and  a  small  hole  is  made  in 
its  centre.  Fig.  295  gives  a 
drawing  of  it ;  a  is  the  piece  j— •*"""£ 
of  glass  tubing,  b  is  the  FIG.  211  o. 

stretched  elastic  film,  which  is 

securely  tied  on  by  means  of  a  groove  in  the  glass  at  d,  and  c  is  the 
aperture.  The  bottom  edge,  e,  should  be  carefully  ground.  This  is 
laid  in  the  position  in  which  it  is  looked  at  in  the  drawing,  on  the 
linen  of  the  stage,  the  aperture  c  being  over  the  centre  of  the  cover- 
ing glass.  The  object-glass  is  now  racked  down  through  the  small 
hole,  c  (fig.  295),  and  adjusted  to  focus.  The  caoutchouc  should  be 
thin  enough  to  afford  no  impediment  to  the  action  of  the  fine 
adjustment,  when  it  will  be  seen  that  it  clasps  the  object-glass  by  its 
elasticity  at  the  aperture  ;  and  the  gentle  pressure  forces  the  under 
edge  of  the  chamber  upon  the  linen,  so  that  little  or  no  air  is 
admitted,  while  if  the  under  edge  of  the  chamber  be  carefully 
ground  it  will  suffer  the  stage,  linen  and  all,  to  move  under  it  when 
the  milled  heads  for  working  the  mechanical  stage  are  in  action. 

A  drawing  of  the  apparatus  in  working  order  is  given  in  perpen- 
dicular section  at  fig.  296.  The  parts  a,  a  in  this  figure  represent 
the  glass  stage  corresponding  to  a,  «,  fig.  293  ;  b  in  both  figures 
stands  for  the  round  aperture  in  the  thick  glass  ;  b,  in  fig.  296,  cor- 
responds to  the  thin  glass  which  covers  this  aperture,  marked  c,  d, 
e,  f  in  fig.  293 ;  but  in  the  form  of  this  device  now  used  by  the 
Editor  the  thin  glass  floor  is  cemented  to  the  bottom  of  the  plate 
glass,  a,  a,  thus  making  a  cell  equal  to  the  thickness  of  the  whole 
stage.  The  linen  is  marked  in  dotted  lines  in  both  figures  :  d, 
fig,  296,  represents  the  covering  glass,  /,  in  fig.  293  ;  e,  e,  fig.  296,  is 
the  piece  of  glass  tubing  shown  in  fig.  295;  /,/,  fig.  296,  is  the 


344  ACCESSORY  APPARATUS 

stretched  caoutchouc  seen  at  b  in  fig.  295,  with  the  object-glass  </, 
penetrating  and  tightly  filling  up  the  aperture  c  in  the  figure,  thus 
forming  the  moist  chamber,  cA,  ch,  by  enclosing  parts  A,  A,  fig.  296, 
of  the  linen,  which  from  the  glass  vessel  to  the  left  of  the  stage  is 
by  capillarity  always  renewing  its  moisture  ;  and  with  6,  fig.  296, 
sunk  as  a  cell,  by  the  attachment  of  the  thin  glass  floor  to  the  under 
side  of  the  stage,  as  described  above,  this  annular  flap  of  linen  over- 
hangs, but  does  not  lie  upon,  the  floor  on  which  the  drop  of  fluid 
with  its  living  inhabitants  is  placed.  This  is  a  great  security 
against  accidental  flooding. 

It  will  be  seen  that  the  microscope  must  be  vertical ;  but  there 
is  no  inconvenience  arising  from  this  if  it  be  placed  on  a  sufficiently 
low  support,  and  it  will  be  found  in  practice  that  it  may  be  worked 
for  a  long  time  without  any  other  change  in  the  arrangement  than 
the  screwing  up  or  down  of  the  fine  adjustment.  The  difficulties  in 
working  are  few,  and  can  be  best  discovered  and  overcome  in 
practice. 

Dr.  Dallinger's  Thermo-static  Stage  for  Continuous  Observations 
at  High  Temperatures. — It  frequently  happens  that,  either  for  the  pur- 


FIG.  297. 

pose  of  experiment  or  the  study  of  special  organisms,  the  student 
needs  a  similar  continuous  stage  to  the  above,  but  one  in  which 
varying  temperatures  may  be  obtained  and  kept  at  any  point  static 
at  the  will  of  the  operator.  This  is  very  satisfactorily  accomplished 
by  the  following  device :  The  stage  was  made  as  described  above, 
but  it  was  made  hollow  and  water-tight.  The  whole  stage  is  seen 
in  perspective  in  fig.  298.  At  A,  a  b  are  two  grooved  pieces  of  solid 
metal  which  permit  the  stage  to  slide  on  to  the  stage  of  an  ordinary 
microscope,  and  partake  of  the  mechanical  movements  effected  by 
the  milled  heads  ;  B  is  a  vessel  for  water  with  a  thermometer  a 
of  sufficient  delicacy  for  indicating  the  temperature  ;  b  is  a  mer- 
curial regulator,  carefully  made,  but  of  the  usual  pattern  ;  c  brings 
the  gas  from  the  main  ;  d  conveys  as  much  of  the  gas  as  is  allowed 
to  escape  from  between  the  top  of  the  mercury  and  the  bottom 
of  the  gas  delivery  tube  to  the  burner  e.  The  regulation  of  this 
apparatus  so  as  to  obtain  a  static  temperature,  as  is  well  known,  is 
a  matter  of  detail  depending  chiefly  on  the  careful  use  of  the  mer- 
curial screw-plug  f  and  the  height  and  intensity  of  the  burner  e. 
A  temperature  quite  as  accurate  as  is  needed  for  the  purpose 
required  can  be  obtained. 


WARM    CONTINUOUS   MOIST   STAGE 


345 


The  stage  A  is  placed  in  position  on  the  instrument,  and  two 
openings  in  this  hollow  stage  at  c  d  (A)  are  connected  with  two 
similar  openings  in  the  water  vessel,  viz.  g  h  (B).  The  whole  is 
carefully  filled  with  water  and  raised  to  the  required  temperature 
and  regulated. 

The  manner  in  which  it  accomplishes  the  end  desired  is  as  follows  : 
On  the  centre  of  the  stage  (A)  will  be  seen  a  small  cylinder  of  glass  ; 
this  is  ground  at  the  end  placed  on  the  stage,  and  covered  with  a 
sort  of  drumhead  of  indiarubber  at  the  upper  end.  By  examining 
C  with  a  lens  it  will  be  seen  that  a  cell  is  countersunk  into  the 
upper  plate  of  the  hollow  stage  at  e",  and  a  thin  plate  of  glass  is 
cemented  on  to  this.  At  e  andtjjer  disc  of  glass  is  cemented  water- 
tight, so  that  a  film  of  warm  water  circulates  between  the  upper  and 


FIG.  298. 

under  surfaces  of  this  glass  aperture.  A  glass  cup  is  placed  in  the 
jacketed  receptacle  f  (A  and  C),  and  this  also  is  filled  with  water. 
A  piece  of  linen  is  now  laid  on  the  stage  (A,  g)  with  an  aperture  cut  in 
its  centre  slightly  less  than  the  countersunk  cell  in  which  the  glass 
disc  e"  is  fixed,  and  a  flap  from  it  is  allowed  to  fall  over  into  the  glass 
vessel  f  (A  and  C).  Thus  by  capillarity  the  water  is  carried  constantly 
over  the  entire  face  of  the  linen.  But  the  glass  cylinder  seen  in  A  i& 
made  of  a  much  larger  aperture  than  the  cell  and  the  opening  in  the 
linen,  and  consequently  a  large  annulus  of  the  linen  is  enclosed  within 
the  cylinder.  The  drop  of  fluid  to  be  examined  is  placed  on  the  small 
circular  glass  plate,  and  covered  with  the  thinnest  glass,  the  drum- 
head cylinder  is  placed  in  position,  the  point  of  a  high-power  lens 
is  gently  forced  upon  the  top  of  the  indiarubber  through  a  small 


34-6  ACCESSOEY   APPAEATUS 

aperture,  thus  forcing  the  lower  ground  surface  of  the  cylinder  upon 
the  linen,  and  making  the  space  within  the  closed  cylinder  practi- 
cally air-tight,  but  still  admitting  of  capillary  action  in  the  linen. 
Thus  the  enclosed  air  becomes  saturated. 

By  complete  circulation  the  water  in  the  vessel  e  (A)  is  but 
slightly  below  that  within  the  jacket  of  the  stage,  and  thus  the 
vapour  as  well  as  the  stage  is  near  the  same  thermal  point. 

For  the  admission  of  illumination  and  for  allowing  the  use  of 
various  illuminating  apparatus,  a  large  bevelled  aperture  e  (C)  is 
made  between  the  lower  and  upper  plates  of  the  stage  jacket,  which 
is  found  to  supply  all  the  accommodation  needed. 

There  are  many  other  forms  of  hot  stage  having  various  special 
purposes,  and  some  of  general  application  ;  a  good  account  of  these 
will  be  found  in  the  *  Journal  Roy.  Micro.  Soc.'  vol.  vii.  ser.  ii. 
pp.  299—316  and  in  subsequent  volumes. 

The  Live-box  and  Compressors. — What  is  now  so  well  known 
even  to  the  tyro  as  the  '  live-box '  was  originally  devised  by  Tully, 
and  it  was  afterwards  improved  by  Yarley,  who,  in  the  place  of  a 
level  disc  of  glass  for  the  floor,  as  well  as  the  top  of  the  *  box,' 
bevelled  a  piece  of  thick  glass  and  burnished  it  into  the  top  of  the 
tube,  where  it  formed  the  floor  of  this  '  animalcule  cage  ; '  this 
prevented  the  draining  oft'  of  the  water  at  the  edge  by  capillary 


FIG.  299. 

attraction.  But  in  that  form  a  condenser  cannot  be  used  successfully 
with  it,  and  therefore  a  dark  ground  cannot  be  employed.  But  as  it 
is  Rotifera  and  Infusoria  generally  that  constitute  the  raison  d'etre 
for  this  piece  of  apparatus,  and  as  a  dark  ground  gives  results  of 
high  value — to  say  nothing  of  their  beauty — with  these  forms,  it 
lost  much  of  its  value. 

Mr.  Rousselet  has  overcome  these  difficulties  by  a  device  which 
is  shown  in  fig.  299. 

In  this  the  glass  plate  bevelled  for  the  floor  is  somewhat  reduced 
in  diameter,  but  the  outer  ring  is  enlarged  sufficiently  to  allow  any 
high  power  to  focus  to  the  very  edge  of  this  glass  floor.  An  object 
lying  anywhere  over  the  floor  can  be  reached  by  the  condenser  from 
below,  and  by  both  high  and  low  powers  from  above,  and  when  well 
made  it  acts  admirably  as  a  compressor.  A  drop  of  water  so  small  that 
a  rotifer  may  be  unable  to  swim  out  of  the  field  of  view  of  a  J-inch 
objective  can  be  readily  arranged  with  it;  and  a  little  practice 
enables  the  operator  to  employ  it  for  many  useful  purposes  in  the 
study  of  '  pond  life.' 

The  compressor  or  compressorium  is  a  more  elaborate  device, 
somewhat  of  the  same  kind,  but  arranged  to  give  the  operator 
more  accurate  control  over  the  amount  of  pressure  to  which  the 
•object  is  subjected.  Mr.  Rousselet  has  constructed  one  of  very 


COMPRESSORS 


347 


efficient  form ;  we  illustrate  it  in  fig.  300,  but  on  a  reduced  scale. 
The  bevelled  glass  in  this  also  is  kept  small,  with  respect  to  the 
size  of  the  cover-glass,  and  it  acts  with  perfectly  parallel  pressure 
between  the  two  glasses,  which  in  delicate  work  is  of  considerable 
importance. 

The  cover-glass  is  held  on  an  arm  which  screws  down  on  a  vertical 
post  against  a  spring ;  as  the  screw  is  raised  the  spring  raises  the 
cover-glass,  and  by  an  ingenious  spring  catch  it  is  kept  central  with 
the  glass-plate  floor.  This  can  nevertheless  be  released,  and  the  entire 
cover  can  be  turned  aside  to  put  on  a 
fresh  object,  clean,  and  so  forth.  It  is 
simple,  light,  and,  being  parallej,  can 
be  used  with  the  highest  powers.  * 

Messrs.  Beck  and  Co.  have  for 
many  years  made  an  admirable  parallel 
compressor,  but  its  weight  and  cost 
were  somewhat  prohibitive  of  its  use 
generally ;  the  firm  have  now  overcome  both  difficulties  by  the  intro- 
duction of  a  new  form  which  is  most  useful  and  fully  accomplishes 
its  work. 

This  compressor  was  designed  by  Mr.  H.  R.  Davis,  and  is 
specially  intended  for  the  examination  of  living  objects.  It  consists, 
as  shown  in  fig.  301,  of  a  lower  ebonite  plate  A,  which  has  a  circular 
hole  in  the  centre,  and  which  is  recessed  to  receive  a  circular  brass 
ring  B.  This  ring  rests  loosely  in  the  recess.  On  the  recessed 
portion  of  this  plate  A  is  carried  an  oblong  thin  glass  which  is 
held  in  position  by  two  screws,  one  of  which  appears  at  C.  Two 
end  plates  D  D  slide  on  to  the  plate  A,  and  hold  the  ring  B  loosely 
in  position,  allowing  it  to  be  revolved  by  means  of  its  milled  flange, 
which  projects  at  E.  Within  the  ring  B  is  screwed  a  brass  disc  F 
which  carries  the  upper  thin  glass  which  is  attached  by  the  screws 


FIG.  300. — Rousselet's  compressor. 


G  G 

FIG.  801. — Beck's  new  compressor. 

G  G.  The  screws  G  G  and  C,  fitting  into  holes  in  the  lower  plate 
A  and  the  disc  F  respectively,  prevent  the  disc  from  revolving,  and 
when  the  ring  E  is  turned,  the  two  thin  glasses  are  moved  towards 
or  away  from  one  another. 

The  slides  D  D  and  the  ring  B,  together  with  the  disc  F,  are 
removed  for  arranging  the  object   on   the   lower  cover-glass,  and 


348  ACCESSORY   APPARATUS 

when  replaced  by  revolving  the  ring  at  E.  any  desired  amount  of 
compression  may  be  obtained.  The  object  having  been  arranged, 
either  side  may  be  examined  with  equal  facility,  as  the  compressor 
is  reversible. 

When  a  very  small  object  is  to  be  examined  a  small  circular 
cover-glass  should  be  cemented  with  Canada  balsam  to  the  lower 
cover-glass,  and  the  object  is  thus  confined  to  the  centre  of  the  field. 
The  zoophyte  trough  is  a  larger  live-box  differently  constructed. 
The  form  that  has  proved  one  of  the  best  up  to  our  own  day  was 
introduced  by  Mr.  Lister  in  1834,  and  is  well  known.  It  is  depicted 
in  fig.  302,  being  formed  of  slips  of  glass,  and  has  a  loose  horizontal 
plate  of  glass  equal  to  the  inside  length  of  the  trough,  so  that  it 
may  be  moved  freely  within  it,  also  a  slip  of  glass  that  will  lie  on 
the  bottom  and  fill  it,  with  the  exception  of  the  thickness  of  this 
loose  plate.  To  use  it,  the  slip  is  put  upon  the  bottom,  the  loose 
plate  is  placed  in  front  of  it  with  its  bottom  edge  touching  the 
inside  of  the  front  glass,  a  small  ivory  wedge  is  inserted  between 
the  front  glass  of  the  trough  and  the  upper  part  of  the  loose  vertical 

plate,  which  it  serves  to  press 
backwards  ;  but  this  pressure 
is  kept  in  check  by  a  small 
strip  of  bent  whalebone,1 
which  is  placed  between  the 
vertical  plate  and  the  back 
glass  of  the  trough.  By 
moving  the  ivory  wedge  up 
and  down,  the  amount  of  space 
left  between  the  upper  part 
of  the  vertical  plate  and  the 
front  glass  of  the  trough  can 
be  precisely  regulated,  and  as 

FlG  302  their  lower  margins  are  always 

in  close  apposition,  it  is  evi- 
dent the  one  will  incline  to  the  other  with  a  constant  diminution 
of  the  distance  between  them  from  above  downwards.  An  object 
dropped  into  this  space  will  descend  until  it  rests  between  the  two 
surfaces  of  glass,  and  it  can  be  placed  in  a  position  of  great  conveni- 
ence for  observation. 

By  very  little  contrivance  these  troughs  with  their  contents  may 
be  kept,  when  not  under  examination,  in  much  larger  aquaria,  ob- 
taining the  advantage  of  aeration  and  coolness. 

Mr.  Botterill  devised  a  trough  which  is  made  of  two  plates  of 
vulcanite  or  metal  which  screw  together,  and  between  them  are  two 
plates  of  glass,  of  the  proper  size,  of  any  desired  thickness,  kept 
apart  by  half  a  ring  of  vulcanised  indiarubber,  the  whole  being 
screwed  tightly  enough  together  by  three  milled  heads  to  prevent 
leakage.  But  leakage  or  the  fracture  of  glasses  is  not  uncommon 
with  this  otherwise  convenient  form. 

An  excellent,  though  shallow,  trough  was  made  by  Mr.  C.  G. 
Dunning,  which  we  illustrate  in  fig.  303.  The  lower  plate  or  trough 

1  Watch-spring  or  other  elastic  metal  should  not  be  used,  on  account  of  oxidation. 


A   SHALLOW   TROUGH 


349 


proper  is  made  of  metal,  3  inches  long  by  1^  wide  and  about  ^(J 
thick,  with  an  oval  or  oblong  perforation  in.  the  centre,  and  the 
under  side  is  recessed,  as  shown  in  fig.  303,  B.  In  this  recess  is  fixed, 
by  means  of  Canada  balsam  or  shellac,  a  piece  of  stout  covering  glass, 
forming  the  bottom  of  the  cell,  the  recess  being  sufficiently  deep  to 
prevent  the  thin  glass  bottom  from  coming  into  actual  contact  with 
the  stage  of  the  microscope  or  with  the  table  when  it  is  not  in  use. 
Two  pieces  are  provided  near  the  bottom  edge  of  the  cell :  the  cover 
(fig.  303,  C)  is  formed  of  a  piece  of  thin  brass,  rather  shorter  than 
the  trough,  but  about  the  same  width  ;  it  has  an  opening  formed  in 
it  to  correspond  with  that  in  the  trough,  and  under  this  opening  is 
cemented  a  piece  of  cover-glass.  >  The  cover-plate  is  notched  at  the 
two  bottom  corners,  and  at  the  two  top  corners  are  formed  a  couple 
of  projecting  ears.  In  order  to  use  this  apparatus  it  must  be  laid 
flat  upon  the  table,  and  filled  quite  full  of  water.  The  object  to  be 
examined  is  then 

placed  in.  the  cell,  and      A    wwvwtfffi/Mwi  ~  ^vmrXMmffiMh 

may  be  properly  ar- 
ranged therein  ;  the 
cover  is  then  lowered 
gently  down,  the  two 
notches  at  the  bottom 
edges  being  first 
placed  against  the 
pins ;  in  this  way  the 
superfluous  water  will 
be  driven  out,  and  the 
whole  apparatus  may 
be  wiped  dry.  The 
capillary  attraction, 
assisted  by  the  weight 
of  the  cover,  will  be 
found  sufficient  to 
prevent  any  leakage ; 
and  the  pins  at  the 

bottom  prevent  the  cover  from  sliding  down  when  the  microscope  is 
inclined.  This  zoophyte  trough  possesses  two  important  qualities  : 
first,  it  does  not  leak  ;  second,  it  is  not  readily  broken  without  gross 
carelessness.  The  shallowness  may  be  overcome  by  placing  an  ebonite 
plate  with  the  required  aperture  between  the  two  mounted  glasses. 

Infusoria,  minute  algae,  &c.,  however,  can  be  well  seen  by 
placing  a  drop  of  the  water  containing  them  on  an  ordinary  slide, 
and  laying  a  thin  piece  of  covering  glass  on  the  top  ;  and  objects 
of  somewhat  greater  thickness  can  be  examined  by  placing  a  loop 
or  ring  of  fine  cotton  thread  upon  an  ordinary  slide  to  keep  the 
covering  glass  at  a  small  distance  from  it ;  and  the  object  to  be  ex- 
amined being  placed  on  the  slide  with  a  drop  of  water,  the  covering 
glass  is  gently  pressed  down  till  it  touches  the  ring.  Still  thicker 
objects  may  be  viewed  in  the  various  forms  of  '  cells '  hereafter  to 
be  described,  and  as,  when  the  cells  are  filled  with  fluid,  their  glass 
covers  will  adhere  by  capillary  attraction,  provided  the  superfluous 


FIG.  303. 


350  ACCESSOKY  APPARATUS 

moisture  that  surrounds  their  edges  be  removed  by  blotting  paper, 
they  will  remain  in  place  when  the  microscope  is  inclined.  An 
annular  cell,  that  may  be  used  either  as  a  '  live-box  or  as  a  '  grow- 
ing slide,'  has  lately  been  devised  by  Mr.  Weber  (U.S.A.).  It  is  a 
slip  of  plate-glass,  of  the  usual  size  and  ordinary  thickness,  out  of 
which  a  circular  *  cell '  of  j  inch  diameter  is  ground,  in  such  a 
manner  that  its  bottom  is  convex  instead  of  concave,  its  shallowest 
part  being  in  the  centre  and  the  deepest  round  the  margin.  A 
small  drop  of  the  fluid  to  be  examined  being  placed  upon  the  central 
convexity  (the  highest  part  of  which  should  be  almost  flush  with  the 
general  surface  of  the  plate),  and  the  thin  glass  cover  being  placed 
upon  it,  the  drop  spreads  itself  out  in  a  thin  film,  without  finding 
its  way  into  the  deep  furrow  around  it ;  and  thus  it  holds-on  the 
covering  glass  by  capillary  attraction,  while  the  furrow  serves  as  an 
air-chamber.  If  the  cover  be  cemented  down  by  a  ring  of  gold  size 
or  dammar,  so  that  the  evaporation  of  the  fluid  is  prevented,  either 
animal  or  vegetable  life  may  thus  be  maintained  for  some  days,  or, 
if  the  two  should  be  balanced  (as  in  an  aquarium),  for  some  weeks. 
Dipping  Tubes. — In  every  operation  in  which  small  quantities 
of  liquid,  or  small  objects  contained  in  liquid,  have  to  be  dealt  with 
by  the  microscopist,  he  will  find  it  a  very  great  convenience  to  be 
provided  with  a  set  of  tubes  of  the  forms  represented  in  fig.  304, 
but  of  somewhat  larger  dimensions.  These  were  formerly  desig1 
nated  '  fishing  tubes,'  the  purpose  for  which  they  were  originally 
devised  having  been  the  fishing  out  of  water  fleas,  aquatic  insect 
larvae,  the  larger  animalcules,  of  other  living  objects  distinguishable 
either  by  the  unaided  eye  or  by  the  assistance  of  a  magnifying  glass 
from  the  vessels  that  may  contain  them.  But  they  are  equally 
applicable,  of  course,  to  the  selection  of  minute  plants ;  and  they 
may  be  turned  to  many  other  no  less  useful  purposes,  some  of  which 
will  be  specified  hereafter.  When  it  is  desired  to  secure  an  object 
which  can  be  seen  either  with  the  eye  alone  or  with  a  magnifying 
glass,  one  of  these  tubes  is  passed  down  into  the  liquid,  its  upper 
orifice  having  been  previously  closed  by  the  forefinger,  until  its  lower- 
orifice  is  immediately  above  the  object ;  the  finger  being  then  re- 
moved, the  liquid  suddenly  rises  into  the  tube,  probably  carrying 
the  object  up  with  it ;  and  if  this  is  seen  to  be  the  case,  by  putting 
the  finger  again  on  the  top  of  the  tube,  its  contents  remain  in  it 
when  the  tube  is  lifted  out,  and  may  be  deposited  on  a  slip  of  glass, 
or  on  the  lower  disc  of  the  aquatic  box,  or,  if  too  copious  for  either 
receptacle,  may  be  discharged  into  a  large  glass  cell.  In  thus 
fishing  in  jars  for  any  but  minute  objects,  it  will  be  generally  found 
convenient  to  employ  the  open-mouthed  tube  0  ;  those  with  smaller 
orifices,  A,  B,  being  employed  for  'fishing'  for  animalcules,  ifec.,  in 
small  bottles  or  tubes,  or  for  selecting  minute  objects  from  the  cell 
into  which  the  water  taken  up  by  the  tube  C  has  been  discharged. 
It  will  be  found  very  convenient  to  have  the  tops  of  these  last 
blown  into  small  funnels,  which  shall  be  covered  with  thin  sheet 
indiarubber,  or  topped  with  indiarubber  nipples,  which  by  com- 
pression and  expansion  can  then  be  regulated  with  the  greatest 
nicety. 


DIPPING  TUBES 


351 


In  dealing  with  minute  aquatic  objects,  and  in  a  great  variety 
of  other  manipulations,  a  small  glass  syringe  of  the  pattern  repre- 
sented in  fig.  305,  and  of  about  double  the  dimensions,  will  be 
found  extremely  convenient.  When 
this  is  firmly  held  between  the  fore 
and  middle  fingers,  and  the  thumb 
is  inserted  into  the  ring  at  the 
summit  of  the  piston-rod,  such 
complete  command  is  gained  over 
the  piston  that  its  motion  may  be 
regulated  with  the  greatest  nicety  ; 
and  thus  minute  quantities  of  fjiiid 
may  be  removed  or  added  in  tlie 
various  operations  which  have  to  be 
performed  in  the  preparation  and 
mounting  of  objects  ;  or  any  minute 
object  may  be  selected  (by  the  aid  of 
the  simple  microscope,  if  necessary) 
from  amongst  a  number  in  the  same 
drop,  and  transferred  to  a  separate 
slip.  A  set  of  such  syringes,  with 
points  drawn  to  different  degrees  of 
fineness,  and  bent  to  different  curva- 
tures, will  be  found  to  be  among  the 
most  useful  '  tools '  that  the  work- 
ing microscopist  can  have  at  his 
command.  It  will  also  be  found 
that  if  a  dipping  tube  with  a  glass 
bulb  have  an  indiarubber  hollow 
ball  or  teat  attached  to  the  top  of 
it,  it  will  act,  for  the  majority  of 
purposes,  as  well  as  a  syringe. 

Forceps. — Another  instrument 
so  indispensable  to  the  microscopist 
as  to  be  commonly  considered  an 
appendage  to  the  microscope  is  the 
forceps  for  taking  up  minute  objects  ; 
many  forms  of  this  have  been  devised,  of  which  one  of  the  most  con- 
venient is  represented  in  fig.  306,  of  something  less  than  the  actual 
size.  As  the  forceps,  in  marine  researches,  have  continually  to  be 


FIG.  304.— Dip- 
ping tubes. 


FIG.  305.— Glass 
syringe. 


FIG.  306. 


plunged  into  sea-water,  it  is  better  that  they  should  be  made  of  brass 
or  of  German  silver  than  of  steel,  since  the  latter  rusts  far  more 
readily ;  and  as  they  are  riot  intended  (like  dissecting  forceps)  to 
take  a  firm  grasp  of  the  object,  but  merely  to  hold  it,  they  may  be 
made  very  light,  and  their  spring  portion  slender.  As  it  is  essential. 


352 


ACCESSOEY  APPAKATUS 


however,  to  their  utility  that  their  points  should  meet  accurately, 
it  is  well  that  one  of  the  blades  should  be  furnished  with  a  guide-pin 
passing  through  a  hole  in  the  other. 

Most  microscopists  have  at  some  time  experienced  the  danger 
that  is  imminent  to  their  instruments  and  mountings  when  exhibit- 
ing delicate  objects  with  high  power  in  mixed  assemblies,  arising 
from  the  inadvertency  or  want  of  knowledge 
of  some  visitor,  who  may  do  terrible  mischief 
by  innocently  using  the  coarse  adjustment. 
Messrs.  Ross  made  an  arrangement  by  which 
the  coarse  adjustment  could  be  '  locked  '  at  a 
given  point ;  but  an  equally  useful  and  simpler 
method  was  long  ago  devised  by  Messrs. 
Powell  and  Lealand,  who  used  a  deep  ring,  as 
is  shown  in  fig.  307.  This  ring  has  two  pins 
and  a  screw  projecting  inwards.  When  the 
screw  is  withdrawn,  the  rings  can  be  slipped 
over  the  milled  heads  of  the  coarse  adiust- 

FIG.  307. — Powell  and  Lea-  j  -,  ,-,  n1  , 

land's  protecting  ring  for   ment,  and  by  screwing  the  small  screw   home 
coarse  adjustment.  the  ring  cannot  be  withdrawn  ;  but  as  they 

are  loose  upon  the  milled  heads,  the  latter 

cannot  be  brought  into  action  ;  the  rings  simply  revolve  upon  the 
heads  without  bringing  them  into  play. 

Other  forms  of  the  same  appliance  have  been  made  by  this  firm ; 
and  Messrs.  Beck  have  made  these  rings  with  slight  modifications 
more  recently.  They  are  the  most  efficient  means  of  counteracting 
the  danger  incident  on  public  exhibition  of  delicate  objects  under 
high  powers. 

The  foregoing  constitute,  it  is  believed,  all  the  most  important 
pieces  of  apparatus  which  can  be  considered  in  the  light  of  accessories 
to  the  microscope.  Those  which  have  been  contrived  to  afford 
facilities  for  the  preparation  and  mounting  of  objects  will  be  described 
in  a  future  chapter  (Chapter  VI.). 


353 


CHAPTER  V 

OBJECTIVES,  EYE-PIECES,  THE  APERTOMETER 


IT  is  manifest  that  everything  in  the  form  and  construction  as  well 
as  in  the  nature  of  the  optical  and  mechanical  accessories  of  the 
microscope  exists  for,  and  to  make  more  efficient,  the  special  work 
of  the  objective,  or  image-forming  lens  combination,  which  constitutes 
the  basis  of  the  optical  properties  of  this  instrument. 

The  development  of  the  modern  objective,  as  we  have  already 
seen,  has  been  very  gradual ;  but  there  are  definite  epochs  of  very 
marked  and  important  improvement.  Our  aim  in  the  study  of 
objectives  is  practical,  not  antiquarian,  and  we  may  avoid  elaborate 
researches  on  the  subject  of  non- achromatic  lenses  and  reflecting 
specula,  which  have  been  sufficiently  indicated  in  the  third  chapter 
of  this  volume.  We  may  also  pass  over  the  earlier  attempts  at 
achromatism  ;  the  true  history  of  the  modern  objective  bey  ins  from  the 
time  that  its  achromatism  had  been  finally  ivorked  out. 

The  first  movement  of  a  definite  character  towards  this  object 
was  made,  it  has  been  recently  shown,1  so  early  as  1808  to  1811  by 
Bernardino  Marzoli,  who  was  Curator  of  the  Physical  Laboratory  of 
the  Lyceum  of  Brescia.  Mr.  May  all  discovered  a  reference  to  this 
effort  to  make  achromatic  lenses,  and,  through  the  courtesy  of  the 
President  of  the  Athenaeum  of  Brescia,  discovered  that  Marzoli 
was  an  amateur  optician,  that  he  had  taken  deep  interest  in  the 
application  of  achromatism  to  the  microscope,  and  that  a  paper  of  his 
on  the  subject  had  been  published  in  the  '  Commentarj  '  for  the  year 
1808,  and  that  he  had  exhibited  his  achromatic  objectives  at  Milan 
in  1811  and  obtained  the  award  of  a  silver  medal  for  their  merits 
under  the  authority  of^  the  Istituto  Reale  delle  Scienze  of  that  city. 
One  of  these  objectives  wTas  found  to  have  been  '  religiously  pre- 
served,' and  was  generously  presented  in  1890  by  Messrs.  Tranini 
Brothers  to  the  Royal  Microscopical  Society  of  London.  With  it 
was  forwarded  the  '  Processo  Yerbale,'  or  official  record  of  the  awards, 
notifying  Marzoli's  exhibits  and  the  award  of  a  silver  medal,  and 
the  actual  diploma,  dated  August  20,  1811,  signed  by  the  Italian 
Minister  of  the  Interior. 

Marzoli's  objective  wras  a  cemented  combination,  having  the  plane 
side  of  the  flint  presented  to  the  object ;  and  if  this  was  a  part  of 
the  intended  construction,  of  which  there  appears  small  room  for 
doubt,  Marzoli  preceded  Chevalier  in  this,  as  we  shall  subsequently 
see,  very  practical  improvement. 

1  Journt  Roy.  Mic.  Soc.  1890,  p.  420. 

A  A 


354  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETER 

It  has  been,  however,  customary  to  accredit  the  first  practicable 
attempts  to  achromatise  object-glasses  to  M.  Selligues.  In  1823 
he  suggested  to  M.  Chevalier  to  superimpose  two,  three,  or 
four  achromatised  plano-convex  'doublets,'  that  is  to  say,  pairs 
of  lenses.  These  objectives  had  their  convex  surfaces  presented  to 
the  object,  which  gave  them  four  times  as  much  spherical  aberration 
as  would  have  been  the  case  had  their  positions  been  reversed,1  and, 
as  we  have  just  seen,  Marzoli  reversed  them.  This  necessitated  an 
excessive  reduction  of  the  apertures,  which,  nevertheless,  still  too 
manifestly  displayed  an  obtrusive  aberration.  Yet  the  conception 
of  an  achromatised  combination  had  been  embodied  in  an  initial 
manner.  In  1825  M.  Chevalier  perceived  the  exact  nature  of  the 
mistake  made  by  M.  Selligues,  and  made  the  lenses  of  less  focal 
length  and  more  achromatic,  and  inverted  them,  placing  the  plane 
side  of  the  flint  towards  the  object. 

It  is  somewhat  important,  as  it  is  interesting,  to  note  that  the 
idea  of  the  superposition  of  a  combination  of  lenses  did  not  originate 
from  theoretical  considerations  of  the  optical  principles  involved. 
It  is  scarcely  conceivable  that  where  there  was  manifest  ignorance 
of  the  position  of  a  plano-convex  lens  for  least  spherical  aberration 
(a  principle  now  thoroughly  understood)  there  could  have  been  in- 
sihgt  enough  either  to  detect  the  presence  of  the  two  aplanatic  foci 
or  to  discover  a  method  of  balancing  them 
by  inductive  reasoning.  Everything  in  the 
history  points  to  happy  accident  as  the  primal 
step  in  achromatised  objectives,  and  this,  with 
very  high  probability,  applies  to  the  work  of 

Chevalier,  for  Selligues'  attempt  was  a  blunder 

PIG.  308.-Tully's  achro-     aPinst  the  commonplace  knowledge  of  his 
matic  triple.  time. 

The  form  of  three  superimposed  similar 

achromatic  doublets  is  precisely  the  combination  of  the  French 
'  buttons,'  which  have  been  sold  in  thousands  until  quite  recently, 
many  of  them  being  mounted  as  English  objectives. 

At  the  suggestion  of  Dr.  Goring,  Mr.  Tully,  in  this  country, 
without  any  knowledge  of  what  was  being  done  on  the  Continent, 
made  an  achromatic  objective  in  1824.  This  was  a  single  combina- 
tion, being  an  achromatic  uncemented  triplet.  It  was,  in  fact,  a 
miniature  telescope  object-glass,  and  is  illustrated  in  fig.  308.  Two 
lenses  made  on  this  principle  by  Tully,  having  T\  and  ^  foci,  wrere 
found  in  practice  too  thick,  and  in  many  ways  imperfect ;  and  he 
was  induced  to  make  another  single  triplet  of  T^y  focus  and  18°  aper- 
ture, and  its  performance  was  said  to  be  nearly  equal  to  that  of 
the  T%. 

Subsequently  a  doublet  was  placed  in  front  of  a  similar  triplet  of 
somewhat  shorter  focus,  forming  a  double  combination  objective  of 
38°  aperture.  This  was  pronounced  to  be  a  great  advance  upon  all 
preceding  combinations,  even  those  which  had  been  produced  upon 
the  Continent. 

A  note  of  Lister's  at  this  time  upon  the  objectives  of  Chevalier 

1  Chapter  I. 


LISTER'S  DISCOVERY 


355 


is  of  interest.  He  found  them  much  stopped  down,  and  in  one 
instance  he  opened  the  stop  and  improved  the  effect.  Lister  says  : 
'  The  French  optician  knows  nothing  of  the  value  of  aperture,  but 
he  has  shown  us  that  fine  performance  is  not  confined  to  triple- 
objectives  ;  and  in  successfully  combining  two  achromatics  he  has: 
given  an  important  hint — probably  without  being  himself  acquainted! 
with  its  worth — that  I  hope  will  lead  to  the  acquisition  of  a  pene- 
trating l  power  greater  than  could  ever  be  reached  with  one  alone.' 

At  this  time  Professor  Amici,  of  Modena,  one  of  the  leading 
minds  who  assisted  in  giving  its  form  to  the  modern  microscope,  had 
been  baffled  by  the  difficulties  presented  by  the 
problem  of  achromatism,  and  ftajl  laid  it  aside 
in  favour  of  the  reflecting  microscope,  but  he 
now  returned  to  the  practical  reconsideration  of 
the  production  of  an  achromatic  lens.  As  a 
result  he  appears  to  have  constructed  objectives 
of  greater  aperture  than  those  of  Chevalier. 
He  visited  London  in  1844,  and  brought  with 
him  a  horizontal  microscope,  the  object-glass 
being  composed  of  three  doublets,  which  pro- 
duced a  most  favourable  impression. 

Meantime,  in  this  country,  Mr.  Lister 
brought  about  an  important  epoch  in  the  evo- 
lution of  the  achromatic  object-glass  by  the  dis- 
covery of  the  two  aplanatic  foci  of  a  combination. 
It  had  occupied  his  mind  for  several  years,  but 
in  January  1830  a  very  important  paper  was 
read  to,  and  published  by,  the  Royal  Society, 
written  by  him,  in  which  he  points  out  how  the 
aberrations  of  one  doublet  may  be  neutralised 
by  a  second. 

As  the  basis  of  a  microscope  objective,  he 
considers  it  eminently  desirable  that  the  flint 
lens  shall  be  plano-concave,  and  that  it  shall  be 
joined  by  a  permanent  cement  to  the  convex 
lens. 

For  an  achromatic  object-glass  so  constructed 
he  made  the  general  inference  that  it  will  have 
on  one  side  of  it  two  foci  in  its  axis,  for  the 
rays  proceeding  from  which  the  spherical  aber- 
ration will  be  truly  corrected  at  a  moderate 
aperture  ;  that  for  the  space  between  these  twro 
points  its  spherical  aberration  will  be  over-corrected,  and  beyond 
them,  either  way,  under-corrected. 

Thus,  let  a,  b,  fig.  309,  represent  such  an  object-glass,  and  be 
roughly  considered  as  a  plano-convex  lens,  with  a  curve,  a  c  5, 
running  through  it,  at  which  the  spherical  and  chromatic  errors 
are  corrected  which  are  generated  at  the  two  outer  surfaces,  and 
let  the  glass  be  thus  free  from  aberration  for  rays,/,  d,  e,  g,  issuing 

1  '  Penetrating  '  meant  '  resolving  '  power  in  those  days  ;  he  alludes,  therefore,  to 
increase  of  aperture. 

A  A  2 


FIG.  309.  — The  two 
aplanatic  foci  of  an 
optical  combination. 


356  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETER 

from  the  radiant  point,  /,  h  e  being  a  normal  to  the  convex 
surface,  and  i  d  to  the  plane  one — under  these  circumstances  the 
angle  of  emergence,  y  e  h,  much  exceeds  that  of  incidence,  f  d  i, 
being  probably  almost  three  times  as  great. 

If  the  radiant  is  now  made  to  approach  the  glass,  so  that  the 
course  of  the  ray,  fd  e  g,  shall  be  more  divergent  from  the  axis,  as 
the  angles  of  incidence  and  emergence  become  more  nearly  equal 
to  each  other,  the  spherical  aberration  produced  by  the  two  will  be 
found  to  bear  a  less  proportion  to  the  opposing  error  of  the  single 
correcting  curve  a  c  b  ;  for  such  a  focus,  therefore,  the  rays  will  be 
•over-corrected.  But  if  /  still  approaches  the  glass,  the  angle  of 
incidence  continues  to  increase  with  the  increasing  divergence  of 
the  ray,  till  it  will  exceed  that  of  emergence,  which  has  in  the  mean- 
while been  diminishing,  and  at  length  the  spherical  error  produced 
by  them  will  recover  its  original  proportion  to  the  opposite  error  of 
the  curve  of  correction.  When  f  has  reached  this  point  f  (at  which 
the  angle  of  incidence  does  not  exceed  that  of  emergence  so  much  as 
it  had  at  first  come  short  of  it),  the  rays  again  pass  the  glass  free 
from  spherical  aberration. 

If/  be  carried  hence  towards  the  glass,  or  outwards  from  its 
original  place,  the  angle  of  incidence  in  the  former  case,  or  of 
•emergence  in  the  latter,  becomes  disproportionately  effective,  and 
either  way  the  aberration  exceeds  the  correction. 

How  far  Lister's  discoveries  were  affected  by  Amici's  work  it  is 
now  quite  impossible  to  say ;  there  can  be  but  little  doubt  that  some 
influence  is  clue  to  it,  but  it  is  equally  clear  that  a  profound  know- 
ledge of  the  optics  of  that  time  was  the  only  foundation  upon  which 
the  facts  in  Lister's  paper  could  have  been  built.  He  was  a  man  of 
application  and  an  enthusiast,  and  it  was  inevitable  that  he  should 
exert  a  powerful  influence  upon  the  early  history  of  the  optics  of  the 
microscope.  This  is  the  more  certain  when  we  remember  how  few 
were  the  men  at  that  time  who  knew  in  any  practical  sense  what  a 
microscope  was  ;  and  we  find  that  in  1831,  being  unable  to  find  any 
optician  who  cared  to  experiment  sufficiently,  Lister  taught  himself 
the  art  of  lens-grinding,  and  he  made  an  objective  whose  front  was 
a  meniscus  pair,  with  a  triple  middle  combination,  and  the  back  a 
plano-convex  doublet.  He  declared  this  to  be  the  best  lens  of  its 
immediate  time,  and  it  had  a  working  distance  of  •!!. 

One  of  the  immediate  consequences  of  the  publication  of  Lister's 
paper  was  the  rapid  production  by  professional  opticians  of  achromatic 
objectives.  The  data  supplied  by  Lister  proved  to  be  of  the  highest 
value  in  the  actual  production  of  these,  and  the  progress  of  improve- 
ment was,  in  consequence,  and  in  comparison  with  the  time  imme- 
diately preceding,  remarkably  rapid. 

Andreiv  Ross  began  their  manufacture  in  1831.  He  was  followed 
T>y  Hugh  Powell  in  1834,  and  in  1839  by  James  Smith.  It  is  of 
more  than  ordinary  interest  to  study  in  detail  the  work  of  this  im- 
mediate time,  and  the  following  table  giving  a  list  of  objectives,  with 
their  foci,  apertures,  and  mode  of  construction,  with  the  dates  of 
their  production,  will  give  a  fair  idea  of  the  work  of  Andrew  Ross 
in  the  manufacture  of  early  lenses.  He  was  the  earliest  of  the  three 


PRIMITIVE   FORM   OF   LENS   CORRECTION 


357 


English  makers,  and  undoubtedly  carried  the  palm  both  here  and  on 
the  Continent  for  the  excellence  of  his  objectives. 

1  inch  14°  two  doublets,  1832.     Made  for  Mr.  R.  H.  Solly. 


18°  single  triple,  1833. 

55°  three  pairs,  1834.     This  belonged  to  Professor  Quekett. 


63o 

44' 
63 

74° 


1  triple 


front  and  two  double  backs 


'},  Lister's  formula 


1842. 


FIG.  310.— A  J-in. 
combination  by 
Andrew  Ross. 


Examples  of  these  old  lenses  are  extant  and  in  perfect  preserva- 
tion, and  for  correction  they  are  comparable  without  detriment  to 
any  ordinary  crown  and  flint  glass  achromatic  of  the  same  aperture 
of  the  present  day. 

An  example  of  the  construction  of  the  J-inch  focus  objective  of  55°, 
consisting  of  three  pairs  of  lenses  arranged  with  their  plane  sides 
to  the  object,  the  position  of  least  aberration,  is  shown 
in  fig.  310.  The  foci  of  these  three  pairs  are  in  the 
proportion  of  1  :  2  :  3.  In  1837  this  maker  had  so 
completely  corrected  the  errors  of  spherical  and 
chromatic  aberration  that  the  circumstance  of  cover- 
ing an  object  with  a  plate  of  the  thinnest  glass  was 
found  to  disturb  the  corrections  ;  that  is  to  say,  the 
corrections  were  so  relatively  perfect  that  if  the 
combination  were  adapted  to  an  uncovered  object, 
covering  the  object  with  the  thinnest  glass  intro- 
duced refractive  disturbances  that  destroyed  the  high  quality  of  the 
objective.1 

Lister's  paper  of  1830  gave  the  obvious  clue  to  a  method  of 
neutralising  this ;  that  is  to  say,  by  lens  distance  ;  and  Ross  applied 
this  correction  by  mounting  the  front  lens  of 
an  objective  in  a  tube  which  slid  over  another 
tube  carrying  the  t\vo  other  pairs.  A  very 
primitive  form  of  this  lens  correction  is  afforded 
us  by  a  J-inch  objective  made  by  Andrew  Ross 
in  1838.  It  belonged  originally  to  Professor 
Lindley,  the  second  President  of  the  Royal 
Microscopical  Society,  and  was  presented  to  the 
society  by  his  son,  the  Master  of  the  Rolls,  in 
1899.  An  illustration  of  this  lens  is  given  in 
fig.  311.  The  tube  carrying  the  front  lens 
slides  on  an  inner  tube ;  it  can  be  clamped  in 
any  position  by  the  screws  at  the  sides  ;  the 
line  in  the  small  hole  in  the  front  indicates  its 
position,  and  is  the  prototype  of  the  '  covered  ' 
and  '  uncovered  '  lines  of  later  times. 

The  larger  cylinder  at  the  base  is  the  lid  of  its  box  upon  which 
it  is  standing. 

Subsequently  this  arrangement  was  modified  by  the  introduction 

i  Vide  Chapter  I. 


FIG.  811. — Primitive 
form  of  lens  correc- 
tion (1838). 


358 


OBJECTIVES,    EYE-PIECES,    THE   APERTOMETEE 


of  a  screw  arrangement,  as  in  fig.  312.  The  front  pair  of  lenses  is 
fixed  into  a  tube  (A)  which  slides  over  an  interior  tube  (B)  by  which 
the  other  two  pairs  are  held ;  and  it  is  drawn  up  or  down  by  means 
of  a  collar  (C),  which  works  in  a  furrow  cut  in  the  inner  tube,  and 
upon  a  screw-thread  cut  in  the  outer,  so  that  its  revolution  in  the 
plane  to  which  it  is  fixed  by  the  one  tube  gives  a  vertical  movement 
to  the  other.  In  one  part  of  the  outer  tube  an  oblong  slit  is  made, 
as  seen  at  D,  into  which  projects  a  small  tongue  screwed  on  the 
inner  tube ;  at  the  side  of  the  former  two  horizontal  lines  are 
engraved,  one  pointing  to  the  word  '  uncovered,'  the  other  to  the 
word  '  covered  ; '  whilst  the  latter  is  crossed  by  a  horizontal  mark, 
which  is  brought  to  coincide  with  either  of  the  two  lines  by  the 
rotation  of  the  screw-collar,  whereby  the  outer  tube  is  moved  up 
or  down.  When  the  mark  has  been 
made  to  point  to  the  line  '  uncovered,' 
it  indicates  that  the  distance  of  the  lenses 


FIG.  312. — Section  of  adjusting  object-glass. 


FIG.  818.— Present  collar 
correction. 


of  the  object-glass  is  such  as  to  make  it  suitable  for  viewing  an 
object  without  any  interference  from  thin  glass  ;  when,  on  the  other 
hand,  the  mark  has  been  brought,  by  the  revolution  of  the  screw- 
collar,  into  coincidence  with  the  line  '  covered,'  it  indicates  that  the 
front  lens  has  been  brought  into  such  proximity  with  the  other  two 
as  to  produce  an  *  under-correction '  in  the  objective,  fitted  to 
neutralise  the  '  over- correction  '  produced  by  the  interposition  of  a 
glass  cover  of  extremest  thickness. 

This  method  of  collar  correction  served  the  purposes  of  micro- 
scopy for  upwards  of  thirty  years,  but  when  more  critical  investiga- 
tions were  undertaken  and  objectives  had  more  aperture  given  to 
them  it  was  found  that  the  method  had  two  great  faults. 

The  first  was  that  the  *  covered '  and  *  uncovered '  marks  were 
too  crude.  To  remedy  this,  the  screw  collar  was  graduated  into 
fitfy  divisions,  a  device  introduced  by  James  Smith  in  1841  so  that 


THE   MODERN   USE    OF   COLLAE   CORRECTION  359 

intervals  between  the  points    *  covered '   and   '  uncovered  '  might  be 
recorded. 

The  second,  a  more  serious  defect,  was  the  movement  of  the 
front  lens  while  the  back  remained  rigid  with  the  body  of  the 
microscope.  The  detriment  of  this  arrangement  was  that  in  cor- 
recting a  wide-angled,  close-working  objective  there  was  a  danger  of 
forcing  the  front  lens  through  the  cover-glass  by  means  of  the  collar 
•correction. 

Now  the  arrangement  as  shown  in  fig.  313  enables  the  front 
lens  to  maintain  a  fixed  position,  while  the  correctional  collar  acts 
on  the  posterior  combinations  only.  This  device  was  introduced 
by  Mr.  F.  H.  Wenham  in  1855. 

On  the  Continent  it  has  beeyi  the  practice  to  graduate  the  cor- 
rectional collar  in  terms  of  the  thickness  of  the  cover-glass  in  deci- 
mals of  a  millimetre.  Thus  if  a  cover-glass  be  0*18  mm.  thick,  the 
correctional  collar  should  be  set  to  the  division  marked  0'18. 

In  England,  on  the  contrary,  the  divisions  are  entirely  empiri- 
cal, so  that  the  operator  has  to  discover  for  himself  the  proper 
adjustment.  It  is  not  to  be  supposed,  however,  that  the  English 
method  is  unscientific,  for  when  an  operator  becomes  expert  he 
would  never  for  an  instant  think  of  adjusting  by  any  other  indi- 
cation than  that  afforded  by  his  own  eye  and  experience.  This  is  a 
very  important  point,  because  the  interpretation  of  structure  to  a 
great  extent  depends  on  accurate  adjustment  of  the  objective,  and 
it  would  be  folly  to  suppose  that  an  eminent  observer  would  sur- 
render his  judgment  to  the  predetermination  of  theory  embodied  in 
what  must  be  the  imperfections  in  even  the  most  conscientious  and 
thorough  work  which  gives  a  practical  form  to  such  theory.  In 
fact,  it  is  the  test  of  accurate  manipulation  that,  however  the  collar 
correction  be  disturbed,  the  microscopist  will,  in  getting  a  critical 
image  of  the  same  object,  always,  by  the  quality  of  the  image  he 
obtains,  bring  the  correction  to  within  the  merest  fraction  of  the 
same  position,  although  the  correction  collar  and  its  divisions  are 
never  looked  at  until  the  desired  image  is  obtained. 

The  fact  that  the  over-correction  caused  by  the  cover-glass  was 
discovered  in  England,  and  that  means  were  at  once  found  for  its 
correction,  while  no  similar  steps  were  taken  on  the  Continent,  is  a 
sufficient  evidence  of  the  advanced  position  of  this  country  in  practi- 
cal optics  at  that  time. 

This  subject  of  under-  and  over-correction  is  one  of  large  impor- 
tance, and  it  may  be  well  at  this  point  to  enable  the  tyro  to  clearly 
understand,  by  evidence,  its  nature,  although  what  it  is  has  been 
fully  shown  in  Chapter  I.  Take  a  single  lens — the  field-lens  of  a 
Huyghenian  eye -piece  will  serve  admirably — and  hold  it  a  couple 
of  yards  from  a  lamp  flame;  the  rays  passing  through  the  peri- 
pheral portion  of  the  lens  will  be  found  by  experiment  with  a  card 
to  be  brought  to  a  focus  at  a  point  on  the  axis  nearer  the  lens  than 
those  passing  through  the  centre.  This  is  under -correction,  vide  fig.  23, 
p.  20.  The  same,  experiment  should  be  repeated  with  the  plane 
.side  and  the  convex  side  of  the  lens  alternately  turned  to  the  flame. 
In  the  former  case,  when  the  image  of  the  flame  is  at  its  best  focus, 


360  OBJECTIVES,  EYE-PIECES,    THE   APERTOMETEK 

it  will  be  surrounded  by  a  corna,  and  even  the  portion  of  the  flame- 
which  is  in  focus  will  lack  brightness.  But  with  the  convex  side  to- 
wards the  flame  it  will  be  found  that  in  the  image  on  the  card  the 
coma  is  greatly  reduced,  and  the  image  of  the  flame  brightened. 
The  reason  for  this  is,  as  already  stated,  that  the  spherical  aberration 
is  four  times  as  great  when  the  convex  side  of  the  lens  is  towards 
the  card. 

The  practice  of  these  simple  tests  will  be  most  instructive  to 
those  unfamiliar  with  the  optical  principles  on  which  an  objective  is 
constructed.  They  make  plain  that  an  over -corrected  lens  is  one 
which  brings  its  peripheral  rays  to  a  longer  focus  than  its  central,  vide 
fig.  24,  p.  20.  But  a  cover-glass  produces  over-correction,  therefore 
the  means  employed  to  neutralise  the  error  is  by  the  under-cor- 
rection of  the  objective.  If,  however,  the  objective  employed 
should  be  unprovided  with  such  means  of  correction,  the  eye-piece 
must  be  brought  nearer  the  objective,  which  will  effect  the  same 
result.1 

Still  confining  our  consideration  to  the  year  1837,  we  find  that  a 
further  improvement  was  made  by  Lister,  who  employed  a  triple 
front  combination.  This  consisted  of  two  crown  piano-con  vexes  with 
a  flint  plano-concave  between  them.  The  result  of  this  was  the 
increase  of  the  aperture  of  an  inch-focus  objective  to  22°. 

An  illustration  of  the  mode  of  construction  of  these  lenses  is 
given  in  fig.  314,  which  is  drawn  from  an  early  £-inch  objective  by 
Andrew  Ross,  having  bayonet-catch  correction  adjustment.  In  1842 
a  ^-inch  of  44°,  a  J-inch  of  63°,  and  a  ^--inch  of  74°  were  made 
upon  the  same  lines.  The  method  for  computing  these  fronts  i& 
given  by  Mr.  Nelson  in  the  'Journ.  R.  M.  S.,'  1898,  p.  160  et  seq. 

In  1841  the  Royal  Microscopical  Society  ordered  a  microscope 
from  each  of  the  before-mentioned  leading  opticians.  The  objectives 
supplied  with  these  are  still  extant,  representing  with  moral  certainty 
the  very  best  work  of  the  several  makers  ;  they  are  consequently 
valuable  as  reliable  specimens  of  the  best  work  of  the  period. 

The  objectives  supplied  by  James  Smith  have  the  peculiarity  of 
being  separating  lenses. 

The  lowest  power  is  about  1  J-inch  focus.  When  this  is  used 
alone  a  diaphragm  is  slid  over  the  front  to  limit  the  aperture,  but 
we  are  unable  to  say  what  that  limit  was,  since  the  diaphragm  has 
been  lost.  By  placing  another  front  where  the  diaphragm  would 
have  been,  the  new  combination  becomes  an  ^-inch  focus,  while 
yet  another  front  may  be  substituted,  making  the  objective  a  J-inch 
focus.  This  latter  front  consists  of  two  pairs,  and  it  is  provided 
with  a  graduated  screw-collar  adjustment  which  separates  these 
pairs,  but  the  arrangement  is  of  a  very  primitive  order. 

This  object-glass  will  divide  the  podura  marks  in  a  milky  field 
with  a  full  cone,  and  the  field  is  much  curved. 

There  is  also  a  separating  IJ-inch  and  f-inch  which  is  good 
while  the  T\-inch  and  the  J-inch  may  be  considered  fair. 

The  lenses  supplied  by  Andrew  Ross  are  a  good  2 -inch  and  a. 

1  Under-correction   is   also  known   as  '  positive   aberration ; '  over-correction  as 
negative  aberration.' 


TRIPLE   BACK   COMBINATION  361 

fail-  1  -inch,  but  we  have  seen  a  better  than  this  of  about  the  same 
period. 

Hugh  Powell  supplied  a  1-inch  of  good  quality,  and  a  ^,  J,  £, 
iVinch  fairly  good.  The  apertures  of  the  £  and  the  ^-inch  are  of 
course  very  low. 

On  the  whole  it  may  be  said  that  the  corrections  are  well 
balanced  in  the  lower  lenses,  and  the  apertures  moderate  ;  but  when 
we  conie  to  the  higher  powers  it  is  the  deficiency  of  aperture  that 
becomes  so  oppressively  apparent.  In  1844  Amici  made  a  -f-inch 
objective  of  112°  and  brought  it  to  England.  It  was  understood 
that  extra  dense  flint  wTas  employed  in  the  construction  of  this 
objective  ;  but  this  is  perishable  ;  and  Mr.  Ross  altered  slightly  the 
curves  of  Amici's  construction^  and  writh  ordinary  flint  succeeded 
in  extending  the  aperture  of  a  J-inch  objective  to  85°,  or  '68  N.A., 
and  a  TV  inch  objective  to  135°,  or  '93  N.A.  Of  this  latter  it  was 
affirmed  that  it  was  '  the  largest  angular  pencil  that  could  be  passed 
through  a  microscope  object-glass.' 

In  1850  object-glasses  were  made  with  a  triple  back  combination  ; 
these  were  attributed  to  Lister ;  but  it  is  also  affirmed  that  they 


FIG.  314.— An  early                  FIG.  315.— A  triple-  FIG.  316.— A  single- 

^-in.  combination                       back       combina-  front  combination 

by  A.  Ross.                                  tion  by  Lister  (or  by  Wenham. 
Amici  ?). 

were  the  previous  device  of  Amici.  It  may  well  be  a  disputed  pointy 
for  it  is  quite  certain  that  this  device  brought  the  dry  achromatic 
objective  potentially  to  its  highest  perfection.  The  combination  is 
illustrated  in  fig.  315,  and  under  the  conditions  of  its  construction  it 
may  be  well  doubted  if  anything  will  ever  surpass  the  results- 
obtained  by  English  opticians  in  achromatic  objectives  constructed 
with  this  triple  front,  double  middle,  and  triple  back  combinations,, 
apart  from  the  use  of  the  new  kinds  of  Jena  glass.  For  the  method 
of  computing  the  triple  back,  vide  '  Journ.  R.  M.  S.,'  1898,  p.  160  etseq. 
It  may  be  noticed  that  Tully's  objective  had  a  triple  back,  but  it  was- 
not  the  result  of  intended  construction  ;  it  was  a  fortunate  combina- 
tion the  real  value  of  which  was  neither  understood  nor  appreciated, 
and  as  a  consequence  its  existence  was  evanescent. 

In  this  same  year  Wenham  produced  another  modification  of  the 
achromatic  objective  of  considerable  value,  but  more  to  the  manu- 
facturer than  the  user  of  the  microscope.  It  consisted  of  a  single 
front ;  the  combination  is  seen  in  fig.  316,  which,  it  will  be  seen,  is  a 
simpler  construction,  but  this  did  not  affect  in  the  least  the  price  of 
the  objectives  produced.  Subsequently,  however,  the  form  was 


362  OBJECTIVES,    EYE-PIECES,    THE   APERTOMETEK 

adopted  on  the  Continent  for  low-priced  objectives,  which  led  to  a 
reduction  of  the  cost  of  English  objectives  of  the  same  construction. 

Manifestly,  the  single  front  lessened  the  risk  of  technical  errors, 
but  we  have  never  been  able  yet  to  find  a  single  front  objective  of 
the  old  achromatic  dry  construction  which  has  shown  any  superiority 
over  a  similar  one  possessing  a  triple  front. 

The  single  front  employed  with  two  combinations  at  the  back 
was  the  form  in  which  the  celebrated  water-immersion  objectives 
of  Powell  and  Lealand  were  made.  It  was  by  one  of  these  that  the 
striae  on  Amphipleura  pellucida  were  first  resolved.  Indeed,  what  is 
known  as  the  water-immersion  system  of  objectives,  devised  by 
Professor  Amici,  was  the  next  advance  upon  the  old  form  ;  it  should, 
however,  be  remembered  that  as  early  as  1813  achromatic  water - 
immersion  lenses  had  been  suggested  by  Sir  D.  Brewster,  but  it  was 
an  advance  the  optical  principles  of  which  were  certainly  not  at  the 
time  understood. 

In  Paris,  Prazmowski  and  Hartnack  brought  these  objectives  to 
great  perfection,  and  were  enabled  to  take  the  premier  place  against 
all  competitors  at  the  Exhibition  of  1867.  The  next  year,  however, 
Powell  and  Lealand  adopted  the  system,  and  in  turn  they  distanced 
the  Paris  opticians  and  produced  some  of  the  finest  objectives  ever 
made.  Their  '  New  Formula '  water-immersions  were  made  after 
the  fine  model  of  Tolles  referred  to  below,  and  had  a  duplex  front, 
a  double  middle,  and  a  triple  back.  In  1877,  wrhen  the  water- 
immersion  system  touched  its  highest  point,  apertures  as  great  as 
1'23  were  reached;  and  in  America,  Spencer,  Tolles,  and  Wales 
produced  some  extremely  fine  lenses  of  large  aperture. 

During  the  year  1869  Wenham  experimented  with  and  sug- 
gested J  the  employment  of  a  duplex  front ;  that  is  to  say,  a  front 
combination  made  up  of  two  uncorrected  lenses  in  contradistinction 
to  an  achromatised  pair.  An  illustration  of  the  plan  suggested  is 
given  in  fig.  317,  which  hardly  appears  to  us  as  a  practicable  form, 
and  which  certainly  was  never  brought  to  perfection  or  put  into 
practice. 

But  in  the  month  of  August,  1873,  Tolles  actually  made,  on 
wrholly  independent  lines,  a  duplex  front  formula  for  a  •!-  glycerine 
immersion  of  110°  balsam  angle,  which  passed  into  the  possession  of 
the  Army  Medical  Museum  at  Washington.  There  can  be  little 
:doubt  but  this  objective  would  have  produced  a  much  deeper  im- 
pression but  for  the  fact  that  it  was  in  advance  of  its  immediate 
time. 

Tolles,  as  we  have  hinted  above,  used  the  duplex  front  in  the 
construction  of  some  of  his  immersion  objectives,  and  was  followed 
in  this  by  the  best  English  makers,  and,  in  the  case  of  a  celebrated 
^-inch  purchased  by  Mr.  Crisp,  Tolles  was  able  to  reach  a  balsam 
angle  of  96°. 

At  the  time  that  the  water-immersion  lenses  were  being  con- 
structed  by  rival   opticians  with  increasing  perfection,   the    great 
theory  of  Professor  Abbe  concerning  microscopic  vision,  the  impor- 
tance of  diffraction  spectra,  and  the  relation  of  aperture  to  power 
1  Monthly  Micro.  Jonrn.  Vol.  I.  p.  172. 


THE   INFLUENCE    OF   THE   DIFFRACTION   THEORY         363 

was  entirely  unknown.     In  the  absence  of  this  knowledge  wholly 
mistaken  value  was  attached  to  power  per  se  in  the  objective. 

With  a  focus  as  short  as  the  J--inch,  it  was  not  uncommon  to 
find  apertures  less  than  1-2,  while  objectives  of  ^  ^  -1TJ,  and  even 
higher  powers,  were  made  with  extremely  reduced  apertures.  This 
was  done  in  the  interests  of  the  common  belief  that  '  power  '- 
devoid  of  its  suitable  concurrent  aperture — could  do  what  was  so 
keenly  wanted. 

This  impression,  however,  was  far  from  universally  relied  oil ; 
there  were  several  earnest  workers  „  who,  without  being  able  to 
explain,  as  Abbe  subsequently  did,  why  it  was  so,  still  urged  the 
opticians,  in  the  manufacture  of  every  new  power,  especially  the 
higher  ones,  to  produce  the  largest  possible  amount  of  aperture ; 
and  the  evidence  of  this  is  still  to  be  found  in  the  objectives  they 
then  succeeded  in  obtaining.  But  there  can  be  no  doubt  that  a 
reckless  desire  for  magnifying  power,  all  other  considerations  apart, 
greatly  obtained ;  and  the  opticians  were  able  to  encourage  it,  for  it 
is  for  easier  to  construct  an  objective  of  high  power  and  low  aperture 
than  it  is  to  make  a  low  power  with  a  large  aperture. 


FIG.   317.— A    suggested                FIG.  318.— Combina-  FIG.  319.— Diagram  of 

combination   by  Wen-                     tion  for  '  homoge-  apochromatic    com- 

ham,  1869.                                           neous '  immersion  bination. 

by  Abbe. 

Thus  a  Tpinch  of  0*65  N.A.  will  be  far  more  expensive,  and  pro- 
bably not  as  well  corrected,  as  £  of  0'7  N".A.  The  Vinch  objective, 
even  if  a  good  one,  is  sure  to  exhibit  spherical  aberration,  while  the 
,V  of  low  aperture  will  show  many  minute  objects  with  considerable 
clearness,  especially  if  a  comparatively  narrow  illuminating  cone  be 
used. 

This  difference  becomes  still  more  conspicuous  as  the  difference 
between  aperture  and  power  grows  relatively  greater,  until  we  obtain 
ultimately  an  amplification  more  than  useless  from  its  utter  inability, 
on  account  of  deficiency  of  aperture,  to  grasp  details.1 

Up  to  1874,  however,  there  was  an  entire  absence  of  knowledge, 
even  on  the  part  of  the  leaders  in  microscopic  theory,  art,  and 
practice,  as  to  the  real  optical  principles  that  enabled  us  to  see  a 
microscopic  image,  and  consequently  to  understand  the  essential 
requirements  to  be  aimed  at  in  the  best  form  of  microscope.  But  in 
1877  Abbe's  great  Diffraction  Theory  of  Microscopic  Vision  appeared, 
which  has  led  to  changes  of  incomparable  value  in  the  principles  of 

1  Vide  Chapter  II. 


364  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETEK 

construction  of  objectives  and  eye-pieces,  and,  as  a  consequence,  has 
to  some  considerable  extent  given  a  new  character  to  the  entire  in- 
strument. Its  promulgation  has  indeed  inaugurated  an  entirely 
new  epoch  in  the  construction  and  use  of  the  microscope. 

The  general  character  and  the  details  of  Abbe's  theory  are  given 
in  the  second  chapter  of  this  treatise  ;  but  its  practical  bearing  upon 
the  theory  and  application  of  the  optical  part  of  the  instrument  was 
soon  manifest ;  for  in  1878  the  homogeneous  system  of  immersion 
objectives  l  was  introduced  as  a  logical  outcome  of  the  diffraction 
theory  of  microscopic  vision.  A  formula  for  a  ^-inch  objective  on 
this  system  was  prepared  by  Abbe,  to  whom,  we  learn  from  himself, 
it  had  been  suggested  by  Mr.  J.  "W.  Stephenson,  of  the  Royal 
Microscopical  Society.2  It  has  been  already  shown  3  that  the  homo- 
geneous system  was  so  called  because  it  employed  the  oil  of  cedar- 
wood  to  unite  the  front  lens  of  the  objective  to  the  cover-glass  of 
the  object,  in  the  same  way  as  water  had  been  employed  in  the 
ordinary  immersion  system, ;  but  as  there  was  a  practical  identity 
between  the  refractive  and  dispersive  indices  of  the  oil  and  those 
of  the  crown  glass  of  the  front  lens,  the  rays  of  light  passed 
through  what  was  essentially  a  homogeneous  substance  in  their 
path  across  from  the  balsam-mounted  object  to  the  front  lens,  and 
a  homogeneous  system  of  objectives  took  the  place  of  the  previous 
water  immersions. 

This  was  the  first  great  step  in  advance  in  optical  construction 
and  application  following  the  theory  of  Abbe. 

As  often  happens  in  matters  of  this  kind,  there  had  been  an 
apparent  anticipation  of  this  system  of  lenses  by  Amici  as  far  back 
as  1844  ;  but  it  is  very  apparent  that  Amici  employed  the  oil  of 
aniseed  without  any  clear  knowledge  of  the  principles  involved  in 
the  homogeneous  system,  being  wholly  unaware  of  either  the  increase 
of  aperture  involved  or  the  cause  of  it.  But  this  cannot  be  said  of 
Tolles,  of  New  York.  We  have  pointed  out  that,  as  early  as  1873, 
he  made  a  j^j-inch,  and  subsequently,  in  the  same  year,  a  |-inch 
objective,  each  with  a  duplex  front  to  work  in  soft  balsam,  and  with 
a  N.A.  of  1'27.  These  objectives  were  examined  by  the  late  Dr. 
Woodward,  of  the  Army  Medical  Department,  New  York,  and  with 
that  examination  were  allowed  to  drop.  For  Tolles  as  an  original 
deviser  of  a  practical  homogeneous  system  this  was  unfortunate  ;  for 
the  actual  introduction  of  the  system  in  a  form  capable  of  universal 
application,  and  worked  out  in  all  its  details  in  an  entirely  inde- 
pendent manner,  we  are  wholly  indebted  to  Abbe. 

The  principle  was  not,  nevertheless,  so  readily  and  warmly 
adopted  in  England  on  its  first  introduction  as  might  have  been 
anticipated.  This  arose  partly,  however,  from  the  fact  that  water 
immersions  had  been  brought  to  so  high  a  point  of  excellence  by 
Messrs.  Powell  and  Lealand  that  the  early  homogeneous  objectives 
were  not  possessed  of  more  aperture,  and  were  not  sensibly 
superior  to  the  best  immersions  made  in  England. 

The  homogeneous  objectives  were  made  with  duplex  fronts  and 

1  Chapter  II.        2  P.  27 ;  also  Journ.  Boy  Micro.  Soc.  Vol.  II.  1879,  p.  257. 
5  Chapter  I. 


THE   EXCLUSION   OF   THE    SECONDAKY   SPECTKUM         365 

two  double  backs.  A  general  diagram  of  their  mode  of  construction 
is  given  in  fig.  318. 

So  long  as  crown  glass  was  employed  in  their  manufacture,  and 
the  anterior  front  lens  was  a  hemisphere,  it  appeared  that  N.A.  1*25 
to  1*27  was  the  aperture  limit  they  could  be  made  to  reach. 
Messrs.  Powell  and  Lealand,  however,  by  making  the  anterior  front 
lens  greater  than  a  hemisphere,  increased  the  aperture  of  a  jV-inch 
objective  to  1'43  KA. 

This  front,  from  being  greater  than  a  hemisphere,  presented 
difficulty  in  mounting ;  this  was  at  first  overcome  by  cementing  its 
plane  surface  to  a  thin  piece  of  glass,  which  was  then  fixed  in  the 
metal.  Eventually,  however,  this  form  of  construction  was  changed 
by  these  makers  in  a  very  ingenious  manner ;  so  to  speak,  they 
entirely  inverted  the  combination,  and  accomplished  the  end  by 
making  the  front  of  flint.  By  this  means  they  obtained  apertures 
which  have  not  as  yet  been  equalled  by  any  other  makers,  reaching 
in  a  ^,  a  rV,  and  a  ^  a  IS". A.  of  1'50  out  of  a  theoretically  possible 
aperture  of  1'52.  Professor  Abbe  has  since,  it  is  true,  made  an 
objective  with  a  numerical  aperture  of  1'63,  but  this  requires  the 
objects  to  be  mounted  and  studied  in  a  medium  of  corresponding 
refractive  index,  and  consequently,  in  the  present  state  of  our  know- 
ledge of  the  subject  of  media,  not  applicable  to  the  investigation  of 
ordinary  organic  structures — certainly  not  of  living  things. 

These  objectives  fully  occupied  the  microscopist  until  1886,  when 
the  most  important  epoch  since  the  discovery  and  application  of 
achromatism  was  inaugurated. 

We  have  already  pointed  out  in  detail x  that  it  was  the  great 
defect  of  the  ordinary  crown  and  flint  achromatics  that  two  colours 
only  could  be  combined  and  that  the  other  colours  caused  out-of- 
focus  images,  which  appeared  as  fringes  round  the  object.  This  was 
what  was  known  as  the  residuary  secondary  spectrum. 

In  like  manner,  it  has  been  shown  that  it  was  not  possible  in  the 
flint  and  crown  achromatic  to  combine  two  colours  in  all  the  zones 
of  the  objective,  so  that  if  two  given  colours  are  combined  in  the  in- 
termediate zone  they  will  not  be  combined  in  the  peripheral  and 
the  central  portions  of  the  objective. 

These  phenomena,  it  has  been  pointed  out,1  arise  from  what  is 
known  as  the  irrationality  of  the  spectrum.  To  correct  this  we  have 
seen  that  Drs.  Abbe,  Schott,  and  Zeiss  directed  their  attention  to 
the  devising  of  vitreous  compounds  which  should  have  their  dis- 
persive powers  proportional  to  their  refractive  indices  for  the  various 
parts  of  the  spectrum.  Only  by  these  means  could  the  outstanding 
errors  of  achromatism  be  corrected. 

It  is  therefore  a  fact  that  the  old  flint  and  crown  objectives, 
whether  for  the  microscope,  the  telescope,  or  the  photographic 
camera,  are,  strictly  speaking,  neither  achromatic  nor  aplanatic. 

Glass  whose  properties  far  more  nearly  approximated  the  theo- 
retical requirement  than  any  previously  attainable  having  been 
manufactured  by  the  Jena  opticians,2  Abbe  was  able  to  produce 
objectives  entirely  cleansed  of  the  secondary  spectrum.  From  calcu- 
1  Chapter  I.  *  Chapter  II. 


366  OBJECTIVES,    EYE-PIECES,   THE   APERTOMETER 

lations  of  a  most  elaborate  and  exhaustive  kind  made  by  Dr.  Abbe, 
objectives  are  made  by  Zeiss  which  not  only  combine  three  parts  of 
the  spectrum  instead  of  two,  as  formerly,  but  are  also  aplanatic 
for  two  colours  instead  of  for  one.  This  higher  stage  of  achromatism 
Abbe  has  called  apochromatiim. 

A  general  plan  of  the  cor. struction  of  an  apochromatic  objective 
as  made  by  Zeiss  is  shown  in  fig.  319,  which,  it  will  be  understood,  is 
diagrammatic,  but  sufficiently  illustrates  the  elaborate  corrections  by 
which  the  perfect  results  given  by  these  objectives  are  accomplished. 
But,  in  addition  to  their  form  of  construction  and  the  special  optical 
glass  of  which  they  are  composed,  it  is  now  known  that  they  owe 
much  of  their  high  quality  to  the  use  of  fluorite  lenses  amongst  the 
combination.  Fluorite  is  a  mineral  which  has  lower  refractive  and 
dispersive  indices  than  any  glass  that  has  yet  been  composed,  and 
therefore  by  its  introduction  the  optician  can  reduce  the  spherical 
and  chromatic  aberrations  greatly  below  that  reached  by  achromatic 
combinations  of  the  known  type. 

It  is  a  somewhat  depressing  fact  that  fluorite  is  very  difficult  to 
procure  in  the  clear  condition  needful  for  the  optician,  but  from  what 
we  have  seen  the  optician  can  do  in  the  manufacture  of  glass,  we 
may  hope  that  an  equivalent  of  this  mineral  in  all  optical  qualities 
may  be  discovered. 

The  medium  for  mounting  and  immersion  contact  has,  of  course, 
to  be  of  a  corresponding  refractive  and  dispersive  index  in  all  ob- 
jectives of  great  aperture,  and  it  is  insisted  by  Abbe  that  the  glass  of 
which  the  mount  is  made,  both  slip  and  cover,  must,  when  the  limit  of 
refraction  by  crown  glass  is  passed  by  the  objective,  be  of  flint  glass. 
This  he  presents  as  a  sine  qua  non  in  the  case  of  the  new  objective 
made  a  few  years  since  by  the  house  of  Zeiss,  and  a  specimen  of 
which  has  been  generously  given  by  the  firm  to  the  Royal  Micro- 
scopical Society.  This  glass  has  a  numerical  aperture  of  1'63;  in 
a  subsequent  chapter  on  the  present  state  of  our  knowledge  as  to 
the  ultimate  structure  of  diatoms  we  are  enabled  to  present  the 
results  of  some  of  the  photo-micrographs  produced  by  its  means. 
But  it  may  be  noted  that  very  much  will  depend  upon  the  N.A.  of 
the  illuminating  cone  which  can  be  employed  with  it — not  theoreti- 
cally, but  practically,  and  it  is  for  practical  purposes  of  no  value  to 
the  student  of  minute  life,  because  the  highly  refractive  and  dis- 
persive medium  needed  to  make  the  object  mounted  homogeneous  is 
destructive  of  life,  and  even  of  organic  tissues.  Such  value  as  it 
may  have  is  therefore  confined  entirely  to  the  examination  of 
silicious  and  other  indestructible  organic  or  inorganic  products. 

Before  leaving  this  part  of  our  subject  we  note  with  pleasure 
that  Mr.  Nelson  has  computed  a  triplex  front  of  minimum  aberra- 
tion suitable  for  an  oil-immersion  condenser.  We  illustrate  it 
in  fig.  320.  The  data  for  this  are  as  follows,  viz.  : — 

O  is  the  object  and  V  its  virtual  image  ;  the  hyperhemispherical 
front  is  aplanatic  for  these  two  points.  The  scale  of  the  drawing  is 
arranged  so  that  the  distance  of  the  vertex  A  of  the  front  lens  to 
the  object  0  is  one  inch.  The  three  lenses  are  made  of  borosilicate 
glass,  No.  5  in  the  Jena  catalogue,  ju  =  l'51  ;  and  as  the  reciprocal  of 


IMMERSION   FRONT   FOR    CONDENSER   BY   NELSON        367 

the  dispersive  power  is  64*0,  the  chromatic  aberration  of  the  triplet 
is  very  small.     Moreover  the  glass  is  hard  and  perfectly  safe  to  use. 

Radii :  curve  A=  +  -602 

B  =    00 

C  =-f3-434 

D=  + 1-280  ' 
E=  — 15-078 
F=  + 2-359 
Diameters  :  lens  FE=2'45 

DC=2-1 

Distance  between  surfaces  :  ED ='05 

CA=-03 


FIG.  320. — Nelson's  new  immersion  front  for  a  condenser. 

Thickness  AB=-683. 

Working  distance  BO=-317. 

Diameter  of  the  plane  surface  B  of  front  lens=  1*192,  AO=1'0, 
AY=1-51. 

The  angle  c  =  62°,  and  <£=35°  47';  the  numerical  aperture  of 
the  combination  is  therefore  1*33  X.A. 

The  front  lens  AB  is  aplanatic ;  the  spherical  aberration  of  the 

yi 

next  two  DC.  FE  only  amounts  to  — '214  "~.     The  back  correcting 


368 


OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 


lens,   which   might  be    a   triplet,    will   require  to  have   +'214   ^ 

F 
of  spherical  aberration  to  render  the  whole  combination  aplanatic. 

On  the  whole,  and  for  the  purposes  of  practical  and  prolonged 
"biological  investigation,  it  is  to  the  dry  apochromatics  that  we  are 
most  indebted,  and  from  their  use  we  shall  derive  the  largest  benefit. 

As  no  subject  is  really  of  more  importance  than  a  clear  under- 
standing of  the  difference  of  action  of  chromatic,  achromatic,  and 
apochromatic  lenses,  we  venture  to  present  a  diagrammatic  illustra- 
tion, which,  while  not  strictly  accurate,  will  carry  with  it  no  error, 
as  a  popular  illustration  of  this  important  subject. 

In  fig.  321,  1,  2,  3,  we  have  representations,  as  truly  as  they  can 
be  drawn,  of  zones  of  equal  light ;  that  is  to  say,  the  peripheral  zone 
will  transmit  an  amount  of  light  equal  to  that  given  either  by  the 
intermediate  zone  or  the  central  circle.  Let  them  therefore  be 
called  equilucent  zones. 


If  we  assign  a  numerical  value  for  the  visual  intensity  of  the 
whole  spectrum,  say  100,  made  up  of  the  following  parts,  viz. : — 


Red 

Orange-yellow 
Yellow-green  . 
Blue 


15 
40 
30 
15 


then  if  in  any  one  of  the  equilucent  zones  the  whole  spectrum  is 
brought  to  a  focus,  we  shall  have  for  that  zone  100  as  its  effective 
value. 

But  the  entire  object-glass  is  divided,  as  in  the  diagram,  into 
three  equilucent  zones ;  consequently  300  will  represent  the  value  of 
the  whole  lens,  provided  the  whole  of  the  spectrum  is  brought  to  the 
same  focus. 

By  referring  to  the  diagrams  we  see  that  in  a  non-achromatic 
lens  (fig.  321,  3)  we  shall  get  only  40,  because  only  one  part  of  the 
spectrum  is  brought  to  the  focus  in  its  intermediate  zone  ;  and  as 
spherical  aberration  causes  the  light  which  passes  through  the  other 
zones  to  be  brought  toother  foci,  they  for  all  practical  purposes  might 
be  stopped  out. 

In  the  achromatic  lens  we  have  (fig.  321,  1)  in  the  intermediate 
zone  two  parts  of  the  spectrum  combined,  as  40  +  30=70,  and  one 


ZEISS'S   APOCHROMATICS  369 

in  each  of  the  other  zones  is  also  brought  to  the  same  focus,  sav  .'>() 
in  the  outer  zone,  and  40  in  the  centre  circle.  The  result  is  that 
the  whole  achromatic  lens  gives  a  total  of  light,  on  the  principle  stated 
above,  of  30  +  70  +  40=140.  In  the  apochromatic  system,  how- 
ever (fig.  323,  2),  we  find  in  the  intermediate  zone  three  parts  of  the 
.spectrum  united  ;  that  is  to  say,  40  +  30  +  15  =  85  ;  and  two  in  each 
of  the  others,  say,  40  +  30=70.  Thus  an  apochromatic  objective 
will  give  70  +  85  +  70=225. 

Recalling  the  suppositions  we  have  made  for  the  purpose  of  this 
graphic  presentation  of  a  difficult  subject,  it  will  be  seen  that  a  non- 
achromatic  objective  would  give  40,  an  achromatic  140,  and  an 
apochromatic  225  out  of  a  possible  iotal  of  300. 

This  illustration  might  be  exceeded  in  severe  accuracy,  but 
scarcely  in  simplicity,  and  it  sufficiently  explains  from  this  point  of 
view  alone  the  vast  gain  of  the  apochromatic  system. 

It  is  interesting  to  note  that,  while  the  microscope  in  its  earlier 
form  took  its  powerful  position  by  borrowing  achromatism  from  the 
telescope,  it  has  now  led  the  way  to  the  apochromatised  state,  which 
without  doubt  it  will  be  the  work  of  the  optician,  in  constructing 
the  telescope  of  the  immediate  future,  to  follow. 

We  would  beg  the  reader  to  bear  in  mind  in  the  purchase  of 
objectives  that,  whilst  the  vitreous  compounds  with  which  Abbe's 
beautiful  objectives  are  constructed  are  now  accessible  to  all  opticians, 
and  whilst  without  these  Abbe's  objectives  could  never  have  been 
constructed,  yet  it  does  not  by  any  means  follow  that  because  an 
objective  is  MADE  with  the  Abbe-Schott  glass  it  is  therefore  apo- 
chromatic ;  the  secondary  spectrum  must  be  removed,  and  the  spherico- 
chromatic  aberration  balanced,  or  it  is  '  apochromatic '  only  by  mis- 
nomer. It  is  another  feature  of  these  objectives,  which  it  is  import- 
ant to  note,  that  they  are  so  constructed  that  the  upper  focal  points 
of  all  the  objectives  lie  in  one  plane,.  Now  as  the  lower  focal  points 
of  the  eye-pieces  are  also  in  one  plane,  it  follows  that,  whatever  eye- 
piece or  whatever  objective  is  used,  the  optical  tube-length  will 
remain  the  same. 

Professor  Abbe  has  found  1  that  in  the  wide-aperture  objective 
of  high  power  there  is  an  outstanding  error  which  there  is  no 
means  of  removing  in  the  objective  alone,  but,  as  we  have  already 
explained,  this  is  left  to  be  balanced  by  an  over-corrected  eye-piece. 
As  this  peculiarity  pertains  only  to  the  higher  powers,  a  correspond- 
ing error  had  to  be  intentionally  introduced  into  the  lower  powers  in 
order  that  the  same  over-corrected  eye-pieces  might  be  available  for 
use  with  them. 

It  appears  worthy  of  note  in  this  relation  that  one  of  the  best 
forms  for  the  combination  of  three  lenses  is  that  kiio\vn  as  Steinheil's 
formula,  which  consists  of  a  bi-convex  lens  encased  in  two  concavo- 
convex  lenses.  It  will  be  observed  by  reference  to  the  figure  illustrat- 
ing the  apochromatic  lens  construction  (fig.  319)  that  this  is  largely 
made  use  of.  In  some  instances  the  encasing  lenses  possess  sufficient 
density,  with  regard  to  the  central  bi  -convex  lens,  to  altogether  over- 
power it,  the  result  being  a  bi-convex  triple  with  a  negative  focus. 

1  Chapter  II. 

B  B 


37O  OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 

It  is  another  distinctive  feature  of  the  3  mm.  objective  that  it 
has  a  triplex  front ;  thus  Zeiss's  3  mm.  (=  £  inch  focus)  had  the 
errors  from  three  unconnected  lenses  balanced  by  two  triple  backs, 
i.e.  nine  lenses  taken  together,  but  it  has  since  been  constructed  on  a 
different  formula. 

The  foci  of  the  set  of  apochromatic  lenses  now  made  by  Zeiss 
are  integral  divisions  of  what  may  be  termed  a  unit  lens  of  24  mm. ; 
24  he  chooses  as  a  means  of  avoiding  the  inconveniences  inseparable 
from  the  use  of  the  decimal  system.1  The  unit  lens  is  therefore  a 
little  higher  than  1  inch  in  power.  In  the  series  of  dry  lenses  there 
are  two  powers  of  the  same  aperture.  Thus  24  mm.  arid  16  mm., 
corresponding  to  English  1  inch  and  f  inch,  each  has  an  aperture  of  '3  ; 
a  12  mm.  and  8  mm. = English  ^  inch  and  £  inch,  have  each  an 
aperture  of  '65 ;  while  a  6  mm.  and  a  4  mm.  =  J  inch  and  ^  inch, 
have  both  an  aperture  of  '95. 

There  are  also  water- immersions  :  a  2'5  mm.  =  -^  inch,  with 
N.A.  T25,  and  two  oil-immersions  respectively  3  mm.  and  2  mm. 
=  £  inch  and  TL  inch,  both  being  made  either  with  1'3  or 
1-4  N.A. 

Apart  from  these,  intended  to  be  used  for  photographic  purposes 
without  an  eye-piece,  is  a  70  mm.  =  a  3-inch,  also  a  35  mm.  or 
1^-inch  objective. 

With  the  exception  of  the  6  mm.,  4mm.,  and  2'5  mm.  objectives, 
which  have  the  screw-collar  adjustment,  this  series  have  rigid  mounts, 
correction  being  secured  by  alteration  of  the  tube-length. 

The  performance  of  these  lenses,  as  they  are  now  made,  is  of  the 
very  highest  order.  They  present  to  the  most  experienced  eye  unsur- 
passed images.  They  are  corrected  with  a  delicate  perfection  which 
only  this  system,  coupled  with  technical  execution  of  the  first  order, 
can  possibly  be  made  to  produce.  The  optical  polish,  the  centring, 
the  setting,  and  the  brass  work  certainly  have  never  been  surpassed. 

It  is  a  matter  also  worthy  of  note  that  Zeiss's  apochromatic 
series  of  objectives  are  true  to  their  designations  as  powers.  The 
^-inch  is  such,  and  not  a  ^-inch  designated  ^-inch.  This  was 
equally  true  of  the  early  achromatics.  A.  Ross  produced  a  J-inch 
under  that  name.  One  now  before  us,  made  fifty  years  ago,  has  an 
initial  power  of  41  ;  and  that  of  ^  inch  has  an  initial  power  of  21. 
But  modern  achromatics  of  fair  aperture  are  always  greatly  in. 
excess  of  their  designated  power ;  J  are  nearly  ^-inch.  A  ^-inch 
of  40°  has  an  initial  power  of  25,  and  is  a  fL-inch ;  -^,-inch 
objectives  are  in  reality  ^-inch ;  and  J-inch  objectives  of  90°  and 
upwards  have  initial  powers  of  50  instead  of  40,  which  they  should 
have,  so  that  they  are  in  reality  iths  ;  some  in  fact — by  no  means 
uncommon — have  an  initial  power  of  60,  and  are  actually  ^th-inch 
objectives. 

This  is  explicable  enough  from  the  maker's  point  of  view  ;  it  is 
far  easier  to  put  power  into  an  object-glass  than  aperture.  It  is 

1  Although  the  foci  of  the  lenses  are  expressed  in  integers,  with  the  single  excep- 
tion of  the  water -immersion  2'5  mm.,  there  are  inconvenient  decimal  fractions  in  the 
initial  magnifying  power  of  all  the  series  except  those  of  2'5  and  2  mm.  focus. 


HISTOLOGICAL  ADVANTAGE    OF   HIGH   POWEE  371 

easier  to  make  a  J-inch  of  100°  than  a  J  with  100°  ;  the  result  is 
that  low  powers  with  suitably  wide  apertures  are  costly. 

In  the  Zeiss  apochromatic  series  of  objectives  the  24  mm.  of  -3 
N.A.  and  12  mm.  of  '65  N.A.  may  be  considered  as  lenses  of  the 
very  highest  order ;  the  relation  of  their  aperture  to  their  power  is 
such  that  everything  which  a  keen  and  trained  eye  is  capable  of 
taking  cognisance  of  is  resolved  when  the  objective  is  yielding  a 
magnification  equal  to  twelve  times  its  initial  power  ;  for  this  purpose 
an  objective  must  have  0'26  N.A.  for  each  hundred  diameters  of 
combined  magnification.  Under  these  conditions  an  object  is  seen  in 
the  most  perfect  manner  possible.  In  this  connection  Mr.  Nelson 
has  suggested  *  that  the  term,V  optical  index '  should  be  added  to 
that  of  the  numerical  aperture.  The  optical  index  or  O.I.  is  the 
ratio  of  the  numerical  aperture  (  X  1000)  to  the  initial  magnifying 
power.  Thus  the  numerical  aperture  of  the  Zeiss  apochromatic 
24  mm.  is  '3,  and  its  initial  power  1 0.  Then  its  O.I.  is  \°J  =  30.  The 
O.I.  of  the  12  mm.  apochromatic  of  '65  N.A.  is  6/V°=  31.  That  of 
the  j-  homogeneous  immersion  of  1'4  N.A.  is  J-|jp=  17.  Compare 
now  these  figures  with  an  old  water-immersion  ^  of  1*1  N.A.  VsV 
=  2'0.  The  value  of  these  figures  will  be  apparent  when  we 
remember  that  any  lens  used  with  a  10  power  eye-piece  must  have 
an  O.I.  of  26  to  resolve  all  detail  visible  to  a  keen  eye. 

The  optical  index  therefore  tells  us  that  the  -^  water-immersion 
of  I'l  N.A.  had  a  vast  amount  of  empty  magnifying  power,  while  on 
the  other  hand  the  24  and  12  mm.  will  both  stand  a  higher  eye- 
piece than  10  ;  nay,  even  require  it  before  the  detail  resolved  by  them 
is  made  visible  to  the  eye.  It  also  shows  that  the  ^  of  1*4  N.A.  will 
stand  a  higher  eye-piece  without  arriving  at  an  empty  magnifying 
power  than  the  j^ef  1*4  N.A.,  whose  O.I.  is  ll'O. 

As  it  is  more  difficult  to  put  aperture  into  a  lens  than  power,  the 
O.I.  becomes  also  an  index  of  the  money  value  of  a  lens.  Thus  the 
J  mentioned  above  that  had  an  initial  magnifying  power  of  60  and 
N.A.  of  '8  ought  to  be  a  cheaper  lens  than  a  true  J  with  an  initial 
magnifying  power  of  40  and  a  N.A.  of  '9,  their  optical  indices 
being  13  and  22  respectively.  The  limit  of  combined  power  for  best 
definition  with  any  objective  of  any  given  aperture  may  be  found  by 
multiplying  its  N.A.  by  400.  Example  :  The  limit  of  power  for  best 
definition  with  a  §  of  '3  N.A.  is  120  diameters.  The  converse  rule 
may  be  stated  thus  :  The  ideal  N.A.  for  any  objective  whose  initial 
power  is  known  can  be  found  by  multiplying  its  power  by  '025. 
Example  :  The  ideal  N.A.  for  a  |  of  power  20  is  20  x  '025  =  '5  N.A. 

It  may  be  well  for  the  student  to  prove  this,  which  may  be 
readily  done. 

Take  a  suitable  object,  such  as  a  well-prepared  proboscis  of  a 
blow-fiy,  and  examine  it  under  critical  illumination  with  the  24  mm. 
•3  N.A.  (=  1-inch)  objective,  and  a  12  compensating  eye-piece. 
Note  with  close  attention  every  particular  of  the  image :  the 
resolution  of  the  points  of  the  minute  hairs,  the  form  of  the  edges  of 
the  cut  suctorial  tubes,  the  extent  of  the  surface  taken  into  the 
t  field,'  and  the  relation  of  all  the  parts  to  the  whole. 

1  Journ.  It.  M.  S.  1893,  p.  12. 

B  B  9. 


372  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETER 

Now  change  the  objective  for  the  16  mm.  '3  N.A.  (=  f,  but 
with  the  same  aperture).  Nothing  more  is  to  be  seen  ;  the  most 
dexterous  manipulation  cannot  bring  out  a  single  fresh  detail ;  the 
resolution  is  in  no  sense  carried  farther  ;  the  cut  suctorial  tubes  were 
in  fact,  in  our  judgment,  better  seen  with  a  lower  power,  while  with 
it  all  of  course  a  smaller  extent  of  the  object  occupies  the  '  field.' 

It  can  in  fact  be  scarcely  doubted  that  the  picture  presented 
by  the  §  is  a  distinct  retrogression  in  every  sense  compared  with 
that  presented  by  the  1-inch  when  both  are  equally  well  made 
and  have  equal  apertures,  viz.  *3.  But  beyond  all  this,  whatever 
may  be  done  by  the  16  mm.  *3  N.A.  can  be  accomplished  in  an 
equally  satisfactory  manner  by  removing  the  1 2  eye-piece  and 
replacing  it,  with  practically  no  other  alteration,  by  [an  18  eye-piece  ; 
and  still  higher  results  can  be  obtained  without  the  slightest  detri- 
ment to  the  image  by  using  an  eye-piece  of  27. 

Not  less  interesting  and  convincing  will  it  be  to  examine  the 
same  object  with  a  12  mm.  '65  N.A.  (=  ^-inch),  and  an  A  Zeiss 
achromatic  of  *20  N.A.  (=  §rds  inch),  using  a  12  eye-piece.  Those 
who  may  still  retain  some  conviction  as  to  the  value  of  '  low-angled 
glasses  to  secure  penetration  '  can  want  no  further  evidence  of  its 
entire  fallacy  than  such  a  simple  experiment  affords. 

For  those  who  prefer  it,  a  true  histological  object  may  be  selected. 
We  choose  a  portion  of  a  frog's  bladder  treated  wTith  nitrate  of 
silver,  in  which  are  some  convoluted  vessels,  enclosed  in  a  muscular 
sheath  which  had  contracted. 

This  object  is  presented  by  photo  -micrograph  in  figs.  7  and  8  of 
the  frontispiece.  In  fig.  7  the  vessel  in  the  frog's  bladder  is  seen 
by  a  Zeiss  A  *2  N.A.  magnified  140  diameters.  The  object  of  the 
photograph  is  to  expose  the  fallacy  which  underlies  the  generally 
accepted  statement  that  low-angled  glasses  are  the  most  suitable  for 
histological  purposes.  The  assumption  is  founded  on  the  fact  that 
the  penetration  of  a  lens  varies  inversely  as  its  aperture,  and  it  is 
taken  for  granted  that  '  depth  of  focus '  will  be  obtained,  not  to  be 
secured  by  large  apertures,  and  therefore  it  is  taken  for  granted 
that  we  are  enabled  to  see  into  the  structure  of  tissues. 

In  examining  the  illustration  (which  will  with  advantage  permit 
the  use  of  a  lens)  it  will  be  seen  that  scarcely  an  endothelium  cell  can 
be  clearly  seen.  A  sharp  outline  is  nowhere  manifest,  because 
the  image  of  one  cell  is  confused  with  the  outlines  of  others  upon 
which  it  is  superposed.  We  have  seen  that  there  is  no  perspective 
proper  in  a  microscopic  image ;  therefore  it  is  better  to  use  high 
apertures  in  objectives,  and  obtain  a  clear  view  of  one  plane  at  one 
time,  and  train  the  mind  to  appreciate  perspective  by  means  of  focal 
adjustment. 

It  will  be  admitted  that  no  clear  idea  of  what  an  endothelium 
cell  is  can  be  obtained  from  fig.  7. 

But  fig.  8  (frontispiece)  represents  the  same  structure  slightly 
less  magnified  (x  138)  by  means  of  an  apochromatic  \  N.A.  '65. 
Here  only  the  upper  surface  of  the  tube  is  seen  ;  but  the  endothe- 
lium cells  can  be  clearly  traced,  and  a  sharp  definition  is  given  to 


HISTOLOGICAL  ADVANTAGE   OF   LARGE    APERTURE       373 

every  cell.     The  circular  elastic  tissue   is  also  displayed,  while  the 
whole  image  has  an  increased  sharpness  and  perfection. 

Thus,  with  the  objective  (A  -20  X.A.  =  frds  inch)  of  lower 
aperture,  the  endothelium  cells  can  be  seen',  but  when  the  image  is 
compared  with  that  of  the  objective  of  wider  aperture  (*65  N.A.), 
the  former  image  is  found  to  be  dim  and  ill-defined.  The  muscular 
sheath  is  so  ill-defined  that  it  would  not  be  noticed  at  all  if  it  had 
not  been  clearly  revealed  by  the  objective  of  wider  aperture.  But, 
011  the  other  hand,  the  objective  of  greater  aperture  not  only  shows  - 
the  muscular  sheath,  but  it  also  shows  the  elongated  nuclei  of  the 
muscle  cells  ;  and  at  the  same  time  brings  out  the  convoluted  vessels 
lying  in  the  muscular  sheath  a§  plainly  as  if  it  were  an  object  of 
sufficient  dimensions  to  lie  upon  the  table  appealing  to  the  unaided 
eye. 

We  have  pointed  out  in  the  proper  place,1  that  although  '  pene- 
trating power  '  varies  inversely  as  the  numerical  aperture,  it  also 
varies  inversely  as  the  square  of  the  power. 

Now,  from  what  we  know  of  histological  teaching  in  this  country, 
\vc  do  not  hesitate  to  say  that  a  histologist  would  not  have  attempted 
to  examine  the  above  object  with  even  a  Zeiss  A  objective.  He 
would  have  advised  the  use  of  '  the  J-inch,'  of,  perhaps,  '65  aperture  ; 
but  by  so  doing  he  would  have  secured  only  one-third  of  the  pene- 
trating power  qua  aperture,  and  one-seventh  of  the  penetrating  power 
qua  power. 

It  is  manifest,  then,  that  pursuing  this  course  in  the  histological 
laboratory  defeats  the  end  sought,  and  which  it  is  so  desirable  to 
attain. 

It  is  absolutely  unwise  to  use  a  higher  power  than  is  needful. 
A  j-inch  where  a  Viiich  would  answer  involves  loss  in  many  ways, 
and  would  never  be  resorted  to  if  the  aperture  of  the  lenses  employed 
n-ere  as  great  as  the  power  used  legitimately  permitted? 

A  given  structure,  to  be  seen  at  all,  must  have  a  given  aperture  ; 
to  obtain  this,  as  objectives  now  made  for  laboratory  purposes  run, 
they  are  obliged  to  use  too  high  a  power.  The  result  is  that  in  seek- 
ing to  avoid  what  is  accounted  the  loss  of  '  penetrating  power '  at 
an  inverse  ratio  to  the  aperture,  it  is  forgotten  that  we  are  losing  it 
inversely  as  the  square  of  the  power ! 

Moreover,  the  two  apochromatic  objectives  we  have  already 
referred  to  as  test  lenses  are  equally  able  to  show  the  value  of 
apochromatism,  not  so  much  on  account  of  the  removal  of  the 
secondary  spectrum  as  for  the  reduction  of  the  aberrations  depend- 
ent  on  the  irrationality  of  the  spectrum  in  ordinary  achromatics. 

Use  the  12  mm.  '65  N.A.  objective.  Place  a  diatom  in  balsam 
in  the  focus  of  it  on  a  dark  ground ;  the  diatom  will  shine  with  a 
silvery  whiteness,  and  the  image  will  be  wholly  free  from  fog. 

Xow  take  one  of  the  best  achromatics  obtainable  of  Vinch 
focus  of  80°  (almost  certainly  a  ^  in  power)  and  examine  the  same 
diatom  in  the  same  circumstances ;  it  will  be  bathed-  in  fog.  If, 
however,  the  achromatic  objective  is  an  exceptionally  good  one,  and 
we  reduce  its  aperture  to  60°,  we  shall  get  a  fair  picture  of  the 
1  Chapter  I.  2  Chapter  II. 


374  OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 

diatom — one  indeed  that  was  considered  critical  until  that  with  the 
apochromatic  was  seen.  But  in  comparison  it  is  dull  and  yellowish. 
From  which  it  follows  that  an  exceptionally  fine  achromatic  -j^-inch 
of  60°  or  '5  N.A.  will  not  suffer  comparison  of  the  image  it  yields 
with  that  of  an  apochromatic  Vinch  of  "65  N.A. 

Speaking  generally  on  the  whole  question,  then,  it  would  be  the 
utmost  folly  for  histologists  or  opticians  to  shut  their  eyes  to  the 
magnificent  character  of  the  series  of  dry  apochromatics  of  Zeiss, 
ranging  from  1  inch  (24  mm.)  to  j?  inch  (4  mm.  '95  N.A.).  They 
are  the  most  perfect  and  efficient  series  of  objectives  ever  placed  in 
the  hands  of  the  worker ;  and,  unless  English  lenses  on  a  truly 
apochromatic  principle  and  equal  quality  are  produced,  it  must  be 
to  the  detriment  of  either  the  opticians  or  the  workers  of  this 
country. 

Nor  need  it  be  supposed  that  the  production  of  objectives 
approximate  to  these  must  be  costly ;  great  steps  have  been  taken 
lately  in  the  reduction  of  their  cost.  The  manufacture  of  the  Jena 
glass  has  indeed  wrought  an  entire  change  in  the  character  of 
objectives  now  produced  ;  and  although  the  very  finest  and  most 
costly  apochromatics  having  nuorite  used  in  their  construction  still 
hold  an  unrivalled  position,  yet  the  new  glass  admits  of  corrections 
so  nearly  perfect  that  some  stronger  word  than  achromatic  appeared 
to  be  needed,  and  the  word  semi -apochromatic  has  crept  in  and 
undoubtedly  designates  a  most  valuable  and  far  from  costly  set  of 
lenses  of  all  powers.  It  is  Leitz,  of  Wetzlar,  that  has  first  and 
efficiently  attacked  this  problem  and  provided  the  student  whose 
means  are  limited  with  objectives  of  a  very  high  class,  and  which 
come  remarkably  near  to  the  best  apochromatics.  We  would 
specially  call  attention  (wholly  in  the  interests  of  students)  to  No.  3 
(j-inch  N.A.  0*28)  at  a  cost  of  15s.  No.  5  is  an  equally  valuable 
and  admirable  objective  which  is  a  J-inch  O77  N.A.,  the  price  of 
which  is  25s.,  and  it  comes  so  near  to  an  apochromatic  as  to  require 
expert  judgment  to  discover  that  it  is  not.  He  also  makes  a  dry 

-iVincn  N-A-  O'8?  and  a  <%  6  of  >82  N-A-  at  a  cost  of  SI-'  which 
is  a  very  low  price  for  so  good  a  piece  of  optical  work.  Also  an 
oil-immersion  T^-inch  N.A.  T30  is  sold  for  31.  15s.  This  glass  is 
corrected  for  the  long  tube,  and  a  similar  ^ih  N.A.  1*30  for 
51.  resolves  secondary  diatom  structure  well,  and  it  is  hardly  dis- 
tinguishable from  an  apochromatic  lens ;  and  we  can  attest,  from 
personal  investigation,  the  value  of  each  of  these,  which  are  only 
selections  from  a  considerable  series,  all  of  which  wre  have  found 
to  be  reliable,  and,  when  examined  in  numbers,  very  few  indeed 
are  below  the  standard  quality.  But  such  work  is  so  much  needed 
that  it  is  not  likely  that,  with  the  glass  accessible  to  all,  it  will 
remain  the  peculiarity  of  one  maker ;  hence  we  find  that  Reichert 
follows  Leitz  so  closely  in  quality  and  price  that  it  is  not  easy  to 
distinguish  the  semi-apochromats  of  one  maker  from  the  other. 
Reichert's  No.  3  (4-inch  N.A.  O30)  is  17s.,  his  7A  (an  admirable 
lens)  i-inch  N.A.  0*87  is  II.  16s.  He  makes  a  high-class  oil- 
immersion  TL-inch  N.A.  1*30  for  SI.  And  of  apochromatic  lenses 
he  makes  a  2-inch  N.A.  O30  and  a  ^-inch  N.A.  O95  for  4£.  each, 


AMERICAN   OBJECTIVES — EYE-PIECE  375 

which,  so  for  as  we  have  seen  them  (and  we  have  examined 
many),  are  excellent.  Reichert's  semi-apochromatic  J  is  also  a  fine 
and  useful  lens,  and  his  jVinch  apochromatic  N.A.  1*30  has 
qualities  fitting  it  for  use  in  any  kind  of  research. 

But  we  confess  tjhat  it  is  a  matter  of  most  pleasant  surprise  to 
us  to  find  that  the  great  American  firm  of  Bausch  and  Lomb  are 
putting  upon  the  English  market  objectives  that  fairly  compete 
with  the  above  in  the  lowness  of  their  price,  while  their  optical 
quality  and  mechanical  work  are  of  the  best  order.  We  have 
examined  these  lenses  with  much  pleasure  ;  they  are  from  the  com- 
putations of  Professor  Hastings,  and,  considering  the  fact  that  they, 
in  all  the  higher  powders  especially,  are  so  low-priced,  their  correc- 
tions and  high  quality  are  beyotfd  all  praise.  We  would  specially 
call  attention  to  a  §-inch,  a  ^-inch,  and  a  ^-inch  which  we  have 
examined  thoroughly  and  with  approval  that  needs  no  quali- 
fication when  it  is  remembered  that  the  most  advanced  Continental 
opticians  have  not  touched  a  lower  price. 

Messrs.  R.  and  J.  Beck  are  making  good  objectives,  oil-immer- 
sion and  other,  and  one  of  their  TL-  oil-immersions  is  sold  at  the 
strikingly  low  figure  of  4£. 

Messrs.  Swift  and  Son  are  making  a  large  number  of  objectives, 
especially  apochromats  and  semi-apochromats,  and  they  have  long 
striven  to  supply  the  student  with  high-quality  lenses  at  the  lowest 
possible  price.  There  can  be  no  doubt  that  the  whole  secret  of 
success  in  this  matter  is  dependent  on  a  sufficiently  large  series  of 
experiments  to  determine  on  the  right  kind  of  glass,  so  as  to  produce 
the  highest  order  of  '  semi-apochromatism.' 

Messrs.  Watson  and  Sons  have  commenced  the  manufacture  of  a 
new  series  of  objectives  based  on  original  computations.  These 
promise  exceedingly  well.  We  have  examined  the  V  inch  and  the 
J-inch.  We  find  that  their  initial  powers  are  21  diameters  0'45  N.A., 
and  40  diameters  074  N.A.,  and  they  depend  for  aplanatic  results, 
which  are  admirable,  on  a  triple  back  lens.  The  objectives,  we 
believe,  will  be  valuable  as  a  series  when  complete.  They  do  not 
claim  to  be  amongst  the  very  low-priced  lenses;  but  they  claim, 
and  we  believe  they  will  possess,  some  of  the  best  qualities  which 
•should  be  aimed  at  in  microscopic  object-glasses. 

These  facts  are  of  importance  to  the  medical  student  and  to 
opticians  generally.  By  apochromatised  and  semi-apochromatised 
objectives  of  the  highest  order  the  work  of  present  and  future 
microscopy  will  be  done — that  is  inevitable.  To  thoroughly  under- 
stand what  its  very  best  results,  theoretically  and  practically,  must 
be  becomes  the  imperative  aim  of  the  optician  who  would  be 
abreast  of  the  direct  wTants  of  his  time ;  and  to  produce  the  nearest 
to  these  in  objectives  and  eye-pieces  at  the  lowest  possible  price 
is,  apart  from  all  other  issues,  to  be  a  direct  benefactor  of  true 
science. 

The  Eye-piece. — The  eye-piece,  sometimes  called  the  ocular,  is  an 
optical  combination,  the  purpose  of  which  is  so  to  refract  the  diverg- 
ing pencils  of  rays  which  form  the  real  object-image  that  they  may 
all  arrive  at  the  pupil  of  the  observer's  eye.  They  have  also  to  form 


376 


OBJECTIVES,    EYE-PIECES,    THE   APEETOMETER 


a  virtual  image  of  the  real  image  which  is  presented  to  them  as  the 
object.  For  this  purpose  a  combination  is  indispensable,  but  this 
may  be  varied.  There  are  ordinary  and  special  eye-pieces.  Those 
in  ordinary  use  separate  into  two  divisions  :  (1)  positive  eye -pieces 
and  (2)  negative  eye-pieces.  These  are  easily  Distinguished ;  with 
a  positive  eye-piece  we  can  obtain  a  virtual  image  of  an  object  by 
using  it  as  a  simple  microscope,  because  its  focus  is  exterior  to  itself. 
This  cannot  be  done  with  the  negative  eye-piece,  because  its  focus  is 
within  itself. 

The  eye-piece  in  common  use  is  negative,  and  is  generally  known 
as  Huyghens's,  and  sometimes  as  Campani's.  Monconys  appears  to 
have  been  the  first  (1665)  to  supply  the  field-lens  to  the  eye-lens  of 
the  microscope,  and  Hooke  in  1665  adopted  his  suggestion  ;  but 
how  far  Monconys  was  indebted  for  this  to  the  compound  eye-piece 
attributed  to  Huyghens  cannot  now  be  determined. 

This  instrument,  as  commonly  used  in  a  telescope,  consists  of 
an  eye-lens  and  a  field-lens,  each  being  plano-convex,  having  their 
convex  sides  towards  the  object,  their  foci  being  in  the  ratio  of 


FIG.  322. — Huyghenian  eye-piece. 


FIG.  823. — Kellner  eye-piece. 


3  :  1,  and  the  distance  between  them  being  equal  to  half  the 
sum  of  their  focal  lengths,  a  diaphragm  being  placed  in  the 
focus  of  the  eye-lens.  In  a  microscope  a  different  ratio  and  lens 
distance  is  employed,  the  fact  being  that  different  tube  lengths 
require  different  formulae.  The  general  form  of  a  Huyghenian  eye- 
piece is  shown  in  longitudinal  section  in  fig.  322.  This  makes  a 
very  convenient  form  of  eye-piece  of  5  and  10  magnifying  power ; 
but  when  the  power  much  exceeds  this  last  amount  the  eye -lens 
becomes  of  deep  curvature  and  short  focus,  so  that  the  eye  must  be 
placed  uncomfortably  near  the  eye-lens.  This,  however,  is  its  chief 
defect,  and  it  may  fairly  be  considered  the  best  ordinary  eye-piece. 

Another  negative  eye-piece  is  that  known  as  the  Keliner,  or 
orthoscopic.  This  consists  of  a  bi-coiivex  field-glass,  and  an  achromatic 
doublet  meniscus  (bi-convex  and  bi-concave)  eye-lens.  A  vertical 
section  of  one  so  constructed  is  seen  in  fig.  323.  These  eye-pieces 
usually  magnify  ten  times,  and  the  advantage  they  are  supposed  to 
give  consists  in  a  large  field  of  view  ;  but  they  are  not  good  in  practice 
for  this  very  reason  ;  they  take  in  a  field  of  view  greater  than  the 


NEW   HUYGHENIAN   EYE-PIECE 


377 


objective  can  stand,  and  as  a  rule  even  the  centre  of  the  field  will 
not  bear  comparison  in  sharpness  with  the  Huygheniaii  form. 

Mr.  ISTelson  has  recently  computed  and  had  made  a  Huygheniaii 
eye-piece  on  a  Avholly  new  formula  l  which  has  the  field  reduced  by 
about  7  inches,  yet  we  can  testify  that  in  use  it  gives  exceedingly 
sharp  images,  and  what  surprises  the  accustomed  worker  is  that  it 
acts  admirably  in  the  place  of  'compensated'  eye-pieces,  giving 
results  that  often  not  only  equal  but  surpass  these. 

The  power  of  this  eye-piece  is  12  ;  equivalent  focus,  -8,  corrected 
for  the  English  tube  (p  =  9'5). 

Fig.  324  is  enlarged  twice. 


FIG.  324. — Nelson's  new  formula  Huyghenian  eye-piece. 

Data:  Glass,  borosilicate  crown,  ^=1*51,  v=64*0,  Jena  cata- 
logue Ko.  5. 

Field-lens,  biconvex  r=  +   '94)    -,.  KK 

s=_2.94j  diameter  '55. 

Eye-lens,   biconvex  r'=-f-    '34)    -,. 

g/__1.Q1f  diameter  '30. 

Distance  of  eye-lens  from  field-lens,  measured  from  their  sur- 
faces, -97. 

Distance  of  diaphragm  from  surface  of  field  lens,  '48. 

Diameter  of  hole  in  diaphragm,  *26. 

Power,  12  ;  equivalent  focus,  '53,  corrected  for  the  Continental 
tube  (p=6-3). 

Data  :  Glass,  same  as  before. 

Field-lens,  biconvex  ^=  +  ^65)   diameter  .g5 

Eye-lens,  biconvex  /=+   -22  J  diameter  .20. 

Distance  of  eye-lens  from  field-lens,  measured  from  their  sur- 
faces, -66. 

Distance  of  diaphragm  from  surface  of  field-lens,  '34. 

Diameter  of  hole  in  diaphragm,  '16. 

These  eye-pieces  should  enter  the  tube  of  the  microscope  as  far 
as  their  diaphragms. 

Positive  Eye-pieces. — In   the  early  compound  microscopes  the 

1  J.B.M.S.  1900,  p.  165. 


378  OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 

eye-pieces  were  all  positive  ;  that  is  to  say,  they  consisted  of  a  single 
bi-convex  eye-lens  and  no  field-glass.  The  definition  with  this  must 
have  been  most  imperfect ;  the  addition  of  a  field-lens,  though  it 
were  a  bi-convex  not  in  the  correct  ratio  of  focus  nor  the  theo- 
retically best  distance,  must  have  been  considered  a  great  advance. 

In  this   way  matters  rested,    however,   until  the   theoretically 
perfect  Huyghenian  form  was   devised.     Object-glasses  have  been 
used  as  eye-pieces,   and  all  forms  of  loups  or  simple  microscopic 
lenses  have  been  employed  for  the  same  pur- 
pose.    Solid  eye-pieces   have   also  been   used 
both  in  England  and  America,  but  with  no 
results    that    surpassed    a    well-made    Huy- 
ghenian combination  ;  but  the  best  form  of  all 
of  the  combinations  which  have  been  tried  by 
us  as  positive  single  eye-pieces  are  the  Stein- 
heil  triple  loups ;  a  section  of  one  of  these  is 
FIG  325.  seen  in  %•  325-     This  combination  also  forms 

one  of  the  best  lenses  for  projection  purposes 

ever  constructed.  But  a  positive  eye-piece  was  devised  by  Bamsden, 
consisting  of  two  plano-convex  lenses  of  equal  foci ;  the  distance 
being  equal  to  two- thirds  the  focal  length  of  one.  The  diaphragm 
was  of  course  exterior. 

Abbe's  Compensating  Eye-pieces. — We  have  already  given  a 
general  description  of  the  nature  and  action,  in  connection  with  the 
apochromatic  objectives,  of  this  form  of  eye-piece.1  In  the  section 
above  on  objectives  we  have  referred  to  the  fact  that  these  eye-pieces 
are  over-corrected  ;  this  may  be  easily  seen  by  observing  the  colour 
at  the  edge  of  the  diaphragm,  which  is  an  orange-yellow.  If  we 
compare  this  with  the  colour  in  the  same  position  with  a  Huyghenian 
eye-piece,  this  will  be  blue,  being  seen  through  the  simple  unconnected 
eye-lens. 

There  are  three  kinds  of  compensating  eye-piece  as  designed  by 
Abbe.  These  are: — 

1.  Searcher  eye-pieces. 

2.  Working        ,, 

3.  Projection      ,, 

1 .  The  searcher  forms  are  negatives  of  very  low  power,  intended 
only  for  the  purpose  of  finding  an  object ;  they  consist  of  a  single 
field -lens  and  a  doublet  eye-lens. 

The  working  forms  are  both  positive  and  negative.  The  eye-piece 
for  the  long  tube  has  a  triplet  eye-lens ;  but  the  remainder,  viz.  8, 
12,  18,  and  27,  when  first  introduced,  were  all  positive.  The  8  was 
subsequently,  however,  changed  for  a  negative.  Having  used  both, 
we  are  glad  to  learn  that  it  is  made  now  both  positive  and  negative. 
It  may  be  convenient  to  have  the  8  a  negative  like  the  4,  but  with 
regard  to  the  12,  18,  27  it  is  important  that  they  should  be  positives. 

These  positive  forms  are  on  a  totally  new  plan,  being  composed  of 
a  triple  with  a  single  plano-convex  over  it ;  the  diaphragm  is,  of  course, 
exterior  to  the  lens  (fig.  326).  With  these  the  definition  is  of  the 
finest  quality  throughout  the  field,  which  has  been  reduced  to  about 
6  inches.  They  present  the  admirable  condition  that  with  the  deeper 
1  Chapter  I.  p.  83. 


"  HOLOSCOPIC  "  EYE-PIECES 


379 


powers  the  proper  position  of  the  eye  is  further  from  the  eye-lens 
than  is  the  case  with  those  of  the  Huygheiiiaii  construction ;  which 
makes  it  as  easy  to  use  an  eye-piece  of  as  great  a  power  as  18  or  27 
as  one  of  4  or  8. 

The  field  of  these  eye-pieces  has,  as  we  .believe,  been  very  wisely 
limited  to  five  or  six  inches.  The  attempt  on  the  part  of  English 
opticians  to  give  to  our  eye-pieces  fields  reaching  eighteen  inches  is 
an  error.  A  microscopic  objective  with  the  lowest  aperture  has  the 
field  greatly  in  excess  of  any  other  optical  instrument ;  and  to  deal 
with  such  eccentrical  pencils  as  must  be  engaged  by  an  eye-piece 
with  a  field  of  eighteen  inches  is  a  strain  not  justified  by  what  is 
gained. 

The  powers  of  the  working  eye-pieces  are  also  arranged  in  a  new 
way.  The  multiplying  powers  for  the  long  tube  are  4,  8, 12, 18,  27  ; 
it  will  be  seen  at  once,  therefore,  that  they  bear  no  definite  ratio  to 
one  another,  and  if  we  seek  to  simplify  the  focal  lengths  we  are,  by 
the  employment  of  the  metrical  system,  confronted  with  decimal 
fractions.  But  without  further  elaboration  it  may  be  well  to  say 
that  12  is  the  most  generally  useful  eye-piece, 
and  if  only  one  compensating  eye-piece  is  to  be 
selected,  there  can  be  no  question,  from  a  prac-% 
tical  point  of  view,  but  this  is  the  best  to  em- 
ploy. The  4  is  too  low,  and 
the  27  is  too  high  for  general 
purposes,  and  the  8  and  18  are 
sufficiently  near  the  1 2  to  give 
the  latter  the  advantage  in 
general  work.  We  cannot, 
however,  refrain  from  the  ex- 
pression of  the  opinion  that  a 
series  of  5,  10,  20,  or  6,  12,  24 
powers  would  be  in  many  senses 
more  useful,  and  would  offer 
facilities  in  application  not  se- 
cured by  the  series  of  Abbe  now  in  use. 

It  may  be  well  to  give  further  emphasis  to  the  fact  that  this  con- 
struction of  eye-piece  is  not  only  essential  to  the  proper  work  of 
apochromatic  objectives,  but  they  greatly  enhance  the  images  given 
by  ordinary  achromatic  lenses ;  and  it  may  be  noted  that  the  8,  12, 
and  18  eye-pieces  for  the  short  tube  are  identical  with  12,  18,  27 
for  the  long  tube.  The  4  eye-piece  for  the  short  tube  makes  a  very 
suitable  6  power  for  the  long  tube. 

A  new  series  of  eye-pieces  has  been  recently  introduced  by 
AY.  Watson  and  Sons,  to  which  they  have  given  the  trade  name  of 
'  Holoscopic.'  What  is  held  to  be  a  very  simple  method  is  employed 
for  rendering  them  either  over-  or  under-corrected,  and  therefore 
suitable  for  either  apochromatic  or  the  ordinary  achromatic  objectives. 

This  eye-piece  is  of  the  Huyghenian  type,  but  unlike  the  ordinary 
pattern  the  eye-lens,  together  with  the  diaphragm,  is  mounted  in  a 
tube  which  slides  telescopically  in  the  body  of  the  eye-piece,  at  the 
lower  end  of  which  the  field-lens  is  fixed.  This  is  shown  in  fig.  327. 
When  the  sliding  tube  is  pushed  home  as  far  as  it  will  go,  the  eye- 


Fio.  826.— Abbe's 
comp  ensating 
eye-piece  of  12 
power. 


FIG.  327.— Watson's 
holoscopic  eye- 
piece. 


380 


OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 


piece  is  an  under-corrected  one  and  suitable  for  use  with  the 
ordinary  achromatic  objectives  ;  by  drawing  out  the  sliding  tube 
and  so  increasing  the  distance  between  the  eye  and  field  lenses,  the 
so-called  over-correction,  which  is  associated  with  the  compensating 
eye-pieces,  can  be  obtained  in  varying  degree  according  to  the 
amount  of  extension.  A  scale  is  provided  on  the  sliding  draw- tube 
for  registering  any  desired  position. 

There   are    theoretically    two    distinct    advantages    with    this 
eye-piece  : — 

(1)  It  obviates  the  necessity  for  being  provided  with  both  Huy- 
ghenian    and    compensating    eye-pieces,    because    it    performs    the 
functions  of  both. 

(2)  It  will  have  been  observed  that  with  some  objectives  the 
compensating   eye-piece   has   appeared   to   possess   too  much  over- 
correction,  producing  the  feeling  in  the  mind  of  the  worker  that  if 
it  were  possible  to  vary  the  correction  of  the  eye-piece  a  little  a 
better  image  could  be  produced  ;  this  can  theoretically  be  done  with 
the  new   '  Holoscopic '   eye-piece,  but  we   prefer  a  definitely  com- 
pensated, or  an  ordinary  eye-piece,. 

The  initial  magnifying  powers  of  this  series  of  eye-pieces  are  : — 
For  the  160  nftn.  tube  length  5,     7,  10,  and  14  diameters. 

„      „    250    „  „  7,  10,  14,    „    20 

For  the  English  tube  length,  where  the  diameter  of  the  eye-piece 
fitting  of  the  microscope  permits  of  it,  special!}' 
large  field-lenses  are  used. 

The  cost  is  very  little  greater  than  that  of  the 
ordinary  Huygheiiian  eye-pieces. 

The  projection  eye-piece  is  mainly  intended 
for  photo- micrography,  but  it  is  also  useful  for 
drawing  and  exhibition  purposes.  It  is  a  negative, 
with  a  single  field-lens  and  a  triple  projection- 
lens.  The  projection-lens  is  fitted  with  a  spiral 
focussing  arrangement  in  order  that  the  diaphragm 
which  limits  the  field  may  be  focussed  on  to  the 
screen  or  paper.  The  field  of  this  eye-piece  is 
small,  but  its  definition  is  exquisitely  sharp. 

It  may  not  be  generally  known  that  good 
photo-micrographs  can  be  obtained  by  projection 
with  the  ordinary  compensating  working  eye- 
pieces, but  this  is  a  fact  worthy  of  note. 

It  will  perhaps  be  of  practical  utility  if  we 
append  a  table  indicating  the  focus  of  the  com- 
pensating eye-pieces  when  used  with  the  long  and 
the  short  body. 

Special  Eye-pieces. — The  most  important  of 
these,  the  micrometer  eye-piece,  we  have  already 
considered,  so  far  as  its  application  to  micrometry  is  concerned.1 
Its  optical  character  may  be  properly  considered  here.  If  it  is  a 
negative  eye-piece  the  micrometer  is  placed  in  the  focus  of  the  eye- 
lens  ;  but  if  a  positive  combination,  it  is  placed  in  the  focus  of  the 
eye-piece  itself.  The  Ramsden  form  described  above  is  thoroughly 

1  Chapter  IV. 


FIG.  328.— Zeiss's 
projection  eye- 
piece No.  2. 


SPECIAL   EYE-PIECES  38 

Focus  of  Eye-pieces  for  Long  Body. 


Power 

. 

2 

4 

s 

12 

18 

27 

Focus 

in  mm.      .  ' 

135 

67-5           33-7 

22-5 

15 

10 

" 

inches  . 

5-3             2-6             1-33 

•89 

•59 

•39 

Focus  of  Eye-pieces  for  Short  Body. 


Power 

2 

4 

4* 

6 

o 

12            18 

Focus  in 

mm.    . 

90 

45 

45 

30      i    22-5 

15 

10 

»• 

inches 

3-54 

1-77 

1>77 

1-18         -89 

•59 

•39 

Projection  Eye-pieces.  —  2  for  short  and  3  for  long  bodies  =  90  mm.  or  3'54 
inches ;  4  for  short  and  6  for  long  bodies  =  45  mm.  or  1*77  in. 

suited  for  this  purpose,  but  a  negative  form  is  often  employed,  the 
micrometer  being  placed  inside  the  eye-piece  in  the  diaphragm,  i.e. 
the  focus  of  the  eye-lens. 

In  order  that  the  micrometer  may  be  susceptible  of  focus  for 
various  sights,  it  is  necessary  that  the  eye-lens  in  the  case  of  a 
negative  eye-piece,  and  the  whole  eye-piece  in  the  case  of  a  positive 
one,  should  be  mounted  in  a  sliding  tube ;  and  one  with  a  spiral  slot 
will  be  preferable,  since  it  makes  the  work  of  focussing  both  facile 
and  accurate. 

If  only  one  micrometer  eye -piece  is  used  it  should  be  of  medium 
power,  such  as  H-inch  focus;  but  it  is  an  inexpensive  and  a  useful 
plan  to  have  an  additional  set  of  lenses  to  screw  on  to  the  same  mount, 
so  as  to  make  the  eye-piece,  say,  a  §-inch  focus. 

Spectroscopic,  polarising,  goniometer,  and  binocular  eye-pieces 
are  each  treated  under  their  respective  subjects. 

Quekett's  index  eye-piece  is  one  which  has  a  pointer  placed  at 
the  diaphragm,  so  constructed  that  it  can  be  turned  in  or  out  of  the 
field,  and  is  used  to  point  to  the  position  of  an  object. 

A  good  plan,  when  the  magnification  is  great,  is  to  have  a  dia- 
phragm with  a  small  aperture  to  drop  into  the  eye-piece  and  dimi- 
nish the  field  of  view.  This  not  only  makes  the  object  to  be  pointed 
out  more  easily  accessible  to  the  eye,  but — as  we  have  by  many 
years  of  observation  proved — it  aids  in  close  observation  upon  minute 
objects  by  cutting  off  a  large  area  of  light  without  altering  the  in- 
tensity of  what  remains,  and  so  makes  close  observation  more  easy. 

Diaphragms  with  a  square  aperture  are  fitted  into  eye-pieces  for 
the  purpose  of  counting  blood-corpuscles  in  a  definite  area.  The 
hole  in  the  diaphragm  must  be  adjusted  for  a  definite  tube  length 
and  for  use  with  a  definite  objective  and  used  with  110  other. 

As  it  is  directly  associated  with  the  eye-piece,  we  shall  find  no 
better  place  to  note  the  curious  and  hitherto  unexplained  fact,  that 
when  resolving  stria?  or  lines  with  oblique  light  the  effect  is  much 
strengthened  by  placing  a  Xicol's  analysing  prism  over  the  eye- 
piece. 

Testing  Object-glasses. — It  will  have  been  noted  by  the  attentive 


382  OBJECTIVES,   EYE-PIECES,   THE   APERTOMETER 

reader  that  many  of  the  more  important  qualities  of  objectives 
are  determined  by  the  principles  of  their  construction,  and  become 
in  fact  questions  simply  of  the  quality  of  the  workmanship  involved 
in  producing  the  optical  and  mechanical  parts  of  the  object-glass. 

The  quality  of  the  workmanship  may  be  tested  by  technical 
means  described  below,  and  by  that  subtle  power  which  comes  with 
experience.  This  can  only  be  imparted  through  the  paths  of  labour 
and  experiment,  by  which  in  every  case  it  is  reached.  But,  granted 
that  an  object  has  been  illuminated  in  an  intelligent  and  satisfactory 
manner,  the  first  complete  view  of  the  image  (which  must  of  course 
be  a  thoroughly  familiar  one)  will  enable  the  expert  to  come  to  a 
conclusion  as  to  the  quality  of  a  given  objective.  The  character  of 
the  image  to  the  expert  determines  at  once  the  character  of  the 
lens.  This  is  the  more  absolute  if  a  series  of  eye-pieces  (up  to  the 
most  powerful  that  can  be  obtained)  are  at  hand.  Nothing  tests 
the  quality  of  an  objective  so  uncompromisingly  as  a  deep  eye-piece. 
For  brilliancy  of  image  a  moderate  power  of  eye-piece  is  of  course 
best ;  but  the  capacity  of  the  object-glass  is  clearly  commensurate 
with  its  ability  to  endure  high  eye-pieces  without  loss  of  character, 
and  even  sharpness  in  the  image.  Unless  the  objective  be  of  high 
quality,  the  sharpness  of  the  image  gradually  disappears  as  the  more 
powerful  eye-pieces  are  used,  until  at  last  either  all  or  part  of  the 
image  breaks  up  into  the  'rotten'  details  of  a  coarse  lithograph. 

A  lens  finely  corrected  (w^ith  large  aperture)  will  bear  the  deepest 
eye-piecing  with  no  detriment.  The  24  mm.  and  the  12  mm.  of  Zeiss 
will  suffer  any  eye-piecing  accessible  to  the  microscopist  without  the 
smallest  surrender  of  the  sharpness  of  the  image.  We  have  in  fact 
tried  in  vain  to  '  break  down  the  image '  yielded  by  these  objectives. 
This  mode  of  testing  is  of  course  to  a  large  extent  subjective,  or 
at  least  is  controlled  by  incommunicable  judgments.  It  is  most 
important  therefore  to  have  a  mode  of  judgment  that  shall  be  acces- 
sible to  the  beginner  and  the  interested  amateur.  Dr.  Abbe  has 
proposed  a  method  which  is  at  least  accessible  to  all. 

In  ordinary  practice  microscope  objectives,  if  tested  at  all  by 
their  possessors,  are  simply  subjected  to  a  comparison  of  perform- 
ance with  other  lenses  tried  upon  the  same  'test-objects.' 

The  relative  excellence  of  the  image  seen  through  each  lens  may, 
however,  depend  in  a  great  part  upon  fortunate  illumination,  and 
not  a  little  upon  the  experience  and  manipulative  skill  of  the  ob- 
server ;  besides  which  any  trustworthy  estimate  of  the  performance 
of  the  lens  under  examination  involves  the  consideration  of  a  suit- 
able test-object,  as  well  as  the  magnifying  power  and  aperture  of  the 
objective.  It  is  knowing  what  is  meant  by  a  ;  critical  image,'  and 
being  able  to  discover  whether  or  not  a  given  objective  will  yield  it. 
Clearly  all  tests  of  optical  instruments,  which  are  not  capable  of 
numerical  expression,  must  be  comparative.  Magnifying  power  can 
be  measured  numerically  ;  it  is  not  comparative.  In  the  same  way 
resolving  power  is  mathematically  measurable ;  so  is  penetrating 
power.  But  definition  and  brilliancy  of  image,  and  evidence  of 
centring,  can  have  no  numerical  expression ;  they  are  consequently 
comparative. 


TESTING  OBJECTIVES  383 

The  structure  of  the  test-object  should  be  well  known,  find  the 
value  of  its  '  markings  ' — if  intended  to  indicate  microscopical  dimen- 
sions— should  be  accurately  ascertained,  care  being  taken  that  the 
minuteness  of  dimensions  and  general  delicacy  and  perfection  of  the 
test-object  should  be  adapted  to  the  power  of  the  lens.  A  fairly 
correct  estimate  of  the  relative  performance  of  lenses  of  moderate 
magnifying  power  may  doubtless  be  thus  made  by  a  competent 
observer ;  but  it  is  not  possible  from  any  comparisons  of  this  kind 
to  determine  what  may  or  ought  to  be  the  ultimate  limit  of  optical 
performance,  or  whether  any  particular  lens  under  examination  has 
actually  reached  this  limit. 

Assuming  the  manipulation  ©¥  ^the  instrument  and  the  illumina- 
tion of  the  object  to  be  as  perfect  as  possible,  and  further  that  the  test- 
object  has  been  selected  with  due  appreciation  of  the  requirements  of 
perfect  optical  delineation,  a  fair  comparison  can  only  be  drawn  be- 
tween objectives  of  the  same  magnifying  power  and  aperture.  Which 
of  two  or  more  objectives  gives  the  better  image  may  be  readily 
enough  ascertained  by  such  comparison,  but  the  values  thus  ascer- 
tained hold  good  only  for  the  particular  class  of  objects  examined. 
The  best  performance  realised  with  a  given  magnifying  power  may 
possibly  exceed  expectation,  yet  still  be  below  what  might,  and 
therefore  ought  to  be  obtained. 

On  the  other  hand,  extravagant  expectations  may  induce  a 
belief  in  performances  which  cannot  be  realised.  The  employment 
of  the  test-objects  most  in  use  is  moreover  calculated  to  lead  to  an 
entirely  one-sided  estimation  of  the  actual  working  power  of  an 
objective — as,  for  example,  when  '  resolving  power  '  is  estimated  by 
its  extreme  limits  rather  than  by  its  general  efficiency,  or  '  denning 
power '  by  extent  of  amplification  rather  than  by  clearness  of  outline. 
So  that  an  observer  is  tempted  to  affirm  that  he  can  discern  through 
his  pet  lens  what  no  eye  can  see  or  lens  show.  This  happens  chiefly 
with  the  inexperienced  beginner,  but  not  unfrequently  also  with 
the  more  experienced  worker  who  advocates  the  use  of  great  amplifi- 
cation, in  whose  mind  separation  of  detail  means  analysis  of  struc- 
ture, and  optically  void  interspaces  prove  the  non-existence  of  any- 
thing which  he  does  not  see. 

As  much  time  is  often  lost  by  frequent  repetition  of  these  com- 
petitive examinations  (which,  after  all,  lead  to  no  better  result  than 
that  the  observer  finds  or  fancies  that  one  lens  performs  in  his  hands 
more  or  less  satisfactorily  than  some  other  lens),  it  seems  worth 
while  to  consider  the  value  of  a  mode  of  testing  which  can  be  readily 
applied  whatever  its  value  may  be.  A  short  and  easy  method  of 
testing  an  objective — not  by  comparison  with  others  only,  but  by 
itself  and  on  its  own  merits — affords  not  only  the  most  direct  and 
positive  evidence  of  its  qualities  to  those  who  are  more  concerned 
in  proving  these  instruments  than  using  them,  but  also  yields  to 
the  genuine  worker  the  satisfying  conviction  that  his  labour  is 
not  frustrated  by  faulty  construction  and  performance  of  his  instru- 
ment. It  is,  however,  to  be  borne  in  mind  that  the  microscopist,  in 
any  scrutiny  of  the  quality  of  his  lenses  which  he  may  attempt,  has 
110  other  object  in  view  than  to  acquire  such  insight  into  the  optical 


384  OBJECTIVES,    EYE-PIECES,    THE   APERTOMETER 

conditions  of  good  performance  as  will  enable  him  to  make  the  best 
use  of  his  instrument,  and  acquire  confidence  in  his  interpretation 
of  what  he  sees,  as  well  as  manipulative  skill  in  examining  micro- 
scopical objects.  To  the  constructor  and  expert  of  optical  science 
are  left  the  severer  investigations  of  optical  effects  and  causes, 
the  difficulties  of  technical  construction,  the  invention  of  new  lens- 
combinations,  and  the  numerous  methods  of  testing  their  labours 
by  delicate  and  exhaustive  processes  which  require  special  aptitude 
^ind  lie  entirely  outside  the  sphere  of  the  microscopist's  usual 
work. 

Professor  Abbe's  mode  of  testing  objectives  is  explained  in  his 
4  Beitrage  zur  Theorie  des  Mikroskops.' 

The  process,  in  our  judgment,  requires  large  experience  and 
much  skill  to  be  of  practical  service  ;  but  it  is  based  on  the  following 
principle : — 

In  any  combination  of  lenses  of  which  an  objective  is  composed 
the  geometrical  delineations  of  the  image  of  any  object  will  be  more 
or  less  complete  and  accurate  according  as  the  pencils  of  light  coming 
from  the  object  are  more  or  less  perfectly  focussed  on  the  conjugate 
focal  plane  of  the  objective.  On  this  depend  fine  definition  and 
exact  distribution  of  light  and  shade.  The  accuracy  of  this  focussing 
function  will  be  best  ascertained  by  analysing  the  course  of  isolated 
pencils  directed  upon  different  parts  or  zones  of  the  aperture,  and 
observing  the  union  of  the  several  images  in  the  focal  plane.  For 
this  purpose  it  is  necessary  to  bring  under  view  the  collective  action 
of  each  part  of  the  aperture,  central  or  peripheral,  while  at  the  same 
time  the  image  which  each  part  singly  and  separately  forms  must  be 
distinguishable  and  capable  of  comparison  with  the  other  images. 

1 .  The  illumination  must  therefore  be  so  regulated  that  each  zone 
of  the  aperture  shall  be  represented  by  an  image  formed  in  the  upper 
focal  plane  of  the  objective  (i.e.  close  behind  or  above  its  back  lens), 
so  that  only  one  narrow  track  of  light  be  allowed  to  pass  for  each 
zone,  the  tracks  representing  the  several  zones  being  kept  as  far  as 
possible  apart  from  each  other. 

Thus,  supposing  the  working  surface  of  the  front  lens  of  an 
objective  to  be  ^  inch  in  diameter,  the  image  of  the  pencil  of  light 
let  in  should  not  occupy  a  larger  space  than  ^  inch.  When 
two  pencils  are  employed  one  of  these  should  fall  so  as  to  extend 
from  the  centre  of  the  field  to  ^  inch  outside  of  it,  and  the  other 
should  fall  on  the  opposite  side  of  the  axis  in  the  outer  periphery 
of  the  field,  leaving  thus  a  space  of  /,.;  inch  clear  between  its  own 
inner  margin  and  the  centre  of  the  field.  The  objective  images  of 
the  pencils  occupy  each  a  quarter  of  the  diameter  of  the  whole  field. 

If  three  pencils  of  light  be  employed,  the  first  should  fall  so  as  to 
extend  from  the  centre  of  the  field  to  ^  inch  outside  of  it ;  the 
second  should  occupy  a  zone  on  the  opposite  side  of  it,  between  the 
J-  and  -jL  inch  (measured  from  the  centre)  ;  and  the  third  the 
peripheral  zone  on  the  same  side  as  the  first  in  fig.  329. 

This  arrangement  places  the  pencils  of  light  in  their  most  sensi- 
tive position  and  exposes  most  vividly  any  existing  defect  in  correc- 
tion, since  the  course  of  the  rays  is  such  that  the  pencils  meet  in 


ABBE'S   MODE    OF  TESTING  385 

the  focal  plane  of  the  image  at  the  widest  possible  angle.  As  many 
distinct  images  will  be  perceived  as  there  may  be  zones  or  portions  of 
the  front  face  of  the  objective  put  in  operation  by  separate  pencils  of 
light.  If  the  objective  be  perfect  all  these  images  should  blend  with 
one  setting  of  focus  into  a  single  clear,  colourless  picture.  Such  a 
fusion  of  images  into  one  is,  however,  prevented  by  faults  of  the 
image-forming  process,  which  (so  far  as  they  arise  from  spherical 
aberration)  do  not  allow  this  coincidence  of  several  images  from 
different  parts  of  the  field  to  take  place  at  the 
same  time,  and  (so  far  as  they  arise  from  dis- 
persion of  colour)  produce  coloured  fringes 
on  the  edges  bordering  the  dark  and  light 
lines  of  the  test-object  and  the  edges  of  each 
separate  image,  as  also  of  the  corresponding  FIG.  329.  FIG.  330 
coincident  images  in  other  parts  of  the  field. 

It  is  to  be  borne  in  mind  that  the  errors  which  are  apparent  with 
two  or  three  such  pencils  of  light  must  necessarily  be  multiplied 
when  the  whole  area  of  an  objective  of  faulty  construction  is  in 
action.  This  would  appear  to  us  to  be  the  strongest  reason  for 
utilising  the  whole  area,  because  what  we  are  seeking  is  the  defects 
— the  errors  of  the  objective — and  to  make  these  as  plain  as  possible 
is  a  sine  qua  non.  Dr.  Abbe  proceeds,  however,  to  consider — 

2.  The  means  by  which  such  isolated  pencils  can  be  obtained. 

As  a  special  illuminating  apparatus,  the  condenser  of  Professor 
Abbe  is  recommended,  or  even  a  hemispherical  lens.  But  we  are 
convinced  that  the  illuminating  apparatus  should  be  as  nearly  apla- 
natic  as  it  can  be.  This  is  certainly  not  true  of  Abbe's  chromatic 
condenser  or  a  hemispherical  lens.  The  reason  is  obvious  :  the 
spherical  aberration  wholly  prevents  the  rays  passing  through  the 
holes  in  the  diaphragm  from  being  focussed  on  the  object — the 
silvered  plate  of  lines — at  the  same  time.  In  the  lower  focal  plane 
of  the  illuminating  lens  must  be  fitted  diaphragms  (easily  made  of 
blackened  cardboard)  pierced  with  two  or  three  openings  of  such  a 
size  that  the  images,  as  formed  by  the  objective,  may  occupy  a 
fourth  or  sixth  part  of  the  diameter  of  the  whole  aperture  (i.e.  of  the 
field  seen  when  looking  down  the  tube  of  the  instrument,  after  re- 
moving the  ocular,  upon  the  objective  image).  The  required  size 
of  these  holes,  which  depends,  first,  on  the  focal  length  of  the  illumi- 
nating lens,  and,  secondly,  on  the  aperture  of  the  objective,  may  be 
thus  found.  A  test-object  being  first  sharply  focussed,  card  dia- 
phragms having  holes  of  various  sizes  (two  or  three  of  the  same  size 
in  each  card)  must  be  tried  until  one  size  is  found,  the  image  of  which 
in  the  posterior  focal  plane  of  the  objective  shall  be  about  a  fourth 
to  a  sixth  part  of  the  diameter  of  the  field  of  the  objective.  Holes 
having  the  dimensions  thus  experimentally  found  to  give  the  required 
size  of  image  must  then  be  pierced  in  a  card,  in  such  a  position  as 
will  produce  images  situate  in  the  field,  as  shown  by  figs.  329  and  330 ; 
the  card  is  then  fixed  in  its  place  below  the  condenser.  We  are, 
however,  strongly  inclined  to  believe,  partly  from  experiment,  that 
better  results  would  be  obtained  by  putting  sections  of  annular  slits 
at  the  back  of  the  objective.  If  the  condenser  be  fitted  so  as  to 

c  c 


386  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETER 

revolve  round  the  axis  of  the  instrument,  and  also  carry  with  it 
the  ring  or  tube  to  which  the  card  diaphragm  is  fixed,  the  pencils 
of  light  admitted  through  the  holes  will,  by  simply  turning  the  con- 
denser round,  sweep  the  face  of  the  lens  in  as  many  zones  as  there 
are  holes.  Supposing  the  condenser  to  be  carried  on  a  rotating 
sub-stage,  no  additional  arrangement  is  required  besides  the 
diaphragm-carrier.  Thus,  for  example,  if  a  Collins  condenser  fitting 
in  a  rotating  sub-stage  be  used,  all  that  is  required  is  to  substitute 
for  the  diaphragm  which  carries  the  stops  and  apertures  as  arranged 
by  the  maker,  a  diaphragm  pierced  with,  say,  three  openings  of  |-inch 
diameter,  in  which  circles  of  card  may  be  dropped,  the  card  being 
pierced  with  holes  of  different  sizes  according  to  the  directions  given 
above.  We  doubt,  however,  if  any  sub-stage  will  revolve  with 
sufficient  accuracy  for  so  delicate  a  test. 

Another  plan  adopted  by  Dr.  Fripp,  and  found  very  convenient 
in  practice,  is  to  mount  a  condensing  lens  (Professor  Abbe's  in  this 
case)  upon  a  short  piece  of  tube,  which  fits  in  the  rotating  sub-stage. 
On  opposite  sides  of  this  tube,  and  at  a  distance  from  the  lower  lens 
equal  to  the  focal  distance  of  the  combinations,  slits  are  cut  out 
through  which  a  slip  of  stout  cardboard  can  be  passed  across  and 


FIG,  381. 

below  the  lens.  In  the  cardboard,  holes  of  various  sizes,  and  at 
various  distances  from  each  other,  may  be  pierced  according  to 
pleasure.  By  simply  passing  the  slip  through  the  tube,  the  pencils 
of  light  admitted  through  the  holes  (which  form  images  of  these 
holes  in  the  upper  focal  plane  of  the  objective)  are  made  to  traverse 
the  field  of  view,  and  by  rotating  the  sub-stage  the  whole  face  of  the 
lens  is  swept,  and  thus  searched  in  any  direction  required.  But  here, 
again,  the  spherical  aberration  of  an  unconnected  condenser  would, 
with  an  objective  of  large  aperture,  cause  the  oblique  pencils 
under  some  conditions  to  pass  under  the  object ;  and  alteration  of 
focus  will  not  properly  alter  this — at  least  without  a  disturbance  of 
the  focus  of  the  objective. 

When  an  instrument  is  not  provided  with  a  rotating  sub-stage, 
it  is  sufficient  to  mount  the  condenser  on  a  piece  of  tubing,  which 
may  slide  in  the  setting  always  provided  for  the  diaphragm  on  the 
under  side  of  the  stage. 

Card  diaphragms  for  experiment  may  be  placed  upon  the  top  of 
a  thin  piece  of  tube  (open  at  both  ends)  made  to  slide  inside  that 
which  carries  the  condenser,  and  removable  at  will.  By  rotating 
this  inner  tube  the  pencils  of  light  will  be  made  to  sweep  round  in 


ABBE'S  TEST-PLATE  38; 

the  field,  and  thus  permit  each  part  of  the  central  or  peripheral  zones 
to  be  brought  into  play.  Against  the  accurate  value  of  this,  again, 
the  spherical  aberration  of  an  uncorrected  condenser  would  strongly 
operate. 

Abbe's  Test-plate. — This  test-plate  is  intended  for  the  examina- 
tion of  objectives  with  reference  to  their  corrections  for  spherical 
and  chromatic  aberration,  and  for  estimating  the  thickness  of  the 
cover-glass  for  which  the  spherical  aberration  is  best  corrected. 

The  test-plate  consists  of  a  series  of  cover-glasses,  ranging  in? 
thickness  from  O09  mm.  to  O24  mm.,  silvered  on  the  under  surface 
and  cemented  side  by  side  on  a  slide,  the 
thickness  of  each  being  marked  on  the  silver 
film.  Groups  of  parallel  lines  are  4-ut  through 
the  films,  and  these  are  so  coarsely  ruled  that 
they  are  easily  resolved  by  the  lowest  powers ; 
yet  from  the  extreme  thinness  of  the  silver 
they  also  form  a  very  delicate  test  for  objectives 
of  even  the  highest  power  and  widest  aperture. 
The  test-plate  in  its  natural  size  is  seen  in  fig. 
331,  and  one  of  the  circles  enlarged  is  seen  in  FIG.  332. 

fig.  332. 

To  examine  an  objective  of  large  aperture,  the  discs  must  be 
focussed  in  succession,  observing  in  each  case  the  quality  of  the 
image  in  the  centre  of  the  field,  and  the  variation  produced  by 
using  alternately  central  and  very  oblique  illumination. 

When  the  objective  is  perfectly  corrected  for  spherical  aberration 
for  the  particular  thickness  of  cover-glass  under  examination,  the 
outlines  of  the  lines  in  the  centre  of  the  field  will  be  perfectly 
sharp  by  oblique  illumination,  and  without  any  nebulous  doubling 
or  indistinctness  of  the  minute  irregularities  of  the  edges.  If,  after 
exactly  adjusting  the  objective  for  oblique  light,  central  illumination 
is  used,  no  alteration  of  the  focus  should  be  necessary  to  show  the 
outlines  with  equal  sharpness. 

If  an  objective  fulfils  these  conditions  with  any  one  of  the  discs 
it  is  free  from  spherical  aberration  when  used  with  cover-glasses  of 
that  thickness.  On  the  other  hand,  if  every  disc  shows  nebulous 
doubling,  or  an  indistinct  appearance  of  the  edges  of  the  lines  with 
oblique  illumination,  or  if  the  objective  requires  a  different  focal  ad- 
justment to  get  equal  sharpness  with  central  as  with  oblique  light, 
then  the  spherical  correction  of  the  objective  is  more  or  less  im- 
perfect. 

Nebulous  doubling  with  oblique  illumination  indicates  over- 
correction  of  the  marginal  zone  ;  indistinctness  of  the  edges  without 
marked  nebulosity  indicates  under-correction  of  this  zone  ;  an 
alteration  of  the  focus  for  oblique  and  central  illumination  (that  is, 
a  difference  of  plane  between  the  image  in  the  peripheral  and  central 
portions  of  the  objective)  points  to  an  absence  of  concurrent  action 
of  the  separate  zones,  which  may  be  due  to  either  an  average  under- 
or  over-correction,  or  to  irregularity  in  the  convergence  of  the  rays. 
The  test  of  chromatic  correction  is  based  on  the  character  of  the 
colour-bands  which  are  visible  by  oblique  illumination.  With  good 

cc  2 


388  OBJECTIVES,   EYE-PIECES,    THE   APERTOMETER 

correction  the  edges  of  the  lines  in  the  centre  of  the  field  should 
show  only  narrow  colour-bands  in  the  complementary  colours  of  the 
secondary  spectrum,  namely,  on  one  side  yellow-green  to  apple-green, 
and  on  the  other,  violet  to  rose.  The  more  perfect  the  correction  of 
the  spherical  aberration,  the  clearer  this  colour-band  appears. 

To  obtain  obliquity  of  illumination  extending  to  the  marginal 
zone  of  the  objective,  and  a  rapid  interchange  from  oblique  to 
central  light,  Abbe's  illuminating  apparatus  is  manifestly  defective 
on  account  of  its  spherical  aberration.  We  want  at  least  his 
achromatic  condenser.  For  the  examination  of  ordinary  immersion 
•objectives,  the  apertures  of  which  are,  as  a  rule,  greater  than  180° 
in  arc  (I'OO  N.A.),  and  those  homogeneous  immersion  objectives 
which  considerably  exceed  this,  it  will  be  necessary  to  bring  the 
Minder  surface  of  the  test-plate  into  contact  with  the  upper  lens 
of  the  illuminator  by  means  of  cedar  oil,  even  if  water-immersion 
objectives  are  used.  We  may  add,  as  a  matter  of  experience,  that 
having  once  centred  the  light  and  the  condenser,  we  hold,  with 
deference  to  Dr.  Abbe,  that  the  light  should  on  no  account  be 
touched,  which,  to  obtain  obliquity,  he  advises  by  mirror  changes. 
We  believe  that  this  should  be  secured  solely  by  the  movement  of  the 
/diaphragm. 

For  the  examination  of  objectives  of  smaller  aperture  (less  than 
40°  to  50°),  we  may  obtain  all  the  necessary  data  for  the  estimation 
of  the  spherical  and  chromatic  collections  by  placing  the  concave 
mirror  so  far  laterally  that  its  edge  is  nearly  in  the  line  of  the  optic 
axis,  the  incident  cone  of  rays  then  only  filling  one-half  of  the  aper- 
ture of  the  objective,  by  which  means  the  sharpness  of  the  outlines 
and  the  character  of  the  colour-bands  can  be  easily  estimated. 

It  is  of  fundamental  importance,  in  employing  the  test-plate,  to 
have  brilliant  illumination  and  to  use  an  eye-piece  of  high  power. 
With  oblique  illumination  the  light  must  always  be  thrown  perpen- 
dicularly to  the  direction  of  the  lines. 

When  from  practice  the  eye  has  learnt  to  recognise  the  finer 
differences  in  the  quality  of  the  outlines  of  the  image,  this  method 
of  investigation  gives  very  trustworthy  results.  Differences  in  the 
thickness  of  cover-glasses  of  O01  or  O02  mm.  can  be  recognised  with 
objectives  of  2  or  3  mm.  focus.  The  quality  of  the  image  outside 
the  axis  is  not  dependent  on  spherical  and  chromatic  correction  in 
the  strict  sense  of  the  term. 

Indistinctness  of  the  outlines  towards  the  borders  of  the  field  of 
arises,  as  a  rule,  from  unequal  magnification  of  the  different 


zones  of  the  objective  ;  colour-bands  in  the  peripheral  portion  (with 
good  colour-correction  in  the  middle)  are  always  caused  by  unequal 
magnification  of  the  different  coloured  images.  Imperfections  of  this 
kind,  improperly  called  '  curvature  of  the  field,'  are  shown  to  a 
greater  or  less  extent  in  the  best  objectives,  when  their  aperture  is 
considerable. 

Testing  an  objective  does  not  mean  seeing  the  most  delicate 
points  in  an  object  ;  it  rather  means  the  manner  in  which  an  object 
-of  some  size  is  defined. 

A  test  for  low  powers  up  to  £  of  80°  or  N.A.  '65  is  an  object  on 


OBJECTS   FOE  LENS-TESTING— APERTOMETER  389 

a  dark  ground.  Nothing  is  so  sensitive.  For  the  lowest  powers 
one  of  the  smaller  and  more  delicate  of  the  Polycistince,  because  it 
takes  light  well,  is  good.  For  medium  powers  a  coarse  diatom,  a 
Triceratium  fimbriatum,  is  excellent ;  for  unless  an  objective  is  well 
corrected  the  image  will  be  fringed  and  surrounded  with  scattered 
light,  and  the  aberration  produced  by  the  cover-glass  is  plainly 
manifest,  and  by  accurate  correction  can  be  done  aw^ay. 

Ei*ror  of  centring  is  one  of  the  special  defects  of  objectives 
which  the  Abbe  method  of  testing  does  not  cover.  But  if  we  place 
a  sensitive  object  in  a  certain  direction,  and  when  the  best  adjust- 
ments have  given  the  best  image,  rotate  that  object  through  an  angle 
of  90°,  only  a  well-centred  objective  will  give  an  unaltered  image 
throughout.  If  not  well  centrefl  it  will  at  certain  parts  grow 
fainter  or  sharper.  The  most  useful  image  for  this  purpose  with 
medium  powers  is  a  hair  of  Polyxenus  lagurus  mounted  in  balsam 
(frontispiece,  fig.  6). 

For  higher  powers  nothing  surpasses  a  podura  scale.  In  this 
particular  it  has  always  been  of  great  value  to  opticians.  It  should 
be  strongly  marked,  and  must  be  in  optical  contact  with  the  cover - 
glass  ;  this  may  be  tested  by  means  of  an  oil-immersion  and  the 
'  vertical  illuminator.' 

The  objectives  of  widest  aperture  are  now  more  easily  tested, 
because  homogeneous  condensers  with  much  wider  aplanatic  areas 
are  now,  as  we  have  seen,  made  by  the  leading  English  and 
Continental  opticians ;  and  there  is  little  doubt  but  that  there  is  a 
considerable  future  before  homogeneous  condensers.  The  best  that 
can  be  done  is  to  take  a  diatom,  such  as  a  Coscinodiscus,  in  balsam 
with  strong  'secondaries'  (Plate  I.  figs.  3  and  4),  with  the  largest 
aplanatic  cone  that  can  be  obtained,  which  at  present  can  be  best 
accomplished  with  a  semi-apochromatic  oil-immersion  condenser  of 
1*3  IS". A.  It  must  be  a  good  objective  indeed  that  does  not  show 
signs  of  breaking  down  under  this  strain.  An  illuminating  cone 
of  N.A.  I'O  is  probably  just  belowT  the  point  of  overstrain  with  the 
best  lenses  at  present  at  our  disposal. 

Testing  lenses  therefore  resolves  itself  into  the  following  methods, 
viz. : — 

1 .  For  low  and  medium  powers  :  dark  ground  with  a  Polycistina 
or  a  diatom,  according  to  the  po\ver. 

2.  Centring  for  medium  powers  (an  ordeal  not  needful  for  very 
low  powers)  should  be  by  means  of  a  hair  of  Polyxenus  lagurus,  em- 
ploying a  J  illuminating  cone. 

3.  Centring  for  high  powers  :  by  means  of  podura  scale. 

4.  Definition :    with  wide-angled   oil   immersions,  Coscinodiscus 
aster 'omphalus   with    wide-angled    cone   obtaining   sharp,    brilliant, 
and  clear   view    of   'secondaries/  or  coarse   specimen  of  Navicula 
rhomboides,   which   may   be   mounted   in    a    dense    medium.       In 
testing  a  lens  it  does  not  so  much  matter  what  the  object  is,  because 
the  real  test  lies  in  the  ability  of  the  lens  to  stand  a  large  direct 
axial  cone.     A  lens  of  very  great  excellence  will  stand  a  Jths  cone, 
an  excellent  lens  a  Jths  cone,  an  indifferent  lens  only  a  \  cone,  while 
a  bad  lens  will  not  even  admit  the  use  of  that.     A  dark  ground  is  a 


390 


OBJECTIVES,    EYE-PIECES,   THE  APEKTOMETEK 


very  severe  test,  as  it  is  of  the  nature  of  a  full  cone,  so  to  speak,  and 
only  the  lower  powers  will  stand  it.  If  a  dark  ground  is  required 
with  the  higher  objectives  it  can  be  obtained  by  using  an  oil-immer- 
sion condenser,  but  the  aperture  of  the  objective  will  have  to  be 
reduced  by  a  stop. 

The  apertometer,  as  its  name  implies,  is  an  instrument  for  mea- 
suring the  aperture  of  a  microscopic  objective.     As  correct  ideas  of 

aperture  have  only  obtained  dur- 
ing the  past  few  years,  it  may  be 
inferred  that  apertometers  con- 
structed before  the  definition  of 
aperture  was  given  and  accepted 
were  crude  and  practically  use- 
less. 

The  controversy  on  the  '  aper- 
ture question,'  which  was  in  full 
operation  some  eighteen  years 
since,  is  not  an  altogether  satis- 
factory page  in  the  history  of 
the  modern  microscope,  and  for 
many  reasons  it  is  well  to  JKISS 
it  unobservantly  by.  It  will 
suffice  to  state  that  during  its 
progress  an  apertometer  was  de- 
vised by  R.  B.  Tolles,  of  America, 
which  accurately  measured  the 
true  aperture  of  an  objective. 
About  the  same  time  Professor 
Abbe  gave  his  attention  to  the 
subject,  and  with  the  result,  as 
we  have  seen,  that  he  has  given 
a  definite  and  permanent  meaning 
to  numerical  aperture,  making 
it,  as  we  have  seen,  the  equiva- 
lent of  the  mathematical  expres- 
sion n  sine  u,  n  being  the  refrac- 
tive index  of  the  medium,  and  u 
half  the  angle  of  aperture.1 

The  application  of  this  for- 
mula to,  and  its  general  bearing 
upon,  the  diffraction  theory  of 
microscopic  vision  has  been  given 
in  its  proper  place ;  but  as  the 
aim  of  this  manual  is  thoroughly 

practical,  we  shall  be  pardoned  for  even  a  small  measure  of  repeti- 
tion in  endeavouring  to  explain  the  use  of  this  formula  in  such  a 
manner  that  only  a  knowledge  of  simple  arithmetic  will  be  required 

1  A  knowledge  of  the  meaning  of  the  trigonometrical  expression  '  sine  '  is  not 
necessary  in  solving  any  of  the  following  questions.  As  the  values  are  all  found  in 
tables,  it  is  only  necessary  to  caution  those  who  are  unacquainted  with  the  use  of 
mathematical  tables  to  see  that  they  have  the  '  natural  sine  '  and  not  the  '  log  sine.' 


FIG.  333. 


SIMPLE   ILLUSTRATIONS    OF   THE   USE   OF   N    SINE  .U      391 

to  enable  the  student  to  work  out  any  of  the  problems  which  are 
likely  to  arise  in  his  practical  work. 

We  can  best  accomplish  this  by  illustration. 

(i)  If  a  certain  dry  objective  has  an  angular  aperture  of  60°,  what 
is  its  N.A.  (i.e.  numerical  aperture)  ? 

All  that  is  needful  is  to  find  the  value  of  n  sine  u  ;  in  this  case 
r<,= the  refractive  index  of  the  medium,  which  is  air,  is  1  ;  and  w, 
which  is  half  of  60°  =  30°  opposite  30°  in  a  table  of  natural  sines,1  is 
•5  ;  sine  w,  therefore  =  '5,  which  multiplied  by  1  gives  '5  as  the 
N.A.  of  a  dry  objective  having  60°  of  angular  aperture. 

(ii)  What  is  the  N.A.  of  a  water-immersion  whose  angular 
aperture =44°? 

n  here=l'33,  the  refractive  inslex  of  wrater  ;  and  u,  or  half  44°, 
is  22°.  Sine  22°  from  tables='375,  which  multiplied  by  1'33='5 
(nearly),  which  is  the  N.A.  required. 

(iii)  What  is  the  X.A.  of  an  oil -immersion  objective  having  38^° 
of  angular  aperture  ? 

n  the  refractive  index  of  oil,  which  is  equal  to  that  of  crown 
glass,  is  1'52 ;  ?t=:19j  and  sine  u  from  table=*329,  which  multi- 
plied by  l-52=-5. 

Thus  it  is  seen  that  a  dry  objective  of  60°,  a  water-immersion  of 
44°,  and  an  oil-immersion  of  38^°  all  have  the  same  N.A.  of  "5. 

It  will  be  well,  perhaps,  to  give  the  converse  of  this  method. 

(iv)  If  a  dry  objective  is  '5  N.A.,  what  is  its  angular  aper- 
ture ? 

Here  because  n  sine  it='5,  sine  u=     ;    the    objective    being 

dry,  ?i=l,  therefore  sine  u='5.  Opposite  -5  in  the  table  of  natural 
sines  is  30°  ;  hence  w=30°.  But  as  u  is  half  the  angular  aperture 
of  the  objective,  2u  or  60°= the  angular  aperture  required. 

(v)  What  is  the  angular  aperture  of  a  water-immersion  objective 
whose  N.A. ='5? 

•5       *5 
Here     9i=l*33,     n     sine     w=*5 ;      sine     u= — =:.— ^='376  • 

71         1  *OO 

?t=22°  (nearly)  from  tables  of  sines;  /.  2^=44°,  the  angle  re- 
quired. 

(vi)  What  is  the  angular  aperture  of  an  oil-immersion  objective 

of  -5  N.A.  ? 

'5       *5 
Here     ?i=l-52,     n     sine     w='5 ;      sine     u=— =^-^=.-329 ; 

w=19J°  (by  tables  of  sines) ;  and  2^=38^,  the  angle  required. 

We  may  yet  further  by  a  simple  illustration  explain  the  use  of 
n  sine  u. 

In  the  accompanying  diagram,  fig.  333,  let  n'  represent  a  vessel 
of  glass ;  let  the  line  A  be  perpendicular  to  the  surface  of  the  water 
C  D ;  suppose  now  that  a  pencil  of  light  impinges  on  the  surface  of 
the  water  at  the  point  where  the  perpendicular  meets  it,  making  an 
angle  of  30°  with  the  perpendicular.  This  pencil  in  penetrating  the 
water  will  be  refracted  or  bent  towards  the  perpendicular.  The 

1  Vide  Appendix  A  to  this  volume. 


3Q2  OBJECTIVES.    EYE-PIECES,   THE   APEKTOMETER 

problem  is  to  find  the  angle  this  pencil  of  light  will  make  with  the 
perpendicular  in  the  water. 

To  do  this  we  must  remember  that  n  sine  u  on  the  air  side  is 
equal  to  n'  sine  u'  on  the  water  side.  Thus  on  the  air  side  n=l. 
^=30°,  and  by  the  tables  of  sines  sine  30°  =  '5;  consequently  on 
the  air  side  we  have  n  sine  ^='5. 

On  the  water  side  ri=l'33,  and  u'  is  to   be   found.     But   as 

,    .                      .                                         ,        n  sine  u          '5 
n  sine  u  ==  n  sine  u,  therefore  sine  n'  = 7 =_      =  -376  ; 

*"  1ft  I'Ou 

which  (as  the  tables  show)  is  the  natural  sine  of  an  angle  of  22° 
(nearly) ;  consequently  u'  =  22°  ;  so  the  pencil  of  light  in  passing 
out  of  air  into  water  has  been  bent  8°  from  its  original  direction. 
Conversely  a  pencil  in  water,  making  an  angle  of  22°  with  the 
perpendicular,  would  on  emerging  from  the  water  be  bent  in  air  8° 
further  away  from  the  perpendicular,  and  so  make  an  angle  of  30° 
with  it. 

Now  if  we  suppose  that  these  pencils  of  light  revolve  round  the 
perpendicular,  cones  would  be  described,  and  we  can  readily  see  that 
a  solid  cone  of  60°  in  air  is  the  exact  equivalent  of  a  solid  cone  of 
44°  in  water. 

If  we  further  suppose  that  the  water  in  the  vessel  is  replaced  by 
cedar  oil,  the  pencil  in  air,  remaining  the  same  as  before,  will,  when 
it  enters  the  oil,  be  bent  more  than  it  was  in  the  water,  because  the 
oil  has  a  higher  refractive  index  than  water ;  n  in  this  case  is  equal 
to  1-52. 

The  exact  position  of  the  pencil  can  be  determined  in  the 
same  manner  as  in  the  previous  case.  On  the  air  side,  as  before, 
n  sine  u='5  ;  on  the  oil  side  n'  sine  u'—n  sine  u ;  sine  u'= 

— 7 =TT^;O  ='329,  which  (by  the  tables)  is  the  natural  sine  of 

n  1  *OZ 

19J°.  It  follows  that  the  pencil  has  been  bent  in  the  cedar  oil  lOf  ° 
out  of  its  original  course,  and  a  cone  of  60°  in  air  becomes  a  cone  of 
38^°  in  cedar  oil  or  crown-glass. 

Finally,  it  is  instructive  to  note  the  result  when  an  incident  pencil 
in  air  makes  an  angle  of  90°  with  the  perpendicular  :  n  sine  u  becomes 
unity,  and  u  in  water  48|°,  in  oil  41°  (nearly) ;  consequently  a  cone 
of  either  97^°  in  water,  or  82  J°  in  oil  or  crown  glass,  is  the  exact 
equivalent  of  the  whole  hemispherical  radiant  in  air.  In  other  words, 
and  to  vary  the  mode  in  which  this  great  truth  has  been  before 
stated,  the  theoretical  maximum  aperture  for  a  dry  lens  is  equiva- 
lent to  a  water-immersion  of  97^°  and  an  oil-immersion  of  82J 
angular  aperture. 

The  last  problem  that  need  occupy  us  is  to  find  the  angular 
aperture  of  an  oil-immersion  which  shall  be  equivalent  to  a  water- 
immersion  of  180°  angular  aperture. 

On  the  water  side  n  =  T33,  u  —  90°,  sine  90°  =  1,  n  sine  u= 
1-33.  On  the  oil  side  n'  =  T52  and  ur  has  to  be  found. 

As  n'  sine  u'  =  n  sine  u,   therefore  sine  u'  —  7^m^i=:  l— 

n'  1-52 

=  -875;  u'  =  61°  (nearly)  by  the  tables;  2w'  =  122°  (nearly), 
the  angle  required. 


THE   APERTOMETER  393 

It  thus  appears  (1)  that  dry  and  immersion  objectives  having 
different  angular  apertures,  if  of  the  same  equivalent  aperture,  are 
designated  by  the  same  term.  Thus  objectives  of  60°  in  air,  or  44° 
in  water,  or  38^°  in  oil,  have  identically  the  same  aperture,  and  are 
known  by  the  same  designation  of  "5  N.A. 

(2)  The  penetrating  power  of  any  objective  is  proportional  to 

•jSr~T~j  an<^   itg  illuminating   power   to    (N.A.)2.     Therefore,    if  we 

double  the  N.A.  we  halve  the  penetrating  power,  and  increase  the 
illuminating  power  four  times. 

In  comparing  the  penetrating  and  illuminating  powers  of  objec- 
tives, however,  care  must  be  ^aken  to  avoid  a  popular  error,  by 
making  them  between  objectives  of  different  foci. 

It  cannot,  for  example,  be  said  that  a  J-inch  objective  of  '8  N.A. 
has  half  the  penetrating  power  of  a  ^-inch  of  '4  N.A.  Neither  can 
it  be  said  that  it  has  four  times  the  illuminating  power.  What  is 
meant  is  that  a  J-inch  of  '8  N.A.  has  half  the  penetrating  and  four 
times  the  illuminating  power- of  a  ^-inch  objective  of  '4  N.A. 

But  because  penetrating  and  illuminating  powers  diminish  as 
the  square  of  the  foci,  a  J-inch  objective  of  '6  N.A.  has  four  times 
the  illuminating  and  nearly  four  times  the  penetrating  power  of  a 
J-inch  of  '6  N.A. ;  but  these  conditions  only  hold  when  a  full 
illuminating  cone  is  employed,  in  other  words,  when  the  back  lens 
of  the  objective,  as  seen  when  the  eye-piece  is  removed,  is  full  of 
light.  Thus  if  a  small  cone  of  illumination  is  used  with  the  ^-inch 
objective  of  *6  N.A.,  its  illuminating  power  would  be  much 
diminished,  while  its  penetrating  power  would  be  much  increased. 

The  old  nomenclature,  in  use  before  numerical  aperture  was  so 
happily  introduced,  did  not  of  course  admit  of  comparisons  of  pene- 
trating and  illuminating  powers  by  inspection  ;  which,  however,  is 
a  manifest  advantage,  contributing  to  accuracy  and  precision  in 
important  directions. 

(3)  It  may  be  well,  for  the  sake  of  completeness,  to  repeat l  here 
that  the  resolving  power  of  an  objective  is  directly  proportional  to 
its  numerical  aperture.     If  we  double  the  N.A.  we  also  double  the 
resolving  power ;  and  this  not  simply  with  objectives  of  the  same 
foci,  as  in  the  case  of  penetrating  and  illuminating  powers.     Thus  it 
is  not  only  true  that  a  J-inch  objective  of  '6  N.A.   resolves  twice  as 
many  lines  to  the  inch  as  a  ^-inch  of  '3  N.A.,   but  so  also  does  a 
^-inch  of  1'4  N.A.  resolve  twice,  and  only  twice,  as  many  as  a  J-inch 
of  -7  N.A. 

Within  certain  limits,  then,  the  advantage  lies  with  long  foci  of 
wide  angle,  because  we  thus  secure  the  greatest  resolving  power 
with  the  greatest  penetrating  and  illuminating  powers. 

From  what  has  here  been  shown,  then,  it  becomes  evident  that 
the  employment  of  the  microscope  as  an  instrument  of  precision  is 
largely  due  to  Abbe's  work,  and  that  the  introduction  of  numerical 
aperture,  with  its  strictly  accurate  meaning,  has  been  a  practical 
gain  of  untold  value.  But  this  has  been  greatly  enriched  by  his 
having  introduced  a  thoroughly  simple  and  useful  apertometer.  This 

1  Chapter  I. 


394 


OBJECTIVES,    EYE-PIECES,    THE   APERTOMETEK 


involves  the  same  principle  as  that  of  Tolles.  but  it  is  carried  out  in 
a  simpler  manner. 

Abbe's  instrument  is  presented  in  fig.  334.  It  will  be  seen  that 
it  consists  of  a  flat  cylinder  of  glass,  about  three  inches  in  diameter 
and  half  an  inch  thick,  with  a  large  chord  cut  off  so  that  the  portion 
left  is  somewhat  more  than  a  semicircle  ;  the  part  where  the  segment 
is  cut  is  bevelled  from  above  downwards  to  an  angle  of  45°,  and  it 
will  be  seen  that  there  is  a  small  disc  with  an  aperture  in  it  denoting 
the  centre  of  the  semicircle.  This  instrument  is  used  as  follows  : — 

The  microscope  is  placed  in  a  vertical  position,  and  the  aperto- 
meter  is  placed  upon  the  stage  with  its  circular  part  to  the  front 
and  the  chord  to  the  back.  Diffused  light,  either  from  sun  or  lamp, 
is  assumed  to  be  in  front  and  on  both  sides.  Suppose  the  lens  to 
be  measured  is  a  dry  J-inch  ;  then  with  a  1-inch  eye-piece  having  a 
large  field,  the  centre  disc  with  its  aperture  on  the  apertometer  is 
brought  into  focus.  The  eye-piece  and  the  draw-tube  are  now 
removed,  leaving  the  focal  arrangement  undisturbed,  and  a  lens 


^GarlZeiss  Apertometer 


FIG.  334. — Abbe's  apertometer. 

supplied  with  the  apertometer  is  screwed  into  the  end  of  the  draw- 
tube.  This  lens  with  the  eye -piece  in  the  draw- tube  forms  a 
low-power  compound  microscope.  This  is  now  inserted  into  the  body- 
tube,  and  the  back  lens  of  the  objective  wrhose  aperture  we  desire 
to  measure  is  brought  into  focus.  In  the  image  of  the  back  lens 
will  be  seen  stretched  across,  as  it  were,  the  image  of  the  circular 
part  of  the  apertometer.  It  will  appear  as  a  bright  band,  because 
the  light  which  enters  normally  at  the  surface  is  reflected  by  the 
bevelled  part  of  the  chord  in  a  vertical  direction,  so  that  in  reality 
a  fan  of  180°  in  air  is  formed.  There  are  two  sliding  screens  seen 
on  either  side  of  the  figure  of  the  apertometer ;  they  slide  on  the 
vertical  circular  portion  of  the  instrument.  The  images  of  these 
screens  can  be  seen  in  the  image  of  the  bright  band.  These  screens 
should  now  be  moved  so  that  their  edges  just  touch  the  periphery  of 
the  back  lens.  They  act,  as  it  were,  as  a  diaphragm  to  cut  the  fan 
and  reduce  it,  so  that  its  angle  just  equals  the  aperture  of  the  objec- 
tive and  no  more. 

This  angle  is  now  determined  by  the  arc  of  glass  between  the 


THE   USE   OF   THE   APERTOMETER  395 

screens  ;  thus  we  get  an  angle  in  glass  the  exact  equivalent  of  the 
aperture  of  the  objective.  As  the  numerical  apertures  of  these  arcs 
are  engraved  on  the  apertometer  they  can  be  read  off  by  inspection. 
Nevertheless  a  difficulty  is  experienced,  from  the  fact  that  it  is  not 
easy  to  determine  the  exact  point  at  which  the  edge  of  the  screen 
touches  the  periphery  of  the  back  lens,  or,  as  we  prefer  to  designate 
it,  the  limit  of  aperture,  for,  curious  as  this  expression  may  appear, 
we  have  found  at  times  that  the  back  lens  of  an  objective  is  larger 
than  the  aperture  of  the  objective  requires.  In  that  case  the  edges 
of  the  screen  refuse  to  touch  the  periphery. 

On  the  whole  we  have  found  that  a  far  better  way  of  employing 
this  instrument  is  to  use  it  in  Connection  with  a  graduated  rotary 
stage,  the  edge  of  the  flame  of  ar  paraffin  lamp  being  the  illumi- 
nator. 

Thus  :  Set  the  lamp  in  a  direction  at  right  angles  to  the  chord 
of  the  apertometer,  and  suppose  that  the  index  of  the  stage  is  at  0°. 
The  edge  of  the  flame  will  be  seen  in  the  centre  of  tne  bright  band. 
The  sliding  screens  being  dispensed  with,  rotation  of  the  stage  will 
cause  the  image  of  the  flame  to  travel  towards  the  edge  of  the 
aperture  ;  rotation  is  continued  until  the  image  of  the  flame  is  half 
extinguished  by  the  edge  of  the  aperture,  the  arc  is  then  read,  and 
the  same  thing  is  repeated  on  the  other  side,  and  the  mean  of  the 
readings  is  taken. 

If  the  stage  rotates  truly,  and  if  the  instrument  is  properly  set 
up,  the  reading  on  the  one  side  ought  to  be  identical  with  that  on 
the  other. 

Suppose  that  the  sum  of  the  readings  on  both  sides  =  60°,  the 
mean  reading  is  consequently  30°,  which  is  the  semi-angle  of  aperture 
of  the  lens  in  glass.  From  this  datum  we  have  to  determine  the  N.A. 
of  the  dry  J-inch  as  well  as  its  angular  aperture  in  air.1 

(i)  As  before,  N.A.  =  n  sine  u,  and  n  sine  u  =  nf  sine  ur  ; 
which  means  that  the  aperture  on  the  air  side  is  equal  to  the  aperture 
on  the  glass  side  ;  n  =  1  for  air  ;  n'  =  T615,  the  refractive  index  of 
the  apertometer  ;  u'  is  the  mean  angle  measured,  which  in  this  case 
is  30°  ;  and  n  sine  u  has  to  be  found. 

Now  sine  30°  =  '5  (by  the  tables)  ;  n'  sine  u'  —  1-615  X  sine  30° 
=  1-615  x  *5  =  '8  =  n  sine  u  =  the  N.A.  required. 

(ii)  Again,  to  find  the  angular  aperture  or  2u.    As  before,  n  sine  u 


»'  sine  «'  and  sine  «  =  _'  =  L61:5  x  J5  =  -8  ;    u  =  53" 

n  1 

nearly  (by  the  tables)  ;  1u  =  106°,  which  is  the  angle  required. 

(iii)  If  it  be  a  water-immersion  we  have  to  deal  with,  suppose 
the  mean  angle  =  45°  =  uf  ;  sine  45°  =  707  (by  the  tables)  ;  n 
=  1-33;  and  w'  =  1-615. 

n  sine  u  =  nf  sine  u'  =  1'615  x  '707  =  1-14,  the  N.A.  required. 

(iv)  Again,  sine  «  =  »'  sine  u'  =  1'615  x  "707  =  -86  ;  u  =  59|° 

n  1'33 

(by  the  tables)  ;  and  2u  =  118^°,  the  angle  required. 

(v)  In  the   case  of   an  oil-  immersion,  suppose  the  mean  angle 

1   Vide  p.  2  et  seq. 


396  OBJECTIVES,   EYE-PIECES,   THE   APEKTOMETEK 

=  60°  =  u  ;  sine  60°  =  '866  (by  the  tables) ;  n  =  1'52  ;  n'  =  1'615  ; 
n  sine  u  =ri  sine  u'  =  1*615  X  '866  =  1*4,  which  is  the  N.A. 
required. 

,  -x    A      .       .  n'  sinew'       1-615  X  '866        Q0 

(vi)  Again,  sine  u=  =  -     - — — -   —  =  92. 

%  1'52 

n  =  67°  (by  the  tables),  2u  =  134°,  the  angle  required. 

It  is  manifest  that  if  the  refractive  index  of  the  apertometer 
equals  that  of  the  oil  of  cedar,  the  mean  angle  measured  is  the  semi- 
angle  of  aperture  of  the  objective,  and  its  sine  multiplied  by  that 
refractive  index  is  the  numerical  aperture. 

This  will  be  found  the  more  accurate  and  universally  applicable 
method  of  measuring  the  apertures  of  objectives,  as  the  extinction 
of  the  light  shows  precisely  when  the  limit  of  aperture  is  reached. 

Powell  and  Lealand's  stands  lend  themselves  admirably  for  use 
with  the  apertometer.  The  body  being  removable,  the  lens  can  be 
placed  in  the  upper  part  of  the  nose-piece,  and  any  measurement 
can  be  accurately  made.  We  would  advise  every  microscopist  to 
master  the  use  of  this  admirable  instrument,  and  to  demonstrate  for 
himself  the  aperture  capacity  of  his  lenses,  that  he  may  know  with 
precision  their  true  resolving  powers.  It  will  facilitate  this  that 
Mr.  Nelson  has  shown  (*  Journ.  R.M.S.'  1896,  p.  592)  that  the  use  of 
the  internal  lens  is  not  required ;  the  point  of  rotation  of  the  stage 
when  the  edge  of  the  flame  is  eclipsed  by  the  limiting  aperture  of 
the  objective  can  be  readily  observed  by  means  of  a  low-power  eye- 
piece. 

When  the  apparatus  is  accurately  set  up  in  the  manner  described 
above,  the  exact  point  is  indicated  by  the  dark  segments  coming 
across  the  field  of  the  eye-piece.  One  dark  segment  will  be  found  to 
advance  slowly  from  one  side,  and  then  when  the  precise  point  of 
rotation  of  the  stage  is  reached  the  other  dark  segment  will  come  in 
from  the  other  side  and  meet  it.  For  this  purpose  the  glass  disc 
with  its  refractive  index  only  engraved  upon  it  is  alone  required. 
Messrs.  Zeiss  supply  this  at  a  much  lower  cost  (25s.)  than  the 
engraved  disc  and  the  supplementary  lens. 

Boucher's  circular  slide  rule  is  a  convenient  adjunct  to  the 
apertometer,  for  the  N.A.  can  be  read  off  by  inspection  without  the 
necessity  of  looking  out  sines  or  making  calculations. 


397 


CHAPTER   YI 

PRACTICAL  MICROSCOPY:  MANIPULATION  AND  PRESERVATION 
OF  THE  MICROSCOPE 

WITHOUT  attempting  to  occupy  space  with  a  discussion  of  the  ques- 
tion of  the  right  of  *  microscopy '  to  be  considered  a  science,  we  may 
venture  to  affirm  that  it  will  be  but  a  recognition  of  practical  facts 
if  we  claim  as  a  definition  of  microscopy  that  it  expresses,  and  is  in- 
tended to  carry  with  it,  all  that  belongs  to  the  science  and  art  of  the 
microscope  as  a  scientific  instrument,  having  regard  equally  to  its 
theoretical  principles  and  its  practical  working.  Hence  '  practical 
microscopy '  will  mean  a  discourse  on,  or  discussion  of,  the  methods 
of  employing  the  microscope  and  all  its  simplest  and  more  complex 
appliances  in  the  most  perfect  manner,  based  alike  and  equally  upon 
theoretical  knowledge  and  practical  experience. 

On  this  condition  a  *  microscopist '  means  (or  at  least  implies) 
one  who,  understanding  'microscopy,'  applies  his  theoretical  and 
practical  knowledge  either  to  the  further  improvement  and  perfec- 
tion of  the  instrument,  or  to  such  branches  of  scientific  research  as 
he  may  profitably  employ  his  '  microscopy '  in  prosecuting.  He  is, 
in  fact,  a  man  employing  specialised  theoretical  knowledge  and 
practical  skill  to  a  particular  scientific  end. 

But  a  '  microscopical  society '  has  a  noble  raison  d'etre,  because 
it  is  established,  on  the  one  hand,  to  promote — without  consideration 
of  nationality  or  origin — improvements  in  the  theory  and  practical 
construction  of  both  the  optical  and  mechanical  parts  of  the  micro- 
scope, and  to  endeavour  to  widen  its  application  as  a  scientific 
instrument  to  every  department  of  human  knowledge,  recording,  in- 
vestigating, and  discussing  every  refinement  and  extension  of  its 
application  to  every  department  of  science,  whether  old  or  new. 

In  this  sense  no  more  practical  definition  of  a  *  microscopical 
society '  can  be  given  than  is  contained  in  the  invaluable  pages  of 
the  '  Journal  of  the  Royal  Microscopical  Society '  from  the  end  of 
1880  to  the  present  day  ;  and  no  better  justification  for  the  existence 
of  such  a  society  can  be  needed  than  is  afforded  by  the  work  done 
directly  or  indirectly  by  it,  in  inciting  to  and  promoting  the  theo- 
retical and  practical  progression  of  the  instrument  and  its  ever- 
widening  applications  to  the  expanding  areas  of  natural  knowledge. 

In  this  chapter  we  propose  to  discuss  the  best  practical  methods 
of  using  the  instrument  and  its  appliances,  the  theory  concerning 
which  has  already  been  discussed,  while  the  mode  of  applying  this 


398   MANIPULATION  AND   PKESEEVATION   OF  THE   MICROSCOPE 

knowledge  to  biological  and  other  investigations  is  entered  upon  in 
the  subsequent  chapters  of  the  book. 

To  begin  his  work  with  success — if  his  object  be  genuine  work — 
the  student  must  be  provided  with  some  room,  or  portion  of  a  room, 
which  he  can  hold  sacred  to  his  purpose.  Unless  special  investiga- 
tions are  undertaken,  it  is  not  a  large  area  that  is  required,  but  a 
space  commanding,  if  possible,  a  north  aspect,  and  which  can  be 
arranged  to  readily  exclude  the  daylight  and  command  complete 
darkness. 

The  first  requirement  will  be  a  suitable  table. 

This  should  be  thoroughly  firm,  and  it  should  be  rectangular  in 
shape.  A  round  table,  if  small  especially,  is  most  undesirable,  as  it 
offers  no  support  for  the  arms  on  either  side  of  the  instrument ;  and 
with  prolonged  work  this  is  not  only  a  serious,  but  an  absolutely 
fatal  defect. 

In  a  rectangular  table  the  centre  may  be  kept  clear  for  micro- 
scopical work,  while  there  are  two  corners  at  the  back,  one  on  the 
left  and  the  other  on  the  right  hand.  The  former  may  be  used  for 
the  locked  case  or  glass  shade  for  protecting  the  instrument  when 
not  in  use ;  and  when  it  is  in  use,  it  in  no  way  interferes  with  the 
usefulness  of  the  table.  In  the  same  way  the  right-hand  corner  may 
be  used  for  the  cabinet  of  objects  which  is  being  worked,  or  the 
apparatus  needful  for  use. 

The  most  important  part  of  the  table — that  is,  the  middle,  from 
front  to  back — should  be  kept  quite  clear  for  the  purposes  of  mani- 
pulation, and  a  sufficient  space  should  be  kept  clear  on  either  side  of 
the  instrument  for  resting  the  arms,  and  no  loose  pieces  of  apparatus 
should  ever  be  deposited  within  those  spaces.  This  soon  becomes  a 
habit  in  practice,  for  experience  teaches — sometimes  painfully,  by  the 
unwitting  destruction  of  a  more  or  less  valuable  appliance. 

The  spaces  to  the  right,  beyond  that  left  for  the  arm  of  the 
operator,  may  be  used  for  the  work  immediately  in  hand — especially 
for  a  second  and  simpler  microscope.  An  instrument  with  only  a 
coarse  adjustment  and  a  1-inch  or  a  ||-inch  objective  wrill  suffice,  or 
a  good  dissecting-stand  will  answer  every  purpose.  Those  who  do 
much  practical  work  will  find  such  a  plan  more  rapid  and  more 
efficient  than  the  cumbrous  method  of  a  rotary  nose-piece,  especially 
wrhere  critical  work  has  to  be  done. 

When  work  is  being  done  in  a  darkened  room  there  should  be 
on  the  extreme  right  a  small  lamp  with  a  paper  shade.  (Special 
shades  for  this  purpose  can  be  obtained  from  Baker,  of  Holborn.) 
This  light  may  be  kept  low  or  used  for  general  illumination  wThen 
required  ;  it  is  never  obtrusive,  and  always  at  hand. 

A  similar  space  on  the  left  hand  should  be  reserved  for  a  small 
round  stand  fitted  with  a  flat  cylindrical  glass  shade  with  a  knob  on 
the  top.  The  stand  should  be  suitably  arranged  to  hold  two  eye- 
pieces, three  objectives,  one  condenser,  a  bottle  of  cedar-oil  (fitted 
with  a  suitable  pointed  dipper),  and  a  box  containing  the  condenser- 
stops.  This  is  a  most  useful  arrangement  for  such  a  table ;  and  it 
need  not  have  a  diameter  greater  than  nine  inches. 

The  size  for  the  top  of  such  a  table  should  be  4^  x  3  feet,  and  as 


MICKOSCOMSTS'   WORK-TABLES 


399 


no  work,  such  as  mounting1  or  dissecting,  may  be  supposed  to  be  done 
at  this  table,  it  is  well  to  cover  the  surface  with  morocco,  that 
being  very  pleasant  and  suitable  to  work  upon. 

It  should  be  remembered  that  for  a  full-sized  microscope  a  depth 
of  three  feet  is  required  for  comfortable  work  When  the  micro- 
scope is  set  up  for  drawing.1  the  lamp  being  used  direct,  2  ft.  5  in. 
is  the  narrowest  limit  in  which  this  can  be  accomplished. 

Another  point  of  much  importance  is  the  height  of  the  table. 
Ordinary  tables,  being  about  2  ft.  4  in.  high,  are  too  low  even 
for  large  microscopes.  Tvo  or  three  inches  higher  than  this  will  be 
found  to  greatly  facilitate  all  the  work  to  be  done.  It  is  best  to 
have  the  table  made  completely %  on  thoroughly  solid  square  legs,  to 
the  height  of  2  ft.  7  in.  ;  but -we  may  employ  the  glass  blocks 
employed  underneath  piano  feet  as  an  expedient.  It  is  further  im- 
portant to  have  the  table  quite  open  underneath,  and  not  with  nests 
of  drawers  on  either 
side,  because  with  this 
particular  table  it  will 
be  frequently  required 
that  two  persons  may 
sit  side  by  side,  which 
is  only  possible  with  a 
cleai-  space  beneath. 

The  accompanying- 
illustration  (fig.  335), 
with  the  appended  re- 
ferences, will  make  quite 
clear  the  character  of 
the  table  which  we  re- 
commend, as  well  as  the 
mode  of  using  it. 

The  table  above  de- 
scribed is  supposed  to 
be  employed  wholly  for 
general  purposes  of  ob- 
servation or  research  on  \\  holly  or  partially  mounted  objects.  But 
the  microscopist  who  aims  at  more  than  this  will  require  an  arrange- 
ment for  dissecting,  mounting,  and  arranging  histological  and  other 
preparations,  and  in  some  cases  a  special  table  for  general  purposes 
of  microscopical  biology.  These  are  certainly  not  essentials,  especi- 
ally if  the  work  done  is  a  mere  occasional  occupation  ;  but  where 
anything  like  continuity  or  periodical  regularity  of  occupation  with 
such  work  is  intended,  these  will  be  of  great  service. 

A  dissecting  and  mounting  table  is  indeed  of  inestimable  value  to 
those  who  aflect  complete  order  and  cleanliness  in  the  accomplishment 
of  such  work. 

We  have  found  in  practice  that  a  table  firmly  made,  with  a  height 
of  2  ft.  6  in.,  semicircular  in  form,  and  a  little  more  than  half  the 
circle  in  area  on  the  outside,  with  the  arc  of  another  circle  cut  out 
from  it  to  receive  the  person  sitting  at  work — much  after  the  fashion 

1  Chap.  IV.  p.  287. 


FIG.  335. — Microscopist's  table. 
(Scale,  A  inch  to  1  foot.) 

1.  Case  for  microscope;  2.  Cabinet  for  objects; 
3.  Microscope  lamp ;  4.  Lamp  with  shade ; 
5.  Stand  of  apparatus  ;  6.  Book  ;  7.  Large  micro- 
scope ;  8.  Second  microscope;  9.  Writing  pad; 
10.  Bull's-eye  stand  ;  11.  Light-modifier. 


400  MANIPULATION  AND   PEESEEVATION   OF   THE   MICKOSCOPE 

of  the  jeweller's  bench — serves  admirably.  A  rough  suggestion  of 
this  is  given  in  fig.  336,  which  presents  the  plan  of  the  top  of  the 
table.  The  whole  area  beneath  should  be  unoccupied,  but  at  A  and 
B  drawers  may  be  put,  not  extending  more  than  four  inches  below 
the  under  surface  of  the  top  of  the  table ;  on  the  side  B  a  couple 
of  shallow  drawers,  with  everything  required  in  the  form  of  scalpels, 
needles,  scissors,  forceps,  pipettes,  life-slides,  &c.  in  the  upper  one, 
and  pliers,  cutting  pliers,  small  shears,  files  of  various  coarsenesses 
and  finenesses,  &c.  in  the  other  ;  on  the  A  side  a  single  drawer  con- 
taining slips,  covers  of  various  thicknesses,  bone,  tin,  glass,  and  other 
cells  of  all  (assorted)  sizes,  watch-glasses,  staining  cups  or  slabs, 
lifters  (if  used),  saiu  with  fine  teeth,  hones  of  various  shapes,  pewter 
plate  for  grinding  and  polishing  glass,  <fec.,  platinum  capsule,  camera 
lucida,  three  '  No.  2  '  sable  brushes  (water-colour),  &c. 

In  this  way  all  that  is  needed  for  dissection  or  mounting  will  be 
within  reach  without  moving  from  the  chair ;  and  if  by  an  arrange- 


FIG.  386. — Dissecting  and  mounting  table. 

ment  which  most  moderately  ingenious  manipulators  could  accom- 
plish, each  of  the  articles  in  the  drawers  has  a  fixed  place,  there  will 
be  no  difficulty  in  finding  by  touch  what  is  wanted. 

The  table  top  may  be  of  pitch  pine  stained  black,  or,  still  better, 
some  very  hard  wood  finished  smoothly,  but  '  grey.' 

The  space  in  the  figure  immediately  in  front  of  the  operator 
may  be  cut  out  to  a  convenient  size  and  thickness.  A  thick  plate - 
glass  slab  whose  edges  on  the  right  and  left  sides  shall  be  slightly 
bevelled,  so  that  it  may  slide  firmly  into  a  prepared  space  cut  into 
the  surface  of  the  table,  should  occupy  this  space,  the  surface  being 
exactly  level  with  the  surface  of  the  table.  This  plate  of  glass  should 
be  made  black  on  its  under  side,  so  as  to  present  a  uniform  black 
surface.  This  is  often  of  great  value  in  certain  kinds  of  work. 
Equally  useful  is  a  purely  white  unabsorbent  surface,  and  a  slab  of 
lohite  porcelain  may  be  easily  obtained  of  the  same  size  and  be  made 
to  fit  exactly  into  the  same  place. 


APPLIANCES   FOE   DISSECTING   TABLE 


4OI 


In  using  this  table  for  dissection  the  arms  have  complete  rest, 
and  1  in  the  figure  would  represent  the  position  of  the  dissecting 
microscope. 

2  is  a  suitable  position  for  a  small   easily  managed  microtome 
for  general  (chiefly  botanical)  purposes.     We  find  that  of  Ryder  *  to 
answer  this  purpose  admirably. 

3  is  a  small  vessel  of  spirit  (dilute)   for   use  with  the  section 
knife. 

4  is  a  stand  of  mounting  media,  in  suitable  bottles,  as  Canada 
balsam  in  paraffin,  or  xylol,  glycerine,  &c.,  as  well  as  small  bottles 
of  reagents  for  botanical  or  zoological  histology,  &c. 

.1  is  a  nest  of  apertures  in  whjch  to  place  partly  mounted  objects, 
to  protect  them  from  dust,  while  "the  balsam,  dammar,  &c.  may  be 
hardening  on  the  cover  so  as  to  be  in  a  suitable  state  for  final  mount- 
ing. A  slide  may  go  over  the  sloping  front  of  this  and  wholly  ex- 
clude dust. 

6  is  a  stand  of  cements,  varnishes,  &c.,  such  as  are  needful ;  and 

7  is  a  turn-table. 

For  the  work  of  dissection,  when  the  subject  requires  reflected 
light,  one  of  the  desiderata  is  a  mode  of  illumination  at  once  con- 
venient and  intense.  Mr.  Frank  R.  Cheshire,  F.L.S.,  &c.,  whose  work 
on  '  Bees  and  Bee-keeping '  is  a  proof  of  knowledge  and  practice  of 
minute  anatomy,  adopts  an 
old  plan  which  we  have 
always  found  admirable.  It 
is  illustrated  in  fig.  337. 
Rays  of  light  from  a  lamp 
are  parallelised  by  a  bull's- 

eye  full  upon  an  Abraham's  FlG  887._Mode  of  illuminati0n  for 

prism  and  focussed  upon  the  dissection, 

object.      The  prism  may  be 

mounted  on  a  long  many -jointed  arm,  and  is  of  mbst  varied  useful- 
ness. A  Stephenson's  binocular  is,  we  believe,  employed  by  this 
gentleman,  but  it  will  serve  admirably  for  any  form  of  dissecting 
instrument. 

For  the  more  general  purpose  of  the  private  laboratory  a  plain, 
firm  table  4  feet  6  inches  x  3  feet  in  area,  of  a  suitable  height  for 
the  worker,  should  be  fitted  as  follows,  viz. :  if  fig.  338  represent  the 
•rough  plan  of  the  table,  1  and  2  are  gas  fittings  attached  to  the  main 
to  supply  blowpipe,  Bunseris  burner,  &c. 

4  is  a  small  tube  of  metal  attached  to  the  water  main,  with  a 
tap,  and  bent  in  the  form  of  an  inverted  fj,  with  the  attached  leg  of 
the  pj  the  longer.     This  affords  a  pleasant  stream  of  water  for  wash- 
ing dissections,  &c.  ;  and  if  the  open  end  be  made  with  a  screiv,  and 
have  a  suitably  made  piece  of  tubing  fitted  to  screw  on  to  it,  this  latter 
may  be  attached  to  an  indiarubber  tube,  at  the  other  end  of  ivhich  we 
may  fasten  fine  glass  nozzles,  which  will  act  as  wash  bottles  of  the 

bore,  and  serve  with  the  finest  dissecting  work. 

5  is  a  glass  trough  for  waste,  with  a  perforated  aperture,  6,  con- 

1  Journ.  R.M.S.  new  series,  1887,  p.  682. 

D  D 


402   MANIPULATION  AND   PRESERVATION   OF   THE   MICROSCOPE 


/      Z 


nected  with  a  waste-pipe,  through  which  the  waste  water,  etc.  flows 
innocuously  away. 

3  represents  the  position  of  a  Thoma  microtome,  and  A,  B  are  two 
well-framed  flat  slides,  which  may  be  drawn  out  eighteen  inches,  or 

pushed  fully  in.     They 

/-N  are  found    at   times   to 

I  \4  be     of    great      service, 

where  the  space  is  some- 
wrhat  confined. 

This  table  may  be 
fitted  on  one  side  (the 
left)  at  least  with  a  set 
of  drawers  and  shelves 
for  receiving  various  ap- 
paratus and  materials, 
with  larger  quantities  of 
stains  and  reagents, 
ha rdeiiing,  macerating, 
and  other  materials; 

I  l  i          ^          i        while  if  a  door  covers 

^  A  "*  -B  the  whole,  the  inner  side 

FIG.  338. — Laboratory  table  for  microscopical  work,      of  this    may  be   readily 

fitted  to  receive  drop- 
bottles  1  containing  all  the  stains,  reagents,  and  similar  materials  in 
constant  use.  If  these  be  labelled  with  paper  labels  saturated  in  a 

solution  of  solid  paraf- 
fin in  turpentine,  and 
after  the  turpentine 
has  evaporated  firmly 
fixed  on  the  bottle, 
they  are  very  perma- 
nent, and,  indeed, 
better  than  anything 
we  have  tried  save 
where  the  name  of 
the  contents  is  en- 
amelled or  engraved 
on  the  bottle. 

It  has  been  al- 
ready pointed  out 
that  there  are  condi- 
tions of  research  in 
which  the  microscope 
has  to  be  in  a  con- 
stantly vertical  position.  This  was  the  case  with  the  researches  on 
the  saprophytic  organisms  made  conjointly  by  the  present  Editor 
and  Dr.  J.  J.  Drysdale.2  It  must  always  be  the  case  where  certain 
forms  of  continuous  life  stages  are  employed  for  prolonged  or  coii- 

1  Chapter  VII. 

2  Monthly  Micro.  Journ.  vols.  x.  to  xviii. ;  Journ.  R.M.S.  vol.  iii.  p.  1 ;   vol.  v. 
series  ii.  p.  177 ;  vol.  vi.  p.  193 ;  vol.  vii.  p.  185  ;  Vol.  viii.  p.  177. 


FIG.  339. — Tripod  for  using  microscope  in  an 
upright  position. 


SPECIAL    USE   OF   MICROSCOPE— UPRIGHT 


403 


tinuous  observations  on  the  development  of  the  minutei  forms  of 
life. 

In  such  cases  the  table  is  quite  unsuitable,  and  special  stands 
have  to  be  employed  that  from  their  form  give  great  stability  to  the 
microscope,  and  afford  the  body  and  head  of  the  observer  as  much 
command  and  ease  in  using  the  instrument  in  this  awkward  position 
as  can  be  obtained. 

This  is  best  done  by  means  of  a  firmly  made  tripod,  with  a  V- 
shapecl  piece  at  the  top  made  to  receive  the  feet  of  the  microscope. 
Fig.  339  is  an  outline  of  the  construction.  The  three  legs  of  the 


FIG.  340. — Using  the  microscope  in  an  upright  position  for  special  investigations 
necessitating  its  use  in  this  position. 

tripod  are  well  made  and  firmly  braced  together  with  metal  rods. 
A,  A  is  the  bed  for  the  tripod  feet  of  Powell  and  Lealand's  large 
stand.  B  is  a  table  which  slides  to  the  level  of  A,  A,  or  down  to  its 
present  position.  This  is  mainly  to  receive  the  lamp. 

By  this  arrangement  the  body  can  so  place  itself  as  to  command 
the  instrument  fully,  and  there  is  an  arrangement  at  the  two  sides, 
A,  A,  to  receive  supports  on  which  the  arms  may  rest  when  any 
other  manipulation  than  that  involved  in  working  the  fine  adjust- 
ment and  the  milled  heads  of  the  stage  is  required.  The  manner  of 

D  D  2 


404   MANIPULATION   AND    PEESERVATION   OF   THE   MICROSCOPE 


using  this  arrangement  is  seen  in  fig.  340.  In  that  case,  however, 
the  whole  is  employed  for  the  making  of  a  camera  lucida  drawing 
with  a  -g^-inch  objective ;  it  is  not  a  desirable  position  for  general 
work,  but  was  absolutely  needful  for  the  kind  of  investigation  being 
pursued  ;  and  the  position  of  the  basal  tripod,  the  microscope  upon 
it,  the  position  of  the  lamp  (partly  seen  in  the  immediate  fore- 
ground to  the  left),  and  the  relative  ease  with  which  the  entire 
instrument  is  at  the  command  of  the  observer,  will  be  manifest. 
In  order  to  use  the  microscope  successfully,  we  must  have  an 

illumination  the  inten- 
sity of  which  we  can 
fully  rely  on.  Dayliyht 
has  certain  qualities  thai 
involve  advantages  at 
times,  and  under  special 
circumstances,  in  its  em- 
ployment, but  this  is  the 
exception  rather  than  the 
rule.  What  is  needed 
is  a  well-made  lamp  with 
a  flat  flame ;  this  we 
should  be  able  to  control 
with  great  ease  as  to 
height  and  distance  from 
the  microscope.  No- 
thing is  equal  practically 
to  a  ^-inch  or  a  1-inch 
paraffin  lamp  ;  this  gives 
the  whitest  light  artifi- 
cially accessible  save  the 
higher  intensities  of  the 
incandescent  electric 
light.  But  there  is  no- 
thing of  this  kind  at 
present  accessible  to  the 
student.  The  employ- 
ment of  the  edge  of  the 
flame  of  a  well-made 
paraffin  lamp  used  with 
good  '  oil '  has  no  present 
rival.  Its  illuminating 
power  should  be  about 
2^  candles.  Gas  is  much  yellower,  and  not  so  easy  in  employment. 
To  get  the  best  form  of  microscopical  lamp  is  a  matter  of  some 
importance.  We  call  the  attention  of  the  reader  to  the  best  simple 
form  of  lamp  which  will  accomplish  every  purpose.  This  is  a  model 
arranged  by  Mr.  Nelson,  the  drawing  of  which  is  given  in  fig. 
341.  The  lamp  burns  paraffin  and  has  an  ordinary  J-inch  wick 
burner.  The  reservoir  is  rectangular  and  flat,  5J  x^4  x  1J;  it 
serves  three  distinct  purposes :  1st,  it  will  hold  sufficient  oil  to  burn 
for  a  whole  day  ;  2nd,  permits  the  lamp  to  be  lowered  near  the 


FIG.  341. — Lamp  devised  by  Mr.  E.  M.  Nelson. 


NELSON'S  LAMP  405 

table ;  3rd,  radiates  the  heat  conducted  by  the  metal  chimney,  and 
prevents  the  oil  boiling.  The  burner  is  placed  at  one  angle  of  the 
reservoir  to  enable  the  flame  to  be  placed  very  near  the  stage  of  the 
microscope,  which  is  exceedingly  useful  with  some  kinds  of  illumina- 
tion, especially  with  reflected  light,  with  the  higher  powers,  and  for 
Powell  and  Lealand's  super-stage  condenser. 

The  hole  for  filling  the  reservoir  is  placed  at  the  diagonal  corner 
for  convenience.  The  chimney  is  metal,  with  an  ordinary  3x1 
glass  slip  in  front ;  the  diameter  of  the  flame-chamber  should  not 
exceed  1^  inch,  and  the  grooves  holding  the  glass  slip  should  project 
J  inch  from  the  flame-chamber  ;  the  aperture  should  be  only  1^  inch 
long;  length  of  chimney  should > be  7  inches.  Chimney  should  be 
dead-black  inside.  This  chimney'  serves  four  purposes  :  1st,  image 
of  flame  is  not  distorted  by  striae  and  specks  common  to  ordinary 
lamp  chimneys ;  2nd,  prevents  reflexion  from  inner  surface  of 
chimney,  which  causes  a  double  image  of  flame ;  3rd,  prevents 
scattered  light  in  room  ;  4th,  is  not  readily  broken ;  slips  can  be 
easily  replaced.1 

By  rotation  of  chimney  either  the  edge  or  flat  side  of  the  flame 
may  be  used.  The  bull's-eye  is  of  Herschel's  form,  viz.  a  meniscus 
and  crossed  convex ;  it  is  mounted  on  an  arm  which  rotates  cen- 
trally with  the  lamp  flame.  Unfortunately,  as  we  have  seen 
(p.  332),  there  are  errors  in  Sir  J.  Herschel's  original  calcula- 
tion, and  with  these  it  has  been  copied  by  many  opticians  ;  a  lens, 
it  has  been  demonstrated,  can  be  made  on  the  Herschel  formula, 
as  calculated  by  Mr.  Nelson,  having  a  minimum  aberration. 
The  arm  is  slotted  so  that  the  bull's-eye  may  be  focussed  to  the 
flame ;  it  can  be  fixed  by  a  clamping  screw.  The  bull's-eye  may 
also  be  elevated  or  depressed  and  fixed  by  a  clamping  screw,  not 
shown  in  the  illustration.  The  bull's-eye,  having  once  been  focussed, 
is  permanently  clamped,  and  it  is  brought  into  or  taken  out  of  posi- 
tion simply  by  rotation  of  the  arm.  There  should  be  a  groove  in 
the  pillar  with  a  steadying  pin  on  the  lamp  to  prevent  rotation 
during  elevation  or  depression. 

The  form  of  the  clamping  screw  is  important ;  it  should  be  at  the 
upper  part  of  the  tube,  and  not  at  the  lower,  as  shown  in  the  figure. 
This  keeps  the  screw  clean  from  oil,  which  always,  to  a  greater  or  less 
extent,  exudes  over  paraffin  lamps.  The  screw  should  be  of  that 
form  which  closes  a  pinching  ring  round  the  rod,  and  not  merely  a 
screw  which  screws  on  to  the  rod  and  bruises  it.  This  lamp,  if 
made,  as  it  should  be,  with  a  japanned  tin  reservoir  and  a  cast- 
iron  tripod  foot,  is  quite  inexpensive.  There  is  no  justification 
for  a  circular  foot,  except  that  it  can  be  readily  and  well  finished 
in  the  lathe  with  better  apparent  results  and  less  labour  than  other 
forms. 

A  small  lamp  is  made  by  Messrs.  B.  and  J.  Beck.  We  illus- 
trate it  in  fig.  342. 

The  base,  A,  consists  of  a  heavy  ring,  into  which  a  square  brass 

1  It  is  very  important  to  remove  the  metal  chimney  after  use,  or  at  least  not  to 
leave  it  on  when  not  in  use,  since  the  evaporating  paraffin  gathers  round  it  and  causes 
undesirable  scent  when  the  lamp  is  again  lit.  The- thinnest  slips  should  be  used. 


406   MANIPULATION  AND   PRESERVATION   OF  THE   MICROSCOPE 

rod,  B,  is  screwed.     The  square  rod  carries  a  socket,  C,  with  an  arm, 
D,  to  which  the  lamp  is  attached. 

On  each  side  of  the  burner,   and   attached   to  the  arm,  D,  is  an 
upright  rod,  G,  to  one  of  which  the  chimney  is  fixed,  independent  of 


FIG,  342. 

the  reservoir  of  the  lamp,  thus  enabling  the  observer  to  revolve  the 
burner  and  reservoir,  and  obtain  either  the  edge  or  the  flat  side  of 
the  flame  without  altering  the  position  of  the  chimney.  The 
chimney,  F,  is  made  of  thin  brass,  with  two  openings  opposite  to 
each  other,  into  which  slide  3x1  glass  slips  of  either  white,  blue,  or 


LAMP   WITH  LATERAL  MOTION 


407 


opal  glass,  the  latter  serving  as  a  reflector  ;  but  we  do  not  consider 
the  reflexion  here  accomplished  as  other  than  an  error ;  it  causes 
double  reflexion  and  confuses  the  condensed  image. 

A  semicircle  swings  from  the  two  uprights,  G,  to  which  it  is 
attached  by  the  pins,  H,  placed  level  with  the  middle  of  the  flame ; 
to  this  semicircle  is  fixed  a  dovetailed  bar,  L,  carrying  a  sliding 
fitting,  O,  which  bears  a  Herschel  bull's-eye,  P.  This  is  complex, 
and  therefore  costly. 

The  bull's-eye  is  fixed  at  any  inclination  by  a  milled  head  working 
in  a  slotted  piece  of  brass,  K, 
fixed  to  the  arm,  D. 

For  use  with  the  micro- 
scope in  an  upright  position, 
when  prolonged  investiga- 
tions have  to  take  place,  the 
lamp  becomes  even  of  more 
importance  than  under  ordin- 
ary circumstances.  The  pre- 
sent Editor  devised  a  some- 
what elaborate  apparatus  of 
this  kind,  which  he  always 
employs  in  this  kind  of  ob- 
servation.1 But  the  essential 
part  of  it  is  only  an  arrange- 
ment by  which  a  milled-head 
movement  of  the  entire  lamp 
may  take  place  to  the  right 
or  the  left  of  the  observer, 
a>  well  as  a  similar  power  to 
elevate  or  depress  the  posi- 
tion of  the  flame.  When  the 
microscope  is  fixed,  and  the 
rectangular  prism  for  illu- 
mination (in  place  of  the 
mirror)  is  fixed  at  right 
angles,  the  centring  of  the 
lamp  flame  upon  the  object 
is  more  readily  done  by  means 
of  motion  in  the  lamp.  A 
very  simple  form  of  this  lamp 
has  been  made  for  the  Editor 
by  Mr.  Charles  Baker,  of 
Holborn ;  it  is  seen  in  fig. 

343,  being  an  ordinary  lamp,  except  that  the  milled  head  to  the 
right  as  we  face  the  flame  racks  up  and  down  the  entire  lamp,  and 
the  milled  head  behind,  and  at  right  angles  to  this,  works  a  rack 
and  pinion  (shown  in  the  engraving)  carrying  the  whole  lamp  to 
the  right  or  left  of  the  middle  position.  This  lamp  would  be 
better,  if  the  student  did  not  object  to  the  cost,  to  be  made  with  a 
metal  reservoir,  or  at  least  to  have  an  arrangement  by  means  of 
1  Monthly  Micro%  Jo  urn.  vcl.  xv.  p.  165f 


FIG.  343. 


408   MANIPULATION   AND   PRESERVATION   OF   THE   MICROSCOPE 

which  the  bull's-eye  (with  a  catch  fixing  its  focus  from  the  flame) 
were  so  affixed  as  to  be  carried  up  and  down  and  to  right  and  left 
with  the  lamp. 

When  the  microscope  is  fixed  in  its  upright  position,  and  the 
prism  is  arranged  to  give  direct  and  not  oblique  reflexion,  the  lamp 
flame,  by  means  of  a  card,  is  arranged  as  nearly  right  for  the  re- 
flexion of  the  image  of  the  flame  into  the  centre  of  the  field  as  may 
be,  and  then  a  little  movement  in  one  or  both  milled  heads  will 
bring  it  accurately  into  the  field. 

We  may  arrange  the  microscope  for  ordinary  transmitted  light, 
that  is,  for  light  caused  to  pass  through  the  object  into  the  object- 
glass,  by  placing  it  upon  the  table,  arranged  as  already  directed ; 
the  instrument  is  then  sloped  to  the  required  position,  and  a  con- 
denser, suitable  to  the  power  to  be  employed,1  is  put  into  the  sub- 
stage.  The  lamp  is  now  put  into  the  right  position,  with  a  bull's- 
eye,  on  the  left  of  the  observer.  The  condenser  is  then,  as  described 
below,  to  be  '  centred,'  when  the  objective  may  be  changed 
as  desired,  and  the  eye-piece  altered  to  suit.  But  it  should  be 
carefully  noted  that,  if  apochromatic  powers  are  being  used,  there 
must  be  accurate  adjustment  of  the  tube  length  if  the  best  results 

are  to  be  obtained ;  and 
with  any  serious  increase 
of  the  power  of  the  objec- 
tive a  condenser  of  higher 
aperture  and  shorter  focus 
must  be  used. 

FIG.  344.— Edge  of  lamp  flame  in  centre  and  Often,  however,  as  good 

focus  of  bull's-eye.  or  better  results  may  be 

obtained  without  the  em- 
ployment of  the  mirror  at  all,  the  light  being  sent  directly  through 
the  condenser  from  the  lamp  flame.  The  mode  of  arrangement  for 
this  kind  of  manipulation  is  presented  in  Plate  V.,  where  it  will  be 
observed  that  the  microscope  is  inclined  more  towards  the  horizontal 
to  suit  the  observer  ;  the  lamp  is  directly  in  front  of  the  sub-stage, 
the  mirror  is  turned  aside,  and  a  frame  (fixed  upon  a  bull's-eye 
stand)  carrying  a  monochromatic  screen  is  placed  between  the  lamp 
flame  and  the  condenser  (sub-stage). 

By  this  means  the  light  is  sent  into  the  condenser  and  upon  the 
object,  and  is  then  treated  as  is  the  case  (for  centring)  when  the 
mirror  is  used.  The  first  step  in  the  direction  of  efficiency  in  the 
use  of  the  microscope  is  to  understand  the  principles  of  illumination, 
and  a  knowledge  of  the  various  effects  produced  by  the  bull's-eye 
lies  on  the  threshold  of  this. 

Having  given  details  as  to  the  forms  of  lamp  which  are  of  most 
service,  we  assume  that  a  paraffin  lamp  with  1-inch  wick  is  used. 

If  we  place  the  edge  of  this  flame  (E,  fig.  344)  in  the  centre  and 
exact  focus  of  the  bull's-eye  B,  A  shows  the  effect  of  doing  so. 

If  a  piece  of  card  were  held  in  the  path  of  the  rays  proceeding 
from  B,  the  picture  as  shown  at  A  would  not  be  seen— instead  of  it 
an  enlarged  and  inverted  image  of  the  flame.  The  image  at  A  is 
,x  Vide.  Chapter  IV.  p,  298. 


^  A 

D 


THE   USE    OF   THE    BULL'S-EYE 


409 


FIG.  845. — Altered  relations  between  lamp 
flame  and  bull's-eye. 


obtained  by  placing  the  eye  in  the  rays  and  by  looking  directly  at 
the  bull's-eye. 

The  light  is  so  intense  that  it  is  more  pleasant  to  take  the  field 
lens  of  a  2-inch  eye-piece  and  place  it  in  the  path  of  the  rays  focus- 
sing the  image  of  the  bull's-eye  on  a  card.  It  should  be  noticed 
with  care  that  the  diameter  of  the  disc  A  depends  upon  the  diameter 
of  the  bull's-eye  B  ;  but  the  in- 
tensity of  the  light  in  A  depends  -D 
on  the  focal  length  of  B.  The 
shorter  the  focus,  the  more  in- 
tense will  be  the  light. 

We  are  here  assuming 
throughout  that  the  field  lens  is 
at  a  fixed  distance  from  the 
bull's-eye  B. 

But  if  we  move  the  flame,  E 
— still  central — within  the  focus 
of  B,  we  get  the  result  shown 
in  D,  fig.  345.  But  by  moving 
E  flthout  the  focus  of  B  we  get 
the  picture  H,  while  K  is  the  picture  when  E  is^focussed  but  not 
centred. 

A  common  error,  one  repeatedly  met  with,  is  that  of  placing  a 
concave  mirror,  C  (fig.  346),  so  that  the  flame,  E,  is  in  its  pi*incipal 
focus.  The  result  of  this  is  that  parallel  rays  are  sent  to  B.  These 
rays  are  brought  to  a  focus  at  a  distance  from  B  about  equal  to  twice 
the  radius  of  the  cur- 
vature of  B  and  then 
scattered,  a  totally 
different  result  from 
what  is  aimed  at.  If 
the  concave  mirror,  C, 
is  to  be  of  any  use  in 
illumination,  it  must 
be  placed  so  that  E  is 
not  at  its  principal  focus,  but  at  its  centre  of  curvature. 

The  bull's-eye  gives  an  illustration  of  what  is  of  wider  application. 
The  method  of  obtaining 
a  critical  image  with  a 
condenser  by  means  of 
transmitted  light  is 
shown  in  fig.  347.  E  is 
the  edge  of  the  flame,  S 
represents  the  sub- stage 
condenser,  and  F  the 
object.  F  is  thus  the 
focal  conjugate  of  E,  and 
F  and  E  are  in  the  prin- 
cipal axis  of  S  ;  that  is 
to  say,  these  are  the  relations  which  exist 
focussed  on  and  centred  to  an  object.  Let 


FIG.  846. — Result  of  placing  flame  in  principal  focus 
of  concave  mirror. 


FIG.  347.— Mode  of  obtaining  critical  image. 


when   £ 
this  be 


condenser   is 
understood  as 


410   MANIPULATION  AND   PRESERVATION   OF  THE   MICROSCOPE 

the  law,  and  there  can  be  but  little  difficulty  remaining  in  getting 
the  best  results  from  a  condenser. 

Fig.  348  illustrates  another  method  of  getting  the  same  result. 
We  may  illuminate  a  condenser  with  light  direct  from  the  name,  as 

in  fig.  347,  or  we  may  interpose  the 
f  mirror  as  in  fig.   348.     M  is  the 

plane  mirror,  and,  properly  used, 
exactly  the  same  result  may  be 
obtained  as  in  the  former  case.  It 
is,  however,  slightly  more  difficult 
to  set  up,  but  the  method  shown 
in  fig.  347  will,  'on  the  whole,  be 
preferable. 

Nothing  can  be  of  more  moment 
to  the  beginner  than  to  understand 
the  practical  use  of  the  condenser. 
We  must  direct  the  student  to  what 
has  been  stated  concerning  it  in 
Chapter  IV.  But  the  following 
should  be  carefully  considered. 
Fig.  349  shows  a  sub-stage  con- 
denser, S,  and  an  objective,  O,  both  focussed  on  the  same,  point. 
The  condenser  has  an  aperture  equal  to  that  of  the  objective.  Now 
if  the  eye-piece  be  removed,  and  we  look  at  the  back  lens  of  the 
objective,  it  will  be  seen  to  be  full  of  light,  as  at  R.  The  same 
thing,  but  with  the  aperture  of  the  condenser  cut  down  by  a  stop,  is 
seen  in  fig.  350.  Now  only  a  part  of  the  back  of  the  objective  is 
filled  with  light,  as  at  T  in  the  same  illustration. 

Now  it  does  not  follow,  because  the  back  lens  of  the  objective  is 
full  of  light,  as  in  fig.  349,  that  therefore  the  field  ought  to  be  full  of 
light.  The  field  only  shows  the  bright  image  of  the  edge  of  the  flame, 


M 


FIG.  348.—  Another  method  of  getting 
critical  image. 


FIG.  349. — Condenser  and  object-glass 
with  the  same  aperture. 


O 


FIG,  350. — The  same,  with  the  aperture 
of  the  condenser  cut  down. 


and  it  is  in  that  alone  that  a  critical  picture  can  be  found.  If  the 
condenser  be  racked  either  within  or  without  the  focus,  the  whole 
field  will  become  illuminated,  but  at  the  same  time  a  far  smaller  por- 
tion of  the  objective  will  be  utilised.  On  removing  the  eye-piece  and 
examining  the  back  lens  of  the  objective,  pictures  like  D,  H,  fig.  345, 
will  be  seen — D  when  within,  and  H  when  without  the  focus. 

The  condition  represented  in  fig.  349  at  R  and  O  is  the  severest 
test  which  can  be  applied  to  the  microscopic  objective  ;  that  is  to 
say,  to  fill  the  whole  objective  with  light  and  so  test  the  marginal 
and  central  portions  at  the  same  time. 

Even  to  obtain  the  state  of  illumination  known  as  '  diffused  day- 


TO   USE   DIFFUSED   DAYLIGHT 


411 


light '  with  the  simple  mirror  when  no  condenser  is  used  is  frequently 
done  in  a  most  inaccurate  manner.  The  correct  method  of  doing 
this  is  shown  in  fig.  351.  F  is  the  plane  of  the  object,  C  is  the  con- 
cave mirror,  the  mirror  being  placed  at  the  distance  of  its  principal 
focus  from  the  object.  But  the  manner  in  which  it  is  usually  done, 
from  want  of  thought  or  knowledge,  or  both,  is  shown  in  fig.  352, 


FIG.  351. — Illumination  for  '  diffused  daylight.' 

where  it  is  manifest  that  there  is  a  total  disregard  of  the  true  focal 
point  of  the  mirror  and  its  incidence  on  the  plane  of  the  object. 
From  the  impracticability  of  this  diagram  as  a  representation  of  a 
working  plan  of  illumination,  we  may  see  at  once  the  importance  of 
having  the  mirror  fixed  upon  a  sliding  tube,  so  that  its  focal  point 
may  be  adjusted 

It  is  also  important  here  to  note  that  in  daylight  illumination  a 


FIG.  352. — Erroneous  method  of  arrangement  for  '  diffused  daylight.' 

plane  mirror  gives  a  cone  of  illumination,  as  in  fig.  353,  when  there 
is  ample  sky -room  ;  but  a  window  acts  as  a  limiting  diaphragm. 

In  regard  to  the  parallelism  of  the  direct  solar  rays  there  is  of 
course  no  question.  But  the  parallelism  of  that  portion  of  the  solar 
light  wrhich  goes  to  form  the  firmament  in  our  own  higher  atmo- 
sphere is  so  completely  broken  up  by  refraction  and  reflexion 
amongst  the  subtle  particles  of  this  higher  atmosphere  that  the  rays 


412    MANIPULATION  AND   PRESERVATION   OF   THE   MICROSCOPE 

which  constitute  our  daylight  fall  from  every  point  of  the  visible 
heavens  (though  with  greatly  diminished  intensity).  That  is  to  say, 
we  have  at  disposal  a  light  source  extending  over  180°,  while  the  sun 
itself  extends  over  a  visual  angle  of  but  half  a  degree.  Being  thus 
surrounded  by  an  illimitable  and  self-luminous  expanse  of  ether  un- 
dulations, the  question  is  no  longer  of  parallel  rays  only,  but  of  light 
emanating  from  an  outer  circle  above  the  earth  upon  every  point  of 
the  earth's  surface ;  and  a  mirror  exposed  to  such  a  luminous  atmo- 
sphere must  both  receive  and  reflect  from  all  sides  and  upon  all 
sides.  If,  however,  it  be  placed  under  the  stage  of  a  microscope, 
all  vertical  light  is  intercepted,  and  there  remains  nothing  but  the 
oblique  incidence  as  the  starting-point  of  the  theory  of  illumination 
by  converging  light ;  for  it  scarcely  needs  repetition  that  obliquity 
of  incidence  gives  inevitable  rise  to  obliquity  of  reflexion ;  and  it 


FIG.  353. — Light  from  the  open  sky  falls  upon  the  mirror  in  all  directions. 

becomes  equally  clear  that  in  order  to  strike  the  object  the  light 
miist  always  fall  obliquely  on  the  mirror. 

Then  it  follows  from  what  has  been  said  that  the  light  falling 
from  the  open  sky  upon  a  mirror  falls  in  all  conceivable  directions. 
Thus  fig.  353  shows  the  lines  1  to  7,  including  an  angle  of  30°.  If 
nothing  intervene,  the  light  of  that  sky  surface  must  fall  upon  the 
mirror,  a  b,  and  be  reflected  on  O.  The  intermediate  rays,  2,  3,  4, 
5,  6,  form  the  converging  illuminating  pencil,  with  of  course  an  in- 
finity of  others  filling  up  the  spaces  between. 

In  other  words,  every  point  of  a  mirror  is  a  radiant  of  a  whole 
hemisphere,  and  this  is  equally  true  whether  the  mirror  be  plane, 
concave,  or  convex,  so  long  as  it  is  exposed  to  a  boundless  sky. 
Therefore  a  plane,  concave,  or  convex  mirror  will  give  a  cone  of 


LIGHT   REFLECTED    TO   A   FOCUS   FROM   THE   OPEN   SKY  413 


FIG.  354.— With  the  open  sky,  light  is 
focussed  at  all  points. 


illumination  of  which  the  object  is  its  apex,  no  matter  what  the  in- 
clination or  distance  of  the  mirror.  The  angle  of  the  cone  will  be 
the  angle  the  mirror  subtends  at  the  object — subject  of  course  to  its 
not  being  cut  down  by  a  stop. 

As  a  matter  of  fact,  the  boundless  sky  is  an  abstraction  which  is 
never  obtained  in  practice  ;  therefore  it  practically  does  make  a 
difference  whether  the  plane  or  concave  mirror  is  used,  and  whether 
the  latter  is  focussed  on  the  object  or  not. 

The  dotted  lines  in  fig.  354  show  rays  falling  on  six  different 
points  on  a  plane  mirror ;  the  continuous  lines  show  the  reflexions 
of  these  rays  on  the  object. 

The   heavy   lines   from    either   fc  \ 

extremity  of  the  mirror  to  the 
object  show  the  maximum  angle 
of  cone  that  mirror  will  give 
in  that  particular  position. 

The  influence  of  a  limita- 
tion (as  by  means  of  a  window) 
should  therefore  be  considered. 
The  extent  to  which  it  is 
limiting,  so  far  as  its  influence 
upon  the  illuminating  cone  is 
concerned,  is  shown  by  an  ex- 
amination of  the  back  of  the 
lens  of  the  objective  when  the 
eye-piece  is  removed.  Fig.  355  shows  the  back  of  the  objective  when 
the  plane  mirror  is  used,  and  fig.  349  R,  when  the  concave  mirror 
is  used,  as  in  fig.  351.  The  beginner  should  study  these  experiments 
by  repeating  them. 

Fig.  356  illustrates  the  method  of  obtaining  dark-ground  illumi- 
nation wThen  the  arrangement  shown  in  fig.  347  or  348  does  not  give 
a  sufficiently  illuminated  area  even  when 
the  flat  of  the  flame  is  used.  Of  course 
it  will  be  understood  that  for  the  dark- 
ground  result  a  suitable  stop  is  inserted 
beneath  the  sub-stage  condenser. 

It  has  been  shown  by  many  illustra- 
tions on  many  subjects  that  certain  results 
in  critical  work  can  be  obtained  with  the 
bull's-eye  which  are  not  so  accessible  with- 
out its'  use.  But  Mr.  T.  F.  Smith  has 
made  this  clear  regarding  the  structure  of  certain  diatoms. 

This,  there  can  be  no  doubt,  is  due  to  the  fact  that  the  parallel 
rays,  falling  on  the  sub-stage  condenser,  shorten  its  focus  and  in- 
crease the  angle  of  the  cone  of  illumination.  It  will  be  noticed  that 
when  the  bull's-eye  is  introduced  the  condenser  will  need  racking- 
up.  At  the  same  time  we  prefer  illumination  as  in  fig.  347  or  348, 
except  in  cases  where  illuminating  cones  of  maximum  angles  are 
required.  Thus  it  will  be  little  needed  with  transmitted  light 
except  when  oil-immersion  objectives  of  large  aperture  are  vised, 
because  illuminating  cones  up  to  *9  N.A.  can  be  obtained  with  good 


FIG.  355.— Image  at  the  back 
of  the  objective  when  day- 
light and  a  plane  mirror  are 
used. 


414  MANIPULATION   AND   PKESERVATION    OF   THE   MICROSCOPE 

condensers  by  the  method  shown  in  fig.  347.  But  when  the  micro- 
scope is  of  necessity  used  upright  the  rectangular  prism  or  the  plane 
mirror  must  be  used,  fig.  34'8. 

The  arrangement  at  fig.  356  is  sometimes  useful  for  photo- 
micrography when  it  is  otherwise  impossible  to  illuminate  the  whole 
field.  But  in  ordinary  cases  it  is  better  to  contract  the  field  than 
use  a  bull's-eye,  as  it  invariably  impairs  the  definition. 


FIG.  356. — Illumination  for  dark  ground  (with 
stop  beneath  the  condenser). 


FIG.  357. — Same  result  with  concave 


In  regard  to  this  last  figure  it  will  be  understood  that  (as  before) 
E  represents  the  edge  of  the  flame,  B  the  bull's-eye,  M  the  mirror,  S 
the  condenser  under  the  stage,  and  F  the  plane  of  the  obejct. 

The  same  result  as  the  above  may  be  obtained  by  the  concave 
mirror  (as  shown  in  fig.  357)  instead  of  the  bull's-eye.  But  this 
is  a  very  difficult  arrangement,  yielding  the  best  results  only  with 
great  application  and  care. 

But  the  supreme  folly  of  using  a  concave  mirror  and  a  bull's-eye 


FIG.  358.— Absurdity  of  using  a  bull's-eye 
and  a  concave  mirror. 


FIG.  359.— Absurdity  of  using  a  bull's- 
eye  with  the  edge  of  the  lamp  flame 
not  in  its  principal  focus. 


is  shown  in  fig.  358,  where  0  is  the  concave  mirror  and  (as  before)  S 
the  sub-stage  condenser ;  this  secures  a  result — as  will  be  seen  by  the 
relation  of  the  light  to  the  condenser  (S) — which  is  as  far  from  what 
is  sought  and  desirable  as  it  can  well  be,  while  another  lesson  of 
great  importance  may  be  learnt  from  fig.  359,  which  illustrates 
the  error  of  not  having  the  edge  of  the  flame  E  in  the  principal  focus 
of  the  bull's-eye  B.  The  rays  converge  on  the  condenser  S,  so  that 
it  will  become  in  all  probability  impossible  to  focus  it  on  the 


DARK-GROUND   ILLUMINATION 


415 


object.  This  is  a  lateral  lesson  on  the  value  of  having  the  bull's- 
eye  fixed  to  the  lamp,  so  that  both  may  be  moved  together ;  and 
there  should  be  a  notch  in  the  slot  or  arm  which  carries  the 
bull's-eye  to  denote  when  the  flame  of  the  lamp  is  in  its  principal 
focus. 

The  above  are  fundamental  principles  of  illumination,  and  if  the 
student  is  to  succeed  as  a  manipulator  he  must  demonstrate  and  re- 
demonstrate  them,  and  become  master  of  their  details  and  what  they 
collaterally  teach.  P--- 

We  may,  however,  with  much  advantage  give  them  a  larger  and 
more  detailed  application  to  the  practical  setting  up  of  a  dark- 
ground  illumination,  as  in  fig.  356. 

Let  an  object  such  as  a  tricerdtinm  (diatom)  be  taken,  and  sup- 
pose that  the  objective  employed  is  a  ^-inch  of  *28  N.A.  We  must 
first  adjust  the  lamp  and  bull's-eye,  as  in  fig.  344,  and  get  the  edge 
of  the  lamp  flame  extended  to  a  disc  as  at  A. 

Xow  let  a  small  aperture  be  put  into  the  condenser  and  a  tri- 
ceratium  on  the  stage  and  the  §  objective  on  the  nose-piece. 

The  microscope  being  put  into  position,  the  lamp  should  be 
placed  on  the  left-hand  side  of  it — a  lamp  with  a  fixed  bull's-eye  is 


FIG.  360. 


FIG.  361. 


FIG.  362. 


FIG.  363. 


assumed— and  it  should  now  be  arranged  as  to  height,  so  that  the 
rays  from  the  bull's-eye  should  fall  fairly  on  the  plane  mirror,  this 
latter  being  inclined  so  as  to  reflect  the  beam  on  the  back  of  the 
sub- stage  condenser. 

Xow,  with  any  kind  of  light,  focus,  and  place  in  the  centre  of  the 
field,  the  triceratium,  as  in  fig.  360  ;  then  rack  the  condenser  until 
the  small  aperture  in  its  diaphragm  comes  into  focus ;  centre  this 
to  the  triceratium,^  in  fig.  361.  Rack  the  condenser  closer  up 
until  the  bull's-eye  is  in  focus,  as  in  fig.  362. 

Here  it  happens  that  the  bull's-eye  is  not  in  the  centre,  and  it  is 
not  uniformly  filled  with  light,  but  has  instead  two  crescents  of  light 

This  is  a  case  which  frequently  repeats  itself,  but  it  is  of  course 
not  inevitable.  The  bull's-eye  may  be  more  or  less  filled  with  light, 
and  may  or  may  not  be  more  nearly  centred.  In  this  case  we  have 
next  to  centre  the  image  of  the  bull's-eye  to  the  triceratium  by 
moving  the  mirror,  as  in  fig.  363. 

But  it  will  be  noticed  that  this  centring  of  the  image  of  the 
bull's-eye  does  not  rectify  the  diffusion  of  the  light.  This  will  be  at 
once  done  by  moving  the  lamp  with  attached  bull's-eye  ;  this  motion 
requires  to  be  a  kind  of  rotation  in  azimuth  round  the  wick  as  an 
axis.  The  relative  positions  of  the  lamp  and  bull's-eye  must  on  no 


4l6   MANIPULATION   AND   PKESERVATION   OF   THE   MICROSCOPE 

account  be  altered,  and  it  is  understood  that  the  Limp  was  adjusted 
to  the  picture  A  in  fig.  344  by  inspection  and  without  the  micro- 
scope. A  very  slight  movement  in  azimuth,  however,  is  enough  to 
effect  the  desired  end  (fig.  364),  and  all  that  now  remains  is  to  open 
the  full  aperture  of  the  condenser  and  put  in  the  smallest  stop  ;  if 
this  does  not  stop  out  all  the  light,  a  larger  one  must  be  tried  ;  but 
it  is  of  the  greatest  importance  that  the  smallest  stop  possible  be 
used,  a  very  little  difference  in  the  size  of  the  stop  making  a  remark- 
able difference  in  the  quality  of  the  picture.  Hence  the  need  of  a 
large  and  varied  supply  of  stops  with  all  condensers. 

On  account  of  some  residual  spherical  aberration  the  condenser 
will  probably  have  to  be  racked  up  slightly  to  obtain  the  greatest 
intensity  of  light. 

In  fig.  364  the  expanded  edge  of  the  flame  covers  the  triceratium. 
When  the  whole  aperture  of  the  condenser  is  opened  the  size  of  that 
disc  will  not  be  altered,  its  intensity  only  will  be 
increased.  When  the  stop  is  placed  at  the  back 
of  the  condenser,  only  in  that  part  of  the  field 
represented  by  the  disc  of  light  will  the  object 
be  illuminated  on  a  dark  ground.  If,  therefore, 
the  disc  of  light  does  not  cover  the  object  or  ob- 
jects, bring  the  lamp  nearer  the  mirror.  The 
size  of  the  disc  of  light  depends  on  three  things  : — 
FIG.  364.  o.  The  diameter  of  the  bull's-eye. 

ft.  The  length  of  the  path  of  the  rays  from 
the  bull's-eye  to  the  sub-stage  condenser. 

y.  The  magnifying  power  of  the  condenser. 

If  a  and  y  are  constants,  the  only  way  of  varying  the  size  of  the 
dark  field  is  by  ft. 

In  the  same  way  the  intensity  of  the  light  in  the  disc  depends  on 
three  things. 

A.  The  initial  intensity  of  the  illumination. 

B.  The  angular  aperture  of  the  bull's-eye. 

C.  The  angular  aperture  of  the  sub-stage  condenser. 

If  the  student  will  thoroughly  and  practically  understand  the 
above  series  of  single  demonstrations,  and  ponder  such  inevitable 
variations  as  practice  will  bring  in  regard  to  them,  the  '  difficulties 
of  illumination '  will  have  practically  passed  away. 

There  are  two  kinds  of  microscopical  work — one,  the  more  usual 
and  comparatively  easy,  is  the  examination  of  an  object  to  see  some- 
thing which  is  known.  The  other  is  the  examination  of  an  object 
in  search  of  the  unknown.  Thus  some  blood  may  be  examined 
for  the  purpose  of  finding  a  white  corpuscle.  It  matters  little 
what  is  the  quality  of  either  the  lens  or  the  illumination  or  the 
microscope,  or  whether  the  room  is  darkened  or  not,  because  the 
observer  knows  that  there  is  such  a  thing  as  a  white  corpuscle.  It 
is  quite  immaterial  as  to  whether  the  observer  had  ever  seen  one  or 
not ;  so  long  as  he  possesses  the  knowledge  that  there  is  such  a  thing, 
the  finding  of  it,  even  under  unfavourable  conditions,  will  be  an 
easy  task. 

But  if  the  observer  has  not  that  knowledge,  he  may  examine 


SEARCH   WORK — LIGHT   AND   THE   EYES  417 

blood  many  times,  under  favourable  conditions,  and  yet  not  notice 
the  presence  of  a  white  corpuscle,  and  that,  too,  with  one  immediately 
in  the  centre  of  the  field  ;  this,  moreover,  is  a  large  object. 

It  is  only  those  in  the  habit  of  searching  for  new  things  who  can 
appreciate  the  enormous  difficulty  in  first  recognising  a  new  point. 
Therefore,  when  critical  work  is  undertaken,  care  should  be  exercised 
to  have  the  conditions  as  favourable  as  possible. 

When  working  with  artificial  light  all  naked  lights  in  the  room 
should  be  avoided. 

It  is  quite  unreasonable  to  expect  the  retina  to  remain  highly 
sensitive  if,  whenever  the  eye  is  removed  from  the  eye-piece,  it  is 
exposed  to  the  glare  of  a  naked  gas  name. 

At  the  same  time  there  should  t>e  ample  light  on  the  microscope 
table,  as  it  is  not  at  all  necessary  or  desirable  that  the  work  should 
be  insufficiently  illuminated.  All  that  is  required  is  that  the  lamps 
should  have  shades  and  be  placed  at  such  a  height  that  the  direct 
rays  do  not  enter  the  observer's  eye. 

If  these  precautions  are  taken,  several  hours'  continued  work 
may  be  carried  on  without  any  injurious  effect. 

Some  observers  use  only  the  left  eye,  some  the  right,  others  the 
right  or  left  indiscriminately. 

It  seems  immaterial  which  is  used,  it  being  merely  a  matter  of 
habit,  as  those  who  are  accustomed  to  use  one  particular  eye  feel 
awkward  with  the  other.  In  continuous  work,  extending  over  many 
months  of  long  daily  observation,  if  the  eye  has  been  accustomed  to 
monocular  vision,  even  with  high  powers,  there  is  no  difficulty 
experienced.  The  effect  of  years  of  work  with  optical  instruments 
on  those  possessed  of  strong  normal  sight  seems  to  be  an  increase 
in  the  defining  perception  accompanied  by  a  decrease  of  the 
perception  of  brightness.  Those  accustomed  to  use  one  particular 
eye  with  microscopical  work,  and  who  have  done  nuich  work,  would, 
if  they  looked  at,  say,  the  moon  with  that  eye,  see  more  detail  in  it 
than  if  the  other  eye  were  used ;  at  the  same  time  it  would  not 
appear  as  bright. 

If  there  is  too  much  light,  as  there  often  is,  when  large-angled 
illuminating  cones  are  used,  it  is  as  well  to  interpose  between  the 
lamp  and  the  microscope  a  piece  or  pieces  of  signal  green  glass ;  this 
softens  the  light  and  removes  the  objectionable  yellowness,  a  feature 
of  illumination  not  due  to  the  light  from  the  edge  of  a  paraffin  lamp, 
which,  as  we  have  stated,  is  not  particularly  yellow.  Great  yellow- 
ness is  a  sign  of  imperfect  achromatism  in  an  objective.  We  may 
with  precisely  the  same  conditions  find  the  images  yielded  by  twro 
objectives  of  the  same  power  and  aperture  differ,  in  so  much  as  one 
is  yellow  and  dim  and  the  other  white  and  bright;  other  things 
being  equal,  the  white  and  bright  image  is  to  be  preferred.  It  is 
necessary  to  say  'other  things  being  equal,'  because  an  objective 
which  gives  a  bright  and  a  white  image  may  nevertheless  be  inferior 
.to  the  one  giving  the  yellowr  and  dim  picture.  Thus  if  the  planes 
of  the  lenses  of  which  the  objective  is  composed  are  not  at  right 
angles  to  the  optic  axis  there  will  be  serious  defects  in  the  image, 
although  it  is  bright  and  white.  This  fault  is  known  in  practice  as 

E  E 


41 8   MANIPULATION   AND   PRESERVATION   OF   THE   MICROSCOPE 

an  error  of  centring,  which  also  means  the  error  of  not  placing  the 
axes  of  the  lenses  in  the  same  straight  line ;  so  both  faults  are 
described  by  the  same  term. 

It  should  be  understood  that  signal  green  glass  will  not  yield 
monochromatic  illumination ;  only  the  Gifford  screen  or  the  filter 
screen  of  Prof.  Miethe  (q.v.)  or  the  Nelson  spectroscopic  arrange- 
ment (q.v.)  can  be  of  real  service. 

Coloured  light  derived  from  a  polariscope  and  a  selenite  is  not 
monochromatic . 

For  critical  work,  such  as  testing  lenses  or  forcing  out  the  greatest 
resolution  with  the  widest-angled  oil-immersion  lenses,  daylight 
illumination  is  inadmissible. 

When  daylight  illumination  is  used,  a  northern  aspect,  or  at  least 
one  away  from  direct  sunlight,  is  to  be  preferred. 

It  is  a  good  plan,  where  it  is  possible,  to  arrange  the  table  so  that 
the  windowr  is  at  the  observer's  left  hand.  The  microscope  should  be 
placed  in  a  direction  parallel  to  the  window,  and  the  light  reflected 
by  the  mirror  through  a  right  angle.  A  screen  may  be  placed  parallel 
to  the  window  which  just  allows  the  mirror  of  the  microscope  to 
project  beyond  it.  This  cuts  off  direct  light  from  the  stage  and 
from  the  observer's  eyes. 

A  concave  mirror  with  the  object  in  its  principal  focus  is  the  best 
for  diffused  daylight  illumination.  The  diaphragm  should  not  be 
close  to  the  stage.  When  delicate  microscopical  work  is  carried  011, 
it  is  important  to  remember  that  the  human  eye  can  work  best  when 
the  body  is  in  a  state  of  ease.  If  there  is  any  strain  on  the  muscles 
of  the  body,  or  if  the  observer  is  in  a  cramped  position,  vision  will 
be  impaired.  Consequently,  where  permissible,  a  microscope  should 
always  be  inclined,  and  the  observer  seated  in  such  a  way  that  the 
eye  can  be  brought  to  the  eye-piece  in  a  perfectly  natural  and  com- 
fortable manner.  The  body  should  also  be  steadied  by  resting  the 
arms  on  the  table. 

It  is  advisable  to  use  the  bull's-eye  as  little  as  possible  ;  even  ivith 
dark- ground  illumination  the  flat  of  the  flame  is  preferable,  reserving 
the  bull's-eye  for  those  cases  where  the  flat  of  the  flame  will  not  cover 
enough  of  the  object.  Generally  speaking,  if  the  whole  field  is  re- 
quired to  be  illuminated  on  a  dark  ground,  a  bull's-eye  will  be  neces- 
sary ;  but  for  an  object  such  as  a  single  diatom  the  flat  side  of  the 
lamp  flame  will  usually  be  large  enough. 

In  examining  diatoms  or  other  objects,  such  as  the  karyokinetic 
figures  in  very  minute  nuclei  of  microscopic  organisms,  or  other  obscure 
and  undetermined  parts  of  such  forms  of  life,  it  is  most  important, 
amongst  other  means,  to  resort  to  the  use  of  large  solid  cones  ;  what 
they  teach  and  suggest  can  scarcely  be  neglected  by  the  searcher  for 
the  unknown.  Professor  Abbe  does  not  advise  their  employment  as 
in  any  way  final;  he  says  that  'the  resulting  image  produced  by 
means  of  a  broad  illuminating  beam  is  always  a  mixture  of  a  multi- 
tude of  partial  images  which  are  more  or  less  different  and  dissimilar 
from  the  object  itself ; '  and  he  does  not  conceive  that  there  is  any 
ground  for  expectation  '  that  this  mixture  should  come  nearer  to  a 
strictly  correct  projection  of  the  object  .  .  .  than  the  image  which 


TO   DISPLAY   OBJECTS   MICROSCOPICALLY  419 

is  projected  by  a  narrow  axial  illuminating  pencil.'  This  is  a  weighty 
judgment,  and  should  receive  full  consideration.  At  the  same  time 
the  use  of  wide  and  solid  cones  is  so  full  of  suggestive  results  that 
we  must  employ  them  with  all  possible  control  by  other  means  of  the 
images  they  present.  This  is  the  more  a  necessity  since  Mr.  Nelson 
has  been  able  to  obtain  the  most  wonderful  results  writh  narrow  cones, 
*  true  ghosts  '  and  '  false  ghosts,'  the  presence  of '  intercostal  markings' 
in  the  image  of  a  fly's  eye  (!),  and  many  complex  and  false  images 
with  the  coarser  diatoms.  But  with  wide  cones  he  has  proved  that 
these  false  images  cannot  be  produced  ;  and  that  when  the  true  image 
is  reached  by  a  wide  cone,  the  image  is  not  altered  by  any  change  of 
focus,  but  simply  fades  in  and  (Kit  of  focus  '  as  a  daisy  under  a 
4-inch  objective.' 

Mr.  Nelson  has  photographed  all  these  results,1  and  we  have  seen 
them  demonstrated.  When  theory  and  practice  are  thus  at  variance 
we  must  pause  for  further  light. 

If  it  is  required  to  accentuate  a  known  structure,  such  as  the  per- 
forated membrane  of  a  diatom,  it  can  be  done  by  annular  illumination, 
which  means  the  same  arrangement  as  for  dark  ground,  but  with  a 
stop  insufficiently  large  to  shut  out  all  the  light.  This  method  is  not 
to  be  recommended  \vheii  a  structure  is  unknown,  as  it  is  also  liable 
to  give  false  images.  It  must  be  remarked  that  diatom  and  other 
delicate  structure,  when  illuminated  with  a  narrow-angled  cone,  gives 
on  slight  focal  alterations  a  variety  of  patterns  like  a  kaleidoscope ; 
with  a  wide-angled  cone  a  single  structure  gives  a  single  focus,  i.e 
it  goes  completely  out  of  focus  on  focal  alteration.  When  a  large- 
angled  and  a  wide-angled  objective  are  used  a  change  of  pattern,  only 
occurs  when  the  structure  is  fine.  This  practical  observation  has  its 
value,  and  must  not  be  forgotten. 

To  properly  display  objects  under  a  microscope  is  to  a  certain  ex- 
tent an  art,  for  it  not  only  demands  dexterity  in  the  manipulation 
of  the  instrument  and  its  appliances,  but  it  also  requires  knowledge 
of  what  sort  of  illumination  is  best  suited  to  the  particular  object. 
At  this  point  we  think  it  advisable,  especially  in  the  interests  of 
beginners,  to  clearly  point  out  the  best  method  of  commencing 
microscopic  work  by  centring  the  condenser  and  arranging  the 
light  for  the  critical  examination  of  an  object. 

1st.  Place  a  powder  of  about  a  §  on  the  nose-piece,  and  a  B  or 
No.  2  eye-piece  in  the  tube. 

2nd.  Use  as  a  source  of  illumination  the  light  from  a  paraffin 
lamp  with  a  ^-inch  wick. 

3rd.  Place  any  suitable  object  on  the  stage,  and,  having  focussed 
it  with  any  kind  of  illumination,  centre  it  to  the  field  of  the  eye- 
piece. 

4th.  Place  a  small  diaphragm  beneath  the  sub -stage  condenser,  or 
close  the  iris. 

5th.  Rack  the  condenser  until  the  hole  in  the  diaphragm  is  in 
focus  (in  the  plane  of  the  object). 

6th.  If  the  hole  in  the  diaphragm  should  not  be  central  to  the 

1  Journ.  E.  M.  S  ,  1891,  p.  90,  pi.  II. 

E  E  2 


42O  MANIPULATION   AND   PRESENTATION   OF  THE   MICROSCOPE 

object  on  the  stage,  it  must  be  centred  by  means  of  the  sub-stage  ad- 
justing screws. 

7th.  Rack  up  the  condenser  until  the  image  of  the  flame  comes 
into  focus. 

8th.  Centre  the  image  of  the  flame  to  the  object  on  the  stage  by 
moving  the  position  of  the  lamp,  and  place  the  lamp  so  that  the 
edge  of  the  flame  is  presented.  In  performing  this  adjustment  the 
sub-stage  centring  screws  must  on  no  account  be  moved.  (If  a  mirror 
is  employed,  the  centring  of  the  image  of  the  flame  upon  the  object 
can  be  effected  by  moving  the  mirror.) 

9th.  The  object  to  be  examined  may  now  be  substituted  for  that- 
used  for  centring  purposes,  and  be  placed  in  the  image  of  the  edge 
of  the  flame. 

10th.  The  objective  by  which  the  object  is  to  be  examined  is 
placed  on  the  nose-piece  and  the  object  brought  into  focus. 

llth.  The  eye-piece  is  removed  and  the  back  lens  of  the  objective 
is  examined.  The  diaphragm  at  the  back  of  the  condenser  is  then 
altered  so  that  three-fourths  of  the  back  lens  of  the  objective  is  filled 
with  an  unbroken  disc  of  light. 

12th.  The  eye-piece  is  replaced  and  the  objective  brought  into 
adjustment  either  by  screw  collar  or  by  altering  the  tube  length. 

13th.  If  it  is  necessary  at  any  time  to  use  a  large  field  for  a  rough 
survey  of  an  object,  or  to  localise  any  particular  portion  of  an 
object,  all  that  is  necessary  is  to  rack  down  the  condenser  until  the 
whole  field  becomes  illuminated  ;  but  when  any  part  requires  critical 
examination  the  condenser  must  be  racked  up  again  and  the  image 
-of  the  edge  of  the  flame  focussed  on  the  object. 

For  learning  the  manipulation  of  the  instrument  no  class  of 
objects  are  as  suitable  as  diatoms  ;  they  are  also  an  excellent  means 
of  training  the  eye  to  appreciate  critical  images.  For  a  general  view 
of  the  larger  diatoms  take  a  spread  slide  in  balsam ;  a  ^  of  80°,  a 
good  binocular,  and  a  dark-ground  illumination  will  give  a  fine  effect. 
This  is  not  merely  a  pretty  object,  but  it  is  also  a  very  instructive 
one,  because  we  obtain  a  far  clearer  idea  of  the  contour  of  various 
•diatoms  than  can  be  obtained  in  any  other  way.  The  diatoms  should 
be  studied  and  worked  at  in  this  manner  most  carefully  and  for  a 
long  time.  The  same  identical  specimens  should  be  then  viewed  with 
transmitted  light.  This  lesson,  if  conscientiously  learnt,  will  teach  a 
student  how  to  appreciate  form  by  focal  alteration.  This  is  a  most 
important  lesson,  and,  if  several  days  are  spent  in  mastering  it,  they 
will  be  far  from  thrown  away.  Diatoms,  especially  the  larger  forms, 
;ire  seen  very  well  when  mounted  dry  on  cover  by  means  of  a  J-inch 
objective  and  a  Lieberkuhn  ;  the  bull's-eye  and  the  plane  mirror  should 
be  used.  Some  objects  are  so  transparent,  or  become  so  transparent 
in  the  medium  in  which  they  are  mounted,  that  they  will  not  bear  a 
large  illuminating  cone,  the  brightness  of  the  illumination  destroying 
the  contrast.  It  will  illustrate  this  w^hen  we  recall  that  dirt  on  an 
eye-piece  which  is  quite  invisible  in  a  strong  light  becomes  im- 
mediately apparent  in  a  feeble  light.  Thus  animalcules  require  a 
small  cone  of  illumination  when  they  are  being  examined,  particularly 
with  a  J-inch  objective;  for  a  general  view  of  '  pond  life'  a  IJ-inch 


•  CE1TICAL  '   AND    UNCEITICAL  IMAGES    - 


421 


objective  with  a  dark-ground  illumination,  employing  a  binocular,  is 
very  suitable.  Stained  bacteria  in  tissue  are  best  seen  with  a  large 
cone,  as  was  pointed  out  by  Dr.  Robert  Koch,  and  is  directly  supported 
by  Dr.  Abbe  as  suitable  in  his  directions  for  the  use  of  the  Abbe 
condenser.1  The  brilliancy  of  the  illumination  obliterated  the  thin 
tissue  which  is  in  a  medium  whose  refractive  index  is  similar  to 
itself.  The  bacteria,  which  are  opaque  with  pigment,  then  stand 
out  boldly.  A  bacterium  not  in  tissue  is  always  better  seen  by 
means  of  a  large  cone, .  provided  that  the  objective  is  properly 
corrected.  The  very  minute  hairs  on  the  lining  membrane  of  the 
blow-fly's  tongue,  if  examined  by  a  ^  objective  and  a  narrow  cone, 
appear  thickened,  shorter,  more  blunted,  and  often  split  into  two 
parts.  This  is  shown  in  figs.  2  rflifl  3  in  the  frontispiece.  Fig.  3  is 
a  critical  image  magnified  510  diameters.  A  lens  should  be  used  to 
examine  this. 

It  will  be  seen  that  the  hairs,  especially  the  long  central  one,  are 
very  fine  and  spinous.  They  have  not  the  ring  socket  common  to 
insect  hairs,  but  grow  directly  from  a  delicate  membrane. 

This  photograph  was  taken  with  an  apochromatic  J  of  '95  N".A. 
and  Xo.  3  projection  eye-piece  ;  and  it  was  illuminated  by  means  of 
a  large  solid  cone  of  '65  N.A.  from  an  achromatic  condenser. 

Fig.  2  is  an  uncritical  image,  with  all  the  conditions  as  above, 
save  that  a  cone  of  small  angle,  i.e.  of  0*1,  was  used  for  illumination. 

The  first  alteration  which  thrusts  itself  upon  the  eye  is  the 
doubling  of  the  hairs  which  are  in  the  least  degree  out  of  focus. 
But,  further,  it  will  be  noted  that  there  is  a  bright  line  with  a  dark 
edge  round  the  hairs  which  are  precisely  in  focus  ;  this  is  a  diffraction 
effect,  always,  in  our  experience,  present  in  objects  illuminated  by 
cones  of  insufficient  angle,  and  it  can  be  easily  made  to  disappear  by 
widening  the  cone.  As  the  illuminating  cone  is  enlarged  they  become 
sharper  and  longer,  and  their  edges  become  more  definite.  But 
nothing  is  gained,  but  rather  a  distinct  loss  is  incurred,  by  making 
the  illuminating  cone  much  larger  than  three-fourths  of  the  objective 
cone. 

As  an  example  of  erroneous  interpretation,  the  representation  of 
the  pygidium  of  a  flea  by  some  leading  sources  of  information  of  a 
few  years  ago  may  be  instanced.  It  was  a  special  test  of  many 
authors,  and  has  been  carefully  figured  ;  this  shows  that  it  is  not  an 
accidental  error,  which  it  might  have  been  if  it  were  merely  an 
ordinary  object ;  it  is  an  error  depending  in  all  probability  on  a 
faulty  system  of  illumination.  Moreover,  the  error  cannot  be  attri- 
buted to  the  object-glasses  of  the  time,  as  it  is  a  low-power  object, 
and  the  low  powers  of  that  day  were  quite  as  good  as  those  lately  in 
use.  In  the  descriptions  and  in  the  drawings,  often  beautifully 
executed,  the  hairs  proceeding  from  the  centre  of  the  wheel-like 
discs  are  represented  as  being  *  stiff  and  longish  bristles/  thick  at 
one  end  and  tapering  off*  to  a  point.  And  the  small  hairs  round 
are  described  as  '  minute  spines  ; '  in  the  drawing  they  are  like  the 
spinous  hairs  of  an  insect,  and  have  the  usual  socket-joint  at  the 

1  Directions  for  the  Use  of  Abbe's  Illuminating  Apparatus — a  leaflet  issued  by 
Carl  Zeiss,  1888. 


422    MANIPULATION   AND   PRESERVATION    OF  THE   MICROSCOPE 

base.  In  reality  the  '  stiff  and  longish  bristle  '  is  an  extremely  long 
and  delicate  filament,  totally  unlike  a  bristle,  being  not  tapered  but 
of  nearly  uniform  thickness.  The  '  minute  spines '  are  in  reality 
very  curious  hairs,  and,  as  far  as  we  at  present  know,  unlike  any 
others.  They  are  delicate,  lambent,  bulbous  hairs.  What  they 
most  resemble  are  the  tentacles  of  a  sea-anemone,  and  there  are  two 
tubes  discoverable  which  are  important  and  comparatively  large  ob- 
jects. There  appears  to  be  considerable  probability  that  this  inte- 
resting object  upon  the  last  ring  of  the  body  of  the  flea,  and  known 
as  its  '  pygidium,'  acts  as  an  auditory  instrument.1  In  the  examina- 
tion of  ordinary  stained  histological  and  pathological  sections  by 
transmitted  light,  unless  some  very  delicate  point  is  sought,  the  con- 
denser should  have  a  stop,  so  that  when  the  back  of  the  objective  is 
examined  the  stop  is  seen  cutting  into  the  back  of  the  objective  by 
about  a  third.  This  in  some  instances  may  be  increased  to  a  half  by 
diminishing  the  cone,  but  it  is  not  advisable  to  use  anything  less 
than  a  half  unless  it  is  absolutely  necessary.  As  we  have  pointed 
out  above,  high-class  objectives  will  stand  a  j  cone  perfectly,  and  very 
special  objectives  will  bear  even  a.-J  cone ;  but  for  the  ordinary  run 
of  objectives  §  will  be  found  as  much  as  they  are  able  to  bear — some 
indeed  will  not  stand  a  ^  cone.  Thus,  to  put  it  in  round  numbers, 
an  illuminating  cone  '2  N.A.  is  very  suitable  for  ordinary  work  with 
the  apochromatic  1-inch  and  §  objectives,  and  one  of '4  N.A.  for  the 
^  and  ^,  and  one  of  '6  N.A.  for  the  J  and  J.  It  is  a  good  plan  to 
have  one  or  two  stops  cut  to  give  special  cones,  the  N.A.  of  which 
should  be  engraved  on  them.  This  subject  is  one  of  great  import- 
ance, as  more  than  nine-tenths  of  all  microscopic  objects  are 
examined  by  means  of  transmitted  light. 

Let  us  now  note  the  effect  of  large  cones  on  the  simplest  object. 
A  microscope  is  set  up  having  an  achromatic  condenser  with  an  iris 
diaphragm ;  let  three  good  wide-angled  objectives  be  chosen,  say 
1-inch,  a  ^-inch,  and  J-inch  dry.  Let  the  object  be  the  one  we  have 
already  studied  to  some  extent  in  this  relation,  viz.  one  of  the  stiff 
hairs  on  the  maxillary  palpus  of  the  blow-fly's  tongue  ;  place  the 
1-inch  on  the  nose-piece,  open  the  full  aperture  of  the  condenser  and 
get  the  instrument  into  perfect  adjustment.  Now  close  the  iris.  The 
haii-  will  be  surrounded  by  a  luminous  border,  which  will  give  it 
a  glazy  appearance,  and  its  fine  point  will  be  blurred  out.  NOWT 
open  the  iris  until  the  last  trace  of  that  glaziness  disappears.  The 
hair  will  appear  as  a  different  object,  its  outline  being  perfectly  clear 
and  sharp.  If  the  eye-piece  is  removed,  about  two-thirds  of  the  ob- 
jective back  will  be  full  of  light.  NOWT,  without  disturbing  any  of 
the  adjustments,  replace  the  1-inch  by  the  ^,  and  it  will  be  found 
that  the  glaziness  or  false  light  will  have  returned.  Let  the  iris 
be  further  opened  until  the  last  trace  of  it  disappears ;  now,  on 
examination  of  the  back  of  the  objective,  two- thirds  of  it  will  be  found 
full  of  light,  and  so  on  with  the  |.  We  call  the  attention  of  the 
student  to  these  facts  as  having  a  direct  bearing  upon  the  question 
of  the  comparative  effects  of  large  and  small  illuminating  cones,  and 

1  Micros.  Journ.  April  24,  1885  :  '  Pygidium  of  Flea  '  (E.  M.  Nelson). 


VARIOUS   MODES   OF   ILLUMINATION— LARGE    CONES      423 

with  no  idea  of  offering  opposing  opinions  to  those  of  Professor  Abbe  ; 
we  have  no  direct  judgment,  but  we  record  these  facts  as  factors  in 
and  for  the  elucidation  of  the  question.  It  is  perhaps  better  to  test 
the  j  on  some  of  the  more  minute  hairs  which  are  studded  over  the 
delicate  lining  membrane.  The  same  results  will  be  obtained.  Thus 
it  would  appear  to  suggest  itself  that  this  glaziness  depends  on  the 
relation  of  the  aperture  of  the  illuminating  cone  to  that  of  the  objective 
cone.  Apochromatic  objectives  behave  precisely  as  achromatic  ob- 
jectives in  this  respect.  Of  course,  if  the  hair  becomes  pale  and  in- 
distinct on  the  opening  of  the  iris,  it  shows  that  there  is  uncorrected 
spherical  aberration  in  the  objective  ;  another  objective  must  there- 
fore be  used ;  that  paleness  has  nothing  whatever  to  do  with  the 
glaze  or  false  light  mentioned  aboye. 

In  photo-micrographs  of  bacteria  one  frequently  sees  a  white  halo 
round  them.  We  have  never  been  able  to  demonstrate  what  this  is  ; 
sometimes  it  denotes  the  presence  of  an  envelope,  and  sometimes  it 
is  the  result  of  the  use  of  too  small  a  cone  of  illumination.  Photo- 
micrography with  a  small  cone  is  quite  easy,  as  great  contrast  can 
be  secured.  With  a  large  cone  the  difficulties  begin — difficulties 
of  adjustment,  difficulties  of  lens  correction,  difficulties  of  exposure, 
and  difficulties  of  development.  If,  so  far  as  our  experience  goes, 
a  good  photo -micrograph  is  required,  these  difficulties  must  be 
mastered. 

It  is  hardly  necessary  to  remind  the  student  that  in  micrometry 
it  is  essential  that  the  edges  of  the  object  should  be  defined ;  conse- 
quently a  large  cone  must  then  be  employed. 

For  the  examination  of  Poly cystines,  Foraminifera,  &c.,  a  binocular 
is  useful ;  illumination  may  be  by  a  Lieberkiihn  if  mounted  dry,  and 
by  dark  ground  by  a  condenser  if  mounted  in  balsam.  Parts  of 
insects  should  be  usually  examined  with  dark-ground  illumination ; 
whole  insects  are  seen  best  with  the  Lieberkiihn,  and  the  binocular 
should  be  used  for  both. 

Some  of  this  class  of  objects  are  best  seen  under  doitble  illumina- 
tion ;  that  is,  a  dark  ground  with  a  condenser  and  light  thrown  from 
above  with  a  silver  side-reflector,  as  the  Lieberkiihn  cannot  be  used  in 
conjunction  with  an  achromatic  condenser.  It  is  a  good  plan  with 
low -power  Lieberkiihn  work  to  interpose  between  the  slip  and  the 
ledge  a  strip  of  plain  glass  Vinch  wide ;  this  prevents  the  ledge 
stopping  out  light  from  the  Lieberkuhn  when  it  is  larger  in  diameter 
than  the  slip.  Mr.  Julius  Rheinberg  has  recently  brought  to  a 
high  state  of  perfection  a  system  of  colour  illumination,  and  the 
special  importance  of  the  choice  of  suitable  colours.  It  is  of  much 
interest,  but  cannot  be  condensed  in  the  space  at  our  disposal.  The 
full  paper  will  be  found  illustrated  in  « Journ.  E.M.S.'  1896,  p.  373, 
and  the  'Journ.  R.  M.  S.'  for  1899,  p.  142. 

Polarised  light  used  with  a  condenser  is  very  useful  for  insect 
work.  For  very  low-power  work — such  as  the  usual  botanical  sec- 
tions— it  is  a  good  plan  to  give  up  the  cone,  and  place  a  piece  of  fine 
ground  glass  at  the  back  of  the  condenser ;  and  with  lamplight  it  is 
as  well  to  use  a  Clifford's  screen  with  it.  With  objectives  of  greater 
angle  than  '6  N.A.  it  is  usually  difficult  to  get  satisfactory  illumina- 


424  MANIPULATION  AND   PRESEKVATION   OF  THE   MICROSCOPE 

tion  with  a  dark  ground.  The  best  that  can  be  done  is  to  use  an 
oil-immersion  condenser  with  a  suitable  stop ;  this  will  give  a  good 
dark  ground  up  to  '65  N.A.,  but  it  will  fail  if  the  object  is  dry  on 
the  cover.  Generally  speaking,  the  only  way  of  accomplishing  this 
with  objectives  of  wider  aperture  is  to  reduce  the  aperture  of  the 
objective  by  a  stop  placed  at  the  back. 

When  a  condenser  is  united  by  a  film  of  oil  to  a  slip,  if  the  slip 
is  thin,  the  oil  invariably  runs  down  when  the  condenser  is  focussed. 

The  following  is  a  method 
by  which  this  may  be  en- 
tirely prevented.  A  piece 
of  thick  cover-glass  about 
•02  inch,  and  1  inch  square, 
has  a  strip  of  thicker  glass, 
jj-  inch  broad,  cemented  by 
shellac  to  one  edge.  This 
piece  of  glass  is  oiled  to 


Thin  slip  of  glass  with  the   slip,    the  ledge  being' 

ledge  to  place  glass  hooked  over  the  top  of  the 

slip  with  oil  contact,         Slide  in  situ  on  thin        ,.,         , ,  . 

so    as    to   vary    the  slip  with  ledge.          slide;   this    not    Only    pre- 

thickness  of  a  slide.  vents     its    slipping    down, 

FIG.  3(i5.  but  also  keeps  the  oil  from 

creeping  out  at  the  bottom, 

which  would  be  the  case  if  the  two  edges  of  the  glass  coincided.1 
This  is  illustrated  in  fig.  365. 

In  its  proper  place  we  have  dealt  with  the  suitable  relation  of 
aperture  to  power,  and  have  pointed  out  the  irresistible  nature  of 
the  contentions  and  teachings  of  Abbe  on  the  subject.  Here  a 
direct  practical  presentation  of  the  matter  may  be  of  service  to  the 
student. 

A  normal  unaided  human  eye  can  divide  ^\-^  inch  at  ten  inches. 
Consequently  a  microscope  with  a  power  of  200  should  be  capable  of 
showing  structure  as  fine  as  S^OTT  inch.  Xow,  as  this  power  can  be 
made  up  by  ^-inch  objective  and  a  1-inch  eye-piece,  it  follows  that 
sufficient  aperture  ought  to  be  given  to  the  i-inch  to  enable  it  to 
resolve  50,000  lines  per  inch.  This  2  will  be  '52  N.A.  The  inch 
objective  should  have  half  this  aperture,  and  the  J  double,  and  the 
^  four  times  as  much,  if  perfect  vision  is  required ;  in  other  words, 
•26  N.A.  for  every  100  diameters.3  These  ideals  have  (as  we  have 
before  indicated)  been  realised,  notably  by  the  Zeiss  apochromatics, 
the  1-iiich  and  the  ^-inch  4  resolving  everything  capable  of  being 
appreciated  by  the  eye  when  the  12  compensating  eye-piece  is  used. 
The  J-inch  is  also  a  near  approach  to  the  ideal,  as  it  has  been  very 
wisely  kept  a  dry  lens.  The  oil-immersion  J-in.  of  1*4  N. A.  with 
a  6  eye-piece  also  attains  the  ideal.  This  relation  of  aperture  to 

1  Q.  M.  C.  Jouriial,  November  1885. 

2  In  reality  it  will  require  more,  because  an  axial  cone  is   assumed  to  be  used 
instead  of  an  oblique  beam. 

3  English  Mechanic,  vol.  xxxviii.  1888,  No.  979.— E.  M.  Nelson. 

4  This   lens,  with   an   8   compensating   eye-piece,   will   resolve   a   Pleurosigma 
angulatum  with  an  axial  cone  ;  this  is  the  lowest  power  with  which  it  has  ever  been 
done. 


THE    QUALITIES   OF   OBJECTIVES  425 

power  is  very  significant,  and  should  be  carefully  pondered  by  those 
who  still  desire  low  apertures  as  the  only  perfect  form  of  objectives. 

It  is  as  well  to  mention  that  objectives  may  be  arranged  in  two 
series — one  the  2,  1,  ^,  J,  and  £,  the  other  1J,  J,  J,  ^,  ^.  One  of 
these  series  will  form  a  complete  battery,  as  it  is  unnecessary  to 
have  objectives  differing  from  the  next  in  the  series  by  less  than 
double  the  power. 

The  most  usual  combination  is  perhaps  the  1  and  the  J  of  one 
series,  or  the  §  and  the  £  of  the  other.  Of  these  two  preference 
might  rather  be  given  to  the  latter.  The  only  exception  would  be 
the  addition  of  a  1  J-inch  for  pond  life. 

Eye-pieces  should  also  double^  the  powyer  thus  :  5,  10,  and  20 
(imcompensated),  or  6,  12,  and  27  (compensated),  the  most  useful  of 
the  three  being  the  10  (uncompensated)  and  the  12  (compensated). 
As  there  is  no  6-power  compensated  eye-piece  for  the  long  tube,  a  4 
for  the  short  tube  admirably  answers  the  purpose. 

In  addition  to  the  explanations  already  given  on  the  subject  of 
testing  objectives,  it  may  be  useful  here  to  note  that  the  qualities  of 
an  objective  are  seven  in  number : — 

1.  Magnifying  power  (initial). 

2.  Aperture  or  N.A. 

3.  Resolving  power. 

4.  Penetrating  power. 

5.  Illuminating  power. 

6.  Flatness  of  field. 

7.  Defining  power. 

1 .  Magnifying  power. — No  test  is  required,  as  the  initial  magni- 
fying power  can  be  directly  measured. 

2.  Aperture  or  N.A.  can  be  directly  measured ;  no  test  is  there- 
fore necessary. 

3.  Resolving  power. — A  lens  illuminated  by  a  large  solid  axial 
cone,  when  a  Gifford's  screen  is  used,  should  resolve  a  number  of 
lines  to  the  inch  expressed  by  its  N.A.  multiplied  by  SOjOOO.1 

4.  Penetrating  poiver  is  the  reciprocal  of  the  resolving  power  of 

•^-  .    .     No  test  needed,  but  penetrating  power  varies  largely  with 

the  combined  magnifying  power,  and  also  with  the  magnitude  of  the 
illuminating  cone  used,  as  already  intimated. 

5.  Illuminating  poiver  is  the  square  of  the  numerical  aperture 
(N.A.)2.     No  test  is  necessary,  but   the   remarks   made  above  in 
regard  to  penetrating  power  apply  equally  here. 

6.  Flatness  of  field  is,  in  the  strict  meaning  of  the  term,  an 
optical  impossibility.      The  best  thing  therefore  is  to  contract  the 
visible  field,  as  is  done  in  the  compensating  eye-pieces.     (Tests  :  For 
low  powers  a  micro-photograph  ;  for  medium  and  high  powers  a  stage 
micrometer.) 

7.  Defining  power    depends    on    (a)  the  reduction   of  spherical 
aberration,  (b)  the  reduction  of  chromatic  aberration,  (c)  the  perfect 
centring  of  the    lenses — by  which  is  meant  (i.)  the  alignment  of 

1  J.B.M.S.  1893,  p.  15.— E.  M.  Nelson. 


426  MANIPULATION   AND   PRESERVATION   OF   THE   MICROSCOPE 

their  optic  axes,  (ii.)  the  parallelism  of  their  planes,  (iii.)  the  setting 
of  their  planes  at  right  angles  to  the  optic  axis. 

Defining  power  can  only  be  tested  by  a  critical  image.  The 
following  is  a  list  of  suitable  objects  of  which  a  critical  image  is  to 
be  obtained,  using  a  solid  axial  cone  of  illumination  equal  to  at  least 
three-fourths  of  the  aperture  of  the  objective. 

Very  low  powers  (3-,  2-.  and  H-inch). — Wing  of  Agrion  pul- 
chelltim  $  (dragon-fly). 

Low  powers  (1  and  §). — Proboscis  of  blow-fly.  Large  diatoms 
on  dark  ground. 

Medium  powers  (^,  ^  ^,  and  low-angled  J). — Minute  hairs  on 
proboscis  of  blow-fly  ;  hair  of  pencil-tail  (Polyxenus  lagurus)  ; 
diatoms  on  a  dark  ground.  This  last  is  a  most  sensitive  test ;  unless 
the  objective  is  good  there  is  sure  to  be  false  light. 

Medium*  potvers  (with  wide  aperture). — Pleurosigma  formosum  ; 
Navicula  lyra  in  balsam  or  styrax ;  Pleurosigma  angulatum  dry  on 
cover  ;  bacteria  and  micrococci  stained. 

High  powers  (wide  aperture  and  oil-immersion  J  and  -^). — The 
secondary  structure  of  diatoms,  especially  the  fracture  through  the 
perforations ;  Navicula  rhomboides  from  Cherry  field  in  balsam  or 
styrax  ;  bacteria  and  micrococci  stained. 

Test  with  a  10  or  12  eye-piece,  and  take  into  account  the  general 
whiteness  and  brilliancy  of  the  picture. 

The  podura  scale  is  not  mentioned  as  a  test,  as  it  may  be  very 
misleading  in  unskilled  hands.  One  great  point  in  testing  objectives 
is  to  know  your  object.  Care  must  be  exercised  to  ascertain  by 
means  of  vertical  illuminator  if  objects  such  as  diatoms  dry  on  the 
cover  are  in  optical  contact  with  the  cover-glass.  Testing  objectives 
is  an  art  which  can  only  be  acquired  in  time  and  with  experience 
gained  by  seeing  large  numbers  of  objectives. 

In  the  manipulation  of  the  microscope  it  is  not  uncommon  to 
observe  the  operator  rolling  the  milled  head  of  ihejine  adjustment 
instead  of  firmly  grasping  it  between  the  finger  and  thumb  and 
governing,  to  the  minutest  fraction  of  arc,  the  amount  of  alteration 
he  desires.  It  is  undesirable  and  an  entirely  inexpert  procedure  to 
roll  the  milled  head,  and  cannot  yield  the  fine  results  which  a  delin- 
eate mastery  of  this  part  of  the  instrument  necessitates  and  implies. 
To  use  aright  the  fine  adjustment  of  a  first-class  microscope  is  not 
the  first  and  easiest  thing  mastered  by  the  tyro.  We  have  already 
intimated  that  the  fine  adjustment  should  never  be  resorted  to  while 
the  coarse  adjustment  can  be  efficiently  employed.  The  focus  should 
always  be  found,  even  with  the  highest  powers,  by  means  of  the 
coarse  adjustment.  It  is  only  a  clumsy  microscopist  who  brings  his 
objective  by  means  of  the  coarse  adjustment  near  the  cover-glass  and 
looks  at  the  distance  he  is  off  it  either  by  the  eye  or  by  the  aid  of  a, 
hand  magnifier,  and  then  completes  his  work  with  the  fine  adjust- 
ment. In  every  case  the  focus  ought  to  be  found  by  the  coarse 
adjustment,  and  the  working  distance  should  be  felt  by  the  finger 
tilting  the  slide  gently  against  the  front  of  the  objective.  Also  the 
examination  of  objects  for  depth  of  structure  with  low  and  medium 
powers  up  to  the  dry  J-  or  ^-inch  objective  should  be  performed  by 


EKRORS   OF   INTERPRETATION 


427 


the  coarse  adjustment ;  only  the  very  finest  details,  such  as  the  podura 
;  exclamation'  marks,  require  the  fine  adjustment. 

Beyond  the  correct  and  judicious  use  of  the  microscope  and  all 
its  appliances,  there  is  the  matter  of  the  elimination  of  errors  of  in- 
terpretation to  be  carefully  considered. 

The  correctness  of  the  conclusions  which  the  microscopist  will 
dr;iw  regarding  the  nature  of  any  object  from  the  visual  appearances 
which  it  presents  to  him  when  examined  in  the  various  modes  now 
specified,  will  necessarily  depend  in  a  great  degree  upon  his  previous 
experience  in  microscopic  observation  and  upon  his  knowledge  of 
the  class  of  bodies  to  which  the  particular  specimen  may  belong. 
Not  only  are  observations  of  any  kind  liable  to  certain  fallacies 
arising  out  of  the  previous  notions  'which  the  observer  may  entertain 
in  regard  to  the  constitution  of  the  objects  or  the  nature  of  the  actions 
to  which  his  attention  is  directed,  but  even  the  most  practised  ob- 
server is  apt  to  take  no  note  of  such  phenomena  as  his  mind  is  not 
prepared  to  appreciate.  Errors  and  imperfections  of  this  kind  can 
only  be  corrected,  it  is  obvious,  by  general  advance  in  scientific 
knowledge  ;  but  the  history  of  them  affords  a  useful  warning  against 
hasty  conclusions  drawn  from  a  too  cursory  examination.  If  the 
history  of  almost  any  scientific  investigation  were  fully  made  known, 
it  would  generally  appear  that  the  stability  and  completeness  of  the 
conclusions  finally  arrived  at  had  only  been  attained  after  many 
modifications,  or  even  entire  alterations,  of  doctrine.  And  it  is 
therefore  of  such  great  importance  as  to  be  almost  essential  to  the 
correctness  of  our  conclusions  that  they  should  not  be  finally  formed 
and  announced  until  they  have  been  tested  in  every  conceivable 
mode.  It  is  due  to  science  that  it  should  be  burdened  with  as  few 
false  facts  and  false  doctrines  as  possible.  It  is  due  to  other  truth- 
seekers  that  they  should  not  be  misled,  to  the  great  waste  of  their 
time  and  pains,  by  our  errors.  And  it  is  due  to  ourselves  that  we 
should  not  commit  our  reputation  to  the  chance  of  impairment  by 
the  premature  formation  and  publication  of  conclusions  w^hich  may 
be  at  once  reversed  by  other  observers  better  informed  than  our- 
selves, or  may  be  proved  to  be  fallacious  at  some  future  time,  per- 
haps even  by  our  own  more  extended  and  careful  researches.  The 
suspension  of  the  judgment  whenever  there  seems  room  for  doubt  is 
a  lesson  inculcated  by  all  those  philosophers  who  have  gained  the 
highest  repute  for  practical  wisdom  ;  and  it  is  one  which  the  micro- 
scopist cannot  too  soon  learn  or  too  constantly  practise.  Besides  these 
general  warnings,  however,  certain  special  cautions  should  be  given 
to  the  young  microscopist  with  regard  to  errors  into  which  he  is 
liable  to  be  led  even  when  the  very  best  instruments  are  employed. 

Errors  of  interpretation  arising  from  the  imperfection  of  the 
focal  adjustment  are  not  at  all  uncommon  amongst  microscopists, 
and  some  of  the  most  serious  arise  from  the  use  of  small  cones  of 
illumination.  With  lenses  of  high  power,  and  especially  with  those 
of  large  numerical  aperture,  it  very  seldom  happens  that  all  the 
parts  of  an  object,  however  minute  and  flat  it  may  be,  can  be  in 
focus  together;  and  hence,  when  the  focal  adjustment  is  exactly 
made  for  one  part,  everything  that  is  not  in  exact  focus  is  not  only 


428   MANIPULATION  ANL>   PRESERVATION   OF   THE   MICROSCOPE 

more  or  less  indistinct,  but  is  often  wrongly  represented.  The  in- 
distinctness of  outline  will  sometimes  present  the  appearance  of  a 
pellucid  border,  which,  like  the  diffraction-band,  may  be  mistaken 
for  actual  substance.  But  the  most  common  error  is  that  which  is 
produced  by  the  reversal  of  the  lights  and  shadows  resulting  from  the 
refractive  powers  of  the  object  itself;  thus,  the  biconcavity  of  the 
blood-discs  of  human  (and  other  mammalian)  blood  causes  their 
centres  to  appear  dark  when  in  the  focus  of  the  microscope,  through 
the  divergence  of  the  rays  which  it  occasions ;  but  when  they  are 
brought  a  little  within  the  focus  by  a  slight  approximation  of  the 
object-glass  the  centres  appear  brighter  than  the  peripheral  parts  of 
the  discs.  The  student  should  be  warned  against  supposing  that  in 
all  cases  the  most  positive  and  striking  appearance  is  the  truest,  for 
this  is  often  not  the  case.  Mr.  Slack's  optical  illusion,  or  silica-crack 
slide,1  illustrates  an  error  of  this  description.  A  drop  of  water  holding 
colloid  silica  in  solution  is  allowed  to  evaporate  011  a  glass  slide,  and 
when  quite  dry  is  covered  with  thin  glass  to  keep  it  clean.  The 
silica  deposited  in  this  way  is  curiously  cracked,  and  the  finest  of 
these  cracks  can  be  made  to  present  a  very  positive  and  deceptive 
appearance  of  being  raised  bodies  like  glass  threads.  It  is  also  easy 
to  obtain  diffraction-lines  at  their  edges,  giving  an  appearance  of 
duplicity  to  that  which  is  really  single. 

A  very  important  and  very  frequent  source  of  error,  which 
sometimes  operates  even  on  experienced  microscopists.  lies  in  the 
refractive  influence  exerted  by  certain  peculiarities  in  the  interim  1 
structure  of  objects  upon  the  rays  of  light  transmitted  through 
them,  this  influence  being  of  a  nature  to  give  rise  to  appearances 
in  the  image,  which  suggest  to  the  observer  an  idea  of  their  cause 
that  may  be  altogether  different  from  the  reality.  Of  this  fallacy 
we  have  a  '  pregnant  instance '  in  the  misinterpretation  of  the  nature 
of  the  lacunae  and  canaliculi  of  bone,  which  were  long  supposed 
to  be  solid  corpuscles  with  radiating  filaments  of  peculiar  opacity, 
instead  of  being,  as  is  now  universally  admitted,  minute  chambers 
with  diverging  passages  excavated  in  the  solid  osseous  substance. 
When  Canada  balsam  fills  up  the  excavations,  being  nearly  of  the 
same  refractive  power  as  the  bone  itself,  it  obliterates  them 
altogether.  So,  again,  if  a  person  who  is  unaccustomed  to  the  use 
of  the  microscope  should  have  his  attention  directed  to  a  preparation 
mounted  in  liquid  or  in  balsam  that  might  chance  to  contain  air- 
bubbles,  he  will  be  almost  certain  to  be  so  much  more  strongly 
impressed  by  the  appearances  of  these  than  by  that  of  the  object, 
that  his  first  remark  will  be  upon  the  number  of  strange -looking 
black  rings  which  he  sees,  and  his  first  inquiry  will  be  in  regard  to 
their  meaning. 

Although  no  experienced  microscopist  could  now  be  led  astray 
by  such  obvious  fallacies  as  those  alluded  to,  it  is  necessary  to 
notice  them  as  warnings  to  those  who  have  still  to  go  through  the 
same  education.  The  best  method  of  learning  to  appreciate  the 
class  of  appearances  in  question  is  the  comparison  of  the  aspect  of 
globules  of  oil  in  water  with  that  of  globules  of  water  in  oil,  or  of 
1  Monthly  Microscopical  Journal,  vol.  v.  1872,  p.  14. 


STUDIES   IN   INTERPRETATION 


429 


bubbles  of  air  in  water  or  Canada  balsam.  This  comparison  may 
be  very  readily  made  by  shaking  up  some  oil  with  water  to  which 
a  little  gum  has  been  added,  so  as  to  form  an  emulsion,  or  by 
simply  placing  a  drop  of  oil  of  turpentine  (coloured  with  magenta 
or  carmine)  and  a  drop  of  water  together  upon  a  slide,  laying  a  thin 
glass  cover  over  them,  and  then  moving  the  cover  backwards  and 
forwards  several  times  on  the  slide.  Equally  instructive  are  the 
appearances  of  an  air-bubble  in  water  and  Canada  balsam. 

The  figures  which  illustrate  the    appearance    at   various  points 


-   ••-•jr^'lJ  -        "^   -^ 

(3 


FIG.  366. — Air-bubbles  in  (1)  water  ;  (2)  Canada  balsam;  (3)  fat-globule8  in  water. 

of  the  focus  of  an  air-bubble  in  water  and  Canada  balsam,  and  of  a 
frit-globule  in  water,  may  be  thus  illustrated,  viz.  a  diaphragm  of 
about  f  of  a  mm.  being  placed  at  a  distance  of  5  mm.  beneath  the 
stage,  and  the  concave  mirror  exactly  centred. 

Air-babbles  in  water. — No.  1  (fig.  366)  represents  the  different 
appearances  of  an  air-bubble  in  water.  On  focussing  the  objective 
to  the  middle  of  the  bubble  (B),  the  centre  of  the  image  is  seen  to  be 
very  bright— brighter  than  the  rest  of  the  field.  It  is  surrounded  by 
a  greyish  zone,  and  a  somewhat  broad  black  ring  interrupted  by  one 


430   MANIPULATION  AND   PRESERVATION   OF   THE   MICROSCOPE 

or  more  brighter  circles.  Round  the  black  ring  are  again  one  or 
more  concentric  circles  (of  diffraction),  brighter  than  the  field. 

On  focussing  to  the  bottom  of  the  bubble  (A)  the  central  white 
circle  diminishes  and  becomes  brighter ;  its  margin  is  sharper,  and 
it  is  surrounded  by  a  very  broad  black  ring,  which  has  on  its 
periphery  one  or  more  diffraction  circles. 

When  the  objective  is  focussed  to  the  upper  surface  of  the 
bubble  (C)  the  central  circle  increases  in  size,  and  is  surrounded  by 
a  greater  or  less  number  of  rings  of  various  shades  of  grey,  around 
which  is  again  found  a  black  ring,  but  narrower  than  those  in  the 
previous  positions  of  the  objective  (A  and  B).  The  outer  circles  of 
diffraction  are  also  much  more  numerous. 

Air-bubbles  in  Canada  balsam. — Canada  balsam  being  of  a 
higher  refractive  index  than  water,  the  limiting  angle,  instead  of 
being  48°  35',  is  41°  only,  so  that  the  rays  which  are  incident  much 
less  obliquely  on  the  surface  of  separation  undergo  total  reflexion, 
and  it  will  be  only  those  rays  which  face  very  close  to  the  lower 
pole  of  the  bubble  that  will  reach  the  eye,  and  the  black  marginal 
zone  will  therefore  be  much  larger.  This  is  shown  in  fig.  366,  No.  2. 

When  the  objective  is  focussed  to  the  bottom  of  the  bubble 
(A'),  we  have  a  small  central  circle,  brighter  than  the  rest  of  the 
field,  all  the  rest  of  the  bubble  being  black,  with  the  exception  of 
some  peripheral  diffraction  rings.  On  focussing  to  the  centre  (B') 
or  upper  part  (C;)  of  the  bubble,  we  have  substantially  the  same 
appearances  as  in  B  and  C,  with  the  exception  of  the  smaller  size 
of  the  central  circle. 

Fat-globules  in  ivater  (fig.  366,  No.  3).— These  illustrate  the 
case  of  a  highly  refracting  body  in  a  medium  of  less  refractive  power. 

When  the  objective  is  adjusted  to  the  bottom  of  the  globule  A", 
it  appears  as  a  grey  disc  a  little  darker  than  the  field,  and  separated 
from  the  rest  of  the  field  by  a  darkish  ring. 

Focussing  to  the  middle  of  the  bubble  (Br/),  the  central  disc 
becomes  somewhat  brighter,  and  is  surrounded  by  a  narrow  black 
ring,  bordered  within  and  without  by  diffraction  circles. 

On  further  removing  the  objective  the  dark  ring  increases  in 
size,  and  when  the  upper  part  of  the  bubble  is  in  focus,  we  have 
(Cr/)  a  small  white  central  disc,  brighter  than  the  rest  of  the  field, 
and  sharply  limited  by  a  broad,  dark  ring  which  is  blacker  towards 
the  centre. 

These  appearances  are  the  converse  of  those  presented  by  the 
air-bubble.  That,  as  we  saw,  has  a  black  ring  and  a  white  centre, 
which  are  the  sharper  as  the  objective  is  approached  to  the  lower 
pole  of  the  bubble.  The  fat-globule  has,  however,  a  dark  ring 
which  is  the  broader,  and  a  centre  which  is  the  sharper,  according 
as  the  objective  is  brought  nearer  to  the  upper  pole. 

These  considerations,  apart  from  their  enabling  us  to  distinguish 
between  air-bubbles  and  fat-globules,  and  preventing  their  being 
confounded  with  the  histological  elements,  enable  two  general 
principles  to  be  established,  viz.  bodies  which  are  of  greater  re- 
fractive power  than  the  surrounding  medium  have  a  white  centre 
which  is  sharper  and  smaller,  and  a  black  ring  which  is  larger  when 


'BKOWNIAN'   MOVEMENT  431 

the  objective  is  withdrawn  ;  whilst  those  which  are  of  less  refractive 
power  have  a  centre  which  is  whiter  and  smaller,  and  a  black  ring 
which  is  broader  and  darker  when  the  objective  is  lowered. 

Monochromatic  light. — The  same  phenomena  are  observed  by 
yellow  monochromatic  light,  except  that  the  diffraction  fringes  are 
more  distinct,  further  apart,  and  in  greater  numbers  than  with 
ordinary  light. 

A  fat-globule,  indeed,  seems  to  be  composed  of  a  series  of  con- 
centric layers  like  a  grain  of  starch.  With  blue  light  these  fringes 
are  also  multiplied,  but  are  closer  together  and  finer,  so  that  they 
are  not  so  easily  visible. 

Yellow  monochromatic  light,  therefore,  constitutes  a  good 
means  for  determining  whether  "the  striae  seen  on  an  object  are 
peculiar  to  it  or  are  only  diffraction  lines.  In  the  former  case 
they  are  not  exaggerated  by  monochromatic  light ;  but  if,  on  the 
contrary,  they  are  found  to  be  doubled  or  quadrupled  with  this 
light,  we  may  be  certain  that  they  are  diffraction  fringes. 

But  there  is  no  source  of  fallacy,  to  a  certain  class  of  workers,  so 
much  to  be  guarded  against  as  that  arising  from  errors  in  the  inter- 
pretation concerning  movements  as  such,  and  especially  concerning 
the  movement  exhibited  by  certain  very  minute  particles  of  matter  in 
a  state  of  suspension  in  fluids.  The  movement  was  first  observed  in 
the  fine  granular  particles  which  exist  in  great  abundance  in  the 
contents  of  pollen  grains  of  plants  known  as  ihefovilla,  and  which 
are  set  free  by  crushing  the  pollen.  It  was  first  supposed  that  they 
indicated  some  special  vital  movement  analogous  to  the  motion  of 
the  spermatozoa  of  animals.  But  it  was  discovered  in  1827,  by  Dr. 
Robert  Brown,  that  inorganic  substances  in  a  state  of  fine  trituration 
would  give  the  same  result ;  and  it  is  now  known  that  all  substances 
in  a  sufficiently  fine  state  of  powder  are  affected  in  the  same  manner, 
one  of  the  most  remarkable  being  the  movement  visible  in  the  con- 
tents of  the  fluid  cavities  in  quartz  in  the  oldest  rocks.  These  have 
probably  retained  their  dancing  motion  for  reons.  A  good  illustra- 
tion is  gamboge,  which  can  be  easily  rubbed  from  a  water-colour 
cake  upon  a  glass  slip  and  covered,  and  will  at  once  show  the 
characteristic  movement ;  so  will  carmine,  indigo,  and  other  similarly 
light  bodies.  But  the  metals  which  are  from  seven  to  twenty  times 
as  heavy  as  water  require  to  be  reduced  to  a  state  of  minuteness 
many  times  greater  ;  but,  triturated  finely  enough,  these  also  show 
the  movement,  for  a  long  time  known,  from  the  name  of  its  dis- 
coverer, as  Brownidn  movement,  but  now  more  generally  called 
pedesis.  , 

The  movement  is  chiefly  of  an  oscillatory  nature,  but  the  particles 
also  rotate  backwards  and  forwards  on  their  axes,  and  gradually  (if 
persistently  watched)  change  their  places  in  the  field  of  view.  It  is  an 
extremely  characteristic  movement,  and  could  not  be  mistaken  for 
any  vital  motion  by  an  observer  acquainted  with  both;  but  the 
student  must  familiarise  himself  with  this  kind  of  motion  or  he  will 
be  utterly  unable  to  distinguish  certain  kinds  of  motion  in  minute 
living  forms  in  certain  stages  of  their  life  from  this  movement,  and 
will  make  erroneous  inferences. 


432   MANIPULATION   AND   PRESERVATION   OF   THE  MICROSCOPE 

The  movement  of  the  smallest  particles  in  pedesis  is  always  the 
most  active,  while  in  the  majority  of  cases  particles  greater  than  the 
-()10()th  of  an  inch  are  wholly  inactive.  A  drop  of  common  ink 
which  has  been  exposed  to  the  air  for  some  weeks,  or  a. drop  of  fine 
clay  (such  as  the  prepared  kaolin  used  by  photographers),  shaken  up 
with  water,  is  recommended  by  Professor  Jevons,1  who  has  recently 
studied  this  subject,  as  showing  the  movement  (which  he  designates 
pedesis)  extremely  well.  But  none  of  the  particles  he  has  examined 
is  so  active  as  those  of  pumice-stone  that  has  been  ground  up  in  an 
agate  mortar ;  for  these  are  seen  under  the  microscope  to  leap  and 
swarm  with  an  incessant  quivering  movement,  so  rapid  that  it  is 
impossible  to  follow  the  course  of  a  particle,  which  probably  changes 
its  direction  of  motion  fifteen  or  twenty  times  in  a  second.  The 
distance  through  which  a  particle  moves  at  any  one  bound  is  usually 
less  than  .-.oWyth  °f  an  incn-  This  'Brownian  movement'  (as  it  is 
commonly  termed)  is  not  due  to  evaporation  of  the  liquid,  for  it 
continues  without  the  least  abatement  of  energy  in  a  drop  of  aqueous 
fluid  that  is  completely  surrounded  by  oil,  and  is  therefore  cut  off 
from  all  possibility  of  evaporation ;  and  it  has  been  known  to  con- 
tinue for  many  years  in  a  small  quantity  of  fluid  enclosed  between 
two  glasses  in  an  air-tight  case  ;  and  for  the  same  reason  it  can 
scarcely  be  connected  with  the  chemical  change.  But  the  observa- 
tions of  Professor  Jevons  (loc.  cit.)  show  that  it  is  greatly  affected 
by  the  admixture  of  various  substances  with  water,  being,  for 
example,  increased  by  a  small  admixture  of  gum,  while  it  is  checked 
by  an  extremely  minute  admixture  of  sulphuric  acid  or  of  various 
saline  compounds,  these  (as  Professor  Jevons  points  out)  being  all 
such  as  increase  the  conducting  power  of  water  for  electricity.  The 
rate  of  subsidence  of  finely  divided  clays  or  other  particles  suspended 
in  water  thus  greatly  depends  upon  the  activity  of  their  '  Brownian 
movement,'  tor  wrhen  this  is  brought  to  a  stand  the  particles  aggre- 
gate and  sink,  so  that  the  liquid  clears  itself.2 

Pedetic  motion  depends  on,  that  is,  is  affected  by — 

1.  The  size  of  the  particles. 

2.  The  specific  gravity  of  the  particles.     Metals,  or  particles  of 
vermilion,  of  similar  size  to  particles  of  silica  or  gamboge,  move  much 
more  slowly  and  less  frequently. 

3.  The  nature  of  the  liquid.     No  liquid  stops  pedesis,  but  liquids 
which  have  a  chemical  action  on  the  substance  do  hinder  it.     This 
action  may  be  very  slow  ;  still  it  tends  to  agglomerate  the  particles. 
For   instance,    barium  sulphate,    when  precipitated    from  the  cold 
solution,  takes  a  long  time  to  settle  ;  whereas,  when  warm  and  in 
presence  of  hydrochloric  acid,  agglomeration  soon  occurs.     Iron  pre- 
cipitated as  hydrate  in  presence  of  salts  of  ammonium,  and  mud  in 
salt  water,  are  other  instances.     The  motion  does  not  cease,  but  the 
particles  adhere  together  and  move  very  slowrly. 

But  besides  the  right  appreciation  of  the    nature    of  pedesis, 
there  is  the  utmost  caution  required  in  the  interpretation  of  the 

1  Quarterly  Journal  of  Micro.  Science,  N.S.  vol.  viii.  1878,  p.  172. 

2  See  also  the  Rev.  J.  Delsaulx,  '  On  the  Thermo- dynamic  Origin  of  the  Brownian 
Motions,'  in  Monthly  Journal  of  Microsc.  Sci.  vol.  xviii.  1877. 


INTEKPKETATIOX   OF  MICKOSCOPIC   MOVEMENT          433 

rapidity  of  movement,  and  kind  of  mo ve ment,  which  living  and  motile 
forms  effect. 

The  observation  of  the  phenomena  of  motion  under  the  microscope l 
has  led  to  many  false  views  as  to  the  nature  of  these  movements. 
If,  for  instance,  swarm-spores  are  seen  to  traverse  the  field  of  view 
in  one  second,  it  might  be  thought  that  they  race  through  the  water 
at  the  speed  of  an  arrow,  whereas  they  in  reality  traverse  in  that 
time  only  a  third  part  of  a  millimetre,  which  is  somewhat  more  than 
a  metre  in  an  hour.  It  must  not,  therefore,  be  forgotten  that  the 
rapidity  of  motion  of  microscopical  objects  is  only  an  apparent  one, 
and  that  its  accurate  estimation  is  only  possible  by  taking  as  our 
standard  the  actual  ratio  between  time  and  space.  If  we  wish,  for 
the  sake  of  exact  comparison,  to  estimate  the  magnitude  of  the  mov- 
ing bodies,  we  may  always  do  so;  the  ascertainment  of  the  real 
rapidity  remains,  however,  with  each  successive  motion,  the  princi- 
pal matter. 

If  a  screw-shaped  spiral  object,  of  slight  thickness,  revolves  on 
its  axis  in  the  focal  plane,  at  the  same  time  moving  forward,  it 
presents  the  deceptive  appearance  of  a  serpentine  motion.  Thus  it 
is  that  the  horizontal  projections  of  an  object  of  this  kind,  corre- 
sponding to  the  successive  moments  of  time,  appear  exactly  as  if  the 
movement  were  a  true  serpentine  one.  As  an  example  of  an  appear- 
ance of  this  nature  we  may  mention  the  alleged  serpentine  motion 
of  Spirillum  and  Vibrio. 

Similar  illusions  are  also  produced  by  swarm-spores  and  sperma- 
tozoa ;  they  appear  to  describe  serpentine  lines,  while  in  reality  they 
move  in  a  spiral.  It  was  formerly  thought  that  a  number  of  differ- 
ent appearances  of  motion  must  be  distinguished,  whereas  modern 
observers  have  recognised  most  of  them  as  consisting  of  a  forward 
movement  combined  with  rotation,  where  the  revolution  takes  place 
sometimes  round  a  central,  and  sometimes  round  an  eccentric,  axis. 
To  this  category  belong,  for  instance,  the  supposed  oscillations  of 
the  oscillatorice,  whose  changes  of  level,  when  thus  in  motion,  were 
formerly  unnoticed. 

In  addition  to  these  characteristics  of  a  spiral  motion  it  must,  of 
course,  be  ascertained  whether  it  is  right-  or  left-handed.  To  dis- 
tinguish this  in  spherical  or  cylindrical  bodies,  which  revolve  round 
a  central  axis,  is  by  no  means  easy,  and  in  many  cases,  if  the  object 
is  very  small  and  the  contents  homogeneous,  it  is  quite  impossible. 
The  slight  variations  from  cylindrical  or  spherical  form,  as  they 
occur  in  each  cell,  are  therefore  just  sufficient  to  admit  of  our  per- 
ceiving whether  any  rotation  does  take  place.  The  discovery  of  the 
direction  of  the  rotation  is  only  possible  when  fixed  points  whose 
position  to  the  axis  of  the  spiral  is  known  can  be  followed  in  their 
motion  round  the  axis.  The  same  holds  good  also,  mutatis  mutandis, 
of  spirally  wound  threads,  spiral  vessels,  &c. ;  we  must  be  able 
to  distinguish  clearly  which  are  the  sides  of  the  windings  turned 
towards  or  turned  away  from  us. 

If  the  course  of  the  windings  is  very  irregular,  as  in  fig.  367,  a 
little  practice  and  care  are  needed  to  distinguish  a  spiral  line  as 

1  Das  Mikroskop,  Naegeli  and  Schwendener,  p.  258  (Eng.  edit.). 

F  F 


434  MANIPULATION   AND   PRESERVATION   OF   THE   MICROSCOPE 


such  in  small  objects.  The  microscopical  image  might  easily  lead 
us  to  the  conclusion  that  we  were  examining  a  cylindrical  body 
composed  of  bells  or  funnels  inserted  one  in  another.  The  spirally 
thickened  threads,  for  instance,  as  they  originate  from  the  epidermis 
cells  of  many  seeds,  were  thus  interpreted,  although  here  and  there 
by  the  side  of  the  irregular  spirals  quite  regular  ones  are  also 
observed.  In  illustration  -of  this  a  very  excellent  example  is  given 
in  the'Quekett  Journal'  for  1899  (No.  44),  p.  166,  where  Mr. 
Nelson  shows  that  a  certain  structure  in  the 
remarkable  diatom  Climacosphenia  moniligera, 
which  for  a  long  time  has  been  regarded  as  inter- 
locking teeth,  is  in  reality  a  spiral  pipe. 

Moreover,  it  must  not  be  forgotten  that  in 
the  microscopical  image  a  spiral  line  always  ap- 
pears wound  in  the  same  manner  as  when  seen 
with  the  naked  eye,  while  in  a  mirror  (the  inver- 
sion being  only  a  half  one)  a  right-handed  screw 
is  obviously  represented  as  left-handed,  and  con- 
versely. If,  therefore,  the  microscopical  image 
is  observed  in  a  mirror,  as  in  drawing  with  the 
Sommering  mirror,  or  if  the  image-forming  pen- 
cils are  anywhere  turned  aside  by  a  single  reflec- 
tion, a  similar  inversion  takes  place  from  right- 
handed  to  left-handed,  and  this  inversion  is  again 
cancelled  by  a  second  reflexion  in  some  micro- 
scopes. All  this  is,  of  course,  well  known,  arid  to 
the  practised  observer  self-evident;  nevertheless  many microscopists 
have  shown  that  they  are  still  entirely  in  the  dark  about  matters  of 
this  kind. 

One  of  Professor  Abbe's  experiments  on  diffraction  phenomena 
proves  that  when  the  diffraction  spectra  of  the  first  order  are  stopped 
out,  while  those  of  the  second  are  admitted,  the  appearance  of  the 
structure  will  be  double  the  fineness  of  the  actual  structure  which  is 
causing  the  interference.1  • 


FIG.  367.— A  spiral 
in  motion. 


FIG.  368. 

Upon  this  law  there  appears  to  depend  a  number  of  possible 
fallacies,  errors  which  may  arise  from  either  its  misapprehension  or 
misinterpretation.  At  least  these  appear  to  us,  from  a  practical 
point  of  view,  to  be  of  sufficient  importance  to  need  either  caution 
or  a  fuller  exposition  of  the  great  law  of  Abbe  in  regard  to  them. 

If,  for  example,  figs.  368,  369,  and  370  may  be  taken  to  represent 
1  See  Chapter  II. 


INTERPRETATION   OF   THE   N.A.   TABLE 


435 


si  square  grating  having  25,000  holes  per  linear  inch  at  the  focus  of  an 
objective  at  P,  P  D  the  dioptric  beam,  P1  P1  diffraction  spectra  of  the 
first  order,  and  P2  P2  those  of  the  second  order,  then  if  the  objective 
is  aplanatic  all  those  spectra  will  be  brought  to  an  identical  focal 
conjugate  ;  and  the  image  of  the  grating  will  be  a  counterpart  of  the 
structure,  characteristic  of  such  a  group  of  spectra.  Let  us  suppose 
our  objective  to  be  over-corrected,  as  in  fig.  369,  then  when  the  grat- 
ing is  focussed  at  P  the  spectra  of  the  first  order  only  will  be  brought 
to  the  focal  conjugate  ;  the  image,  however,  will  not  be  materially 
affected  on  that  account,  as  the  diffraction  elements  of  the  first  order 
are  alone  sufficient  to  give  a  truthful  representation  of  the  25,000 
per  inch  grating.  If,  however^the  objective  be  raised  so  that  the 
grating  lies  at  P',  the,  diffraction'  elements  of  the  second  order  only  are 
brought  to  the  focal  conjugate;  consequently  by  the  hypothesis  the 
image  will  have  50,000  holes  per  linear  inch,  or  double  that  of  the- 
original.  In  other  words,  placing  a  grating  at  the  longer  focus  of  an 
over-corrected  objective  is  apparently  tantamount  to  cutting  out  the 
diffraction  spectra  of  the  first  order  by  a  stop  at  the  back  of  the 
objective. 

The  effect  of  this  is  to  give  an  impression  that  there  is  a  strong 
grating  with  25,000  holes  per  linear  inch  ;  and  over  it  another  grat- 
ing with  50,000  holes  per  linear  inch.  The  raising  the  focus  so  as 
to  bring  P  to  P7  necessarily  gives  the  idea  of  the  fine  structure  being 
superimposed  on  the  coarse.  Therefore  the  microscopist  should 
beware,  whenever  he  notices  a  structure  of  double  fineness  over 
another  one,  lest  he  has  a  condition  of  things  similar  to  fig.  369,  The 
following  is  a  test  which  may  be  applied  to  confirm  the  genuineness  of 
any  such  structure.  First  measure  by  means  of  the  divided  head 
of  the  fine-adjustment  screw,  as  accurately  as  possible,  the 
movement  required  to  bring  P  to  P'  in  fig.  369  ;  next  by  means 
of  the  draw- tube  increase  the  distance  between  the  eye-piece  and  the 
objective  :  this  will  have  the  effect  of  increasing  the  over-correction 
of  the  objective,  and  a  state  of  things  will  be  obtained  as  in  fig.  370. 
Hence  it  will  require  a  larger  movement  of  the  fine-adjustment 
screw  to  bring  P  to  P'.  This  will  make  the  distance  between  the 
50,000  grating  and  the  25,000  grating  appear  greater  than  it  was 
before.  If  this  takes  place  the  50,000  grating  is  a  mere  diffraction 
ghost.  It  is  well  to  note  that  we  have  seen  a  photograph  by 
Mr.  Comber  of  a  diatom  surface  which  is  uneven.  In  those  parts 
where  the  focus  is  correct  the  structure  is  single,  but  in  the  parts 
where  the  focus  is  withdrawn  it  is  double. 

A  precisely  similar  condition  of  things  exists  with  an  under- 
corrected  objective,  only  in  that  case  the  false  finer  grating  will 
appear  below  the  original  coarse  grating,  and  to  increase  the  distance 
between  them  the  draw-tube  must  be  shortened. 

It  may  therefore  be  of  service  to  give  an  example  of  the  use  of 
the  numerical  aperture  table  as  a  check  in  the  inter pretation  of 
structure. 

Fig.  371  gives  six  illustrations  of  the  back  of  an  objective  (the 
eye-piece  being  removed)  of  -83  X.A.,  or  112°  in  air.  D  stands  for 

F  F  2 


436   MANIPULATION  AND   PRESERVATION   OF  THE   MICROSCOPE 


dioptric  beam ;   1  for  diffraction  spectrum  of  the  first  order ;    2  for 
diffraction  spectrum  of  the  second  order. 

When  the  back  of  an  objective  of  -83  N.  A.  shows  an  arrange- 
ment as  in 

No.  1,  then,  although  the  structure  will  be  invisible, 

it  cannot  be  coarser  than  .         .         .  40,000  per  inch. 

No.  2  „  „  „  80,000 

No.  3,  then  the  structure  does  not  differ  greatly  from  40,000 

No.  4  „  „  „  80,000 

No.  5  „  „  „  20,000 

No.  6  „  „  „  40,000 

It  will  be  understood  by  the  student  that  the  preservation  of  the 
microscope  and  its  apparatus  is  a  matter  that  must  largely  depend 
upon  his  own  action.  The  stand  should  be  kept  from  dust,  generally 
wiped  with  a  soft  chamois  leather  after  use,  and  when  needful  a 
minute  quantity  of  watchmaker's  oil  may  be  put  to  a  joint  working 
.stiffly.  There  is  no  better  way  to  preserve  this  stand  than  to  keep 


it  either  under  a  bell-glass  or  in  a  cabinet  which  is  easily  accessible. 

All  objectives  should  be  examined  after  use,  and  all  oils  or  other 
fluids  carefully  wiped  away  from  them  with  old  cambric  which  has 
been  thoroughly  washed  with  soda,  well  rinsed  and  not  '  ironed '  or 
finished  in  any  way,  but  simply  dried. 

If  chemical  reagents  are  employed  the  cessation  of  their  use 
should  become  the  moment  for  wiping  with  care  the  lenses  employed  ; 
and  all  processes  involving  the  use  of  the  vapours  of  volatile  acids, 
or  which  develop  sulphuretted  hydrogen,  chlorine,  &c.,  must  never 
take  place  in  a  room  in  which  a  microscope  of  any  value  is  placed. 

Dry  elder-pith  and  Japanese  paper  are  by  some  workers  sug- 
gested for  cleaning  the  front  lenses  of  homogeneous  objectives ;  but 
while  these  are  excellent,  especially  the  former,  we  find  nothing 
better  than  the  simple  cambric  we  suggest. 

Two  or  three  good  chamois  leathers  should  be  kept  by  the 
worker  for  specific  purposes  and  not  interchanged.  Cleanliness, 
care,  delicacy  of  touch,  and  a  purpose  to  be  accurate  in  all  that  he 
does  or  seeks  to  do  are  essentials  of  the  successful  microscopist. 


DUST   ON   THE   EYE-PIECE 


437 


It  may  be  noted  that  dust  on  the  eye -piece  can  be  detected  in  a 
dim  light,  and  can  be  discovered  by  closing  the  iris  diaphragm.  The 
lens  of  the  eye-piece  on  which  the  dust  appears  may  be  localised  by 
rotation  ;  and  this  should  be  done  before  wiping.  In  reference  to 
dust  on  the  back  of  the  objective,  it  should  be  observed  that  if  the 
eye-piece  be  removed,  dust  sometimes  appears  to  be  upon  it  which 
comes  really  from  the  focus  of  the  sub-stage  condenser,  and  is,  in  fact, 
not  on  the  back  of  the  objective  at  all.  To  find  this  condition,  remove 
the  light  modifier  (if  in  use),  for  the  dust  may  be  on  it,  and  rotate 
the  condenser  ;  else  there  will  be  needless  and  injurious  rubbing  of 
the  back  lens  of  the  objective. 

With  oil-immersion  objectives  dust  or  air-bubbles  in  the  oil  must 
be  carefully  avoided. 

If  chamois  leather  be  used  for  cleaning  the  lenses,  it  should  be 
previously  well  beaten  and  shaken,  and  then  kept  constantly  in  a 
well-made  box. 


438 


CHAPTER   VII 

PREPARATION,   MOUNTING,  AND   COLLECTION  OF  OBJECTS 

UNDER  this  head  it  is  intended  to  give  an  account  of  those  materials, 
instruments,  and  appliances  of  various  kinds  which  have  been  found 
most  serviceable  to  microscopists  engaged  in  general  biological  re- 
search, and  to  describe  the  most  approved  methods  of  employing 
them  in  the  preparation  and  mounting  of  objects  for  the  display  of 
the  minute  structures  thus  brought  to  our  knowledge.  Not  only  is 
it  of  the  greatest  advantage  that  the  discoveries  made  by  microscopic 
research  should — as  far  as  possible — be  embodied  (so  to  speak)  in 
*  preparations,'  which  shall  enable  them  to  be  studied  by  every  one 
who  may  desire  to  do  so,  but  it  is  now  universally  admitted  that 
such  '  preparations '  often  show  so  much  more  than  can  be  seen  in 
the  fresh  organism  that  no  examination  of  it  can  be  considered  as 
complete  in  which  the  methods  most  suitable  to  each  particular 
case  have  not  been  put  in  practice.  It  must  be  obvious  that  in  a 
comprehensive  treatise  like  the  present  such  a  general  treatment  of 
this  subject  is  all  that  can  be  attempted,  excepting  in  a  few  instances 
of  peculiar  interest ;  and  as  the  histological  student  can  find  all 
the  guidance  he  needs  in  the  numerous  manuals  now  prepared  for 
his  instruction,  the  Author  will  not  feel  it  requisite  to  furnish  him 
with  the  special  directions  that  are  readily  accessible  to  him  else- 
where. 

MATERIALS,  INSTRUMENTS,  AND  APPLIANCES. 

Glass  Slides. — The  kind  of  glass  best  suited  for  mounting  objects 
is  that  which  is  known  as  *  patent  plate,'  and  it  is  now  almost  in- 
variably cut,  by  the  common  consent  of  microscopists  in  this  country, 
into  slips  measuring  3  in.  by  1  in.  For  objects  too  large  to  be 
mounted  on  these  the  size  of  3  in.  by  1^  in.  may  be  adopted.  Such 
slips  may  be  purchased,  accurately  cut  to  size,  and  ground  at  the 
edges,  for  so  little  more  than  the  cost  of  the  glass  that  few  persons 
to  whom  time  is  an  object  would  trouble  themselves  to  prepare 
them ;  it  being  only  when  glass  slides  of  some  unusual  dimensions 
are  required,  or  when  it  is  desired  to  construct  *  built-up  cells,'  that 
a  facility  in  cutting  glass  with  a  glazier's  diamond  becomes  useful. 
The  glass  slides  prepared  for  use  should  be  free  from  veins,  air-bubbles, 
or  other  flaws,  at  least  in  the  central  part  on  which  the  object  is 
placed  ;  and  any  whose  defects  render  them  unsuitable  for  ordinary 
purposes  should  be  selected  and  laid  aside  for  uses  to  which  the 
working  microscopist  will  find  no  difficulty  in  putting  them.  As 


COVERING   GLASS 


439 


the  slips  vary  considerably  in  thickness,  it  will  be  advantageous  to 
determine  on  a  gauge  for  thin,  thick,  and  medium  glass.  The  first 
may  be  employed  for  mounting  delicate  objects  to  be  viewed  by  the 
high  powers  with  which  the  apochromatic  and  achromatic  condensers 
are  to  be  used,  so  as  to  allow  plenty  of  room  for  the  focal  point  of  an 
optical  combination  with  great  aperture  to  be  fixed  readily  upon  the 
plane  of  the  object;  the  second  should  be  set  aside  for  the  attach- 
ment of  objects  which  are  to  be  ground  down,  and  for  which,  there- 
fore, a  stronger  mounting  than  usual  is  desirable ;  and  the  third  are 
to  be  used  for  mounting  ordinary  objects.  Great  care  should  be 
taken  in  washing  the  slides,  and  in  removing  from  them  every  trace 
of  greasiness  by  the  use  of  a  little  soda  or  potass  solution.  If  this 
should  not  suffice  they  may  be  immersed  in  the  solution  recommended 
by  Dr.  Seiler,  composed  of  2  oz.  of  bichromate  of  potass,  3  fl.  oz.  of 
sulphuric  acid,  and  25  oz.  of  water,  and  afterwards  thoroughly  rinsed. 
(The  same  solution  may  be  advantageously  used  for  cleansing  cover- 
glasses.)  Before  they  are  put  away  the  slides  should  be  wiped 
perfectly  dry,  first  with  an  ordinary  '  glass  cloth,'  and  afterwards 
with  an  old  cambric  handkerchief;  and  before  being  used  each 
slide  should  be  washed  in  methylated  spirit  to  ensure  freedom  from 
greasiness.  Where  slides  that  have  been  already  employed  for 
mounting  preparations  are  again  brought  into  use,  great  care  should 
be  taken  in  completely  removing  all  trace  of  adherent  varnish  or 
cement — first  by  scraping  (care  being  taken  not  to  scratch  the  glass), 
then  by  using  an  appropriate  solvent,  and  then  by  rubbing  the  slide 
with  a  mixture  of  equal  parts  of  alcohol,  benzole,  and  liquor  sodaB, 
finishing  with  clean  water. 

Thin  Glass. — The  older  microscopists  were  obliged  to  employ  thin 
laminae  of  talc  for  covering  objects  to  be  viewed  with  lenses  of  short 
focus ;  but  this  material,  which  was  in  many  respects  objectionable, 
is  entirely  superseded  by  the  thin  glass  manufactured  by  Messrs. 
Chance,  of  Birmingham,  which  may  be  obtained  of  various  degrees  of 
thickness,  down  to  the  --^th  of  an  inch.  This  glass,  being  unannealed, 
is  very  hard  and  brittle,  and  much  care  and  some  dexterity  are  re- 
quired in  cutting  it ;  hence  covers  should  be  purchased,  as  required, 
from  the  dealers,  who  usually  keep  them  in  several  sizes  and  supply 
any  others  to  order.  Save  the  fact  that  '  cover-glass '  is  made  by 
Messrs.  Chance,  there  is  no  definite  information  as  to  the  mode  of  its 
manufacture  and  the  conditions  upon  which  it  is  most  satisfactorily 
.produced.  It  wrould  be  an  advantage  to  the  microscopist  to  possess 
information  on  this  point.  The  different  thicknesses  are  usually 
ranked  as  1.2,  and  3  ;  the  first,  which  should  not  exceed  in  thickness 
the  '006  in.,  being  used  for  covering  objects  to  be  viewed  with  low 
powers ;  the  second,  which  should  not  exceed  '005  in.  in  thickness, 
for  objects  to  be  viewed  with  medium  powers ;  and  the  third,  which 
ought  never  to  exceed  '004  in.  in  thickness,  for  objects  which  either 
require  or  may  be  capable  of  being  used  with  high  powers.  It  must, 
however,  be  remembered  that  the  achromatic  objectives  of  great 
power  and  great  aperture  (1*5)  will  require  much  thinner  covers 
than  even  this.  The  thinnest  glass  is  of  course  most  difficult  to 
handle  safely,  and  is  most  liable  to  fracture  from  accidents  of  various 


44O    PEEPAEATION,    MOUNTING,   AND   COLLECTION   OF   OBJECTS 

kinds  ;  and  hence  it  should  only  be  employed  for  the  purpose  for 
which  it  is  absolutely  needed.  The  thickest  pieces,  again,  may  be 
most  advantageously  employed  as  covers  for  large  cells,  in  which 
objects  are  mounted  in  fluid  to  be  viewed  by  the  low  powers  whose 
performance  is  not  sensibly  affected  by  the  aberration  thus  produced. 
The  working  microscopist  will  find  it  desirable  to  provide  himself 
with  some  means  of  measuring  the  thickness  of  his  cover-glass ;  and 
this  is  especially  needed  if  he  is  in  the  habit  of  employing  objectives 
without  adjustment,  which  are  corrected  to  a  particular  standard. 
A  small  screw-gauge  of  steel,  made  for  measuring  the  thickness  of 
rolled  plates  of  brass,  and  sold  at  the  tool-shops,  answers  this  purpose 
very  well ;  but  Ross's  lever  of  contact  (fig.  372),  devised  for  this 
express  purpose,  is  in  many  respects  preferable.  This  consists  of  a 
small  horizontal  table  of  brass,  mounted  upon  a  stand,  and  having 
at  one  end  an  arc  graduated  into  twenty  divisions,  each  of  which  re- 
presents the  r  oVo^h  of  an  inch,  so  that  the  entire  arc  measures  the 
5-fi th  of  an  inch  ;  at  the  other  end  is  a  pivot  on  which  moves  a  long  and 
delicate  lever  of  steel,  whose  extremity  points  to  the  graduated  arc, 
whilst  it  has  very  near  its  pivot  a  sort  of  projecting  tooth,  which 
bears  it  against  a  vertical  plate  of  steel  that  is  screwed  to  the 


-pa 

_£j 


FIG.  372. — Ross's  lever  of  contact. 

horizontal  table.  The  piece  of  thin  glass  to  be  measured  being  in- 
serted between  the  vertical  plate  and  the  projecting  tooth  of  the 
lever,  its  thickness  in  thousandths  of  an  inch  is  given  by  the  number 
on  the  graduated  arc  to  which  the  extremity  of  the  lever  points. 
Thus,  if  the  number  be  8,  the  thickness  of  the  glass  is  '008,  or  the  -^th 
of  an  inch.  It  will  be  found  convenient  to  sort  the  covers  according 
to  their  thicknesses,  and  to  keep  the  sortings  apart,  so  that  there 
may  be  a  suitable  thickness  of  cover  for  each  object.  But  it  is  well 
to  remember  that,  with  the  exception  of  objects  to  which  from  their 
size  or  nature  it  is  impossible  to  apply  high  powers,  it  is  better  to 
mount  the  object  so  that,  if  it  be  required  or  desirable,  high  powers 
may  be  used  upon  it. 

Another  simple  and  very  efficient  cover-glass  tester  is  made  by 
Zeiss,  of  Jena,  and  illustrated  in  fig.  373.  It  will  be  seen  that  the 
measurement  is  effected  by  a  clip  projecting  from  a  box,  between  the 
jaws  of  which  the  cover  to  be  measured  is  placed  ;  the  reading  is 
given  by  an  indicator  moving  over  a  divided  circle  on  the  upper  face 
of  the  box.  The  divisions  show  hundredths  of  a  millimetre,  and  the 
instrument  measures  to  upwards  of  5  mm. 


MICROMETERS   FOR   COVERING   GLASS 


441 


One  of  the  continuous  aims  of  the  working  microscopist  is  to 
save  or  utilise  to  its  utmost  his  time.  Complicated  measurements  and 
calculations  are  to  be  avoided  where  possible,  and  a  very  beautiful 
and  ingenious  instrument,  capable  of  being  used  as  a  meter  for 
cover-glass,  has  been  devised  by  Mr.  J.  Ciceri  Smith,  of  61  Hatton 
Garden,  London.  It  is  a  perfect  direct-reading  micrometer,  and  i& 
constructed  to  take  measurements  in  thousandths  of  an  inch,  and 
may  be  used  in  gauging  the  thickness  of  microscopical  glass,  metal 
and  other  sheets,  balls 
for  bearings,  needles, 
wire,  &c.  Its  advan- 
tages over  the  ordinary 
micrometer  consist  in 
the  measurements 
being  automatically 
and  accurately  re- 
corded in  clear  figures 
on  the  index,  thus 
avoiding  the  strain  on 
the  eyes  caused  by 
reading  the  fine  lines 
on  the  old  form  of 

gauge;  in  there  being  no  liability  to  errors  through  miscalcula- 
tions, and  in  its  being  possible  to  take  any  number  of  various 
readings  with  ease,  accuracy,  and  rapidity.  We  illustrate  this  appa- 
ratus in  fig.  374. 

As  in  the  ordinary  decimal  gauge  the  glass  or  other  article  to  be 
measured  is  placed  between  the  '  anvil '  (or  hexagonal  nut)  and  the 
face  of  the  spindle,  the  thimble  being  rotated  in  either  direction 


FIG.  373. — Zeiss's  cover- glass  tester. 


FIG.  374. — Mr.  J.  Ciceri  Smith's  direct-reading  micrometer. 

until  the  required  adjustment  is  obtained,  the  exact  measurement  in 
decimal  parts  of  an  inch  being  at  the  same  instant  automatically  and 
accurately  recorded  on  the  index,  these  readings  responding  in  either 
direction  with  the  most  delicate  movements  of  the  screw. 

To  avoid  the  screw  being  unduly  strained,  the  spindle  is  rotated 
by  friction  from  the  cuter  spring-tight  thimble,  the  inner  thimble 
being  rigidly  fixed  to  the  spindle.  Hence  it  is  impossible  to  strain 
the  screw,  since  as  soon  as  the  pressure  becomes  too  great  the  spring 


442    PKEPARATION,    MOUNTING,    AND   COLLECTION    OF   OBJECTS 

allows  the  outer  thimble  to  slip.  The  connection  of  the  spindle  to 
the  measuring  wheels  is  effected  by  means  of  a  stop.  This  takes 
into  a  slot  on  a  sleeve,  on  which  is  mounted  the  thousandths  wheel, 
which  in  turn  drives  the  hundredths  and  tenths  wheels  through  the 
intermediate  pinions.  These  latter  have  a  step-by-step  motion,  as 
in  an  ordinary  counter.  The  cover  of  the  cage  in  which  the 
mechanism  is  placed  is  pierced  to  show  the  numbers  on  the  dials,  but 
these  openings  are  covered  with  glass,  with  a  view  to  excluding  dust 
and  dirt.  It  must  be  understood  that  gauges  of  this  kind  are 
expensive,  but  there  is  one  made  by  G.  Boley,  reading  to  '01,  which 
answers  all  purposes  and  can  be  purchased  for  five  shillings  at  a 
watchmaker's  tool  shop. 

It  is  well  to  keep  assorted,  measured,  and  cleaned  cover-glasses  in 
small  separate  wide -stoppered  bottles  of  methylated  spirit,  each 
bottle  being  labelled  with  the  gauge  of  thickness  of  the  covers  it 
contains.  What  is  then  required  is  a  simple  apparatus  for  cleaning 
the  delicate  covers  with  the  least  risk  of  breakage.  This  can  be 
well  accomplished  by  having  two  blocks  of  boxwood,  shaped  so  as  to 
be  easily  held  one  in  each  hand,  turned  with  perfect  trueness  on  the 
faces  opposite  to  the  respective  handles,  so  that  when  the  surfaces 
so  flattened  are  laid  upon  and  pressed  towards  each  other  they  are 
every  where  in  perfect  contact.  They  should  be  from  two  to  four 
inches  in  diameter,  and  these  flattened  surfaces  should  each  have, 
very  tightly  stretched  upon  them,  a  firm,  even-textured,  moderately 
thick  piece  of  chamois  leather.  If  covers  be  slightly  moistened — 
even  breathed  upon — and  laid  on  one  of  these  blocks  and  pressed 
down  with  the  other,  breath,  or  moisture  applied  by  a  small  camel- 
hair  brush  to  the  upper  surface  of  the  cover,  may  be  applied,  and  a 
few  twists  of  these  blocks  upon  each  other  when  firmly  pressed 
together  will  effectually  clean  without  breaking  the  thinner  covers. 
It  will  be  often  needful  to  treat  both  sides  of  the  covers  thus,  as  one 
side  generally  adheres  while  the  other  is  subject  to  the  friction. 

For  cleaning  slips  and  covers  by  hand,  finishing  should  be  done 
with  old  fine  cambric  handkerchiefs.  These  should  not  be  washed 
with  soap,  but  with  common  soda  and  hot  water,  plenty  of  the  latter 
being  subsequently  employed  to  get  rid  of  every  trace  of  the  alkali. 
But  when  dry  these  cloths  must  not  be  '  ironed '  or  smoothed  in  any 
way,  the  '  rough-dry '  surface  acting  admirably  for  wiping  delicate 
glass. 

Varnishes  and  Cements.  — There  are  three  very  distinct  purposes 
for  which  cements  which  possess  the  power  of  holding  firmly  to  glass, 
and  of  resisting  not  merely  water  but  other  preservative  liquids, 
are  required  by  the  microscopist,  these  being  (1)  the  attachment  of 
the  glass  covers  to  the  slides  or  cells  containing  the  object,  (2)  the 
formation  of  thin  '  cells  '  of  cement  only,  and  (3)  the  attachment  of 
the  *  glass  plate '  or  '  tube-cells '  to  the  slides.  The  two  former  of 
these  purposes  are  answered  by  liquid  cements  or  varnishes,  which 
may  be  applied  without  heat ;  the  last  requires  a  solid  cement  of 
greater  tenacity,  which  can  only  be  used  in  the  melted  state.  Among 
the  many  such  cements  that  have  been  recommended  by  different 
workers,  two  or  three  will  be  selected  by  the  worker  for  general 


VARNISHES   AND    CEMENTS  443 

purposes,  and  perhaps  three  or  four  for  special  purposes,  and  the  re- 
mainder will  be  in  practice  neglected.  We  do  not  hesitate  to  say 
that  the  two  cements  on  which  the  most  complete  trust  may  be  re- 
posed are  japamier's  gold  size  and  Bells  cement.  This  opinion  is  the 
result  of  over  twenty  years  of  special  observation. 

A  good  varnish  may  easily,  in  a  general  way.  be  tested  :  when  it  is 
thoroughly  hard  and  old,  if  scraped  off  it  comes  away  in  shreds  ;  un- 
safe varnishes  break  under  the  scraper  in  flakes  and  dust.  To  those 
who  put  up  valuable  preparations  and  objects  of  value  the  risk 
should  never  be  run  of  using  a  new  and  unknown  varnish  or  cement. 
Neither  appearance  nor  facility  nor  cheapness  in  use  should  for  one 
moment  weigh  against  a  varnish  qr  cement  of  known  and  tested 
worth. 

Japanned  s  gold  size  may  be  obtained  from  the  colour  shops.  It 
may  be  used  for  closing-in  mounted  objects  of  almost  any  description. 
It  takes  a  peculiarly  firm  hold  of  glass,  and  when  dry  it  becomes 
extremely  tough  without  brittleness.  When  new  it  is  very  liquid 
and  '  runs '  rather  too  freely  ;  so  that  it  is  often  advantageous  to  leave 
open  for  a  time  the  bottle  containing  it  until  the  varnish  is  some- 
what thickened.  By  keeping  it  still  longer,  with  occasional  exposure 
to  air,  it  is  rendered  much  more  viscid,  and  though  such  '  old '  gold- 
size  is  not  fit  for  ordinary  use,  yet  one  or  two  coats  of  it  may  be  ad- 
vantageously laid  over  the  films  of  newer  varnish,  for  securing  the 
thicker  covers  of  large  cells.  Whenever  any  other  varnish  or  cement 
is  used,  either  in  making  a.  cell  or  in  closing  it  in,  the  rings  of  these 
should  be  covered  with  one  or  two  layers  of  gold-size  extending 
beyond  it  on  either  side,  so  as  to  form  a  continuous  film  extending 
from  the  marginal  ring  of  the  cover  to  the  adjacent  portion  of  the 
glass  slide. 

Asphalte  Varnish. — This  is  a  black  varnish  made  by  dissolving 
half  a  drachm  of  caoutchouc  in  mineral  naphtha,  and  then  adding  4  oz. 
of  asphaltum,  using  heat  if  necessary  for  its  solution.  It  is  very 
important  that  the  asphaltum  should  be  genuine,  and  the  other 
materials  of  the  best  quality.  Some  use  asphalte  as  a  substitute  for 
gold  size ;  but  the  Author's  experience  leads  him  to  recommend  that 
it  should  only  be  employed  either  for  making  shallow  '  cement  cells ' 
or  for  finishing  off  preparations  already  secured  with  gold- size.  For 
the  former  purpose  it  may  advantageously  be  slightly  thickened  by 
evaporation. 

Bell's  cement  is  sold  by  J.  Bell  and  Co.,  chemists,  Oxford  Street, 
London ;  they  are  the  sole  makers,  and  retain  the  secret  of  its  com- 
position. It  is  of  great  service  for  glycerin  mounts  ;  but  the  edge 
of  the  cover  should  be  ringed  with  glycerin  jelly  before  this  cement 
is  applied.  It  is  an  extremely  useful  and  reliable  varnish,  which  is 
extremely  easy  of  manipulation.  It  can  be  readily  dissolved  in 
either  ether  or  chloroform. 

Canada  balsam  is  the  oleo-resin  from  Abies  balsamea  and  Pinus 
canadensis ;  it  is  so  brittle  when  hardened  by  time  that  it  cannot 
be  safely  used  as  a  cement,  except  for  the  special  purpose  of  attaching 
hard  specimens  to  glass,  in  order  that  they  may  be  reduced  by 
grinding,  &c.  Although  fresh,  soft  balsam  may  be  hardened  by  heating 


444    PREPARATION,    MOUNTING,   AND   COLLECTION   OF   OBJECTS 

it  on  the  slide  to  which  the  object  is  to  be  attached,  yet  it  may  be 
preferably  hardened  en  masse  by  exposing  it  in  a  shallow  vessel  to 
the  prolonged  but  moderate  heat  of  an  oven,  until  so  much  of  its 
volatile  oil  has  been  driven  off  that  it  becomes  almost  (but  not  quite) 
resinous  on  cooling.  If,  when  a  drop  is  spread  out  on  a  glass  and 
allowed  to  become  quite  cold,  it  is  found  to  be  so  hard  as  not  to  be 
readily  indented  by  the  thumb-nail,  and  yet  not  so  hard  as  to  '  chip/ 
it  is  in  the  best  condition  to  be  used  for  cementing.  If  too  soft,  it 
will  require  a  little  more  hardening  on  the  slide,  to  which  it  should 
be  transferred  in  the  liquid  state,  being  brought  to  it  by  the  heat  of 
a  water-bath ;  if  too  hard  it  may  be  dissolved  in  chloroform  or  ben- 
zole for  use  as  a  mounting  '  medium  ; '  we  do  not  recommend  its  use 
for  mounts  with  glycerin. 

Brunswick  black  is  a  very  useful  cement,  obtainable  at  the  op- 
tician's as  prepared  for  the  use  of  microscopists.  It  is  one  of  the  best 
cements  for  the  purpose  of  ringing  mounts,  and  it  may  be  recom- 
mended for  turning  cells.  x  We  have  already  stated  that  we  do 
not,  as  a  rule,  recommend  opaque  or  black-ground  mounting ;  but  if 
this  is  desired  or  needful  no  better  '  ground '  can  be  obtained  than 
by  putting  on  the  centre  of  the  slide  a  disc  of  Brunswick  black  the 
size  of  the  outside  of  the  cell  or  cover-glass,  and  while  it  is  wet 
putting  a  thin  cover-glass  upon  it.  The  cover-glass  becomes  quickly 
fixed,  and  a  pleasant  surface  is  formed  to  receive  the  object  which  it 
is  intended  to  mount.  Should  it  be  desirable  to  have  the  floor  of  the 
opaque  cell  dead  instead  of  bright,  this  can  be  quickly  accomplished 
with  a  little  emery-powder  and  water  applied  to  the  surface  by  a 
flattened  block  of  tin  fixed  in  boxwood. 

Brunswick  black  is  soluble  in  oil  of  turpentine,  and  it  dries 
quickly. 

Glue  and  honey  mixed  in  equal  parts  is  very  valuable  for  special 
purposes,  and  softens  with  heat. 

Shellac  cement  is  made  by  keeping  small  pieces  of  picked  shel- 
lac in  a  bottle  of  rectified  spirit,  and  shaking  it  from  time  to  time. 
It  cannot  be  recommended  as  a  substitute  for  any  of  the  preceding, 
but  it  may  be  employed  to  put  a  thin  film  upon  the  edge  of  all 
mounts — however  closed  and  finished — that  are  to  be  used  with  homo- 
geneous lenses.  It  is  a  sure  protection  against  the  otherwise  in- 
jurious action  of  the  cedar  oil.  Hollis's  liquid  glue  may  also  be 
employed  with  confidence  for  this  purpose. 

Sealing-wax  varnish,  which  is  made  by  digesting  powdered 
sealing-wax  at  a  gentle  heat  in  alcohol,  should  never  be  used  as  a 
cement ;  it  is  serviceable  only  as  a  varnish,  and  resists  cedar  oil. 

Venice  turpentine  is  the  liquid  resinous  exudation  of  Abies  larix. 
It  must  be  dissolved  in  enough  alcohol  to  filter  readily,  and  after 
filtering  must  be  placed  in  an  evaporating  dish,  and  by  means  of  a 
sand-bath  must  be  reduced  by  evaporation  one-fourth. 

This  cement  is  used  for  closing  glycerin  mounts.  Square  covers 
are  used,  and  we  find  it  best  to  edge  the  cover  with  glycerin  jelly. 
A  piece  of  copper  wire  of  No.  10  to  No.  12  gauge  is  taken,  and  one 
end  of  it  is  bent  just  the  length  of  one  of  the  sides  of  the  cover  at 
right  angles  to  the  length  of  the  wire.  This  end  is  now  heated  in  a 


COLOURED   VARNISHES— DRY  MOUNTING  445 

spirit  lamp,  plunged  into  the  cement,  which  adheres  in  fair  quantity, 
and  is  instantly  brought  down  upon  the  slide  and  the  margin  of  the 
cover.  The  fluid  turpentine  distributes  itself  evenly  along  the  cover 
and  slide  and  hardens  at  once.  We  have  no  long  experience  of  it, 
but  from  some  of  its  characteristics  we  are  inclined  to  believe  it  will 
prove  a  useful  cement  for  this  purpose. 

Marine  glue,  which  is  composed  of  shellac,  caoutchouc,  and 
naphtha,  is  distinguished  by  its  extraordinary  tenacity,  and  by  its 
power  of  resisting  solvents  of  almost  every  kind.  Different  qualities 
of  this  substance  are  made  for  the  several  purposes  to  which  it  is 
applied,  and  the  one  most  suitable  to  the  wants  of  the  microscopist 
is  known  in  commerce  as  G  K  4.  The  special  value  of  this  cement, 
which  can  only  be  applied  hot,  is  iixtottaching  to  glass  slides  the  glass 
or  metal  rings  which  thus  form  *  cells **  for  the  reception  of  objects 
to  be  mounted  in  fluid,  no  other  cement  being  comparable  to  it 
either  for  tenacity  or  for  durability.  The  manner  of  so  using  it  will 
be  presently  described. 

Various  coloured  varnishes  are  used  to  give  a  finish  to  mounted 
preparations,  or  to  mark  on  the  covering  glasses  of  large  preparations 
the  parts  containing  special  kinds  of  noteworthy  structure.  A 
very  good  black  varnish  of  this  kind  is  made  by  working  up  very 
finely  powdered  lamp-black  with  gold-size.  For  red,  sealing-wax 
varnish  may  be  used ;  but  it  is  very  liable  to  chip  and  leave  the  glass 
when  hardened  by  time.  The  red  varnish  specially  prepared  for 
microscopic  purposes  by  Messrs.  Thompson  and  Capper,  of  Liverpool, 
spems  likely  to  stand  better.  For  \vhite,  '  zinc  cement '  answers 
well,  which  is  made  of  benzole,  gum  dammar,  oxide  of  zinc,  and 
turpentine.  But  it  is  inexpensive,  and  either  in  Cole's  or  Ziegler's 
formula  may  be  obtained  at  the  optician's.  Blue  or  green  pigments 
may  be  worked  up  with  this  if  cements  of  those  colours  be  desired. 

For  attaching  labels  to  slides  either  of  glass  or  wood,  and  for 
fixing  down  small  objects  to  be  mounted  'dry'  (such  as  foraminifera, 
parts  of  insects,  &c.),  the  Author  has  found  nothing  preferable  to  a 
rather  thick  mucilage  of  gum  arabic,  to  which  enough  glycerin  has 
been  added  to  prevent  it  from  drying  hard,  with  a  few  drops  of  some 
essential  oil  to  prevent  the  development  of  mould.  The  following 
formula  has  also  been  recommended  :  Dissolve  2  oz.  of  gum  arabic 
in  2  oz.  of  water,  and  then  add  J  oz.  of  soaked  gelatin  (for  the 
solution  of  which  the  action  of  heat  will  be  required),  30  drops  of 
glycerin,  and  a  lump  of  camphor.  The  further  advantage  is  gained 
by  the  addition  of  a  slightly  increased  proportion  of  glycerin  to 
either  of  the  foregoing,  that  the  gum  can  be  very  readily  softened 
by  water,  so  that  covers  may  be  easily  removed  (to  be  cleansed  if 
necessary)  and  the  arrangement  of  objects  (where  many  are  mounted 
together)  altered. 

Cells  for  Dry-mounting. — Where  the  object  to  be  mounted  '  dry ' 
(i.e.  not  immersed  either  in  fluid  or  in  any  *  medium ')  is  so  thin  as  to 
require  that  the  cover  should  be  but  little  raised  above  the  slide, 
a  '  cement  cell '  answers  this  purpose  very  well ;  and  if  the  ap- 
plication of  a  gentle  warmth  be  not  injurious,  the  pressing  down  of 
the  cover  on  the  softened  cement  will  help  both  to  fix  it  and  to 


446    PREPARATION,   MOUNTING,    AND    COLLECTION    OF   OBJECTS 

prevent  the  varnish  applied  round  its  border  from  running  in. 
Where  a  somewhat  deeper  cell  is  required,  Prof.  H.  L.  Smith 
(U.S.A.)  suggests  the  following  specially  for  the  mounting  of 
diatoms.  A  sheet  of  thin  writing-paper  dipped  into  thick  shellac 
varnish  is  hung  up  to  dry ;  and  rings  are  then  cut  out  from  it  by 
punches  of  two  different  sizes.  One  of  these  rings  being  laid  on 
a  glass  slide,  and  the  cover,  with  the  object  dried  upon  it,  laid  on  the 
ring,  it  is  to  be  held  in  its  place  by  the  forceps  or  spring-clip,  and 
the  slide  gently  warmed  so  as  to  cause  a  slight  adhesion  of  the 
cover  to  the  ring,  and  of  the  ring  to  the  slide  ;  and  this  adhesion  may 
then  be  rendered  complete  by  laying  another  glass  slide  on  the  cover 
and  pressing  the  two  slides  together,  with  the  aid  of  a  continued 
gentle  heat.  Still  deeper  cells  may  be  made  with  rings  punched  out 
of  tinfoil  of  various  thicknesses  and  cemented  with  shellac  varnish 
on  either  side.  And  if  yet  deeper  cells  are  needed,  they  may  be 
made  of  turned  rings  of  vulcanite  or  ebonite,  cemented  in  the  same 
manner.  There  is,  however,  a  tendency  in  shellac-formed  cells  to 
throw  off  a  cloudiness  inside  the  cell,  usually  called  'sweating/ 
which  is  very  undesirable.  It  has  been  found  that  a  ring  of  solid 
paraffin,  to  which  the  cover  is  attached,  if  first  '  ringed '  with  the 
same  material  and  afterwards  with  a  finishing  varnish,  makes  a 
useful  and  permanently  clean  dry  shallow  cell ;  or  paper  may  be 
saturated  with  paraffin  and  treated  as  described  for  shellac. 

Cement-cells. — Cells  for  mounting  thin  objects  in  any  watery 
medium  may  be  readily  made  with  asphalte  or  Brunswick  black 
varnish  by  the  use  of  Mr.  Shadbolt's  '  turn -table '  or  one  of  its  modi- 
fications.  The  glass  slide  being  placed  under  its  spring  in  such 
a  manner  that  its  two  edges  shall  be  equidistant  from  the  centre  (a 
guide  to  which  position  is  afforded  by  the  circles  traced  on  the  brass), 
and  its  four  corners  equally  projecting  beyond  the  circular  margin 
of  the  plate,  a  camel's-hair  pencil  dipped  in  the  varnish  is  held  in  the 
right  hand,  so  that  its  point  comes  into  contact  with  the  glass  over 
whichever  of  the  circles  may  be  selected  as  the  guide  to  the  size 
of  the  ring.  The  turn-table  being  made  to  rotate  by  the  application 
of  the  left  forefinger  to  the  milled  head  beneath,  a  ring  of  varnish 
of  a  suitable  breadth  is  made  upon  the  glass  ;  and  if  this  be  set  aside 
in  a  horizontal  position,  it  will  be  found,  when  hard,  to  present  a  very 
level  surface.  If  a  greater  thickness  be  desired  than  a  single  appli- 
cation will  conveniently  make,  a  second  layer  may  be  afterwards 
laid  on.  It  will  be  found  convenient  to  make  a  considerable  number 
of  such  cells  at  once,  and  to  keep  a  stock  of  them  ready  prepared  for 
use.  If  the  surface  of  any  ring  should  not  be  sufficiently  level  for  a 
covering  glass  to  lie  flat  upon  it,  a  slight  rubbing  upon  a  piece  of 
fine  emery  paper  laid  upon  a  flat  table  (the  ring  being  held  down- 
wards) will  make  it  so. 

Ring-cells. — For  mounting  objects  of  greater  thickness  it  is 
desirable  to  use  cells  made  by  cementing  rings,  either  of  glass  or  metal, 
to  the  glass  slides,  with  marine  glue.  Glass  rings  of  any  size,  dia- 
meter, thickness,  and  breadth  are  made  by  cutting  transverse  sections 
of  thick-walled  tubes,  the  surfaces  of  these  sections  being  ground 
flat  and  parallel.  Not  only  may  round  cells  (fig.  375,  A,  B)  of  vari- 


MOUNTING  IN   CELLS 


447 


ous  sizes  be  made  by  this  simple  method,  but,  by  flattening  the  tube 
(when  hot)  from  which  they  are  cut,  the  sections  may  be  made  qua- 
drangular, or  square,  or  oblong  (C,  D).  For  intermediate  thicknesses 
between  cement-cells  and  glass  ring-cells,  the  Editor  has  found  no 
kind  more  convenient  than  the  rings  stamped  out  of  tin,  of  various 
thicknesses.  These,  after  being  cemented  to  the  slides,  should  have 
their  surfaces  made  perfectly  flat  by  rubbing  on  a  piece  of  fine  grit 
or  a  corundum-file,  and  then  smoothed  on  a  Water-of-Ayr  stone; 
to  such  surfaces  the  glass  covers  will  be  found  to  adhere  with  great 
tenacity.  The  ebonite  and  bone  cells  are  cheap,  and  also  easy  of 
manipulation.  They  are  specially  useful  for  dry  mounts. 

The  glass  slides  and  cells  whicij  are  to  be  attached  to  each  other 
must  first  be  heated  on  the  mountiifg  plate  ;  and  some  small  cuttings 
of  marine  glue  are  then  to 
be  placed  either  upon  that 
surface  of  the  cell  which  is 
to  bo  attached,  or  upon  A 
that  portion  of  the  slide 
on  which  it  is  to  lie,  the 
former  being  perhaps  pre- 
ferable. When  they  begin 
to  melt,  they  may  be  B 
worked  over  the  surface  of 
attachment  by  means  of  a 
needle  point ;  and  in  this 
manner  the  melted  glue 
may  be  uniformly  spread,  c 
care  being  taken  to  pick 
out  any  of  the  small  gritty 
particles  which  this  cement 
sometimes  contains.  When 
the  surface  of  attachment 
is  thus  completely  covered  D 
with  liquefied  glue,  the  cell 
is  to  be  taken  up  with  a 
pair  of  forceps,  turned 
over,  and  deposited  in  its 
proper  place  on  the  slide ;  and  it  is  then  to  be  firmly  pressed  down 
with  a  stick  (such  as  the  handle  of  the  needle),  or  with  a  piece  of 
flat  wrood,  so  as  to  squeeze  out  any  superfluous  glue  from  beneath. 
If  any  air-bubbles  should  be  seen  between  the  cell  and  the  slide, 
these  should  if  possible  be  got  rid  of  by  pressure,  or  by  slightly 
moving  the  cell  from  side  to  side ;  but  if  their  presence  results,  as  is 
sometimes  the  case,  from  deficiency  of  cement  at  that  point,  the  cell 
must  be  lifted  off  again,  and  more  glue  applied  at  the  required 
spot.  Sometimes,  in  spite  of  care,  the  glue  becomes  hardened  and 
blackened  by  overheating  ;  and  as  it  will  not  then  stick  well  to 
the  glass,  it  is  preferable  not  to  attempt  to  proceed,  but  to  lift  off 
the  cell  from  the  slide,  to  let  it  cool,  scrape  off  the  overheated  glue, 
and  then  repeat  the  process.  When  the  cementing  has  been  satis- 
factorily accomplished,  the  slides  should  be  allowed  to  cool  gradually 


FIG.  375. — Glass  ring-cells. 


448     PKEPAKATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 


in  order  to  secure  the  firm  adhesion  of  the  glue  ;  and  this  is  readily 
accomplished,  in  the  first  instance,  by  pushing  each,  as  it  is  finished, 
towards  one  of  the  extremities  of  the  plate.  If  two  plates  are  in 
use,  the  heated  plate  may  then  be  readily  moved  away  upon  the  ring 
which  supports  it,  the  other  being  brought  down  in  its  place  ;  and  as 
the  heated  plate  will  be  some  little  time  in  cooling,  the  firm  attach- 
ment of  the  cells  will  be  secured.  If,  on  the  other  hand,  there  be 
only  a  single  plate,  and  the  operator  desire  to  proceed  at  once  in 
mounting  more  cells,  the  slides  already  completed  should  be  carefully 
removed  from  it,  and  laid  upon  a  wooden  surface,  the  slow  conduc- 
tion of  which  will  prevent  them  from  cooling  too  fast.  Before  they 
are  quite  cold,  the  superfluous  glue  should  be  scraped  from  the  glass 
with  a  small  chisel  or  awl,  and  the  surface  should  then  be  carefully 
cleansed  with  a  solution  of  potash,  which  may  be  rubbed  upon  it 
with  a  piece  of  rag  covering  a  stick  shaped  like  a  chisel.  The  cells 

should  next  be  washed 
with  a  hard  brush  and 
soap  and  water,  and 
may  be  finally  cleansed 
by  rubbing  with  a  little 
weak  spirit  and  a  soft 
cloth.  In  cases  in  which 
appearance  is  not  of 
much  consequence,  and 
especially  in  those  in 
which  the  cell  is  to  be 
used  for  mounting  large 
opaque  objects,  it  is  de- 
cidedly preferable  not 
to  scrape  off  the  glue 
too  closely  round  the 
edges  of  attachment,  as 
the  '  hold '  is  much 
firmer,  and  the  proba- 
bility of  the  penetra- 
tion of  air  or  fluid  much 

less,  if  the  immediate  margin  of  glue  be  left  both  outside  and 
inside  the  cell.  To  those  to  whom  time  is  of  value,  it  is  recom- 
mended that  all  cells  which  require  marine  glue  cementing  be 
purchased  from  the  dealers  in  microscopic  apparatus,  and  it  is 
well  to  note  that  all  cells  cemented  with  marine  glue  should  be 
well  '  payed,'  as  the  nautical  expression  is,  or  well  surrounded 
with  shellac  varnish  or  gold-size  as  indicated  by  the  nature  of 
the  enclosed  fluid.  Many  media,  saline  fluids  especially,  work  their 
way  between  the  cell  and  the  slide,  and  at  length  destroy  the  marine 
glue. 

Plate-glass  Cells,— Where  large  shallow  cells  with  flat  bottoms  are 
required  (as  for  mounting  zoophytes,  small  medusce,  &c.),  they  may  be 
made  by  drilling  holes  in  pieces  of  plate-glass  of  various  sizes, 
shapes,  and  thicknesses  (fig.  376,  A),  which  are  then  cemented 
to  the  slide  with  marine  glue.  By  drilling  two  holes  at  a 


D 


FIG.  376.— Plate-glass  cells. 


MOUNTING    CELLS 


449 


suitable  distance,  and  cutting  out  the  piece  between  them,  any 
required  elongation  of  the  cavity  may  be  obtained  (B,  C,  D). 

Sunk-cells. — This  name  is  given  to  round  or  oval  hollows,  exca- 
vated by  grinding  in  the  substance  of  glass  slides,  which  for  this 

purpose      should       be 

thicker  than  ordinary. 
They  are  shown  in  fig. 
377,  A,  B,  C.  Such  A 
cells  have  the  advan- 
tage not  only  of  com- 
parative cheapness,  but 
also  of  durability,  as 
they  are  not  liable  to 
injury  by  a  sudden  jar, 
such  as  sometimes 
causes  the  detachment 
of  a  cemented  plate  or 
ring.  For  objects  whose 
shape  adapts  them  to 
the  form  and  depth  of 
the  cavity,  such  cells 
will  be  found  very  con- 
venient. It  naturally 
suggests  itself  as  an 
objection  to  the  use  of 
such  cells  that  the  con- 
cavity of  their  bottom 
must  so  deflect  the 

light-rays  as  to  distort  or  obscure  the  image  ;  but  as  the  cavity  is 
filled  either  with  water  or  some  other  liquid  of  higher  refractive 
power,  the  deflection  is  so  slight  as  to  be  practically  inoperative. 
Before  mounting  objects  in  such  cells  the  microscopist  should  see 
that  their  concave  surfaces  are  free  from  scratches  or  roughnesses. 

Built-up  Cells. — When  cells  are  required  of  forms  or  dimensions 
not  otherwise  procurable, 
they  may  be  built  xp  of 
separate  pieces  of  glass 
cemented  together.  Large  A 
shallow  cells,  suitable  for 
mounting  zoophytes  or 
similar  flat  objects,  may  be 
easily  constructed  after 
the  following  method  :  A 
piece  of  plate-glass,  of  a 
thickness  that  shall  give 
the  desired  depth  to  the 

cell,    is  to  be    cut  to    the  FIG.  378.— Built-up  cells, 

dimensions  of   its   outside 

wall  ;  and  a  strip  is  then  to  be  cut  oft'  with  the  diamond  from 
each  of  its  edges,  of  such  breadth  as  shall  leave  the  interior  piece 
equal  in  its  dimensions  to  the  cavity  of  the  cell  that  is  desired. 

G  G 


FIG.  377.— Plate-glass  sunk-cells. 


450    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

This  piece  being  rejected,  the  four  strips  are  then  to  be  cemented 
upon  the  glass  slide  in  their  original  position,  so  that  the  diamond-cuts 
shall  fit  together  with  the  most  exact  precision;  and  the  upper 
surface  is  then  to  be  ground  flat  with  emery  upon  a  pewter  plate 
and  left  rough.  The  perfect  construction  of  large  deep  cells  of  this 
kind,  as  shown  in  fig.  378,  A,  B,  however,  requires  a  nicety  of  work- 
manship which  few  amateurs  possess,  and  the  expenditure  of  more 
time  than  microscopists  generally  have  to  spare  ;  and  as  it  is  conse- 
quently preferable  to  obtain  them  ready-made,  directions  for  making 
them  need  not  be  here  given. 

Wooden  Slides  for  Opaque  Objects. — Such  'dry'  objects  as/bra- 
minifera,  the  capsules  of  mosses,  parts  of  insects,  and  the  like,  may 
be  conveniently  mounted  in  a  very  simple  form  of  wooden  slide  (first 
devised  by  the  Author  and  now  come  into  general  use),  which  also 
serves  as  a  protective  '  cell.'  Let  a  number  of  slips  of  mahogany  or 
cedar  be  provided,  each  of  the  3-inch  by  1-inch  size,  and  of  any 
thickness  that  may  be  found  convenient,  with  a  corresponding 
number  of  slips  of  card  of  the  same  dimensions,  and  of  pieces  of 
dead-\)\Sick  paper  rather,  larger  than  the  aperture  of  the  slide.  A 
piece  cf  this  paper  being  gummed  to  the  middle  of  the  card,  and 
some  stiff  gum  having  been  previously  spread  over  one  side  of  the 
wooden  slide  (care  being  taken  that  there  is  no  superfluity  of  it 
immediately  around  the  aperture),  fchis  is  to  be  laid  down  upon  the 
card,  and  subjected  to  pressure.1  An  extremely  neat '  cell'  will  thus 
be  formed  for  the  reception  of  the  object,  as  we  see  in  fig.  37i>.  the 

depth   of  which    will  be  deter- 

£^g^^~                  \        mined    by  the  thickness  of  the 
^MJHItl                    \       slide,  and  the  diameter  by  the 
^tijfJr \       size  of  the  perforation  ;  and   it 
•  =3      will    be    found    convenient    to 

FIG.  379.— Slip  made  of  wood.  provide  slides  of  various  thick- 

nesses, with  apertures  of  diffe 

rent  sizes.  The  cell  should  always  be  deep  enough  for  its  wall  to 
rise  above  the  object ;  but,  on  the  other  hand,  it  should  not  be  too 
deep  for  its  walls  to  interfere  with  the  oblique  incidence  of  the  light 
upon  any  object  that  may  be  near  its  periphery.  The  object,  if  fiat 
or  small,  may  be  attached  by  gum-mucilage  ;  if,  however,  it  be  large, 
and  the  part  of  it  to  be  attached  have  an  irregular  surface,  it  is 
desirable  to  form  a  '  bed '  to  this  by  gum  thickened  with  starch.  If, 
on  the  other  hand,  it  should  be  desired  to  mount  the  object  edgeways 
(as  when  the  mouth  of  nforaminifer  is  to  be  brought  into  view),  the 
side  of  the  object  may  be  attached  with  a  little  giun  to  the  wall  of 
the  cell.  The  complete  protection  thus  given  to  the  object  is  the 
great  recommendation  of  this  method.  But  this  is  by  no  means 
its  only  convenience.  It  allows  the  slides  not  only  to  range  in 
the  ordinary  cabinets,  but  also  to  be  laid  one  against  or  over 
another,  and  to  be  packed  closely  in  cases,  or  secured  by  elastic 
1  It  will  be  found  a  very  convenient  plan  to  prepare  a  large  number  of  such  slides 


at  once,  and  this  may  be  done  in  a  marvellously  short  time  if  the  slips  of  card  have 
been  previously  cut  to  the  exact  size  in  a  bookbinder's  press.  The  slides,  when  put 
together,  should  be  placed  in  pairs,  back  to  back,  and  every  pair  should  have  each 


TUR  N  -TABLE  S— FINISHING 


451 


bands;  which  plan  is  extremely  convenient  not  merely  for  the 
sa\-ing  of  space.  1  nit  also  for  preserving  the  objects  from  dust.  Should 
any  more  special  protection  be  required,  a  thin  glass  cover  may  be 
laid  over  the  top  of  the  cell,  and  secured  there  either  by  a  rim  of 
gum  or  by  a  perforated  paper  cover  attached  to  the  slide ;  and  if 
it  should  be  desired  to  pack  these  covered  slides  together,  it  is  only 
necessary  to  interpose  ytiards  of  card  somewhat  thicker  than  the 
glass  covers. 

Turn-table. — This  simple  instrument  (fig.  380),  devised  by 
Mr.  Shadbolt,  is  almost  indispensable  to  the  microscopist  who  desires 
to  preserve  prepara- 
tions that  are 
mounted  in  any 
'  medium  '  beneath 
circular  covers  ;  since 
it  not  only  serves 
for  the  making  of 
those  '  cemeilt-cells  '  FIG.  380. — Shadbolt's  turn-table, 
ill  which  thin  trans- 
parent objects  can  be  best  mounted  in  any  kind  of  *  medium,'  but 
also  enables  him  to  apply  his  varnish  for  the  securing  of  circular 
cover-glasses  not  only  with  greater  neatness  and  quickness,  but  also 
with  greater  certainty  than  he  can  by  the  hand  alone.  The  only 
special  precaution  to  be  observed  in  the  use  of  this  instrument  is 
that  the  cover-glass,  not  the  slide,  should  be  '  centred ; '  which  can 
be  readily  done,  if  several  concentric  circles  have  been  turned  on  the 
rotating-table,  by  making  the  cover-glass  correspond  with  the  one 
having  its  own  diameter.  A  num- 
ber of  ingenious  modifications  have 
been  devised  in  this  simple  instru- 
ment with  a  view  to  securing  exact 
centring.  The  most  practicable 
and  inexpensive  of  these  is  an 
application  of  Mr.  E.  H.  Griffith's 
device  shown  in  its  improved  form 
in  fig.  381. 

The  centre  of  the  table  marked 
with  circles  has  a  straight  spring 
attached  to  it  beneath.  The  slide, 
being  placed  between  the  two  pins 
A  and  B  in  this  centre,  is  partially 
rotated  against  the  spring  and 
pushed  forward,  when  the  spring 
keeps  it  between  the  two  pins  and  a  third  fixed  pin,  D,  at  the  upper 
side  of  the  slide,  centring  it  perfectly  for  width.  The  fourth  pin, 
E,  at  the  left  end,  1^  in.  from  the  centre,  is  for  length,  and  allows 
the  slide  to  be  always  placed  in  the  same  relative  position.  The 
recent  improvements  add  much  to  the  value  of  the  table.  One  of 
them  is  a  countersunk  decent-ring  wheel  and  pin,  C,  which  may  be 
seen  at  the  upper  right-hand  side  of  the  slide.  The  axle  of  the 
wheel  passes  through  the  table  and  is  furnished  underneath  with  a 

G  G  2 


FIG.  381.— Griffith's  turn-table. 


452     PREPARATION,   MOUNTING,    AND    COLLECTION   OF   OBJECTS 

short  bar  with  which  the  decentring  wheel  may  be  turned,  forcing 
the  pin  against  the  slide,  pushing  it  as  far  out  of  centre  as  may  be 
desired.  Another  improvement  is  in  making  the  end-pin  a  screw, 
which  may  be  turned  down  out  of  the  way  if  desired. 

Mounting  Plate  and  Water-Bath. — Whenever  heat  has  to  be 
applied  either  in  the  cementing  of  cells  or  in  the  mounting  of 
objects,  it  is  desirable  that  the  slide  should  not  be  exposed  direct  to 
the  flame,  but  that  it  should  be  laid  upon  a  surface  of  regulated 
temperature.  As  cementing  with  marine  glue  or  hardened  Canada 
balsam  requires  a  heat  above  that  of  boiling  water,  it  must  be 


FIG.  382. — Apparatus  for  preparing  mounting  media,  paraffin,  &c.,  for  imbedding 

by  heat. 

supplied  by  a  plate  of  metal ;  and  the  Author's  experience  leads  him 
to  recommend  that  this  should  be  a  piece  of  iron  not  less  than  six 
inches  square  and  half  an  inch  thick,  and  that  it  should  be 
supported,  not  on  legs  of  its  own,  but  on  the  ring  of  a  retort-stand, 
so  that  by  raising  or  lowering^ the  ring  any  desired  amount  of  heat 
may  be  imparted  to  it  by  tne  lamp  or  gas-flame  beneath.  The 
advantage  of  a  plate  of  this  size  and  thickness  consists  in  the 
(jradational  temperature  which  its  different  parts  afford,  and  in  the 
slowness  of  its  cooling  when  removed  from  the  lamp.  When  many 
cells  are  being  cemented  at  once,  it  is  convenient  to  have  two  such 
plates,  that  one  may  be  cooling  while  the  other,  is  being  heated. 


WATEK-BATH— SPRING-PRESSES  45  3 

It  is  also  needful  to  Have  a  smaller  plate,  much  thinner,  of  brass, 
having  a  groove  cut  in  it  into  which  the  ordinary  3x1  in.  mounting 
slip  can  easily  slide,  but  so  grooved  as  to  leave  a  space  between  a 
ledge  on  each  side  on  which  the  slip  rests,  and  the  main  surface  of 
the  brass  under  the  slip.  In  this  way  there  is  always  a  film  of 
heated  air  between  the  main  surface  of  the  heated  brass  and  that  of 
the  glass,  giving  more  facility  for  rapid  and  delicate  heating.  This 
may  be  either  a  separate  '  table '  or  a  plate  fitted  to  a  retort-stand. 

Beyond  this,  however,  heat  of  various  kinds,  dry  and  moist,  of 
variable  but  determinate  temperatures,  w^ill  be  required  for  various 
purposes,  especially  for  melting  the  various  mounting  media,  such 
as  gelatin,  agar-agar,  &c.,  and  also,  as  we  shall  shortly  see,  for  the 
preparation  of  imbedding  masses>for  section  cutting  and  a  variety 
of  other  purposes.  One  of  the  many  pieces  of  apparatus  which 
have  been  devised  to  combine  as  large  a  number  of  the  requirements 
of  the  mounter  in  one  construction  as  can  be  conveniently  clone  was 
devised  by  Dr.  P.  Mayer  and  his  colleagues.  It  is  illustrated  in 
fig.  382.  * 

W  is  the  bath ;  Z  the  tube  by  which  it  is  filled  with  water;  1, 
2,  3,  4  are  glass  tubes ;  a  is  a  pot  for  melting  and  clarifying  the 
paraffin,  and  this  may  be  replaced  by  others  for  other  needful 
purposes  ;  b  and  c  are  half-cylinders  with  handles  for  imbedding  ;  t 
is  a  thermometer  bent  at  a  right  angle  ;  the  horizontal  leg  ends  in  the 
air-bath,  and  can  be  closed  with  a  glass  plate,  which  is  of  service  for 
biological  as  well  as  mounting  purposes.  The  temperature  in  the 
air-bath  will  be  always  about  10°  less  than  that  in  the  water-bath.  It 
serves  wrell  for  evaporating  chloroform,  etc. ;  ^  is  the  thermometer  for 
the  water-bath  ;  K,  is  a  Reichert's  thermo-regulator.  The  variation 
in  temperature  is  less  than  1°  C. ;  r  is  the  tube  in  wThich  the  gas 
and  air  mix,  and  f  a  mica  chimney.  There  is  a  small  independent 
and  removable  water-bath,  r,  filled  with  water  by  means  of  rubber 
tubes  attached  to  lateral  openings.  It  is  supplied  with  a  thermo- 
meter, £2,  is  warmed  on  the  platform,  F,  and  is  intended  chiefly  for 
fixing  objects  which  are  small  in  the  right  position  in  the  imbedding 
mass,  usually  known  as  '  orienting'  objects,  under  a  simple  lens  or 
dissecting  microscope. 

Slide-forceps,  Spring-clip,  and  Spring-press.  —  For  holding 
slides  to  which  heat  is  being  applied,  especially  while  cementing 
objects  to  be  ground  down  into  thin  sections,  the  wooden  slide- 
forceps,  seen  in  fig.  383,  will  be  found  extremely  convenient.  This, 
by  its  elasticity,  affords  a  secure  grasp  to  a  slide  of  any  ordinary 
thickness,  the  wooden  blades  being  separated  by  pressure  upon  the 
1  >i-ass  studs  ;  while  the  lower  stud,  with  the  bent  piece  of  brass  at 
the  junction  of  the  blades,  affords  a  level  support  to  the  forceps, 
which  thus,  while  resting  upon  the  table,  keeps  the  heated  glass  from 
contact  with  its  surface.  For  holding  down  cover-glasses  whilst 
the  balsam  or  other  medium  is  cooling,  if  the  elasticity  of  the  object 
should  tend  to  make  them  spring  up,  the  wire  spring-clip  (fig.  384), 
sold  at  a  cheap  rate  by  dealers  in  microscopic  apparatus,  will  be 
found  extremely  convenient.  Or  if  a  stronger  pressure  be  required, 
recourse  may  be  had  to  a  simple  spring-press  made  by  a  slight 


454    PREPARATION,   MOUNTING,   AND   COLLECTION   OF  OBJECTS 

alteration  of  the  '  American  clothes-peg,'  which  is  now  in  general 
use  in  this  country  for  a  variety  of  purposes,  all  that  is  necessary 
being  to  rub  down  the  opposed  surfaces  of  the  '  clip '  with  a  flat  file, 
so  that  they  shall  be  parallel  to  each  other  when  an  ordinary  slide 
with  its  cover  is  interposed  between  them  (fig.  385).  One  of  those 


FIG.  383.— Slide-forceps. 

convenient  little  implements  may  also  be  easily  made  to  serve  the 
purpose  of  a  slide-forceps  by  cutting  back  the  upper  edge  of  the 
clip,  and  filing  the  lower  to  such  a  plane  that  when  it  rests  on  its 
flat  side  it  shall  hold  the  slide  parallel  to  the  surface  of  the  table,  as 
in  fig.  383. 


FIG.  384.— Spring-clip. 


FIG.  885. — Spring-press. 


Mounting  Instrument. — A  simple  mode  of  applying  graduated 
pressure  concurrently  with  the  heat  of  a  lamp,  which  will  be  found 
very  convenient  in  the  mounting  of  certain  classes  of  objects,  is 
afforded  by  the  mounting  instrument  devised  by  Mr.  James  Smith. 
This  consists  of  a  plate  of  brass  turned  up  at  its  edges,  of  the  proper- 
size  to  allow  the  ordinary  glass  slide  to  lie  loosely  in  the  bed  thus 
formed ;  this  plate  has  a  large  perforation  in  its  centre,  in  order  to 
allow  heat  to  be  directly  applied  to  the  slide  from  beneath  ;  and  it 


FIG.  386. — Smith's  mounting  instrument. 

is  attached  by  a  stout  wire  to  a  handle  shown  in  fig.  386.  Close  to 
this  handle  there  is  attached  by  a  joint  an  upper  wire,  which  lies 
nearly  parallel  to  the  first,  but  makes  a  downward  turn  just  above 
the  centre  of  the  slide-plate,  and  is  terminated  by  an  ivory  knob ; 
this  wire  is  pressed  upwards  by  a  spring  beneath  it.  whilst,  on  the 
other  hand,  it  is  made  to  approximate  the  lower  by  a  milled  head 
turning  on  a  screw,  so  as  to  bring  its  ivory  knob  to  bear  with  greater 
or  less  force  on  the  covering-glass.  The  special  use  of  this  arrange- 
ment will  be  explained  hereafter. 


ARRANGEMENTS    FOE    DISSECTING 


455 


Dissecting  Apparatus.— The  mode  of  making  a  dissection  for 
microscopic  purposes  must  be  determined  by  the  size  and  character 
of  the  object.  *  Generally  speaking,  it  will  be  found  advantageous  to 
carry  on  the  dissection  under  water,  with  which  alcohol  should  be 
mingled  where  the  substance  has  been  long  immersed  in  spirit.  The 
size  and  depth  of  the  vessel  should  be  proportioned  to  the  dimensions 
of  the  object  to  be  dissected  ;  since,  for  the  ready  access  of  the  hands 
and  dissecting  instruments,  it  is  convenient  that  the  object  should 


FIG.  387. — Swift's  Stephenson  binocular  dissecting  microscope. 

neither  be  far  from  its  walls  nor  lie  under  any  great  depth  of  water. 
Where  there  is  no  occasion  that  the  bottom  of  the  vessel  should  be 
transparent,  no  kind  of  dissecting  trough  is  more  convenient  than 
that  which  every  one  may  readily  make  for  himself,  of  any  dimen- 
sion he  may  desire,  by  taking  a  piece  of  sheet  gutta-percha  of  adequate 
size  and  stoutness,  warming  it  sufficiently  to  render  it  flexible,  and 
then  turning  up  its  four  sides,  drawing  out  one  corner  into  a  sort  of 
spout,  which  serves  to  pour  away  its  contents  when  it  needs  empty- 
ing. The  dark  colour  of  this  substance  enables  it  to  furnish  a  back- 


i     I'KKI'AKATION.    MOI'NTINO,    AM>   COLLECTION    OF   OBJECTS 

Around,  \\hivh  a>siMs  the  ohsrrxer  in  distinguishing  delicate  mew 

hianos.  tihivs.  fa  .  ,>sp,viall\    \\hon  niapnt'x  ini;  lenses  are  employed  ; 
and  it  is  hard  enough  ^  \\ithont  hoin^  too  hard)  t.oallou  of  pins  Ivinu 
1  into  it.  hoth  tor  soenrini;  tho  ohjeet  ami  for  keeping  apart  such 
portions  as  it   is  useful  to  put  on  the  stretch.      NY  hen  j^lass  or  earthen 

ware  troughs  are  employed,  :\  piece  of  sinvt  cork  loaded  \\ithlead 

must  l»e  prox  ided  to  answer  tho  same  purposes.  In  rarrvinu  on 
.lis^j'i-tiiMiN  in  Mirh  a  tron^li.  it  is  tVt»(|iuMtll\  <K>>i\al>K>  to  i-t»mvut  i  atr 
additional  \\g\\\  UJHM\  tlu>  part  \\hu-h  is  IUMIIU  oporatod  on  l>\  nu\in> 
.>!  t  lio  sinalltT  i'«>n«K%nsiMi;-  UMIS  ;  and  \\lu»n  a  U>\v  uiMii'nit'vinc  |o\\ri 
JN  \\antrd  it  \na\  l>c  Mippliod  t»itlu«r  1>\  a  siu^U-  K-us.  nunintod  at't<>r 
the  IDAIUier  of  R068*8  sinipir  inirio^ropc.  or  1»\  a  j  air  of  s 
inonnttHl  \\  ith  tln>  •srnulr:  linarilv  n>od  for  st»-i  r 

of  thebodl  nndor  disst'i-tiiui.  IUMUJJ  tl(vt!««d  otV  uln-n  dctarhod, 
uu»\  IH>  ron\onu«ntl\  icikon  up  t'roui  tlu>  trough  1»\  plarinu  a  >lij»  ot' 
^la^-  InMioath  tlu-iu  ^  \vlurh  is  ot'itMi  lluMmlv  modo  in  \\hirh  d«>livato 
Ukembranee  ran  lv  >atistactoril\  spread  out),  and  inav  lv  thon  plarod 
un»l»M-  tlu4  mu-nvM-op<>  tor  ininuti-  o\aniinat  ioi\.  U»int:  lifst  co\  <M  od 
\\ith  thin  j^lnss.  l>onoath  tho  tMluos  *>('  \\hu-h  is  to  l>o  intiodiu-^l  a 
littlo  of  tlu»  li»|uid  \\luMvin  th<>  vlissort  ion  i>  IHMJII;  rarriod  on. 
\\'htM't>  tlu>  body  mitlor  d  to  tiansparont  that  nioif 

•Ua^o  is  ^ainod  hy  t  ransiuit  t  im;  li^ht  through  it  than  l»\   looking 
at  it  M  Uk  OJM^QA  ol^jeot,  tlu>   trough    sliould    l\a\t>    a    ul:»^    hot  torn: 
and  for  this  purpus<\  unlos*  tho  l>od>   h«>  ,»f  unusual  Ed  «'.  s(>mo  of  llu> 
gl»8S  oeUfi  iJreaidQ    drsml>t>d  ^ti^s.  :>7^    ."77)  \\ill  usuall\   ans\\cr  \ 
\\oll.      Tho  tiniest  dissections  may  oft  on   ho  host   mado  npiMi  ordinary 

stips  of  glass,  o*re  being  takm  to  k«vp  tlu«  i^hjtH-t  MitUi-iontly  su»- 

nnuidod  hy  thii»l.      For    \\i»rk    of  this    kind    no    instrument    is  moro 
v  uvahlo   than   tht>    onvtin^'    hiiuvular   form  of  stand  as 
itly  nuHlit'uHl  for  «U»ivtin%c  pin;  S  It  is  an  instrn 

niont  \vhioh  OOmbineS  OOnvenienoefl  and  supplies  \\auts  \\hii-h  oi\ly 
rk«M  at  dixstvrion  ouiUl  havo  kno\\n.  It  is  illust  rated  in  tiu- 
and  \\ill  Iv  thoroughly  siiitahlo  for  all  thr  \\ork  in  \vliioh  it  \\ill 

ho  r»H|niriHU  fnmi  diatom  mount  in^  to  ;lu>  most    vlelirato  disMVtions. 

Tho  suppoits  for  tlio  haiuis  on  either  sido  of  tin*    siago   ha\o   ai 

tivmolv  suitiihlo  i%urv»».  and  tho  instriuuont  K'luU  il>olf  admii-ahly  tv^ 

tho  ^ 

The  tMtffrtfiMttt/jt  usod  in  mionvsti>p-,  the  most 

part  of  tho  SMIUO  kind  as  thuso  \\hioli  aro  mvdod  in  ordinai-y  minnto 

anatomioal  research)  such  as  soalpels^  .  tho  tim> 

instruments  usod  in  operations  UJH^U  thoox  r.  ho\\o\  <M  .  \\ill  ixnumonly 
U»  t\nuu)  nuvst  suitablo.      A    (lair  of  ilolit  ,',ii\t>d  to 

moly     i-vn\voniont    for    out  tins;    opon     tubular     ports  ; 
tl»oso  should   bav»»    tboir  points  hlnntod,   but    oil  t8  should 

lia\v  tino  {nnnts.      A   pair  of  \or\    tino  {H»intod   scissors  (fi^ 
»»nt^  U\ii  of  which  n  a   Hirlit    haiullc.  and   the    other   kept 


;»\  U^  rvHvnxiuonded  »s  useful  in  «  gn%t  viuriely  <rf  «»«>••.  •  \\iv\\. 

«w  boat  ivrforint^  uiult»r  A  low  ma£mfyin£  |H>>wr»  w»U\  lh»  conjoin: 
\VluMv   :t  lu^h  )w>w«r  is  n««cl«d»  recount  n-.  •>    hail   to  M 

li  s\ohivnu  ,  v,   \\hioh 

IM-iuviplo.  «Uo\\  i»\jr  tho  object  to  be  brought  v^ry  nww  the^yes,  without  r«\uuu-. 
Ukcomfortob)«  COATWCWD  AXM. 


ANALYSIS     (>!•      MoTNTINfi      M  KTIH  >!>S  457 

:i|i:irt  IVnin  it  hy  :i  spring.  so  MS  In  close  |,\  the  |  .ressn  n-  (,fl|n. 
linger  ;ilid  to  open  of  itself,  \\ill  In-  found  (if  j|H.  M:M|e.-  )„•  \\ell 
sharpened)  much  superior  to  :\\\y  kind  of  kni\es  for  cutting  throu:_:h 

delicate  tissues  with  MS  little  disturlunce  of  tii<>m  MS  possihle.     A 

I'.-iir  ol'  sni.-ill  slr.-ii-lit  forceps  \\itli  line  points.  Mini  .-mother  pnirof 
curved  forceps.  will  !H>  found  useful  in  addition  to  tin-  oi-ilin.-n  v 
dissect  inu  forceps. 

(  )f  :ill  1  lie  i  list  rn  nielits  ci  u  it  ri  \  e<|  for  <  lei  ic:it  e  d  issect  ioi  is,  lio\\  i  '\  er. 
fe\\  :il'e  more  sel'\  ice;i  I  »le  tlliili  tliox-  \\liicli  tin-  In  icl'oscopist  III.MV 
in.-ike  for  himself  out  of  ordin.-irv  tti-<-(//cx.  These  should  he  lixed  in 
liirllt  \\  .....  lell  h;illdles  (the 
cednr  sticks  used  for 
c.-iliiel  h:iir  pencils,  or  t  he 
h.-indles  of  steel  pen 
holders,  or  siniill  porcii  l-'n..  :IHH.—  Spi-ii. 

pine    (piills   will    answer 

extremely  \\ell)  in  such  ;i  in:iimer  that  their  points  slionld  not 
project  i:ir.  since  they  will  other\\i>e  h;i\e  too  lunch  'spring;' 
much  IIKIV  l)edoiieli\  t  hei  r  mere  fi-nriiiif  ;ict  ion  :  hut  if  it  be  desired 
to  use  them  '.\srnffinif  inst  rnineiit  s.  .-ill  tlint  is  neces>;i  ry  is  to  Imrden 
:ilid  temper  them.  :illd  t  hen  •;  'i  \  e  them  ;in  ed^e  Ujioil  .M  liolie.  It  U  i  1  1 
solliet  ilnes  he  desir.'lhle  to  ^i\e  ;i  liner  |ioint  to  such  needles  thiili 
they  oi-i^iiuilly  possess;  this  ;dso  ni:iy  he  done  Upon  ;i  holie.  A 
needle  \\ith  its  point  In-lit,  ton  ri^ht  .-in^le.  or  m-.-irly  so.  is  often  use 
fill  ;  :nnl  this  iimy  he  shnped  hy  simply  he;itini:  the  point  in  M  l:imp 
or  c:indle.  ^hini;  to  it  the  recpiired  turn  with  :i  p:iir  <>f  pliers.  :m«l 
then  h:n  deiiin--  the  point  ;i^:iin  hy  re  he.-itin^  it  :ind  |ilinii:iMir  it  into 

cold    \\;i!er  or  t;illo\\  . 

Analysis  of    Methods  of   Preparation  and  Mounting   which 
follow  . 

1.  Descriptions    of     microtomo.    ;md     knife-holdere    :«iul     /////'' 

position. 

2.  Mounting  objects  in  i;-eiier:d. 

.'5.    I  'rep.-i  ration   of  wj't   tissues,  under  the    following  Subtitles  : 

Fixation, 

I  )ehdr:it  ion. 


Staining, 
This  lust  is  further  subdivided  .-is  follo\\s: 

Sl;iili-   for   li\  ilii:'  ohjects. 

St;iins  for  fresh  tissues. 

St.-iins  for  lixed  :ind  preserved  entire  ohjects. 

Nucle.-ir  st.  -i  ins  for  >ect  ions. 

lM:ism:it  ic  st.-iin-. 

[mbed,ding  methods  under  the  following  subtitles  :— 

Imheddin^  methods  in  ^eiier;d. 
'Die  p;ir;illin  method. 

This  lust   is  further  subdivided  MS  follows: 
1.   S.-iturMtion  with  :i  solvent. 
-.   SM!  iir.-it  ion  with  p:ir.Mtlin. 


458    PKEPARATION,    MOUNTING,   AND    COLLECTION   OF   OBJECTS 

3.  Arranging  for  cutting. 

4.  Cutting. 

5.  Flattening  sections  and  mounting,  with  description  of  the 

best  serial  section  methods. 

The  celloidin  method,  further  subdivided  as  follows  : — 
Celloidin  imbedding  in  general. 
Hardening  the  mass. 
Fixing  to  microtome  and  cutting. 
Staining  and  mounting,  with  description  of  (i]>/>i-o/,,-i<it>',  wild 

section  methods. 

4.  Preparation  of  hard  tissues,  under  the  following  titles  : -  — 
Grinding  and  polishing  sections,  with  descriptions  of  lathes. 
Decalcificatioii. 

Desilioification. 

5.  Sections  dealing  with 

(a)  Vegetable  tissues. 

(b)  Staining  bacteria. 

(c)  Staining  flagella. 

(d)  Chemical  testing. 

(e)  Preservative  media. 

(f)  Cleanliness,  and  labelling. 

Microtomes  are  machines  devised  for  the  purpose  of  obtaining 
extremely  thin  and  uniform  slices,  or  'sections'  as  they  arc 
technically  called,  of  animal  or  vegetable  tissues,  hard  or  soft, 
Some  of  the  purposes  to  which  these  are  adapted  will  be  found 
to  be  answered  by  a  very  simple  and  inexpensive  little  instrument, 
which  may  either  be  held  in  the  hand,  or  (as  is  preferable)  may  In- 
firmly attached  by  means  of  a  "T-shaped  piece  of  wood  (fig.  381))  to 
the  end  of  a  table  or  work-bench,  or  may  be  provided  with  a  clamp 
for  firm  attachment  to  the  work-table,  as  in  fig.  390.  This  instru- 
ment essentially  consists  of  an  upright  hollow  cylinder  of  brass, 
with  a  kind  of  piston  which  is  pushed  from  belowT  upwards  by  a  fine- 
threaded  or  '  micrometer '  screw  turned  by  a  large  milled  head  ;  at 
the  upper  end  the  cylinder  terminates  in  a  brass  table,  which  is 
planed  to  a  flat  surface,  or  (which  is  preferable)  has  a  piece  of  plate- 
glass  cemented  to  it,  to  form  its  cutting  bed.  At  one  side  is  seen  a 
small  milled  head,  wrhich  acts  upon  a  '-binding  screw",'  whose  ex- 
tremity projects  into  the  cavity  of  the  cylinder,  and  serves  to  com- 
press and  steady  anything  that  it  holds.  For  this  is  now  generally 
substituted  a  pair  of  screws,  working  through  the  side  of  the 
cylinder,  instead  of  one  as  in  fig.  390.  A  cylindrical  stem  of  wood, 
a  piece  of  horn,  whalebone,  cartilage,  &c.,  is  to  be  fitted  to  tl it- 
interior  of  the  cylinder,  so  as  to  project  a  little  above  its  top,  and  is 
to  be  steadied  by  the  '  binding  screw  ; '  it  is  then  to  be  cut  to  a  level 
by  means  of  a  sharp  knife  or  razor  laid  flat  upon  the  table.  Tlic 
large  milled  head  is  next  to  be  moved  through  such  a  portion  of  a 
turn  as  may  very  slightly  elevate  the  substance  to  be  cut,  so  as  to 
make  it  project  in  an  almost  insensible  degree  above  the  table,  and 
this  projecting  part  is  to  be  sliced  off  with  a  knife  previously  dipped 


SECTION    CUTTERS 


459 


FIG.  389.— Simple  microtome. 


in  water  or,  preferably,  methylated  spirit  and  water  in  equal  parts. 
An  ordinary  razor  will  answer  for  cutting.  The  motion  given  to 
its  edge  should  be  a  combination  of  drawing  and  pressing.  (It  will 
be  generally  found  that 
better  sections  are  made 
by  working  the  knife  from 
the  operator  than  towards 
him.)  When  one  slice  has 
been  thus  taken  off,  it 
should  be  removed  from 
the  blade  by  dipping  it  into 
spirit  and  water,  or  by  the 
use  of  a  camel-hair  brush ; 
the  milled  head  should  be 
again  advanced,  •  and  an- 
other section  taken,  and  so 
on.  It  is  advantageous  to 
have  the  large  milled  head 
graduated,  and  furnished 
with  a  fixed  index,  so  that 
this  amount  having  been 
once  determined,  the  screw  shall  be  so  turned  as  to  always  produce 
the  exact  elevation  required.  Where  the  substance  of  which  it  is 
desired  to  obtain  sections  by  this  instrument  is  of  too  small  a  size  or 
of  too  soft  a  texture  to  be 
held  firmly  in  the  manner 
just  described,  it  may  be 
placed  between  the  two  ver- 
tical halves  of  a  piece  of  carrot 
of  suitable  size  to  be  pressed 
into  the  cylinder,  and  the 
carrot  with  the  object  it 
grasps  is  then  to  be  sliced 
in  the  manner  already  de- 
scribed, the  small  section  of 
the  latter  being  carefully 
taken  off  the  knife,  or  floated 
away  from  it,  on  each  occa- 
sion, to  prevent  it  from  being 
lost  among  the  lamellae  of 
carrot  which  are  removed  at 
the  same  time.  Vertical 
sections  of  many  leaves  may 
be  successfully  made  in  this 
way,  and  if  their  texture  be 
so  soft  as  to  be  injured  by 
the  pressure  of  the  carrot, 
they  may  be  placed  between  two  half-cylinders  of  elder-pith,  or  be 
imbedded  in  any  of  the  ways  employed  with  the  more  elaborate 
microtomes  about  to  be  described. 

The  modern  art  of  section-cutting,  as  practised   by  the   most 


FIG.  390.— Microtome. 


460    PREPARATION,   MOUNTING,    AND    COLLECTION   OF   OBJECT 

accomplished  experts,  with  the  most  complete  of  the  many  almost 
perfect  recent  microtomes,  is  one  of  the  most  refined  and  beautiful 
with  which  the  scientific  mind  can  concern  itself.  The  combined 
cutting,  staining,  and  mounting  of  the  most  delicate  organic  tissues  in 
almost  every  conceivable  state  has  thrown  a  light  upon  histological 
and  pathological  matters,  the  present  and  prospective  value  of  which 
we  can  scarcely  estimate  too  highly  ;  while  some  of  the  profoundest 
and  most  interesting  questions  of  biology  are  opening  themselves  to 
renewed  research  by  its  means. 

Throughout  this  chapter  we  only  seek  to  give  the  possessor  of  a 
good  microscope  a  fair  outline  of  the  principal  methods  employed, 
and  clues  to  the  finest  processes  in  detail,  for  histological,  patholo- 
gical, and  embryological  work.  For  full  details  we  may  refer  him 
to  the  more  or  less  exhaustive  handbooks  which  the  several  subjects 
have  called  forth,  the  fullest  account  of  the  subject'  being  that  given 
in  Mr.  A.  Bolles  Lee's  '  The  Microtomist's  Yade-Mecum.'  But  we 
are  at  the  same  time  convinced  that  if  the  student  be  but  rightly 
directed  as  to  instruments  and  the  best  way  of  employing  them,  and 
at  the  same  time  have  the  best  general  processes  concisely  indicated 
to  him,  he  will  soon  discover  what  to  him  will  be  the  most  facile  and 
satisfactory  method  of  obtaining  the  best  results.  In  the  hands  of 
an  original  worker  prescriptions  are  only  satisfactory  starting-points 
to  better  methods.  We  shall  therefore  describe  one  microtome 
which  we  believe,  on  the  whole,  to  be  the  best,  and  sufficiently 
indicate  the  character  and  peculiarities  of  two  or  three  others,  to 
enable  the  student,  as  we  believe,  to  judge  for  himself  in  considera- 
tion of  his  future  purpose  as  to  which  will  best  serve  him  in  the 
object  he  has  in  view. 

It  will  be  as  well,  however,  to  note  that  extremely  thin  sections 
are  not  the  supreme  purpose  of  microtomes.  Good  sections,  treated 
with  success  from  beginning  to  end,  are  the  first  consideration.  The 
tenuity  of  a  section  must  be  proportional  to  the  character  of  the 
tissue. 

Manifestly  a  tissue  with  injected  arteries  or  veins  must  be  thick 
enough  to  contain  some  of  these  vessels  with  their  branches  entire. 
If  we  require  to  study  the  hepatic  cells  or  the  renal  tubules  we  must 
give  depth  enough  in  the  sections  to  include  these.  But  it  will  be 
found  that  the  hardening  and  imbedding  agents  contract  greatly, 
without  distorting,  the  anatomical  elements,  and  sections  much 
thinner  than  would  be  normally  required  to  completely  disclose  what 
is  sought  may  be  often  successfully  made  in  tissues  so  prepared. 

It  is  none  the  less  true  that  a  mere  race  for  extreme  attenuation 
in  sections  is  in  every  sense  undesirable  ;  and  for  extremely  thin 
sections — say  the  -^L^th  of  an  inch  in  thickness,  or  less — only  small 
sections  should  be  attempted. 

Here  it  may  be  advisable  to  state  that  the  standard  unit  in 
microscopy,  as  accepted  by  the  Council  of  the  Royal  Microscopical 
Society,1  is  the  -j^^th  of  a  millimetre,  which  is  indicated  by  the 
sign  /;,  being  known  as  a  micron. 

1  Jowrn.  Boy.  Micro.  Soc.  ser.  ii.  vol.  vii.  pp.  502,  526  ;  Nat.  xxxviii.  p.  221. 


THE    THOMA  MICROTOME 


461 


The  choice  of  microtomes,  English,  Continental,  and  American,  is 
very  large,  and  high  merit  is  characteristic  of  many.  But  one  of 
these,  devised  by  Thoma  and  made  by  Jung  of  Heidelberg,  entered 
the  field  early,  having  from  the  first  been  based  on  thoroughly 
sound  practical  principles ;  and  as  a  result  it  has  been  susceptible 
of,  and  has  lent  itself  to,  every  improvement  suggested  by  the 
advancing  refinements  of  this  beautiful  art  of  microtomy.  In  its 
latest  form  we  describe  and  illustrate  it,  satisfied  that  it  will  in 
an  almost  perfect  manner  meet  the  general  wants  of  the  biologist's 
laboratory. 

This  (the  Thoma)  microtome  is  based  upon  the  model  of  Rivet  ;  but 
that  has  been  immensely  expanded  in  detail.  The  body  of  the 
instrument  consists  of  three  plate's,*- the  middle  plate,  M,  and  the 
side  plates,  S  and  0,  fig.  391.  These  are  fastened  to  the  bottom 
plate  by  screws.  S  supports  the  knife-carriage,  M  S,  which  rests  at 


FIG.  391. — Jung's  Thoma  microtome. 

three  points  on  a  planed  and  polished  track  ;  whilst  on  the  side  of 
the  knife-carriage  two  other  points  slide  upon  the  middle  plate. 
Thus  in  the  angle  in  which  the  block  carrying  the  knife  slides  there 
are  five  points  of  contact  on  polished  surfaces,  the  block  itself  having 
weight  enough  to  keep  the  whole  steady,  so  that  at  a  touch  it  glides 
to  and  fro  with  a  firmness  and  precision  that  could  scarcely 'be 
attained  in  any  other  way. 

The  plate  6  is  an  inclined  plane,  its  highest  point  being  in  the 
direction  of  M.  The  inclination  of  the  angle  is  1  :  20  ;  it  supports 
the  object-holder,  O  S,  which  rests  in  its  place  exactly  as  does  the 
knife -carriage,  M  S. 

This  plate  also  bears  the  scale  TA,  which,  by  means  of  a  vernier 
011  the  object-holder,  enables  the  thickness  of  the  section  to  be  read 
off. 

The  bottom  plate  is  at  once  a  base  and  a  re  ceiver  for  the  dripping 
spirit,  oil,  &c. 


462    PKEPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

For  fastening  the  knife  a  thumb-screw,  C,  fig.  391,  serves;  but 
in  the  modified  form  of  the  instrument  designed  by  the  Zoological 
Station,  Naples,  this  is  replaced  by  a  single  head-screw,  C,  fig.  392, 
which  is  provided  with  holes  and  tightened  by  means  of  a  lever ; 
and  to  give  greater  freedom  to  the  use  of  the  knife  there  are  several 
holes  drilled  and  tapped  into  which  this  screw  fits. 

The  knives  of  the  form  A,  fig.  391,  are  generally  screwed  directly 
to  the  knife-carriage,  and  are  used  for  cutting  very  large  sections,  the 
oblique  position  shown  in  the  figure  being  the  one  that  is  generally 
indicated  for  the  cutting  of  very  large  objects.  This  knife  is  now 
seldom  used  except  in  pathological  observations  and  in  studies  on  the 
central  nervous  system. 


FIG.  392. — The  Thoma  microtome  with  the  usual  zoologist's  knife. 

The  knife,  however,  is  also  made  upon  another  model,  E,  fig,  392  ; 
it  then  has  a  special  holder  a,  in  which  it  is  secured  in  a  conical  slit 
by  the  screws  5,  ft1,  and  firmly  held.  i  , 

For  deep  objects  requiring  considerable  length  to  cut  from,  there 
are  plates  provided  for  elevating  the  knives  and  the  knife-holders. 

The  knife -holder  shown  in  fig.  392  can  be  rotated  round  the  axis 
formed  by  the  screw  c.  This  allows  of  any  degree  of  slant  or 
obliquity  of  direction  being  given  to  the  knife,  from  the  str  ctly 
transversal  position  shown  in  fig.  392  up  to  and  beyond  the  slanting 
position  shown  in  fig.  391.  But  it  provides  no  means  of  altering 
the  tilt  of  the  blade,  that  is,  of  elevating  or  depressing  the  back  of 
the  blade  relatively  to  its  edge — a  point  of  considerable  importance, 
to  which  we  shall  return  later  on.  To  meet  this  difficulty,  the 
maker  (R.  Jung,  1 2  Landhausstrasse,  Heidelberg  ;  his  instruments, 
as  well  as  price  lists,  may  be  obtained  from  Mr.  C.  Baker,  244  High 
Holborn,  London)  supplies  wedges  to  be  inserted  under  the  knife- 


POSITION    OF    KNIFE    IN    SECTION    CUTTING  463 

holder.  These  (Xeumayer's)  wedges,  are  horseshoe-shaped,  so  that 
they  may  be  slipped  round  the  central  screw.  They  are  made  in  pairs, 
one  member  of  each  pair  having  the  opening  of  the  horseshoe  at 
the  thin.  end.  the  other  having  it  at  the  thick  end.  The  wedge  with 
the  opening  at  the  thin  end  is  slipped  under  the  knife-holder 
(tltin  end  towards  the  operator),  and  operates  to  tilt  up  the  back  of 
the  knife. 

The  sister  wedge  is  then  placed  over  the  slotted  stem  or  handle 
of  the  carrier,  thick  end  towards  the  operator,  in  order  that  the 
binding-screw  may  have  a  horizontal  surface  to  bear  on.  The 
wedges  are  sold  in  sets  of  three  pairs,  of  different  degrees  of  bevel. 

This  simple  device  is  quite  suijicient  so  long  as  the  utmost  pre- 
cision of  section-cutting  is  not  required.  For  more  elaborate  work 
it  is  convenient  to  employ  a  special  knife-holder,  which  provides  a 
means  of  elevating  or  depressing  the  back  of  the  blade  by  rotating 
the  blade  round  its  axis.  Similar  contrivances  have  been  described 
by  Dr.  Hesse  (in  the  '  Zeitschrift  fur  wissenschaftliche  Mikroskopie,' 
xiv.  1,  1897,  p.  13;  see  'Journal  of  the  Royal  Microscopical  Soc.' 
1897,  p.  441),  and  by  Prof.  Apathy  (' Zeitschr.,'  xiv.  2,  p.  15,  and 
'Journal,'  1897,  p.  582).  This  last  is  rather  complicated  to  work 
with,  and  consequently  the  Naples  Zoological  Station  has  worked 
out  a  new  device,  made  by  Jung,  which  it  is  hoped  will  meet  all 
requirements.  This  is  the  '  Model  L '  of  his  price-list,  and  is 
figured  in  the  'Journal,'  1899,  p.  546.  That  of  Hesse  is  very 
simple,  and  ought  to  be  quite  sufficient  where  no  considerable 
change  of  tilt  is  likely  to  be  required.  It  is  made  by  Jung. 

Before  leaving  this  part  of  the  subject  it  appears  advisable  to 
consider  briefly  the  question  of  knife-position  in  general — a  matter 
011  which  success  or  failure  in  section-cutting  may  often  entirely 
depend. 

The  position  of  the  knife  should  be  varied  according  to  circum- 
stances, both  according  as  to  its  slant  or  obliquity  in  relation  to  the 
line  of  section,  and  as  to  its  tilt,  or  the  elevation  of  its  back  relatively 
to  its  edge. 

As  regards  slant — the  slanting  position,  fig.  391,  is  adapted  for 
cutting  soft  and  watery  objects,  not  imbedded,  and  tissues  imbedded 
in  celloidin,  or  the  like  ;  for  these  cannot  be  cut  with  the  knife 
placed  transversely.  It  is  also  frequently  indicated  for  paraffin 
objects ;  but  011  this  head  no  general  rule  can  be  laid  down.  The 
transverse  position,  fig.  392,  is  indicated  for  cutting  paraffin  sections 
by  the  ribbon  method  (see  below,  Imbedding  Methods,  Paraffin),  and 
also  frequently  for  cutting  loose  sections  by  the  paraffin  method. 

As  regards  tilt :  (1)  The  knife  must  always  be  tilted  enough  to 
lift  the  under  facet  of  the  edge  clear  of  the  tissue  as  it  passes  over 
it,  for  if  not  the  tissues  will  be  crushed  by  it  as  it  passes  over 
them.  (2)  It  must  not  be  too  much  tilted,  or  it  will  not  bite,  but 
will  act  as  a  scraper.  Prof.  Apathy,  who  has  investigated  the 
subject  in  an  instructive  paper  in  the  '  Sitzber.  d.  med.-naturw. 
Section  d.  Siebenburgischen  Museumvereins,  Kolozsvar,'  xix.  1897, 
H.  7,  concludes  as  follows:  (1)  The  knife  should  always  be  tilted 
somewhat  more  than  enough  to  bring  the  under  cutting-facet  of  the 


464    PREPARATION,    MOUNTING,   AND    COLLECTION    OF   OBJECTS 

edge  clear  of  the  object.  (2)  It  should  in  general  be  less  tilted  for 
hard  and  brittle  objects  than  for  soft  ones,  therefore,  cmteris  paribus, 
less  for  paraffin  than  for  celloidin.  (3)  The  extent  of  useful  tilt 
varies  (according  to  the  angle  to  which  the  knife  is  ground,  amongst 
other  factors)  between  0°  and  16°.  (Jung's  ordinary  knife-holders 
have  mostly  a  tilt  of  about  9°,  which  is  only  enough,  with  the  usual 
plane-concave  knives,  for  cutting  ribbons  of  sections  with  hard 
paraffin.)  (4)  Excessive  tilt  causes  paraffin  sections  to  roll,  and  may 
produce  longitudinal  rifts  in  them.  It  may  also  set  up  vibrations  in 
the  blade,  which  are  heard  as  a  humming  tone,  and  which  give  an 
undulatory  surface  to  the  sections.  Excessive  tilt  may  often  be 
recognised  by  the  knife  giving  out  a  short  metallic  note  just  as  it 
leaves  the  object.  For  knives  with  plane  under-surfaces  it  is  seldom 
advisable  to  give  less  than  10°  tilt;  whilst  knives  with  concave 
under-surfaces  on  the  contrary  may  require  to  be  placed  almost 
horizontal.  A  knife  with  too  little  tilt  will  cut  a  second  section, 
or  a  portion  of  one,  without  the  object  having  been  raised  ;  showing 


FIG.  393.— Object-holder  with  jaws. 

that  during  the  first  cut  the  object  was  pressed  down  by  the  knife 
and  recovered  itself  afterwards.  This  fault  is  denoted  by  the  ringing 
tone  given  out  by  the  knife  on  passing  back  over  the  object  before  it 
is  raised.  Ribbon-cutting  requires  a  relatively  hard  '  paraffin  and 
less  tilt.  With  celloidin  it  is  very  important  to  avoid  insufficient 
tilt,  as  the  elastic  celloidin,  with  too  little  tilt,  yields  before  the 
knife  and  is  not  cut. 

The  exigencies  of  section-cutting  have  given  rise  to  a  great  rar  !>'/</ 
of  object-holders  in  this  instrument.  The  simplest  is  seen  in  O  S, 
fig.  391,  which  is  a  pair  of  jaws  clamped  by  screws  and  fixed  upon 
the  pivot  St  by  the  milled  head  a.  At  n  is  the  vernier,  which  indi- 
cates the  position  on  the  mm.  scale,  Th,  and  t  is  an  agate  highly 
polished,  upon  which  the  micrometer  screw  m  works  to  drive  forward 
the  object-carrier,  O  S. 

The  Zoological  Station  at  Naples  employs  a  holder  specially  de- 
signed for  use  with  paraffin  ;  the  object  is  soldered  with  paraffin  on 
to  the  cylinder,  b  y,  fig.  392.  This  is  supported  on  gimbals  and  may 


THE   THOMA   MICROTOME 


465 


be  shifted  vertically  and  horizontally  by  means  of  the  small  screw  a, 
and  is  fastened  by  means  of  the  milled  head,  m.  By  the  pinion  n  it 
may  be  displaced  over  90°,  and  as  great  an  inclination  can  be  taken 
in  a  plane  perpendicular  to  this  by  the  supporting  metal  frames  by 
means  of  the  pinion  p.  In  this  way  every  desired  inclination  of  the 
object  to  the  knife  can  be  readily  secured. 

Fig.  393  presents  the  same  object-holder,  but  instead  of  the 
cvlinder  a  simple  pair  of  jaws  with  the  screw  m  to  secure  objects  of 
every  variety.  A  cylinder-holder  as  in  fig.  393  can  be  placed  in 
these  jaws  from  which  the  benefits  of  the  Neapolitan  holder  can  be 
secured.  But  fig.  396  shows  a  still  greater  improvement  which  can  be 
applied  to  both  object-holders,  v&.ja  perpendicular  displacement  by 
means  of  a  coy  and  pinion  governing  the  height  of  the  mass  from 
which  the  sections  are  to  be  cut. 

The  elevator  in  fig.  393  is  supported  on  one  side  by  the  prism  P, 
;  i  nd  on  the  other  by  the  rod  C;  these  are  joined  by  the  bridged, 


=  cf 


FIG.  394. — Object-holder  movable  about  two  horizontal  axes  at  right  angles 
to  each  other. 

to  which  a  cogged  bar  is  fastened,  into  which  a  pinion  catches,  which 
is  moved  by  the  lever  Y,  allowing  a  perpendicular  displacement  of  the 
object  of  12  mm.  At  O  is  the  millimetre  scale  on  which  the  perpen- 
dicular displacement  can  be  read  off  by  means  of  the  index  x. 

An  object-holder  movable  about  two  horizontal  axes  situated 
perpendicularly  to  each  other  is  seen  in  fig.  394.  These  positions 
are  fixed  by  the  milled  heads  &1,  b ;  e  shows  the  jaws  for  holding  the 
object,  into  which,  however,  cylinders  like  fig.  396  may  be  intro- 
duced. This  object-holder  has  a  perpendicular  displacement  con- 
trolled by  a  screw.  The  part.  K,  which  supports  the  chief  axis  of  the 
j;i\vs,  is  fitted  on  to  the  triangular  prism  Si,  the  lower  part  of  which  is 
furnished  with  hinges  ;  on  the  hinge  the  screw  V  moves,  which  at  its 
upper  end  lies  close  to  K,  and  is  sustained  in  this  position  by  the 
steel  plate  gr,  so  that  K  is  carried  up  and  down  with  it,  and  this 
movement  is  read  off  by  a  scale  under  S. 

H   II 


466    PREPARATION,   MOUNTING,    AND   COLLECTION   OF   OBJECTS 

Fig.  395  presents  an  object-holder  intended  to  analyse  by  diversified 
section  objects  which  are  wedged  or  fan-shaped  in  form  on  a  fixed 
axis,  but  may  be  applied  to  other  purposes. 

B  is  a  prism-shaped,  semicircularly  bent  bar,  moving  in  the  slot 
F  F1  ;  at  b  and  bl  the  jaws  occupy  the  position  common  to  those  of 
the  ordinary  form. 


FIG.  395. — Object-holder  for  analysis  by  diversified  section. 

On  the  circumference  of  B  a  spiral  is  cut,  which  becomes  slightly 
visible  at  g ;  into  this  spiral  a  screw  passes  at  H,  which  is  turned  bv 
the  milled  head  S,  which  can  alter  the  position  of  the  arc  to  the 
horizontal  to  the  extent  of  1  mm. ;  and  the  amount  of  the  change  of 
position  can  be  read  off  on  the  graduated  circle  K. 

In  a  fixed  position  the  middle  of  this  section-holder  is  the  plane 
of  action  of  the  knife.  If  an  object  be  fixed  in  the  jaws  so  that  the 


FIG.  396.— Cylinder  for  u 


fixed  axis  of  it  lies  in  this  plane,  it  will  only  be  required  that  the 
screw  S  be  brought  into  action  to  obtain  wedge-shaped  sections  of 
whatever  thickness  is  required,  which  will  all  be  made  in  this  axis. 

The  set  of  cylinders  which  may  be  used  with  these  and  other 
jaws  is  represented  in  fig.  396  :  b  y  is  the  cylinder,  G  the  compressing 
screw  for  it,  the  block  W  being  held  in  the  jaws. 

The  object-slide  with  its  vernier  may  be  sliddeii  up  the  incline  by 


THE   THOMA   MICROTOME  467 

1  uiiid:  but  it  is  much  more  accurate  to  control  its  movement  with 
the  micrometer-screw.  The  point  of  this  screw  in  fig.  392,  t,  works 
on  the  polished  plane  of  an  agate  cone.  The  clamp  on  which  the 
screw  is  mounted  is  held  firmly  in  its  place  by  the  milled  head  "VV  in 
ftch.  It  may  stretch  up  as  far  as  O,  being  refastened  by  W. 

The  screw  m  is  so  cut  that  a  single  rotation  moves  the  slide  011 
the  y,f'{,"0-  mm.,  which  in  the  inclination  of  the  plane  of  1  :  20  gives 
an  elevation  of  the  object  of  i}^  mm.  The  barrel  or  drum, 'K, 
situated  on  the  axis  of  the  screw,  is  divided  into  fifteen  parts ;  con- 
sequently the  interval  of  each  division  corresponds  to  an  elevation  of 

nAnrmm-. 

There  is  also  an  action  by  means  of  a  spring  which  gives  the  ear 

as  well  as  the  eye  cognisance  of  tne  amount  of  elevation  which  has 
taken  place,  which  greatly  relieves  the  eye.  This,,  however,  can  be 
brought  into  action  or  not  at  the  option  of  the  operator. 

Besides  these  object-holders  a  freezing  apparatus  can  be  added 
which  is  simply  placed  on  the  object-slide  as  shown  in  fig.  397. 


FIG.  397. — Freezing  apparatus  for  the  Thoma  microtome. 

The  freezing  is  effected  by  ether-spray.  A  specially  favourable 
effect  is  obtained  if  the  cylinder  g  is  mica  and  not  glass.  A  layer  of 
water  freezes  in  from  thirty  to  thirty-five  seconds. 

An  arrangement  of  the  Thoma  for  cutting  large  objects  has  also 
been  devised  which  is  illustrated  in  fig.  398. 

The  knife  is  to  be  placed  considerably  higher  in  front  than 
behind,  in  order  to  lessen  the  pressure  on  the  objects.  In  order  to 
satisfy  all  demands,  the  knife-rest  is  adjustable. 

The  knife  is  so  arranged  that  the  whole  length  of  blade  can  be 
used,  and  then  the  screw  c  is  fairly  tightly  screwed  down.  As  strong 
knives,  even  of  a  length  of  36  cm.,  easily  give,  a  knife-support  has 
been  constructed  ;  this  is  fastened  by  the  screw  c'  to  the  carrier. 
The  support  is  arranged  parallel  with  the  back  of  the  knife  M  ;  if 
the  extremity  n  be  slightly  pressed  backwards,  so  that  it  touches  the 

H  H  2 


THE  ROCKING  MICROTOME  469 

knife,  it  is  then  fixed  in  this  position  by  the  screw  o  (scarcely  evident 
in  the  illustration). 

This  done,  the  spirit-vessel  S/>  can  be  arranged  in  a  position 
which  will  not  interfere  with  the  free  movement  of  the  knife.  In 
order  that  a  stream  of  spirit  may  follow  the  knife  over  the  object, 
the  following  arrangement,  is  adopted.  The  spirit-vessel  Sp  turns 
round  an  axis  on  the  column  h  ;  to  it  is  joined  the  arm  L,  which 
carries  in  front  the  fine  tube  r  (connected  with  t  #),  and  also  the  rod 
p ;  the  latter  is  movable  perpendicularly,  and  to  its  lower  end  a 
bridge  or  grip  with  two  small  rollers  i  and  i'  is  fastened.  The  rod 
;;  is  so  placed  that  on  each  side  of  the  metal  strip  b,  screwed  on  to 
the  knife-support,  there  is  one  o£  the  rollers.  By  the  adjusting 
screws  Stf,  the  whole  apparatus  is  so*arranged  that,  when  the  knife- 
carrier  is  in  motion,  no  other  friction  occurs  than  that  of  the  rollers 
on  the  strip  b  b  b. 

The  vessel  is  filled  by  screwing  off  the  head  Z.  As  the  tube  r 
acts  as  a  siphon,  it  is  necessary,  when  the  cock  is  turned  on,  to  blow 
down  the  tube.  The  stream  of  spirit  should  be  directed  at  a  right 
angle  to  the  knife,  and  about  the  middle  of  the  object.  This  done, 
the  object  06,  by  means  of  the  screw  K,  is  firmly  grasped  in  the  fangs 
of  the  object-carrier ;  the  correct  direction  for  the  position  of  the 
knife  is  given  to  its  surface  by  the  screws  at  f  and  f\,  and  then 
the  axes  of  the  fangs  are  tightened  up  by  the  levers  q  and  q1 '.  If 
the  height  of  the  object  is  not  quite  correct,  adjustment  is  made  by 
the  screwr  m.  By  turning  the  screws  S,  S  the  holder  is  fixed. 

Y  is  a  wheel  with  cranked  axle  E?c,  and  this  by  means  of  a  cat- 
gut band  moves  the  knife. 

For  the  r«i>'xl  production  of  ribbons  of  sections,  however,  the 
instrument  par  excellence  is  the  Cambridge  rocking  microtome.  It 
is  illustrated  in  fig.  399. 

The  principle  is  the  employment  of  a  rotary  instead  of  a  sliding 
movement  of  the  parts.  Two  uprights  are  cast  on  the  base-plate, 
and  are  provided  with  slots  at  the  top,  into  which  the  razor  is  placed 
and  clamped  by  two  screws  with  milled  heads.  The  inner  face  of 
the  slot  is  so  made  as  to  give  the  razor  that  inclination  which  has  in 
practice  been  found  most  advantageous.  The  razor  is  thus  clamped 
between  a  flat  surface  and  a  screw  acting  in  the  middle  of  the  blade, 
and  the  edge  of  the  razor  is  consequently  in  no  way  injured. 

The  imbedded  object  is  cemented  with  paraffin  into  a  brass  tube 
which  fits  tightly  on  to  the  end  of  a  cast-iron  lever.  This  tube  can 
lie  made  to  slide  backwards  or  forwards,  so  as  to  bring  the  imbedded 
object  near  to  the  razor  ready  for  adjusting.  It  is  now  furnished 
with  a  mechanical  arrangement  for  accurately  adjusting  the  position 
of  the  object.  The  cast-iron  lever  is  pivoted  at  about  3  in.  from  the 
end  of  the  tube.  To  the  other  end  of  this  lever  is  attached  a  cord 
by  which  the  motion  is  given,  and  the  object  to  be  cut  brought 
across  the  edge  of  the  razor.  The  bearings  of  the  pivot  are 
V-shaped  grooves,  which  themselves  form  part  of  another  pivoted 
system. 

Immediately  under  the  first  pair  of  V's  is  another  pair  of  inverted 
Vs.  which  rest  on  a  rod  fixed  to  two  uprights  cast  on  the  base-plate. 


470    PREPARATION,    MOUNTING,   AND   COLLECTION   OF   OBJECTS 

A  horizontal  arm  projects  at  right  angles  to  the  plane  of  the  two 
sets  of  V's,  the  whole  being  parts  of  the  same  casting.  On  the  end 
of  the  horizontal  arm  is  a  boss  with  a  hole  in  it,  through  which  a 


USING   THE   ROCKINO   MICROTOME  471 

screw  passes  freely.  The  bottom  ot  the  boss  is  turned  out  spheri- 
cally, and  into  it  fits  a  spherical  nut  working  on  the  screw.  The  nut 
is  pi-evented  from  tinning  by  a  pin  passing  loosely  through  a  slot 
in  the  boss.  The  bottom  of  the  screw  rests  on  a  pin  fixed  in  the 
base-plate. 

It  will  be  seen  that  the  effect  of  turning  the  screw  is  to  raise  or 
lowTer  the  end  of  the  horizontal  arm,  and  therefore  to  move  backwards 
or  forwards  the  upper  pair  of  V's,  and  with  them  the  lever  and 
object  to  be  cut.  The  top  of  the  screw  is  provided  with  a  milled  head, 
which  may  be  used  to  adjust  the  object  to  the  cutting  distance.  The 
distance  between  the  centres  of  the  two  pivoted  systems  is  1  in.  and 
the  distance  of  the  screw  from  thft  fixed  rod  is  6J  in.  The  thread 
of  the  screw  is  25  to  the  inch  ;  thus,  if  the  screw  is  turned  once  round, 

the  object  to  be  cut  will  be  moved  forward  —  of          or   -  -   in. 

25       6^          156 

The  turning  of  the  screw  is  effected  automatically  as  follows  : 
A  wheel  with  a  milling  on  the  edge  is  fixed  to  the  bottom  of  the 
screw ;  an  arm  to  which  a  pawl  is  attached  rotates  about  the  pin 
which  supports  the  screw.  This  arm  is  moved  backwards  and  for- 
wards by  hand  or  by  a  cord  attached  to  any  convenient  motor. 
When  the  arm  is  moved  forward  the  pawl  engages  in  the  milling 
and  turns  the  wheel ;  when  the  arm  is  moved  back  the  pawl  slips  over 
the  milling  without  turning  the  wheel.  A  stop  acting  against  the 
pawl  itself  prevents  any  possibility  of  the  wheel  turning,  by  its  own 
momentum,  more  than  the  required  amount.  The  arm  is  always 
moved  backwards  and  forwards,  between  two  stops,  a  definite 
amount,  but  the  amount  the  wheel  is  turned  is  varied  by  an  adjustable 
sector,  which  engages  a  pin  fixed  to  the  pawl  and  prevents  the  pawl 
from  engaging  the  milling  of  the  wheel.  By  adjusting  the  position 
of  this  sector,  the  feed  can  be  varied  from  nothing  to  about  ^V  of  a 
turn  ;  and  hence,  since  the  screw  has  25  threads  to  the  inch,  the 
thickness  of  the  sections  cut  can  be  varied  from  a  minimum, 
depending  on  the  perfection  with  which  the  razor  is  sharpened,  to  a 

511  1 

maximum  of  -^  of  —  of  --^  or  y~~~  of  a  turn.  The  practical  mini- 
mum thickness  obtainable  with  a  good  razor  is  approximately  -4^7  ^"o 
inch.  The  values  of  the  teeth  on  the  milled  wheel  are  as  follows  : — 

1  tooth  of  the  milled  wheel  =  ^^^  in.  =  -000625  mm. 

2  teeth          „  „  =20000 in-  =  '001250  mm. 
4     .,              „           „             =  y^oo in-  =  '°025  mm- 

16     „  „  „  =551oo  in.  =  -01  mm. 

The  movement  of  the  lever  which  carries  the  imbedded  object  is 
effected  by  a  string  attached  to  one  end  of  the  lever.  This  string 
passes  under  a  pulley  and  is  fastened  to  the  arm  carrying  the  pawl. 
Attached  to  the  other  end  of  the  lever  is  a  spring  pulling  downwards. 
When  the  arm  is  moved  forward  the  feed  takes  place,  the  string  is 
pulled,  the  imbedded  object  is  raised  past  the  razor,  and  the  spring 
is  stretched.  When  the  arm  is  allowed  to  move  back,  the  spring 
draws  the  imbedded  object  across  the  edge  of  the  razor,  and  the  sec- 
tion is  cut.  The  string  is  attached  to  the  lever  by  a  screw  which 


472    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

allows  the  position  of  the  imbedded  object  to  be  adjusted,  so  that  at 
the  end  of  the  forward  stroke  it  is  only  just  past  the  edge  of  the 
razor.  This  is  an  important  adjustment,  as  it  causes  the  razor  to 
commence  the  cut  when  the  object  is  travelling  slowly,  and  produces 
the  most  favourable  conditions  for  the  sections  to  adhere  to  each 
other. 

The  following  are  perhaps  the  most  prominent  advantages  of  this 
instrument:  (1)  The  price  is  low.  (2)  Manipulation  is  simple. 
(3)  The  work  is  rapid,  and  extremely  accurate.  (4)  There  are  no 
delicate  working  parts  which  can  get  out  of  order,  and  the  whole 
instrument  is  easily  taken  apart  for  packing,  and  is  very  portable. 

The  above  description  refers  to  the  original  form  of  the  instru- 
ment. Later,  the  Cambridge  Scientific  Instrument  Company  ha\v 
brought  out  an  improved  form,  at  a  higher  price.  For  most 
purposes  the  original  form  will  suffice.  The  instrument  is  said  by 
the  makers  to  cut  celloidin  objects ;  but  for  this  purpose  a  sliding 
microtome  will  certainly  be  found  preferable. 

The  Minot  microtome,  of  which  a  description  may  be  found  in 
the  'Journal  of  the  Royal  Microscopical  Society,'  1889,  p.  143,  is  a 
neat  instrument  designed,  like  the  Cambridge  rocker,  for  cutting- 
ribbons  of  paraffin  imbedded  objects.  It  is  worked  on  the  sewing- 
machine  principle,  and  cuts  very  rapidly.  But  its  work  is  not  so 
fine  as  that  of  the  Cambridge  instrument,  possibly  on  account  of  in- 
sufficient compensation  in  the  working  parts.  This  defect  is  said  to 
have  been  satisfactorily  overcome  in  the  beautiful  instrument,  con- 
structed on  the  same  principle,  of  Reinhold,  a  description  of  which 
may  be  found  in  the  journal  above  quoted,  1893,  p.  706.  The  work 
afforded  by  this  instrument  is  certainly  of  the  highest  order,  but 
the  price  is  against  it,  as  it  costs  about  201.  Both  of  these  instru- 
ments are  said  to  be  able  to  cut  celloidin  sections  ;  but  it  is  self- 
evident  that  they  are  not  so  well  adapted  for  that  purpose  as  the 
sliding  microtome. 

It  is  unnecessary  here  to  do  more  than  allude  to  the  large  and 
cumbrous  instruments  specially  designed  for  cutting  sections  of 
brain.  Such  is  the  microtome  of  Strasser,  of  which  a  description 
may  be  found  in  the  '  Journal  of  the  Royal  Microscopical  Society,' 
1892,  p.  703,  and  that  of  Gudden  and  others.  They  are  only 
required  for  certain  very  special  neurological  researches,  and  are 
not  at  all  adapted  to  the  wants  of  the  zoologist  or  histologist  in 
general.  For  these,  we  may  here  repeat,  the  all-round  instrument 
par  excellence  is  Jung's  medium-sized  Thoma  microtome,  No.  IV.,  to 
which,  if  lengthy  series  of  paraffin  sections  be  frequently  required,  a 
Cambridge  rocker  may  conveniently  be  added. 

But  it  is  needful  also  to  describe  one  or  more  of  the  best  instru- 
ments designed  specially  for  cutting  sections  by  congelation  or  freer./ it;/ 
of  the  imbedding  mass.  Dr.  R.  A.  Hayes  designed  an  ether  freezing 
microtome  with  the  object  of  affording  to  those  who  have  occasional 
need  to  cut  sections  of  tissues  for  pathological  investigations,  &c.. 
the  means  of  doing  so  quickly,  conveniently,  and  accurately.  It  is 
illustrated  in  fig.  400.  It  is  very  compact,  solidly  constructed,  and 
simple  in  plan.  It  freezes  rapidly,  and  permits  sections  of  large 


ETHER   FREEZINO   .MICROTOMES  473 

surface  to  be  made  with  precision,  sections  1  in.  x  f  in.  having 
been  cut  by  it  without  difficulty. 

It  consists  of  a  solid  cast-iron  base,  A,  10  in.  x  4J  in.,  which 
rests  upon  a  mahogany  block.  Extending  the  whole  length  of  the 
upper  surface  of  the  base  is  a  V-shaped  gutter,  on  the  planed  sides 
of  which  slides  a  heavy  metal  block,  B,  on  the  flat  top  of  which 
the  razor  is  secured  (any  ordinary  razor  can  be  used),  the  tang 
being  grasped  between  two  flat  pieces  of  iron,  which  are  press* M! 
together  by  a  winged  nut,  C.  The  razor  by  this  arrangement  can 
be  secured  at  any  desired  angle  to  the  direction  of  its  motion  to 
and  fro. 

The  freezing-chamber  is  formed  by  a  short  vulcanite  cylinder,  ] ), 
its  lower  end  being  screwed  into  a' "brass  base,  E.  To  its  upper  end 
is  fastened  by  two  bayonet-catches  a  brass  plate,  F,  on  which  the 
tissue  to  be  cut  is  placed.  Inside  the  cylinder,  I),  and  rising  from 
the  base,  E,  is  an  ordinary  spray,  the  air  and  ether  being  supplied 
through  tubes,  g  and  H,  passing  outside  through  the  base.  There 


FIG.  400. — Dr.  Hayes's  ether  freezing  microtome. 

is  also  an  opening  in  the  floor  of  the  chamber  communicating  with 
the  tube,  to  allow  the  overflow  of  ether  in  case  of  any  accumulation 
inside  the  cylinder  ;  any  such  overflow  may  be  returned  by  the  tube 
to  the  ether  supply  bottle,  K.  The  freezing- chamber  is  secured  to 
the  top  of  the  micrometer-screw  arrangement.  Z,  which  is  of  the 
simplest  form,  but  has  a  perfectly  smooth  and  regular  motion.  The 
nut  is  divided  to  indicate  a  section  O01  mm.  in  thickness,  but  half 
this  thickness  can  be  cut  without  difficulty. 

The  method  of  using  the  microtome  is  very  simple.  The  slide 
and  block,  D,  having  been  carefully  rubbed  clean  and  well  oiled,  the 
razor  is  clamped  at  any  desired  angle,  the  bottle,  K,  is  filled  with 
ether  (good  dry  methylated  ether  answers  perfectly),  and  the  piece 
of  tissue  to  be  cut,  having  been  previously  saturated  with  thick  gum 
solution,  is  placed  upon  the  plate  F,  and  the  spray  which  plays  upon 
the  under  surface  of  the  plate,  F,  set  working  by  the  hand-pump, 
M  ;  in  a  short  time  the  tissue  will  be  frozen  quite  through,  and  if  a 
number  of  sections  are  required,  an  occasional  stroke  or  two  of  the 


474    PREPARATION,    MOUNTING,   AND    COLLECTION   OF   OBJECTS 

pump  will  keep  the  gum  in  proper  condition  for  cutting.  The 
sections  are  easily  cut,  as  in  other  microtomes  of  this  class,  by 
alternate  movements  of  the  screw,  Z,  and  strokes  of  the  razor. 

The  instrument  may  also  be  used  for  cutting  tissue  imbedded  in 
paraffin  or  other  mass,  the  object  to  be  cut  being  secured  in  position 
either  by  being  gently  heated  at  its  under  surface  and  pressed  on  the 
plate,  F,  to  which  it  firmly  adheres  on  cooling,  or  by  a  simple  clamp- 
ing arrangement,  which  can  be  substituted  for  the  freezing-chamber. 
When  used  in  this  way  large  numbers  of  sections  may  be  cut  in  series 
by  attaching  to  the  razor  a  light  support  to  receive  the  sections  as 
they  are  cut. 


FIG.  401. — Cathcart's  freezing  microtome. 

Another  most  serviceable  and  admirable,  because  inexpensive 
and  efficient,  microtome,  especially  for  freezing  purposes,  was 
devised  by  Mr.  Cathcart ;  and  it  is  now  presented  in  a  simplified 
and  improved  condition.  The  instrument  is  illustrated  in  fig.  401. 

In  this  form  the  clamping  arrangements  are  much  more  perfect 
than  in  the  old  form  ;  the  principal  screw  and  its  milled  head  are 
larger  and  more  convenient ;  the  freezing-plate .  is  circular,  and  is 
provided  with  an  arrangement  for  preventing  the  ether,  with  which 
the  freezing  is  effected,  from  reaching  the  upper  side  of  the  plate ; 
and  the  instrument  is  now  so  modified  that  it  can  be  used  for  ordinary 
imbedding  as  well  as  freezing. 


ETHER   FREEZING   MICROTOMES  475 

The  increased  size  of  the  screw  gives  a  more  steady  movement 
than  was  possessed  by  the  older  and  smaller  microtome,  while  the 
greater  circumference  of  the  screw-head  enables  an  operator  to  im- 
part a  finer  movement  to  the  screw.  The  relation  between  the  pitch 
of  the  screw  and  the  circumference  of  its  head  is  such  that  if  the 
edge  be  moved  forward  a  quarter  of  an  inch,  an  object  will  be  raised 
one-thousandth  of  an  inch  ;  and  if  it  be  moved  an  eighth  of  an  inch, 
the  object  will  be  raised  a  two-thousandth  of  an  inch. 

In  the  original  instrument  the  plate  was  supported  on  two 
pillars,  in  order  that  as  little  heat  as  possible  might  be  conveyed  to 
the  freezing- plate  from  the  body  of  the  instrument.  In  the  new 
instrument  the  size  of  the  three  supporting  pillars  and  screws  is  so 
much  reduced  that  the  conducting  surface  is  not  greater  than  in  the 
old  microtome.  The  arrangement  for  cutting  imbedded  sections 
consists  of  a  tube  wrhich  fits  the  principal  well  of  the  microtome,  and 
wdthin  which  fits  a  hinged  part  similar  to  an  ordinary  vice.  With 
the  instrument  are  provided  the  means  of  preparing  paraffin  blocks 
for  imbedding  sections. 

When  it  is  intended  to  use  the  microtome  for  imbedding,  the 


FIG.  402. — Holder  for  Cathcart's  microtome.  FIG.  403.— Dropping-bottle. 

ether  spray,  spray-bellows,  and  ether-bottle  should  be  removed,  and 
the  freezing-tube,  having  been  raised  as  far  as  possible  by  means  of 
the  principal  screw,  should  then  be  withdrawn  from  the  well.  The 
imbedding  tube,  fig.  402,  is  now  placed  in  the  well,  and,  having  been 
pushed  down  until  it  rests  upon  the  point  of  the  large  screw,  it  may 
be  lowered  to  a  convenient  height  by  working  the  large  screw  back- 
wards. 

Mr.  Cathcart  recommends  in  freezing  with  this  instrument  that 
a  few  drops  of  mucilage  (1  part  gum  to  3  parts  water)  be  placed  on 
the  zinc  plate,  and  that  a  piece  of  the  tissue  be  cut,  of  about  a  quarter 
of  an  inch  in  thickness,  and  pressed  into  the  gum  ;  the  ether-bottle, 
filled  with  anhydrous  methylated  ether,  is  taken  and  the  spray  points 
pushed  into  their  socket.  All  spirit  must  of  course  have  been  pre- 
viously removed  by  soaking  for  a  night  in  water.  The  tissue  should 
afterwards  be  soaked  in  gum  for  a  like  time  before  being  cut.  The 
operator  must  now  work  the  spray- bellows  briskly  until  the  gum 
begins  to  freeze  ;  after  this,  work  more  gently.  Raise  the  tissue  by 
turning  the  milled  head,  and  cut  by  sliding  the  knife  along  the  glass 
plates. 


4/6    PREPARATION,   MOUNTING,    AND    COLLECTION   OF   OBJECTS 

Mounting. — By  the  term  '  mounting '  is  meant  the  arranging  of 
specimens  on  slides  in  such  media  and  in  such  a  manner  as  are  most 
favourable  for  the  demonstration  of  their  minute  structure  by  the 
microscope.  In  the  case  of  the  most  numerous  and  important  class 
of  objects  that  it  is  the  function  of  the  microscope  to  scrutinise, 
namely,  those  derived  from  the  substance  of  animal  or  vegetable 
organisms,  it  is  found  that  no  methods  of  mounting  will  avail  to  re- 
veal their  minute  structure  unless  the  specimens  have  first  been 
submitted  to  the  frequently  very  elaborate  processes  of  previous 

Preparation  to  be  hereafter  described  under  the  heads  of  Fixing, 
mbedding,  Section-cutting,  Staining,  and  the  like.  But  still  there 
are  many  objects  of  interest  and  beauty  that  can  be  satisfactorily 
mounted  without  the  aid  of  these  elaborate  processes  of  previous 
preparation.  And  as  also  the  manipulations  of  mounting  sensu 
stricto  are  in  principle  the  same  in  both  cases,  it  appears  advisable 
to  make  the  description  of  the  processes  of  mounting  precede  that 
of  the  processes  of  previous  preparation ;  merely  warning  the 
beginner  that  in  the  case  of  the  majority  of  specimens  intended  to 
illustrate  the  minute  structure  of  the  tissues  of  either  animals  or 
plants,  such  previous  preparation  is  a  sine  qua  non. 

The  manipulations  of  mounting  will  alone  be  described  here,  the 
most  useful  mounting  media  being  described  later  on  ('  Preserva- 
tive and  Mounting  Media  ') . 

In  dealing  with  the  small  quantities  of  fluid  media  required  in 
mounting  microscopic  objects,  it  is  essential  for  the  operator  to  be 
provided  with  the  means  of  transferring  very  small  quantities  from 
the  vessels  containing  them  to  the  slide,  as  well  as  of  taking  up  from 
the  slide  what  may  be  lying  superfluous  upon  it.  Where  some  one 
fluid,  such  as  glycerin,  is  in  continual  use,  it  will  be  found  very  con- 
venient to  keep  it  in  the  small  dropping-bottle  represented  in  fig.  403. 
The  stopper  is  perforated,  and  is  elongated  below  into  a  fine  tube, 
whilst  it  expands  above  into  a  bulbous  funnel,  the  mouth  of  which  is 
covered  with  a  piece  of  thin  vulcanised  indiarubber  tied  firmly 
round  its  lip.  If  pressure  be  made  on  this  cover  with  the  point  of 
the  finger,  and  the  end  of  the  tube  be  immersed  in  the  liquid  in  the 
bottle,  this  will  rise  into  it  on  the  removal  of  the  finger ;  if,  then, 
the  funnel  be  inverted,  and  the  pressure  be  reapplied,  some  of  the 
residual  air  will  be  forced  out,  so  that  by  again  immersing  the  end 
of  the  tube,  and  removing  the  pressure,  more  fluid  will  enter.  This 
operation  may  be  repeated  as  often  as  may  be  necessary,  until  the 
bulb  is  entirely  filled  ;  and  when  it  is  thus  charged  with  fluid,  as 
much  or  as  little  as  may  be  needed  is  then  readily  expelled  from  it 
by  the  pressure  of  the  finger  on  the  cover,  the  bulb  being  always 
refilled  if  care  be  taken  to  immerse  the  lower  end  of  the  tube  before 
the  pressure  is  withdrawn.  We  speak  from  large  experience  of  the 
value  of  this  little  implement,  which  is  very  clean,  simple,  and  use- 
ful. But  the  small  pipettes  now  used  so  commonly  for  filling  the 
stylographic  pens,  fitted  into  the  centre  of  a  cork  and  placed  in  any 
wide-mouthed  bottle,  will  be  found  to  be,  though  less  elegant, 
equally  useful  and  much  less  costly. 

Solutions   of  Canada  balsam  and  gum-dammar  in  volatile  fluids 


DROP-BOTTLES— MOUNTING   THIN    SECTIONS 


477 


are  best  kept  in  wide-mouthed  cupped  jars,  the  liquid  being  taken 
out  on  a  pointed  glass  rod,  cut  to  such  a  length  as  will  enable  it 
to  stand  in  the  jar  when  its  cap  is  in  place.  Great  care  should  be 
taken  to  keep  the  inside  of  the  cap  and  the  part  of  the  neck  of  the 
jar  on  which  it  fits  quite,  clean,  so  as  to  prevent  the  fixation  of  the 
neck  by  the  adhesion  between  these  two  surfaces.  Should  such 
adhesion  take  place,  the  cautious  application  of  the  heat  of  a  spirit- 
lamp  will  usually  make  the  cap  removable.  In  taking  out  the 
liquid  care  should  be  taken  not  to  drop  it  prematurely  from  the  rod 
— a  mischance  which  may  be  avoided  by  not  taking  up  more  than  it 
will  properly  carry,  and  by  holding  it  in  a  horizontal  position,  after 
drawing  it  out  of  the  bottle,  until  >its  point  is  just  over  the  slip  or 
cover  on  which  the  liquid  is  to  be  deposited. 

A  bottle  for  use  with  reagents,  enabling  the  operator  to  pour  out 
<  )iily  the  quantity  he  desires,  is  invaluable.  Small  capped  and  stoppered 
bottles,  the  stoppers  of 
which  are  tubes,  and  the 
well-fitting  caps  of  which 
prevent  evaporation,  arc 
very  valuable  for  aqueous 
and  thin  fluids.  We  illus- 
trate this  bottle  in  fig.  404. 
All  that  is  needful  is  to 
take  the  bottle,  with  the 
cap  off,  in  the  warm  hand, 
and  by  slight  expansion  a 
drop  or  more  as  required 
is  exuded.  These  bottles 
are  easily  procurable. 

But  we  like  still  better 
the  small  German  bottles, 
shown  in  fig.  405,  contain- 
ing about  30  grammes,  in 

which  two  deep  grooves  are  cut  on  opposite  sides  of  the  stopper,  so 
arranged  that  by  giving  the  stopper  half  a  turn  one  groove  is 
connected  with  a  hole  in  the  neck  of  the  bottle  :  this  will  be  seen  at 
a  in  fig.  405  ;  the  air  travels  down  this  groove,  and  by  inverting  the 
bottle  the  fluid  enters  the  other  groove  of  the  stopper  and  finds  its 
way  to  a  third  groove  cut  in  the  inside  of  the  neck  and  extending  to 
the  lip.  The  figure  shows  the  bottle  complete. 

Mounting  Thin  Sections. — It  is  customary  to  recommend  the  use 
of  '  section  lifters  '  in  order  to  raise  delicate  sections  out  of  the  fluid 
in  which  they  finally  are  placed  into  the  position  in  which  they  are 
to  be  mounted.  For  very  large  sections  they  are  probably  essential  ; 
but  from  personal  experience,  supported  by  the  most  accomplished 
histological  mounters  of  our  time,  we  believe  them  to  be  adverse  to, 
rather  than  promotive  of,  good  section-mounting.  One  of  the 
many  patterns  recommended  is  shown  in  fig.  406,  where  it  will  be 
seen  that  one  end  of  the  *  lifter '  is  perforated,  for  the  purpose  of 
drainage,  and  the  other  is  plain. 

The  present  writer  cannot  endorse  the  recommendation  of  this 


FIG.  404. 
Expansion  drop- 
bottle. 


FIG.  405. 
(TCVDUUI  drop-bottle. 


478     PREPARATION,    MOUNTING.    AND    COLLECTION   OF   OBJECTS 

instrument,  but  prefers  a  smooth  glass  rod  or  tube  ;  the  section  in 
fluid  can  easily  be  made  to  wrap  itself  round  the  rod,  from  which  it 
may  be  rolled  off  into  a  drop  of  liquid  placed  on  the  slide.  It  must 
be  manifest  that  the  less  we  have  to  manipulate  such  delicate  sections 
as  we  are  now  considering,  the  better  ;  to  get  a  section  on  and  off 
the  '  lifter '  is  a  needless"  process.  We  should,  as  stated  above, 
mount  on  the  cover-glass,  and  this  cover  should  be  the  only  lifter 
employed. 

The  cover  must  be  carefully  cleaned,  and  properly  selected  as  to 
size  and  tenuity.  By  means  of  a  needle  or  the  handle  of  an  ivory 
dissecting-knife  the  clearing  fluid  in  which  the  section  is  resting 
prior  to  mounting  is  gently  disturbed,  in  a  good-sized  vessel  or 
saucer,  until  the  section  desired  is  in  its  proper  position  on  the 
cover.  Now  lay  the  cover,  section  upwards,  on  fresh  blotting-paper, 
to  take  off  the  superfluous  liquid  from  the  free  side  of  the  cover,  and 
then  hold  the  edge  of  the  slip  at  an  angle,  more  or  less  acute,  with 
the  section  towards  the  blotting-paper,  But  never  suffering  the 
former  to  touch  the  latter  ;  when  this  has  removed  the  superfluous 

liquid  from  the  section,  lay  the 
cover,  section  upwards,  on  a 
glass  slip,  put  on  (say)  the 
benzol  balsam  until  it  stands  in 
an  evenly  diffused  mound  cover- 
ing the  section,  and  lay  it  aside 
absolutely  protected  from  dust 
for  twenty-four  hours  in  order 
that  the  benzol  may  evaporate. 

Now  take  it  out,  place  upon 
the  centre  of  the  section  one 
small  drop  of  fresh  benzol 
balsam,  and  turn  the  cover  over 

FIG.  406.  on  to  a  warm  slip,  being  careful 

to  have  guides  to  the  position  on 

the  slip  on  which  it  should  be  fixed  ;  and  in  an  hour  or  so  we  may 
clean  off  superfluous  balsam  and  finish  the  slide. 

To  those  who  mount  much  this  will  prove  the  quicker  plan,  as, 
for  fine  results,  it  is  undoubtedly  the  better. 

The  above  considerations  refer  only  to  loose  sections  in  fluid, 
or  thin  membranes,  or  other  thin  and  isolated  objects.  It  is  one  of 
the  advantages  of  the  paraffin  process  that  with  paraffin  sections  no 
lifter  is  required,  as  these  are  cut  dry,  and  being  stiffened  by  the 
paraffin  may  be  lifted  by  means  of  a  flat  camel's-hair  brush,  or  a 
scalpel  or  forceps.  The  manipulations  of  mounting  series  of  sections 
on  one  slide  are  described  under  '  Imbedding  Methods.' 

When  the  preparation  has  been  previously  immersed  in  aqueous 
liquids,  and  is  to  be  mounted  in  glycerin,  glycerin  jelly,  or 
Fan-ants'  medium,  the  best  mode  of  placing  it  on  the  slide  is  to  float 
it  in  a  saucer  or  shallow  capsule  of  water,  to  place  the  slide  or  cover 
beneath  it,  and,  when  the  object  lies  in  a  suitable  position  above  it, 
to  raise  the  slide  or  cover  cautiously,  holding  the  object  in  place  by 
a  needle,  until  it  is  entirely  out  of  the  water  ;  and  the  small  quantity 


MOUNTING-  479 

of  liquid  still  surrounding  the  object  is  to  be  carefully  drawn  oft' by 
blotting-paper,  care  being  taken  not  to  touch  the  object  with  it  (as 
its  fibres  are  apt  to  adhere)  or  to  leave  any  loose  fibres  on  the  slide. 
Before  the  object  is  covered,  it  should  be  looked  at  under  a 
dissecting  or  mounting  microscope,  for  the  purpose  of  improving  (if 
desirable)  its  disposition  on  the  slide,  and  of  removing  any  foreign 
particles  that  may  be  accidentally  present.  A  drop  of  the  medium 
(liquefied,  if  necessary,  by  a  gentle  warmth)  is  then  to  be  placed  upon 
it,  and  another  drop  placed  011  the  slip  or  cover  and  allowed  to  spread 
out.  The  cover  being  then  taken  up  with  a  pair  of  forceps  must  be 
inverted  over  the  slide,  and  brought  to  touch  it  at  one  part  of  its 
margin,  the  slide  being  itself  inclined  in  the  direction  of  the  place 
of  contact,  so  that  the  medium  dc.Qumulates  there  in  a  little  pool. 
By  gently  letting  down  the  cover,  a  little  wave  of  the  medium  is 
pressed  before  it,  and,  if  enough  of  the  medium  has  been  deposited, 
the  whole  space  beneath  the  cover  will  be  filled,  and  the  object  com- 
pletely saturated.  If  air-bubbles  should  unfortunately  show  them- 
selves, the  cover  must  be  raised  at  one  margin,  and  a  further  quantity 
of  the  medium  deposited. 

If,  again,  there  are  no  air-bubbles,  bu^  the  medium  does  not 
extend  itself  to  the  edge  of  the  cover,  the  cover  need  not  be  raised, 
but  a  little  may  be  deposited  at  its  edge,  whence  it  will  soon  be  drawn 
in  by  capillary  attraction,  especially  if  a  gentle  warmth  be  applied 
to  the  slide.  It  will  then  be  advantageous  again  to  examine  the 
preparation  under  the  dissecting  microscope  ;  for  it  will  often  happen 
that  an  opportunity  may  thus  be  found  of  spreading  it  better  by  the 
application  of  gentle  pressure  to  one  part  or  another  of  the  covering- 
glass,  which  may  be  done  without  injurious  effect  either  with  a  stiff 
needle  or  by  a  pointed  stick  ;  a  method  whose  peculiar  value,  when 
viscid  media  are  employed,  was  first  pointed  out  by  Dr.  Beale.  The 
slide  should  then  be  set  aside  for  a  few  days,  after  which  its  mount- 
ing may  be  completed.  Any  excess  of  the  medium  must  first 
be  removed.  If  glycerin  has  been  employed,  much  of  it  may  be 
drawn  off  by  blotting-paper  (taking  care  not  to  touch  the  edge  of  the 
cover,  as  it  will  be  very  easily  displaced) ;  and  the  remainder  may  be 
washed  away  with  a  camel's-hair  brush  dipped  in  water,  which  may 
be  thus  carried  to  the  edge  of  the  cover.  The  water  having  been 
drawn  off,  a  narrow  ring  of  liquefied  glycerin  jelly  may  be  made 
around — not  on — the  margin  of  the  cover  (according  to  the  suggestion 
of  Dr.  S.  Marsh)  for  the  purpose  of  fixing  it  before  the  cement  is 
applied ;  and  when  this  has  set,  the  slide  may  be  placed  on  the  turn- 
table, and  the  preparation  '  sealed '  by  a  ring  either  of  gold-size  or 
of  Bell's  cement,  which  should  be  carried  a  little  over  the  edge  of  the 
cover,  and  outside  the  margin  of  the  ring  of  glycerin  jelly.  This 
'  ringing '  should  be  repeated  two  or  three  times  ;  and  if  the  pre- 
paration is  to  be  viewed  with  '  oil-immersion  '  lenses,  it  should  be 
finished  off  with  a  coat  of  HolhVs  glue  or  Bell's  cement,,  which  are 
not  attacked  by  cedar  oil.  Until  the  cover  has  been  perfectly  secured, 
a  slide  carrying  a  glycerin  preparation  should  never  be  placed  in  an 
inclined  position,  as  its  cover  will  be  almost  sure  to  slide  by  its  own 
weight.  If  glycerin  jelly  or  Fan-ants'  medium  has  been  employed, 


480    PKEPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

less  caution  need  be  used,  as  the  cover-glass,  after  a  few  days'  setting, 
will  adhere  with  sufficient  firmness  to  resist  displacement.  The 
superfluous  medium  having  been  removed  by  the  cautious  use  of  a 
knife,  the  slide  and  the  margin  of  the  cover  may  be  completely 
cleansed  by  a  camel's-hair  brush  dipped  in  warm  water  ;  and,  when 
quite  dried,  the  slide,  placed  on  the  turn-table,  may  be  sealed  with 
gold-size — any  other  cement  being  afterwards  added,  either  for 
additional  security  or  for  '  appearance.' 

It  is  well  in  mounting  in  glycerin  jelly  to  soak  the  object 
previously  in  dilute  glycerin,  and  we  prefer  to  '  ring '  with  benzole 
and  balsam,  which  should  harden.  Then  coat  the  ring  with  shellac 
varnish  two  or  three  times  and  permanently  finish  with  thin,  coats  of 
gold-size. 

When,  on  the  other  hand,  the  section  or  other  preparation  is  to 
be  mounted  in  a  resinous  medium,  it  must  have  been  previously  pre- 
pared for  this  in  the  modes  described  further  on,  which  will  present 
it  to  the  mounter  either  in  some  essential  oil,  or  in  xylol  or  benzol 
or  the  like,  or  in  alcohol.  From  either  of  these  it  may  be  transferred 
to  the  cover  or  slide  in  the  manner  already  described. 

The  thin  sections  cut  by  the  microtome,  or  membranes  obtained 
by  dissection,  do  not  require  to  be  placed  in  cells  when  mounted  in 
any  viscid  medium  ;  since  its  tenacity  will  serve  to  keep  off  injurious 
pressure  by  the  cover-glass. 

Mounting  Objects  in  *  Natural '  Balsam. — Although  it  is  pre- 
ferable for  histological  purposes  to  employ  a  solution  of  hardened 
balsam,  as  directed  under  '  Mounting  Media,'  yet  as  there  are  main 
objects  for  mounting  for  which  the  use  of  the  '  natural '  balsam  is 
preferable,  it  will  be  well  to  give  some  directions  for  its  use.  When 
sections  of  hard  substances  have  been  ground  down  on  the  slides  to 
which  they  have  been  cemented,  it  is  much  better  that  they  should 
be  mounted  without  being  detached,  unless  they  have  become  clogged 
with  the  abraded  particles,  and  require  to  be  cleansed  out — as  is 
sometimes  the  case  with  sections  of  the  shells,  spines,  £c.,  of  echino- 
derms,  when  the  balsam  by  which  they  have  been  cemented  is  too 
soft.  If  the  detachment  of  a  specimen  be  desirable,  it  may  be 
loosened  by  heat,  and  lifted  off  with  a  camel's-hair  brush  dipped  in 
oil  of  turpentine.  But,  where  time  is  not  an  object,  it  is  far  better 
to  place  the  slide  to  steep  in  ether  or  chloroform  in  a  capped  jar 
until  the  object  falls  off  of  itself  by  the  solution  of  its  cement.  It 
may  then  be  thoroughly  cleansed  by  boiling  it  in  methylated  spirit, 
and  afterwards  laid  upon  a  piece  of  blotting-paper  to  dry,  after 
which  it  may  be  mounted  in  fresh  balsam  011  a  slide,  just  as  if  it 
had  remained  attached.  The  slide  having  been  warmed  011  the 
water-bath  lid,  a  sufficient  quantity  of  balsam  should  be  dropped  on 
the  object,  and  care  should  be  taken  that  this,  if  previously  loosened, 
should  be  thoroughly  penetrated  by  it.  If  any  air-bubbles  arise, 
they  should  be  broken  with  the  needle-point.  The  cover  having 
been  similarly  warmed,  a  drop  of  balsam  should  be  placed  on  it,  and 
made  to  spread  over  its  surface  ;  and  the  cover  should  then  be 
turned  over  and  let  down  on  the  object  in  the  manner  already  de- 
scribed. If  this  operation  be  performed  over  the  water-bath,  instead 


MOUNTING— IN  BALSAM— IN  AQUEOUS   LIQUIDS  481 

of  over  the  spirit-lainp,  there  will  be  little  risk  of  the  formation  of 
air-bubbles.  However  large  the  section  may  be,  care  should  be  taken 
that  the  balsam  is  well  spread  both  over  its  surface  and  that  of 
its  cover  ;  and  by  attending  to  the  precaution  of  making  it  accumu- 
late on  one  side  by  sloping  the  slide,  and  letting  down  the  cover 
so  as  to  drive  a  wave  before  it  to  the  opposite  side,  very  large  sections 
may  thus  be  mounted  without  a  single  air-bubble.  (The  Author  has 
thus  mounted  sections  of  Eozoon  three  inches  square.)  In  mounting 
minute  balsam  objects,  such  as  diatoms,  polycystince,  sponge-spicules, 
and  the  beautiful  minute  spines  of  ophiurida,  no  better  plan  can  be 
adopted  than  to  arrange  these  objects  carefully  upon  the  cover, 
either  by  '  scattering  '  or  '  arrangement,'  and  then  to  drop  on  to  the 
whole  cover  and  its  arranged  objects  as  much  balsam  as  the  cover 
will  receive  without  overflow ;  this  should  stand  free  from  dust  for 
some  hours,  after  which  the  partly  hardened  balsam  may  receive  a 
small  drop  of  fresh  balsam,  and  being  placed  upon  the  -slip  in  proper 
position,  may  by  the  use  of  gentle  heat  be  pressed  finally  into  position. 
When  the  chitinous  textures  of  insects  are  to  be  thus  mounted,  they 
must  be  first  softened  by  steeping  in  oil  of  turpentine  ;  and  a  large 
drop  of  balsam  being  placed  on  a  warmed  slide,  the  object  taken  up 
in  the  forceps  is  to  be  plunged  in  it,  and  the  cover  (balsamed  as  before) 
let  down  upon  it.  It  is  with  objects  of  this  class  that  the  spring- 
clip  and  the  spring-press  prove  most  useful  in  holding  down  the  cover 
until  the  balsam  has  hardened  sufficiently  to  prevent  its  being  lifted 
by  the  elasticity  of  the  object.  Various  objects  (such  as  the  palates 
of  gasteropods)  which  have  been  prepared  by  dissection  in  water  or 
weak  spirit  may  be  advantageously  mounted  in  balsam ;  for  which 
purpose  they  must  be  first  dehydrated,  and  then  transferred  from 
rectified  spirit  into  turpentine  or  one  of  the  other  '  clearing  agents ' 
mentioned  below.  Sections  of  horns,  hoofs,  &c.,  which  afford  most 
beautiful  objects  for  the  polariscope,  are  best  mounted  in  natural 
balsam,  which  has  a  remarkable  power  of  increasing  their  trans- 
parence. It  is  better  to  set  aside  in  a  warm  place  the  slides  which 
have  been  thus  mounted  before  attempting  to  clean  off  the  super- 
fluous balsam  in  order  that  the  covers  may  be  fixed  by  the  gradual 
hardening  of  what  lies  beneath  them. 

Mounting  Objects  in  Aqueous  Liquids. — By  far  the  greater 
number  of  preparations  which  are  to  be  preserved  in  liquid,  however, 
should  be  mounted  in  a  cell  of  some  kind,  which  forms  a  well  of 
suitable  depth,  wherein  the  preservative  liquid  may  be  retained. 
This  is  absolutely  necessary  in  the  case  of  all  objects  whose  thickness 
is  such  as  to  prevent  the  glass  cover  from  coming  into  close  approxi- 
mation with  the  slide ;  and  it  is  desirable  whenever  that  approxima- 
tion is  not  such  as  to  cause  the  cover  to  be  drawn  to  the  glass  slide 
by  capillary  attraction,  or  whenever  the  cover  is  sensibly  kept  apart 
from  the  slide  by  the  thickness  of  any  portion  of  the  object.  Hence 
it  is  only  in  the  case  of  objects  of  the  most  extreme  tenuity  that 
the  cell  can  be  advantageously  dispensed  with ;  the  danger  of  not 
employing  it,  in  many  cases  in  wrhich  there  is  no  difficulty  in 
mounting  the  object  without  it,  being  that  after  a  time  the  cement 
is  apt  to  run  in  beneath  the  cover,  which  process  is  pretty  sure  to 

I  I 


482    PREPARATION,    MOUNTING,   AND   COLLECTION   OF   OBJECTS 

continue  when  it  may  have  once  commenced.  When  cement- cells 
are  employed  for  this  purpose,  care  must  be  taken  that  the  surface 
of  the  ring  is  perfectly  flat,  so  that  when  the  cover-glass  is  laid  on 
no  tilting  is  produced  by  pressure  on  any  part  of  its  margin.  As  a 
general  rule,  it  is  desirable  that  the  object  to  be  mounted  should  be 
steeped  for  a  little  time  previously  in  the  preservative  fluid  employed. 
A  sufficient  quantity  of  this  fluid  being  deposited  to  overfill  the 
cell,  the  object  is  to  be  introduced  into  it  either  with  the  forceps  or 
the  dipping  tube ;  and  the  slide  should  then  be  examined  on  the 
dissecting  microscope  that  its  entire  freedom  from  foreign  particles 
and  from  air-bubbles  may  be  assured,  and  that  its  disposition  may 
be  corrected  if  necessary.  The  cover  should  then  be  laid  on  very 
cautiously,  so  as  not  to  displace  the  object ;  which  in  this  case  is 
best  done  by  keeping  the  drop  highest  in  the  centre,  and  keeping 
the  cover  parallel  to  the  slide  whilst  it  is  being  lowered,  so  as  to 
expel  the  superfluous  fluid  all  round.  This  being  taken  up  by  the 
syringe,  the  cement  ring  and  the  margin  of  the  cover  are  to  be 
dried  with  blotting-paper,  especial  care  being  taken  to  avoid  drawing 
off  too  much  liquid,  which  will  cause  the  gold-size  to  run  in.  It  is 
generally  best  to  apply  the  first  coat  of  gold-size  thin,  with  a  very 
small  and  flexible  brush  worked  with  the  hand  ;  this  will  dry  suffi- 
ciently in  an  hour  or  two  to  hold  the  cover  whilst  being  'ringed' 
on  the  turn-ta,ble.  And  it  is  safer  to  apply  a  third  coat  a  day  or 
two  afterwards  ;  old  gold-size,  which  lies  thickly,  being  then  applied 
so  as  to  raise  the  ring  to  the  level  of  the  surface  of  the  cover.  As 
experience  shows  that  preparations  thus  mounted,  which  have 
remained  in  perfectly  good  order  for  several  years,  maybe  afterwards 
spoiled  by  leakage,  the  Author  strongly  recommends  that  to  prevent 
the  loss  of  valuable  specimens  an  additional  coating  of  gold-size  be 
laid  on  from  time  to  time.  But  a  device  of  much  greater  value  in 
all  fluid  mounting  is  that  adopted  by  Mr.  Enock,1  who  puts  a 
metallic  ring  of  angular  section  round  the  outside  of  the  cell. 
slightly  overlapping  the  cover-glass  and  enclosing  the  rim  made 
good  with  cement ;  this  proves  perfect. 

Mounting  of  Objects  in  Deep  Cells, — The  objects  which  require 
deep  cells  are,  as  a  rule,  such  as  are  to  be  viewed  by  reflected  light, 
and  are  usually  of  sufficient  size  and  substance  to  allow  of  air  being 
entangled  in  their  tissues.  This  is  especially  liable  to  occur  where 
they  have  under-gone  the  process  of  decalcification,  which  will  very 
probably  leave  behind  it  bubbles  of  carbonic  acid.  For  the  extrac- 
tion of  such  bubbles  the  use  of  an  air-pump  is  commonly  recommended ; 
but  the  Editor  has  seldom  found  this  answer  the  purpose  satisfactorily, 
and  is  much  disposed  to  place  confidence  in  a  method  lately  recom- 
mended— steeping  the  specimen  in  a  stoppered  jar  filled  with  freshly 
boiled  water,  which  has  great  power  of  drawing  into  itself  either  air 
or  carbonic  acid.  Where  the  structure  is  one  which  is  not  injured 
by  alcohol,  prolonged  steeping  in  this  will  often  have  the  same  effect. 
The  next  point  of  importance  is  to  select  a  cover  of  a  size  exactly 
suitable  to  that  of  the  ring,  of  whose  breadth  it  should  cover  about 
two-thirds,  leaving  an  adequate  margin  uncovered  for  the  attachment 
1  Quekett  Journ.  second  series,  vol.  i.  p.  40. 


MOUNTING   IX   DEEP   CELLS  483 

of  the  cement.  And  the  perfect  flatness  of  that  ring  should  then  be 
carefully  tested,  since  on  this  mainly  depends  the  security  of  the 
mounting.  It  is  to  secure  this  that  we  prefer  rings  of  tin  or  bone, 
to  those  of  glass,  for  cells  of  moderate  depth  ;  for  their  surface  can 
be  easily  made  perfectly  flat  by  grinding  with  water,  first  on  a  piece 
of  grit,  and  then  on  a  Water-of- Ayr  stone,  these  stones  having  been 
previously  reduced  to  a  plane  surface,  or  still  better  with  a  good  flat 
file.  If  glass  rings  are  not  found  to  be  '  true,'  they  must  be  ground 
down  with  fine  emery  on  a  plate  of  lead.  When  the  cell  has  been 
thus  finished  off,  it  must  be  carefully  cleaned  out  by  dropping  into  it 
some  of  the  mounting  fluid  ;  and  should  be  then  examined  under  the 
dissecting  microscope  for  minute  fur-bubbles,  which  often  cling  to 
the  bottom  or  sides.  These  having  "been  got  rid  of  by  the  needle, 
the  cell  should  be  finally  filled  with  the  preservative  liquid,  and  the 
object  immersed  in  it,  care  being  taken  that  no  air-bubbles  are 
carried  down  beneath  it.  The  cell  being  completely  filled  so  that  the 
liquid  is  running  over  its  side,  the  cover  may  then  be  lowered  down 
upon  it  as  in  the  preceding  case  ;  or,  if  the  cell  be  quadrangular, 
the  cover  may  be  sloped  so  as  to  rest  one  margin  on  its  wall,  and' 
fresh  liquid  may  be  thrown  in  by  the  syringe,  while  the  other  edge- 
is  lowered.  When  the  cover  is  in  place,  and  the  liquid  expelled  from 
it  has  been  taken  up  by  the  syringe,  it  should  again  be  examined 
under  a  lens  for  air-bubbles ;  and  if  any  of  these  troublesome 
intruders  should  present  themselves  beneath  the  cover,  the  slide 
should  be  inclined,  so  as  to  cause  them  to  rise  towards  the  highest 
part  of  its  circumference,  and  the  cover  slipped  away  from  that  part,  so 
as  to  admit  of  the  introduction  of  a  little  additional  fluid  by  the  pipette 
or  syringe  ;  and  when  this  has  taken  the  place  of  the  air-bubbles  the 
cover  may  be  slipped  back  into  its  place.  The  surface  of  the  ring  and 
the  edge  of  the  cover  must  then  be  thoroughly  dried  with  blotting- 
paper,  care  being  taken  that  the  fluid  be  not  drawn  away  from 
between  the  cover  and  the  edge  of  the  cell  on  which  it  rests.  These 
minutiae  having  been  attended  to,  the  closure  of  the  cell  may  be  at 
once  effected  by  carrying  a  thin  layer  of  gold-size  or  dammar  around 
and  upon  the  edge  of  the  glass  cover,  taking  care  that  it  touches 
every  point  of  it,  and  fills  the  angular  channel  which  is  left  along  its 
margin.  The  Author  has  found  it  advantageous,  however,  to  delay 
closing  the  cell  for  some  little  time  after  the  superfluous  fluid  has 
been  drawn  off;  for  as  soon  as  evaporation  from  beneath  the  edge 
of  the  cover  begins  to  diminish  the  quantity  of  fluid  in  the  cell,  air- 
bubbles  often  begin  to  make  their  appearance  which  were  previously 
hidden  in  the  recesses  of  the  object;  and  in  the  course  of  half  an 
hour  a  considerable  number  are  often  collected.  The  cover  should 
then  be  slipped  aside,  fresh  fluid  introduced,  the  air-bubbles  removed, 
and  the  cover  put  on  again ;  and  this  operation  should  be  repeated 
until  it  fails  to  draw  forth  any  more  air-bubbles.  It  will  of  course 
be  observed  that  if  the  evaporation  of  fluid  should  proceed  far  air- 
bubbles  will  enter  beneath  the  cover  ;  but  these  will  show  themselves 
on  the  surface  of  the  fluid,  whereas  those  which  arise  from  the 
object  itself  are  found  in  the  deeper  parts  of  the  cell.  When  all  thesa 

ii  2 


484    PREPARATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 

have  been  successfully  disposed  of,  the  cell  may  be  '  sealed '  and 
'  ringed  '  in  the  manner  already  described. 

Preparation  of  Soft  Tissues, — It  is  impossible  in  the  limited 
space  at  disposal  here  to  do  more  than  give  a  sketch  of  the  very 
elaborate  art  of  histological  preparation.  The  reader  who  desires  to 
pursue  the  subject  further  will  find  all  necessary  information  in  Mr. 
A.  Bolles  Lee's  '  The  Microtomist's  Vade-mecum '  (London  :  J.  &  A. 
Churchill),  from  which  work  the  information  here  given  is  for  the 
most  part  abridged  (the  passages  in  quotation  marks  in  the  following 
pages  are  taken  therefrom  verbatim). 

Fixation. — '  The  first  thing  to  be  done  with  any  structure  is  to 
fix  its  histological  elements.  Two  things  are  implied  by  the  word 
*  fixing : '  first,  the  rapid  killing  of  the  element,  so  that  it  may  not 
have  time  to  change  the  form  it  had  during  life,  but  is  fixed  in 
death  in  the  attitude  it  normally  had  during  life ;  and  second,  the 
hardening  of  it  to  such  a  degree  as  may  enable  it  to  resist  without 
further  change  of  form  the  action  of  the  reagents  with  which  it  may 
.subsequently  be  treated.'  For  instance,  if  you  were  to  take  a  living 
rotifer  and  throw  it  into  one  of  the  usual  staining  fluids  or  preser- 
vative liquids,  it  would  at  once  contract  into  a  shapeless  mass,  the 
•elements  of  its  tissues  would  be  neither  properly  stained  nor  properly 
preserved,  and  the  result  would  be  an  unrecognisable  caricature  of 
the  living  organism.  But  if  it  be  first  properly  killed  and  slightly 
hardened  in  the  proper  manner,  it  may  be  permanently  mounted  in 
.such  a  way  as  to  show,  uninjured  and  undistorted,  even  the  most 
delicate  details  of  its  structure. 

Fixation  is  generally  performed  by  immersing  the  object  to  be 
fixed  in  an  appropriate  liquid,  and  leaving  it  therein  until  the 
•desired  degree  of  hardening  has  been  obtained.  After  that  the 
•object  is  well  washed  to  remove  all  excess  of  the  fixing  liquid.  The 
object  may  then  be  further  prepared  by  the  wet  method,  in  which 
all  subsequent  operations  are  performed  by  means  of  aqueous  media. 
It  may  be  mounted  at  once  in  an  aqueous  mounting  medium,  or  it 
may  be  stained  (see  below),  or  it  may  be  put  away  till  wanted,  with- 
out mounting,  in  some  preservative  medium. 

Or  '  the  object  may  be  further  prepared  by  the  dehydration 
method '  (see  below),  *  which  consists  in  treatment  with  successive 
alcohols  of  gradually  increasing  strength,  final  dehydration  with 
absolute  alcohol,  clearing '  (see  below)  '  with  an  essential  oil  or  other 
clearing  agent,  and  lastly  either  mounting  in  balsam  or  imbedding 
in  paraffin  for  the  purpose  of  making  sections/ 

Corrosive  sublimate  is  the  fixing  agent  that  is  most  to  be  recom- 
mended for  general  work.  A  good  formula  consists  of  a  saturated 
.solution  in  water  containing  1  per  cent,  of  acetic  acid.  The  present 
writer  adds  a  little  nitric  acid,  say  1  per  cent.,  which  helps  to 
make  the  solution  keep  without  precipitating.  Another  good  solu- 
tion is  a  saturated  solution  in  alcohol  of  50  per  cent.,  or  even  70  per 
•cent.,  also  with  addition  of  1  per  cent,  of  acetic  acid. 

Whatever  solution  is  taken,  the  objects  should  be  removed  from 
it  soon  after  they  have  become  thoroughly  penetrated  by  it.  For 
.sublimate  hardens  very  rapidly,  and  makes  tissues  brittle  if  they  are 


PICRIC   AND    OSMIC   ACIDS  485: 

allowed  to  remain  too  long  in  it.  The  objects  should  be  well  washed 
out,  after  fixing,  with  alcohol,  beginning  with  alcohol  of  50  per  cent, 
or  70  per  cent.,  and  passing  gradually  to  stronger  alcohols.  In  order 
to  facilitate  the  removal  of  the  sublimate  from  the  tissues,  the 
alcohol  should  have  added  to  it  enough  tincture  of  iodine  to  make  it 
of  a  good  port- wine  colour,  and  the  objects  should  remain  in  it  till 
they  themselves  have  acquired  the  same  colour.  They  may  then  be 
washed  with  pure  alcohol,  and  further  treated  as  desired. 

Solutions  of  sublimate,  or  the  objects  in  them,  must  never  be 
touched  with  steel  implements,  as  these  produce  at  once  precipitates 
that  may  injure  the  preparations.  To  manipulate  the  objects,  wood 
or  glass  implements  may  be  emplctyed  ;  for  dissecting  them,  hedge- 
hog spines,  or  quill  pens,  or  cactus  needles. 

Tissues  become  of  an  opaque  whiteness  on  fixation  with  sublimate, 
which  in  the  case  of  small  transparent  objects  is  a  good  guide  for 
controlling  the  duration  of  the  fixing  bath.  The  fixing  action  is 
extremely  rapid. 

Picric  acid  is  a  reagent  that  gives  very  fair  results  for  general 
work,  and  is  especially  to  be  recommended  where  great  power  of 
penetration  is  required,  as  is  the  case  in  work  with  chitinous 
organisms.  A  saturated  solution  in  water  with  the  addition  of 
1  per  cent,  of  acetic  acid  may  be  taken,  or  the  picro-nitric  acid  of 
Mayer.  This  consists  of  water  100  parts,  nitric  acid  of  25  per  cent. 
N2O.5,  5  parts,  and  picric  acid  to  saturation. 

Objects  should  remain  in  these  liquids  much  longer  than  in  sub- 
limate liquids ;  for  though  the  penetration  is  extremely  rapid  the 
hardening  power  is  slight.  They  may  remain  for  twenty-four  hours 
without  hurt,  but  in  many  cases  three  or  four  hours  will  sufiice.  After 
fixation  the  objects  should  be  brought  into  alcohol  of  70  per  cent, 
(never  water),  in  which  they  should  remain  for  a  few  hours,  and 
then  be  transferred  to  alcohol  of  90  per  cent.,  in  which  they  should 
remain,  the  alcohol  being  frequently  changed  for  fresh,  until  the 
yellow  tint  of  the  picric  acid  has  disappeared  or  at  least  become 
greatly  attenuated.  Objects  prepared  in  this  way  are  best  stained 
in  alcoholic  staining  solutions. 

Mixtures  of  picric  acid  solution  with  sublimate  in  various  pro- 
portions have  lately  been  much  used,  with  good  results. 

Osmic  acid  is  a  useful  reagent  for  fixing  small  objects.  It  pre- 
serves the  forms  of  cells  admirably,  and  at  the  same  time  imparts  to 
tissues  a  grey  stain  that  is  frequently  of  the  greatest  value  in  bring- 
ing out  delicate  structures.  This  substance  is  sold  in  the  solid  state, 
in  sealed  tubes  containing  from  ^  grm.  to  1  grm.  It  is  extremely 
volatile.  Care  should  be  taken  to  avoid  exposure  to  the  vapours  given 
oft'  from  it,  as  they  are  exceedingly  irritating  to  mucous  mem- 
branes and  may  easily  give  rise  to  serious  catarrh,  conjunctivitis,  &c. 
Its  solution  in  pure  water  keeps  very  badly,  as  the  slightest  con- 
tamination with  any  organic  dust  will  cause  it  to  reduce  and  precipi- 
tate. It  is  recommended,  therefore,  that  only  a  small  quantity  be 
kept  in  stock  in  the  shape  of  aqueous  solution,  whilst  another 
quantity  may  be  preserved  in  the  shape  of  a  2  per  cent,  solution  in 
chromic  acid  of  1  per  cent.,  or,  better,  in  platinic  chloride  of  the 


486    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

same  strength.  These  solutions  do  not  precipitate  so  readily,  and: 
may  be  used  for  fixation  by  the  vapours. 

For  it  is  one  of  the  advantages  of  osmic  acid  that  it  may  be 
employed  for  fixation  in  the  form  of  vapour,  and  its  employment  in 
this  form  is  indicated  in  most  of  the  cases  in  which  it  is  possible  to 
expose  the  tissues  to  be  fixed  directly  to  the  action  of  the  vapour. 
For  fixation  in  this  way  '  the  tissues  are  pinned  out  on  a  cork  which 
must  fit  well  into  a  wide  mouthed  bottle  in  which  is  contained  a 
little  solid  osmic  acid  (or  a  small  quantity  of  1  per  cent,  solution  will 
do).  Very  small  objects,  such  as  isolated  cells,  are  simply  placed  on 
a  slide,  which  is  inverted  over  the  mouth  of  the  bottle.  They 
remain  there  until  they  begin  to  turn  brown  (isolated  cells  will 
generally  be  found  to  be  sufficiently  fixed  in  thirty  seconds,  whilst 
in  order  to  fix  the  deeper  layers  of  relatively  thick  objects,  such  as 
retina,  an  exposure  of  several  hours  may  be  desirable).  It  is  well  to 
wash  the  objects  with  water  before  staining,  but  a  very  slight  wash- 
ing will  suffice.  For  staining,  methyl-green  may  be  recommended 
for  objects  destined  for  study  in  an  aqueous  medium,  and,  for  per- 
manent preparations,  alum-carmine,  picro-carmine,  or  hsematoxylin/ 

*  The  reasons  for  preferring  the  process  of  fixation  by  vapour  of 
osmium,  where  practicable,  are  that  osmium  is  more  highly  penetra- 
ting when  employed  in  this  shape  than  when  employed  in  solution, 
and  produces  a  more  equal  fixation,  and  that  the  arduous  washing 
out  required  by  the  solutions  is  here  done  away  with.  In  many 
cases  delicate  structures  are  better  preserved,  all  possibility  of 
deformation  through  osmosis  being  here  eliminated/  (From  Mr.  Lee's 
*  The  Microtomist's  Vade-mecum.') 

For  fixation  by  solutions,  strengths  of  from  ^  to  ^  per  cent,  may 
be  taken,  which  may  in  general  with  advantage  be  acidified  with 
about  1  per  cent,  of  acetic  acid.  Small  Crustacea.,  such  as  the 
copepods  and  the  larvae  of  decapods,  may  be  very  well  prepared  in 
this  way.  After  fixation,  the  osmic  acid  should  be  very  thoroughly 
washed  out  with  water. 

If  it  be  desired  to  intensify  the  grey  stain  of  the  osmium,  this 
may  be  easily  done  by  putting  the  objects  into  a  weak  solution  of 
pyrogallic  acid  or  tannin,  which  will  turn  them  of  a  fine  black. 

Osmic  acid  stains  most  fatty  substances  of  an  intense  black. 

Osmic  acid  is  now  not  so  much  used  in  the  form  of  a  pure 
aqueous  solution  as  in  that  of  the  mixture  known  as  liquid  of  Flem- 
ming.  This  consists  of  25  parts  of  1  per  cent,  solution  of  chromic 
acid,  10  parts  of  1  per  cent,  osmic  acid,  10  parts  of  1  per  cent,  acetic 
acid,  and  55  of  water.  This  mixture  blackens  tissues  much  less  than 
the  pure  aqueous  solution.1 

1  Bleaching.  —Tissues  that  have  been  blackened  or  browned  by  osmic  or  chromic 
acid  or  the  like  may  often  with  advantage  be  bleached  by  Mayer's  chlorine  method, 
and  will  then  be  found  to  stain  much  more  readily. — '  Put  into  a  glass  tube  a  few 
crystals  of  chlorate  of  potash,  add  two  or  three  drops  of  hydrochloric  acid,  and  as 
soon  as  the  green  colour  of  the  evolving  chlorine  has  begun  to  show  itself,  add  a  few 
cubic  centimetres  of  alcohol  of  50  to  70  per  cent.  Now  put  the  objects  (which  must 
have  previously  been  soaked  in  alcohol  of  70  to  90  per  cent.)  into  the  tube.  They 
float  at  first,  but  eventually  sink.  They  will  be  found  bleached  in  from  a 
quarter  of  an  hour  to  one  or  two  days,  without  the  tissues  having  suffered. 
Only  in  obstinate  cases  should  the  liquid  be  warmed  or  more  acid  taken. 


CLEARING-  487 

For  the  very  numerous  other  fixing  reagents  and  mixtures  now 
in  use,  and  the  manner  of  their  employment,  the  reader  must  be 
referred  to  Mr.  Lee's  '  The  Microtomist's  Vade-mecum.' 

After  due  fixation  and  washing,  objects  may  be  stained  and 
mounted  in  an  aqueous  medium  in  the  manner  directed  above 
(p.  481),  if  it  be  desired  to  prepare  them  in  the  wet  way.  But  if 
they  are  destined  to  be  preserved  in  balsam,  they  must  first,  after 
staining  if  required,  be  dehydrated  and  cleared. 

Dehydration  is  performed  as  follows  : — '  The  objects  are  brought 
into  weak  alcohol,  and  are  then  passed  through  successive  alcohols 
of  gradually  increased  strength,  remaining  in  each  the  time  neces- 
sary for  complete  saturation,  and  ^he  last  bath  consisting  of  absolute 
or  at  least  very  strong  alcoliol.'  'For  instance,  alcohol  first  of  30  per 
cent,  or  50  per  cent.,  then  70  per  cent.,  then  95  per  cent.,  or,  if  the 
objects  be  very  delicate,  80  per  cent.,  before  the  95  per  cent.,  the 
last  to  be  changed  at  least  once. 

Clearing. — '  The  water  having  been  thus  sufficiently  removed,  the 
alcohol  is  in  its  turn  removed  from  the  tissues,  and  its  place  taken 
by  some  anhydrous  substance,  generally  an  essential  oil,  which  is 
miscible  with  the  material  used  for  imbedding.  This  operation  is 
known  as  clearing.  It  is  very  important  that  the  passage  from  the 
last  alcohol  to  the  clearing  agent  be  made  gradual.  This  is  effected 
by  placing  the  clearing  medium  under  the  alcohol.  A  sufficient 
quantity  of  alcohol  is  placed  in  a  tube  (a  watch-glass  will  do,  but 
tubes  are  generally  better),  and  then  with  a  pipette  a  sufficient 
quantity  of  clearing  medium  is  introduced  at  the  bottom  of  the 
alcohol.  Or  you  may  first  put  the  clearing  medium  into  the  tube, 
and  then  carefully  pour  the  alcohol  on  to  the  top  of  it.  The  two 
fluids  mingle  but  slowly.  The  objects  to  be  cleared,  being  now 
quietly  put  into  the  supernatant  alcohol,  float  at  the  surface  of 
separation  of  the  two  fluids,  the  exchange  of  fluids  takes  place 
gradually,  and  the  objects  slowly  sink  down  into  the  lower  layer. 
When  they  have  sunk  to  the  bottom  (and  the  wavy  refraction-lines 
at  first  visible  round  them  have  disappeared)  the  alcohol  may  be 
drawn  off  with  a  pipette,  and  the  objects  will  be  found  to  be  com- 
pletely penetrated  by  the  clearing  medium.  (It  may  be  noted  here 
that  this  method  of  making  the  passage  from  one  fluid  to  another 
applies  to  all  cases  in  which  objects  have  to  be  transferred  from  a 
lighter  to  a  denser  fluid — for  instance,  from  alcoliol  or  from  water 
to  glycerine.)'  From  '  The  Microtomist's  Vade-mecum.' 

Another  method  of  passing  the  objects  from  the  alcohol  to  the 
clearing  agent  consists  in  giving  them  baths  of  mixtures  of  the 
alcohol  and  the  clearer,  made  gradually  to  contain  a  higher  propor- 
tion of  the  latter. 

All  clearing  agents  are  liquids  of  high  refraction,  having  indices 
of  refraction  not  greatly  inferior  to  that  of  the  elements  of  tissues 

Sections  on  slides  may  be  bleached  in  this  way.  Instead  of  hydrochloric  acid,  nitric 
acid  may  be  taken;  in  which  case  the  active  agent  is  evolved  oxygen  instead  of 
chlorine.  This  method  serves  also  for  removing  natural  pigments,  such  as  those  of 
the  skin,  or  of  the  eyes  of  Arthropods.  For  bleaching  chitin  of  insects,  not  alcohol 
but  water  should  be  added  to  the  chlorate  and  acid.'  (From  '  The  Microtomist's 
Vade-mecum.') 


488    PREPAKATION,   MOUNTING,   AND    COLLECTION   OF   OBJECTS 

in  the  fixed  state.  Hence,  by  penetrating  amongst  these  highly 
refractive  elements,  they  render  the  tissues  transparent  arid  clear,, 
which  is  the  reason  of  their  being  called  ;  clearing  agents.' 

The  best  clearing  agent  for  general  use  is  oil  of  cedar  wood.  Oil 
of  cloves  is  a  very  good  one ;  it  should  be  known  that  it  make& 
objects  brittle,  which  is  sometimes  to  be  desired,  sometimes  the 
reverse.  Oil  of  bergamot  is  useful ;  it  will  clear  from  alcohol  of  no 
more  than  90  per  cent,  strength. 

It  should  be  noted  that  the  proper  stage  for  performing  minute 
dissections  in  is  the  one  at  which  the  objects  have  now  arrived,  a 
drop  of  clearing  agent  being  a  most  helpful  medium  for  carrying 
out  such  dissections  in.  Oil  of  cedar  is  very  good  for  this  purpose. 
But  oil  of  cloves  is  sometimes  to  be  preferred,  not  only  on  account  of 
its  property  of  making  tissues  brittle,  which  is  often  very  helpful, 
but  also  on  account  of  the  property  it  has  of  forming  very  convex 
drops  on  the  slide. 

Staining. — Good  histological  stains  can  in  general  only  be 
obtained  with  properly  fixed  tissues.  But  it  is  possible  to  obtain  with 
unfixed  and  even  with  living  tissues  a  stain  which  though  imperfect 
and  not  '  fast '  may  be  of  considerable  utility  in  research,  either  as  a 
means  of  controlling  the  results  obtained  by  the  examination  of  fixed 
and  prepared  specimens,  or  as  a  means  of  revealing  delicate  traits  of 
structure  that  may  be  masked  or  destroyed  by  the  action  of  fixing 
and  preserving  reagents,  and  only  visible  in  the  living  or  perfectly 
fresh  object. 

It  goes  without  saying  that  staining  is  performed  by  immersing 
the  tissues  in  the  colouring  solution  employed.  After  the  tissue  has 
become  duly  stained,  all  superfluous  colour  is  removed  from  it  by 
'  washing  out '  with  an  appropriate  liquid. 

Stains  for  Living  Objects  (Intra  Vitam  Stains). — The  most 
widely  used  of  these  stains  is  methylen-blue  (to  be  obtained  from 
Griibler  and  Hollborn,1  and  not  to  be  confounded  with  methyl-blue, 
which  is  a  totally  different  dye).  Small  aquatic  organisms  (such  as- 
rotifers,  infusoria,  small  annelids,  tadpoles)  are  stained  by  adding 
a  small  quantity  of  the  dye  (best  previously  dissolved  in  distilled 
water)  to  the  water  in  which  they  are  kept,  and  leaving  them  till 
the  stain  has  taken  effect.  Enough  of  the  dye  should  be  added  to- 
make  the  water  of  a  good  blue,  the  proportion  required  varying 
roughly  between  1  part  of  the  dye  to  10,000  of  the  water,  and 
1  part  to  100,000.  Most  aquatic  organisms  will  live  in  the 
coloured  water  for  many  hours,  some  for  days  or  weeks.  They 
should  be  examined  as  soon  as  the  required  intensity  of  stain  has 
been  attained.  For  if  they  are  allowed  to  remain  longer  the 
elements  that  have  taken  up  the  dye  will  begin  to  yield  it  up  again 
to  the  water,  and  the  objects  may  become  quite  pale  again  even 
though  they  have  riot  been  removed  from  the  coloured  water.  The 
stain  is  an  imperfect  one,  being  mostly  confined  to  certain  granules 
of  the  protoplasm  of  cells,  and  taking  effect  capriciously  now  on  one 
tissue  and  now  on  another.  It  is  difficult  to  preserve  the  stain  in  a 

1  63  Bayerische  Strasse,  Leipzig ;  or  through  Mr.  C.  Baker,  243  High  Holborn. 


STAINS   FOR   UNFIXED   TISSUES  489- 

satisfactory  manner,  as  it  will  not  bear  mounting  in  the  usual  media 
without  deterioration. 

Weak  solutions  of  Bismarck  brown,  quinole  in-blue,  anilin-llacl •, 
Congo  red,  and  neutral  red  (Neutralroth)  may  be  used  in  the  same 
way. 

Methylen-blue,  used  as  an  intra  vitam  stain,  is  an  important 
reagent  for  the  study  of  nerve-endings.  For  the  details  of  this  very 
difficult  branch  of  technique,  as  well  as  for  the  methods  for  preserv- 
ing the  stain  obtained  with  entire  living  organisms,  the  reader  must 
be  referred  to  Mr.  A.  Bolles  Lee's  '  The  Microtomist's  Vade-mecum,' 
in  which  an  entire  chapter  is  devoted  to  the  subject. 

Stains  for  Fresh  (Unfixed)  Tissues  or  Organisms. — The  stains  to- 
be  mentioned  under  this  heading  resemble  the  intra  vitam  stains 
described  in  the  last  paragraph  in  that  they  may  be  applied  to  living 
tissues  or  organisms.  But  they  differ  from  them  in  that  they  do- 
not  take  effect  on  the  objects  without  impairing  their  vitality ;  on  the 
contrary  they  first  kill  them,  then  stain  them. 

The  most  important  of  this  class  of  stains  is  methyl-green.  A 
strong  solution  in  water  acidified  with  from  ^  to  1  per  cent, 
of  acetic  acid  is  employed.  The  objects  are  soaked  in  the  solution 
until  they  are  penetrated  by  it,  then  washed  with  pure  water,  or,, 
better,  acidified  water,  and  either  studied  therein  or  mounted.  They 
in; iv  be  permanently  preserved  in  any  of  the  usual  aqueous  mount- 
ing media,  provided  that  the  medium  be  acid  or  at  most  strictly 
neutral,  and  that  it  contain  a  little  of  the  dye  in  solution.  Liquid 
of  Ripart  and  Petit,  or  Brun's  glucose  medium  may  be  recommended 
for  mounting.  It  is  difficult  to  mount  the  stained  objects  in  balsam,, 
on  account  of  the  great  solubility  of  the  dye  in  alcohol. 

The  stain  is  an  extremely  rapid  one ;  tissues  are  stained  almost 
as  soon  as  they  are  penetrated  by  it.  It  is,  generally  speaking,  a 
nuclear  stain,  nuclei  being  stained  more  rapidly  than  cytoplasm,, 
though  some  kinds  of  cytoplasm  and  formed  material  are  stained  by 
it.  It  preserves  the  forms  of  cells  well.  It  does  not  overstain,. 
and  requires  little  washing  out.  This,  if  required,  is  best  done  with 
water  acidified  with  acetic  acid. 

Bismarck  brown  is  also  a  useful  stain  for  fresh  tissues.  It  may 
be  used  in  solution  in  acidified  water,  as  directed  for  methyl -green. 
But  as  the  dye  is  not  very  soluble  in  wrater  it  is  not  easy  to  get  a 
good  solution  in  this  way,  and  the  solutions  when  made  keep  very 
badly.  Some  persons  dissolve  the  dye  in  dilute  glycerin  (glycerin 
diluted  with  one  or  two  volumes  of  water).  This  makes  a  good  solu- 
tion, but  on  account  of  the  shrinking  action  of  the  glycerin  should 
only  be  employed  with  objects  that  have  been  previously  well  fixed. 
Bismarck  brown  stains  quickly,  and  does  not  overstain.  The  stain 
is  permanent  both  in  aqueous  mounting  media  and  in  balsam.  It  is 
a  nuclear  stain  in  so  far  as  nuclei  are  stained  by  it  more  than  proto- 
plasm. 

The  once  celebrated  mixture  known  as  Ranvier's  picro -carmine 
is  irrational  in  composition,  and  inconstant  and  frequently  injurious- 
in  its  effects,  and  is  now  generally  abandoned. 


490    PKEPAKATION,    MOUNTING-,   AND    COLLECTION   OF   OBJECTS 

Stains  for  Fixed  and  Preserved  Entire  Objects  or  Material  to 
be  Stained  in  Bulk. — These  fall  naturally  into  the  two  classes  of 
aqueous  stains  and  alcoholic  stains.  The  aqueous  stains  are 
generally  the  more  precise,  and  are  generally  preferable  for  small 
and  permeable  objects,  but  the  alcoholic  stains  are  absolutely' 
necessary  where  great  penetration  is  required,  as  for  instance  in  the 
case  of  organs  or  organisms  enclosed  in  thick  chitinous  investments, 
as  is  so  generally  the  case  amongst  the  Arthropoda. 

The  most  precise  and  the  safest  of  the  stains  of  this  class  are  the 
alum-carmines — a  general  term  including  the  divers  formulae  that 
have  been  recommended  under  the  names  of  alum-carmine, 
carmalum,  alum- cochineal.  One  of  these  will  suffice. 

Partschs  alum-cochineal. — '  Powdered  cochineal  is  boiled  for  some 
time  in  a  5  per  cent,  solution  of  alum,  the  decoction  filtered,  and  a 
little  salicylic  acid  added  to  preserve  it  from  mould.' 

An  extremely  precise  nuclear  stain,  and  one  with  which  it  is  hardly 
possible  to  overstain.  It  is  permanent  in  balsam  and,  it  is  believed, 
in  aqueous  media  if  not  acid.  Objects  may  be  left  in  it  for  several 
hours.  They  should  not  be  very  large,  as  the  stain  has  no  great 
power  of  penetration.  Objects  containing  calcareous  elements  that 
it  is  desired  to  preserve  must  not  be  treated  with  this  stain,  nor 
with  any  other  stain  containing  alum. 

Mayers  carmalum  is  made  with  carminic  acid  1  grin.,  alum 
10  grm.,  and  distilled  water  200  c.c.  It  has  the  advantage  of  being 
much  more  penetrating  than  the  other  stains  of  this  class. 

All  the  alum-carmine  solutions  are  rather  weak  stains.  If  a 
more  powerful  stain  be  desired,  take  the  following  : — 

Mayer's  hwmalum. — This  is  made  with  hsematein,  the  essential 
colouring  principle  of  hrematoxylin  (obtainable  from  0  rubier  and 
Hollborn).  One  grm.  of  haematein  is  either  dissolved  with  heat 
in  50  c.c.  of  90  per  cent,  alcohol,  or  rubbed  up  in  a  mortar  with  a 
little  glycerin,  and  added  to  a  solution  of  50  grm.  of  alum  in  a  litre 
of  water.  This  liquid  may  be  used  for  staining  either  concentrated 
or  diluted.  Concentrated  it  stains  almost  instantaneously.  For 
ordinary  purposes  it  may  be  diluted  with  from  ten  to  twenty 
volumes  of  distilled  water,  and  will  then  stain  through  small  objects 
in  an  hour  or  so.  Large  objects  will  require  an  hour  or  more.  The 
solution  is  admirable  for  staining  in  bulk.'  Objects  should  be  well 
washed  out  (for  as  long  a  time  as  they  have  taken  to  stain)  either 
with  distilled  water  or  tap  water.  One  per  cent,  alum  solution  is 
also  a  good  medium  to  wash  out  in.  Overstains  may  be  corrected 
by  washing-out  with  O'l  to  0'5  per  cent,  of  hydrochloric  acid.  In 
.this  case  the  acid  should  be  neutralised  afterwards  by  treatment  with 
O'l  per  cent,  solution  of  bicarbonate  of  soda  (or  other  weak  alkali). 

Passing  now  to  the  alcoholic  solutions,  Grenacher's  alcoholic  borax- 
carmine  may  be  recommended  as  affording  a  convenient,  safe,  and 
brilliant  stain.  Dissolve  2  or  3  per  cent,  of  carmine  in  a  4  per  cent, 
solution  of  borax  in  water  ;  boil  the  solution  for  half  an  hour  ; 
-dilute  it  with  an  equal  volume  of  70  per  cent,  alcohol,  allow  it  to 
for  twenty-four  hours,  and  filter. 

Objects  are  put  into  this  solution  and  allowed  to  remain  in  it 


STAINING  ENTIRE    OBJECTS  491 

until  they  are  thoroughly  penetrated  (for  days  if  necessary).  They 
are  then  put  into  alcohol  of  70  per  cent,  acidified  with  from  four  to 
six  drops  of  hydrochloric  acid  for  every  100  c.c.  of  the  alcohol.  The 
acid  alcohol  at  once  begins  to  remove  the  excess  of  colour  from  the 
objects,  which  may  be  seen  to  give  it  off  in  rosy  clouds.  They 
remain  in  it  until  the  colour  no  longer  comes  away  freely  and  they 
have  exchanged  their  primitive  opaque  red  coloration  for  a  brilliant 
transparent  coloration.  This  may  require  days  (the  acid  alcohol 
should  be  changed  frequently). 

The  staining  is  now  complete,  and  the  objects  arc  washed  in  pure 
neutral  alcohol,  cleared  and  mounted  in  balsam  or  any  other  desired 
medium.  The  result  is  a  brilliant  nuclear  stain,  quite  permanent. 
The  process  must  not  be  used  for  objects  containing  calcareous 
elements  that  it  is  desired  to  preserve. 

For  delicate  objects,  and  for  very  impermeable  objects,  it  may  be 
well  to  increase  the  proportion  of  70  per  cent,  alcohol  in  the 
solution ;  the  proportion  of  alcohol  may  be  brought  up  to  about 
50  per  cent.,  but  should  not  exceed  60  per  cent,  in  any  case. 

This  process  is  an  example  of  what  is  known  as  regressive  or 
indirect  staining ;  the  objects  are  first  overstained  in  the  carmine 
solution,  and  the  excess  of  stain  is  then  removed  to  the  required 
•degree  in  the  acid  alcohol. 

If,  as  is  frequently  the  case,  especially  in  studies  on  the 
Arthropoda,  a  still  more  highly  alcoholisecl  stain  be  desired,  Mayers 
alcoholic  cochineal  may  be  tried.  Cochineal  in  coarse  powder  is 
macerated  for  several  days  in  70  per  cent,  alcohol.  For  each 
gramme  of  the  cochineal  there  is  required  8  to  10  c.c.  of  alcohol. 
Stir  frequently.  Filter,  and  the  solution  is  ready  for  staining. 

The  objects  to  be  stained  must  previously  be  well  imbibed  with 
70  per  cent,  alcohol.  They  may  remain  for  almost  any  length  of 
time  in  the  staining  bath.  After  staining  they  are  washed  in 
70  per  cent,  alcohol,  which  is  frequently  changed  until  it  takes  up  no 
more  colour  from  the  objects.  Overstating  seldom  happens  :  it 
may  be  corrected  by  means  of  70  per  cent,  alcohol  containing  1  per 
<3ent.  of  acetic  acid  or  T^  per  cent,  of  hydrochloric  acid. 

Small  objects  or  thin  sections  are  stained  in  a  few  minutes  ;  large 
objects  require  hcurs  or  days ;  a  nuclear  stain,  either  red  or  blue, 
according  to  the  chemical  composition  of  the  tissues  stained.  It 
does  not  succeed  with  all  objects.  The  best  stains  are  obtained  with 
objects  that  have  been  prepared  with  chromic  or  picric  acid 
combinations,  or  with  absolute  alcohol.  Osmic  acid  preparations 
stain  very  weakly  unless  they  have  been  previously  bleached.  All 
acids  should  be  carefully  washed  out  of  the  objects  before  staining. 
The  stain  is  permanent  in  oil  of  cloves  and  balsam. 

Kleinenberg1  s  Alcoholic,  Hftimatoxylin,  once  very  much  vised,  is 
highly  irrational  and  very  inconstant  in  its  composition  and  its 
effects,  and  is  now  with  reason  generally  abandoned. 

Nuclear  Stains  for  Sections. — Any  of  the  foregoing  stains  may 
of  course  be  used  for  sections  if  desired.  But  in  many  cases  other 
stains  are  indicated,  as  being  more  powerful,  or  more  precise,  or  of 
a  richer  selectivity. 


492    PREPARATION,   MOUNTING,   AND    COLLECTION   OF   OBJECTS 

The  solution  known  as  Kernschwarz  may  be  confidently  recom- 
mended as  a  powerful,  precise,  and  very  safe  stain.  It  is  a  black 
liquid  imported  from  Russia  by  Grubler  and  Hollborn,  and  consists, 
of  an  iron  base  united  to  some  gallic  acid.  Sections  may  be  stained 
in  it,  either  concentrated  or  diluted  to  the  required  intensity. 
Overstaining  seldom  occurs.  If  it  should  occur  it  may  be  corrected 
by  means  of  any  weak  acid  (solution  of  liquor  ferri  sidfarici  oxydati, 
diluted  twentyfold — see  the  iron-haematoxylin  of  Benda,  below — is  a 
very  fitting  decolorant). 

The  result  is  a  nuclear  stain,  sometimes,  though  by  no  means 
always,  also  taking  effect  on  protoplasm,  of  a  brownish  grey  or 
black,  powerful  and  precise,  and  well  adapted  for  photography.  It  is 
permanent  in  balsam,  presumably  also  in  aqueous  mounting  media. 
Being  a  progressive  stain,  it  is  possible  that  it  might  give  good 
results  for  staining  in  bulk. 

The  present  writer  obtains  a  very  similar  stain  by  '  mordanting " 
for  a  few  hours  in  Benda's  liquor  ferri,  and  then  bringing  the 
sections  directly  for  some  hours  into  a  2  per  cent,  solution  of 
pyrogallol  in  water.  Similar  results  are  also  obtained  by  mordanting 
in  2  per  cent,  solution  of  tincture  of  perchloride  of  iron  in  70  per 
cent,  alcohol,  and  then  treating  with  2  per  cent,  solution  of  pyro- 
gallol in  spirit :  a  process  which  is  applicable  to  staining  in  bulk. 

Benda's  iron  hcematoxylin  is  a  still  more  powerful  and  precise 
stain.  Sections  of  material  that  has  been  fixed  in  any  way  may  be 
employed.  They  are  '  mordanted '  by  soaking  for  half  an  hour  or 
for  some  hours  (as  much  as  twenty-four,  if  a  very  strong  stain  be 
required)  in  liquor  ferri  sulfurici  oxydati,  P.G.,  diluted  with  one  or 
two  volumes  of  water.1  They  are  then  well  washed,  first  with 
distilled  water,  then  with  tap  water,  and  are  brought  into  a  1  per 
cent,  solution  of  heematoxylin  in  water,  in  which  they  remain  till 
they  have  become  thoroughly  black.  They  are  now  overstained, 
and  must  be  '  differentiated.'  To  this  end  they  are  washed  and  put 
either  into  some  of  the  sulphate  solution  strongly  diluted  with  water 
(say  twenty  or  thirty  fold),  or  into  30  per  cent,  acetic  acid,  the 
progress  of  the  decoloration  being  followed  in  either  case  under  the 
microscope.  They  are  then  mounted  in  the  usual  way. 

This  gives  an  extremely  powerful  blue-black  stain,  purely  nuclear 
if  the  differentiation  has  been  pushed  far  enough,  or  nuclear  and  at 
the  same  time  plasmatic  if  the  differentiation  is  stopped  before  the 
protoplasm  has  become  decoloured.  The  stain  is  absolutely 
permanent  in  balsam. 

The  results  obtained  by  this  process  are  practically  identical  with 
those  obtained  by  the  iron  hcematoxylin  process  of  Heidenhain,  with 
this  advantage,  that  Benda's  iron  solution  is  easily  made  and  keeps 
indefinitely,  whereas  Heidenhain's  process  involves  the  employment 

1  This  preparation  consists  of  sulphate  of  iron  80  parts,  water  40,  sulphuric  acid 
15,  and  nitric  acid  18.  The  ingredients  should  be  mixed,  and  give  at  first  a  black 
liquid  which  gradually  acquires  a  red  colour.  The  operation  should  be  performed 
out  of  doors,  or  in  a  chemical  laboratory,  as  during  the  process  of  solution  voluminous 
nitrous  vapours  are  given  off,  which  would  be  hurtful  to  lenses  and  delicate  instru- 
ments. 


EEGKESSIVE  STAINING  493 

of  ferric  alum,  which  can  only  be  obtained  from  large  chemical 
works,  and  does  not  keep  well  either  in  substance  or  in  solution. 

Owing  to  the  precision  and  depth  of  the  stain,  preparations 
made  by  this  process  will  bear  study  with  higher  microscopic 
powers  than  those  made  by  any  other  means  ;  that  is  to  say,  it  is 
certainly  found  in  practice  that  they  will  bear  notably  higher 
eye-piecing. 

It  will  be  observed  that,  as  with  borax-carmine,  this  is  a 
'  regressive '  stain.  The  progress  of  decoloration,  being  slow,  may 
be  controlled  under  the  microscope,  and  a  little  practice  with  this 
process  may  serve  as  an  introduction  to  the  art  of  regressive 
staining  with  safranin  and  other  tar-colours,  with  which  the 
progress  of  decoloration  is  so  rapid  that  it  cannot  be  controlled 
under  the  microscope. 

Safranin  is  perhaps  the  most  beautiful  stain  of  this  class.  The 
first  requisite  to  success  in  staining  with  this  colour  is  to  obtain  a 
good  sample  of  the  dye.  This  is  absolutely  essential.  There  are  at 
least  a  score  of  brands  of  safranin  on  the  market,  many  of  which 
cannot  be  made  to  afford  a  good  stain  by  any  means  whatever.  The 
brand  '  Safranin  O  '  supplied  by  Griibler  and  Hollborn  is  an  excellent 
one. 

The  dye  is  employed  in  the  form  of  a  saturated  or  at  least  very 
concentrated  solution  in  water  or  alcohol.  Perhaps  the  best  plan  in 
general  is  to  make  a  saturated  solution  in  water,  and  another 
saturated  solution  in  strong  alcohol,  and  then  mix  the  two  in  equal 
parts.  Sections  are  soaked  in  the  solution  until  thoroughly  over- 
stained — the  longer  the  better.  Good  stains  can  often  be  obtained 
after  half  an  hour  in  the  staining  bath,  but  for  many  objects  it  is 
necessary,  in  order  to  ensure  good  results,  to  stain  for  twenty-four 
hours,  or  even  for  many  days. 

After  the  staining  comes  the  '  differentiation '  of  the  stain.  The 
sections  are  just  rinsed  with  water  and  brought  into  strong  alcohol, 
either  in  a  watch-glass,  if  they  be  loose  sections,  or  in  a  flat-bottomed 
tube  if  they  be  affixed  to  a  slide.  '  The  sections  in  the  watch-glass 
are  seen  to  give  up  their  colour  to  the  alcohol  in  clouds,  which  are 
at  first  very  rapidly  formed,  afterwards  more  slowly.  The  sections 
on  the  slide  are  seen,  if  the  slide  be  gently  lifted  above  the  surface 
of  the  alcohol,  to  be  giving  off  their  colour  in  the  shape  of  rivers 
running  down  the  glass.  In  a  short  time  the  formation  of  the  clouds 
or  of  the  rivers  is  seen  to  be  on  the  point  of  ceasing  ;  the  sections 
have  become  pale  and  somewhat  transparent,  and  (in  the  case  of  some 
objects)  have  changed  colour,  owing  to  the  coming  into  view  of  the 
general  ground-colour  of  the  tissues,  from  which  the  stain  has  now 
been  removed.  At  this  point  the  differentiation  is  complete,  and 
the  extraction  of  the  colour  must  be  stopped  instantly! 

This  may  be  done  if  desired  by  simply  putting  the  sections  into 
water  ;  but  the  more  usual  practice  is  to  proceed  at  once  to  mount 
them  in  balsam.  To  this  end  they  may  be  cleared  bv  being  put  into 
clove  oil  (or  by  pouring  the  oil  over  them  on  the  slide).  This  will 
extract  slowly  a  little  more  colour,  and  may  thus  serve  to  complete 
the  differentiation  in  a  frequently  very  desirable  manner.  Or  you 


494    PREPARATION,    MOUNTING,   AND    COLLECTION   OF   OBJECTS 

may  clear  or  remove  the  alcohol  with  an  agent  that  does  not  remove 
any  more  colour,  such  as  cedar  oil,  or  bergamot  oil,  or  xylol,  toluol, 
or  benzol.  This  being  done,  nothing  more  remains  but  to  add  a  drop 
of  xylol-balsam  or  dammar,  and  a  cover  (chloroform  is  best  avoided, 
either  as  a  clearer  or  as  a  menstruum  for  the  mounting  medium). 

The  result  is  a  pure  nuclear  stain,  of  exceeding  brilliancy,  and 
perfectly  permanent  in  balsam. 

The  process  is  not  available  for  staining  in  bulk,  but  besides 
sections  such  material  as  is  thin  enough  to  behave  like  a  section — 
portions  of  thin  membranes,  for  instance — may  be  stained  in  this  way. 
The  process  of  differentiation  takes  about  a  couple  of  minutes  with 
most  thin  sections,  but  in  some  cases  considerably  more  is  required. 

Besides  safranin,  many  others  of  the  coal-tar  dyes  may  be  used 
in  the  same  way  :  for  instance,  basic  fuchsin  (magenta),  also  a  red 
stain,  or  gentian  violet  or  thionin,  both  these  being  blue.  Thionin  is 
peculiarly  resistent  to  alcohol,  which  is  an  important  quality  in  some 
cases. 

Plasma  Stains,  or  Plasmatic  Stains, — All  the  stains  we  have 
hitherto  considered  (with  the  exception  of  the  infra  vitam  stains) 
have  been  nuclear  stains — that  is,  such  as-  stain  nuclei  either 
exclusively,  or  at  least  more  energetically  than  protoplasm  or  formed 
material.  In  very  many  cases  they  perform  all  that  the  histologist 
requires  in  the  way  of  rendering  structure  visible.  But  still  there 
are  other  cases  in  which  it  is  desirable  to  obtain  a  separate  stain  of 
extra-nuclear  parts.  For  this  purpose  the  so-called  plasma  stains 
are  employed. 

Picric  acid  is  a  useful  one,  especially  when  employed  after  a 
carmine  or  hsematoxylin  nuclear  stain.  The  modus  operandi  is  as 
simple  as  possible  :  it  consists  merely  in  adding  picric  acid  to  the 
alcohol  employed  for  dehydrating  the  objects,  and  leaving  them 
therein  until  the  desired  intensity  of  stain  is  obtained.  '  It  has  the 
great  quality,  shared  by  very  few  plasma  stains,  that  it  can  be  used 
for  staining  entire  objects.  And  as  it  is  extremely  penetrating,  it  is 
very  much  indicated  for  the  preparation  of  such  objects  as  small 
arthropods  or  nematodes,  mounted  whole.' 

Lyons  blue  (Bleu  de  Lyori)  is  a  good  plasma  stain  that  will  work 
well  after  carmine-  (borax-carmine  for  instance).  It  may  be  used 
for  staining  in  bulk,  in  a  very  dilute  alcoholic  solution  ;  or  for 
staining  sections,  in  a  strong  aqueous  solution.  The  objects  must 
not  remain  too  long  in  alcohol  after  staining. 

The  dye  known  as  Wasserblau  (water -blue]  gives  with  sections 
a  similar  but  perhaps  more  delicate  stain.  It  is  a  good  stain  to 
use  in  conjunction  with  safranin,  using  the  Wasserblau  first.  The 
process  is,  first,  to  stain  rather  strongly  in  a  concentrated  aqueous 
solution  of  the  blue,  and  then  for  from  half  an  hour  to  four  or  five 
hours  in  the  safranin,  as  described  above. 

Either  of  these  stains  will  probably  be  found  safer  than  indigo- 
carmine,  which  wras  once  much  employed  for  similar  purposes. 

A  still  more  precise  and  delicate  plasma  stain  is  /Sdurefuchsin 
(also  known  under  the  synonyms,  or  names  of  brands,  of  acid 
fuchsin,  Xaurerubin,  Fuchsin  S,  Rubin  8,  and  others).  It  is 


IMBEDDING  METHODS  495 

important  not  to  confound  it  with  basic  fuchsin,  as  appears  to 
have  been  done  by  some  writers.  For  staining  sections  a  \  per- 
cent, solution  in  water  may  be  employed,  and  allowed  to  act  on 
sections  for  from  one  to  five  minutes.  A  red  stain,  very  resistent  to 
alcohol  and  acids,  and  permanent  in  balsam.  It  is  an  excellent  stain 
for  use  after  a  blue  nuclear  stain,  such  as  haematoxylin,  thioninr 
gentian  violet,  or  the  like. 

The  celebrated  mixture  known  as  the  Ehrlich-Biondi-Heidenhain 
stain  involves  such  complicated  and  delicate  manipulations  as  to  be 
totally  unsuitable  for  ordinary  histological  work. 

Imbedding  Methods, —  'The  beautiful  processes  known  as 
imbedding  methods  are  employed  for  a  threefold  end.  Firstly,  they 
enable  us  to  surround  an  object*  too  small  or  too  delicate  to  be 
firmly  held  by  the  fingers  or  by  any  instrument,  with  some  plastic 
substance  that  will  support  it  on  all  sides  with  firmness  but  without 
injurious  pressure,  so  that  by  cutting  sections  through  the 
composite  body  thus  formed,  the  included  object  may  be  cut  into 
sufficiently  thin  slices  without  distortion.  Secondly,  they  enable  us 
to  fill  out  with  the  imbedding  mass  the  natural  cavities  of  the  object, 
so  that  their  lining  membranes  or  other  structures  contained  in 
them  may  be  duly  cut  in  situ.  And,  thirdly,  they  enable  us  not 
only  to  surround  with  the  supporting  mass  each  individual  organ  or 
part  of  any  organ  that  may  be  present  in  the  interior  of  the  object, 
but  also  to  impregnate  with  it  each  separate  cell  or  other  anatomical 
element,  thus  giving  to  the  tissues  a  consistency  they  could  not  other- 
wise possess,  and  ensuring  that  in  the  thin  slices  cut  from  the  mass 
all  the  details  of  structure  will  precisely  retain  their  natural  relations 
of  position.' 

'  These  ends  are  usually  attained  in  one  of  two  ways.  Either  the 
object  to  be  imbedded  is  saturated  by  soaking  with  some  material 
that  is  liquid  while  warm  and  solid  when  cold,  which  is  the  principle 
of  the  paraffin  process ;  or  the  object  is  saturated  with  some 
substance  which  whilst  in  solution  is  sufficiently  fluid  to  penetrate 
the  object  to  be  imbedded,  whilst  at  the  same  time,  after  the 
evaporation  or  removal  by  other  means  of  its  solvent,  it  acquires  and 
imparts  to  the  imbedded  object  sufficient  firmness  for  the  purpose 
of  cutting,'  which  is  the  principle  of  the  celloidin  process.  (From 
Mr.  Lee's  '  Microtomist's  Vade-mecum.') 

Any  substance  used  for  imbedding  is  technically  termed  an 
'  imbedding  mass.' 

The  older  workers  were  not  aware  of  the  importance  of 
thoroughly  saturating  the  objects  to  be  cut  with  the  imbedding 
mass,  a  point  which  is  very  important  in  order  to  the  production 
of  thin  and  undistorted  sections.  They  were  content  with  simply 
surrounding  the  objects  to  be  cut  with  the  mass.  This  primitive 
procedure  is  now  rightly  abandoned,  except  in  cases  in  which,  on 
account  of  the  large  size  or  other  peculiarities  of  the  object,  it  is 
impossible  to  procure  due  saturation. 

Among  the  numerous  methods  of  imbedding  that  have  been 
advocated,  only  two  are  in  general  use  at  the  present  day.  These 
are  the  paraffin  method,  and  the  collodion  or  celloidin  method.  And 


.496    PREPARATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 

of  these,  it  is  the  paraffin  method  that  is  by  far  the  most  usually 
•employed.  It  is  the  most  convenient  for  ordinary  work,  the 
collodion  method  only  presenting  points  of  superiority  in  special 
cases,  such  as  the  sectioning  of  extremely  large  objects,  or  very 
brittle  tissues,  and  other  special  circumstances. 

The  Paraffin  Method. — The  first  step  in  the  paraffin  method 
•consists  in  saturating  the  objects  with  a  solvent  of  paraffin.  The 
second  consists  in  saturating  them  with  molten  paraffin,  which 
gradually  takes  the  place  of  the  solvent.  The  third  consists  .in 
causing  the  paraffin  to  solidify,  and  arranging  the  solidified  mass  in 
ti  suitable  form  for  cutting  sections.  The  fourth  consists  in  cutting 
the  sections  and  freeing  them  from  the  solid  paraffin  with  which 
they  are  saturated,  and  if  desired  affixing  them  in  serial  order  to  a 
slide  for  the  purpose  of  mounting. 

1.  Saturation  ivith  a  solvent. — The  solvents  employed  are  either 
chloroform,  or  one  of  the  volatile  hydrocarbons,  such  as  benzol, 
toluol,  or  naphtha,  or  an  essential  oil,  such  as  oil  of  cedar  or  oil  of 
cloves.  None  of  these  are  miscible  with  water,  but  all  of  them  are 
miscible  with  alcohol.  Therefore  the  objects  to  be  imbedded  are  in 
the  first  place  thoroughly  dehydrated  with  alcohol,  according  to  the 
principles  set  forth  above,  p.  487.  The  alcohol  is  then  removed 
from  the  objects,  and  the  solvent  is  made  to  take  its  place  gradually 
by  one  of  the  substitution  methods  described  above,  p.  487,  under 
*  Clearing.'  Cedar  oil  is  one  of  the  most  convenient  solvents ;  and 
.as  it  is  at  the  same  time  one  of  the  best  of  clearing  agents,  it  follows 
that  any  object  that  has  been  cleared  in  it  is  at  once  ready  for 
saturation  with  paraffin.  Other  essential  oils,  such  as  clove  oil,  may 
.also  be  employed.  But  the  two  best  saturation  agents  are  certainly 
oil  of  cedar  and  chloroform.  It  will  be  noticed  that  the  best  way  to 
saturate  objects  with  chloroform  is  to  place  the  chloroform  under 
the  alcohol,  and  allow  the  substitution  of  liquids  to  take  place  just 
as  in  clearing  with  a  non-volatile  clearing  agent,  as  directed 
above,  under  '  Clearing.' 

2.  Saturation  with  paraffin. — If  cedar  oil,  or  other  non-volatile 
medium,  has  been  employed,  proceed  as  follows : — Melt  some 
paraffin  in  a  suitable  vessel — a  watch-glass  will  do  for  small  objects 
— and  keep  it  as  nearly  as  possible  at  melting-point  on  a  water-bath 
or  in  a  stove,  taking  care  to  keep  it  protected  from  vapour  of  water. 
Remove  the  object  from  the  oil,  and  put  it  into  the  paraffin,  and 
leave  it  there  till  thoroughly  saturated.  The  length  of  time 
required  for  this  must  be  found  by  experience.  A  piece  of 
soft  tissue  of  J  inch  thickness  is  generally  well  saturated  in  an 
hour.  If  the  objects  be  at  all  large,  the  paraffin  should  be 
changed  for  fresh  once  or  twice,  so  that  none  of  the  oil  may  remain 
"to  contaminate  it  and  render  it  soft  after  cooling. 

Some  persons  prefer  to  bring  the  objects  gradually  from  the  oil 
into  the  paraffin  by  passing  them  through  graduated  mixtures  of  oil 
-and  paraffin  ;  but  with  cedar  oil,  at  all  events,  that  is  not  necessary. 

If  chloroform,  or  other  volatile  medium,  has  been  employed, 
the  procedure  may  be  modified  in  the  following  manner,  which  is 
very  advantageous  for  delicate  objects  : — 


ARRANGEMENTS  FOE   SECTION-CUTTING  497 

'  The  chloroform  and  the  objects  in  it  are  gradually  warmed  up 
to  the  melting-point  of  the  paraffin  employed,  and  during  the 
warming  small  pieces  of  paraffin  are  by  degrees  added  to  the 
chloroform.  So  soon  as  it  is  seen  that  no  more  bubbles  are  given 
off  from  the  objects,  the  addition  of  paraffin  may  cease,  for  that  is  a 
sign  that  the  paraffin  has  entirely  displaced  the  chloroform  in  the 
objects.  This  displacement  having  been  a  gradual  one,  the  risk  of 
shrinkage  of  the  tissues  is  reduced  to  a  minimum.'  After  this, 
however,  the  w^hole  must  be  kept  warm  on  the  water-bath, 'at  the 
temperature  of  the  melting-point  of  the  pure  paraffin  employed, 
until  all  the  chloroform  has  been  driven  off  from  it,  as,  if  even  a 
trace  of  chloroform  remain  in  the  paraffin,  it  will  render  it  soft  after 
cooling.  As  this  is  a  very  long  process  (it  may  take  days  for  large 
objects),  it  is  frequently  better  to  simply  transfer  the  objects  from 
the  paraffin  solution  to  a  bath  of  pure  paraffin. 

3.  Arranging  for  cutting. — After  the  objects  have  been  duly 
saturated,  they  are  arranged  in  a  suitable  position  for  cutting,  and 
the  paraffin  is  caused  to  solidify  as  quickly  as  possible.  It  must  not 
be  allovied  to  cool  slowly,  as  slow  cooling  allows  the  paraffin  to 
crystallise,  and  gives  a  mass  less  homogeneous  and  of  a  consistency 
less  favourable  for  cutting  than  after  rapid  cooling. 

Very  small  objects  may  be  taken  out  of  the  paraffin  with  a 
needle  or  small  spatula,  and  put  to  cool  on  a  block  of  glass,  then 
imbedded  in  position  for  cutting  on  a  cone  of  paraffin  already 
soldered  to  the  object-carrier  of  the  microtome,  or  to  a  cork  or 
cylinder  of  wood  fitted  into  it.  This  is  done  as  follows  : — 

;  A  piece  of  stout  wire,  or  a  mounted  needle,  is  heated  in  the 
name  of  a  spirit-lamp,  and  with  it  a  hole  is  melted  in  the  end  of  the 
cone  of  paraffin  ;  the  specimen  is  pushed  into  the  melted  paraffin, 
and  placed  in  any  desired  position.  In  the  use  of  the  needle  or  wire 
it  should  be  noted  that  it  is  important  to  melt  as  little  paraffin  as 
possible  at  one  time,  in  order  that  that  which  is  melted  may  cool 
again  as  rapidly  as  possible.  The  advantages  of  the  method  lie  in 
the  quickness  and  certainty  with  which  it  can  be  performed.' 

If  the  paraffin  bath  has  been  given  in  a  watch-glass,  float  the 
watch-glass  with  the  paraffin  and  objects  on  to  cold  water.  Do  not 
let  it  sink  till  all  the  paraffin  has  solidified.  When  cool,  warm  the 
bottom  slightly  and  cut  out  blocks  containing  the  objects ;  do  thi& 
with  a  slightly  warmed  scalpel.  Then  fix  the  blocks  to  the  object- 
carrier  by  means  of  a  heated  needle  as  above  described. 

For  many  objects,  other  methods  of  arrangement  are  preferable. 
These  consist  chiefly  in  causing  the  paraffin  to  solidify  in  a  mould  of 
any  desired  shape.  Paper  trays  are  often  used  as  moulds. 

To  make  paper  trays,  proceed  as  follows.  Take  a  piece  of  stout 
paper  or  thin  cardboard,  of  the  shape  of  the  annexed  figure  (fig.  407)  ; 
thin  (foreign)  post-cards  do  very  well  indeed.  Fold  it  along  the 
lines  a  a'  and  b  V,  then  along  c  cf  and  d  d' ,  taking  care  to  fold 
always  the  same  way.  Then  make  the  folds  A  A',  B  B' ,  C  C",  D  D', 
still  folding  the  same  way.  To  do  this  you  apply  A  c  against  A  a, 
;m<l  pinch  out  the  line  A  A',  and  so  on  for  the  remaining  angles. 
This  done,  you  have  an  imperfect  tray  with  dogs'  ears  at  the  angles. 

K  K 


498    PREPARATION,    MOUNTING,   AND    COLLECTION   OF   OBJECTS 

To  finish  it,  turn  the  dogs'  ears  round  against  the  ends  of  the  box, 
turn  down  outside  the  projecting  flaps  that  remain,  and  pinch  them 
down.  A  well-made  post-card  tray  will  last  through  several  im- 

beddings,  and  will  generally 
work  better  after  having 
been  used  than  when  new. 
(From  Mr.  Lee's  '  Microto- 
mist's  Vade-mecum.') 

To  imbed  in  such  a  tray, 
or  similar  receptacle,  some 
melted  paraffin  (or  other 
'  mass  ')  is  poured  into  it ; 
at  the  moment  when  the 
mass  has  cooled  so  far  as  to 
have  a  consistency  that  will 
not  allow  the  object  to  sink 
to  the  bottom,  the  object  is 
placed  on  its  surface,  and 
more  melted  mass  poured  on 
until  the  object  is  covered 
by  it.  Or,  the  paper  tray 
being  placed  on  cork,  the 
object  may  be  fixed  in  posi- 
tion in  it  whilst  empty  by 
means  of  pins,  and  the  tray 
filled  with  melted  mass  at 
h1  one  pour.  (The  pins  can  be 

removed  from  the  mass 
when  cold.) 

In  either  case,  when  the 
mass  is  cold  the  paper  is  removed  from  it  before  cutting. 

As  soon  as  the  tray  is  filled,  and  the  object  in  position,  cool  it  on 
water,  holding  it  above  the  surface  with  only  the  bottom  immersed 
until  all  the  paraffin  has  solidified,  as  if  you  let  it  go  to  the  bottom 
at  once  you  will  probably  get  cavities  filled  with  water  formed  in 
your  paraffin.  Or  you  may  put  it  to  cool  on  a  block  of  cold  metal  or 
stone. 

A  better  plan  is  to 
employ  sets  of  two 
pieces  of  type-metal, 
cast  in  rectangular  form 
of  various  heights  and 
capable  of  being  placed 
together  as  in  fig.  408  ; 
in  this  way  a  suitable 
box  is  formed,  and,  the 
end  of  the  shorter  arm 
being  triangularly  en- 


v 

A 

N 

/' 

C1 

A  "           B 

,7 

C               D 

r/' 

(L 

X 

^^^ 

X 

N 

FIG.  407. 


FIG.  408.  -  Type-metal  case  for  imbedding. 


larged  outwards,  it  is  closed  sufficiently  to  retain  the  mass.  Placed 
in  this  way,  with  the  short  arms  nearer  to  or  farther  from  each 
other  as  a  less  or  greater  imbedding  mass  is  required,  they  are  set 


CUTTING   SECTIONS 


499 


on  a  plate  of  glass  which  has  been  wetted  with  glycerin  and  gently 
warmed.  The  melted  paraffin  is  now  poured  into  this  mould  and 
the  object  is  imbedded  in  it  as  described  for  the  paper  tray. 

Still  another  plan  is  to  take  a  common  flat  medicine-bottle,  as  in 
fig.  409,  fitted  with  a  cork  through  which  two  tubes  pass,  or,  if  the 
mouth  is  small,  one  tube  may  be  fastened  into  a  hole  drilled  into  the 
bottle.  One  of  these  tubes,  A,  is  connected  with  hot  and  cold  water  ; 
the  other,  JB,  is  a  discharge -pipe  for  the  water  entering  the  bottle 
by  A,  and  raising  or  lowering  its  temperature  as  warm  or  cold  water 
is  allowed  to  flow  in.  On  the  smooth,  flat  side  of  the  bottle  four 
pieces  of  glass  rods  or  strips  are  cemented  fast,  so  as  to  inclose 
a  rectangular  space,  C,  which  for*ms  a  receptacle 
for  the  melted  paraffin. 

As  long  as  the  warm  water  circulates  through 
the  bottle  the  paraffin  remains  fluid,  and  objects 
in  it  may  be  arranged  under  the  microscope  by 
light  from  above  or  below,  and  can  be  oriented 
with  reference  to  the  sides  of  the  paraffin  recep- 
tacle or  with  reference  to  lines  drawn  upon  the 
surface  of  the  bottle.  When  the  cold  water  is 
allowed  to  enter  in  place  of  the  warm,  the  paraffin 
congeals  rapidly,  and  may  be  easily  removed  as 
one  piece.  The  discharge-pipe  should  open  near 
the  upper  surface  of  the  bottle,  to  drawr  off  any 
air  which  may  accumulate  there. 

In  using  any  form  of  microtome  where  the 
object  is  held  in  jaws,  the  imbedding  mass  must 
either  be  cast  a  suitable  shape,  and  placed  directly 
in  the  jaws,  or  be  cemented  to  pieces  of  soft 
wood  which  may  be  placed  in  the  jaws. 

The  mould  obtained  by  either  of  these  pro- 
cesses is  then  fixed  to  the  carrier  of  the  micro- 
tome, and  finally  pared  into  a  convenient  shape, 
and  oriented  for  cutting. 

4.  Cutting. — Paraffin  sections  are  always  cut 
dry — that  is,  the  knife  is  not  \vetted  with  either 
alcohol  or  any  other  liquid. 

;  If  the  knife  be  set  square — that  is,  with  its 
axis  at  right  angles  to  the  line  of  motion  (of  the 
knife  for  sliding  microtomes,  and  of  the  object- 
carrier  for  rocking  microtomes) — and  if  the  paraffin  block  be 
cut  into  a  rectangle,  and  also  set  square — that  is,  with  one  edge 
parallel  to  the  edge  of  the  knife — sections  may  be  cut  in  "  ribbons." 
The  sections  not  being  removed  from  the  knife  one  by  one  as  they 
are  cut,  but  allowed  to  lie  undisturbed  on  the  blade,  adhere  to  one 
another  by  the  edges  so  as  to  form  a  chain  or  ribbon,  which  may  be 
taken  up  and  transferred  to  a  slide  without  breaking  up,  thus  greatly 
lightening  the  labour  of  mounting  a  series.' 

Difficult  objects  are  in  general  better  cut  in  isolated  sections 
with  an  oblique  knife.  In  this  case  it  is  best  to  cut  the  paraffin  into 
the  shape  of  a  three-sided  prism,  and  arrange  it  so  that  the  knife- 

K  K  2 


FIG.  409. — Arrange- 
ment for  the  orien- 
tation of  objects  in 
paraffin. 


5OO    PREPAEATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 

edge  enters  it  at  one  angle  and  leaves  it  at  another  angle  (in  fig.  410, 
the  knife  enters  at  a  and  leaves  at  c).  The  prism  should  be  so  cut 
as  to  leave  the  imbedded  object  near  to  the  side  which  is  furthest 
from  the  angle  a  which  is  first  touched  by  the  knife.  Then  if  the 
section  should  roll,  at  all  events  the  section  of  the  object  will  come 
to  lie  in  the  most  open  spire  of  the  coil,  and  can  thus  be  more  easily 
unrolled. 

The  rolling    of    sections    above    referred    to    is    an    annoying 
phenomenon  of  very  frequent  occurrence.     Its  most  usual  cause  is 
over-hardness   of  the   paraffin,    but    it   is   favoured    by    excessive 
obliquity  of  the  knife,  and  other  circumstances.     With  large  sections 
it  is  not  difficult  to  catch  them  by  the  edge 
as  they  begin  to  roll,  and  hold  them  down 
with  a  camel' s-hair  brush.     Or  a  section- 
stretcher  may  be  used.1 

If  the  paraffin  be  too  soft,  the  sections 
will  not  roll,  but  will  become  creased. 

Either  of  these  defects  may  be  dimi- 
nished, sometimes  even  totally  cured,  by 
simple  means.  Firstly,  due  attention  must 
be  paid  to  the  position  of  the  knife;  not 
only  to  its  obliquity,  but  also  to  its  tilt,  as 
explained  above. 

Secondly,  if  the  paraffin  should  be  too 
hard,  it  may  be  softened  by  setting  up  a 
lamp  near  it,  or  even  by  closing  the  win- 
dow, if  this  should  happen  to  be  open,  or 
by  carrying  the  microtome  to  a  warmer 
place,  or  by  any  device  that  will  have  the 
effect  of  exposing  the  paraffin  block  to  an 
increase  of  temperature.  An  incredibly 
slight  increase  will  sometimes  suffice. 
Thirdly,  if  it  should  be  too  soft,  an  opposite 

treatment  must  be  tried.     The  microtome  is  removed  to  a  cooler 
place,  or  the  window  is  opened,  or  the  like. 

If  none  of  these  manoeuvres  suffice  to  obtain  sufficiently  good 
sections,  the  object  must  be  re-imbedded  in  a  harder  or  softer 
paraffin.  But  it  will  generally  be  possible  to  save  the  sections  by 
flattening  them  out  by  the  water  method,  to  be  presently  described. 
The  paraffin  employed  for  imbedding  must  be  of  a  hardness 
determined  by  the  temperature  of  the  workroom  :  hard  paraffin  for  a 
warm  room,  soft  paraffin  for  a  cold  room.  For  the  Thoma  microtome, 
a  paraffin  melting  at  45°  C.  (or  113°  F.)  gives  good  results  so  long  as 

1 '  Section- stretchers  are  instruments  consisting  essentially  of  a  little  metallic  roller 
suspended  over  the  object  to  be  cut  in  such  a  way  as  to  rest  on  its  free  surface  with 
a  pressure  that  can  be  delicately  regulated  so  as  to  be  sufficient  to  keep  the  section 
flat  without  in  any  way  hindering  the  knife  from  gliding  beneath  it.'  They  are  made  in 
various  forms,  the  most  convenient  being  that  of 'Mayer,  Andres  and  Giesbi  echt,  of  which 
a  description  and  figure  maybe  found  \\\i\\e  Journal  of  the  Hoy.  Microscopical  Soc. 
1883,  p.  916.  Now  that  the  water  flattening  process  (see  below,  Flattening)  has  been 
perfected,  section- stretchers  are  not  so  necessary  as  they  were  formerly,  and  for  most 
work  may  be  dispensed  with. 


FIG.  410. 


FLATTENING   SECTIONS   AND   MOUNTING  501 

the  temperature  of  the  laboratory  lies  between  15°  and  17°  C.  (59° 
and  62°  F.)  ;  though  many  workers  prefer,  even  with  this  instrument, 
a  much  harder  mass.  For  microtomes  tuith  fixed  knives,  such  as 
the  Cambridge  rocker,  harder  paraffins  may  be  used  than  with  sliding 
microtomes,  paraffins  of  from  55°  to  60°  C.  (131°  to  140°  F.)  being 
used  by  many  workers.  For  cutting  ribbons  with  these  hard 
masses  it  is  frequently  necessary  to  coat  the  face  of  the  block 
nearest  to  the  knife  with  a  softer  paraffin,  in  order  that  the  sections 
may  cohere. 

Masses  of  intermediate  consistency  may  be  made  by  mixing  a 
hard  and  a  soft  paraffin.  Two  parts  of  paraffin  of  50°  C.  (122°  F.) 
with  one  of  36°  C.  (97°  F.)  meltjng-point,  give  a  mass  melting  at 
48°  C.  (119°  F.). 

Mixtures  of  paraffin  with  vaseline  and  with  various  fatty  and 
other  substances  have  been  recommended.  They  are  now  generally 
abandoned. 

5.  Flattening  the  sections,  and  mounting. — If  the  sections  have 
<3ome  off  either  rolled  or  creased,  they  must  be  flattened  before  the 
paraffin  is  removed. 

If  they  are  large  sections,  float  them  on  to  warm  water  in  a 
suitable  dish.  They  will  flatten  out  perfectly  in  a  few  seconds,  and 
they  may  then  be  lifted  out  011  a  slide  or  cover-glass  slid  under 
them.  The  water  must  not  be  ivarm  enough  to  melt  the  paraffin,  which 
must  only  be  warmed,  not  melted,  till  the  sections  have  been 
securely  fixed  to  the  slide  or  cover.  A  temperature  of  about  40°  C. 
(104°  F.)  is  about  right. 

Or  take  a  clean  slide,  free  from  grease,  spread  on  it  with  a 
brush  enough  water  to  float  the  sections,  lay  the  sections  on  it,  and 
warm,  either  on  the  water-bath,  or  on  a  hot  plate,  or  over  a  small 
flame,  taking  care  not  to  melt  the  paraffin. 

If  the  sections  are  numerous  and  small,  take  a  perfectly  clean 
slide,  so  clean  that  water  will  readily  spread  on  it.  Breathe  on  it, 
and  smear  on  it  with  a  brush  a  streak  of  water  as  wide  as  the 
sections  and  of  the  length  of  the  first  intended  row.  Lay  the  first 
row  of  sections  on  this  streak.  Breathe  on  the  slide  again,  and 
draw  on  it  another  streak  of  water  under  the  first  one.  Lay  a 
second  row  of  sections  on  this  ;  and  so  on  until  the  slide  is  full. 
Then  warm  as  before. 

The  chief  difficulty  connected  with  this  process  lies  in  the  diffi- 
culty of  getting  the  water  to  spread  evenly  on  the  slide.  The  slide 
should  be  wrell  freed  from  grease,  by  means  of  xylol  or  some  good 
solvent  of  fats,  and  then  cleaned  with  alcohol.  The  test  for  suffi- 
cient freedom  from  grease  is,  that  on  breathing  on  the  slide  the 
moisture  of  the  breath  should  condense  on  it  evenly,  and  evaporate 
evenly.  The  slide  should  also  be  well  rubbed  with  a  clean  cloth 
wetted,  or  rather  moistened,  with  water,  before  the  water  is  defi- 
nitely spread  on  it  with  the  brush.  Some  sorts  of  slides  cannot  be 
got  to  spread  the  water  evenly  by  any  means. 

The  following  is  said  by  De  Groot  ('  Zeitschrift  f.  wiss.  Mikro- 
skopie,'  xv.  1,  p.  62)  to  be  infallible.  Wrap  the  corner  of  a  clean 
-cloth  round  two  fingers  and  rub  it  with  a  piece  of  chalk.  Moisten 


502    PREPARATION,    MOUNTING-,    AND    COLLECTION    OF   OBJECTS 

it  with  a  drop  of  water  and  rub  the  slide  with  the  chalked  part, 
then  finish  with  pure  water  and  a  clean  part  of  the  cloth. 

6.  The  flattening  having  been  accomplished  by  either  of  these  pro- 
cesses, the  sections  must  now  be  fixed  to  the  slide  or  cover  before  the 
paraffin  is  removed. 

The  most  elegant  method  of  accomplishing  this  is  by  what  is 
known  as  the  water  method.  It  consists  simply  in  drying  the  sec- 
tions on  the  slide  (or  cover).  After  they  have  been  got  on  the  slide 
and  flattened  out  by  water  and  warming  as  above  described,  the 
superfluous  water  is  drained  off,  and  the  slide  put  away  to  dry.  A  s 
soon  as  the  water  has  entirely  evaporated  off,  the  sections  will  be 
found  to  be  so  firmly  affixed  to  the  glass  that  they  will  bear  the 
melting  of  the  paraffin,  treatment  with  solvents,  with  alcohol  or 
stains,  &c.,  without  moving.  A  convenient  plan  is  to  dry  the  slides 
on  the  top  of  the  stove  or  water-bath  at  a  temperature  somewhat 
under  the  melting-point  of  the  paraffin.  This  will  take  from  half  an 
hour  to  three  or  four  hours.  When  dry  the  sections  will  have 
assumed  a  certain  horny  transparent  look.  The  paraffin  must  not  be 
allowed  to  melt  before  the  sections  are  perfectly  dry.  If  they  are  left 
to  dry  at  the  temperature  of  the  room,  they  should  be  left  overnight. 
As  soon  as  the  sections  are  quite  dry,  the  paraffin  may  be  melted 
by  holding  the  slide  for  a  few  seconds  over  a  small  flame,  after  which 
it  is  plunged  at  once  into  a  tube  of  xylol  or  benzol  or  chloroform  or 
the  like,  which  in  a  few  seconds  or  minutes  dissolves  out  all  the 
paraffin  from  the  sections. 

The  water  method  is  very  safe  for  sections  that  present  a  sufficient 
uninterrupted  surface  capable  of  affording  adhesion  at  all  points  to 
the  slide.  But  sections  of  hollow  organs,  offering  only  a  relatively 
small  surface  for  attachment,  adhere  very  badly.  Sections  of  such 
things  as  tubular  chitinous  organs,  for  instance,  will  generally  not 
allow  of  mounting  at  all  in  this  way. 

In  such  cases,  Mayer's  albumen  fixative  should  be  employed. 
Take  50  c.c.  of  white  of  egg,  50  c.c.  of  glycerin,  and  1  grm.  of 
salicylate  of  soda,  shake  them  up  well  together,  and  filter  into  a  clean 
bottle.  The  filtering  may  take  days.  A  little,  very  little  of  this  is 
now  painted  on  to  the  part  of  the  slide  destined  to  receive  the  sec- 
tions, and  the  layer  smoothed  by  drawing  the  edge  of  a  slide  over  it 
(some  persons  rub  off  the  excess  with  the  ball  of  a  finger).  Place  a 
drop  of  water  on  the  prepared  surface,  lay  the  sections  on  it  and 
flatten  by  warming,  drain  and  evaporate  as  in  the  water  process, 
with  this  difference,  however,  that  the  evaporation  need  not  be 
carried  to  the  point  of  perfect  drying.  The  slides  will  be  sufficiently 
evaporated  at  a  temperature  of  40°  C.  in  ten  minutes  or  a  quarter  of 
an  hour.  And  if  the  evaporation  be  conducted  by  waving  the  slide 
to  and  fro  over  a  flame,  from  three  to  five  minutes  may  suffice.  The 
paraffin  is  then  melted  and  removed  by  xylol  or  other  solvent,  as 
before.  This  process  has  the  advantage  over  the  water  process  of 
greater  safety  and  greater  rapidity,  but  has  the  disadvantage  that 
the  layer  of  albumen  stains  obstinately  in  some  plasma  stains,  thus 
producing  an  inelegant  mount. 

If  the  sections  be  neither  rolled  nor  creased,  it  is  not  necessary 


CELLOIDIN   IMBEDDING  503 

to  flatten  them  on  water.  They  may  be  laid  down  on  Mayer's 
albumen,  without  water,  gently  pressed  down  with  a  brush,  and  the 
paraffin  melted  and  dissolved  at  once,  the  whole  process  taking  only  a 
few  seconds.  But  for  delicate  histological  work  it  is  well  to  employ 
the  water  method  in  any  case,  as  the  flattening  on  water  serves  to 
somewhat  expand  the  sections,  which,  unless  cut  from  extremely 
hard  paraffin,  are  generally  somewhat  compressed  by  the  impact  of 
the  knife. 

As  soon  as  the  paraffin  has  been  removed,  all  that  is  necessary, 
in  the  pure  water  process,  is  to  add  a  drop  of  balsam  and  a  cover,  if 
the  material  has  been  already  stained.  If  not.  the  solvent  of  the 
paraffin  is  removed  by  alcohol,  ai^d  the  sections  are  stained  in  any 
manner  that  may  be  desired. 

But  if  Mayer's  albumen  has  been  employed  the  sections  must  be 
thoroughly  washed  with  alcohol  before  the  definitive  clearing  and 
mounting.  This  is  necessary  in  order  to  remove  the  glycerin,  which 
would  otherwise  cause  turbidity  in  the  mount. 

Tubes  for  Handling  Serial  Sections. — The  most  convenient 
vessels  for  performing  the  various  operations  of  washing,  dehydrating, 
clearing,  staining,  &c.,  with  sections  fixed  to  the  slide,  are  flat 
bottomed  corked  tubes.  They  should  have  an  internal  diameter 
slightly  over  1  inch,  so  as  to  be  able  to  take  two  slides  placed  back 
to  back  ;  and  they  should  be  nearly  4  inches  high,  so  as  not  only 
to  take  the  slides  in  an  upright  position,  but  to  allow  room  for  the 
cork.  A  stand  is  easily  made  for  them  by  taking  a  piece  of  inch 
deal  board,  and  boring  in  it  with  a  centrebit  holes  about  ^  inch 
deep,  large  enough  to  take  the  bottoms  of  the  tubes,  and  about  1  inch 
apart.  A  board  with  three  rows  of  seven  holes  each  does  not  take 
up  too  much  room  on  the  work-table. 

The  Collodion  or  Celloidin  Imbedding  Method. — Celloidin  is  a 
patent  collodion,  sent  out  in  semi-dry  tablets.  It  may  be  obtained 
through  Griibler  and  Hollborn.  To  prepare  it  for  use  for  imbedding 
it  may  either  be  dissolved  at  once  in  a  mixture  of  equal  parts  of 
ether  and  absolute  alcohol,  or,  which  is  held  by  some  workers  to  be 
preferable,  it  may  be  cut  up  into  thin  shavings,  which  are  allowed  to 
dry  in  the  air  until  they  have  assumed  a  horny  consistency,  and  are 
then  dissolved  in  the  ether  and  alcohol.  It  is  held  that  by  thus 
drying  the  celloidin  all  water  is  removed  from  it,  and  a  more  favour- 
able imbedding  mass  obtained.  Either  celloidin  or  common  collodion 
may  be  used  for  imbedding,  celloidin  having  merely  the  advantage 
stated. 

A  thin  celloidin  solution  is  made  by  dissolving  from  4  to 
6  per  cent,  of  the  dried  shavings  in  the  alcohol  and  ether  mixture ; 
a  thick  one  by  dissolving  from  10  to  12  per  cent,  of  them. 
Thicker  solutions  than  this  are  not  necessary.  If  common  collodion 
be  taken,  a  thin  solution  should  be  prepared  by  diluting  it  with 
ether. 

The  objects  to  be  imbedded  must  first  be  thoroughly  dehydrated 
with  absolute  alcohol.  They  are  then  soaked,  till  thoroughly  pene- 
trated, in  ether,  01%  which  is  better,  in  a  mixture  of  ether  and 
absolute  alcohol.  They  are  then  brought  into  the  collodion. 


504    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

They  should  be  soaked  first  in  a  thin  solution,  until  thoroughly 
impregnated  with  it,  for  days,  even  for  small  objects  ;  weeks  or 
months  for  large  ones.  When  well  saturated  with  this  they  should 
be  brought  into  a  thick  solution,  and  soaked  in  it  for  a  long  time,  the 
longer  the  better. 

When  it  is  deemed  that  they  are  saturated,  they  may  be  imbedded, 
In  many  cases  this  may  be  efficiently  done  by  simply  gumming  the 
object  by  means  of  a  drop  of  thick  collodion  to  a  cork,  or,  better,  a 
piece  of  soft  wood,  adapted  to  be  afterwards  fitted  to  the  microtome. 
But  for  the  purpose  of  accurate  orientation  it  is  preferable  to  imbed 
in  a  mould.  This  is  done  in  the  manner  described  for  paraffin.  A 
convenient  mould  for  celloidin  is  made  by  taking  a  cork,  and  winding 
a  strip  of  paper  several  times  round  one  end  of  it,  so  as  to  form  a 
projecting  collar,  which  is  fixed  with  a  pin.  Before  using  this,  or 
•any  paper  tray,  it  should  be  dressed  by  having  the  inside  painted 
with  collodion,  which  is  allowed  to  dry  before  the  imbedding  mass  is 
poured  into  it.  The  object  of  this  is  to  prevent  bubbles  of  air 
coming  in  through  the  bottom  or  sides  of  the  mould.  Watch-glasses, 
deep  water-colour  moulds,  and/  the  like,  also  make  convenient 
imbedding  receptacles.  Care  should  be  taken  to  have  them  perfectly 
dry. 

If  bubbles  should  appear  after  the  mass  has  been  poured  in,  they 
should  be  got  rid  of  before  proceeding  further  by  exposing  the  whole 
to  the  vapour  of  ether  for  an  hour  or  two  in  a  closed  vessel. 

The  next  step  consists  in  the  hardening  of  the  mass.  One  of  the 
best  ways  of  doing  this  is  as  follows  : — 

'  Put  the  preparation  into  a  desiccator  or  other  suitable  closed 
vessel,  on  the  bottom  of  which  a  teaspoonful  of  chloroform  has  been 
poured.  As  soon  as  the  mass  has  attained  sufficient  superficial  hard- 
ness, it  is,  of  course,  well  to  turn  it  out  of  its  recipient  and  turn  it- 
over  from  time  to  time,  in  order  that  it  may  be  equally  exposed  on  all 
sides  to  the  action  of  the  vapour.  Small  objects  may  be  sufficiently 
hardened  in  from  one  hour  to  overnight.  When  fairly  hard  (it  is  not 
necessary  to  wait  till  the  mass  has  attained  all  the  hardness  of  which  it 
is  susceptible),  throw  it  into  a  mixture  of  one  part  of  chloroform  with 
one  or  two  parts  of  cedar  oil.  From  time  to  time  more  cedar  oil  should 
be  added,  so  as  to  bring  the  mixture  up  gradually  to  nearly  pure  cedar 
oil.  As  soon  as  the  object  is  cleared  throughout,  the  mass  may  be 
exposed  to  the  air,  and  the  rest  of  the  chloroform  will  evaporate 
gradually.  The  block  may  now  be  mounted  on  the  holder  of  the 
microtome  with  a  drop  of  thick  collodion  (which  may  be  allowed  to 
dry,  or  may  be  hardened  by  putting  back  into  chloroform  vapour), 
and  may  either  be  cut  at  once,  or  may  be  preserved  indefinitely 
without  change  in  a  stoppered  bottle.  Cut  with  a  dry  knife,  the  cut 
surface  will  not  dry  injuriously  under  several  hours.  The  cutting 
quality  of  the  mass  is  often  improved  by  allowing  it  to  evaporate  in 
the  air  for  some  hours. 

4  The  hardening  may  be  done  at  once  in  the  chloroform  and  cedar- 
wood  mixture,  instead  of  the  chloroform  vapour,  but  the  latter 
process  is  preferable  as  giving  a  better  hardening.  And  clearing  may 
be  done  in  pure  cedar  oil  instead  of  the  mixture,  but  then  it  will  be 


HARDENING  505 

very  slow,  whereas  in  the  mixture  it  is  extremely  rapid.'     (From 
Mr.  Lee's  *  Microtomist's  Vade-mecum.') 

Instead  of  cedar  oil,  white  oil  of  thyme  may  be  employed  ;  and 
some  workers  use  glycerin. 

The  above  process  is  recommended  as  giving  good  results  with 
small  objects.  For  large  ones  the  alcohol  process  is  more  generally 
employed. 

In  this  the  mass  is  first  subjected  to  a  preliminary  hardening. 
The  mass,  with  the  imbedded  object,  is  set  under  a  glass  shade  or 
put  into  a  loosely  closed  vessel,  so  as  to  allow  of  just  enough  com- 
munication with  the  air  to  set  up  a  slow  evaporation.  It  is  some- 
times a  good  plan  to  set  it  under  a  bell-jar  with  a  dish  containing 
alcohol,  so  that  the  evaporation  is  gone  through  in  an  atmosphere  of 
alcohol.  As  soon  as  the  mass  (of  which  only  enough  to  just  cover 
the  object  should  have  been  taken)  has  so  far  sunk  down  that  the 
object  begins  to  lie  dry,  fresh  thick  solution  is  added,  and  the  whole 
is  left  as  before.  The  process  is  repeated  every  few  hours  for,  if 
need  be,  two  or  three  days. 

When  the  mass  has  attained  a  consistency  such  that  the  ball  of 
a  finger  (not  the  nail)  no  longer  leaves  an  impress  on  it,  it  should 
be  scooped  out  of  the  dish  or  mould,  or  have  the  paper  removed  if 
it  has  been  imbedded  in  paper,  and  be  submitted  to  the  next  stage 
of  the  hardening  process. 

This,  the  definitive  hardening,  consists  in  putting  the  preparation 
into  alcohol,  and  leaving  it  till  it  has  attained  the  right  consistency 
(one  day  to  several  wreeks).  The  strength  of  alcohol  used  by 
different  workers  varies  between  70  per  cent,  and  85  per  cent.,  the 
latter  strength  being  probably  the  best.  The  vessel  containing  the 
alcohol  ought  not  to  be  tightly  closed,  but  should  be  left  at  least  slightly 
open. 

'  To  fix  the  hardened  preparation  to  the  microtome,  proceed  as 
follows.  Take  a  piece  of  soft  wood,  or,  for  very  small  objects,  pith, 
of  a  size  and  shape  adapted  to  fit  the  holder  of  the  microtome. 
Cover  it  with  a  layer  of  collodion,  which  you  allow  to  dry.  Take  the 
block  of  collodion,  or  the  impregnated  and  hardened  but  not 
imbedded  object ;  cut  a  slice  off  the  bottom,  so  as  to  get  a  clean 
surface  ;  wet  this  surface  first  with  absolute  alcohol,  then  with  ether 
(or  allow  it  to  dry),  place  one  drop  of  very  thick  collodion  on  the 
prepared  wood  or  pith,  and  press  down  tightly  on  to  it  the  wetted 
or  dried  surface  of  the  block  of  collodion.  Then  throw  the  whole 
into  weak  (70  per  cent.)  alcohol  for  a  few  hours  (or  even  less),  or 
into  chloroform,  or  vapour  of  chloroform,  for  a  few  minutes,  in 
order  that  the  joint  may  harden.'  (From  Mr.  Lee's  '  Microtomist's 
Vade-mecum.') 

Sections  of  material  prepared  in  this  way  are  cut  with  a  knife 
kept  abundantly  wetted  with  alcohol  (of  50  to  85  or  even  95  per 
cent.).  Some  kind  of  drip  arrangement  may  be  found  very  useful 
here.  The  knife  is  set  in  as  oblique  a  position  as  possible.  These 
two  points  are  illustrated  in  fig.  398. 

Another  method  of  definitive  hardening  and  cutting  is  the 
freezing  method.  '  After  preliminary  hardening  by  alcohol  the  mass 


506    PKEPAEATION,    MOUNTING,    AND   COLLECTION   OF   OBJECTS 

is  soaked  for  a  few  hours  in  water  in  order  to  get  rid  of  the  greater 
part  of  the  alcohol  (the  alcohol  should  not  be  removed  entirely,  or 
the  mass  may  freeze  too  hard).  It  is  then  dipped  for  a  few 
moments  into  gum  mucilage  in  order  to  make  it  adhere  to  the 
freezing  plate,  and  is  frozen.  The  sections  are  brought  into  warm 
water.  If  the  mass  have  frozen  too  hard,  cut  with  a  knife  warmed 
with  warm  water.' 

Staining  and  mounting. — The  sections  are  brought  into  alcohol 
of  not  more  than  95  per  cent,  as  fast  as  they  are  cut,  and  may  now 
either  be  stained  or  mounted  at  once.  It  is  not  in  general 
necessary  nor  even  desirable  to  remove  the  mass  from  the  sections 
before  staining  or  mounting.  It  is,  no  hindrance  to  staining,  and  on 
being  mounted  in  glycerin  or  balsam  it  becomes  perfectly  invisible. 

To  mount  in  glycerin,  nothing  more  is  necessary  than  to  add  a 
drop  of  glycerin  and  a  cover. 

To  mount  in  balsam,  dehydrate  in  alcohol  of  not  more  than  95 
per  cent.,  and  clear  with  an  oil  that  does  not  dissolve  collodion,  such 
as  oil  of  origanum,  bergamot  oil,  cedar  oil,  or  with  chloroform  or 
xylol. 

The  foregoing  relates  to  single  sections.  If  it  be  desired  to 
mount  a  series  of  small  sections  under  one  cover,  arrange  them  on 
the  slide  and  expose  it  for  a  few  minutes  to  the  vapours  of  a 
mixture  of  ether  and  alcohol  in  a  closed,  tube.  Then  treat  with  95 
per  cent,  alcohol,  clear  and  mount. 

If  the  sections  are  to  be  stained  on  the  slide,  care  should  be 
taken  when  arranging  them  to  let  the  celloidin  of  each  section  over- 
lap that  of  its  neighbour  at  the  edges,  so  that  the  ether  vapour  may 
fuse  them  all  into  a  continuous  sheet.  Then  on  passing  the  slide 
into  any  aqueous  liquid  the  sheet  will  be  detached,  and  may  then 
'be  treated  as  a  single  section. 

If  the  sections  should  come  off  the  knife  creased,  they  may  be 
flattened  by  floating  them  on  to  oil  of  bergamot,  after  which  they 
may  be  got  on  to  the  slide  and  gently  pressed  on  to  it  with  a 
cigarette  paper  or  a  piece  of  glossed  tissue  paper,  after  which  they 
may  be  exposed  to  the  vapour  of  ether  and  alcohol  as  before. 

Series  may  also  be  affixed  to  the  slide  by  means  of  Mayer's 
albumen,  as  described  above  for  paraffin  sections. 

For  the  complicated  manipulations  involved  in  the  methods  of 
Weigert,  Obregia,  and  others,  which  are  only  necessary  in  very 
special  cases,  the  reader  must  be  referred  to  Mr.  A.  Bolles  Lee's 
'  The  Microtomist's  Vade-mecum.' 

Grinding  and  Polishing  Sections  of  Hard  Substances. — Sub- 
stances which  are  too  hard  to  be  sliced  in  a  microtome — such  as 
bones,  teeth,  shells,  corals,  fossils  of  all  kinds,  and  even  some  dense 
vegetable  tissues — can  only  be  reduced  to  the  requisite  thinness  for 
microscopical  examination  by  grinding  down  thick  sections  until 
they  become  so  thin  as  to  be  transparent.  General  directions  for 
making  such  preparations  will  be  here  given  j1  but  those  special 

1  The  following  directions  do  not  apply  to  siliceous  substances,  as  sections  of 
these  can  only  be  prepared  by  those  who  possess  a  regular  lapidary's  apparatus,  and 
have  been  specially  instructed  in  the  use  of  it. 


GKINDING  AND   POLISHING   SECTIONS  507 

details  of  management  which  particular  substances  may  require  will 
be  given  when  these  are  respectively  described.  The  first  thing  to 
be  done  will  usually  be  to  procure  a  section  of  the  substance,  as  thin 
as  it  can  be  safely  cut.  Most  substances  not  siliceous  may  be  divided 
by  the  fine  saws  used  by  artisans  for  cutting  brass ;  and  these  may 
be  best  worked  either  by  a  mechanical  arrangement  such  as  that 
devised  by  Dr.  Matthews,1  or,  if  by  hand,  between  '  guides,'  such  as 
are  attached  for  this  purpose  to  Hailes's  and  some  other  microtomes. 
But  there  are  some  bodies  (such  as  the  enamel  of  teeth,  and  porcel- 
lanous  shells)  which,  though  merely  calcareous,  are  so  hard  as  to 
make  it  very  difficult  and  tedious  to  divide  them  in  this  mode ;  and 
it  is  much  the  quicker  operation  fco  slit  them  with  a  disc  of  soft  iron 
(resembling  that  used  by  the  lapidary)  charged  at  its  edge  with 
diamond  dust,  which  disc  may  be  driven  in  an  ordinary  lathe.  Where 
waste  of  material  is  of  no  account,  a  very  expeditious  method  of 
obtaining  pieces  fit  to  grind  down  is  to  detach  them  from  the  mass 
with  a  strong  pair  of  *  cutting  pincers,'  or,  if  they  be  of  small 
dimensions,  with  '  cutting  pliers  ; '  and  a  flat  surface  must  then  be 
given  to  it,  either  by  holding  them  to  the  side  of  an  ordinary  grind- 
stone, or  by  rubbing  on  a  plate  of  lead  (cast  or  planed  to  a  perfect 
level)  charged  with  emery,  or  by  a  strong-toothed  file,  the  former 
being  the  most  suitable  for  the  hardest  substances,  the  latter  for  the 
toughest.  There  are  certain  substances,  especially  calcareous  fossils 
of  wood,  bone,  and  teeth,  in  which  the  greatest  care  is  required  in 
the  performance  of  these  preliminary  operations,  on  account  of  their 
extreme  friability — the  vibration  produced  by  the  working  of  the 
saw  or  the  file,  or  by  grinding  on  a  rough  surface,  being  sufficient  to 
disintegrate  even  a  thick  mass  so  that  it  falls  to  pieces  under  the 
hand  ;  such  specimens,  therefore,  it  is  requisite  to  treat  with  great 
caution,  dividing  them  by  the  smooth  action  of  the  wheel,  and  then' 
rubbing  them  down  upon  nothing  rougher  than  a  very  fine  '  grit,'  or 
on  the  '  corundum  files '  now  sold  in  the  tool  shops,  which  are  made 
.by  imbedding  corundum  of  various  degrees  of  fineness  in  a  hard, 
resinous  substance.  Where  (as  often  happens)  such  specimens  are 
sufficiently  porous  to  admit  of  the  penetration  of  Canada  balsam,  it 
will  be  desirable,  after  soaking  them  in  turpentine  for  a  while,  to 
lay  some  liquid  balsam  upon  the  parts  through  which  the  section  is 
to  pass,  and  then  to  place  the  specimen  before  the  fire  or  in  an  oven 
for  some  little  time,  so  as  first  to  cause  the  balsam  to  run  in,  and 
then  to  harden  it ;  by  this  means  the  specimen  will  be  rendered 
much  more  fit  for  the  processes  it  has  afterwards  to  undergo.  It 
not  unfrequently  happens  that  the  small  size,  awkward  shape,  or 
extreme  hardness  of  the  body  occasions  a  difficulty  in  holding  it 
either  for  cutting  or  grinding  ;  in  such  a  case  it  is  much  better  to 
attach  it  to  the  glass  in  the  first  instance  by  any  side  that  happens 
to  be  flattest,  and  then  to  rub  it  down,  by  means  of  the  '  hold '  of  the 
glass  upon  it,  until  the  projecting  portion  has  been  brought  to  a 
plane,  and  has  been  prepared  for  permanent  attachment  to  the  glass. 
This  is  the  method  which  it  is  generally  most  convenient  to  pursue 
with  regard  to  small  bodies ;  and  there  are  many  which  can  scarcely 

1  Journ.  Quekett  Microsc.  Club,  vol.  vi.  1880,  p.  83. 


508    PKEPAKATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 

be  treated  in  any  other  way  than  by  attaching  a  number  of  them 
to  the  glass  at  once  in  such  a  manner  as  to  make  them  mutually 
support  one  another.1 

The  mode  in  which  the  operation  is  then  to  be  proceeded  with 
depends  upon  whether  the  section  is  to  be  ultimately  set  up  in  Canada 
balsam,  or  is  to  be  mounted  '  dry,'  or  in  fluid.  In  the  former  case 
the  following  is  the  plan  to  be  pursued  : — The  flattened  surface  is  to 
be  polished  by  rubbing  it  with  water  on  a  '  Water-of-Ayr  '  stone,  or 
on  a  hone  or  '  Turkey  '  stone,  or  on  an  '  Arkansas  '  stone  ;  the  first  of 
the  three  is  the  best  for  all  ordinary  purposes,  but  the  two  latter, 
being  much  harder,  may  be  employed  for  substances  which  resist  it.2 
When  this  has  been  sufficiently  accomplished,  the  section  is  to  be 
attached  with  hard  Canada  balsam  to  a  slip  of  thick,  well -annealed 
glass  ;  and  as  the  success  of  the  final  result  will  often  depend  upon 
the  completeness  of  its  adhesion  to  this,  the  means  of  most  effectually 
securing  that  adhesion  will  now  be  described  in  detail.  The  slide 
having  been  placed  on  the  cover  of  the  water-bath,  and  the  previously 
hardened  balsam  having  been  softened  by  the  immersion  of  the  jar 
containing  it  in  the  bath  itself,  a  sufficient  quantity  of  this  should  be 
laid  on  the  slide  to  form,  when  spread  out  by  liquefaction,  a  thick 
drop,  somewhat  larger  than  the  surface  of  the  object  to  be  attached. 
The  slide  should  then  be  allowed  to  cool  in  order  that  the  hardness 
of  the  balsam  should  be  tested.  If  too  soft,  as  indicated  by  its 
ready  yielding  to  the  thumbnail,  it  should  be  heated  a  little  more, 
care  being  taken  not  to  make  it  boil  so  as  to  form  bubbles ;  if  too 
hard,  which  will  be  shown  by  its  chipping,  it  should  be  remelted 
and  diluted  with  more  fluid  balsam,  and  then  set  aside  to  cool  as 
before.  When  it  is  found  to  be  of  the  right  consistence,  the  section 
should  be  laid  upon  its  surface  with  the  polished  side  downwards ; 
the  slip  of  glass  is  next  to  be  gradually  warmed  until  the  balsam  is 
softened,  special  care  being  taken  to  avoid  the  formation  of  bubbles  ; 
and  the  section  is  then  to  be  gently  pressed  down  upon  the  liquefied 
balsam,  the  pressure  being  at  first  applied  rather  on  one  side  than 
over  its  whole  area,  so  as  to  drive  the  superfluous  balsam  in  a  sort 
of  wave  towards  the  other  side,  and  an  equable  pressure  being  finally 

1  Thus,  in  making  horizontal  and  vertical    sections  of  Foraminifera,  as  it  would 
be  impossible  to  slice  them  through,  they  must  be  laid  close  together  in  a  bed  of 
hardened  Canada  balsam  on  a  slip  of  glass,  in  such  positions  that  when  rubbed  down 
the  plane  of  section  shall  traverse  them  in  the  desired  directions ;  and  one  flat  surface 

.  having  been  thus  obtained  for  each,  this  must  be  turned  downwards,  and  the  other 
side  ground  away.  The  following  ingenious  plan  was  suggested  by  Dr.  Wallich  (Ann. 
of  Nat.  Hist.  July  1861,  p.  58)  for  turning  a  number  of  minute  objects  together,  and 
thus  avoiding  the  tediousness  and  difficulty  of  turning  each  one  separately  : — The 
specimens  are  cemented  with  Canada  balsam,  in  the  first  instance,  to  a  thin  film  of 
mica,  which  is  then  attached  to  a  glass  slide  by  the  same  means  ;  when  they  have 
been  ground  down  as  far  as  may  be  desired,  the  slide  is  gradually  heated  just  suffi- 
ciently to  allow  of  the  detachment  of  the  mica  film  and  the  specimens  it  carries  ;  and 
a  clean  slide  with  a  thin  layer  of  hardened  balsam  having  been  prepared,  the  mica 
film  is  transferred  to  it  with  the  ground  surface  downwards.  When  its  adhesion  is 
complete,  the  grinding  may  be  proceeded  with  ;  and  as  the  mica  film  will  yield  to  the 
stone  without  the  least  difficulty,  the  specimens,  now  reversed  in  position,  may  be 
reduced  to  requisite  thinness. 

2  As  the  flatness  of  the  polished  surface  is  a  matter  of  the  first  importance,  that 
of  the  stones  themselves  should  be  tested  from  time  to  time ;  and  whenever  they  are 
found  to  have  been  rubbed  down  on  any  one  part  more  than  on  another,  they  should 
be  flattened  on  a  paving- stone  with  fine  sand,  or  on  the  lead-plate  with  emery. 


GRIXDIXG  AXD   POLISHIXG-  509 

made  over  the  whole.  If  this  be  carefully  done,  even  a  very  large 
section  may  be  attached  to  glass  without  the  intervention  of  any  air- 
bubbles.  If,  however,  they  should  present  themselves,  and  they 
cannot  be  expelled  by  increasing  the  pressure  over  the  part  beneath 
which  they  are,  or  by  slightly  shifting  the  section  from  side  to  side, 
it  is  better  to  take  the  section  entirely  off,  to  melt  a  little  fresh 
balsam  upon  the  glass,  and  then  to  lay  the  section  upon  it  as  before. 
When  the  section  has  been  thus  secured  to  the  glass,  and  the 
attached  part  thoroughly  saturated  (if  it  be  porous)  with  hard 
Canada  balsam,  it  may  be  readily  reduced  in  thickness,  either  by 
grinding  or  filing,  as  before,  or,  if  the  thickness  be  excessive,  by 
taking  off  the  chief  part  of  it  a£  once  by  the  slitting  wheel.  So 
soon,  however,  as  it  approaches  the- thinness  of  a  piece  of  ordinary 
card,  it  should  be  rubbed  down  with  water  on  one  of  the  smooth 
stones  previously  named,  the  glass  slip  being  held  beneath  the 
fingers  with  its  face  downwards,  and  the  pressure  being  applied 
with  such  equality  that  the  thickness  of  the  section  shall  be  (as 
nearly  as  can  be  discerned)  equal  over  its  entire  surface.  As  soon 
as  it  begins  to  be  translucent,  it  should  be  placed  under  the  micro- 
scope (particular  regard  being  had  to  the  method  of  illumination 
so  as  not  to  flood  the  object  with  light),  and  note  taken  of  any 
inequality ;  and  then  when  it  is  again  laid  upon  the  stone,  such 
inequality  may  be  brought  dowrn  by  making  special  pressure  with 
the  forefinger  upon  the  part  of  the  slide  above  it.  When  the 
thinness  of  the  section  is  such  as  to  cause  the  water  to  spread 
around  it  between  the  glass  and  the  stone,  an  excess  of  thick- 
ness on  either  side  may  often  be  detected  by  noticing  the  smaller 
distance  to  which  the  liquid  extends.  In  proportion  as  the  sub- 
stance attached  to  the  glass  is  ground  away,  the  superfluous 
balsam  wThich  may  have  exuded  around  it  will  be  brought  into  con- 
tact with  the  stone  ;  and  this  should  be  removed  with  a  knife, 
care  being  taken,  however,  that  a  margin  be  still  left  round  the  edge 
of  the  section.  As  the  section  approaches  the  degree  of  thinness 
which  is  most  suitable  for  the  display  of  its  organisation,  great  care 
must  be  taken  that  the  grinding  process  be  not  carried  too  far  ;  and 
frequent  recourse  should  be  had  to  the  microscope,  which  it  is 
convenient  to  have  always  at  hand  when  work  of  this  kind  is  being 
carried  on.  There  are  many  substances  whose  intimate  structure 
can  only  be  displayed  in  its  highest  perfection  when  a  very  little 
more  reduction  would  destroy  the  section  altogether ;  and  every 
microscopist  who  has  occupied  himself  in  making  such  preparations 
can  tell  of  the  number  which  he  has  sacrificed  in  order  to  attain 
this  perfection.  Hence,  if  the  amount  of  material  be  limited,  it  is 
advisable  to  stop  short  as  soon  as  a  good  section  has  been  made,  and 
to  lay  it  aside — '  letting  well  alone  ' — whilst  the  attempt  is  being 
made  to  procure  a  better  one  ;  if  this  should  fail,  another  attempt 
may  be  made,  and  so  on,  until  either  success  has  been  attained  or 
the  whole  of  the  material  has  been  consumed  ;  the  first  section, 
however,  still  remaining,  whereas,  if  the  first,  like  every  subsequent 
section,  be  sacrificed  in  the  attempt  to  obtain  perfection,  no  trace 
will  be  left  '  to  show  what  once  has  been.'  In  judging  of  the 


5IO    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

appearance  of  a  section  in  this  stage  under  the  microscope,  it  is  to 
be  remembered  that  its  transparence  will  subsequently  be  consider- 
ably increased  by  mounting  in  Canada  balsam  :  this  is  particularly 
the  case  with  fossils  to  which  a  deep  hue  has  been  given  by  the 
infiltration  of  some  colouring  matter,  and  with  any  substances 
whose  particles  have  a  molecular  aggregation  that  is  rather  amor- 
phous than  crystalline.  When  a  sufficient  thinness  has  been  attained 
the  section  may  generally  be  mounted  in  Canada  balsam  ;  and  the 
mode  in  which  this  must  be  managed  will  be  detailed  hereafter. 

By  a  slight  variation  in  the  foregoing  process,  sections  may  be 
made  of  structures  in  which  (as  in  corals)  hard  and  soft  parts  are 
combined,  so  as  to  show  both  to  advantage.  Small  pieces  of  the 
substance  are  first  to  be  stained  thoroughly  and  are  then  to  be 
'  dehydrated '  by  alcohol.  A  thin  solution  of  copal  in  chloroform  is 
to  be  prepared,  in  which  the  pieces  are  to  be  immersed  ;  and  this 
solution  is  to  be  concentrated  by  slow  evaporation,  until  it  can  be 
drawn  out  in  threads  which  become  brittle  on  cooling.  The  pieces 
are  then  to  be  taken  out,  and  laid  aside  to  harden  ;  and  when  the 
copal  has  become  so  firm  that  the  edge  of  the  finger-nail  makes  no 
impression,  they  are  to  be  cut  into  slices  and  ground  down  attached 
to  glass  in  the  manner  already  described,  the  sections  being  finally 
mounted  in  Canada  balsam.  The  sections  (attached  to  glass)  may 
be  partially  or  completely  decalcified,  the  soft  parts  remaining  in 
situ,  by  first  dissolving  out  the  copal  with  chloroform ;  when,  after 
being  well  washed  in  water,  they  should  be  again  stained,  and 
mounted  either  in  weak  spirit  or  (after  having  been  dehydrated)  in 
Canada  balsam.1 

A  different  mode  of  procedure,  however,  mu  •*;  be  adopted  when 
it  is  desired  to  obtain  sections  of  bone,  tooth,  or  other  finely  tubular 
structures,  impenetrated  by  Canada  balsam.  If  tolerably  thin  sec- 
tions of  them  can  be  cut  in  the  first  instance,  or  if  they  are  of  a  size 
and  shape  to  be  held  in  the  hand  whilst  they  are  being  roughly  ground 
down,  there  will  be  no  occasion  to  attach  them  to  glass  at  all ;  it  is 
frequently  convenient  to  do  this  at  first,  however,  for  the  purpose  of 
obtaining  a  '  hold '  upon  the  specimen  ;  but  the  surface  which  has 
been  thus  attached  must  afterwards  be  completely  rubbed  away  in 
order  to  bring  into  view  a  stratum  which  the  Canada  balsam  shall 
not  have  penetrated.  As  none  but  substances  possessing  considerable 
toughness,  such  as  bones  and  teeth,  can  be  treated  in  this  manner, 
and  as  these  are  the  substances  which  are  most  quickly  reduced  by 
a  coarse  file,, and  are  least  liable  to  be  injured  by  its  action,  it  will 
be  generally  found  possible  to  reduce  the  secticms  nearly  to  the 
required  thinness  by  laying  them  upon  a  piece  of  cork  or  soft  wood 
held  in  a  vice,  and  operating  upon  them  first  with  a  coarser  and  then 
with  a  finer  file.  When  this  cannot  safely  be  carried  farther,  the 
section  must  be  rubbed  down  upon  that  one  of  the  fine  stones  already 
mentioned  which  is  found  best  to  suit  it ;  as  long  as  the  section  is 
tolerably  thick,  the  finger  may  be  used  to  press  and  move  it ;  but  as 

1  See  Koch  in  Zoologischer  Anzeig.  Bd.  i.  p.  36.  The  Author,  having  seen  (by 
the  kindness  of  Mr.  H.  N.  Moseley)  some  sections  of  corals  prepared  by  this  process, 
can  testify  to  its  complete  success. 


CUTTING  HARD   SECTIONS  511 

soon  as  the  finger  itself  begins  to  come  into  contact  with  the  stone, 
it  must  be  guarded  by  a  flat  slice  of  cork,  or  by  a  piece  of  gutta-percha 
a  little  larger  than  the  object.  Under  either  of  these,  the  section 
may  be  rubbed  down  to  the  desired  thinness  ;  but  even  the  most 
careful  working  on  the  finest-grained  stone  will  leave  its  surface 
covered  with  scratches,  which  not  only  detract  from  its  appearance, 
but  prevent  the  details  of  its  internal  structure  from  being  as  readily 
made  out  as  they  can  be  in  a  polished  section.  This  polish  may  be 
imparted  by  rubbing  the  section  with  putty-powder  (peroxide  of  tin) 
and  water  upon  a  leather  strap  made  by  covering  the  surface  of  a 
board  with  buff  leather,  having  three  or  four  thicknesses  of  cloth, 
flannel,  or  soft  leather  beneath  it  ^  this  operation  must  be  performed 
on  both  sides  of  the  section,  until' aH  the  marks  of  the  scratches  left 
by  the  stone  shall  have  been  rubbed  out,  when  the  specimen  will  be 
fit  for  mounting  '  dry,'  after  having  been  carefully  cleansed  from  any 
adhering  particles  of  putty-powder. 

Greater  facility  in  the  grinding  of  hard  sections,  as  well  as  supe- 
riority of  result,  is  attainable  by  simple  mechanical  means. 

A  cutting  machine  will  greatly  facilitate  the  process  of  preparing 


FIG.  411. — Hand  machine  for  cutting  hard  sections. 

rock  slices.  The  thickness  of  each  slice  must  be  mainly  regulated 
by  the  nature  of  the  rock,  the  rule  being  to  make  it  as  thin  as  can 
be  conveniently  cut,  so  as  to  save  labour  in  grinding  down  afterwards. 
Perhaps  the  thickness  of  a  shilling  may  be  taken  as  a  fair  average. 
This  thickness  may  be  still  further  reduced  by  cutting  and  polishing 
a  face  of  the  specimen,  cementing  that  on  glass,  and  then  cutting  as 
close  as  possible  to  the  cemented  surface.  The  thin  slice  thus  left 
on  the  glass  can  then  be  ground  down  writh  comparative  ease. 

The  first  (fig.  411)  is  a  hand  machine.  The  specimen  is  cemented 
to  the  carrier,  «,  which  is  movable  on  the  axis,  5,  and  can  also  be 
rotated  in  two  directions.  The  object  is  pressed  by  the  weight,  c. 
against  the  steel  disc,  tZ,  which  is  revolved  by  the  wheel,  e,  acting  on 
a  smaller-toothed  wheel  on  the  axis  of  d. 

The  second  (fig.  412)  is  intended  to  be  worked  by  the  foot.  The 
pails  a,  b,  c,  and  d  are  the  same  as  before.  The  wheel  and  treadle  at 
f  and  g  work  the  pulley,  e,  by  which  the  steel  disc,  cZ,  is  revolved  ;  h 
is  part  of  the  cover  for  the  disc,  tc  prevent  the  emery  flying  about. 
A  box  beneath  also  catches  the  powder  that  falls. 

(This  arrangement  is  also  supplied  with  fig.  411,  though  not  shown 
in  the  woodcut.)  A  second  wheel  at  i,  with  a  cord  passing  over  &, 


512    PREPARATION,   MOUNTING,   AND    COLLECTION   OF   OBJECTS 

actuates  a  vertical  spindle,  /,  which   rotates  a  horizontal   cast-iron 
plate  at  m  for  polishing. 

Decalcification. — When  it  is  desired  to  examine  the  structure  of 
the  organic  matrix  in  which  the  calcareous  salts  are  deposited  that 
give  hardness  to  many  animal  and  to  a  few  vegetable  struct  vires  (such 
as  the  true  corallines),  these  salts  must  be  dissolved  away  by  the 
action  of  some  acid,  such  as  nitric  or  hydrochloric.  This  should  be 
employed  in  a  very  dilute  state,  in  order  that  it  may  make  as  little 
change  as  possible  in  the  soft  tissue  it  leaves  behind.  When  jthe 


FIG.  412. — Treadle  machine  for  cutting  hard  sections. 

lime  is  in  the  state  of  carbonate  (as,  for  example,  in  the  skeletons  of 
echinodenns),  the  body  to  be  decalcified  should  be  placed  in  a  glass 
jar  or  wide-mouthed  bottle  holding  from  4  to  6  oz.  of  water,  and  the 
acid  should  be  added  drop  by  drop,  until  the  disengagement  of  air- 
bubbles  shows  that  it  is  taking  effect ;  and  the  solvent  process  should 
be  allowed  to  take  place  very  gradually,  more  acid  being  added  as 
required.  When,  on  the  other  hand,  much  of  the  lime  is  in  the 
state  of  phosphate,  as  in  bones  and  teeth,  the  strength  of  the  acid 
solvent  must  be  increased  ;  and  for  the  hardening  of  the  softer  parts 
of  the  organic  matrix  it  is  desirable  that  chromic  acid  .should  be 


DESILICIFICATION  5  1 3 

used.  In  the  case  of  small  bones,  or  delicate  portions  of  large 
(such  as  the  cochlea  of  the  ear),  a  ^  per  cent,  solution  of  chromic 
acid  will  itself  serve  as  the  solvent ;  but  larger  masses  require  either 
nitric  or  hydrochloric  acid  in  addition,  to  the  extent  of  2  per  cent, 
of  the  former  or  5  per  cent,  of  the  latter.  By  some  the  chromic  and 
the  nitric  or  hydrochloric  acid  are  mixed-in  in  the  first  instance, 
while  by  others  it  is  recommended  that  the  bone  should  lie  first  in 
the  chromic  acid  solution  for  a  week  or  ten  days,  and  that  the 
second  acid  should  be  then  added.  If  the  softening  be  not  com- 
pleted in  a  month,  more  acid  must  be  added.  When  thoroughly 
decalcified,  the  bone  should  be  transferred  to  rectified  spirit ;  and 
it  may  then  be  either  sliced  in  the  microtome'  or  torn  into  shreds 
for  the  demonstration  of  its  lamelbe.  Acid  solvents  may  also  be 
employed  in  removing  the  outer  parts  of  calcareous  skeletons,  for  the 
display  of  their  internal  cavities  (a  plan  which  the  Author  has  often 
found  very  useful  in  the  study  of  Foraminifera),  or  for  getting  rid  of 
them  entirely,  so  as  to  bring  into  complete  view  any  '  internal  cast ' 
which  may  have  been  formed  by  the  silicification  of  its  originally 
soft  contents.  It  has  been  in  this  mode,  even  more  than  by  the 
cutting  of  thin  sections,  that  the  structure  of  Eozoon  canadense  has 
been  elucidated  by  Professor  Dawson  and  the  Author.  For  the  first 
of  these  purposes  strong  acid  should  be  applied  (under  the  dissecting 
microscope)  with  a  fine  camel's-hair  pencil ;  and  another  such  pencil 
charged  with  water  should  be  at  hand,  to  enable  the  observer  to  stop 
the  solvent  action  whenever  he  thinks  it  has  been  carried  far  enough. 
For  the  second  it  is  better  that  the  acid  should  only  be  strong  enough 
for  the  slow  solution  of  the  shelly  substance,  as  the  too  rapid  disen- 
gagement of  bubbles  often  produces  displacement  of  delicate  parts 
of  the  substituted  mineral ;  whilst,  if  the  acid  be  too  strong,  the 
'  internal  cast '  may  be  altogether  dissolved  away. 

Busch  suggests  nitric  acid  as  the  best  of  all  agents  for  decalcifica- 
tion,  insomuch  as  it  does  not  cause  '  swelling  up,'  nor  injuriously 
attack  the  tissue  elements. 

One  volume  of  chemically  pure  nitric  acid  of  specific  gravity  1'25 
diluted  with  ten  volumes  of  water  may  be  employed  for  large  and 
tough  bones ;  but  it  may  be  diluted  to  1  per  cent,  for  young  bones. 

The  method  given  is  that  fresh  bones  should  be  laid  in  alcohol 
of  95  per  cent,  for  three  days ;  they  must  then  be  placed  in  the 
nitric  acid,  which  must  be  changed  daily  for  eight  days.  They  must 
not  remain  after  the  decalcification  is  complete,  or  they  will  become 
yellow.  On  removal  the  bones  must  be  washed  for  a  couple  of  hours 
in  running  water  and  placed  again  in  95  per  cent,  alcohol,  and  in  a 
few  days  placed  again  in  fresh  alcohol. 

Desilicification. — It  is  desirable  to  be  able  to  remove  siliceous  as 
well  as  calcareous  elements  from  objects.  To  do  this  a  glass  vessel 
should  be  carefully  coated  with  paraffin  internally,  to  prevent  the 
action  of  the  acid  used  taking  place  on  the  sides  of  the  vessel.  The 
subject  to  be  cleared  of  its  silica  is  placed  in  alcohol  in  the  coated 
vessel,  and  hydrofluoric  acid  is  added  drop  by  drop.  As  the  mucous 
membranes  are  fiercely  attacked  by  this  acid,  great  care  must  be 
exercised  in  its  use  ;  but  small  sponges  and  other  similar  siliceous 

L  L 


5 14    PREPARATION,   MOUNTING,    AND    COLLECTION   OF   OBJECTS 

objects  by  remaining  a  few  hours  or  a  day  in  this  are  wholly  deprived 
of  their  silica,  while  the  tissues  do  not  suffer. 

Preparation  of  Vegetable  Substances. — Little  preparation  is 
required,  beyond  steeping  for  a  short  time  in  distilled  water  to  get 
rid  of  saline  or  other  impurities,  for  mounting  in  preservative  media 
specimens  of  the  minuter  forms  of  vegetable  life,  or  portions  of  the 
larger  kinds  of  algca,  fungi,  or  other  succulent  cryptogams.  But 
the  woody  structures  of  phanerogams  are  often  so  consolidated  by 
gummy,  resinous,  or  other  deposits  that  sections  of  them  should  not 
be  cut  until  they  have  been  softened  by  being  partially  or  wholly 
freed  from  these.  Accordingly,  pieces  of  stems  or  roots  should  be 
soaked  for  some  days  in  water,  with  the  aid  of  a  gentle  heat  if  they 
are  very  dense,  and  should  then  be  steeped  for  some  days  in  methy- 
lated spirit,  after  which  they  should  again  be  transferred  to  water. 
The  same  treatment  may  be  applied  to  hard-coated  seeds,  the  '  stones  ' 
of  fruit, '  vegetable  ivory,'  and  other  like  substances.  Some  vegetable 
substances,  on  the  other  hand,  are  too  soft  to  be  cut  sufficiently  thin 
without  previous  hardening,  either  by  allowing  them  to  lose  some  of 
their  moisture  by  evaporation,  or  by  drawing  it  out  by  steeping  them 
in  spirit.  Either  treatment  answers  very  well  with  such  substances 
as  that  which  forms  the  tuber  of  the  potato,  sections  of  which 
display  the  starch-grains  in  situ.  Where,  on  the  other  hand,  it  is 
desired  to  preserve  colour,  spirit  must  not  be  used  ;  and  recourse  nrny 
be  had  to  gum-imbedding,  which  is  particularly  serviceable  where 
the  substance  is  penetrated  by  air-cavities,  as  is  the  case  with  the 
stem  of  the  rush,  the  thick  leaves  of  the  water -tity,  &c.  The  tissue 
is  well  soaked  in  a  syrupy  solution  of  gum  arable,  and  this  is  then 
hardened,  either  by  allowing  it  to  slowly  evaporate,  or  by  throwing 
it  into  strong  alcohol,  or  by  freezing  it.  But  where  staining  processes 
are  to  be  employed,  the  substance  should  be  previously  bleached  by 
the  action  of  chlorine  (preferably  by  Labarraque's  chlorinated  soda), 
and  then  treated  with  alcohol  for  a  few  hours. 

For  the  rest,  the  minute  structure  of  the  higher  plants  is  studied 
by  means  of  the  methods  of  fixing,  staining,  and  section-cutting 
above  described  for  the  tissues  of  animals.  For  plants,  absolute 
alcohol  is  much  used  as  a  fixing  agent,  the  other  reagents  employed 
in  their  preparation  being  in  general  the  same  as  those  used  in 
animal  histology. 

Staining  Bacteria. — It  is  needful  to  employ  somewhat  special- 
ised methods  for  staining  the  saprophytic,  pathogenic,  and  other 
schizomycetes.  Some  of  these  stain  admirably,  but  others,  especially 
the  somewhat  larger  forms,  are  much  altered,  and  unless  observa- 
tions are  controlled  with  accurate  and  constant  observations  on  the 
organisms  in  a  living  condition  the  most  egregious  errors  may  arise. 

(1)  Take  half  a  dozen  cases  of  putrescence  in  which  solid  tissues 
are  decomposing,  but  which  are  in  different  states  of  decomposition. 
From  each  take  out  with  a  pipette  a  small  quantity,  and  transfer  to 
a  carefully  prepared  and  well-filtered  decoction  of  veal  in  a  small 
glass  vessel,  at  the  temperature  of  the  respective  putrefactions  ;  leave 
this  for  half  an  hour.  Then  with  a  fine  pipette  take  out  a  minute 
drop  from  each  vessel  and  diffuse  each  drop  upon  a  cover-glass  ;  let 


STAINING  BACTERIA  515 

evaporation  go  on  in  a  -warm  room  for  twenty  minutes,  then  fix  the 
film  of  saprophytes  by  means  of  fairly  strong  osmic  acid  vapour  ; 
float  the  cover  with  the  surface  of  bacteria  downwards  on  a  vessel 
of  solution  of  violet  of  methyl-anilin  for  an  hour  or  less,  drain  the 
edge  of  the  cover-glasses  on  blotting-paper,  and  mount  in  glycerin. 

(2)  Now  take  drops  of  the  fluid  from  the  several  vessels  and  in 
a  moist  growing  cell  examine  the  living  forms,  and  compare  these 
with  your  dried  and  stained  preparations. 

(3)  By  another  method,  which  will  apply  also  to  the  bacillus  of 
tuberculosis,  a  layer  of  sputum  or  of  putrefactive  fluid  may  be  spread 
as  before  upon  a  cover-glass,  dried  in  an  air-oven  at  about  100°  F., 
and  then  passed  three  times,  moderately  slowly,  through  the  flame 
of  a    spirit-lamp,    so    as   to  thoroughly   '  fix '    the    preparation    by 
coagulating   its  albumen.     Mix   1  c.c.  of  concentrated  solution  of 
methylen-blue  in  alcohol,  O2  c.c.  of  10  per  cent,  solution  of  potash, 
and  200  c.c.  of  distilled  water.     On  to  this  float  the  cover  with  its 
surface  of  bacteria  downwards  and  leave  for  twenty-four  hours  ;  the 
film  will  be  coloured  blue  ;  place  a  few  drops  of  a  solution  of  vesuvin 
all  over  the  film,  which  drives  out  the  methylen-blue  from  all  but 
the  bacteria.     Finish  with  alcohol  and  oil  of  cloves,  and  mount  in 
balsam. 

For  the  same  purpose  Professor  Heneage  Gibbes  gives  a  method 
which  has  proved  of  great  value.  Take  of  rosanilin  hydrochloride 
2  grms.,  methylen-blue  1  grm.  ;  rub  them  up  in  a  glass  mortar.  Then 
dissolve  anilin  oil,  3  c.c.,  in  rectified  spirit,  15  c.c.  ;  add  the  spirit 
slowly  to  the  stains  until  all  is  dissolved,  then  slowly  add  distilled 
water,  15  c.c.  Keep  in  a  stoppered  bottle. 

In  the  usual  way  dry  the  sputum,  &c.,  on  a  cover-glass  and  fix  in 
a  flame  as  a  few  drops  of  the  stain  are  poured  into  a  test-tube  and 
warmed.  As  soon  as  steam  rises  pour  into  a  watch-glass  and  float 
the  cover-glass  on  the  warm  stain  ;  allow  it  to  remain  four  or  five 
minutes  ;  or  if  we  do  not  heat  the  stain  but  use  it  cold,  let  it  remain 
for  at  least  half  an  hour.  Wash  in  methylated  spirit  until  no 
colour  comes  off ;  drain,  and  then  dry  in  an  air-oven,  and  mount  in 
balsam. 

Staining  Bacteria  in  Tissues  (Lofner's  solution). — To  100  parts 
of  solution  of  caustic  potash  of  1  :  10,000  add  30  parts  of  saturated 
alcoholic  solution  of  methylen-blue.  Filter.  Stain  section  for  one 
or  two  hours,  wash  out  with  acetic  acid  of  \  per  cent.,  followed  by 
water.  Dehydrate  with  absolute  alcohol,  clear  with  cedar  oil,  and 
mount  in  balsam. 

A  process  of  differential  staining  of  bacillus  tuberculosis  which 
was  devised  by  MM.  Pittion  and  Roux  was  presented  recently  (1889) 
to  the  Societe  de  Medecine  de  Lyon,  and  has  met  with  high  com- 
mendation. It  requires  three  solutions  : — 

A.  Ten  parts  of  fuchsin  dissolved  in  100  parts  of  absolute  alcohol. 

B.  Three  parts  of  liquid  ammonia  dissolved  in  100  parts  of  distilled 
water. 

C.  Alcohol  50  parts,  water  30  parts,  nitric  acid  20  parts,  anilin- 
green  to  saturation.     In  preparing  this  solution  dissolve  the  green 
in  the  alcohol,  add  the  water,  and  lastly  the  acid. 

L2 


516    PEEPAKATION,   MOUNTING,    AND   COLLECTION   OF   OBJECTS 

It  is  used  thus,  viz.  to  10  parts  of  solution  B  add  one  part  of 
solution  A,  and  heat  until  vapour  shows  itself,  then  immerse  the 
whole  cover-glass  prepared  as  in  the  ordinary  way  for  staining.  One 
minute  suffices  to  stain  the  bacilli.  Wash  with  plenty  of  water,  and 
after  rinsing  with  distilled  water  drop  on  the  film  side  of  the  cover- 
glass  a  small  quantity  of  solution  C,  which  is  not  to  remain  more 
than  forty  seconds.  Wash  off  with  plenty  of  water,  dry,  and  mount 
in  xylol  balsam. 

The  bacilli  will  be  found  to  be  stained  a  fine  rose-red  upon  a  pale- 
green  ground. 

Staining  Flagella. — The  following  is  the  latest  form  of  the  cele- 
brated method  of  Loffler.  A  mordant  is  made  as  follows  :  To  10  c.c. 
of  a  20  per  cent,  aqueous  solution  of  tannin  are  added  5  c.c.  of 
cold  saturated  solution  of  ferrous  sulphate  and  1  c.c.  of  (either 
aqueous  or  alcoholic)  solution  of  fuchsin,  methyl-violet,  or  '  Woll- 
schwarz.'  Cover-glass  preparations  are  made  and  fixed  in  a  flame 
in  the  manner  described  above,  special  care  being  taken  not  to  over- 
heat. Whilst  still  warm  the  preparation  is  treated  with  the  above 
described  mordant,  and  is  heated  in  contact  with  it  for  half  si 
minute,  until  the  liquid  begins  to  vaporise,  after  which  it  is  washed 
in  distilled  water  and  then  in  alcohol.  It  is  then  treated  in  a 
similar  manner  with  the  stain,  which  consists  of  a  saturated  solution 
of  fuchsin  in  anilin  water  (water  in  which  a  little  anilm  oil  has 
been  shaken  up  and  filtered),  the  solution  being  preferably 
neutralised  to  the  point  of  precipitation  by  cautious  addition  of 
O'l  per  cent,  soda  solution.  For  some  further  details  concerning  this 
process,  the  'Journal  of  the  Royal  Microscopical  Society'  for  1890, 
p.  678,  may  be  consulted. 

Chemical  Testing. — It  is  often  requisite,  alike  in  biological  and 
in  mineralogical  investigations,  to  apply  chemical  tests  in  minute 
quantity  to  objects  under  microscopic  examination.  Various  con- 
trivances have  been  devised  for  this  purpose  ;  but  the  Author  would 
recommend,  from  his  own  experience,  the  small  glass  syringe  already 
described,  or  preferably  the  drop  bottle,  pp.  475-477,  with  a  fine- 
pointed  nozzle,  as  the  most  convenient  instrument.  One  of  its  advan- 
tages is  the  very  precise  regulation  of  the  quantity  of  the  test  to  be 
deposited  which  can  be  obtained  by  the  dexterous  use  of  it ;  whilst 
another  consists  in  the  power  of  withdrawing  any  excess.  Care  must 
be  taken  in  using  it  to  avoid  the  contact  of  the  test-liquid  with  the 
packing  of  the  piston.  Whatever  method  is  employed,  great  care 
should  be  taken  to  avoid  carrying  away  from  the  slide  to  which  the 
test-liquid  is  applied  any  loose  particles  which  may  lie  upon  it,  and 
which  may  be  thus  transferred  to  some  other  object,  to  the  great 
perplexity  of  the  microscopist.  For  testing  inorganic  substances  the 
ordinary  chemical  reagents  are  of  course  to  be  employed  ;  but  certain 
special  tests  are  required  in  biological  investigation,  the  following 
being  those  most  frequently  required  : — 

a.  Solution  of  iodine  in  water  (1  gr.  of  iodine,  3  grs.  of  iodide  of 
potassium,  1  oz.  of  distilled  water)  turns  starch  blue  and  'cellulose 
brown ;  it  also  gives  an  intense  brown  to  albuminous  substances. 

/3.  Chlor-iodide  of  zinc  (Schultze's  solution)  is  perhaps  best  made 


CHEMICAL  TESTING— PRESERVATIVE   MEDIA  517 

as  follows: — Evaporate  100  c.c.  of  liquor  zinci  chloridi  (B.P.)  to 
70  c.c.  ;  dissolve  in  it  10  grms.  of  iodide  of  potassium  ;  then  add 
0'2  grm.  iodine ;  shake  at  intervals  till  saturated. 

This  is  extremely  useful  for  the  detection  of  pure  cellulose.  The 
zinc  chloride  converts  cellulose  into  amyloid,  which  is  then  turned 
blue  by  free  iodine.  Wood-cells,  cork-cells,  the  extine  of  pollen 
grains,  and  all  lignified  or  corky  membranes,  are  coloured  yellow. 
Starch  colours  blue,  but  is  rapidly  disorganised. 

A  very  weak  solution  will  instantly  detect  tannin,  the  cell  con- 
tents in  which  it  forms  a  part  becoming  reddish  or  violet. 

y.  Solution  of  caustic  potass  or  soda  (the  latter  being  generally 
preferable)  has  a  remarkable  solvent  effect  upon  many  organic  sub- 
stances, both  animal  and  vegetable,  and  is  extremely  useful  in 
rendering  some  structures  transparent,  whilst  others  are  brought 
into  view,  its  special  action  being  upon  horny  textures,  whose 
component  cells  are  thus  rendered  more  clearly  distinguishable. 

8.  Dilute  sulphuric  acid  (one  of  acid  to  two  or  three  parts  of 
water)  gives  to  cellulose  that  has  been  previously  dyed  with  iodine 
a  blue  or  purple  hue ;  also,  when  mixed  with  a  solution  of  sugar,  it 
gives  a  rose-red  hue,  more  or  less  deep,  with  nitrogenous  substances 
and  with  bile  (Pettenkofer's  test). 

Sulphuric  acid  causes  starch  grains  to  swell  and  similarly  affects 
cellulose. 

c.  Concentrated  nitric  acid  gives  to  albuminous  substances  an 
intense  yellow. 

£.  Acid  nitrate  of  mercury  (Millon's  test)  (ten  parts  of  mercury, 
ten  of  fuming  nitric  acid,  and  twenty  of  water)  colours  albuminous 
substances  red. 

rj.  Acetic  acid,  which  should  be  kept  both  concentrated  and  diluted 
with  from  three  to  five  parts  of  water,  is  very  useful  to  the  animal 
histologist  from  its  power  of  dissolving,  or  at  least  of  reducing  to  such  a 
stage  of  transparence  that  they  can  no  longer  be  distinguished,  certain 
kinds  of  membranous  and  fibrous  tissues,  so  that  other  parts  (especially 
nuclei}  are  brought  more  strongly  into  view. 

0.  Ether  dissolves  resins,  fats,  and  oils  ;  but  it  will  not  act  on 
these  through  membranes  penetrated  with  watery  fluid.     For  the 
same  purpose  chloroform,  benzol,  oil  of  turpentine,  and  carbon  bisul- 
phide are  used. 

1.  Alcohol  dissolves  resins  and  some  volatile  oils,  but  it  does  not 
act  on  ordinary  oils  and  fats.     It  coagulates  albuminous  matters,  and 
consequently  renders  more  opaque  such  textures  as  contain  them. 

K.  Osmic  acid  is  a  test  for  fatty  matters,  which  it  stains  black 
in  varying  degrees  ;  and  in  like  manner  for  gallic  and  tannic  acids. 

Preservative  and  Mounting  Media. — We  have  now  to  consider 
the  various  modes  of  preserving  the  preparations  that  have  been 
made  by  the  several  methods  indicated  above,  and  shall  first  treat 
of  such  as  are  applicable  to  those  minute  animal  and  vegetable 
organisms,  and  to  those  sections  or  dissections  of  large  structures, 
which  are  suitable  for  being  mounted  as  transparent  objects.  A 
broad  distinction  may  be  in  the  first  place  laid  down  between 
resinous  and  aqueous  preservative  media ;  to  the  former  belong 


518    PREPARATION,   MOUNTING,   AND   COLLECTION   OF   OBJECTS 

Canada  balsam  and  dammar,  while  the  latter  include  all  the  mix- 
tures of  which  water  is  a  component ;  while  partly  dehydrating 
media,  such  as  glycerin  and  alcohol,  occupy  an  intermediate  position. 
The  choice  between  the  three  kinds  of  media  will  partly  depend 
upon  the  nature  of  the  processes  to  which  the  object  may  have  been 
previously  subjected  and  partly  upon  the  degree  of  transparence 
which  may  be  advantageously  imparted  to  it.  Sections  of  substances 
which  have  been  not  only  imbedded  in  but  penetrated  by  paraffin, 
and  have  been  stained  (if  desired)  previously  to  cutting,  are,  as  a  rule, 
most  conveniently  mounted  in  Canada  balsam  or  dammar ;  since 
they  can  be  at  once  transferred  to  either  of  these  from  the  menstruum 
by  which  the  imbedding  material  has  been  dissolved  out.  The  dura- 
bility of  this  method  of  mounting  makes  it  preferable  in  all  cases  to 
which  it  is  suitable,  the  exception  being  where  it  renders  a  very 
thin  section  too  transparent.  In  such  cases  sections  or  other 
objects  may  sometimes  be  more  advantageously  mounted  in  some  of 
those  aqueous  preparations  of  glycerin  which  approach  the  resinous 
media  in  transparence  and  permanence.  "When  Canada  balsam  was 
first  employed  for  mounting  preparations  it  was  employed  in  its 
natural  semi-fluid  state,  in  which  it  consists  of  a  solution  of  resin  in 
volatile  oil  of  turpentine  ;  and  unless  a  large  proportion  of  the  latter 
constituent  was  driven  off  by  heat  in  the  process  of  mounting 
(bubbles  being  thus  formed  of  which  it  was  often  difficult  to  get 
rid),  or  the  mounted  slide  was  afterwards  subjected  to  a  more 
moderate  heat  of  long  continuance,  the  balsam  would  remain  soft, 
and  the  cover  liable  to  displacement.  This  is  avoided  by  the  method 
now  generally  adopted  of  previously  getting  rid  of  the  turpentine  by 
protracted  exposure  of  the  balsam  to  a  heat  not  sufficient  to  boil 
it,  and  dissolving  the  resin  thus  obtained  either  in  xylol,  benzol,  or 
chloroform,  but  far  preferably  the  former,  the  solution  being  made 
of  such  viscidity  as  will  allow  it  to  '  run  '  freely.  Either  of  these 
solvents  evaporates  so  much  more  quickly  than  turpentine  that  the 
balsam  left  behind  hardens  in  a  comparatively  short  time.  Xylol- 
balsam  is  now  preferred  by  most  mounters.  It  is  made  of  equal 
volumes  of  xylol  and  balsam.  The  natural  balsam,  however,  may  be 
preferably  used  (with  care  to  avoid  the  liberation  of  bubbles  by 
overheating)  in  mounting  sections  already  cemented  to  the  slides 
by  hardened  balsam,  and  also  for  mounting  the  chitirious  textures 
of  insects,  which  it  has  a  peculiar  power  of  rendering  transparent, 
and  which  seem  to  be  penetrated  by  it  more  thoroughly  than  they 
are  by  the  artificially  prepared  solution.  The  solution  of  dammar 
in  xylol  is  very  convenient  to  work  with,  and  hardens  quickly. 

The  following  are  the  principal  aqueous  media  whose  value  has 
been  best  tested  by  general  and  protracted  experience : — 

a.  Fresh  specimens  of  minute  protophytes  can  often  be  very  well 
preserved  in  distilled  water  saturated  with  camphor,  the  complete 
exclusion  of  air  serving  both  to  check  their  living  actions  and  to 
prevent  decomposing  changes.  When  the  preservation  of  colour 
is  not  a  special  object  about  a  tenth  part  of  alcohol  may  be  added,  and 
this  will  be  found  a  suitable  medium  for  the  preservation  of  many 
delicate  animal  textures. 


PRESERVATIVE   MOUNTING-  MEDIA  519 

/3.  Salt  solution,  075  per  cent,  sodium  chloride  in  water.  Use- 
ful as  a  medium  for  temporary  examination,  but  not  for  permanent 
preservation. 

y.   White  of  an  egg. — Simply  filter. 

8.  Syrup  in  which  is  dissolved  1  to  5  per  cent,  of  chloral 
hydrate,  or  1  per  cent,  of  carbolic  acid. 

€.  Liquid  of  Ripart  and  Petit. — Camphor  water  (not  saturated), 
75  grms. ;  distilled  water,  75  grms. ;  glacial  acetic  acid,  1  grm. ; 
acetate  of  copper,  O30  grm. ;  chloride  of  copper,  0'30  grm.  Maybe 
added  to  preparations  stained  with  methyl-green,  which  it  does  not 
precipitate,  and  may  be  used  for  preserving  either  vegetal  or  animal 
tissues. 

£.  Fabre-D  ornery  lie's  Glucose  ^tedium. — Glucose  syrup  of  specific 
gravity  1-1968,  1,000  parts;  methyl  alcohol  (wood  spirit),  200; 
glycerin,  100;  camphor  to  saturation.  The  glucose  to  be  dissolved 
in  warm  water  and  the  other  ingredients  added,  and  the  mixture, 
which  is  always  acid,  neutralised  with  a  little  potash  or  soda. 

rj.  Chloral  Hydrate. — A  5  per  cent,  solution  in  water,  or  12  grains 
chloral  hydrate  to  1  fluid  ounce  of  camphor  water.  (Mount  in  strong 
glycerin  jelly.) 

6.  Bruris  Glucose  Medium. — Distilled  water,  140  parts;  cam- 
phorated spirit,  10  parts ;  glucose,  40 ;  glycerin,  10.  Mix  the 
water,  glucose,  and  glycerin,  then  add  the  spirit,  and  filter  to  remove 
the  excess  of  camphor  which  is  precipitated.  This  medium  preserves 
the  colour  of  preparations  stained  with  anilin  dyes,  methyl-green 
included. 

L.  Gum  and  Syrup. — Gum-mucilage  (B.P.)  five  parts,  syrup  three 
parts.  Add  5  grains  of  pure  carbolic  acid  to  each  ounce  of  the  medium. 

B.P.  gum-mucilage  is  made  by  putting  4  oz.  of  picked  gum  acacia 
in  6  oz.  of  distilled  water  until  dissolved. 

Syrup  is  made  by  dissolving  a  pound  of  loaf  sugar  in  a  pint  of 
distilled  water  and  boiling. 

K.  The  glycerin  jelly  prepared  after  the  manner  of  Mr.  Lawrence 
may  be  strongly  recommended  as  suitable  for  a  great  variety  of 
objects,  animal  as  well  as  vegetable,  subject  to  the  cautions  already 
given  : — 'Take  any  quantity  of  Nelson's. gelatin,  and  let  it  soak  for 
two  or  three  hours  in  cold  water,  pour  off  the  superfluous  water,  and 
heat  the  soaked  gelatin  until  melted.  To  each  fluid  ounce  of  the 
gelatin  add  one  drachm  of  alcohol  and  mix  well ;  then  add  a  fluid 
drachm  of  the  white  of  an  egg.  Mix  well  while  the  gelatin  is  fluid, 
but  cool.  Now  boil  until  the  albumen  coagulates,  and  the  gelatin 
is  quite  clear.  Filter  through  fine  flannel,  and  to  each  fluid  ounce  of 
the  clarified  gelatin  add  six  fluid  drachms  of  Price's  pure  glycerin, 
and  mix  well.  For  the  six  fluid  drachms  of  glycerin  a  mixture  of 
two  parts  of  glycerin  to  four  of  camphor-water  may  be  substituted. 
The  objects  intended  to  be  mounted  in  this  medium  are  best  prepared 
by  being  immersed  for  some  time  in  a  mixture  of  one  part  of  glycerin 
with  one  part  of  diluted  alcohol  (one  of  alcohol  to  six  of  water).' J  A 
small  quantity  of  absolute  phenol  may  be  added  to  it  with  advantage. 

1  A  very  pure  glycerin  jelly,  of  which  the  Author  has  made  considerable  use,  is 
prepared  by  Mr.  Rimmington,  chemist,  Bradford,  Yorkshire. 


520    PREPARATION,    MOUNTING,    AND    COLLECTION    OF   OBJECTS 

When  used,  the  jelly  must  be  liquefied  by  gentle  warmth,  and  it  is 
useful  to  warm  both  the  slide  and  the  cover-glass  previously  to 
mounting.  This  takes  the  place  of  what  was  formerly  known  as 
Dean's  medium,  in  which  honey  was  used  to  prevent  the  hardening 
of  the  gelatin.  i 

A.  For  objects  which  would  be  injured  by  the  small  amount  of 
heat  required  to  liquefy  the  last-mentioned  medium,  the  glycerin  and 
gum  medium  of  Mr.  Farrants  will  be  found  very  useful.  This  is 
made  by  dissolving  four  parts  (by  weight)  of  picked  gum  arabic  in  four 
parts  of  cold  distilled  water,  and  then  adding  two  parts  of  glycerin. 
The  solution  must  be  made  without  the  aid  of  heat,  the  mixture  being 
occasionally  stirred,  but  not  shaken,  whilst  it  is  proceeding ;  after  it 
has  been  completed  the  liquid  should  be  strained  (if  not  perfectly 
free  from  impurity)  through  fine  cambric  previously  well  washed  out 
by  a  current  of  clean  cold  water  ;  and  it  should  be  kept  in  a  bottle, 
closed  with  a  glass  stopper  or  cap  (not  with  cork),  containing  a  small 
piece  of  camphor.  The  great  advantage  of  this  medium  is  that  it 
can  be  used  cold,  and  yet  soon  viscifies  without  cracking ;  it  is  well 
suited  to  preserve  delicate  animal  as  well  as  vegetable  tissues,  and  in 
most  cases  increases  their  transparence. 

Of  late  years  glycerin  has  been  largely  used  as  a  preservative, 
either  alone,  according  to  the  method  of  Dr.  Beale,  or  diluted 
with  water,  or  mixed  with  gelatinous  substances.  It  is  much  more 
favourable  to  the  preservation  of  colour  than  most  other  media,  and 
is  therefore  specially  useful  as  a  constituent  of  fluids  used  for 
mounting  vegetable  objects  in  their  natural  aspects.  It  has  also  the 
property  of  increasing  the  transparence  of  animal  structures,  though 
in  a  less  degree  than  resinous  substances,  and  may  thus  be  advan- 
tageously employed  as  a  component  of  media  for  mounting  objects 
that  are  rendered  too  transparent  by  balsam  or  dammar.  Two 
cautions  should  be  given  in  regard  to  the  employment  of  glycerin  : 
first,  that,  as  it  has  a  solvent  power  for  carbonate  of  lime,  it  should 
not  be  used  for  mounting  any  object  having  a  calcareous  skeleton  ; 
and  second,  that,  in  proportion  as  it  increases  the  transparence  of 
organic  substances,  it  diminishes  the  reflecting  power  of  their 
surfaces,  and  should  never  be  employed,  therefore,  in  the  mounting  of 
objects  to  be  viewed  by  reflected  light,  although  many  objects 
mounted  in  the  media  to  be  presently  specified  are  beautifully 
shown  by  *  dark-ground '  illumination. 

1.  A   mixture   of    one    part    of    glycerin    and    two    parts   of 
camphor-water  may  be  used  for  the  preservation  of  many  vegetable 
structures. 

2.  For  preserving  soft  and   delicate  marine  animals  which  are 
shrivelled  up,  so  to  speak,  by  stronger  agents,  the  Author  has  found 
a  mixture  of  one  part  of  glycerin  and  one  of  spirit  with  eight  or  ten 
parts  of  sea- water  the  most  suitable  preservative. 

3.  For  preserving  minute  vegetable  preparations  the  following 
method,  devised  by  Hantsch,  is  said  to  be  peculiarly  efficient :  A  mix- 
ture is  made  of  three  parts  of  pure  alcohol,  two  parts  of  distilled  water, 
and  one  part  of  glycerin  ;  and  the  object,  laid  in  a  cement-cell,  is 
to  be  covered  with  a  drop  of  this  liquid,  and  then  putaside  under  a  bell- 


PRESERVATIVE   MOUNTING  MEDIA  521 

glass.  The  alcohol  and  water  soon  evaporate,  so  that  the  glycerin 
alone  is  left ;  and  another  drop  of  the  liquid  is  then  to  be  added, 
and  a  second  evaporation  permitted,  the  process  being  repeated,  if 
necessary,  until  enough  glycerin  is  left  to  fill  the  cell,  which  is 
then  to  be  covered  and  closed  in  the  usual  mode.1 

Canada  balsam  is  one  of  the  most  universally  employed  mounting 
media  ;  very  old  hard  balsam  should  be  dissolved  in  enough  pure 
xylol  or  chloroform  to  make  a  thin  solution,  which  should  be  care- 
fully filtered. 

Dammar. — Dissolve  gum -dammar  with  heat  in  a  mixture  of 
equal  parts  of  benzole  and  turpentine,  and  evaporate  to  a  syrupy 
consistency.  This  is  pleasant  to  use,  but  treacherous.  Dammar 
dissolved  in  pure  xylol  in  the  cold  gives  a  beautiful  solution,  but 
on  the  score  of  permanency  is  not  so  trustworthy  as  balsam. 

Gum  Styrax. — This  is  a  resin  which  must  be  dissolved  in  benzole, 
chloroform,  or  ether.  It  should  have  the  consistency  of  olive  oil ; 
all  the  benzole  must  be  evaporated  before  putting  the  cover  on  the 
slip;  its  refractive  index  is  said  to  be  then  1'583.  Its  value  is  in 
the  mounting  of  diatoms,  where  a  marked  difference  between  the 
refractive  index  of  the  siliceous  frustules  and  the  medium  in  which 
they  are  mounted  facilitates  the  discovery  of  obscure  details.  There  is 
a  marked  increase  of  visibility  in  proportion  as  the  mounting  medium 
has  a  refractive  index  higher  than  the  object  (diatom)  mounted. 

Now  the  refractive  index  of  the  silex  of  diatoms  is  1'43.  But 
Canada  balsam  is  1'52  :  hence  the  '  index  of  visibility'  in  obscure 
markings  is  9,  while  styrax  by  comparison  is  15. 

Monobromide  of  naphihalin  is  another  of  the  media  which 
may  be  used  with  a  high  refractive  index.  It  is  colourless  and  oil- 
like,  soluble  in  alcohol  and  ether.  It  has  a  refractive  index  of 
1  '658,  and  therefore  a  splendid  index  of  visibility  above  balsam  or 
styrax ;  but  after  a  lapse  of  many  months  some  change  takes  place 
which  leaves  the  preparation  as  apparently  perfect  as  before,  but 
having  lost  all  the  benefit  of  great  refractive  index. 

The  cover-glass  should  be  run  round  with  a  ring  of  wax,  then 
with  a  ring  of  Heller's  porcelain  cement,  and  be  finally  closed  with 
shellac. 

But  with  the  exception  of  some  media  of  very  high  refractive 
index  not  by  any  means  easy  to  use,  devised  by  Professor  H.  L. 
Smith,  there  is  no  medium  of  such  high  value  as  that  suggested  and 
very  successfully  employed  by  Mr.  J.  W.  Stephenson,  viz. 

Phosphorus. — Its  refractive  index  is  2*1,  and  its  consequent 
increase  of  visibility  is  of  immense  value  in  some  objects. 

Phosphorus,  it  need  hardly  be  said,  is  difficult  and  somewhat 
dangerous  to  handle  on  account  of  its  spontaneous  combustion  in 
air,  and  the  severe  nature  of  the  burns  it  inflicts.  But  it  is  with 
slight  practice  by  no  means  an  unmanageable  medium. 

To  prepare  it,  take  a  2 -drachm  bottle  with  no  contraction  for  the 

1  See  the  Rev.  W.  W.  Spicer's  Handy-book  to  the  Collection  and  Preparation 
of  Freshwater  and  Marine  Algce,  &c..  pp.  57-59.  '  Nothing,'  says  Mr.  Spicer,  '  can 
exceed  the  beauty  of  the  preparations  of  .Desw^'ace^  prepared  after  Herr  Hantsch's 
method,  the  form  of  the  plant  and  the  colouring  of  the  endochrome  having  under- 
gone no  change  whatever." 


522  .PREPARATION,    MOUNTING,    AND   COLLECTION   OF   OBJECTS 

neck.  Make  a  cylinder  of  wood  that  will  just  fit  the  inside  of  the 
neck.  Fold  some  filter  paper  down  and  around  this  cylinder  so  that 
it  will  just  fit  tightly  into  the  neck  of  the  bottle,  to  the  bottom  of 
which  it  is  forced,  and  the  cylinder  of  wood  withdrawn,  leaving  the 
filter  in  its  place.  Now  moisten  the  filter  carefully  with  a  few  drops 
of  bisulphide  of  carbon,  and  a  piece  of  stick  phosphorus  from  a 
quarter  to  three-eighths  of  an  inch  long  should  be  placed  in  the  filter, 
and  the  bottle  corked.  The  vapour  of  the  bisulphide  instantly  acts 
on  the  phosphorus,  and  in  about  half  an  hour  it  will  be  in  a  fluid 
state  remaining  in  the  filter.  By  releasing  the  cork  and  taking 
hold  of  the  filter  tube  with  a  pair  of  pliers  and  slowly  drawing  it 
upwards  a  partial  vacuum  is  formed  beneath  it,  and  the  pressure  of 
the  air  on  the  surface  of  the  fluid  phosphorus  forces  it  through  the 
filter,  leaving  the  now  brilliant  fluid  in  the  bottle. 

With  care,  rapidity,  and  firmness  withdraw  the  filter  and  plunge 
instantly  into  a  vessel  of  water  close  at  hand. 

In  mounting  we  assume  that  the  best  course  as  advised  above 
has  been  adopted,  and  that  the  diatoms  to  be  mounted  are  either- 
arranged  or  diffused  upon  the  cover-glass. 

Make  a  ring  upon  the  slip  of  glue  and  honey  cement  used 
warm  and  allowed  to  cool.  It  is  now  a  stiff  jelly.  Lay  the  cover 
in  its  place,  with  the  diatoms  downwards,  touching  the  ring  at  one 
side,  but  raised  by  a  fine  wire  on  the  side  next  the  operator.  A 
pipette  may  also  be  used  made  of  glass  tubing  an  eighth  of  an  inch  in 
external  diameter,  drawn  to  a  fine  point  at  one  end,  and  somewhat 
enlarged  at  the  other,  and  to  which  an  indiarubber  cap  or  nipple  is 
fastened  airtight.  This  pipette  must  be  passed  through  the  centre 
of  a  cork  fitting  the  bottle  of  phosphorus  solution,  and  the  fine  end 
should  plunge  into  the  fluid  and  nearly  touch  the  bottom  of  the  bottle. 
By  squeezing  the  rubber  cap  before  the  insertion  of  the  pipette  and 
releasing  it  after  the  point  is  well  down,  a  small  quantity  of  phos- 
phorus rises  in  the  pipette.  It  is  withdrawn  and  inserted  rapidly 
beneath  the  tilted  end  of  the  cover  ;  the  slightest  pressure  on  the 
cap  ejects  enough  phosphorus  to  fill  the  space  between  the  cover 
and  the  slide  ;  gently  and  firmly  press  it  down  and  ring  it  with  warm 
glue  and  honey. 

In  half  an  hour  points  of  superfluous  phosphorus  may  have 
exuded.  With  a  pair  of  tweezers  wet  a  piece  of  blotting-paper  with 
bisulphide  and  absorb  these  away,  plunging  the  paper  at  once  into 
water.  The  slides  should  now  be  put  aside  for  a  day  or  two,  then 
they  may  receive  two  or  three  ring-coatings  of  gold-size,  and  finally 
be  finished  with  sealing-wax  or  shellac  varnish. 

It  often  is  quite  impossible  to  predicate  beforehand  what  preserva- 
tive medium  will  answer  best  for  a  particular  kind  of  preparation..; 
and  it  is  consequently  desirable,  where  there  is  no  lack  of  material, 
to  mount  similar  objects  in  two  or  three  different  ways,  marking  on 
each  slide  the  method  employed,  and  comparing  the  specimens  from 
time  to  time,  so  as  to  judge  the  condition  of  each. 

Importance  of  Cleanliness. — The  success  of  the  result  of  any  of 
the  foregoing  operations  is  greatly  detracted  from  if,  in  consequence 
of  the  adhesion  of  foreign  substances  to  the  glasses  whereon  the 


LABELLING   MOUNTED   OBJECTS  523 

objects  are  mounted,  or  to  the  implements  used  in  the  manipulations, 
any  extraneous  particles  are  brought  into  view  with  the  object  itself. 
Some  such  will  occasionally  present  themselves,  even  under  careful 
management ;  especially  fibres  of  silk,  wool,  cotton,  or  linen,  from 
the  handkerchiefs,  &c.,  with  which  the  glass  slides  may  have  been 
wiped  ;  fibres  of  the  blotting-paper  employed  to  absorb  superfluous 
fluid  ;  and  grains  of  starch,  which  often  remain  obstinately  adherent 
to  the  thin  glass  covers  kept  in  it.  But  a  careless  and  uncleanly 
manipulator  will  allow  his  objects  to  contract  many  other  impurities 
than  these ;  and  especially  to  be  contaminated  by  particles  of  dust 
floating  through  the  air,  the  access  of  which  may  be  readily  prevented 
by  proper  precautions.  It  is  desirable  to  have  at  hand  a  well-closed 
cupboard  furnished  with  shelves,  'or-  a  cabinet  of  well -fitted  drawers, 
or  a  number  of  bell-glasses  upon  a  flat  table,  for  the  purpose  of 
securing  glasses,  objects,  &c.,  from  this  contamination  in  the  intervals 
of  the  work  of  preparation  ;  and  the  more  readily  accessible  these 
receptacles  are,  the  more  use  wrill  the  microscopist  be  likely  to  make 
of  them.  Great  care  ought,  of  course,  to  be  taken  that  the  media 
employed  for  mounting  should  be  freed  by  effectual  filtration  from  all 
floating  particles,  and  that  they  should  be  kept  in  well-closed  bottles. 

Labelling  and  Keeping  Mounted  Objects.— The  object  of  labels 
on  mounted  objects  is  of  course  to  give  clear  and  instant  indication 
of  the  nature  of  the  mount.  But  we  must,  if  our  cabinet  shave  any- 
thing like  scientific  pretensions,  not  only  know  what  the  object  may 
be,  but  some  (perhaps  many)  other  particulars  about  it.  In  fact,  a 
thoroughly  scientific  cabinet  must  not  rely  on  the  labels  on  the 
mounts  for  all  the  information  which  it  is  desirable  and  even 
essential  to  have  concerning  them.  One  of  the  desiderata  of  every 
label  should  be  the  presence  of  a  number,  and  this  number  should  be 
at  once  placed  in  a  book,  arranged  in  columns  to  suit  the  requirements 
of  the  student,  and  most  of  the  details  should  be  placed  in  this  book 
in  association  with  the  number. 

For  this  to  be  of  permanent  service,  however,  the  label  011  which 
the  number  is  placed  should  be  as  permanent  and  immovable  as  the 
slip  itself.  We  know  of  cabinets  in  which  only  numbers  are 
marked  on  slides,  and  all  details  are  recorded  in  '  the  book.'  We 
do  not  advise  this  ;  but  all  who  keep  cabinets  know  how  in  the  course 
of  years  ^paper  labels  become  displaced  and  lost,  and  in  many 
instances  the  value  of  slides  is  greatly  diminished. 

What  is  wanted  is  a  permanently  fixed  label,  capable  of  receiving 
the  chief  points  of  character  as  well  as  the  name  and  number  of  an 
object. 

The  present  Editor  has  found  the  following  plan  to  be  hitherto, 
after  twenty-three  years'  trial,  quite  faultless. 

Let  the  slips  which  are  to  be  used  for  mounting  have  the  two 
ends  of  the  upper  surface  finely  ground ;  at  one  end  the  ground 
surface  may  be  three  quarters  of  an  inch,  and  at  the  other  end  half 
an  inch.  On  the  ground  surface  we  can  write  with  a  hard  pencil  as 
clearly  and  sharply  as  with  a  fine  pen  on  cardboard. 

On  the  broader  ground  surface  let  the  principal  facts  as  to  the 
nature  of  the  object  be  written  and  the  number  of  the  slide  with  a 


524    PREPARATION,    MOUNTING,    AND   COLLECTION   OF   OBJECTS 

Faber  pencil  marked  H  H  H  H.  On  the  narrower  and  opposite 
ground  surface  should  be  written  what  the  object  is  mounted  in, 
how  stained,  or  whence  obtained,  the  date  of  mounting,  &c. 

Xow  when  all  this  is  written  take  thin  covers,  cut  respectively 
1  X  J  inch  and  1  x  \  inch,  and  by  means  of  benzol  balsam,  applied 
with  or  without  heat,  the  ground  surfaces  should  have  these  thin 
glasses  put  on  over  the  writing  and  the  entire  ground  surfaces ;  the 
result  of  course  will  be  that  the  transparency  of  what  was  a  ground 
and  opaque  surface  will  be  wholly  restored,  and  the  writing  will  be 
clear  and  ineffaceable.  If  the  bottom  of  the  trays  of  the  cabinets  be 
whitened  it  will  render  still  more  easy  the  instant  reading  of  the 
contents  of  the  label. 

The  grinding  of  the  slips  is  by  no  means  difficult,  and  could  not 
be  costly  if  there  arose  a  demand  for  them. 

It  is  easy,  however,  to  do  all  that  is  required.  A  block  of  wood 
to  receive  the  slide  in  an  excavation  of  its  own  shape  and  size,  and  a 
piece  of  wood  half  an  inch  thick,  of  the  exact  length  (1J  inch) 
of  the  space  between  the  labels,  enables  a  lead  '  buff '  to  be  freely 
used  with  fine  emery  and  the  work  is  speedily  done.  Of  course  the 
finer  the  emery  the  finer  the  surface  ;  and  the  finer  the  surface  the 
more  delicate  the  writing  may  be  made.  The  label  may  in  fact  be 
as  ornate  and  elegant  as  we  please.  Nor  need  we  be  confined  to  an 
oblong  shape.  Oval  or  round  spaces  could  be  ground  on  the  slips 
and  thin  covers  of  corresponding  size  could  be  accordingly  used. 
This  method  gives  a  little  more  trouble  and  is  slightly  more 
expensive,  but  in  elegance  and  above  all  in  durability  we  believe  it 
has  no  equal. 

For  the  preservation  of  objects,  the  pasteboard  boxes  now  made 
at  a  very  reasonable  cost,  with  wooden  racks,  to  contain  six,  twelve, 
or  twenty-four  slides,  will  be  found  extremely  useful.  For  the 
management  of  a  large  collection  the  following  has  proved  itself  to 
be  thoroughly  practical,  and  can  be  universally  employed.  The 
species,  genus,  and  character  of  the  slides  may  be  disregarded.  Place 
the  slides  in  the  cabinet  just  as  they  come,  numbering  each  consecu- 
tively. The  exterior  of  cabinets  should  show  from  what  number  to 
what  number  the  cabinet  contains  :  thus,  527  to  842.  The  porcelain 
slab  on  the  drawer  may  indicate  from  what  number  to  what  number 
the  drawer  contains  :  thus,  527  to  539.  Now  a  number  of  note- 
books should  be  procured,  so  that  there  may  be  a  separate  notebook 
for  each  subject ;  the  size  of  the  notebook  must  be  regulated  to  the 
importance  of  the  special  department  the  collector  has  taken  up. 
Thus  a  diatomist  would  have  probably  a  thick  ledger  for  his  diatom 
collection,  whereas  an  entomologist  would  have  a  thin  notebook  for 
his  diatoms  and  a  thick  ledger  for  his  insects,  and  so  on.  The  note- 
books might  be  distinguished  from  one  another  by  a  letter  of  the 
alphabet. 

In  the  event  of  a  second  notebook  being  required  for  the  same 
subject  or  class  of  objects,  it  might  be  identified  by  doubling  the 
letter — thus,  D  D.  Now  a  large  index  notebook  will  be  required  in 
which  one  line  is  given  to  each  slide.  This  notebook  contains 


COLLECTION   OF   OBJECTS  525 

merely  the  number  of  the  slide  and  the  letter  and  page  of  the  special 
notebook  wherein  all  about  the  slide  will  be  found.  Thus  : — 

'649,  F  127.' 

This  means  that  in  notebook  F  on  page  127  we  shall  find  an 
account  of  slide  No.  649. 

On  turning  to  notebook  F  we  find  (say)  that  the  subject  is 
geology.  The  following  will  be  a  facsimile  of  the  page  : — 

Slide  No.  649  127 

Section  of  porphyry  from  Peterhead,  Aug.  1886. — The  quartz 
crystals  in  this  section  have  minute  cavities  containing  a  liquid,  CO2. 
In  each  cavity  there  is  a  bubble  ;  some  of  these  bubbles  are  ex- 
tremely minute,  and  exhibit  rapid  Brownian  movement.  A  good 
example  of  which  is — 

No.  2  (referring  to  a  second  microscope  when  used),  46-51. 

A  large  bubble  with  no  Brownian  movement. 

No.  2  (microscope),  44-47.  ' 

Section  too  thick  for  oil  immersion. 

Best  seen  dry  J  '95  N.A.  deep  eye-piece ;  condenser  aperture 
•6  N.A. 

At  the  back  of  each  notebook  there  is  an  alphabetical  index. 
In  this  instance  if  we  look  up  '  Porphyry'  we  shall  find  127,  and  if 
we  look  up  '  Quartz  (cavities  in) '  we  shall  find  127,  and  if  we  look  up 
'  Carbonic  acid  (in  quartz)'  we  shall  find  127,  and  if  we  look  up '  Bubbles 
(in  quartz)'  we  shall  find  127. 

By  this  means  the  collector  can  find  a  slide  if  he  know  the 
subject,  and  also  the  subject  if  he  have  the  slide. 

This  is  the  only  scientific  method  we  know  of  dealing  with  a 
microscopical  collection  ;  it  is  one  of  the  greatest  practical  mistakes 
to  make  the  cabinet  its  own  index.  It  always  ends  in  supreme 
confusion.  But  for  the  purposes  of  the  man  of  science  a  large 
cabinet  made  with  a  view  to  the  reception  of  his  own  slides  is  far 
preferable.  The  majority  of  slides  are  3x1  inches  ;  but  all  are  not — 
some  geological  and  mineralogical  sections,  sections  of  coal,  &c.,  are 
often  much  larger.  Many  objects,  again,  are  in  deeper  cells  than 
the  ordinary  cabinet  drawer  or  slide-box  will  admit  of;  all  this  may 
be  provided  for,  and  if  money  be  not  a  special  object,  a  design  with 
two  or  three  special  and  smaller  cabinets  may  be  made  for  the 
reception  of  special  series  of  mounts.1 

COLLECTION  OF  OBJECTS. 

A  large  proportion  of  the  objects  with  which  the  microscopist 
is  concerned  is  derived  from  the  minute  parts  of  those  larger 
organisms,  whether  vegetable  or  animal,  the  collection  of  which  does 
not  require  any  other  methods  than  those  pursued  by  the  ordinary 
naturalist.  With  regard  to  such,  therefore,  no  special  directions 
are  required.  But  there  are  several  most  interesting  and  important 
groups,  both  of  plants  and  animals,  which  are  themselves,  on  account 

1  It  will  be  understood  that  there  are  many  forms  of  cabinet  which  space  prevents 
our  describing ;  they  are  made  suitable  for  the  pocket,  for  postal  transmission,  &e., 
and  may  be  readily  seen  at  the  opticians'. 


526    PREPARATION,    MOUNTING,    AND    COLLECTION   OF   OBJECTS 

of  their  minuteness,  essentially  microscopic  ;  and  the  collection  of 
these  requires  peculiar  methods  and  implements,  which  are,  however, 
very  simple,  the  chief  element  of  success  lying  in  the  knowledge  where 
to  look  and  what  to  look  for.  In  the  present  place,  general  direc- 
tions only  will  be  given  ;  the  particular  details  relating  to  the  several 
groups  being  reserved  for  the  account  to  be  hereafter  given  of  each. 
Of  the  microscopic  organisms  in  question,  those  which  inhabit 
fresh  water  must  be  sought  for  in  pools,  ditches,  or  streams,  through 
which  some  of  them  freely  move,  whilst  others  attach  themselves 
to  the  stems  and  leaves  of  aquatic  plants,  or  even  to  pieces  of  stick 
or  decaying  leaves,  &c.,  that  may  be  floating  on  the  surface  or  sub- 
merged beneath  it ;  while  others,  again,  are  to  be  sought  for  in  the 
muddy  sediments  at  the  bottom.  Of  those  which  have  the  power  of 
free  motion,  some  keep  near  the  surface,  whilst  others  swim  in  the 
deeper  waters  ;  but  the  situation  of  many  depends  entirely  upon  the 
light,  since  they  rise  to  the  surface  in  sunshine,  and  subside  again 
afterwards.  The  collector  will  therefore  require  a  means  of  obtaining 
samples  of  water  at  different  depths,  and  of  drawing  to  himself 
portions  of  the  larger  bodies  to  which  the  microscopic  organisms  may 
be  attached.  For  these  purposes  nothing  is  so  convenient  as  the  pond* 
stick,  which  is  made  in  two  lengths,  one  of  them  sliding  within  the 
other,  so  as  when  closed  to  serve  as  a  walking-stick.  Into  the 
extremity  of  this  may  be  fitted,  by  means  of  a  screw  socket,  (1)  a 
cutting-hook  or  curved  knife,  for  bringing  up  portions  of  larger 
plants  in  order  to  obtain  the  minute  forms  of  vegetable  or  animal 
life  that  may  be  parasitic  upon  them  ;  (2)  a  broad  collar,  with  a 
screw  in  its  interior,  into  which  is  fitted  one  of  the  screw-topped 
bottles  made  by  the  York  Glass  Company  ;  (3)  a  ring  or  hoop  for  a 
muslin  ring-net.  When  the  bottle  is  used  for  collecting  at  the  sur- 
face, it  should  be  moved  sideways  with  its  mouth  partly  below  the 
water ;  but  if  it  be  desired  to  bring  up  a  sample  of  the  liquid  from 
below,  or  to  draw  into  the  bottle  any  bodies  that  may  be  loosely 
attached  to  the  submerged  plants,  the  bottle  is  to  be  plunged  into 
the  water  with  its  mouth  downwards,  carried  into  the  situation  in 
which  it  is  desired  that  it  should  be  filled,  and  then  suddenly  turned 
with  its  mouth  upwards.  By  unscrewing  the  bottle  from  the  collar, 
and  screwing  on  its  cover,  the  contents  may  be  securely  preserved. 
The  net  should  be  a  bag  of  fine  muslin,  which  may  be  simply  sewn 
to  a  ring  of  stout  wire.  But  it  is  desirable  for  many  purposes  that 
the  muslin  should  be  made  removable  ;  and  this  may  be  provided 
for  by  the  substitution  of  a  wooden  hoop,  grooved  on  its  outside,  for 
the  wire  ring ;  the  muslin  being  strained  upon  it  by  a  ring  of 
vulcanised  indiarubber,  which  lies  in  the  groove,  and  which  may  be 
readily  slipped  off  and  on,  so  as  to  allow  a  fresh  piece  of  muslin  to  be 
put  in  the  place  of  that  which  has  been  last  used.  At  the  end  of  the 
muslin  bag  is  tied  a  small  rimmed  tube-bottle  of  thin  clear  glass 
three  inches  long  by  one  inch  in  diameter.  In  this,  objects  can  be 
fairly  seen.  The  collector  should  also  be  furnished  with  a  number 
of  bottles,  into  which  he  may  transfer  the  samples  thus  obtained, 
and  none  are  so  convenient  as  the  screw-topped  bottles  made  in  all 
sizes  by  the  York  Glass  Company.  It  is  well  that  the  bottles  should 


COLLECTING  527 

be  fitted  into  cases,  to  avoid  the  risk  of  breakage.  When  animalcules 
are  being  collected,  the  bottles  should  not  be  above  two-thirds 
filled,  so  that  adequate  air-space  may  be  left.  Whilst  engaged  in 
the  search  for  microscopic  objects,  it  is  desirable  for  the  collector  to 
possess  a  means  of  at  once  recognising  the  forms  which  he  may 
gather,  where  this  is  possible,  in  order  that  he  may  decide  whether  the 
'  gathering  '  is  or  is  not  worth  preserving ;  and  for  this  purpose  we  know 
of  nothing  better,  unless  a  small  travelling  microscope  be  required,  than 
a  couple  of  Steinheil  loups,  magnifying  six  and  ten  diameters. 

Mr.  J.  D.  Hardy  suggests  what  we  have  found  of  great  use,  viz. 
aflat  bottle,  as  a  very  valuable  piece  of  apparatus  for  collecting.1  It  is 
made  by  cutting  a  Lj  -shaped  piece  out  of  a  flat  and  solid  piece  of  india- 
rubber,  about  6  inches  long  by  2|  inches  broad,  and  j  inch  thick  ; 
against  each  side  is  cemented  (by  means  of  Miller's  caoutchouc 
cement)  a  piece  of  good  thin  plate-glass,  and  the  bottle  is  complete. 
A  small  portion  cut  from  the  inner  piece  makes  a  naturally  fitting 
cork.  One  or  two  more,  and  smaller,  bottles  can  be  made  with  the 
remaining  indiarubber.  It  is  essential  that  the  material  should  be 
at  least  J  inch  thick  in  order  to  make  a  wide  bottle,  and  allow  pond- 
weeds  to  be  put  inside  without  difficulty  and  pressure.  A  flat  bottle  is 
made  by  Mr.  Stanley,  London  Bridge,  which  we  have  good  reason  to 
write  favourably  of.  It  is  ground  on  its  outer  surfaces,  and  internal 
irregularities  almost  wholly  disappear  when  filled  with  water  ;  an 
objective  from  3  inches  to  1^  inch  may  be  well  employed  with  it. 

Even  with  the  best  ordinary  round  dipping  bottles  it  is  very 
difficult  to  see  minute  animals  clearly,  whilst  with  this  flat  bottle 
one  can  see  at  a  glance  almost  everything  the  dip  contains,  and 
every  object  can  be  examined  with  the  pocket  lens  with  ease. 

For  collecting  purposes  the  objects  sought  in  pond  or  stream  are 
divisible  into  free -swimming,  and  attached  or  fixed  to  water-plants,  &c. 

The  free-swimming  are  to  be  secured  with  the  net,  the  bottle 
attached  to  which  should  be  examined  after  each  sweep  of  the  net ; 
and  the  flat  bottle  may  be  also  filled  for  examination.  The  mud  at 
the  bottom  of  the  pond  must  not  be  stirred  by  the  net,  since  of 
course  it  obscures  the  objects. 

The  infusoria,  rotifera,  &c.,  are  best  found  with  the  flat  bottle. 
Collect  a  lot  of  the  '  weeds  '  growing  in  pond  or  stream,  and  place 
these  in  the  bottle  ;  then,  Mr.  Rousselet  says  :  '  The  tree-like  colo- 
nies of  Yorticellae  ;  Epistylis,  Zoothamium,  and  Carchesium,  the 
trumpet-shaped  Stentors,  the  crown  Rotifer  Stephanoceros,  the 
tubes  of  Melicerta,  Lymnias,  the  various  Polyzoa,  also  Hydra, 
and  many  more,  can  at  once  be  seen  with  the  naked  eye,  when 
present,  and  in  this  way  the  good  branches  can  be  selected.  Some 
creatures,  however,  such  as  the  beautiful  floscules,  cannot  be  seen 
easily,  even  with  the  lens,  not  so  much  on  account  of  their  small 
size,  as  of  the  perfect  transparency  of  their  bodies.  Experience 
will  soon  teach  one  how  to  see  which  branches  are  likely  to  prove 
prolific.  As  a  general  rule,  old-looking  but  still  sound  and  green 
branches  will  be  the  best.  The  Water  Milfoil  (Myriophyllum)  is 
decidedly  the  best  of  water  plants  to  examine  and  collect,  on  account 
1  Q.M.  Journ.  ser.  ii.  vol.  ii.  p.  55. 


528    PKEPARATION,   MOUNTING,    AND   COLLECTION   OF   OBJECTS 

of  the  ease  with  which  its  leaves  can  subsequently  be  placed  under  the 
microscope.  Anacharis  is  much  more  difficult  of  manipulation,  and 
I  mostly  only  take  it  now  to  aerate  my  aquaria. 

*  In  placing  a  weed  in  the  flat  bottle,  do  not  put  in  more  than  one 
branch  at  a  time,  otherwise  the  branches  will  only  obscure  each  other 
and  render  examination  more  difficult. 

'  When  searching  for  Polyzoa,  such  as  Lophopus,  Plumatella, 
Fredericella,  it  is  advisable  to  examine  the  rootlets  of  trees  growing 
at  the  edge  of  the  water,  and  also  to  drag  up  weeds  from  the  middle 
of  the  pond  or  canal  by  means  of  a  loaded  hook  and  line. 

'  A  good  collection  thus  made  is  transferred  to  small  aquaria  6  to 
8  inches  high,  5  to  6  inches  long,  and  1  to  1J  inch  wide  ;  these  we 
have  used  for  at  least  ten  years  and  can  attest  their  great  value  in 
making  the  best  possible  use  of  a  good  day's  collecting,  and  studying 
in  the  most  intelligent  way  the  objects  collected.  f 

'  Rotifers  can  generally  be  kept  a  week  or  a  fortnight,  some 
species  much  longer  ;  their  lives,  as  well  as  those  of  Polyzoa,  can  be 
prolonged  by  feeding  them  about  twice  daily  with  a  green  soup 
made  by  crushing  some  anachari^,  or  other  green  weed,  in  a  small 
mortar  in  a  little  water,  which  is  then  filtered  through  muslin. 
They  can  be  seen  to  feed  on  this  under  the  microscope,  their  tiny 
stomachs  soon  becoming  filled  with  little  balls  of  chlorophyll. 

'  Under  favourable  conditions  Melicerta,  Stephanoceros,  the  Flos- 
cules,  and  also  Asplanchna,  and  other  forms,  breed  and  multiply  in 
the  aquarium,  and  can  then  be  preserved  for  a  considerable  time. 
A  little  mud  taken  from  a  pond  in  winter  or  early  spring,  and 
put  in  a  tank  at  home,  will  often  produce  an  unexpected  number 
and  variety  of  rotifers  and  infusoria,  which  are  hatched  from 
winter  eggs  and  dormant  germs. '  l 

There  must  of  course  be  a  balance  in  every  tank  between  the 
animal  and  vegetable  life,  or  aeration  must  be  artificially  maintained. 
So  also  food  must  be  obtainable  by  the  organisms,  however  small. 
But  experience  alone  is  the  perfect  teacher  in  this  matter. 

The  same  general  method  is  to  be  followed  in  the  collection  of 
such  marine  forms  of  vegetable  and  animal  life  as  inhabit  the 
neighbourhood  of  the  shore,  and  can  be  reached  by  the  pond-stick. 
But  there  are  many  which  need  to  be  brought  up  from  the  bottom 
by  means  of  the  dredge,  and  many  others  which  swim  freely 
through  the  waters  of  the  ocean,  and  are  only  to  be  captured  by  the 
tow-net.  As  the  former  is  part  of  the  ordinary  equipment  of  every 
marine  naturalist,  whether  he  concern  himself  with  the  microscope 
or  not,  the  mode  of  using  it  need  not  be  here  described ;  but  the 
use  of  the  latter  for  the  purposes  of  the  microscopist  requires 
special  management.  The  net  should  be  of  fine  muslin,  firmly  sewn 
to  a  ring  of  strong  wire  about  ten  or  twelve  inches  in  diameter. 
This  may  be  either  fastened  by  a  pair  of  strings  to  the  stern,  of  a 
boat,  so  as  to  tow  behind  it,  or  it  may  be  fixed  to  a  stick  so  held  in 
the  hand  as  to  project  from  the  side  of  the  boat.  In  either  case  the 
net  should  be  taken  in  from  time  to  time,  and  held  up  to  allow  the 

1  '  On  some  Methods  of  Collecting  and  Keeping  Pond  Life  for  the  Microscope,' 
from  the  Trans.  Middlesex  Nat.  Hist.  Soc. 


COLLECTING  529 

water  it  contains  to  drain  through  it ;  and  should  then  be  turned 
inside  out  and  moved  about  in  a  bucket  of  water  carried  in  the 
boat,  so  that  any  minute  organisms  adhering  to  it  may  be  washed 
off  before  it  is  again  immersed.  It  is  by  this  simple  method  that 
marine  animalcules,  the  living  forms  of  Radiolaria,  the  smaller 
Medusa  ids  (with  their  allies  Beroe  and  Cydippe),  Xoctiluca,  the 
free-swimming  larvae  of  Echinodermata,  some  of  the  most  curious 
of  the  Tunicata,  the  larvae  of  Mollusca,  Turbellaria,  and  Annelida, 
some  curious  adult  forms  of  these  classes,  Entomostraca,  and  the 
larvae  of  higher  Crustacea,  are  obtained  by  the  naturalist ;  and 
the  great  increase  in  our  knowledge  of  these  forms  which  has  been 
gained  within  recent  years  is  mainly  due  to  the  assiduous  use 
which  has  been  made  of  it  by  qualified  observers.  It  is  important 
to  bear  in  mind  that,  for  the  collection  of  all  the  more  delicate  of 
the  organisms  just  named  (such,  for  instance,  as  echinoderm  larvae), 
it  is  essential  that  the  boat  should  be  rowed  so  slowly  that  the  net 
may  move  gently  through  the  water,  so  as  to  avoid  crushing  its  soft 
contents  against  its  sides.  Those  of  firmer  structure  (such  as  the 
Entomostraca),  on  the  other  hand,  may  be  obtained  by  the  use  of  a 
tow-net  attached  to  the  stern  of  a  sailing-vessel,  or  even  of  a 
steamer,  in  much  more  rapid  motion.1  When  this  method  is 
employed,  it  will  be  found  advantageous  to  make  the  net  of  conical 
form,  and  to  attach  to  its  deepest  part  a  wide-mouthed  bottle, 
which  may  be  prevented  from  sinking  too  deeply  by  suspending  it 
from  a  cork  float ;  into  this  bottle  many  of  the  minute  animals 
caught  by  the  net  will  be  carried  by  the  current  produced  by  the 
motion  of  the  vessel  through  the  water,  and  they  will  be  thus 
removed  from  liability  to  injury  It  will  also  be  useful  to  attach  to 
the  ring  an  inner  net,  the  cone  of  which,  more  obtuse  than  that  of 
the  outer,  is  cut  off"  at  some  little  distance  from  the  apex ;  this 
serves  as  a  kind  of  valve,  to  prevent  objects  once  caught  from  being 
washed  out  again.  The  net  is  to  be  drawn  in  from  time  to  time, 
and  the  bottle  to  be  thrust  up  through  the  hole  in  the  inner  cone ; 
and  its  contents  being  transferred  to  a  screw-capped  bottle  for 
examination,  the  net  may  be  again  immersed.  This  form  of  net, 
however,  is  less  suitable  for  the  most  delicate  objects  than  the  simple 
stick-net  used  in  the  manner  just  described.  The  microscopist  on 
a  visit  to  the  seaside,  wrho  prefers  a  quiet  row  in  tranquil  waters 
to  the  trouble  (and  occasional  malaise)  of  dredging,  will  find  in  the 
collection  of  floating  animals  by  the  careful  use  of  the  stick-net 
or  tow-net  a  never-ending  source  of  interesting  occupation. 

1  In  the  Challenger  Expedition  tow-nets  were  almost  constantly  kept  in  use, 
not  only  at  the  surface,  but  at  various  depths  beneath  it,  being  attached  to  a  line 
which  was  made  to  hang  vertically  in  the  water  by  the  attachment  of  heavy  weights 
at  its  extremity.  The  collections  thus  made  showed  the  enormous  amount  of  minute- 
animal  life  pervading  the  upper  waters  of  the  ocean. 


M  M 


530   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


CHAPTER   VIII 

MICROSCOPIC   FORMS   OF   VEGETABLE   LIFE—THALLOPHYTES 

THOSE  who  desire  to  make  themselves  familiar  with  microscopic 
appearances,  and  to  acquire  dexterity  in  microscopic  manipulation, 
cannot  do  better  than  educate  themselves  for  more  difficult  inquiries 
by  the  study  of  those  humblest  types  of  vegetation  which  present 
organic  structure  under  its  most  elementary  aspect.  And  such  as 
desire  to  search  out  the  nature  and  conditions  of  living  action  will 
find  in  the  study  of  its  simplest  manifestations  the  best  clue  to  the 
analysis  of  those  intricate  and  diversified  combinations  under  which 
it  presents  itself  in  the  highest  animal  organisms.  For  it  has  now 
been  put  beyond  question  that  the  fundamental  phenomena  of  life 
are  identical  in  plants  and  in  animals,  and  that  the  living  substance 
which  exhibits  them  is  of  a  nature  essentially  the  same  throughout 
both  kingdoms.  The  determination  of  this  general  fact,  which  forms 
the  basis  of  the  science  of  BIOLOGY,  is  the  most  important  result  of 
modern  microscopic  inquiry ;  and  the  illustration  of  it  will  be  kept 
constantly  in  view  in  the  exposition  now  to  be  given  of  the  chief 
applications  of  the  microscope  to  the  study  of  those  minute  proto- 
phytes  (or  simplest  forms  of  plant-life)  with  whose  form  and  structure, 
and  with  whose  very  existence  in  many  cases,  we  can  only  acquaint 
ourselves  by  its  aid. 

It  was  formerly  supposed  that  living  action  could  only  be 
exhibited  by  organised  structure.  But  we  now  know  that  all  the 
essential  functions  of  life  maybe  carried  on  by  minute  'jelly-specks,' 
in  whose  apparently  homogeneous  semi-fluid  substance  nothing  like 
'  organisation  '  can  be  detected  ;  and,  further,  that  even  in  the  very 
highest  organisms,  which  present  us  with  the  greatest  variety  of 
*  differentiated '  structures,  the  essential  part  of  the  life-work  is  clone 
by  the  same  material — these  structures  merely  furnishing  the 
mechanism  (so  to  speak)  through  which  its  wonderful  properties 
exert  themselves.  Hence  this  substance,1  known  in  vegetable 
physiology  as  protoplasm,  but  often  referred  to  by  zoologists  as 

1  Attention  was  drawn  in  1835  by  Dujardin  (the  French  zoologist  to  whom  we  owe 
the  transfer  of  the  Foraminifera  from  the  highest  to  the  lowest  place  among  inverte- 
brate animals)  to  the  fact  that  the  bodies  of  some  of  the  lowest  members  of  tlie 
animal  kingdom  consist  of  a  structureless,  semi-fluid,  contractile  substance,  to  which 
he  gave  the  name  sarcode  (rudimentary  flesh).  In  1851  the  eminent  botanist  Von 
Molil  showed  that  a  similar  substance  forms  the  essential  constituent  of  the  cells  of 
plants,  and  termed  it  protoplasm  (primitive  plastic  or  organisable  material).  And  in 
1868  it  was  pointed  out  by  Prof.  Max  Schultze,  who  had  made  a  special  study  of  the 
rhizopod  group,  that  the  '  sarcode  '  of  animals  and  the  '  protoplasm '  of  plants  are 
identical.  See  his  memoir  Ueber  das  Protoplasma  der  Rliizopodcn  itnd  Pflanzen- 
zellen. 


SIMPLEST  FORMS   OF  VEGETABLE    LIFE  53  £ 

sarcode,  has  been  appropriately  designated  by  Professor  Huxley  '  the 
physical  basis  of  life.'  In  its  typical  state  (such  as  it  presents 
among  rhizopods)  it  is  a  semi-fluid,  tenacious,  glairy  substance, 
resembling — alike  in  aspect  and  in  composition — the  albumen  (or 
micoagulated  *  white  ')  of  an  unboiled  egg.  But  it  is  fundamentally 
distinguished  from  that  or  any  other  form  of  dead  matter  by  two 
attributes,  which  (as  being  peculiar  to  living  substances)  are  desig- 
nated vital:  (1)  its  power  of  increase,  by  assimilating  (that  is,  con- 
verting into  the  likeness  of  itself,  and  endowing  with  its  own  pro- 
perties) nutrient  material  obtained  from  without ;  (2)  its  power  of 
spontaneous  movement,  which  shows  itself  in  an  extraordinary  variety 
of  actions,  sometimes  slow  and  progressive,  sometimes  rapid,  some- 
times wave-like  and  continuous^  and  sometimes  rhythmical  with 
regular  intervals  of  rest.  When  examined  under  a  sufficiently  high 
magnifying  power,  multitudes  of  minute  granules  are  usually  seen  to 
be  diffused  through  it,  which  have  been  termed  *  microsoines.' 
Protoplasm,  whether  living  or  dead,  has  a  great  power  of  absorbing 
water;  but  the  distinction  between  these  two  states  is  singularly 
marked  by  its  behaviour  in  regard  to  any  colouring  matter  which  the 
wrater  may  contain.  Thus,  if  living  protoplasm  be  treated  with  a 
solution  of  carmine,  it  will  remain  unstained  so  long  as  it  retains 
its  vitality.  But  if  the  protoplasm  be  dead,  the  carmine  will  at  once 
pervade  its  whole  substance,  and  stain  it  throughout  with  a  colour 
even  more  intense  than  that  of  the  solution  ;  thus  furnishing  (as 
was  first  pointed  out  by  Dr.  Beale)  a  ready  means  of  distinguishing 
the  *  germinal  matter,'  or  protoplasmic  component  of  the  tissues  of 
higher  animals,  from  the  '  formed  material '  which  is  the  most  con- 
spicuous part  of  their  structure. 

All  those  minute  and  simple  forms  of  life  with  which  the  micro- 
scope brings  us  into  acquaintance  consist  essentially  of  particles  of 
protoplasm,  each  kind  having  usually  a  tolerably  definite  size  and 
shape,  and  showing  (at  least  in  some  stage  of  its  existence)  some- 
thing distinctive  in  its  habit  of  life.  And  it  is  rather  according  to 
the  manner  in  which  they  respectively  live,  grow,  and  multiply, 
than  on  account  of  any  structural  peculiarities,  that  they  are  assigned 
to  the  vegetable  or  to  the  animal  kingdom  respectively.  It  is 
impossible,  in  the  present  state  of  our  knowledge,  to  lay  down  any 
definite  line  of  demarcation  between  the  two  kingdoms  ;  since  there 
is  no  single  character  by  which  the  animal  or  vegetable  nature  of 
any  organism  can  be  tested.  Probably  the  one  which  is  most 
generally  applicable  among  those  that  most  closely  approximate  to 
one  another  is  not,  as  formerly  supposed,  the  presence  or  absence  of 
spontaneous  motion,  but,  on  the  one  hand,  the  dependence  of  the 
organism  for  nutriment  upon  organic  compounds  already  formed 
which  it  takes  (in  some  way  or  other)  into  the  interior  of  its  body, 
or,  on  the  other,  its  possession  of  the  power  of  producing  the  organic 
<•< impounds  which  it  applies  to  the  increase  of  its  fabric,  at  the 
expense  of  the  inorganic  elements  with  which  it  is  supplied  by  air 
and  water.  The  former,  though  perhaps  not  an  absolute,  is  &  general 
characteristic  of  the  animal  kingdom  ;  the  latter,  but  for  the  exist- 
ence of  which  animal  life  would  be  impossible,  is  certainly  the 

M   M  2 


532   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

prominent  attribute  of  the  vegetable.  We  shall  find  that  the  protozoa 
(or  simplest  animals)  are  supported  as  exclusively  either  upon 
other  protozoa  or  upon  protophytes,  as  are  the  highest  animals  upon 
the  flesh  of  other  animals  or  upon  the  products  of  the  vegetable 
kingdom ;  whilst  many  protophytes,  in  common  with  the  highest 
plants,  draw  their  nourishment  from  the  atmosphere  or  the  water  in 
which  they  live,  and,  like  them,  are  distinguished  by  their  power  of 
decomposing  carbonic  acid  (CO2)  under  the  influence  of  light — 
setting  free  its  oxygen,  and  combining  its  carbon  with  the  elements 
of  water  to  form  the  carbohydrates  (starch,  cellulose,  &c.),  and  with 
those  of  atmospheric  ammonia  to  form  nitrogenous  (albuminoid) 
compounds.  And  we  shall  find,  moreover,  that  even  such  protozoa 
as  have  neither  stomach  nor  mouth  receive  their  alimentary  matter 
direct  into  the  very  substance  of  their  bodies,  in  which  it  under- 
goes a  kind  of  digestion ;  whilst  protophytes  absorb  through  their 
external  surface  only,  and  take  in  no  solid  particles  of  any  descrip- 
tion. With  regard  to  motion,  which  was  formerly  considered  the 
distinctive  attribute  of  animality,  we  now  know,  not  merely  that 
many  protophytes  (perhaps  all,  at  some  period  or  other  of  their  lives) 
possess  a  power  of  spontaneous  movement,  but  also  that  the  instru- 
ments of  motion  (when  these  can  be  discovered)  are  of  the  very  same 
character  in  the  plant  as  in  the  animal,  being  little  hair-like  fila- 
ments, termed  cilia  (from  the  Latin  word  cilium,  an  eyelash),  or 
longer  whip-like  flagella,  by  whose  rhythmical  vibrations  the  body 
of  which  they  form  part  is  propelled  in  definite  directions.  The 
peculiar  contractility  of  these  organs  seems  to  be  an  intensification 
of  that  of  the  general  protoplasmic  substance,  of  which  the}'  an- 
special  extensions. 

There  are  certain  plants,  however,  which  resemble  animals  in 
their  dependence  upon  organic  compounds  prepared  by  other 
organisms,  being  themselves  unable  to  effect  that  fixation  of  carbon 
by  the  decomposition  of  the  CO.2  of  the  atmosphere,  which  is  the 
first  stage  in  their  production.  Such  is  the  case,  &mong  phanerogams 
(flowering  plants),  with  the  leafless  '  parasites '  which  draw  their 
support  from  the  tissues  of  their  *  hosts.'  And  it  is  the  case  also, 
among  the  lower  cryptogams,  with  the  entire  group  of  FUXGI  ; 
which,  however,  in  a  large  number  of  cases,  depend  rather  for  their 
nutritive  materials  upon  organic  matter  in  a  state  of  decomposition, 
many  of  them  having  the  power  of  promoting  that  process  by  their 
zymotic  (fermentative)  action.  Among  animals,  again,  there  are 
several  in  whose  tissues  are  found  organic  compounds,  such  as  chloro- 
phyll, starch,  and  cellulose,  which  are  characteristically  vegetable ; 
but  it  has  not  yet  been  proved  that  they  generate  these  compounds 
for  themselves  by  the  decomposition  of  C02. 

The  plan  of  organisation  recognisable  throughout  the  vegetable 
kingdom  presents  this  remarkable  feature  of  uniformity,  that  the 
fabric,  alike  in  the  highest  and  most  complicated  plants  and  in  the 
lowest  and  simplest  forms  of  vegetation,  consists  of  nothing  else 
than  an  aggregation  of  the  bodies  termed  cells,  every  one  of  which 
(save  in  the  forms  that  lie  near  the  border-ground  between  animal 
and  vegetable  life)  has  its  little  particle  of  protoplasm  enclosed  by  a 


THE    VEGETABLE    CELL  533 

casing  of  the  substance  termed  cellulose — a  non-nitrogenous  substance 
identical  in  chemical  composition  with  starch.  The  entire  mass  of 
cells  of  which  any  vegetable  organism  is  composed  has  been  gene- 
rated from  one  ancestral  cell  by  processes  of  multiplication  to  be 
presently  described ;  and  the  difference  between  the  fabrics  of  the 
lowest  and  of  the  highest  plants  essentially  consists  in  this,  that  whilst 
the  cells  produced  by  the  repeated  multiplication  of  the  ancestral 
cell  of  the  protophyte  are  all  mere  repetitions  of  it  arid  of  one  an- 
other each  living  by  said  for  itself,  those  produced  by  the  like  multi- 
plication of  the  ancestral  cell  in  the  oak  or  palm  not  only  remain  in 
mutual  connection,  but  go  through  a  progressive  '  differentiation,' 
the  ordinary  type  of  the  cell  undergoing  various  modifications  to  be 
described  in  their  proper  place.  A  composite  structure  is  thus 
developed,  which  is  made  up  of  a  number  of  distinct  '  organs '  (stem, 
leaves,  roots,  flowers,  <fec.),  each  of  them  characterised  by  specialities 
not  merely  of  external  form,  but  of  internal  structure  ;  and  each 
performing  actions  peculiar  to  itself,  which  contribute  to  the  life 
of  the  plant  as  a  ivhole.  Hence,  as  was  first  definitely  stated  by 
Schleiden,  it  is  in  the  life-history  of  the  individual  cell  that  we 
find  the  true  basis  of  the  study  of  vegetable  life  in  general. 

We  have  now  to  consider  in  more  detail  the  structure  and  life- 
history  of  the  typical  plant-cell,  and  shall  begin  by  treating  of  the 
cell-wall.  This  cell-wall  is  composed,  as  long  as  the  cell  is  in  a 
living  state,  chiefly  of  the  substance  known  as  cellulose,  one  of  the 
group  of  compounds  called  *  carbohydrates,'  and  bearing  the  definite 
chemical  composition  CGH10O5.  From  a  physical  point  of  view  it 
consists  of  particles  or  micellce  of  cellulose  surrounded  by  water. 
In  addition  to  cellulose,  recent  observations  have  shown  that  pectic 
substances  enter  largely  into  the  composition  of  the  wall  of  the 
living  cell,  especially  in  its  early  stages.  In  fungi  it  is  doubtful 
whether  there  is  any  true  cellulose  in  the  cell-walls.  With  regard 
to  the  mode  of  growth  of  the  cell-wall,  two  hypotheses  have  been 
proposed :  one,  that  it  is  formed  by  apposition,  that  is,  by  the 
constant  addition  of  fresh  layers  to  the  inner  surface  of  the  cell-wall ; 
the  other  that  it  increases  by  intussusception,  or  the  intercalation  of 
fresh  particles  of  cellulose  between  those  already  in  existence.  The 
results  of  modern  researches  tend  in  the  direction  of  the  former 
being  the  more  usual  process  ;  but  it  is  probable  that  the  two  co- 
operate in  producing  the  total  growth  of  the  cell-wall. 

The  contents  of  the  plant-cell,  which  may  be  collectively  termed 
the  endoplasm  (answering  to  the  *  endosarc '  of  rhizopods),  or,  when 
strongly  coloured  throughout  (as  in  many  algae),  the  endochrome, 
consist  in  the  first  place  of  an  outer  layer  of  protoplasmic  substance 
called  the  ectoplasm ,  primordial  utricle,  or  parietal  utricle.  This  is  an 
extremely  thin  and  delicate  layer,  so  that  it  escapes  attention  so  long 
as  it  remains  in  contact  with  the  cell-wall ;  and  it  is  only  brought 
into  view  when  separated  from  this,  either  by  developmental  changes 
(fig.  415),  or  by  the  influence  of  reagents  which  cause  it  to  con- 
tract by  drawing  forth  part  of  its  contents  (fig.  413,  C).  It  is  not 
sharply  defined  on  its  internal  face,  but  passes  gradually  into  the 
inner  mass  of  protoplasm,  from  which  it  is  chiefly  distinguishable  by 


534  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

the  absence  of  granules  ;  and  it  is  shown  by  the  effects  of  reagents  to 
have  the  all> miiin rms  composition  of  protoplasm.  It  may  tlius  l>e 
regarded  as  the  slightly  condensed  external  film  of  the  protoplasmic 
layer  with  which  the  inner  surface  of  the  cell-wall  is  in  contact  ;  and 
it "  essentially  corresponds  to  the  'ectosarc'  of  Antnlxi  or  any  other 
rhizopod.  The  '  ectoplasm  '  and  '  cellulose  wall '  can  be  readily  dis- 
tinguished from  each  other  by  chemical  tests,  and  also  by  the  action 
of  carmine,  which  stains  the  protoplasmic  substance  (when  dead) 
without  affecting  the  cellulose  wall.  The  further  contents  of  the  cell 
consist  of  a  watery  fluid  called  cell  sap,  which  holds  in  solution  sup ir. 
vegetable  acids,  saline  matters,  &c.  ;  the  peculiar  body  termed  the 
nucleus ;  and  chlorophyll  corpuscles  (enclosing  starch  granules), 
oil  particles,  &c.  In  the  young  state  of  the  cell  the  whole  cavity 
is  occupied  by  the  protoplasmic  substance,  which  is,  however,  viscid 
and  granular  near  the  cell-wall,  but  more  watery  towards  the  interior. 
With  the  enlargement  of  the  cell  and  the  imbibition  of  water,  clear 
spaces  termed  vacuoles,  filled  with  watery  cell-sap,  are  seen  in  the 
protoplasmic  substance;  and  these  progressively  increase  in  size  and 
number,  until  they  come  to  occupy  a  considerable  portion  of  the 
cavity,  the  protoplasm  stretching  across  it  as  an  irregular  network 
of  bands.  Each  of  the  vacuoles  is  enclosed  in  a  very  delicate  con- 
tractile membrane,  the  tonoplast.  When,  as  usually  happens,  the 
nucleus  lies  imbedded  in  the  outer  protoplasmic  layer,  these  bands 
are  gradually  withdrawn  into  it,  so  that  the  separate  vacuoles  unite 
into  one  large  general  vacuole  which  is  filled  with  watery  cell-sap. 
But  where  the  nucleus  is  situated  nearer  to  the  centre  of  the  cell, 
part  of  the  protoplasm  collects  around  it,  and  bands  or  threads  of 
protoplasm  stretch  thence  to  various  parts  of  the  parietal  layer.  It 
is  by  the  contractility  of  the  protoplasmic  layer  that  the  curious 
'  cyclosis'  hereafter  to  be  described  is  carried  on  within  the  plant- 
cell,  which  is  the  most  interesting  to  the  microscopist  of  all  its 
manifestations  of  vital  activity.  The  nucleus  is  a  small  body, 
usually  of  lenticular  or  subglobose  form  (fig.  413,  A.  fi).  and  of 
albuminous  composition,  that  lies  imbedded  in  protoplasmic  sub- 
stance, either  close  to  the  cell-wall  or  nearer  the  centre  of  the 
cavity.  Cells  containing  a  number  of  nuclei,  or  '  multinucleaied  cells J 
are  not  uncommon.  They  occur,  for  example,  in  many  alga\  in  the 
'suspensor'  and  'embryo-sac'  of  the  ovule  of  phanerogams,  and  in 
the  '  laticiferous '  tubes.  Within  the  nucleus  are  often  seen  one  or 
more  small  distinct  particles  termed  nucleoli  (fig.  413,  A,  b),  which 
can  be  best  distinguished  by  the  strong  coloration  they  receive  from 
a  twenty-four  hours'  immersion  in  carmine,  and  subsequent  washing 
in  water  slightly  acidulated  with  acetic  acid.  Though  in  some  points 
the  precise  function  of  the  nucleus  is  still  unknown,  there  can  be  no 
doubt  of  its  essential  relation  to  the  vital  activity  of  the  cell,  at  least 
in  all  the  higher  plants,  although  in  the  cells  of  some  of  the  lower 
cryptogams  it  has  not  at  present  been  distinguished  with  certainty 
at  any  stage  of  their  existence.  In  the  nucleated  cells  which 
exhibit  '  cyclosis,'  it  may  be  observed  that  if  the  nucleus  remains 
attached  to  the  cell -wall,  it  constitutes  a  centre  from  which  the 
protoplasmic  streams  diverge,  and  to  which  they  return ;  whilst  if 


CONTENTS   OF  THE   CELL  535 

it  retains  its  freedom  to  wander  about,  the  course  of  the  -treams 
alters  in  conformity  with  its  position.  /But  it  is  in  the  multiplication 
of  cells  by  binary  subdivision,  which  will  be  presently  described,  that 
the  speciality  of  the  nucleus  as  the  centre  of  the  ,-itfil  m-tir'dy  of  the 
cell  is  most  strongly  manifested.  The  chlorophyll  cr>rj,»sdes,  which 
are  limited  to  the  cells  of  the  parts  of  plants  acted  on  by  light,  are 
specialised  particles  of  protoplasm  through  which  a  green  colouring 
matter  is  diffused  ;  and  it  is  by  them  that  the  work  of  decomposing 
CO2,  and  of*  fixing  '  its  carlxm  by  union  with  the  oxygen  and  hydrogen 
of  water  into  starch,  is  effected.  The  characteristic  green  of 
chlorophyll  often  gives  place  to  other  colours,  which  seem  to  be  pro- 
duced from  it  by  chemical  action.  ,  Starch  grains  are  always  formed 
in  the  first  instance  in  the  interior  T>f  the  chlorophyll  corpuscles  and 
gradually  increase  in  size  until  they  take  the  places  of  the  corpuscles 
that  produced  them.  So  long  as  they  continue  to  grow,  they  are 
always  imbedded  in  the  protoplasm  of  the  cell ;  and  it  is  only  when 
fully  formed  that  they  lie  free  within  its  cavity. 

But  although  these  component  parts  may  be  made  out  without 
any  difficulty  in  a  large  proportion  of  vegetable  cells,  yet  they  cannot 
be  distinguished  in  some  of  those  humble  organisms  which  are 
nearest  to  the  border-line  between  the  two  kingdoms.  For  in  them 
we  find  the  '  cell-wall '  very  imperfectly  differentiated  from  the  '  cell- 
contents  ; '  the  former  not  having  by  any  means  the  firmness  of  a 
perfect  membrane,  and  the  latter  not  possessing  the  liquidity  which 
elsewhere  characterises  them.  And  in  some  instances  the  cell  is 
represented  only  by  a  mass  of  endoplasm,  so  viscid  as  to  retain  its 
external  form  without  any  limiting  membrane,  though  the  superficial 
layer  seems  to  have  a  firmer  consistence  than  the  interior  substances  ; 
and  this  may  or  may  not  be  surrounded  by  a  gelatinous-looking 
envelope,  which  is  equally  far  from  possessing  a  membranous  firmness, 
and  yet  is  the  only  representative  of  the  cellulose  wall.  This  viscid 
endoplasm  consi>t>.  as  elsewhere,  of  a  colourless  protoplasm,  through 
which  minute  colouring  particles  may  be  diffused,  sometimes  uni- 
formly, sometimes  in  local  aggregations,  leaving  parts  of  the  proto- 
plasm uncoloured.  The  superficial  layer  in  particular  is  frequently 
destitute  of  colour  ;  and  the  partial  solidification  of  its  surface  gives 
it  the  character  of  an  i  ectoplasm.'  Such  individualised  masses  of 
protoplasm,  destitute  of  a  true  cell-wall,  have  sometimes  been 
termed  *  primordial  cells.'  It  is  an  extremely  curious  feature  in 
the  cell-life  of  certain  protophytes  that  they  not  only  move  like 
animalcules  by  cilia  or  flagella,  but  that  they  exhibit  the  rhythmically 
contracting  vacuoles  which  are  specially  characteristic  of  protozoic 
organisms. 

So  far  as  we  yet  know,  every  vegetable  cell  derives  its  existence 
from  a  pre-existing  cell :  and  this  derivation  may  take  place  (in  the 
ordinary  process  of  growth  and  extension,  as  distinguished  from 
•-•xual"  multiplication')  in  one  of  two  modes:  either  (1)  binary 
«"h(lh'ision  of  the  parent-cell,  or  (2)  free-cell  formation  within  the 
parent-cell.  The  first  stage  of  the  former  process  consists  in  the 
elongation  and  transverse  constriction  of  the  nucleus;  and  this  con- 
striction becomes  deeper  and  deeper,  until  the  nucleus  divides  itself 


5  36  MICKOSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPflYTES 


into  two  halves  (fig.  413,  B,  a,  a').  These  then  separating  from 
each  other,  the  endoplasm  of  the  parent-cell  collects  round  the  two 
new  centres,  so  as  to  divide  itself  into  two  distinct  masses  (C,  a,  af) ; 
and  by  the  investment  of  these  two  secondary  '  endoplasms '  with 
cellulose-walls  a  complete  pair  of  new  cells  (D,  #,  a')  is  formed 
within  the  cavity  of  the  parent-cell.  The  process  of  free-cell  forma- 
tion is  always  connected,  directly  or  indirectly,  with  a  process  of 
reproduction  rather  than  of  growth,  and  takes  two  different  forms, 
the  one  occurring  in  the  production  of  the  '  zoospores  '  or  '  swarm- 
spores  '  of  alga?,  the  other  in  the  formation  of  pollen-grains,  or  of 

the  '  endosperm '  within 
the  embryo-sac  of  flowering 
plants.  In  the  former 
case,  the  endosperm^  in- 
stead of  dividing  itself  into 
two  halves,  usually  breaks 
up  into  numerous  segments 
corresponding  with  one 
another  in  size  and  form, 
each  of  which,  escaping 
from  the  parent  -  cavity, 
becomes  an  independent 
cell,  without  any  investing 
cell-wall  of  cellulose,  hence 
a  '  primordial  cell,'  en- 
dowed with  a  power  of 
rapid  motion  by  means  of 
cilia  or  flagella.  In  the 
second  case  the  endoplasm 
groups  itself,  more  or  less 
completely,  round  several 
centres,  each  of  which  has 
its  own  nucleus,  formed  by 
subdivision  of  the  nucleus 
of  the  parent-cell ;  and 
these  secondary  cells,  in 
various  stages  of  develop- 
ment, lie  free  within  the 
cavity  of  the  parent-cell, 
imbedded  in  its  resi<lu;il 
endoplasm,  each  proceeding  to  complete  itself  as  a  cell  by  the 
formation  of  a  limiting  wall  of  cellullose  (fig.  414).  As  a  'new 
generation'  in  any  phanerogamic  plant  has  its  origin  in  the 
fertilisation  of  a  highly  specialised  '  germ-cell '  (contained  within 
the  ovule)  by  the  contents  of  a  '  sperm-cell '  (the  pollen-grain), 
so  do  we  find,  among  all  save  the  lowest  cryptogams,  a  provision 
for  the  union  of  the  contents  of  two  highly  specialised  cells, 
the  'germ-cells'  being  fertilised  by  the  access  of  motile  proto- 
plasmic bodies  (antherozoids),  set  free  from  the  cavities  of  the 
'  sperm-cells '  within  which  they  were  developed.  But  although 
the  sexual  process  can  be  traced  downwards  under  this  form  into 


FIG.  413. — Binary  subdivision  of  cells  in  endo- 
sperm of  seed  of  scarlet-runner :  A,  ordinary 
cell,  with  nucleus  a,  and  nucleolus  b,  imbedded 
in  its  protoplasm ;  B,  cell  showing  subdivision 
of  nucleus  into  two  halves,  a  and  a' ;  C,  cell  in 
same  stage,  showing  contraction  of  endoplasm 
(produced  by  addition  of  water)  into  two  sepa- 
rate masses  round  the  two  segments  of  original 
nucleus ;  D,  two  complete  cells  within  mother- 
cell,  divided  by  a  partition. 


PROTOPLASM   OF   THE   LIVING   CELL 


537 


the  group  of  thallophytes,  we  find  among  the  lower  types  of  that 
group  a  yet  simpler  mode  of  bringing  it  about ;  for  there  is  strong 
reason  to  regard  the  act  of  '  conjugation '  which  takes  place  in 
the  Conjugate  and  in  some  fungi  in  the  same  light,  and  to  look 
upon  the  '  zygospore,' l  which  is  its  immediate  product,  as  the 
originator  (like  the  fertilised  embryo-cell  of  the  phanerogamic  seed) 
of  a  '  new  generation.' 
Great  attention 
has  recently  been 
paid  by  Strasburger 
and  others  to  the  con- 
stitution of  the  endo- 
plasm  and  to  the 
processes  connected 
with  cell-division.  On 
both  these  subjects  it 
is  impossible  here  to 
give  more  than  the 
barest  outlines.  Stras- 
burger distinguishes 

between  the  following  pIG<  414.— Successive  stages  of  free-cell  formation 
differentiated  parts  of  in  embryo-sac  of  seed  of  scarlet-runner;  a,  a,  a, 
the  protoplasm  of  the  completed  cells,  each  having  its  proper  cell-wall, 
i-  •  r  ,,  ™,  nucleus,  and  endoplasm,  lying  in  a  protoplasmic 

living  cell  I  -  J.ne  mass,  through  which  are  dispersed  nuclei  and  cells 
protoplasm  outside  the  in  various  stages  of  development. 

nucleus  he  terms  the 

cytoplasm ;  the  portion  which  constitutes  the  nucleus  is  the 
n  acleoplasm ;  that  which  enters  into  the  composition  of  the 
chlorophyll  corpuscles  and  other  allied  substances  is  the  chromato- 
plasm.  Each  of  these  three  portions  of  protoplasm  is  composed  of  a 
hyaline  matrix  or  hyaloplasm  and  of  imbedded  granular  structures 
or  microsomes.  A  distinct  substance,  known  as  nuclein,  absent  from 
the  cytoplasm,  appears  to  enter  into  the  composition  of  the  nucleus. 
The  various  substances  imbedded  in  the  cytoplasm  are  known  under 
the  general  name  of  plastids.  If  colourless,  they  are  leucoplasts,  and 

1  The  term  '  spore  '  has  been  long  used  by  cryptogamists  to  designate  the  minute 
reproductive  particles  (such  as  those  set  free  from  the  '  fructification '  of  ferns,  mosses, 
&c.)  which  were  supposed — in  the  absence  of  all  knowledge  of  their  sexual  relations— 
to  be  the  equivalents  of  the  seeds  of  flowering  plants.  But  it  is  now  known  that  such 
'  spores  '  have  (so  to  speak)  very  different  values  in  different  cases,  being,  in  by  far  the 
larger  proportion  of  cryptogams,  but  the  remote  descendants  of  the  fertilised  cell  which 
is  the  immediate  product  of  the  sexual  act  under  any  of  its  forms.  This  cell,  which 
will  be  distinguished  throughout  the  present  treatise  as  the  oosphere,  is  the  real  repre- 
sentative of  the  '  germinal  cell '  of  the  '  embryo  '  developed  within  the  seed  of  the 
flowering  plant.  On  the  other  hand,  the  various  kinds  of  non-sexual  spores  emitted 
by  cryptogams,  which  have  received  a  great  variety  of  designations,  are  all  to  be 
regarded  (as  will  be  presently  explained)  as  equivalents  of  the  leaf-buds  of  flower- 
ing plants. 

[The  different  interpretations  placed  upon  the  term  '  spore  '  and  its  derivatives  by 
different  writers  on  cryptogamic  botany  present  a  great  difficulty  to  the  student.  A 
different  terminology  for  the  one  followed  here  is  now  employed  by  some  of  the  best 
authorities ;  but,  in  order  to  avoid  the  great  alteration  in  the  use  of  terms  which 
would  otherwise  be  necessary,  it  has  been  thought  best,  in  the  present  edition,  to 
retain  Dr.  Carpenter's  terminology,  at  all  events  until  a  greater  agreement  has  been 
arrived  at  than  is  at  present  the  case.— ED.] 


538   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE^THALLOPHYTES 

these  are  the  special  seat  of  the  formation  of  the  starch  grains.  If 
coloured  they  are  chromoplasts  or  chromatof)hores,  the  origin  of  the 
various  colouring  matters  of  the  cell ;  those  which  give  birth  to  the 
chlorophyll  corpuscles  being  distinguished  by  the  special  term  chloro- 
plasts.  Minute  bodies  termed  physodes,  endowed  with  an  amoeboid 
motion,  have  been  observed  within  the  protoplasm  filaments.  In 
some  of  the  lower  plants,  at  present  exclusively  in  the  green  algae, 
there  are  found  within  the  chlorophyll  corpuscles  homogeneous 
proteid  substances  known  as  pyrenoids ;  they  are  often  surrounded 
by  starch  grains. 

The  division  of  the  nucleus  may  take  place  either  directly,  when 
the  process  is  known  as  fragmentation,  or  indirectly,  when  it  is  known 
as  mitosis  or  karyokinesis  (see  fig.  415).  In  the  process  of  indirect 
division,  the  protoplasm  of  which  the  nucleus  is  composed  undergoes 
a  great  variety  of  changes,  in  the  course  of  which  it  assumes  the 
beautiful  appearance  known  as  the  nuclear  spindle,  consisting  of  an 
equatorial  disc,  the  nuclear  plate,  and  delicate  spindle  fibres  which 
converge  towards  the  two  poles  of  the  spindle.  Apparently  con- 
nected with  the  process  of  cell-division  are  the  peculiar  bodies 
known  as  centrospheres,  directing  spheres,  or  attracting  spheres,  corre- 
sponding to  similar  bodies  found  in  animal  cells,  but  at  present 
detected  only  in  the  lower  forms  of  vegetable  life.  They  form  two 
small  homogeneous  spheres  lying  near  the  nucleus,  one  on  each  side 
of  it,  and  imbedded  in  the  cytoplasm.  Each  centrosphere  has  in  its 
centre  a  body  termed  the  centrosome,  composed  of  one  or  more  small 
granules.  To  follow  out  all  the  processes  of  karyokinesis  requires 
very  high  magnifying  powers  of  the  microscope,  great  skill  in  mani- 
pulation, and  the  use  of  very  delicate  staining  reagents. 

The  older  conception  of  the  vegetable  cell  regarded  it  as  a  com- 
pletely closed  vesicle,  the  eiidoplasm  of  which  is  entirely  shut  off 
from  contact  with  that  of  the  adjacent  cells.  Recent  observations 
require  the  modification  of  this  conception.  It  has  been  shown  that 
in  many  cases  the  cell-wall  is  perforated  by  very  minute  orifices, 
through  wThich  excessively  fine  strings  of  protoplasm  pass  from  one 
cell-cavity  to  another  (fig.  416).  This  continuity  of  protoplasm  has 
been  observed  in  some  seaweeds  and  other  algae,  in  the  endosperm 
of  the  ovule,  in  the  pulvinus  or  motile  organ  of  the  leaves  of  the 
sensitive  plant,  and  in  many  other  instances,  and  is  regarded  by 
some  authorities  as  probably  a  universal  phenomenon  in  living  cells. 
In  the  case  of  the  sensitive  plant  it  is  undoubtedly  connected  with 
the  remarkable  phenomenon  of  sensitiveness  or  irritability  displayed 
by  the  leaves. 

In  the  lowest  forms  of  vegetation  every  single  cell  is  not  only 
capable  of  living  in  a  state  of  isolation  from  the  rest,  but  even 
normally  does  so ;  and  thus  the  plant  may  be  said  to  be  unicellular, 
every  cell  having  an  independent  *  individuality.'  There  are  others, 
again,  in  which  amorphous  masses  are  made  up  by  the  aggregation 
of  cells,  which,  though  quite  capable  of  living  independently,  remain 
attached  to  each  other  by  the  mutual  fusion  (so  to  speak)  of  their 
gelatinous  investments ;  and  there  are  others,  moreover,  in  which  a 
definite  adhesion  exists  between  the  cells,  and  in  which  regular 


CELL-DIVISIOX 


539 


plant-like  structures  are  thus  formed,  notwithstanding  that  every 
cell  is  but  a  repetition  of  every  other,  and  is  capable  of  living  inde- 
pendently if  detached,  so  as  still  to  answer  to  the  designation  of  a 
*  unicellular  '  or  single-celled  plant.  These  different  conditions  we 
shall  find  to  arise  out  of  the  mode  in  which  each  particular  species 
multiplies  by  binary  subdivision  ;  for  where  the  cells  of  the  new  pair 
that  is  produced  by  division  of  the  previous  cell  undergo  a  complete 
separation  from  one  another,  they  will  henceforth  live  indepen- 


J£  I 

FIG.  415. — Division  of  the  pollen-mother-cells  of  Fritillaria  persica.     (From  Stras- 
burger  and  Hillhouse's  '  Practical  Botany,'  published  by  Sonnenschein.) 

dently ;  but  if,  instead  of  undergoing  this  complete  fission,  they  are 
held  together  by  the  intervening  gelatinous  envelope,  a  shapeless 
mass  results  from  repeated  subdivisions  not  taking  place  on  any 
determinate  plan  ;  and  if,  moreover,  the  binary  subdivision  always 
^  takes  place  in  one  direction  only,  a  long,  narrow  filament  (fig.  424,  D), 
or  if  in  two  directions  only,  a  broad,  flat,  leaf-like  expansion  (G), 
may  be  generated.  To  such  extended  fabrics  the  term  '  unicellular ' 
plants  can  scarcely  be  applied  with  propriety ;  since  they  may  be 
built  up  of  many  thousands  or  millions  of  distinct  cells,  which  have 


nt 


540  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

110  disposition  to  separate  from  each  other  spontaneously.  Still 
they  correspond  with  those  which  are  strictly  unicellular,  as  to  the 
absence  of  differentiation,  either  in  structure  or  in  function,  between 
their  component  cells,  each  one  of  these  being  a  repetition  of  the 
rest,  and  no  relation  of  mutual  dependence  existing  among  them ; 
and  all  such  simple  organisms,  therefore,  may  still  be  included 
under  the  general  term  of  Thallophytes. 

Excluding  lichens,  for  the  reasons  to  be  stated  hereafter,  botanists 
now  rank  these  thallophytes  under  two  series  : — algce,  which  form 
chlorophyll,  and  can  support  themselves  upon  air,  water,  and  mineral 
matters;  and  fungi,  which,  not  forming  chlorophyll  for  themselves, 
depend  for  their  nutriment  upon  materials  drawn  from  other  organ- 
isms. Each  series  contains  a  very  large  variety  of  forms,  which, 
when  traced  from  below  upwards,  present  gradually  increasing  com- 
plexities of  structure  ; 
and  these  gradations 
show  themselves  espe- 
cially in  the  provisions 
made  for  the  genera- 
tive process.  Thus,  in 
some  forms,  a  'zygo- 
spore '  is  produced  by 
the  fusion  of  the  con- 
tents of  two  cells, 
which  neither  present 
any  apparent  sexual 
difference  the  one 
from  the  other,  nor 
can  be  distinguished 
in  any  way  from  the 
rest.  In  the  next 
highest  forms,  while 
the  'conjugating'  cells 
are  still  apparently 
undifferentiated  from 
the  rest  of  the  structure,  a  sexual  difference  shows  itself  between 
them ;  the  contents  of  one  cell  (male)  passing  over  into  the  cavity 
of  the  other  (female),  within  which  the  'zygospore'  is  formed. 
The  next  stage  in  the  ascent  is  the  resolution  of  the  contents 
of  the  male  cell  into  motile  bodies  ('  antherozoids '),  which,  escaping 
from  it,  move  freely  through  the  water,  and  find  their  way  to 
the  female  cell,  whose  contents,  fertilised  by  coalescence  with  the 
material  they  bring,  form  an  '  oospore.'  In  the  lower  forms  of  this 
stage,  again,  the  generative  cells  are  not  distinguishable  from  the 
rest  until  the  contents  begin  to  show  their  characteristically  sexual 
aspect;  but  in  the  higher  they  are  developed  in  special  organs, 
constituting  a  true  '  fructification.'  This  must,  however,  be  dis- 
tinguished from  organs  which,  though  commonly  spoken  of  as  the 
*  fructification,'  have  no  real  analogy  with  the  generative  apparatus 
of  flowering  plants,  their  function  being  merely  to  give  origin  to 


I *^1 


FIG.  416. — Continuity  of  protoplasm.     (From  Vines's 
'  Physiology  of  Plants.'    Cambridge  University  Press.) 


STRUCTURE    OF   PALMOGLCEA 


541 


gonidial  l  cells  or  groups  of  cells,  which  simply  multiply  the  parent 
stock,  in  the  same  manner  that  many  flowering  plants  (such  as  the 
potato)  can  be  propagated  by  the  artificial  separation  of  their  leaf- 
buds.  It  frequently  happens  among  cryptogams  that  this  gonidial 
fructification  is  by  far  the  more  conspicuous,  the  sexual  fructifica- 
tion being  often  so  obscure  that  it  cannot  be  detected  without 
great  difficulty  ;  and  we  shall  presently  see  that  there  are  some 
thallophytes  in  which  the  production  of  govids  seems  to  go  on 
indefinitely,  no  form  of  sexual  generation  having  been  detected 
in  them.  These  general  statements  will  now  be  illustrated  by 
sketches  of  the  life-history  of  some  of  those  humble  thallophytes 
which  present  the  phenomena  qf  cell-division,  conjugation,  and 


FIG.  417. — Development  of  Palmoglcea  macrococca. 

gonidial  multiplication,  under  their  simplest  and  most  instructive 
aspect. 

The  first  of  these  lowly  forms  of  life  to  which  we  call  the 
attention  of  the  reader  is  Palmogloea  macrococca,  Ktz.,2  one  of 
those  humble  kinds  of  vegetation  which  spread  themselves  as  green 
slime  over  damp  stones,  walls,  &c.  When  this  slime  is  examined 
with  the  microscope,  it  is  found  to  consist  of  a  multitude  of  green 
cells  (fig.  417,  A),  each  surrounded  by  a  gelatinous  envelope  ;  the  cell, 
which  does  not  seem  to  have  any  distinct  membranous  wall,  is  filled 
with  a  granular  '  endochrome,'  consisting  of  green  particles  diffused 
through  colourless  protoplasm ;  and  in  the  midst  of  this  a  nucleus 

1  The  term  gonids,  originally  applied  to  certain  green  cells  in  the  lichen-crusts 
that  are  capable,  when  detached,  of  reproducing  the  vegetable  portion  of  the  plant, 
is  used  by  some  writers  as  a  designation  of  the  non-sexual  spores  of  cryptogams 
generally,  which  it  is  very  important  to  discriminate  from  the  genitative  '  oospheres.' 
If  possessed  of  motile  powers,  they  are  spoken  of  as  :  zoospores,'  or  sometimes  (011 
account  of  the  appearance  they  present  when  a  number  are  set  free  at  once)  as 
1  swarm-spores.'  In  contradistinction  to  '  motile  '  gonids  or  '  zoospores,'  those  which 
show  no  movement  are  often  termed  resting  spores,  or  hypnospores ;  but  such  may 
be  either  sexual  oi'>S2)lieres  or  non-sexual  'gonids,  the  latter,  like  the  former,  often 
'  encysting '  themselves  in  a  firm  envelope,  and  then  remaining  dormant  for  long  periods 
of  time. 

-  [Most  of  the  species 'of  Kiitzing's  genus  Palmoglcea  are  now  regarded  as  belong- 
ing to  the  Desmldiacece,  and  are  included  under  the  genus  Mesotceninni. — ED.] 


542    MICKOSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

may  sometimes  be  distinguished,  and  can  always  be  brought  into 
view  by  tincture  of  iodine,  which  turns  the  *  endochrome '  to  a 
brownish  hue,  and  makes  the  nucleus  (G)  dark  brown.  Other  cells 
are  seen  (B),  which  are  considerably  elongated,  some  of  them 
beginning  to  present  a  sort  of  hour  glass  contraction  across  the 
middle ;  and  when  cells  in  this  condition  are  treated  with  tincture 
of  iodine,  the  nucleus  is  seen  to  be  undergoing  the  like  elongation 
and  constriction  (H).  A  more  advanced  state  of  the  process  of 
subdivision  is  seen  at  C,  in  which  the  constriction  has  proceeded  to  the 
extent  of  completely  cutting  off  the  two  halves  of  the  cell,  as  well  as  of 
the  nucleus  (I),  from  each  other,  though  they  still  remain  in  mutual 
contact ;  in  a  yet  later  stage  they  are  found  detached  from  each 
other  (D),  though  still  included  within  the  same  gelatinous  envelope. 
Each  new  cell  then  begins  to  secrete  its  own  gelatinous  envelope,  so 
that  by  its  intervention  the  two  are  usually  soon  separated  from 
each  other  (E).  Sometimes,  however,  this  is  not  the  case,  the 
process  of  subdivision  being  quickly  repeated  before  there  is  time  for 
the  production  of  the  gelatinous  envelope,  so  that  a  series  of  cells 
(F)  hanging  on  one  to  another  is  produced.  There  appears  to  be  no 
definite  limit  to  this  kind  of  multiplication,  and  extensive  areas 
may  be  quickly  covered,  in  circumstances  favourable  to  the  growth 
of  the  plant,  by  the  products  of  the  binary  subdivision  of  one 
original  cell.  This,  as  already  shown,  is  really  an  act  of  yrowth. 
which  continues  indefinitely  so  long  as  moisture  is  abundant  and 
the  temperature  lowr.  But  under  the  influence  of  heat  and  dryness 
the  process  of  cell- multiplication  gives  place  to  that  of  '  conjugation,' 
in  which  two  cells,  apparently  similar  in  all  respects,  fuse  together 
for  the  production  of  a  *  zygospore,'  which  (like  the  seed  of  a 
flowering  plant)  can  endure  being  reduced  to  a  quiescent  state  for 
an  unlimited  time,  and  may  be  so  completely  dried  up  as  to  seem 
like  a  particle  of  dust,  yet  resumes  its  vegetative  activity  whenever 
placed  in  the  conditions  favourable  to  it.  The  conjugating  process 
commences  by  the  putting  forth  of  protrusions  from  the  boundaries 
of  two  adjacent  cells,  which  meet,  fuse  together  (thereby  showing 
the  want  of  firmness  of  their  '  ectoplasms  '),  and  form  a  connecting 
bridge  between  their  cavities  (K).  The  fusion  extends  before  long 
through  a  large  part  of  the  contiguous  sides  of  the  two  cells  (L)  ; 
and  at  last  becomes  so  complete  that  the  combined  mass  (M)  shows 
no  trace  of  its  double  origin.  It  soon  forms  for  itself  a  firm  cellulose 
envelope,  which  bursts  when  the  'zygospore'  is  wetted;  and  the 
contained  cell  begins  life  as  a  new  generation,  speedily  multiplying, 
like  the  former  ones,  by  binary  subdivision.  It  is  curious  to  observe 
that  during  this  conjugating  process  a  production  of  oil  particles 
takes  place  in  the  cells;  these  are  at  first  small  and  distant,  but 
gradually  become  larger  and  approximate  more  closely  to  each  other, 
and  at  last  coalesce  so  as  to  form  oil-drops  of  various  sizes,  the  green 
granular  matter  disappearing ;  and  the  colour  of  the  conjugated 
body  changes,  with  the  advance  of  this  process,  from  green  to  a  light 
yellowish  brown.  When  the  zygospore  begins  to  vegetate,  on  the 
other  hand,  a  converse  change  occurs;  the  oil-globules  disappear, 
and  green  granular  matter  takes  their  place. 


STRUCTURE    OF  PROTOCOCCUS 


543 


If  this  (as  seems  probable)  constitutes  the  entire  life-cycle  of 
Palmogl&a,  it  affords  no  example  of  that  curious  *  motile '  stage 
which  is  exhibited  by  most  algal  protophytes  in  some  stage  of  their 
existence,  and  which  constitutes  a  large  part  of  the  life -history  of 
the  minute  unicellular  organism  nowr  to  be  described,  Protococcus 
pluvialis,  Ktz.  (Chlamydococcus  pluvialis,  A.  Br.)  (fig.  418).  which 
is  not  uncommon  in  collections  of  rain-water.  Xot  only  lias  this 
protophyte,  in  its  motile  condition,  been  very  commonly  regarded  as 
an  animalcule,  but  its  different  states  have  been  described .  under 
several  different  names.  In  the  first  place,  the  colour  of  its  cells 
varies  considerably  ;  since,  although  they  are  usually  green  at  the 
period  of  their  most  active  life,  they  are  sometimes  red ;  and  their 
red  form  has  received  the  distinguishing  appellation  of  Hcemato- 


FIG.  418. — Development  of  Protococcus  pluvialis. 

coccus.  Very  commonly  the  red  colouring  matter  forms  only 
a  central  mass  of  greater  or  less  size,  having  the  appearance  of 
a  nucleus  (as  shown  at  E,  fig.  418)  ;  and  sometimes  it  is  reduced 
to  a  single  granular  point,  which  has  been  described  by  Professor 
Ehrenberg  as  the  eye-spot  of  these  so-called  animalcules.  It  is 
quite  certain  that  the  red  colouring  substance  is  very  nearly  related 
in  its  chemical  character  to  the  green,  and  that  the  one  may  be 
converted  into  the  other,  though  the  conditions  under  which  this 
conversion  takes  place  are  not  precisely  known.  In  the  '  still '  form 
of  the  cell,  with  which  we  may  commence  the  history  of  its  life,  the 
eiidoplasm  consists  of  a  colourless  protoplasm,  through  which  red  or 
green  coloured  granules  are  more  or  less  uniformly  diffused  ;  and  the 
surface  of  the  colourless  protoplasm  is  condensed  into  an  ectoplasm, 
which  is  surrounded  by  a  tolerably  firm  cell-wall,  consisting  of  cellulose 


544  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

or  of  some  modification  of  it.  Outside  this  (as  shown  at  A),  when 
the  '  still '  cell  is  formed  by  a  change  in  the  condition  of  a  cell  that 
lias  been  previously  'motile/  we  find  another  envelope,  which  seems 
to  be  of  the  same  nature,  but  which  is  separated  by  the  interposition 
of  aqueous  fluid  ;  this,  however,  may  be  altogether  wanting.  The 
multiplication  of  the  '  still '  cells  by  subdivision  takes  place  as  in 
Palmoglcea,  the  endoplasm  first  undergoing  separation  into  two 
halves  (as  seen  at  B),  and  each  of  these  halves  subsequently  developing 
a  cellulose  envelope  around  itself,  and  undergoing  the  same  division 
in  its  turn.  Thus  two,  four,  eight,  or  sixteen  new  cells  are  succes- 
sively produced ;  and  these  are  sometimes  set  free  by  the  complete 
dissolution  of  the  envelope  of  the  original  cell ;  but  they  are  more 
commonly  held  together  by  its  transformation  into  a  gelatinous 
investment,  in  which  they  remain  imbedded.  Sometimes  the  endo- 
plasm subdivides  at  once  into  four  segments  (as  at  D),  of  which  every 
one  forthwith  acquires  the  character  of  an  independent  cell ;  but 
this,  although  an  ordinary  method  of  multiplication  among  the  '  mo- 
tile '  cells,  is  comparatively  rare  in  the  '  still '  condition.  Sometimes, 
again,  the  endoplasm  of  the  *  still '  form  subdivides  at  once  into 
eight  portions,  which,  being  of  small  size,  and  endowed  with  motile 
power,  may  be  considered  as  zoospores.  As  far  as  the  complete  life- 
history  of  Protococcus  is  at  present  known,  some  of  these  zoospores 
retain  their  motile  powers,  and  develop  themselves  into  the  ordinary 
*  motile '  cells ;  others  produce  a  firm  cellulose  envelope  and  become 
'  still '  cells ;  and  others  (perhaps  the  majority)  perish  without  any 
further  change. 

When  the  ordinary  division  of  the  ;  still '  cells  into  two  segments, 
has  been  repeated  four  times,  so  as  to  produce  sixteen  cells — and 
sometimes  at  an  earlier  period — the  new  cells  thus  produced  assume 
the  *  motile  '  condition,  being  liberated  before  the  development  of  the 
cellulose  envelope,  and  becoming  furnished  with  two  long  vibratile 
flagella  which  seem  to  be  extensions  of  the  colourless  protoplasm 
layer  that  accumulates  at  their  base  so  as  to  form  a  sort  of  trans- 
parent beak  (H).  In  this  condition  it  seems  obvious  that  the  colour- 
less protoplasm  is  more  developed  relatively  to  the  colouring  matter 
than  it  is  in  the  *  still '  cells ;  and  it  usually  contains  '  vacuoles ' 
occupied  only  by  clear  aqueous  fluid,  which  are  sometimes  so 
numerous  as  to  take  in  a  large  part  of  the  cavity  of  the  cell,  so  that 
the  coloured  contents  seem  only  like  a  deposit  on  its  walls.  Before 
long  this  '  motile '  cell  acquires  a  peculiar  saccular  investment,  wrhich 
seems  to  correspond  with  the  cellulose  envelope  of  the  '  still '  cells, 
but  is  not  so  firm  in  its  consistence  (I,  K,  L) ;  and  between  this  and 
the  surface  of  the  ectoplasm  a  considerable  space  intervenes,  tra- 
versed by  thread-like  extensions  of  the  latter,  which  are  rendered 
more  distinct  by  iodine,  and  can  be  made  to  retract  by  means  of 
reagents.  The  flagella  pass  through  the  cellulose  envelope,  which 
invests  their  base  with  a  sort  of  sheath,  and  in  the  portion  that  is 
within  this  sheath  no  movement  is  seen.  During  the  active  life  of 
the  '  motile  '  cell  the  vibration  of  these  flagella  is  so  rapid  that  they 
can  be  recognised  only  by  the  currents  they  produce  in  the  water 
through  which  the  cells  are  quickly  propelled ;  but  when  the  motion 


STRUC'TUKE   OF  PROTOCOCCUS  545 

becomes  slacker  the  flagella  themselves  are  readily  distinguishable, 
and  they  may  be  made  more  obvious  by  the  addition  of  iodine,  which, 
however,  it  should  be  noted,  always  kills  the  plant. 

The  multiplication  of  these  '  motile '  cells  may  take  place  in 
various  modes,  giving  rise  to  a  great  variety  of  appearances.  Some- 
times they  undergo  a  regular  binary  subdivision  (B),  whereby  a  pair 
of  motile  cells  is  produced  (C),  each  resembling  its  single  predecessor 
in  possessing  the  cellulose  investment,  the  transparent  beak,  and  the 
vibratile  flagella,  before  the  dissolution  of  the  original  investment. 
Sometimes,  again,  the  contents  of  the  original  cell  undergo  a  seg- 
mentation in  the  first  instance  into  four  divisions  (D)  ;  which  may 
either  become  isolated  by  the  dissolution  of  their  envelope,  and  may 
separate  from  each  other  in  the  condition  of  '  free  primordial  cells ' 
(H),  developing  their  cellulose  investments  at  a  future  time,  or 
may  acquire  their  cellulose  investments  (as  in  the  preceding  case) 
before  the  solution  of  that  of  the  original  cell ;  while  sometimes, 
even  after  the  disappearance  of  this,  and  the  formation  of  their  own 
independent  investments,  they  remain  attached  to  each  other  at  their 
beaked  extremities,  the  primordial  cells  being  connected  with  each 
other  by  peduncular  prolongations,  and  the  whole  compound  body 
having  the  form  of  a  +.  This  quaternary  segmentation  appears  to 
be  a  more  frequent  mode  of  multiplication  among  the  '  motile '  cells 
than  the  subdivision  into  two,  although,  as  we  have  seen,  it  is  less 
common  in  the  *  still '  condition.  So  also  a  primary  segmentation  of 
the  entire  endochrome  of  the  '  motile '  cells  into  eight,  sixteen,  or  even 
thirty-two  parts,  may  take  place  (E,  F),  thus  giving  rise  to  as  many 
minute  gonidial  cells.  These,  when  set  free,  and  possessing  active 
powers  of  movement,  are  true  zoospores  (G)  ;  they  may  either  develop 
a  loose  cellulose  investment  or  cyst,  so  as  to  attain  the  full  dimensions 
of  the  ordinary  motile  cells  (I,  K),  or  they  may  become  clothed  with 
a  dense  envelope  and  lose  their  flagella,  thus  passing  into  the  '  still ' 
condition  (A)  ;  and  this  last  transformation  may  even  take  place 
before  they  are  set  free  from  the  envelope  within  which  they  were 
produced,  so  that  they  constitute  a  mulberry-like  mass,  which  fills 
the  whole  cavity  of  the  original  cell,  and  is  kept  in  motion  by  its 
flagella 

To  what  extent  Protococcus  is  an  autonomous  organism  is  still 
doubtful,  but  it  appears  to  be  more  or  less  closely  connected  with 
many  forms  of  life  which  have  been  described,  not  merely  as  dis- 
tinct species,  but  as  distinct  genera  of  animalcules  or  of  protophytes, 
such  as  Ghlamydomonas,  Euglena,  Trachelomonas,  Gyges,  Gonium, 
Pandorina,  Jlotryocystis,  Uvella,  Syncrypta,  Nonas.  Astasia,  Bodo,  and 
many  others.  Certain  forms,  such  as  the  *  motile '  cells  I,  K,  L, 
appear  in  a  given  infusion,  at  first  exclusively  and  then  principally ; 
they  gradually  diminish,  become  more  and  more  rare,  and  finally 
disappear  altogether,  being  replaced  by  the  *  still '  form.  After 
some  time  the  number  of  the  *  motile '  cells  again  increases,  and 
reaches,  as  before,  an  extraordinary  amount ;  and  this  alternation 
may  be  repeated  several  times  in  the  course  of  a  few  weeks.  The 
process  of  segmentation  is  often  accomplished  with  great  rapidity. 
If  a  number  of  '  motile  '  cells  be  transferred  from  a  larger  glass  into  a 

N  N 


546   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTE8 

smaller,  it  will  be  found,  after  the  lapse  of  a  few  hours,  that  most  of 
them  have  subsided  to  the  bottom  ;  in  the  course  of  the  day  they 
will  all  be  observed  to  be  upon  the  point  of  subdivision ;  on  the 
following  morning  the  divisional  brood  will  have  become  quite  free  ; 
and  on  the  next  the  bottom  of  the  vessel  will  be  found  covered  with 
a  new  brood  of  dividing  cells,  which  again  proceed  to  the  forma- 
tion of  a  new  brood,  and  so  on.  The  activity  of  motion  and  the 
activity  of  multiplication  seem  to  stand,  in  some  degree,  in  a  relation 
of  reciprocity  to  each  other  ;  for  the  dividing  process  takes  place 
with  greater  rapidity  in  the  '  still '  cells  than  it  does  in  the  '  motile.' 
What  are  the  precise  conditions  which  determine  the  transition 
between  the  i  still '  and  the  i  motile '  states  cannot  yet  be  precisely 
defined,  but  the  influences  of  certain  agencies  can  be  predicted  with 
tolerable  certainty.  Thus  it  is  only  necessary  to  pour  the  water 
containing  these  organisms  from  a  smaller  and  deeper  into  a  larger 
and  shallower  vessel  in  order  at  once  to  determine  segmentation  in 
numerous  cells — a  phenomenon  which  is  observable  also  in  many  other 
protophytes.  The  '  motile '  cells  seem  to  be  favourably  affected  by 
light,  for  they  collect  themselves  at  the  surface  of  the  water  and  at 
the  edges  of  the  vessel,  but  when  they  are  about  to  undergo  segmen- 
tation or  to  pass  into  the  '  still '  condition,  they  sink  to  the  bottom 
of  the  vessel,  or  retreat  to  that  part  of  it  in  which  they  are  least 
subjected  to  light.  When  kept  in  the  dark  the  '  motile  '  cells  undergo 
a  great  diminution  of  their  chlorophyll,  which  becomes  very  pale, 
and  is  diffused,  instead  of  forming  definite  granules  ;  they  continue 
their  movement,  however,  uninterruptedly  without  either  sinking 
to  the  bottom,  or  passing  into  the  'still  '  form,  or  undergoing  seg- 
mentation. A  moderate  warmth,  particularly  that  of  the  vernal  sun, 
is  favourable  to  the  development  of  the  '  motile '  cells  ;  but  a  tempe- 
rature of  excessive  elevation  prevents  it.  Rapid  evaporation  of  the 
water  in  which  the  '  motile  '  forms  may  be  contained  kills  them  at 
once ;  but  a  more  gradual  loss,  such  as  takes  place  in  deep  glasses, 
causes  them  merely  to  pass  into  the  '  still '  form  ;  and  in  this  condi- 
tion— especially  when  they  have  assumed  a  red  hue — they  may  be 
completely  dried  up,  and  may  remain  in  a  state  of  dormant  vitality 
for  many  years.  It  is  in  this  state  that  they  are  wafted  about  in 
atmospheric  currents,  and  that,  being  brought  down  by  rain  into 
pools,  cisterns,  &c.,  they  may  present  themselves  where  none  had 
been  previously  known  to  exist ;  and  there  under  favourable  circum- 
stances they  may  undergo  a  very  rapid  multiplication,  and  may 
maintain  themselves  until  the  water  is  dried  up,  or  some  other 
change  occurs  which  is  incompatible  with  the  continuance  of  their 
vital  activity.  They  then  very  commonly  become  red  throughout, 
the  red  colouring  substance  extending  itself  from  the  centre  towards 
the  circumference,  and  assuming  an  appearance  like  that  of  oil- 
drops  ;  and  these  red  cells,  acquiring  thick  cell-walls  and  a  mucous 
envelope,  float  in  flocculent  aggregations  on  the  surface  of  the  water. 
This  state  seems  to  correspond  with  the  '  resting-spores '  of  other 
protophytes ;  and  it  may  continue  until  warmth,  air,  and  moisture 
cause  the  development  of  the  red  cells  into  the  ordinary  '  still '  cells, 
green  matter  being  gradually  produced,  until  the  red  substance  forms 


PROTOCOCCUS  ;    CYANOPHYCE.E  547 

only  the  central  part  of  the  enclochrome.  After  this  the  cycle  of 
changes  occurs  which  has  been  already  described ;  and  the  plant 
may  pass  through  a  long  series  of  these  before  it  returns  to  the  state 
of  the  red  thick-walled  cell,  in  which  it  may  again  remain  dormant 
for  an  unlimited  period.  Even  this  cycle,  however,  cannot  be 
regarded  as  completing  the  history  of  Protococcus,  since  it  does  not 
include  the  performance  of  any  true  generative  act.  There  can  be 
little  doubt  that,  in  some  stage  of  its  existence,  a  '  conjugation  '  of 
two  cells  occurs,  as  in  Palmoglcea ;  and  the  attention  of  observers 
should  be  directed  to  its  discovery,  as  well  as  to  the  detection  of 
other  varieties  in  the  condition  of  this  interesting  little  plant,  which 
will  probably  be  found  to  present  themselves  before  and  after  the 
performance  of  that  act.1 

The  Cyanophycese  or  Phycochromaceae  constitute  another  group 
of  lowly  forms  of  vegetable  life,  distinguished  by  their  blue-green 
colour,  differing  from  the  Protococcaceae  in  not  containing  true 
chlorophyll  grains,  the  cell-sap  being,  on  the  other  hand,  coloured  by 
a  soluble  blue-green  pigment  known  as  '  phycocyanin.'  They  live 
either  isolated,  or  a  number  congregated  together  and  enclosed  in  a 
more  or  less  dense  colourless  jelly.  They  multiply  by  binary 
division,  and  do  not  in  any  case  produce  zoospores.  To  the  lowest 
family  of  this  group,  which  strongly  resemble  the  Protococcaceae, 
except  in  the  colour  of  the  cells,  the  Chroococcacece,  belong  the  genera 
Chroococcus,  Glceocapsa,  Aphunocapsa,  Merismopedia,  and  many 
others,  the  life-history  of  which  is  but  very  imperfectly  known. 

The  Oscillator  iacece  constitute  a  family  of  Cyanophycese  of  great 
interest  to  the  microscopist,  on  account  both  of  the  extreme  sim- 
plicity of  their  structure  and  of  the  peculiar  animal-like  movements 
which  they  exhibit.  They  consist  of  fine,  usually  microscopic 
threads,  containing  a  blue-green  eiidochrome,  sometimes  replaced  by 
a  red  or  violet,  and  occur  singly  or  in  thick  strata  in  fresh  running 
or  more  abundantly  in  stagnant  water.  The  threads  are  unbranched 
and  usually  straight,  and  either  each  separate  thread  or  a  number 
together  are,  in  most  of  the  genera,  enclosed  in  a  gelatinous  sheath. 
Some  illustrations  of  these  are  seen  on  Plate  VII.  The  contents  of 
the  sheaths  are  imperfectly  divided  into  cells  by  transverse  divi- 
sion ;  small  pieces  of  the  threads,  consisting  of  a  few  cells,  occasion- 
ally break  off,  round  themselves  off  at  both  ends,  move  about  with  a 
slow  undulating  motion,  and  finally  develop  into  new  threads;  these 
portions  are  known  as  hormoyones.  The  most  abundant  genus,  Oscil- 
lator ia,  has  been  so  named  from  the  peculiar  oscillating  or  waving  motion 
with  which  the  threads  are  endowed.  This  consists  of  a  creeping 
motion  in  the  direction  of  the  length  of  the  thread,  now  backwards, 
now  forwards,  accompanied  by  a  curvature  of  the  thread  and  rotation 
round  its  own  axis.  The  cause  of  this  motion  is  still  a  matter  of 

1  In  the  above  sketch  the  Author  has  presented  the  facts  described  by  Dr.  Cohn 
under  the  relation  which  they  seemed  to  him  naturally  to  bear,  but  which  differs  from 
that  in  which  they  will  be  found  in  the  original  memoir ;  and  he  is  glad  to  be  able 
to  state,  from  personal  communication  with  its  able  author,  that  Dr.  Cohn's  later 
observations  led  him  to  adopt  a  view  of  the  relationship  of  the  '  still '  and  '  motile  ' 
forms  which  is  in  essential  accordance  with  his  own. 

N  X  2 


548   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


controversy.  Professor  Colin  l  observed  that  the  oscillating  move- 
ments take  place  only  when  the  thread  is  in  contact  with  a  solid 
substratum.  Zukal  2  compares  the  motion  of  Spirulina  to  that  of  a 
growing  tendril,  and  asserts  that  it  is  intimately  connected  with  the 
growth  of  the  filament.  Hansgirg,3  on  the  other  hand,  considers  the 
twisting  and  nodding  movements  to  be  due,  not  to  the  growth  of  the 
thread,  but  to  osmotic  changes  in  the  cell-contents.  He  regards  them 
as  being  of  the  same  nature  as  the  movements  of  the  sarcode  in  the 
pseudopodia  of  rhizopods  and  other  protozoa.  Schnetzler  4  describes 
the  movements  in  Oscillator ia  as  of  six  different  kinds  :  (1)  rotation  of 
the  thread  or  of  its  segments  round  its  axis  ;  (2)  creeping  or  gliding 
over  a  solid  substratum  ;  (3)  a  free-swimming  movement  in  the  water  ; 
(4)  rotation  or  flexion  of  the  entire  thread  ;  (5)  sharp  tremblings  or 

concussions  ;  and  (6)  a  radiating  arrange- 
ment of  the  entangled  threads.  The. 
movements  are  greatly  influenced  by 
temperature  and  light,  being  much  more 
active  in, warmth  and  sunshine  than  in 
cold  and  shade.  There  are  no  zoospores 
produced,  nor  is  any  sexual  mode  of 
generation  known.  The  Rivulariacece 
and  Scytonemacece  (Pis.  VII  and  VIII) 
are  exceedingly  common  organisms  in 
stagnant  water,  resembling  the  Oscilla- 
toriace?e  in  their  blue-green  colour,  and 
in  their,  reproduction  by  means  of 
'  hormogones.' 

Nearly  allied  to  the  preceding  is  the 
family  of  Nostocacece,  consisting  of 
distinctly  beaded  filaments,  which,  in 
the  most  familiar  genus,  Nostoc,  lie  in 
firmly  gelatinous  envelopes  of  definite 
outline  (fig.  419).  The  filaments  are 
usually  simple,  though  sometimes  densely 
interwoven,  and  are  almost  always  curved 
or  twisted,  often  taking  a  spiral  direction. 
The  masses  of  jelly  in  which  they  are  imbedded  are  sometimes 
globular  or  nearly  so,  and  sometimes  extend  in  more  or  less 
regular  branches ;  they  frequently  attain  a  very  considerable 
size  ;  and  as  they  occasionally  present  themselves  quite  suddenly 
(especially  in  the  latter  part  of  autumn  on  damp  gar  den- walks), 
they  have  received  the  name  of  *  fallen  stars.'  They  are  not 
always  so  suddenly  produced,  however,  as  they  appear  to  be  ;  for 
they  shrink  up  into  mere  films  in  dry  weather  and  expand  again 
with  the  first  shower.  Other  species  are  not  unfrequent  among  wet 
moss  or  011  the  surface  of  damp  rocks.  Species  of  Anabcmia  and 
Aphanizomenon,  genera  of  ISTostocaceae,  constitute  a  large  portion  of 

1  Arch.  MikrosJc.  Anatomie,  1867,  p.  48. 

2  Oesterreichische  Bot.  Zeitsclir.  1880,  p.  11. 
5  See  Bot.  Centralblatt,  vol.  xii.  1882,  p.  361. 
4  Arch.  Sci.  Phys.  et  Nat.  1885,  p.  164. 


FIG.  419. — Portion  of  gelatinous 
frond  of  Nostoc. 


CYANOPHYCE^:  ;     CONJUGATE  549 

the  bluish-green  scum  which  floats  on  the  surface  of  stagnant  water. 
Colonies  of  species  of  Xostoc  and  Anabcena  are  frequently  endophytic 
within  the  cells  of  Marchantia  and  other  Hepaticre,  the  prothallia  of 
ferns,  or  other  aquatic  or  moisture-loving  plants.  3/~ostoc multiplies, 
like  the  Oscillatoriacese,  by  the  subdivision  of  its  filaments,  portions 
of  which  escape  from  the  gelatinous  mass  wTherein  they  were 
imbedded,  and  move  slowly  through  the  water  in  the  direction  of  their 
length.  These  are  '  hormogones,'  similar  to  those  of  the  Oscilla- 
toriacese.  After  a  time  they  cease  to  move,  and  a  new  gelatinous 
envelope  is  formed  around  each  piece,  which  then  begins  to  increase 
in  length  by  the  transverse  subdivision  of  its  segments.  By  the 
repetition  of  this  process  a  mass  of  new  filaments  is  produced,  the 
parts  of  which  are  at  first  coniiised,  but  afterwards  become  more 
distinctly  separated  by  the  interposition  of  the  gelatinous  substance 
developed  between  them.  Besides  the  ordinary  cells  of  the  beaded 
filaments,  two  other  kinds  are  known,  both  larger  than  the  ordinary 
cells,  and  called  respectively  heterocysts  and  resting -spores.  The 
function  of  the  former  is  unknown  ;  the  latter  develop  directly  into 
new  individuals  by  division  in  the  transverse  direction  only,  with- 
out any  sexual  process. 

Resembling  the  Protococcacese  in  the  independence  of  their 
individual  cells  are  the  two  groups  Desmidiacece  and  Diatomacece, 
forms  of  such  special  interest  to  the  microscopist  as  to  require 
separate  treatment,  and  a  detailed  description  of  which  will  be  found 
later  on.  The  Desmidiacece  constitute  a  group  of  the  family 
Conjugatae,  so  called  from  their  mode  of  reproduction  by  conjugation, 
a  process  best  exemplified  in  the  higher  group,  the  Zygnemacece,  in 
which  the  cells  produced  by  binary  subdivision  remain  attached  to 
each  other,  end  to  end,  so  as  to  form  long  unbranched  filaments 
(fig.  420),  whose  length  is  continually  being  increased  by  a  repetition 
of  the  same  process,  which  may  take  place  in  any  part  of  the  filaments, 
and  not  at  their  ends  alone.  The  plants  of  this  group  are  not  found 
so  much  in  running  streams  as  in  waters  that  are  perfectly  still,  such 
as  those  of  ponds,  of  reservoirs,  ditches,  bogs,  or  marshy  grounds ; 
and  they  are  for  the  most  part  unattached,  floating  freely  at  or  near 
the  surface,  especially  when  buoyed  up  by  the  bubbles  of  gas  which 
are  liberated  from  the  midst  of  them  under  the  influence  of  solar 
light  and  heat.  In  the  early  stage  of  their  growth,  whilst  as  yet  the 
cells  are  undergoing  multiplication  by  division,  the  endochrome  is 
frequently  diffused  pretty  uniformly  through  their  cavities  (fig.  420, 
A) ;  but  as  they  advance  towards  the  stage  of  conjugation,  it 
ordinarily  arranges  itself  into  regular  spirals  (B,  Spirogyrd),  a  couple 
of  star-like  discs  in  each  cell  (Zygnema),  or  a  single  plate  running 
through  it  in  an  axile  direction  (Mesocarpus) .  The  act  of  conjugation 
usually  occurs  between  the  cells  of  two  distinct  filaments  that  happen 
to  lie  in  proximity  to  each  other,  and  all  the  cells  of  each  filament 
generally  take  part  in  it  at  once.  The  adjacent  cells  put  forth  little 
protuberances,  which  come  into  contact  with  each  other,  and  then 
coalesce  by  the  breaking  down  of  the  intervening  partitions,  so  as  to 
establish  a  free  passage  between  the  cavities  of  the  conjugating  cells. 
In  some  genera  of  this  family  (such  as  Mesocarpus)  the  conjugating 


5  50   MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

cells  pour  their  endochromes  into  a  dilatation  of  the  passage  that 
has  been  established  between  them  ;  and  it  is  there  that  they  com- 
mingle so  as  to  form  the  zygospore.  But  in  the  various  species  of 
Spiroyyra  (fig.  420,  B),  which  are  among  the  commonest  and  best 
known  of  Conjugate,  the  endochrome  of  one  cell  passes  over  entirely 
into  the  cavity  of  the  other ;  and  it  is  within  the  latter  that  the 
zygospore  is  formed  (C),  the  two  endochromes  coalescing  into  a 
simple  mass,  around  which  a  firm  envelope  gradually  makes  its 
appearance.  Further,  it  may  be  generally  observed  that  all  the 
cells  of  one  filament  thus  empty  themselves,  whilst  all  the  cells  of 
the  other  filament  become  the  recipients.  Here,  therefore,  we'^seem 
to  have  a  foreshadowing  of  the  sexual  distinction  of  the  generative 
cells  into  *  sperm-cells '  and  '  germ-cells,'  which  we  shall  presently 
see  in  the  filamentous  Confervacece,  Conjugation  between  c  two 
adjacent  cells  of  the  same  individual  also  occurs  in  some  species. 


FIG.  420.  —Various  stages  of  the  history  of  a  Spirogyra  :  A,  three  cells,  a,  b,  c,  of  a 
young  filament,  of  which  b  is  undergoing  division ;  B,  two  filaments  in  the  first 
stage  of  conjugation,  showing  the  spiral  disposition  of  their  endochromes  and  the 
protuberances  from  the  conjugating  cells  ;  C,  completion  of  the  act  of  conjugation, 
the  endochromes  of  the  cells  of  the  filament  a  having  entirely  passed  over  to  those 
of  filament  b,  in  which  the  zygospores  are  formed. 

Although  the  two  conjugating  filaments  are  nearly  or  quite  morpho- 
logically alike,  there  must  clearly  be  a  physiological  differentiation, 
since  the  conjugation  takes  place  in  one  direction  only.  Where 
conjugation  occurs  between  cells  in  the  same  filament,  this  sexual 
differentiation  must  be  ascribed  to  the  individual  cells.  Multipli- 
cation by  zoospores  does  not  take  place  among  the  Conjugate. 

From  the  composite  motile  forms  of  Protococcus  the  transition 
is  easy  to  the  group  of  Volvocineae,  an  assemblage  of  minute  plants 
of  the  greatest  interest  to  the  microscopist,  on  account  both  of  the 
animalcule -like  activity  of  their  movements  and  of  the  great  beauty 
and  regularity  of  their,  forms.  The  most  remarkable  example  of  this 
group  is  the  well-known  Volvox  globator  (Plate  VI),  which  is  not 
uncommon  in  fresh- water  pools,  and  which,  attaining  a  diameter  of 
about  ^o  or  even  3^  of  an  inch,  may  be  seen  with  the  naked  eye 
when  the  drop  containing  it  is  held  up  to  the  light,  swimming  through 


PLATE  VI . 


chromo . 


Volvox  globa 


VOLVOCINE^E  5  5  i 

the  water  which  it  inhabits.  Its  onward  motion  is  usually  of  a  roll- 
ing kind  ;  but  it  sometimes  glides  smoothly  along,  without  turning 
on  its  axis ;  whilst  sometimes,  again,  it  rotates  like  a  top,  without 
changing  its  position.  When  examined  with  a  sufficient  magnifying 
power  the  Volvox  is  seen  to  consist  of  a  hollow  sphere,  composed  of 
a  very  pellucid  material,  which  is  studded  at  regular  intervals  with 
minute  green  spots,  and  which  is  often  (but  not  constantly)  traversed 
by  green  threads  connecting  these  spots.  From  each  of  the  spots 
proceed  two  long  nagella,  so  that  the  entire  surface  is  beset  with 
these  lashing  filaments,  to  whose  combined  action  its  movements 
are  due.  Within  the  external  sphere  may  generally  be  seen  from 
two  to  twenty  other  globes,  of  a  darker  colour,  and  of  varying  sizes ; 
the  smaller  of  these  are  attached'  to  the  inner  surface  of  the  investing 
sphere,  and  project  into  its  cavity ;  but  the  larger  lie  freely  within 
the  cavity,  and  may  often  be  observed  to  revolve  by  the  agency  of 
their  own  nagella.  After  a  time  the  original  sphere  bursts,  and  the 
contained  spherules  swim  forth  and  speedily  develop  themselves 
into  the  likeness  of  that  within  which  they  have  been  evolved, 
their  coloured  particles,  which  are  at  first  closely  aggregated  together, 
being  separated  from  each  other  by  the  interposition  of  the  trans- 
parent pellicle.  It  was  long  supposed  that  Volvox  is  a  single 
animal ;  and  it  was  first  shown  to  be  a  composite  fabric,  made  up  of  a 
repetition  of  organisms  in  all  respects  similar  to  each  other,  by  Pro- 
fessor Ehrenberg,  who,  however,  considered  these  organisms  as 
monads,  and  described  them  as  each  possessing  a  mouth,  several 
stomachs,  and  an  eye !  Our  present  knowledge  of  their  nature, 
however,  leaves  little  doubt  of  their  vegetable  character ; 1  and  the 
peculiarity  of  their  history  renders  it  desirable  to  describe  it  in  some 
detail. 

Each  of  the  so-called  *  monads '  (fig.  421,  Nos.  9, 1 1)  is  a  somewhat 
flask-shaped  plant-cell,  about  ^y^th  of  an  inch  in  diameter,  consist- 
ing, as  in  the  previous  instances,  of  green  chlorophyll  granules  diffused 
through  a  colourless  protoplasm,  constituting  an  endochrome  (which 
commonly  includes  also  a  red  spot — '  eye-spot ' — of  altered  chlorophyll), 
and  bounded  by  an  ectoplasm  formed  of  the  condensed  and  colourless 
surface-layer  of  the  protoplasmic  mass.  It  is  prolonged  outwardly 
(or  towards  the  circumference  of  the  sphere)  into  a  sort  of  colourless 
beak  or  proboscis,  from  which  proceed  two  nagella  (fig.  421,  No.  11) ; 
and  it  is  invested  by  a  pellucid  or  hyaline  envelope  (No.  9,  d)  of 
considerable  thickness,  the  borders  of  which  are  flattened  against 
those  of  other  similar  envelopes  (No.  5,  c,  c),  but  which  does  not  appear 
to  have  the  tenacity  of  a  true  membrane.  It  is  impossible  not  to 
recognise  the  close  similarity  between  the  structure  of  this  body 
and  that  of  the  motile  encysted  cell  of  Protococcus  pluvialis  (fig. 
418,  K).  There  is  not,  in  fact,  any  perceptible  difference  between 
them,  save  that  which  arises  from  the  regular  aggregation,  in  Volvox, 

1  Professor  Stein,  however,  in  hit;  great  work  on  the  Infusoria  (Organismus  der 
Infusionsthiere,  Abtheilung  III.,  Leipzig,  1878),  still  ranks  the  Volvocinece  among 
the  flagellate  animalcules,  to  which  they  undoubtedly  show  a  remarkable  parallelism 
in  structure,  the  chief  evidence  of  their  vegetable  nature  lying  in  their  physiological 
conformity  to  undoubted  thallophytes. 


552   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE- Til ALLOPHYTES 

of  the  cells  which  normally  detach  themselves  from  one  another  in 
Protococcus.  The  presence  of  cellulose  in  the  hyaline  substance  is 
not  indicated,  in  the  ordinary  condition  of  Volvox  ylobator,  by  the 
iodine  and  sulphuric  acid  test,  though  the  use  of  '  Schultz's  solution  ' 
gives  to  it  a  faint  blue  tinge ;  there  can  be  no  doubt  of  its  existence, 
however,  in  the  hyaline  envelope  of  Volvox  aureus.  The  flagella 
and  endoplasm,  as  in  the  motile  forms  of  Protococcus,  are  tinged 
a  deep  brown  by  iodine,  with  the  exception  of  one  or  two  starch 
particles  in  each  cell,  which  are  turned  blue  ;  and  when  the  contents 
of  the  cell  are  liberated,  bluish  flocculi,  apparently  indicative  of  the 
presence  of  cellulose,  are  brought  into  view  by  the  action  of  sulphuric 
acid  and  iodine.  All  these  reactions  are  characteristically  vegetable 
in  their  nature.  When  the  cell  is  approaching  maturity,  its  endo- 
plasm always  exhibits  one  or  more  vacuoles  (fig.  421,  No.  9,  a,  a)  of  a 
spherical  form,  and  usually  about  one-third  of  its  own  diameter  ; 
and  these  vacuoles  (which  are  the  so-called  '  stomachs  '  of  Ehrenberg) 
have  been  observed  to  undergo  a  very  curious  rhythmical  contraction 
and  dilatation  at  intervals  of  about  forty  seconds  ;  the  contraction 
(which  seems  to  amount  to  complete  obliteration  of  the  cavity  of  the 
vacuole)  taking  place  rapidly  or  suddenly,  whilst  the  dilatation  is  slow 
and  gradual.  This  curious  action  ceases,  however,  as  the  cell 
arrives  at  its  full  maturity ; 1  a  condition  which  seems  to  be 
marked  by  the  greater  consolidation  of  the  ectoplasm,  by  the 
removal  or  transformation  of  some  of  the  chlorophyll,  and  by  the 
formation  of  the  red  spot  (&),  which  obviously  consists,  as  in  Proto- 
coccus,  of  a  peculiar  modification  of  chlorophyll. 

Each  cell  normally  communicates  with  the  cells  in  nearest 
proximity  with  it  by  extensions  of  its  own  endochrome,  which  are 
sometimes  single  and  sometimes  double  (fig.  421,  No.  5,  b,  b)  ;  and 
these  connecting  processes  necessarily  cross  the  lines  of  division 
between  their  respective  hyaline  investments.  The  thickness  of  these 
processes  varies  very  considerably  ;  for  sometimes  they  are  broad 
bands,  and  in  other  cases  mere  threads  ;  whilst  they  are  occasionally 
wanting  altogether.  This  difference  seems  partly  to  depend  upon  the 
age  of  the  individual,  and  partly  upon  the  abundance  of  nutriment 
which  it  obtains ;  for,  as  we  shall  presently  see,  the  connection  is 
most  intimate  at  an  early  period,  before  the  hyaline  investments  of 
the  cells  have  increased  so  much  as  to  separate  the  masses  of  endo- 
chrome to  a  distance  from  one  another  (fig.  421,  Nos.  2,  3,  4) ;  whilst 
in  a  mature  individual,  in  which  the  separation  has  taken  place  to 
its  full  extent  and  the  nutritive  processes  have  become  less  active,  the 
masses  of  endochrome  very  commonly  assume  an  angular  form,  and 
'the  connecting  processes  are  drawn  out  into  threads  (as  seen  in  No.  5), 
or  they  retain  their  globular  form,  and  the  connecting  processes 
altogether  disappear.  The  influence  of  reagents,  or  the  infiltration 
of  water  into  the  interior  of  the  hyaline  investment,  will  sometimes 
cause  the  connecting  processes  (as  in  Protococcus)  to  be  drawn  back 

1  The  existence  of  rhythmically  contracting  vacuoles  in  Volvox  (though  confirmed 
by  the  observations  of  Prof.  Stein)  is  denied  by  Mr.  Saville  Kent  (Manual  of  the 
Infusoria,  p.  47) ;  but  it  may  be  fairly  presumed  that  he  has  not  looked  for  them  at 
the  stage  of  development  at  which  their  action  was  witnessed  by  Mr.  Busk. 


554  MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

composed  of  an  aggregation  of  somewhat  angular  masses  of  endochrome 
(6),   separated  by  the  interposition  of  hyaline  substance ;  and  the 
whole  seems  to  be  enclosed  in  a  distinctly  membranous  envelope, 
which  is  probably  the  distended  hyaline  investment  of  the  original 
cell,  within  which,  as  will  presently  appear,  the  entire  aggregation 
originated.     In  the  midst  of  the  polygonal  masses  of  endochrome, 
one    mass    (a),  rather  larger  than  the  rest,  is  seen  to  present  a 
circular  form  ;  and  this,  as  will  presently  appear,  is  the  originating 
cell  of  what  is  hereafter  to  become  a  new  sphere.     The  growing 
Volvox  at  first  increases  in  size  not  only  by  the  interposition  of  new 
hyaline  substance  between   its  component  masses  of  endochrome, 
but  also  by  an  increase  in  these  masses  themselves  (No.  2,  a),  which 
come  into  continuous  connection  with  each  other  by  the  coalescence 
of  processes  (b)  which  they  severally  put  forth  ;  at  the  same  time 
an  increase  is  observed  in  the  size  of  the  globular  cell  (c),  which  is 
preliminary  to  its  binary  subdivision.     A  more  advanced  stage  of 
the  same  developmental  process  is  seen  in  No.  3,  in  which  the  con- 
necting processes  (a,  a)  have  so  much  increased  in  size  as  to  establish 
a  most  intimate  union  between  the  masses  of  endochrome,  although 
the   increase   of  the   intervening   hyaline   substance  carries  these 
masses   apart  from  one  another;    whilst   the   endochrome   of  the 
central  globular  cell  has  undergone  segmentation  into  two  halves. 
In  the  stage  represented  in  No.  4  the  masses  of  endochrome  have 
been  still  more  widely  separated  by  the  interposition  of  hyaline 
substance ;  each  has  become  furnished  with  its  pair  of  flagella,  and 
the  globular  cell  has  undergone  a  second  segmentation.     Finally,  in 
No.  5,  which  represents  a  portion  of  the  spherical  wall  of  a  mature 
Volvox,  the   endochrome  masses  are  observed   to  present  a   more 
scattered  aspect,  partly  on  account  of  their  own  reduction  in  size, 
and  partly  through  the  interposition  of  a  greatly  increased  amount 
of  hyaline  substance,  which  is  secreted  from  the  surface  of  each  mass ; 
and  that  portion  which  belongs  to  each  cell,  standing  to  the  endo- 
chrome mass  in  the  relation  of  the  cellulose  coat  of  an  ordinary  cell 
to  its  ectoplasm,  is  frequently  seen  to  be  marked  out  from  the  rest 
by  delicate  lines  of  hexagonal  areolation  (c,  c),  which  indicate  the 
boundaries  of  each.     Of  these  it  is  often  difficult  to  obtain  a  sight, 
a  nice  management  of  the  light  being  usually  requisite  with  fresh 
specimens ;  but  the  prolonged  action  of  water  (especially  when  it 
contains  a  trace  of  iodine)  or  of  glycerin  will  often  bring  them  into 
clear  view.     The  prolonged  action  of  glycerin,  moreover,  will  often 
show  that   the   boundary-lines   are   double,  being   formed   by  the 
coalescence  of  two  contiguous  cell-walls  ;  and  they  sometimes  retreat 
from  each  other  so  far  that  the  hexagonal  areolse  become  rounded. 
As  the  primary  sphere  approaches  maturity,  the  large  secondary 
germ-mass,  or  zoosporange,  whose  origin  has  been  traced  from  the 
beginning,  also  advances  in  development,  its  contents  undergoing 
multiplication  by  successive  segmentations,  so  that  we  find  it  to 
consist   of    eight,    sixteen,    thirty-two,    sixty-four,    or    still    more 
numerous  divisions,  as  shown  in  fig.  421,  Nos.  6,  7,  8.     Up  to  this 
stage,  at  which  the  sphere  first  appears   to  become   hollow,  it  is 
retained  within  the  hyaline  envelope  of  the  cell  within  which  it  has 


PLATE  VIII . 


#est. Newman  ehromo 

Desrmdiaceas,  Rivuiariaceae  and  Scytoneinaceae. 


VOLVO  CINE  JE  555 

been  produced  ;  a  similar  envelope  can  be  easily  distinguished,  as 
shown  in  No.  10,  just  when  the  segmentation  has  been  completed, 
and  at  that  stage  the  flagella  pass  into  it,  but  do  not  extend  beyond 
it ;  and  even  in  the  mature  Volvox  it  continues  to  form  an  invest- 
ment around  the  hyaline  envelopes  of  the  separate  cells,  as  shown  in 
the  same  figure  at  No.  11.  It  seems  to  be  by  the  adhesion  of  the 
hyaline  investment  of  the  new  sphere  to  that  of  the  old  that  the 
secondary  sphere  remains  for  a  time  attached  to  the  interior  wall  of 
the  primary ;  at  what  exact  period,  or  in  what  precise  manner,  the 
separation  between  the  two  takes  place  has  not  yet  been  determined. 
At  the  time  of  the  separation  the  developmental  process  has  gene- 
rally advanced  as  far  as  the  stage  represented  in  No.  1,  the  foundation 
of  one  or  more  tertiary  spheres  ,being  usually  distinguishable  in  the 
enlargement  of  certain  of  its  cells! 

The  development  and  setting-free  of  these  composite  zoosporanges, 
which  is  essentially  a  process  of  cell-subdivision  or  gemmiparous  exten- 
sion, is  the  ordinary  mode  of  multiplication  in  Volvox,  taking  place 
at  all  times  of  the  year,  except  when  the  sexual  generation  (now  to 
be  described)  is  in  progress.  The  mode  in  which  this  process  is 
here  performed  (for  our  knowledge  of  which  we  are  indebted  to 
the  persevering  investigations  of  Professor  Cohn)  shows  a  great 
advance  upon  the  simple  conjugation  of  two  similar  cells,  and 
closely  resembles  that  which  prevails  not  only  among  the  higher 
algae,  but  (under  some  form  or  other)  through  a  large  part  of  the 
cryptogamic  series.  As  autumn  advances  the  Volvox  spheres  usually 
cease  to  multiply  themselves  by  the  formation  of  zoosporanges,  and 
certain  of  their  ordinary  cells  begin  to  undergo  changes  by  which 
they  are  converted,  some  into  male  or  '  sperm-cells,'  others  into 
female  or  '  germ-cells,'  the  greater  number,  however,  remaining 
sterile.  Each  sphere  of  Volvox  globator  (Plate  VI,  fig.  1)  contains 
both  kinds  of  sexual  cells,  so  that  this  species  ranks  as  monoecious ; 
but  V.  aureus  is  dioecious,  the  sperm-cells  and  germ-cells  occurring 
in  separate  spheres.  Both  kinds  of  sexual  cells  are  at  first  dis- 
tinguishable from  the  ordinary  sterile  cells  by  their  larger  size 
(fig.  2,  a),  in  this  respect  resembling  zoosporanges  in  an  early 
stage ;  but  their  subsequent  history  is  altogether  different.  The 
sperm-cells  begin  to  undergo  subdivision  when  they  attain  about 
tliree  times  the  size  of  the  sterile  cells ;  this,  however,  takes 
place,  not  on  the  binary  plan,  but  in  such  a  manner  that  the 
endochrome  of  the  primary  cell  resolves  itself  into  a  cluster  of  very 
peculiar  secondary  cells  (fig.  1,  a,  a2,  fig.  5),  each  consisting  of  an 
elongated  '  body '  containing  an  orange-coloured  endochrome  with  a 
red  corpuscle,  and  of  a  long,  colourless  beak  from  the  base  of  which 
proceeds  a  pair  of  long  flagella  (figs.  6,  7),  as  in  the  antherozoids 
of  the  higher  cryptogams.  As  the  sperm-cells  approach  maturity, 
the  aggregate  clusters  may  be  seen  to  move  within  them,  at  first 
slowly,  and  afterwards  more  rapidly;  the  bundles  then  separate 
into  their  component  antherozoids,  which  show  an  active,  indepen- 
dent movement  whilst  still  within  the  cavity  of  the  primary  cell 
(fig.  1,  a3),  and  finally  escape  by  the  giving-way  of  its  wall  (a4), 
diffusing  themselves  through  the  cavity  of  the  Volvox  sphere.  The 


556   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

<7erm-cells  (fig.  1,  b,  6),  on  the  other  hand,  continue  to  increase  in 
size  without  undergoing  subdivision ;  at  first  showing  large  vacuoles 
in  their  protoplasm  (62,  &2),  but  subsequently  becoming  filled  with 
dark-green  endochrome.  The  form  of  the  germ-cell  gradually 
changes  from  its  original  flask-shape  to  the  globular  (63)  ;  and  it 
projects  into  the  cavity  of  the  Volvox  sphere,  at  the  same  time 
acquiring  a  gelatinous  envelope.  Over  this  the  swarming  antherozoids 
diffuse  themselves  (fig.  3),  penetrating  its  substance,  so  as  to  find 
their  way  to  the  interior ;  and  in  this  situation  they  seem  to  dissolve 
away,  so  as  to  become  incorporated  with  the  oosphere.  The  product 
of  this  fusion  (which  is  only  conjugation  under  another  form)  is  a 
reproductive  cell  or  oospore,  which  speedily  becomes  enveloped  by 
an  internal  smooth  membrane,  and  with  a  thicker  external  coat, 
which  is  usually  beset  with  conical  pointed  processes  (fig.  4)  ;  and 
the  contained  chlorophyll  gives  place,  as  in  Palmoglcea,  to  starch 
and  a  red  or  orange  coloured  oil.  As  many  as  forty  of  such  oospores 
have  been  seen  by  Cohn  in  a  single  sphere  of  Volvox,  which  thus 
acquires  the  peculiar  appearance  that  has  been  distinguished  by 
Ehrenberg  by  a  different  specific  name,  Volvox  stellatus.  Soon 
after  the  oospores  reach  maturity,  the  parent  sphere  breaks  up, 
and  the  oospores  fall  to  the  bottom,  where  they  remain  during 
the  winter.  Their  further  history  has  since  been  traced  out 
by  Kirchner,  who  found  that  their  germination  commenced  in 
February  with  the  liberation  of  the  spherical  endospore  from  its 
envelope,  and  with  its  division  into  four  cells  by  the  formation 
of  two  partitions  at  right  angles  to  each  other.  These  partially 
separate,  holding  together  only  at  one  end,  which  becomes  one  pole 
of  the  globular  cluster  subsequently  formed  by  cell-multiplication, 
the  other  pole  only  closing  in  when  a  large  number  of  cells  have 
been  formed.  The  cells  are  then  carried  apart  from  one  another  by 
the  hyaline  investment  formed  by  each,  and  the  characteristic  Volvox 
sphere  is  thus  completed.1 

Another  phenomenon  of  a  very  remarkable  nature,  namely,  the 
conversion  of  the  contents  of  an  ordinary  vegetable  cell  into  a  free 
moving  mass  of  protoplasm  that  bears  a  strong  resemblance  to  the 
animal  Amceba,  has  been  affirmed  by  Dr.  Hicks  2  to  take  place  in 
Volvox,  under  circumstances  that  leave  no  reasonable  ground  for  that 
doubt  of  its  reality  which  has  been  raised  in  regard  to  the  accounts 
of  similar  phenomena  occurring  elsewhere.  The  endochrome-mass  of 
one  of  the  ordinary  cells  increases  to .  nearly  double  its  usual  size  ; 
but,  instead  of  undergoing  binary  subdivision  so  as  to  produce  a 
zoosporange,  it  loses  its  colour  and  its  regularity  of  form,  and 

The  doctrine  of  the  vegetable  nature  of  Volvox,  which  had  been  suggested  by 
Siebold,  Braun,  and  other  German  naturalists,  was  first  distinctly  enunciated  by 
Prof.  Williamson,  on  the  basis  of  the  history  of  its  development,  in  the  Transactions 
of  the  Philosophical  Society  of  Manchester,  vol.  ix. 

[The  most  recent  and  detailed  accounts  of  the  development  of  the  various  forms 
of  Volvox  are  by  Klein  (Pringsheim's  Jahrbucher  fur  wissenschaftliche  Botanik, 
vol.  xx.  1889,  p.  133)  and  Overtoil  (Botanisches  Centralblatt,  vol.  xxxix.  1889),  which 
do  not  differ  in  any  material  point  from  the  description  given  in  the  text.  See  also 
Bennett  and  Murray's  Handbook  of  Cryptogamic  Botany,  p.  292.— ED.] 

2  Trans,  of  Microsc.  Society,  n.s.  vol.  viii.  1860,  p.  99 ;  and  Quart.  Jo-urn,  of 
Microsc.  Science,  n.s  vol.  ii.  1862,  p.  96. 


VOLVOCINE^E  ;   PALMELLACE^E  557 

becomes  an  irregular  mass  of  colourlsss  protoplasm,  containing  a 
number  of  brown  or  reddish-brown  granules,  and  capable  of  altering 
its  form  by  protruding  or  retracting  any  portion  of  its  membranous 
wall,  exactly  like  a  true  Amoeba.  By  this  self-moving  power,  each 
of  these  bodies  (of  which  twenty  may  sometimes  be  counted  within 
a  single  Volvox)  glides  independently  over  the  inner  surface  of  the 
sphere  among  its  unchanged  green  cells,  bending  itself  round  any 
one  of  these  with  which  it  may  come  into  contact,  precisely  after 
the  manner  of  an  Amoeba.  After  the  'amoeboid'  has  begun  to 
travel,  it  is  always  noticed  that  for  every  such  moving  body  in  the 
Volvox  there  is  the  empty  space>of  a  missing  cell ;  and  this  confirms 
the  belief — founded  on  observation  of  the  gradational  transition 
from  the  one  condition  to  the  other,  and  on  the  difficulty  of  sup- 
posing that  any  such  bodies  could  have  entered  the  sphere  parasiti- 
cally  from  without — that  the  *  amoeboid  '  is  really  the  product  of  the 
metamorphosis  of  a  mass  of  vegetable  protoplasm.  This  meta- 
morphosis may  take  place,  according  to  Dr.  Hicks,  even  after  the 
process  of  binary  subdivision  has  commenced.  What  is  the  sub- 
sequent destination  of  these  amoeboid  bodies  has  not  yet  been  ascer- 
tained.1 

In  other  organisms  allied  to  Volvox,  and  included  in  the  family 
Volvocineos,  we  find  a  very  interesting  and  instructive  transition 
between  the  various  modes  of  multiplication  already  described.  In 
Eudorina,  a  common  organism  in  still  water,  a  sexual  process  similar 
to  that  in  Volvox  has  been  observed.  In  Pandovina  inorum  the 
generative  process  is  performed,  according  to  the  observations  of 
Pringsheim,  in  a  manner  curiously  intermediate  between  the  lower 
and  the  higher  types  referred  to  above.  For  within  each  cell  of  the 
original  sixteen  of  which  its  mulberry-like  mass  is  composed,  a  brood 
of  sixteen  secondary  cells  is  formed  by  ordinary  binary  subdivision ; 
and  these,  when  set  free  by  the  dissolution  of  their  containing  cell- 
wall,  swim  forth  as  'swarm-spores,'  each  being  furnished  with  a 
pair  of  flagella.  Among  the  crowrd  of  these  swarm-spores  may  be 
observed  some  which  approach  in  pairs,  as  if  seeking  one  another ; 
when  they  meet,  their  points  at  first  come  together,  but  gradually 
their  whole  bodies  coalesce,  and  a  globular  zygospore  is  thus  formed 
which  germinates  after  a  period  of  rest,  reproducing  by  binary 
subdivision  the  original  sixteen-celled,  mulberry-like  Pandorina. 
We  have  here,  therefore,  a  true  process  of  conjugation  between 
motile  protoplasm  masses,  each  of  which  is  in  itself  indistinguish- 
able from  a  zoospore.  A  similar  process  takes  place  also  in 
Conferva,  Ulothrix,  Hydrodictyon,  and  a  number  of  fresh-water  algae 
(fig,  422). 

Included  by  many  writers  under  the  general  term  Palmellaceae 
are  a  number  of  minute  organisms  of  very  simple  structure,  the 
relationship  of  which  to  the  Protococcacece  is  not  yet  fully  known. 
They  all  grow  either  on  damp  surfaces  or  in  fresh  water ;  and  they 
may  either  form  (1)  a  mere  powdery  layer,  of  which  the  component 

1  A  similar  production  of  '  amceboids '  has  been  observed  by  Mr.  Archer  in 
Steplianosplicera  pluvialis,  and  is  scarcely  now  to  be  considered  an  exceptional 
phenomenon. 


558   MICEOSCOPIC  FOEMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


particles  have  little  or  no  adhesion  to  each  other ;  or  they  may  pre- 
sent themselves  (2)  in  the  condition  of  an  indefinite  slimy  film,  or  (3) 
in  that  of  a  tolerably  firm  and  definitely  bounded  membranous 
1  frond.'  The  first  of  these  states  we  have  seen  to  be  characteristic 
of  Palmoglcea  and  Protococcus  ;  the  new  cells  which  are  originated 
by  the  process  of  binary  subdivision  usually  separating  from  each 
other  after  a  short  time,  and,  even  where  they  remain  in  cohesion, 
not  forming  a  *  frond '  or  membranous  expansion.  The  '  red  snow,' 
which  sometimes  colours  extensive  tracts  in  Arctic  or  Alpine  regions, 
penetrating  even  to  the  depth  of  several  feet,  and  vegetating  actively 
at  a  temperature  which  reduces  most  plants  to  a  state  of  torpor,  is 
generally  considered  to  be  a  species  of  Protococcus ;  but  as  its  cells 
are  connected  by  a  tolerably  firm  gelatinous  investment,  it  would 
rather  seem  to  be  a  Palmella.  The  second  is  the  condition  of  Pal- 
mella  proper,  of  which  one  species,  P.  cruenta,  usually  known  under 
the  name  of  '  gory  dew/  is  common  on  damp  walls  and  in.  shady 
places,  sometimes  extending  itself  over  a 
considerable  area  as  a  tough  gelatinous 
mass,  of  the  colour  and  general  appearance 
of  coagulated  blood.  A  characteristic 
illustration  of  it  is  also  afforded  by  the 
Hcematococcus  sanguineus  (fig.  423),  which 
chiefly  differs  from  Palmella  in  the  partial 
persistence  of  the  walls  of  the  parent-cells, 
so  that  the  whole  mass  is  subdivided  by 
partitions,  which  enclose  a  larger  or  smaller 
number  of  cells  originating  in  the  sub- 
division of  their  contents.  Besides  in- 
creasing in  the  ordinary  mode  of  binary 
multiplication,  the  Palmella  cells  seem 
occasionally  to  rupture  and  diffuse  their 
granular  contents  through  the  gelatinous 
stratum,  and  thus  to  give  origin  to  a  whole 
cluster  at  once,  as  seen  at  e,  after  the 
manner  of  other  simple  plants  to  be  pre- 
sently described,  save  that  these  minute 
segments  of  the  endochrome,  having  no 

power  of  spontaneous  motion,  cannot  be  ranked  as  zoospores. 
The  gelatinous  masses  of  the  Palmella  are  frequently  found  to  con- 
tain parasitic  growths  formed  by  the  extension  of  other  plants 
through  their  substance;  but  numerous  branched  filaments  some- 
times present  themselves,  which,  being  traceable  into  absolute 
continuity  with  the  cells,  must  be  considered  as  properly  appertaining 
to  them.  Sometimes  these  filaments  radiate  in  various  directions  from 
a  single  central  cell,  and  must  at  first  be  considered  as  mere  exten- 
sions of  this ;  their  extremities  dilate,  however,  into  new  cells  ;  and, 
when  these  are  fully  formed,  the  tubular  connections  close  up,  and  the 
cells  become  detached  from  each  other.1  Of  the  third  condition  we 

\This  fact,  first  made  public  by  Mr.  Thwaites  (Ann.  of  Nat.  Hist.  2nd  series, 
vol.  n.  1848,  p.  318),  is  one  of  fundamental  importance  in  the  determination  of  the 
real  character  of  this  group. 


FIG.  422.— A,  conjugating 
microzoospores  of  Ulo- 
thrix ;  B,  megazoospore 
of  Ulothrix,  from  Vines's 
'  Physiology  of  Plants.' 


PALMELLACE.E  ;    ULVACE^E 


559 


have  an  example  in  the  curious  Palmodictyon  described  by  Kiitzing, 
the  frond  of  which  appears  to  the  naked  eye  like  a  delicate  network, 
consisting  of  anastomosing  branches,  each  composed  of  a  single  or 
double  row  of  large  vesicles,  within  every  one  of  which  is  produced 
a  pair  of  elliptical  cellules  that  ultimately  escape  as  zoospores.  The 
alternation  between  the  motile  form  and  the  still  or  resting  form, 
which  has  been  described  as  occurring  in  Protococcus,  has  been  ob- 
served in  several  other  forms  of  this  group  ;  and  it  seems  obviously 
intended,  like  the  production  of  zoospores,  to  secure  the  dispersion 
of  the  plant  and  to  prevent  it  from  choking  itself  by  overgrowth  in 
any  one  locality.  It  is  very  commonly  by  plants  of  this  group  that 
the  algal  portions  of  lichens  are  formed.1 

Notwithstanding  the  very  definite  form  and  large  size  attained 
by  the  fronds  or  leafy  expansions  of  the  UlvaceaB,  to  which  group 


FIG.  423. — Hcematococcus  sanguineus,  in  various  stages  of  development ;  a,  single 
cells,  enclosed  in  their  mucous  envelope ;  &,  c,  cluster  formed  by  subdivision  of 
the  parent-cell ;  d,  more  numerous  cluster,  its  component  cells  in  various  stages  of 
division ;  e,  large  mass  of  young  cells,  formed  by  the  subdivision  of  the  parent 
endochrome,  and  enclosed  within  a  common  mucous  envelope. 

belong  some  of  the  most  common  grass-green  seaweeds  ('  laver ') 
found  on  every  coast,  yet  their  essential  structure  differs  but  very 
little  from  that  of  the  preceding  group  ;  and  the  principal  advance 
is  shown  in  this,  that  the  cells,  when  multiplied  by  binary  sub- 
division, not  only  remain  in  firm  connection  with  each  other,  but 
possess  a  very  regular  arrangement  (in  virtue  of  the  determinate 
plan  on  which  the  subdivision  takes  place),  and  form  a  definite  mem- 
branous expansion.  The  mode  in  which  this  frond  is  produced  may 
be  best  understood  by  studying  the  history  of  its  development,  some 
of  the  principal  phases  of  which  are  seen  in  fig.  424.  The  isolated 
cells  A,  in  which  it  originates,  resembling  in  all  points  those  of  a 

1  [The  Palmellacece  are  not  now  regarded  by  the  best  authorities  as  a  distinct 
family  from  the  Protococcacece,  and  the  genus  Hcematococcus  is  sunk  in  Proto- 
coccus.— ED.] 


560  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPITYTES 


Protococcus,  give  rise,  by  their  successive  subdivisions  in  determinate 
directions,  to  such  regular  clusters  as  those  seen  at  B  and  C,  or  to 
such  confervoid  filaments  as  that  shown  at  D.  A  continuation  of 
the  same  regular  mode  of  subdivision,  taking  place  alternately  in 
two  directions,  may  at  once  extend  the  clusters  B  and  C  into  leaf- 
like  expansions  ;  or,  if  the  filamentous  stage  be  passed  through 
(different  species  presenting  variations  in  the  history  of  their  develop- 
ment), the  filament  increases  in  breadth  as  well  as  in  length  (as  seen 
at  E),  and  finally  becomes  such  a  '  frond '  as  is  shown  at  F,  G.  In 
the  simple  membranous  expansion  or  thallus  thus  formed,  there  is 

but  little  approach  to  a 
differentiation  of  parts  in 
the  formation  of  root,  stem, 
and  leaf,  such  as  the  higher 
algae  present ;  every  portion 
is  the  exact  counterpart  of 
every  other,  and  every 
portion  seems  to  take  an 
equal  share  in  the  opera- 
tions of  growth  and  repro- 
duction. Each  cell  is  very 
commonly  found  to  exhibit 
an  imperfect  partitioning 
into  four  parts  preparatory 
to  multiplication  by  double 
bipartition,  and  the  entire 
frond  usually  shows  the 
groups  of  cells  arranged  in 
clusters  containing  some 
multiple  of  four.  ' 

Besides  this  continuous 
increase   of  the  individual 

Si!  KM  :;3S!«/  ft  frond,  however,  we  find,  in 

most  species  of  Utoa,  a 
provision  for  extending  the 
plant  by  the  dispersion  of 
zoospores.  The  endochrome 
(fig.  425,  a)  subdivides  into 
numerous  segments  (as  at 
b  and  c),  which  at  first  are 
seen  to  lie  in  close  contact 

within  the  cell  that  contains  them,  then  begin  to  exhibit  a  kind 
of  restless  motion,  and  at  last  escape  by  the  bursting  of  the 
cell- wall,  and  swim  freely  through  the  water  as  zoospores  (d)  by 
means  of  their  flagella,  each  zoospore  having  become  endowed 
with  either  two  or  four  flagella  during  its  formation  within  its 
mother-cell.  At  last,  however,  they  come  to  rest,  attach  them- 
selves to  some  fixed  point,  and  begin  to  grow  into  clusters  or 
filaments  (e)  in  the  manner  already  described.  The  walls  of  the 
cells  which  have  thus  discharged  their  endochrome  remain  as 
colourless  spots  on  the  frond  ;  sometimes  these  are  intermingled  with 


FIG.  424. — Successive  stages  of  development 
of  Ulva. 


ULVACE.E 


56l 


the  portions  still  vegetating  in  the  usual  mode  ;  but  sometimes  the 
whole  endochrome  of  one  portion  of  the  frond  may  thus  escape  in 
the  form  of  zoospores,  leaving  behind  it  nothing  but  a  white  flaccid 
membrane.  If  the  microscopist  who  meets  with  a  frond  of  an  Ulva 
in  this  condition  examines  the  line  of  separation  between  its  green 
and  its  coloured  portions,  he  may  not  improbably  meet  with  cells  in 
the  very  act  of  discharging  their  zoospores,  which  '  swarm '  around 
their  points  of  exit  very  much  in  the  manner  that  animalcules  are 
often  seen  to  do  around  particular  spots  of  the  field  of  view,  and 
which  might  easily  be  taken  for -true  Infusoria ;  but  on  carrying  his 
observations  further,  he  would  see  that  similar  bodies  are  moving 
ivithin  cells  a  little  more  remote  from  the  dividing  line,  and  that  a 


FIG.  425. — Formation  of  zoospores  in  Viva  latissima :  a,  portion  of  the  ordinary 
frond ;  6,  cells  in  which  the  endochrome  is  beginning  to  break  up  into  segments  ; 
c,  cells  from  the  boundary  between  the  coloured  and  colourless  portions,  some  of 
them  containing  zoospores,  others  being  empty ;  d,  flagellate  zoospores,  as  in  active 
motion ;  e,  subsequent  development  of  the  zoospores. 

little  farther  still  they  are  obviously  but  masses  of  endochrome  in 
the  act  of  subdivision.1 

More  recent  observation  has  brought  out  the  interesting  fact 
that  in  Ulva  and  its  allies  there  are  two  kinds  of  swarm-spore,  a  larger 
kind,  *  megazoospores,'  with  four,  and  a  smaller  kind, '  microzoospores,' 
with  two  cilia  each  (see  fig.  422).  Of  these  the  megazoospores 
germinate  directly,  as  above  described,  while  the  microzoospores  or 
'zoogametes'  have  been  observed  to  conjugate  in  pairs,  producing 
zygospores,  by  the  germination  of  which  a  new  generation  is 
produced.  The  two  kinds  of  zoospore  may  be  produced  on  the  same 
or  on  different  individuals. 

1  Such  an  observation  the  Author  had  the  good  fortune  to  make  in  the  year  1842, 
when  the  emission  of  zoospores  from  the  Ulvacece,  although  it  had  been  described 
by  the  Swedish  algologist  Agardh,  had  not  been  seen  (he  believes)  by  any  British 
naturalist. 

O  O 


562   MICKOSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


r> 


Although  many  of  the  plants  belonging  to  the  family  Siphonaceae 
attain  a  considerable  size,  and  resemble  the  higher  seaweeds  in  their 
general  mode  of  growth,  yet  they  retain  a  simplicity  of  structure  so 
extreme  as  to  require  them  to  be  ranked  among  the  simpler  thallo- 

phytes.  They  are  inhabitants 
both  of  fresh  water  and  of  the 
sea,  and  consist  of  very  large 
tubular  cells,  which  often  ex- 
tend themselves  into  branches, 
so  as  to  form  an  arborescent 
frond.  These  branches,  how- 
ever, are  not  separated  from 
the  stem  by  any  intervening 
partition,  except  those  parts 
where  the  generative  organs 
are  produced ;  but  the  whole 
frond  is  composed  of  a  simple 
continuous  tube,  the  entire 
contents  of  which  may  be 
readily  pressed  out  through  an 
orifice  made  by  wounding  any 
part  of  the  wall.  The  genus 
Vaucheria  may  be  selected  as  a 
particularly  good  illustration  of 
this  family,  its  history  having 
been  pretty  completely  made 
out.  Most  of  its  species  are 
inhabitants  of  fresh  water,  but 
some  are  marine ;  and  they 
commonly  present  themselves 
in  the  form  of  cushion-like 
masses,  composed  of  irregularly 
branching  filaments,  which,  al- 
though they  remain  distinct, 
are  densely  tufted  together  and 
variously  interwoven.  Some 
species  form  dense  green  mats 
on  damp  soil  in  flower-pots,  &c. 
The  formation  of  motile  gonids 
or  zoospores  may  be  readily 
observed  in  these  plants,  the 
whole  process  usually  occupying 
but  a  very  short  time.  The 
extremity  of  one  of  the  filaments 
usually  swells  up  in  the  form  of 
club, 


FIG.  426. — Successive  phases  of  generative 
process  in  Vaucheria  sessilis :  at  A  are 
seen  one  of  the  '  horns  '  or  antherids  (a) 
and  one  of  the  oogones  (6),  as  yet  un- 
opened; at  B  the  antherid  is  seen  in 
the  act  of  emitting  the  antherozoids  (c), 
of  which  many  enter  the  opening  at  the 
apex  of  the  oogone,  whilst  others  (d) 
which  do  not  enter  it  display  their  cilia 
until  they  become  motionless ;  at  C  the 
orifice  of  the  oogone  is  closed  again  by 
the  formation  of  a  cellulose  coat  around 
the  ob'sphere,  thus  constituting  an  oospore. 


a   club,    and    the    endochrome 

accumulates  in  it  so  as  to  give  it  a  darker  hue  than  the  rest  ^ 
a  separation  of  this  part  from  the  remainder  of  the  filament, 
by  the  interposition  of  a  transparent  space,  is  next  seen ;  a  new 
envelope  is  then  formed  around  the  mass  thus  cut  off;  and  at  last 
the  membranous  wall  of  the  investing  tube  gives  way,  and  the  zoo- 


SIPHONACE^E  563 

spore  escapes,  not,  however,  until  it  has  undergone  marked  changes 
of  form,  and  exhibited  curious  movements.  Its  motions  continue 
for  some  time  after  its  escape,  and  are  then  plainly  seen  to  be  due 
to  the  action  of  the  cilia,  which  form  a  complete  fringe  round  it. 
If  it  be  placed  in  water  in  which  some  carmine  or  indigo  has  been 
rubbed,  the  coloured  granules  are  seen  to  be  driven  in  such  a 
manner  as  to  show  that  a  powerful  current  is  produced  by  their 
propulsive  action,  and  a  long  track  is  left  behind  it.  When  it 
meets  with  an  obstacle,  the  ciliary  action  not  being  arrested,  the 
zoospore  is  flattened  against  the  object ;  and  it  may  thus  be  com- 
pressed, even  to  the  extent  of  causing  its  endochrome  to  be  dis- 
charged. The  cilia  are  best  seen  when  their  movements  have  been 
retarded  or  entirely  arrested  by  means  of  opium,  iodine,  or  other 
chemical  reagents.  The  motion  of  the  spore  continues  for  about 
two  hours  ;  but  after  the  lapse  of  that  time  it  soon  comes  to  an 
end,  and  the  spore  begins  to  develop  itself  into  a  new  plant.  It  has 
been  observed  by  linger  that  the  escape  of  the  zoospores  generally 
takes  place  towards  8  A.M.  ;  to  watch  this  phenomenon,  therefore, 
the  plant  should  be  gathered  the  day  before,  and  its  tufts  examined 
early  in  the  morning.  The  same  filament  may  give  off  two  or  three 
zoospores  successively. 

In  addition  to  this  mode,  there  exists  also  in  this  humble 
plant  a  true  process  of  sexual  generation.  The  branching  filaments 
are  often  seen  to  bear  at  their  sides  peculiar  globular  or  oval 
capsular  protuberances,  sometimes  separated  by  the  interposition  of 
a  stalk,  which  are  filled  with  dark  endochrome  ;  and  from  these, 
after  a  time,  new  plants  arise.  In  the  neighbourhood  of  these 
bodies  are  found,  in  most  species,  certain  other  projections,  which, 
from  being  usually  pointed  and  somewhat  curved,  have  been  named 
'  horns  '  (fig.  426,  A,  a) ;  and  these  have  been  shown  by  Pringsheim 
to  be  antherids,  which  produce  antherozoids  in  their  interior  ;  whilst 
the  capsule-like  bodies  (A,  6)  are  oogones  or  a/rckegones,  each  con- 
taining a  mass  of  endochrome  which  constitutes  an  oosphere  that  is 
destined  to  become,  when  fertilised,  the  original  cell  of  a  new 
generation.  The  antherozoids  (B,  c,  d),  when  set  free  from  the 
antherid  a,  swarm  about  the  oogone  b,  and,  attracted  by  a  drop  of 
mucilage  formed  at  the  mouth  of  the  oogone,  enter  it,  one  or  more 
antherozoids  becoming  absorbed  into  the  substance  of  the  oosphere. 
This  hitherto  naked  mass  of  protoplasm  now  becomes  invested  by 
an  envelope  of  cellulose  (C,  b),  which  increases  in  thickness  and 
strength,  until  it  has  acquired  such  a  density  as  enables  it  to  afford 
a  firm  protection  to  its  contents.  While  in  Vaucheria  the  separate 
filaments  are  so  slender  as  to  be  scarcely  discernible  to  the  naked 
eye,  the  frond  of  other  genera  of  Siphonaceae,  mostly  natives  of 
shallow  seas  in  the  warmer  parts  of  the  globe,  attains  very  large 
dimensions.  Thus  in  C  odium  it  is  a  spongy  spherical  or  cylindrical 
floating  mass,  as  much  as  a  foot  in  length  ;  in  Caulerpa  it  has  the 
appearance  of  a  branched  leaf  springing  from  a  stem,  which  puts 
out  roots  from  its  under  side  ;  in  Acetabularia  it  takes  a  mushroom- 
like  form  with  a  cap  or  '  pileus,'  a  quarter  of  an  inch  in  diameter, 
divided  into  regular  chambers,  at  the  summit  of  a  cylindrical  stalk, 

o  o  2 


564  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


1^  to  3  inches  in  height.  Munier-Charles  l  believes  that  many 
fossils  generally  regarded  as  Foraminifera  are  in  reality  the 
calcareous  skeleton  of  algae  belonging  or  nearly  allied  to  the 
Siphonacese. 

The  microscopist  who  wishes  to  study  the  development  of  zoo- 
spores,  as  well  as  several  other  phenomena  of  this  low  type  of  vege- 
tation, may  advantageously  have  recourse  to  the  little  plant  termed 
Achlya  prolifera,2  which  grows  parasitically  upon  the  bodies  of  dead 
flies  lying  in  water.  Its  tufts  are  distinguishable  by  the  naked  eye 
as  clusters  of  minute  colourless  filaments  ;  and  these  are  found, 

when  examined  by  the 
microscope,  to  be  long 
tubes,  devoid  of  all  parti- 
tions, extending  them- 
selves in  various  direc- 
tions. The  tubes  contain 
a  colourless  slightly  gra- 
nular protoplasm,  the 
particles  of  which  are 
seen  to  move  slowly  in 
streams  along  the  walls, 
as  in  Okara,  the  currents 
occasionally  anastomosing 
with  each  other  (fig.  427, 
C).  Within  about  thirty- 
six  hours  after  the  first 
appearance  of  the  parasite 
on  any  body,  the  proto- 

Elasm  begins  to  accumu- 
ite  in  the  dilated  ends 
of  the  filaments,  each  of 
which  is  then  cut  off  from 
the  remainder  by  the 
formation  of  a  partition  ; 
and  within  this  dilated 
cell  the  movement  of  the 
protoplasm  continues  for 
a  time  to  be  distinguish  - 


FIG.  427. — Development  of  Achlya  prolifera :  A, 
dilated  extremity  of  a  filament  &,  separated 
from  the  rest  by  a  partition  a,  and  containing 
zoospores  in  progress  of  formation ;  B,  end  of 
filament  after  the  cell-wall  has  burst,  and 
setting  free  zoospores,  a,  b,  c ;  C,  portion  of 
filament,  showing  the  course  of  the  circulation 


of  granular  protoplasm. 


able.  Very  speedily,  how- 
ever, its  endoplasm  shows 
the  appearance  of  being  broken  up  into  a  large  number  of  distinct 
masses,  which  are  at  first  in  close  contact  with  each  other  arid 
with  the  walls  of  the  cell  (fig.  427,  A),  but  which  gradually 
become  more  isolated,  each  seeming  to  acquire  a  proper-  cell-wall ; 
they  then  begin  to  move  about  within  the  parent-cell ;  and,  when 

1  Comptes  Bendus,  vol.  Ixxx.  1877,  p.  814. 

2  [This  plant,  though,  as  an  inhabitant  of  water,  formerly  ranked  among  Alga;,  is 
now   generally   regarded    as  belonging  to  the  group  of   Fungi,  on  account   of   its 
incapacity  for  the  production  of  chlorophyll,  and  its  parasitism  on  the  bodies   of 
animals,  from  whose  juices  its  cells  seem  to  draw  their  nourishment.     It  is  very 
closely  allied  to  Saprolegnia  (see  p.  640),  a  fungus  parasitic  on  the  bodies  of  living 
fish,  and  causing  the  very  destructive  disease  to  which  salmon  are  liable. — ED.] 


ACHLYA;   HYDKODICTYON  565 

quite  mature,  they  are  set  free  by  the  rupture  of  its  wall  (B),  and, 
after  swarming  about  for  a  time,  develop  into  tubiform  cells  resem- 
bling those  from  which  they  sprang.  Each  of  these  zoospores  is 
possessed  of  two  flagella  ;  their  movements  are  not  so  powerful  as 
those  of  the  zoospores  of  Vaucheria,  and  come  to  an  end  sooner. 
The  generative  process  in  this  type  is  performed  in  a  manner  that 
may  be  regarded  as  an  advance  upon  ordinary  conjugation.  The 
end  of  one  of  the  long  tubiform  cells  enlarges  into  a  globular  dilata- 
tion, the  cavity  of  which  becomes  shut  off  by  a  transverse  partition. 
its  contained  endoplasm  divides  into  two,  three,  or  four  segments, 
each  of  which  takes  a  globular  form,  and  is  then  fertilised  by  the 
penetration  of  an  antheridial  tube  which  comes  off  from  the  filament 
a  little  below  the  partition.  The  oospores  thus  produced,  escaping 
from  the  globular  cavities,  acquire  firm  envelopes,  and  may  remain 
unchanged  for  a  long  time  even  in  water,  when  no  appropriate  nidus 
exists  for  them  ;  but  will  quickly  germinate  if  a  dead  insect  or  other 
suitable  object  be  thrown  in. 

One  of  the  most  curious  forms  of  the  lower  algae  is  the  '  water- 
net/  Hydrodictyon  reticulatum,  which  is  found  in  fresh- water  pools 
in  the  midland  and  southern  counties  of  England.  Its  frond  con- 
sists of  a  green  open  network  of  filaments,  acquiring,  when  full 
grown,  a  length  of  from  four  to  six  inches,  and  composed  of  a  vast 
number  of  cylindrical  tubular  cells,  which  attain  the  length  of  four 
lines  or  more,  arid  adhere  to  each  other  by  their  rounded  extremi- 
ties, the  points  of  junction  corresponding  to  the  knots  or  intersections 
of  the  network.  Each  of  these  cells  may  form  within  itself  an 
enormous  multitude  (from  7,000  to  20,000)  of  zoospores,  which  at  a 
certain  stage  of  their  development  are  observed  in  active  motion  in 
its  interior,  but  come  to  rest  in  the  course  of  about  half  an  hour, 
and  then  arrange  themselves  in  such  a  way  that  by  their  elongation 
they  again  form  a  net  of  the  original  kind,  which  is  set  free  by  the 
dissolution  of  the  wall  of  the  mother-cell,  and  attains  in  the  course 
of  three  or  four  weeks  the  size  of  the  mother-colony.  Besides  these 
bodies,  however,  certain  cells  produce  from  30,000  to  100,000 
'  microzoospores '  of  longer  shape,  each  furnished  with  four  long 
ttagella  and  a  red  '  eye-spot ; '  these  escape  from  the  cell  in  a  swarm, 
and  move  freely  in  the  water  for  some  time.  Conjugation  between 
these  smaller  zoospores  has  been  observed  to  take  place  sometimes 
even  with  the  mother-cell.  The  resulting  body  or  *  zygospore ' 
retains  its  green  colour,  but  becomes  invested  with  a  firm  cell-wall 
of  cellulose.  In  this  condition  these  bodies  may  remain  dormant 
for  a  considerable  time,  and  are  described  as  '  hypnospores '  or 
'  resting-spores ; '  and  in  this  state  they  are  able  to  endure  being 
completely  dried  up  without  the  loss  of  their  vitality,  provided  that 
they  are  secluded  from  the  action  of  light,  which  causes  them  to 
wither  and  die.  In  this  state  they  bear  a  strong  resemblance  to  the 
cells  of  Protococcus.  The  first  change  that  manifests  itself  in  them, 
when  they  begin  to  germinate,  is  a  simple  enlargement ;  next  the 
endochrome  divides  itself  successively  into  distinct  masses,  usually 
two  or  four  in  number  ;  and  these,  when  set  free  by  the  giving  way 
of  the  enveloping  membrane,  present  the  characters  of  ordinary 


566   MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

zob'spores,  each  of  them  possessing  two  flagella  at  its  anterior  semi- 
transparent  extremity.  Their  motile  condition,  however,  does  not 
last  long,  often  giving  place  to  the  motionless  stage  before  they  have 
quite  freed  themselves  from  the  parent-cell ;  they  then  project  long 
angular  processes,  so  as  to  assume  the  form  of  irregular  polyhedra, 
at  the  same  time  augmenting  in  size  ;  and  the  endochrome  contained 
within  each  of  these  breaks  up  into  a  multitude  of  zoospores,  which 
are  at  first  quite  independent  and  move  actively  within  the  cell- 
cavity,  but  soon  unite  into  a  network  that  becomes  invested  with 
a  gelatinous  envelope,  and  speedily  increases  so  much  in  size  as  to 
rupture  the  containing  cell-wall,  on  escaping  from  which  it  presents 
all  the  essential  characters  of  a  young  Hydrodictyon.  The  rapidity 
of  the  growth  of  this  curious  organism  is  not  one  of  the  least 
remarkable  parts  of  its  history.  The  individual  cells  of  which  the 
net  is  composed,  at  the  time  of  their  emission  as  zoospores,  measure 


FIG.  428.— Various  phases  of  development  of  Pediastrum  granulatum. 

no  more  than  ^^ th  of  an  inch  in  length  ;  but  in  the  course  of  a 
few  hours  they  grow  to  a  length  of  from  ^th  to  ^rd  of  an  inch. 

The  members  of  the  family  Pediastreae  were  formerly  included 
in  the  Desmidiacece ;  but,  though  doubtless  related  to  them  in 
certain  particulars,  they  present  too  many  points  of  difference  to  be 
properly  associated  with  them.  Their  chief  point  of  resemblance 
consists  in  the  firmness  of  the  outer  covering,  and  in  the  frequent 
interruption  of  its  margin  either  by  the  protrusion  of  '  horns  '  (fig. 
428,  A),  or  by  a  notching  more  or  less  deep  (fig.  429,  B) ;  but  they 
differ  in  these  two  important  particulars — that  the  cells  are  not 
made  up  of  two  symmetrical  halves,  and  that  they  are  always  found 
in  aggregation,  which  is  not,  except  in  such  genera  as  Scenedesmus 
which  connect  this  group  with  the  Desmids,  in  linear  series,  but  in 
the  form  of  discoidal  fronds.  In  this  tribe  we  meet  with  a  form  of 
multiplication  by  motile  '  megazoospores  '  which  reminds  us  of  the 
formation  of  the  motile  spheres  of  Volvox,  and  which  takes  place  in 


PEDIASTEEJE  567 

such  a  manner  that  the  resultant   product  may  vary  greatly  in  the 
number  of  its  cells,  and  consequently  both  in  size  and  in  form. 
Thus  in  Pecliastrum  granulatuin  (fig.  428)  the  zoospores  formed  by 
the  subdivision  of  the  endochrome  of  one  cell,  which  may  be  four, 
•eight,  sixteen,  thirty-two,  or  sixty-four  in  number,  escape  from  the 
parent -frond  still  enclosed  in  the  inner  layer  of  the  cell-wall ;  and  it 
is  within  this  that  they  develop  themselves  into  a  cluster  resembling 
that  in  which  they  originated,  so  that  the  frond  may  be  composed  of 
either  of  the  just-mentioned  multiples  or  sub-multiples  of  16.     At 
A  is  seen  an  old  disc,  of  irregular  shape,  nearly  emptied  by  the 
emission  of  its  zoospores,  which  had  been  seen  to  take  place  within 
a  few  hours  previously  from  the  cells  a,  b,   c,  d,  e  ;  most  of  the 
empty  cells  exhibit  the  cross  slit  through  which  their  contents  had 
been  discharged  ;  and  where  this  does  not  present  itself  on  the  side 
next  the  observer,  it  is  found  on  the  other.      Three  of  the  cells  still 
possess  their  coloured  contents,  but  in  different  conditions.     One  of 
them  exhibits  an  early  stage  of  the  subdivision  of  the  endochrome — 
namely,  into  two  halves,  one  of  which  already  appears  halved  again. 
Two  others  are  filled  by  sixteen  very  closely  crowded  zoospores,  only 
half  of  which  are  visible,  as  they  form  a  double  layer.     Besides 
these,  one  cell  is  in  the  very  act  of  discharging  its  zoospores,  nine  of 
which  have  passed  forth  from  its  cavity,  though  still  enveloped  in  a 
vesicle  formed  by  the  extension  of  its  innermost  membrane  ;  whilst 
seven   yet   remain   in   its   interior.     The   new-born   family,   as   it 
appears   immediately  on  its  complete  emission,  is  shown  at  B ;  the 
zoospores  are  actively  moving  within  the  vesicle,  and  they  do  not  as 
yet  show  any  indication  either  of  symmetrical  arrangement  or  of 
the  peculiar  form  which  they  are  subsequently  to  assume.     Within 
a  quarter  of  an  hour,  however,  the  zoospores  are  observed  to  settle 
down  into  one  plane,  and  to  assume  some  kind  of  regular  arrange- 
ment, most  commonly  that  seen  at  C,  in  wThich  there  is  a  single 
central  body  surrounded  by  a  circle  of  five,  and  this  again  by  a 
circle  of  ten ;  they  do  not,  however,  as  yet  adhere  firmly  together. 
The   zoospores   now  begin   to   develop  themselves  into  new  cells, 
increase  in  size,  and  come  into  closer  approximation  (D) ;  and  the 
edge  of  each,  especially  in  the  marginal  row,  presents  a  notch  which 
foreshadows  the  production  of  its  characteristic  '  horns.'     Within 
about  four  or  five  hours  after  the  escape  of  the  zoospores,  the  cluster 
has  come  to  assume  much  more  of  the  distinctive  aspect  of  the 
species,  the  marginal  cells  having  grown  out  into  horns  (E) ;  still, 
however,  they  are  not  very  closely  connected  with  each  other,  and 
between  the  cells  of  the  inner  row  considerable  spaces  yet  intervene. 
It  is  in  the  course  of  the  second  day  that  the  cells  become  closely 
applied  to  each  other,  and  that  the  growth  of  the  horns  is  completed, 
so  as  to  constitute  a  perfect  disc  like  that  seen  at  F,  in  which,  how- 
ever, the  arrangement  of  the  interior  cells  does   not  follow  the 
typical  plan.1     The  formation  of  *  microzoospores '  has   also   been 
observed,  which  have  been  seen  to  conjugate. 

1  See  Prof.  Braun  on  The  Phenomenon  of  Rejuvenescence  in  Nature,  published 
by  the  Ray  Society  in  1853 ;  and  its  subsequent  memoir,  Algarum  Unicellularum 
Genera  nova  aut  minus  cognita,  1855. 


568   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

The  varieties  which  present  themselves,  indeed,  both  as  to  the 
number  of  cells  in  each  cluster  and  the  plan  on  which  they  are  dis- 
posed, are  such  as  to  baffle  all  attempts  to  base  specific  distinctions 
on  such  grounds  ;  and  the  more  attentively  the  life-history  of  any 
one  of  these  plants  is  studied,  the  more  evident  does  it  appear  that 
many  reputed  '  species '  have  no  real  existence.  Some  of  these, 
indeed,  are  nothing  else  than  mere  transitory  forms  ;  thus  it  can  be 
scarcely  doubted  that  the  specimen  represented  in  fig.  429,  D,  under 
the  name  of  Pediastrum  pertusum,  is  in  reality  nothing  else  than  a 
young  frond  of  P.  granulatum  in  the  stage  represented  in  fig.  428,  E, 
but  consisting  of  thirty-two  cells.  On  the  other  hand,  in  fig.  429,  E, 
we  see  an  emptied  frond  of  P.  granulatum,  exhibiting  the  peculiar 
surface-marking  from  which  the  name  of  the  species  is  derived,  but 
composed  of  no  more  than  eight  cells.  And  instances  every  now 
and  then  occur  in  which  the  frond  consists  of  only  four  cells,  each  of 


FIG.  429. — Various  species  (?)  of  Pediastrum:  A,  P.  tetras;  B,  C,  P.  Ehrenbergii'y 
D,  P.  pert usum ;  E,  empty  frond  of  P.  granulatum. 

them  presenting  the  two-horned  shape.  So,  again,  in  fig.  429,  B  and 
C,  are  shown  two  varieties  of  Pediastrum  Ehrenbergii,  whose  frond 
is  normally  composed  of  sixteen  cells ;  whilst  at  A  is  figured  a  form 
which  is  designated  as  P.  tetras,  but  which  may  be  strongly  suspected 
to  be  merely  a  four-celled  variety  of  B  and  C.  Many  similar  cases 
might  be  cited  ;  and  the  Author  would  strongly  urge  those  micro- 
scopists  wrho  have  the  requisite  time  and  opportunities,  to  apply 
themselves  to  the  determination  of  the  real  species  of  these  groups 
by  studying  the  entire  life-history  of  whatever  forms  may  happen  to 
lie  within  their  reach,  and  noting  all  the  varieties  which  present  them- 
selves among  the  offsets  from  any  one  stock.  The  characters  of  such 
varieties  are  diffused  by  the  process  of  binary  subdivision  amongst 
vast  multitudes  of  so-called  individuals.  Thus  it  happens  that,  as 
Mr.  Ralfs  has  remarked,  '  one  pool  may  abound  with  individuals  of 
Staurastrum  dejectum  or  Arthrodesmus  incus  having  the  mucra 


PEDIASTEE^  ;    CONFEHVACEJE 


569 


curved  outwards  ;  in  a  neighbouring  pool  every  specimen  may  have 
it  curved  inwards  ;  and  in  another  it  may  be  straight.  The  cause 
of  the  similarity  in  each  pool  no  doubt  is  that  all  its  plants  are  off- 
sets from  a  few  primary  fronds.'  Hence  the  universality  of  any 
particular  character  in  all  the  specimens  of  one  gathering  is  by  no 
means  sufficient  to  entitle  these  to  take  rank  as  a  distinct  species  ; 
since  they  are,  properly  speaking,  but  repetitions  of  the  same  variety 
by  a  process  of  simple  multiplication,  really  representing  in  their 
entire  aggregate  the  one  plant  or  tree  that  grows  from  a  single  seed. 
Almost  every  pond  and  di^ch  contains  some  members  of  the 
family  Confervaceae  ;  but  they  £ife  especially  abundant  in  moving 
water,  and  they  constitute  the  greater  A 

part  of  those  green  threads  which 
are  to  be  seen  attached  to  stones, 
with  their  free  ends  floating  in  the 
direction  of  the  current,  in  every 
running  stream,  and  upon  almost 
every  part  of  the  sea-shore,  and 
which  are  commonly  known  under 
the  name  of  '  silk- weeds,'  or  '  crow- 
silk.'  Their  form  is  visually  very 
regular,  each  thread  being  a  long 
cylinder  made  up  by  the  union  of  a 
single  filament  of  short  cylindrical 
cells  united  to  each  other  by  their 
flattened  extremities ;  sometimes 
these  threads  give  off  lateral 
branches,  which  have  the  same 
structure.  The  endochrome,  though 
usually  green,  is  occasionally  of  a 
brown  or  purple  hue,  and  is  usually 
distributed  uniformly  throughout  the 
cell  (as  in  fig.  430).  The  plants  of 
this  family  are  extremely  favourable 
subjects  for  the  study  of  the  method 
of  cell -multiplication  by  binary  sub- 
division. This  process  usually,  but 
not  always,  takes  place  only  in  the 
terminal  cell ;  and  it  may  be  almost 
always  observed  there  in  some  one  of 
its  stages.  The  first  step  is  seen  to  be  the  subdivision  of  the 
endochrome,  and  the  inflexion  of  the  ectoplasm  around  it 
(fig.  430  A,  a) ;  and  thus  there  is  gradually  formed  a  sort  of 
hour-glass  contraction  across  the  cavity  of  the  parent-cell,  by 
which  it  is  divided  into  two  equal  halves  (B).  The  two  surfaces 
of  the  infolded  utricle  produce  a  double  layer  of  cellulose  mem- 
brane between  them.  Sometimes,  however,  as  in  Cladophora 
glomerata  (a  common  species),  new  cells  may  originate  as  branches 
from  any  part  of  the  surface  by  a  process  of  budding,  which, 
notwithstanding  its  difference  of  mode,  agrees  with  that  just 
described  in  its  essential  character,  being  the  result  of  the  sub- 


FIG.  430.— Process  of  cell-multipli- 
cation in  Cladophora  glomerata : 
A,  portion  of  filament  with  incom- 
plete separation  at  a,  and  complete 
partition  at  b ;  B,  the  separation 
completed,  a  new  cellulose  parti- 
tion being  formed  at  a  ;  C,  forma- 
tion of  additional  layers  of  cellulose 
wall,  c,  beneath  the  mucous  in- 
vestment, d,  and  around  the 
ectoplasm,  «,  which  encloses  the 
endochrome,  b. 


5/0  MICROSCOPIC  FOEMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

division  of  the  original  cell.  A  certain  portion  of  the  ectoplasm 
seems  to  undergo  increased  nutrition,  for  it  is  seen  to  project, 
carrying  the  cellulose  envelope  before  it,  so  as  to  form  a  little 
protuberance,  and  this  sometimes  attains  a  considerable  length 
before  any  separation  of  its  cavity  from  that  of  the  cell  which  gave 
origin  to  it  begins  to  take  place.  This  separation  is  gradually 
effected,  however,  by  the  infolding  of  the  ectoplasm,  just  as  in  the 
preceding  case  ;  and  thus  the  endochrome  of  the  branch  cell  becomes 
completely  severed  from  that  of  the  stock.  The  branch  then  begins 
to  elongate  itself  by  the  subdivision  of  its  first-formed  cell ;  and  this 
process  may  be  repeated  for  a  time  in  all  the  cells  of  the  filament, 
though  it  usually  comes  to  be  restricted  at  last  to  the  terminal  cell. 
The  very  elongated  cells  of  some  species  of  Confervaceae  are 
characterised  by  the  possession  of  a  large  number  of  nuclei.  They 
are  multiplied  by  zoospores,  produced  apparently  indifferently  from 
any  cell  of  a  filament,  by  free-cell  formation.  These  zoospores  are 
of  two  kinds,  larger  or  smaller ;  the  larger  kind  have  either  two  or 
four  cilia,  and  germinate  directly ;  the  smaller  are  biciliated,  and 
conjugation  between  them  has  been  observed. 

Nearly  allied  to  the  Confer vacese  is  a  very  interesting  plant  in 
which  a  true  sexual  mode  of  reproduction  has  been  observed,  Sphaero- 
plea  ammlina,  the  development  and  generation  of  which  have  been 
specially  studied  by  Dr.  F.  Cohn.1  The  oospore,  which  is  the  pro- 
duct of  the  sexual  process  to  be  presently  described,  is  filled  when 
mature  with  a  red  oil,  and  is  enveloped  by  two  membranes,  of  which 
the  outer  one  is  furnished  with  stellate  prolongations  (fig.  431,  No.  1). 
When  it  begins  to  vegetate,  its  endochrome  breaks  up — first  into 
two  halves  (No.  2),  and  then,  by  successive  subdivisions,  into  numerous 
segments  (Nos.  3,  4),  at  the  same  time  becoming  green  towards  its 
margin.  These  segments,  set  free  by  the  rupture  of  their  containing 
envelope,  escape  in  the  form  of  motile  zoospores,  which  are  at  first 
rounded  or  oval,  each  having  a  semi-transparent  beak  whence  proceed 
two  cilia;  but  they  gradually  elongate  so  as  to  become  fusiform 
(No.  5),  at  the  same  time  changing  their  colour  from  red  to  green. 
These  move  actively  for  a  time,  and  then,  losing  their  motile  power, 
begin  to  develop  themselves  into  filaments.  The  first  stage  in  this 
development  consists  in  the  elongation  of  the  cell,  and  the  separation 
of  the  endochrome  of  its  two  halves  by  the  interposition  of  a  vacuole 
(No.  6),  and  in  more  advanced  stages  (Nos.  7,  8)  a  repetition  of  the 
like  interposition  gives  to  the  endochrome  that  annular  arrange- 
ment from  which  the  plant  derives  its  specific  name.  This  is  seen 
at  No.  9,  a,  as  it  presents  itself  in  the  filaments  of  the  adult  plant ; 
whilst  at  b,  in  the  same  figure,  we  see  a  sort  of  frothy  appearance 
which  the  endochrome  comes  to  possess  through  the  multiplication 
of  the  vacuoles.  The  next  stage  in  the  development  of  the  filaments 
that  are  to  produce  the  oospheres  consists  in  the  aggregation  of  the 
endochrome  into  definite  masses  (as  seen  at  No.  10,  a),  which  soon 
become  star-shaped  (as  seen  at  6),  each  one  being  contained  within  a 
distinct  compartment  of  the  cell.  In  a  somewhat  more  advanced 
stage  (as  seen  at  No.  11,  a),  the  masses  of  endochrome  begin  to  draw 
1  Ann.  des  Sci.  Nat.  4eme  ser.,  Bot.,  torn.  v.  1856,  p.  187. 


SPH^EROPLEA  ANNULIXA 


571 


themselves  togetlier  again ;  and  they  soon  assume  a  globular  or 
ovoidal  shape  (6),  whilst  at  the  same  time  definite  openings  (c)  are 
formed  in  their  containing  cell-wall.  Through  these  openings  the 
antherozoids  developed  within  other  cells  gain  admission,  as 
shown  at  No.  12,  d ;  and  they  become  absorbed  into  the  before-men- 


FIG.  431. — Development  and  reproduction  of  Sphceroplea. 

tioned  masses,  which  soon  afterwards  become  invested  with  a  firm 
membranous  envelope,  as  shown  in  the  lower  part  of  No.  12.  These 
undergo  further  changes  whilst  still  contained  within  their  tubular 
parent-cells,  their  colour  passing  from  green  to  red ;  and  a  second 
investment  is  formed  within  the  first,  which  extends  itself  into 
stellate  prolongations,  as  seen  in  No.  13;  so  that  when  set  free 


5/2   MICEOSCOPIC  FORMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

they  precisely  resemble  the  mature  oospores  which  we  have  taken  as 
the  starting-point  in  this  curious  history.  Certain  of  the  cells  (as 
in  No.  14),  instead  of  giving  origin  to  oospores,  have  their  annular 
collections  of  endochrome  converted  into  antherozoids,  which,  as 
soon  as  they  have  disengaged  themselves  from  the  mucilaginous 
sheath  that  envelopes  them,  move  about  rapidly  in  the  cavity  of  their 
containing  cell  (a,  6)  around  the  large  vacuoles  which  occupy  its 
interior,  and  then  make  their  escape  through  apertures  (c,  d)  which 
form  themselves  in  its  wall,  to  find  their  way  through  similar  aper- 
tures into  the  interior  of  the  oogones,  as  already  described.  These 
antherozoids  are  shown  in  No.  15,  as  they  appear  when  swimming 
actively  through  the  water  by  means  of  the  two  cilia  which  each 
possesses.  The  peculiar  interest  of  this  history  consists  in  the  entire 
absence  of  any  special  organs  for  the  generative  process,  the  ordinary 
filamentous  cell  developing  oospheres  on  the  one  hand  and  anthero- 
zoids on  the  other,  and  in  the  simplicity  of  the  means  by  which  the 
fecundating  process  is  accomplished. 

The  (Edogoniaceae  resemble  Confervacece  in  general  aspect  and 
habit  of  life,  but  differ  from  them  in  some  curious  particulars.  As 
the  component  cells  of  the  filaments  extend  themselves  longitudi- 
nally, new  rings  of  cellulose  are  formed  successively,  and  are  inter- 
calated into  the  cell-wall  at  its  upper  end,  giving  it  a  ringed  appear- 
ance. Only  a  single  large  zoospore  is  set  free  from  each  cell  ;  and 
its  liberation  is  accomplished  by  the  almost  complete  fission  of  the 
wall  of  the  cell  through  one  of  these  rings,  a  small  part  only  remain- 
ing uncleft,  which  serves  as  a  kind  of  hinge  whereby  the  two  parts 
of  the  filament  are  prevented  from  being  altogether  separated. 
Sometimes  the  zoospore  does  not  completely  extricate  itself  from 
the  parent-cell ;  and  it  may  begin  to  grow  in  this  situation,  the 
root-like  processes  which  it  puts  forth  being  extended  into  the 
cavity.  The  zoospores  are  the  largest  known  in  any  class  of  algje  ; 
each  has  a  nucleus,  a  red  '  eye-spot,'  and  an  anterior  hyaline  spot  to 
which  is  attached  a  tuft  of  cilia  visible  even  before  its  escape  from 
its  mother-cell. 

In  their  generative  process,  also,  the  (Edogoniacew  show  a  curious 
departure  from  the  ordinary  type ;  for  whilst  the  oospheres  are 
formed  within  certain  dilated  cells  of  the  ordinary  filament  (fig.  432, 
A,  No.  1),  which  may  be  termed  oogones,  and  are  fertilised  by  the 
penetration  of  antherozoids  (No.  2),  these  antherozoids  are  not,  in  all 
the  species,  the  immediate  product  of  the  sperm-cells  of  the  same  or 
of  another  filament,  but  are  developed  within  a  body  termed  an 
androspore  (No.  5),  which  is  set  free  from  within  a  special  cell  (No. 
4),  and  which,  being  furnished  with  a  terminal  tuft  of  cilia,  and  having 
motile  powers,  very  strongly  resembles  an  ordinary  zoospore.  This 
androspore,  after  its  period  of  activity  has  come  to  an  end,  attaches 
itself  to  the  outer  surface  of  an  oogone,  or  of  a  cell  in  close  proxi- 
mity to  an  oogone,  as  shown  at  No.  1,  &  ;  it  then  developes  into  a 
very  small  male  plant,  known  as  a  dwarf -male,  consisting  of  two  or 
three  cells ;  the  terminal  of  these  cells  is  an  antherid,  from  the  apex 
of  which  a  sort  of  lid  drops,  as  seen  in  the  upper  part  of  No.  1,  by 
which  its  contained  antherozoids  (No.  2)  are  set  free ;  and  at  the 


(EDOGONIACE^:  ;    CHyETOPHO  RACEME 


573 


same  time  an  aperture  is  formed  in  the  wall  of  the  oogone  by 
which  the  antherozoid  enters  its  cavity  and  fertilises  its  ob'sphere  by 
becoming  absorbed  into  it.  This  mass  then  becomes  an  obspore  (No.  3), 
invested  with  a  thick  wall  of  its  own,  but  still  retains  more  or  less 
of  the  envelope  derived  from  the  cell  within  which  it  was  developed. 
The  offices  of  these  different  classes  of  reproductive  bodies  are  only 
now  beginning  to  be  understood,  arid  the  inquiry  is  one  so  fraught 
with  physiological  interest,  and,  from  the  facility  of  growing  these 
plants  in  aquaria,  can  be  so  easily  pursued,  that  it  may  be  hoped 


FIG.  432. — A,  Sexual  generation  of  CEdogonium  ciliatum :  1,  filament  with  two 
oogones  in  process  of  formation,  the  lower  one  having  two  androspores  attached  to 
its  exterior,  the  contents  of  the  upper  oogone  in  the  act  of  being  fertilised  by  the 
entrance  of  an  antherozoid  set  free  from  the  interior  of  its  androspore ;  2,  free 
antherozoids ;  3,  mature  oospore,  still  invested  with  the  cell-membrane  of  the 
parent-filament ;  4,  portions  of  a  filament  bearing  special  cells,  from  one  of  which 
an  androspore  is  being  set  free  ;  5,  liberated  androspore. 

B,  Branches  of  Chcetophora  elegans,  in  the  act  of  discharging  ciliated  zob'spores, 
which  are  seen  as  in  motion  on  the  right. 

that  the  zeal  of  microscopists  will  not  long  leave  any  part  of  it  in 
obscurity. 

The  Chaetophoraceae  constitute  a  beautiful  and  interesting  little 
group  of  confervoid  plants,  of  which  some  species  inhabit  the  sea, 
whilst  others  are  found  in  fresh  and  pure  water — rather  in  that  of 
gently  moving  streams,  however,  than  in  strongly  flowing  currents. 
Generally  speaking,  their  filaments  put  forth  lateral  branches,  and 
extend  themselves  into  arborescent  fronds ;  one  of  the  distinc- 
tive characters  of  the  group  is  afforded  by  the  fact  that  the 
extremities  of  these  branches  are  usually  prolonged  into  bristle- 


574  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE -THALLOPHYTES 

shaped  processes  (fig.  432,  B).  As  in  many  preceding  cases,  these 
plants  multiply  themselves  by  the  conversion  of  the  endochrome  of 
certain  of  their  cells  into  zobspores,  and  these,  when  set  free,  are 
seen  to  be  furnished  with  either  two  or  four  cilia.  *  Resting- 
spores'  have  also  been  seen  in  many  species.  One  of  the  most 
beautiful  objects  under  the  microscope  is  Draparnaldia  glomerata, 
not  uncommon  in  still  water.  It  consists  of  an  axis  composed  of  a 
single  row  of  large  transparent  cells  containing  but  a  small  quantity 
of  chlorophyll.  From  this  proceed  at  regular  intervals  whorls  of 
slender  branches,  the  endochrome  of  which  is  deep  green,  and  every 
branch  ends  in  a  delicate  hyaline  hair  of  extraordinary  length.  The 
mode  of  reproduction  of  the  Chcetophoracece  closely  resembles  that  of 
the  Gonfervacece. 

The  Batrachospermese,  whose  name  is  indicative  of  the  strong 
resemblance  which  their  beaded  filaments  bear  to  frog-spawn,  are 
now  ranked  as  humble  fresh-water  forms  of  a  far  higher,  chiefly 
marine,  group  of  algae,  the  Rhodospermece,  or  red  sea-weeds.  But 
they  deserve  special  notice  here  on  account  of  the  simplicity  of  their 
structure,  and  the  extreme  beauty  of  the  objects  they  afford  to  the 
microscopist  (fig.  433).  They  are  chiefly  found  in  water  which  is 
pure  and  gently  flowing.  *  They  are  so  extremely  flexible,'  says  Dr. 
Hassall,  '  that  they  obey  the  slightest  motion  of  the  fluid  which 
surrounds  them  ;  and  nothing  can  surpass  the  ease  and  grace  of 
their  movements.  When  removed  from  the  water  they  lose  all 
form,  and  appear  like  pieces  of  jelly,  without  trace  of  organisation  ; 
on  immersion,  however,  the  branches  quickly  resume  their  former 
disposition.'  Their  colour  is  for  the  most  part  of  a  brownish  green, 
but  sometimes  they  are  of  a  reddish  or  bluish  purple.  The  central 
axis  of  each  plant  is  at  first  composed  of  a  single  filament  of  large 
cylindrical  cells  laid  end  to  end ;  but  this  is  subsequently  invested 
by  other  cells,  in  the  manner  to  be  presently  described.  It  bears 
at  pretty  regular  intervals  whorls  of  short  radiating  branches,  each 
of  which  is  composed  of  rounded  cells,  arranged  in  a  bead-like  row, 
and  sometimes  subdividing  again  into  two,  or  themselves  giving  off 
lateral  branches.  Each  of  the  primary  branches  originates  in  a  little 
protuberance  from  the  primitive  cell  of  the  central  axis,  precisely 
after  the  manner  of  the  lateral  cells  of  Cladophora  glomerata  ;  as  this 
protuberance  increases  in  size,  its  cavity  is  cut  off  by  a  septum,  so 
as  to  render  it  an  independent  cell ;  and  by  the  continual  repetition 
of  the  process  of  binary  subdivision  this  single  cell  becomes  con- 
verted into  a  beaded  filament.  Certain  of  these  branches,  however 
instead  of  radiating  from  the  main  axis,  grow  downwards  upon  it, 
so  as  to  form  a  closely  fitting  investment  that  seems  properly  to 
belong  to  it.  Some  of  the  radiating  branches  grow  out  into  long- 
transparent  bristles,  like  those  of  the  Chcetophoracece;  and  within 
those  are  produced  antherozoids,  which,  though  not  endowed  with 
the  power  of  spontaneous  movement,  find  their  way  to  the  oospheres 
contained  in  other  parts  of  the  filaments ;  and  by  the  fertilisation 
of  the  contents  of  these  are  produced  the  somewhat  complicated 
fructifications  known  as  cystocarps,  placed  in  the  axils  of  the 
branches  (fig.  433). 


BATRACHOSPEEME2E ;   COLEOCH^TACE^:  ;   CHARACE^E       575 


A  very  singular  relationship,  called  by  some  writers  an  i  alter- 
nation of  generations/  exists  between  Batrachospermum  and  Chan- 
trtnisia,  a  genus  of  fresh-water  alga?  previously  placed  in  a  totally 
different  section.  This  relationship  was  first  described  by  Sirodot,1 
and  his  observations  have  since  been  confirmed  by  others.  The 
germinating  spores  of  fiatrackospermum  put  out,  under  certain 
conditions,  a  kind  of  filament,  known  as  a  jrrotoneme,  which  develops 
into  a  Chantransia,  a  non-sexual  form  of  Batrachospermum,  which 
can  reproduce  itself  from  generation  to  generation  by  simple 
budding,  or  by  means  of  non-sexual  spores,  without  producing 
sexual  organs.  Chantransia  is  especially  found  in  water  where  very 
little  light  reaches  it.  When  more  exposed  to  light  it  undergoes 
metamorphosis,  and  then  a  branch  springs  up  from  the  protoneme 
which  is  in  every  respect  a  Batrachosperiniiim,  bearing  true  sexual 
organs,  as  above  described. 
This  may  then  go  on  repro- 
ducing itself,  or  revert  to  the 
Chantransia  form. 

The  Coleochsetaceae  are  a 
small  order  of  fresh  -  water 
Algae,  chiefly  represented  by 
the  genus  Coleochcete,  which 
forms  minute  discs  or  cushions 
attached  to  submerged  plants, 
from  -jL  to  J  inch  in  diameter, 
consisting,  in  the  simplest 
forms,  of  a  single  layer  of  cells, 
often  arranged  in  rays  proceed- 
ing from  a  common  centre. 
Reproduction  takes  place  non- 
sexually,  by  means  of  zoospores, 
or  sexually,  by  the  fertilisation 
of  an  oogone  by  motile  anthero- 
zoids,  through  the  agency  of  a 
peculiar  tube  known  as  a  trichoyyne,  a  forecast  of  the  more  com- 
plicated process  which  we  shall  presently  meet  with  in  the  Florideae 
or  Rhodospermeae,  the  highest  class  of  Algae. 

Among  the  highest  of  the  Algae  in  regard  to  the  complexity  of 
their  generative  apparatus,  which  contrasts  strongly  with  the  general 
simplicity  of  their  structure,  is  the  family  of  Characeae,2  some 
members  of  which  have  received  a  large  amount  of  attention  from 
micToscopists  on  account  of  the  interesting  phenomena  they  exhibit. 
These  plants  are  for  the  most  part  inhabitants  of  fresh  waters^ 
and  are  found  rather  in  such  as  are  still  than  in  those  which 
are  in  motion ;  a  few  species,  however,  may  be  met  with  in 
ditches  whose  waters  are  rendered  salt  by  communication  with  the 
sea.  They  may  be  easily  grown  for  the  purposes  of  observation  in 

1  Sirodot,  Les  Batrachospermees,  fo.  1884. 

2  [Many  of  the  best  authorities  regard  the  Characece,  in  consequence  of  their 
mode  of  reproduction,  as  a  group  of  primary  character,  of  equal  rank  with  the  Algae, 
and  superior  to  them  in  organisation. — ED.] 


FIG.  433. 
B a  fra  chosperm  um  moniliform  e. 


576   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

large  glass  jars  exposed  to  the  light,  all  that  is  necessary  being  to 
pour  off  the  water  occasionally  from  the  upper  part  of  the  vessel 
(thus  carrying  away  a  film  that  is  apt  to  form  on  its  surface),  and 
to  replace  this  by  fresh  water.  Each  plant  is  composed  of  an 
assemblage  of  long  tubiform  cells  placed  end  to  end,  with  a  distinct 
central  axis,  around  which  the  branches  are  disposed  at  intervals 
with  great  regularity  (fig.  434,  A).  In  Nitella  the  stem  and 
branches  are  composed  of  simple  cells,  which  sometimes  attain 
the  length  of  several  inches  ;  whilst  in  most  species  of  Char  a  each 
central  tube  is  surrounded  by  an  envelope  of  smaller  ones,  which  is 
formed  as  in  jBatrachospermum,  save  that  the  investing  cells  grow 
upwards  as  well  as  downwards  from  each  node,  and  meet  each  other 
on  the  stem  halfway  between  the  nodes,  their  ends  dovetailing 
into  one  another.  These  investing  tubes  constitute  what  is  termed 
the  *  cortex  '  of  Chara.  They  are  of  smaller  diameter  than  the  central 
tube,  and  are  arranged  spirally  round  it,  giving  the  stem  a  twisted 
appearance.  Each  '  node,'  or  zone  from  which  the  branches  spring, 
consists  of  a  single  plate  or  layer  of  small  cells,  which,  in  Chara,  are 
a  continuation  of  the  cortical  layer  of  the  '  internode.'  The  branches 
are  altogether  similar  in  structure  to  the  primary  axis,  and 
terminate  in  a  large  elongated  pointed  cell,  which  is  not  covered  by 
the  cortex.  From  the  lower  part  of  the  stem  '  rhizoids '  or  rooting 
filaments  are  put  out,  which  attach  the  plant  to  the  soil.  Some 
species  have  the  power  of  secreting  carbonate  of  lime  from  the 
water  in  which  they  grow,  if  this  be  at  all  impregnated  with 
calcareous  matter ;  and  by  the  deposition  of  it  beneath  their  tegu- 
ment they  have  gained  their  popular  name  of  '  stoneworts.'  The 
long  tubiform  cells  of  Nitella,  and  the  terminal  uncorticated  cells  of 
the  branches  of  Chara,  afford  a  very  beautiful  and  instructive  display 
of  the  phenomenon  of  cydosis,  or  rotation  of  protoplasm  in  their 
interior.  Each  cell,  in  the  healthy  state,  is  lined  by  a  layer  of 
chlorophyll  grains,  which  cover  every  part,  except  two  longitudinal 
lines  that  remain  nearly  colourless  (fig.  434,  B)  ;  and  a  constant 
stream  of  semi-fluid  protoplasm,  containing  starch  grains  and 
chlorophyll  granules,  is  seen  to  flow  over  the  green  layer,  the 
current  passing  up  one  side,  changing  its  direction  at  the  extremity, 
and  flowing  down  the  other  side,  the  ascending  and  descending 
spaces  being  bounded  by  the  transparent  lines  just  mentioned.  In 
the  young  cells  the  rotation  may  be  seen  before  this  granular 
lining  is  formed.  The  rate  of  the  movement  is  affected  by  anything 
that  influences  the  vital  activity  of  the  plant ;  thus  it  is  accelerated 
by  moderate  warmth,  whilst  it  is  retarded  by  cold  ;  and  it  may  be 
at  once  checked  by  a  slight  electric  discharge  through  the  plant. 
Carried  along  by  the  protoplasmic  stream  are  a  number  of  solid 
particles,  which  consist  of  starchy  matter,  and  are  of  various  sizes, 
being  sometimes  very  small  and  of  definite  figure,  whilst  in  other 
instances  they  are  seen  as  large  irregular  masses,  which  appear  to 
be  formed  by  the  aggregation  of  the  smaller  particles.  The  produc- 
tion of  new  cells  for  the  extension  of  the  stem  or  branches,  or  for 
the  origination  of  new  whorls,  is  not  here  accomplished  by  the 
.subdivision  of  the  parent-cell,  but  takes  place  by  the  method  of  out- 


CHARACE.T: 


577 


growth  (fig.  434.  B,  e,f,  </,  h),  which,  as  already  shown,  is  nothing 
but  a  modification  of  the  usual  process  of  cell -multiplication  ;  in 
this  manner  the  extension  of  the  individual  plant  is  effected  with 
considerable  rapidity.  When  these  plants  are  well  supplied  with 
nutriment,  and  are  actively  vegetating  under  the  influence  of  light, 
warmth,  A:c..  they  not  unfrequently  develop  '  bulbils,'  which  are 
little  clusters  of  cells,  filled  with  starch,  that  sprout  from  the  sides 
of  the  central  axis,  and  then,  falling  off,  evolve  the  long  tubiform 
cells  characteristic  of  the  plan>  from  which  they  were  produced. 
There  are  also  several  other  non-sexual  ways  in  which  these  plants 


FIG.  434.—Nitc1la  flexilis  :  A,  Stem  and  branches  of  the  natural  size  :  a,  b,  c,  d,  our 
whorls  of  branches  issuing  from  the  stem ;  e,  /,  subdivision  of  the  branches. 
B,  Portion  of  the  stem  and  branches  enlarged  :  a,  b,  joints  of  stem;  c,  d,  whorls  ; 
e,  f,  new  cells,  sprouting  from  the  sides  of  the  branches ;  g,  h,  new  cells  sprouting 
at  the  extremities  of  the  branches. 

are  reproduced,  but  they  are  peculiar  among  cryptogams  in  not 
producing  true  spores,  either  stationary  or  motile.  The  Characece 
may  be  multiplied  by  artificial  subdivision,  the  separated  parts 
continuing  to  grow  under  favourable  circumstances,  and  gradually 
developing  themselves  into  the  typical  form. 

The  generative  apparatus  of  Characew  consists  of  two  sets  of 
bodies,  both  of  which  grow  at  the  bases  of  the  branches  (fig.  435, 
A,  B),  either  on  the  same  or  on  different  individuals  ;  one  set, 
formerly  known  as  *  globules,'  are  really  anther  ids ;  whilst  the 
other,  known  as  'nucules,'  contain  the  oospheres,  and  are  true 
oogones  or  archegones.  The  globules,  which  are  nearly  spherical, 

p  P 


578    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — THALLOPHYTES 

and  often  of  a  bright  red  colour,  have  an  envelope  made  up  of  eight 
triangular  plates  or  '  shields '  (B,  C),  often  curiously  marked,  which 
encloses  a  central  portion  of  a  light  reddish  colour ;  this  central 
portion  is  principally  composed  of  a  mass  of  filaments  rolled  up 
compactly  together.  From  the  centre  of  the  inner  face  of  each 
shield  a  cylindrical  cell  termed  a  manubrium  projects  inwards  nearly 
to  the  centre  of  the  sphere.  The  antherid  is  supported  on  a  short 


FIG.  435.— Generative  organs  of  Chara  fragilis:  A,  antherid  or  globule  developed 
at  the  base  of  archegoiie  or  nucule ;  B,  nucule  enlarged,  and  globule  laid  open  by 
the  separation  of  its  valves ;  C,  one  of  the  valves,  with  its  group  of  antheridial 
filaments  each  composed  of  a  linear  series  of  cells,  within  every  one  of  which  an 
antherozoid  is  formed  ;  in  D,  E,  and  F  the  successive  stages  of  this  formation  are 
seen  ;  and  at  G  is  shown  the  escape  of  the  mature  antherozoids,  H. 

flask -shaped  pedicel,  which  also  projects  into  the  interior.  At  the 
apex  of  each  of  the  eight  manubria  is  a  roundish  hyaline  cell,  called 
a  capitulum,  and  at  the  apex  of  each  capitulum  six  smaller  cells  or 
'secondary  capitula.'  From  the  centre  of  each  of  these  secondary 
capitula  grow  four  long  whip-shaped  filaments  (C),  constituting  the 
mass  already  referred  to.  The  number  of  these  filaments  in  each 
antherid  is  about  200,  and  each  of  these  filaments  divides  by 


CHABACE^E;   DESMIDIACE.E  579 

transverse  septa  into  from  100  to  200  small  disc-shaped  cells,  which 
number,  therefore,  from  20,000  to  40,000  in  each  antherid.  In 
every  one  of  these  cells  there  is  formed,  by  a  gradual  change  in  its 
contents  (the  successive  stages  of  which  are  seen  at  D,  E,  F),  an 
antherozoid,  a  spiral  thread  of  protoplasm  consisting  of  two  or  three 
coils,  which,  at  first  motionless,  after  a  time  begins  to  move  and 
revolve  within  the  cell,  and  at  last  the  cell- wall  gives  way,  and  the 
spiral  thread  makes  its  escape  ^G),  partially  straightens  itself,  and 
moves  actively  through  the  water-for  some  time  (H)  in  a  tolerably 
determinate  direction,  by  the  lashing  action  of  two  long  and  very 
delicate  cilia  with  which  it  is  furnished.  The  exterior  of  the  nucule 
(A.  B)  is  formed  by  five  or  ten  spirally  twisted  tubes  that  give  it  a 
very  peculiar  aspect ;  and  these  enclose  a  central  sac  containing 
protoplasm,  oil,  and  starch  grains.  Each  of  these  tubes  consists,  in 
its  lower  part,  of  a  very  long  unsegmented  cell;  while  at  its  upper 
part  two  small  cells  are  segmented  off;  and  these  small  cells  of  all 
the  tubes  form  together  the  '  crown '  of  the  nucule.  When  ready 
for  fertilisation  the  branches  of  the  crown  part  slightly,  forming  an 
open  passage  or  '  neck  '  down  to  the  central  germ-cell  or  oosphere  ; 
and  through  this  canal  the  antherozoids  make  their  way  down  to 
perform  the  act  of  fertilisation  by  becoming  absorbed  into  the 
substance  of  the  oosphere.  Ultimately  the  nucule,  which  has  now 
become  a  hard  black  body,  falls  off,  and  the  fertilised  germ-cell,  or 
oospore,  gives  origin  to  a  new  plant  after  the  nucule  has  remained 
dormant  through  the  winter.1 

Among  those  simple  Algse  whose  generative  process  consists  in 
the  '  conjugation '  of  two  similar  cells,  there  are  two  groups  of  such 
peculiar  interest  to  the  microscopist  as  to  need  a  special  notice ; 
ihrse  are  the  Desmidiacece  and  the  Diatomacece.  Both  of  them 
were  ranked  by  Ehrenberg  and  some  other  naturalists  as  animal- 
cules ;  but  the  fuller  knowledge  of  their  life-history  and  the  more 
extended  acquaintance  with  the  parallel  histories  of  other  simple 
forms  of  vegetation  which  have  been  gained  during  the  last  twenty 
years,  are  now  generally  accepted  as  decisive  of  their  vegetable 
nature. 

The  Desmidiaceae  2  are  minute  plants  of  a  bright  green  colour 
growing  in  fresh  water  ;  generally  speaking,  the  cells  are  inde- 
pendent of  each  other  (figs.  436—439)  ;  but  sometimes  those  which 

1  A  full  account  of  the  Characece  will  be  found  in  Prof.  Sachs's  Text-Book  of 
Botany,  2nd  English  edition,  p.  292.  Various  observers  have  asserted  that  particles 
of  the  protoplasmic  contents  of  the  cells  of  the  Characece,  when  set  free'  by  the 
rupture  of  their  cells,  may  continue  to  live,  move,  and  grow  as  independent  rhizopods. 
But  the  writer  is  disposed  to  think  that  the  phenomena  thus  represented  are  rather 
to  be  regarded  as  cases  of  parasitism,  the  decaying  cells  of  Nitella  having  been  found 
by  Cienkowski  (Beitrage  zur  Kenntniss  der  Mo)iaden,in  Arch.  f.  Mikr.Anat.  Bd.  i. 
1865,  p.  203)  to  be  inhabited  by  minute,  spindle-shaped,  ciliated  bodies,  which  seem 
to  correspond  with  the  '  spores '  of  the  Myxomycetes,gomg  through  an  amoeboid  stage, 
and  then  producing  a,  plasmode  which,  after  undergoing  a  sort  of  encysting  process, 
finally  breaks  up  into  spindle-shaped  particles  resembling  those  found  in  the  Nitella 
cells. 

-  Our  first  accurate  knowledge  of  this  group  dates  from  the  publication  of  Mr. 
Ralfs's  admirable  monograph  of  the  British  Desmids  in  1848.  Later  information  in 
regard  to  it  will  be  found  in  the  section  contributed  by  Mr.  W.  Archer  to  the  fourth 
edition  of  Pritchard's  Infusoria,  and  in  Cooke's  British  Desmids,  1887. 

p  p  2 


580    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — THALLOPHYTES 

have  been  produced  l>y  binary  subdivision  from  a  single  parent- 
cell  remain  adherent  one  to  another  in  linear  series,  so  as  to  form  a 
filament  (fig.  440;  Plate  IX,  fig.  3).  They  are  distinguished  by  two 
peculiar  features,  one  of  these  being  the  semblance  of  a  division 
of  each  cell  into  two  symmetrical  halves  by  a  '  sutural  line,'  which 
is  sometimes  so  decided  as  to  have  led  to  the  belief  that  the  cell 
is  really  double  (Plate  VIII,  figs.  2,  6),  though  in  other  cases 
it  is  merely  indicated  by  a  slight  notch  ;  the  other  feature  is  the 
frequency  of  projections  from  the  surface,  which  are  sometimes 
short  and  inconspicuous,  but  are  often  elongated  into  spines 
(Plate  VIII,  fig.  6),  presenting  a  very  symmetrical  arrangement. 
These  projections  are  generally  formed  by  the  cellulose  envelope 
alone,  which  possesses  an  almost  horny  consistence,  so  as  to  retain 
its  form  after  the  discharge  of  its  contents  (fig.  436,  B,  D) ;  while,  in 
other  instances,  they  are  formed  by  a  notching  of  the  margin  of 
the  cell  (Plate  IX,  fig.  1),  which  may  affect  only  the  outer  casing,  or 
may  extend  into  the  cell-cavity.  The  outer  coat  is  surrounded  1>\  ,-i 
very  transparent  sheet  of  gelatinous  substance,  which  is  sometimes 
very  distinct  (as  shown  in  fig.  440 ;  Plate  IX,  fig.  6) ;  but  in 
other  cases  its  existence  is  only  indicated  by  its  preventing  the  con- 
tact of  the  cells.  Klebs  states  1  that  in  Desmids,  as  in  the  other 
Conjugates,  this  mucilaginous  sheath  is  composed  of  two  portions — a 
homogeneous  substance  which  is  but  slightly  refringent,  and  a  por- 
tion which  consists  of  minute  rods  at  right  angles  to  the  cell- wall. 
He  regards  the  sheath  as  entirely  independent  of  the  substance  of 
the  cell-wall,  and  as  derived  from  the  protoplasmic  contents  of  the 
cell  by  diffusion  through  the  cell- wall.  The  true  cell- wall  encloses 
a  parietal  utricle,  which  is  not  always  closely  adherent  to  it ;  and 
this  immediately  surrounds  the  endochrome,  which  occupies  nearly 
the  whole  interior  of  the  cell,  and  in  certain  stages  of  its  growth  is 
found  to  contain  starch  granules.  The  endochrome  and  starch 
grains  are  arranged  symmetrically  in  the  two  halves  of  the  cell, 
often  in  very  beautiful  patterns,  such  as  bands  or  stars. 

Many  species  of  desmids  have  a  power  of  slow  movement  in  the 
water,  the  cause  of  which  is  not  obvious,  these  organisms  being 
entirely  destitute  of  vibratile  cilia.  Klebs  2  describes  this  movement 
as  being  of  four  kinds,  viz. : — (1)  a  forward  movement  on  the 
surface,  one  end  of  the  cell  touching  the  bottom,  wliile  the  other  end 
is  more  or  less  elevated,  and  oscillates  backwards  and  forwards ; 
(2)  an  elevation  in  a  direction  vertical  to  the  substratum,  the  free 
end  making  wide  circular  movements;  (3)  a  circular  motion, 
followed  by  an  alternate  sinking  of  the  free  end  and  elevation  of 
the  other  end  ;  and  (4)  an  oblique  elevation  so  that  both  ends 
touch  the  bottom,  lateral  movements  in  this  position,  then  an  ele- 
vation and  circular  motion  of  one  end,  and  a  sinking  again  to  an 
oblique  or  horizontal  position.  Klebs  regards  all  these  movements 
as  due  to  an  exudation  of  mucilage,  and  the  first  two  to  the  forma- 
tion during  the  motions  of  a  filament  of  mucilage  by  which  the 
desmid  is  temporarily  attached  to  the  bottom,  and  which  gradually 

1    Untermiclnnnjai  fins  dem  Bot.  Inst.  Tiibiiiyoi,  1886,  p  33H. 
-  li/ologi.schcfi  Centralblatt,  1885,  p.  ;->r>;->. 


PLATE  IX 


Desmidiaceae. 


VT«st. Newman  chroirio. 


1)ESMII)].\CK.-K 


58l 


lengthens.  Tlie  movements  of  desmids  are  especially  active  when 
they  are  in  tlie  process  of  dividing.  Staid  found  that,' like  the  move- 
ments of  zol.spores,  they  are  affected  by  light,  and  always  move 
towards  the  light. 

A  -cyclosis'  may  1,,.  readily  observed  in  many  Desmidiacw. 
and  is  particularly  obvious  along  the  convex  and  concave  edges  «>f 
the  cell  of  any  vigorous  specimen  of  Closer '»>„,.  with  a  magnifying 
powr  of  2:>0  or  300  diameters  (fig.  436,  A,  B).  By  careful  focus- 
sing the  flow  may  be  seen  in  bro>*l  streams  over  the  whole  surface 
of  the  endochrome;  and  these  streams  detach  and  carry  with  them, 
from  time  to  time,  little  oval  or  globular  bodies  (A,  b)  which  are  put 
forth  from  it,  and  are  carried  by  the  course  of  the  flow  to  the  trans- 
parent spaces  at  the  extremities,  where  they  join  a  crowd  of  similar 
bodies.  In  each  of  these  spar.-*  (IV)  a  protoplasmic  flow  proceeds 
from  the  somewhat  abrupt  termination  of  the  endochrome  towards 
the  obtuse  end  of  the  cell  (as  indicated  by  the  interior  arrows). 


FIG.  43(5. — Cyclosis  in  Closterium  lunula  :  A,  cell  showing  central  separation  at  a, 
in  which  the  large  particles,  b,  are  not  seen ;  B,  one  extremity  enlarged,  showing 
the  movement  of  particles  in  the  colourless  space  ;  D,  cell  in  a  state  of  division. 

and  the  globules  it  contains  are  kept  in  a  sort  of  twisting  movement 
on  the  inner  side  (a)  of  the  parietal  utricle.  Other  currents  are 
seen  apparently  external  to  it,  which  form  three  or  four  distinct 
courses  of  particles,  passing  towards  and  away  from  c  (as  indicated 
by  the  outer  arrows).  Another  curious  movement  is  often  to  be 
witnessed  in  the  interior  of  the  cells  of  members  of  this  family, 
which  has  been  described  as  *  the  swarming  of  the  granules,'  from 
the  extraordinary  resemblance  which  the  mass  of  particles  in  active 
vibratory  motion  bears  to  a  swarm  of  bees.  It  is  especially 
observable  in  the  hyaline  terminal  portions  of  the  cells  of  species  of 
Closteriam,  as  shown  in  fig.  436,  B.  This  motion  continues  for 
some  time  after  the  particles  have  been  expelled  by  pressure  from 
the  interior  of  the  cell  ;  and  it  appears  to  be  an  active  form  of  the 
molecular  movement  common  to  other  minute  particles  freely  sus- 
pended in  fluid.  This  movement  of  minute  particles  affords  an 
instance  of  the  phenomenon  known  as  i  Brownian  movement,'  and 
is  probably  of  a  purely  mechanical  nature. 


582    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — THALLOPHYTES 

When  the  single  cell  has  come  to  its  full  maturity  it  commonly 
multiplies  itself  by  binary  subdivision ;  but  the  plan  011  which  this 
takes  place  is  often  peculiarly  modified,  so  as  to  maintain  the 
symmetry  characteristic  of  the  tribe.  In  a  cell  of  the  simple 
cylindrical  form  of  those  of  Desniidiutn  (fig.  440),  little  more  is 
necessary  than  the  separation  of  the  two  halves  at  the  sutural  line, 
and  the  formation  of  a  partition  between  them  by  the  infolding  of 
the  primordial  utricle  ;  in  this  manner,  out  of  the  lowest  cell  of  the 
filament  A,  a  double  cell,  B,  is  produced.  But  it  will  be  observed 
that  each  of  the  simple  cells  has  a  bifid  wart-like  projection  of  the 
cellulose  wall  on  either  side,  and  that  the  half  of  this  projection, 
which  has  been  appropriated  by  each  of  the  two  new  cells,  is  itself 
becoming  bifid,  though  not  symmetrically;  in  process  of  time,  how 
ever,  the  increased  development  of  the  sides  of  the  cells  which  re- 
main in  contiguity  with  each  other  brings  up  the  smaller  projections 
to  the  dimensions  of  the  larger,  and  the  symmetry  of  the  cells  is 
restored.  In  Closterium  (fig.  436  ;  Plate  IX,  fig.  2)  the  two  halves  of 
the  endochrome  first  retreat  from  one  another  at  the  sutural  line,  and 
a  constriction  takes  place  round  the  cellulose  wall ;  this  constriction 
deepens  until  it  becomes  an  hourglass-like  contraction,  which  pro- 
ceeds until  the  cellulose  wall  entirely  closes  round  the  primordial 
utricle  of  the  two  segments  ;  in  this  state  one  half  commonly  remains 
passive,  whilst  the  other  has  a  motion  from  side  to  side,  which 
gradually  becomes  more  active  ;  and  at  last  one  segment  quits  the 
other  with  a  sort  of  jerk.  At  this  time  a  constriction  is  seen  across 
the  middle  of  the  primordial  utricle  of  each  segment,  indicating  the 
formation  of  the  sutural  band  ;  but  there  is  110  division  of  the  cell- 
cavity,  which  is  that  belonging  to  one  of  the  halves  of  the  original 
entire  cell.  The  cyclosis,  for  some  hours  previously  to  subdivision, 
arid  for  a  few  hours  afterwards,  runs  quite  round  the  obtuse  end.  «., 
of  the  endochrome ;  but  gradually  a  transparent  space  is  formed, 
like  that  at  the  opposite  extremity,  by  the  retreat  of  the  coloured 
layer  ;  whilst  at  the  same  time  its  obtuse  form  becomes  changed  to 
a  more  elongated  and  contracted  shape.  Thus,  in  five  or  six  hours 
after  the  separation,  the  aspect  of  each  extremity  becomes  the  same, 
and  each  half  resembles  the  cell  by  the  division  of  which  it 
originated. 

The  process  is  seen  to  be  performed  after  nearly  the  same  method 
in  Staurastrum,  the  division  taking  place  across  the  central  con- 
striction, and  each  half  gradually  acquiring  the  symmetry  of  the 
original.  In  such  forms  as  Cosmarium,  however,  in  which  the  cell 
consists  of  two  lobes  united  together  by  a  narrowr  isthmus,  the  divi- 
sion takes  place  after  a  different  method  ;  for  when  the  two  halves 
of  the  outer  wall  separate  at  the  sutural  line,  a  semi-globular  protru- 
sion of  the  endochrome  is  put  forth  from  each  half;  these  protru- 
sions are  separated  from  each  other  and  from  the  two  halves  of  the 
original  cell  (which  their  interposition  carries  apart)  by  a  narrow 
neck  ;  and  they  progressively  increase  until  they  assume  the  appear- 
ance of  the  half-segments  of  the  original  cell.  In  this  state,  there- 
fore, the  plant  consists  of  a  row  of  four  segments  lying  end  to  end. 
the  two  old  ones  forming  the  extremes,  and  the  two  new  ones  (which 


DESMIDIACE* 


583 


do  not  usually  acquire  the  full  size  or  the  characteristic  markings  of 
the  original  before  the  division  occurs)  occupying  the  intermediate 
place.  At  last  the  central  fission  becomes  complete,  and  two  bi- 
partite fronds  are  formed,  each  having  one  old  and  one  young  seg- 
ment ;  the  young  segment,  however,  soon  acquires  the  full  size  and 
characteristic  aspect  of  the  old  one ;  and  the  same  process,  the 
whole  of  which  may  take  place  within  twenty-four  hours,  is  repeated 
ere  long.  The  same  general  plan  is  followed  in  Micrasterias ;  but 
as  the  small  hyaline  hemisphere,  put  forth  in  the  first  instance  from 
each  half-cell  (fig.  437,,  A),  enlarges  with  the  flowing  in  of  the  endo- 


D 


4|yj||fcL 


FIG.  437. — Successive  stages  of  binary  subdivision  of  Micrasterias  deMiculata. 

chrome,  it  undergoes  progressive  subdivision  at  its  edges,  first  into 
three  lobes  (B),  then  into  five  (C),  then  into  seven  (D),  then  into 
thirteen  (E).  and  finally  at  the  time  of  its  separation  (F)  acquires 
the  characteristic  notched  outline  of  its  type,  being  only  distinguish- 
able from  the  older  half  by  its  smaller  size.  The  whole  of  this 
process  may  take  place  within  three  hours  and  a  half.  In 
Xphcerozosma  the  cells  thus  produced  remain  connected  in  rows 
within  a  gelatinous  sheath,  like  those  of  Desmidium  (fig.  440)  ; 
and  different  stages  of  the  process  may  commonly  be  observed  in 
the  different  parts  of  any  one  of  the  filaments  thus  formed.  In  any 


584     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — THALLOPHYTES 


such  filament  it  is  obvious  that  the  two  oldest  segments  are  found  at 
its  opposite  extremities,  and  that  each  subdivision  of  the  inter- 
mediate cells  must  carry  them  farther  and  farther  from  each  other. 
This  is  a  very  different  mode  of  increase  from  that  of  the  Confervaceas, 
in  which  commonly  the  terminal  cell  alone  undergoes  subdivision, 
and  is  consequently  the  one  last  formed. 

The  sexual  generative  process  in  the  Desmidiacece,  which  occurs 
but  rarely  compared  with  that  of  binary  division,  always  consists  of 
an  act  of  '  conjugation.'  It  commences  with  the  dehiscence  of  the 
firm  external  envelope  of  each  of  the  conjugating  cells,  so  as  to 
separate  it  into  two  valves  (fig.  438,  0,  D ;  fig.  439,  C).  The 
contents  of  each  cell  thus  set  free  without  any  distinct  investment 
blend  with  those  of  the  other ;  and  a  zygospore  is  formed  by  their 
union,  which  soon  acquires  a  truly  cellulose  envelope.1  This  enve- 
lope is  at  first  very  delicate,  and 
is  filled  with  green  and  granular 
contents  ;  by  degrees  the  envelope 
acquires  increased  thickness,  and 
its  contents  become  brown  or  red. 
Ultimately  the  envelope  becomes 
differentiated  into  three  layers,  of 
which  the  innermost  and  outer- 
most are  colourless,  while  the 
middle  one  is  firmer  and  brown. 
The  outer  surface  is  sometimes 
smooth,  as  in  Closterium  and  its 
allies  (fig.  439  ;  Plate  IX,  fig.  8) ; 
but  in  Cosmarium  it  becomes 
granular,  tuberculated,  or  spiiious 
(fig.  438,  D;  Plate  VIII,  figs.  1, 
4),  the  spines  being  sometimes 
simple  and  sometimes  forked  at 
their  extremities.  The  mode  in 
which  conjugation  takes  place  in 
the  filamentous  species  constitut- 
ing the  Desmidiece  proper  is,  how- 
The  filaments  first  separate  into 
their  component  joints,  and  when  two  cells  approach  in  conjugation, 
the  outer  cell-wall  of  each  splits  or  gapes  at  that  part  which  adjoins 
the  other  cell,  and  a  new  growth  takes  place  which  forms  a  sort  of  con- 
necting-tube that  unites  the  cavities  of  the  two  cells  (fig.  440, 1),  E). 
Through  this  tube  the  entire  endochrome  of  one  cell  passes  over 
into  the  cavity  of  the  other  (D) ;  and  the  two  are  commingled  so  as 
to  form  a  single  mass  (E),  as  is  the  case  in  many  of  the  Conjugate. 
The  joint  which  contains  the  zygospore  can  scarcely  be  distinguished 
at  first  (after  the  separation  of  the  empty  cell),  save  by  the  greater 
density  of  its  contents;  but  the  proper  coats  of  the  zygospore 
gradually  become  more  distinct,  and  the  enveloping  cell-wall  disap- 
pears. 

1  In  certain  species  of  Closterium,  as  in  many  of  the  Diatomacece,  the  act  of 
conjugation  gives  origin  to  tivo  zygospores. 


FIG.  438. — Conjugation  of  Cosmarium 
botrytis:  A,  mature  cell;  B,  empty 
cell-envelope ;  C,  transverse  view  ; 
D,  zygospore  with  empty  cell  enve- 
lopes. 

ever,  in  many  respects  different. 


DESMIDIACEjE 


585 


The  subsequent  history  of  the  zygospore  has  been  followed  out 
in  the  case  of  Cosmariuni  botrytis.  After  remaining  at  rest  for  a 
considerable  time,  it  germinates  by  the  bursting  of  the  two  outer 
coats,  the  protoplasmic  contents  escaping  while  still  enclosed  in  the 
innermost  coat.  In  this  body  the  protoplasm  and  endochrome  are 
already  divided  into  two  halves,  which  contract  somewhat,  and  the 
whole  becomes  enveloped  in  a  new7  cell-wall.  A  constriction  has, 
in  the  meantime,  made  its  appearance  between  the  two  halves, 
which  are  of  somewhat  unequal  size,  and  thus  the  new  desmid  is 
formed.  <*  .„ 

The  subdivision  of  this  family  into  genera,  according  to  the 
method  of  Mr.  Ralfs  ('  British  Desmidiese '),  as  modified  by  Mr. 
Archer  (Pritchard's  '  Infusoria '),  is  based  in  the  first  instance  upon 
the  connection  or  disconnection  of  the  individual  cells,  two  groups 
being  thus  formed,  of  which  one  includes  all  the  genera  whose  cells, 
when  multiplied  by  binary  division,  remain  united  into  an  elongated 
filament ;  whilst  the 
other  and  much  larger 
one  comprehends  all  those 
in  which  the  cells  become 
separated  by  the  comple- 
tion of  the  fission.  The 
further  division  of  the 
filamentous  group,  in 
which  the  zygospores  are 
always  globular  and 
smooth  (Plate  IX,  fig.  8), 
is  based  on  the  fact  that 
in  one  set  of  genera  the 
joints  are  many  times 
longer  than  they  are 
broad,  and  that  they  are 
neither  constricted  nor 
furnished  with  lateral 
teeth  or  projections  ; 
whilst  in  the  other  set 
(fig.  440 ;  Plate  IX,  fig.  3)  the  length  and  breadth  of  each 
joint  are  nearly  equal,  and  the  joints  are  more  or  less  con- 
stricted, or  have  lateral  teeth  or  projecting  angles,  or  some  other 
figure  ;  and  it  is  for  the  most  part  upon  the  variations  in  these  last 
particulars  that  the  generic  characters  are  based.  The  solitary 
group  presents  a  similar  basis  for  primary  division  in  the  marked 
difference  in  the  proportions  of  its  cells,  such  elongated  forms  as 
Closterium  (figs.  436,  439  ;  Plate  IX,  fig.  2),  in  which  the  length  is 
many  times  the  breadth,  being  thus  separated  from  those  in  which, 
as  in  Micrasteria*  (fi$.  437  ;  Plate  IX,  fig.  1),  Cosmarium  (fig.  438  ; 
Plate  VIII,  fig.  2),  and  Staurastrum  (Plate  VIII ;  figs.  5,  6,  10), 
the  breadth  more  nearly  equals  the  length.  In  the  former  the 
zygospores  are  smooth,  whilst  in  the  latter  they  are  very  commonly 
spinous  (Plate  VIII,  figs.  1,  4)  and  are  sometimes  quadrate.  In  this 
group  the  chief  secondary  characters  are  derived  from  the  degree  of 


FIG.  439. — Conjugation  of  Closterium  striolatiun  : 
A,  ordinary  cell ;  B,  empty  cell ;  C,  two  cells  in 
conjugation,  with  zygospore. 


5  86    MICROSCOPIC  FORMS  ( )F  VEGETABLE  LIFE — THALLOPHYTES 


constriction  between  the  two  halves  of  the  cell,  the  division  of  its 
margin  into  segments  by  incisions  more  or  less  deep,  and  its  exten- 
sion into  teeth  or  spines. 

The  Desmldiacece  are  not  found  in  running  streams,  unless  the 
motion  of  the  water  be  very  slow,  but  are  to  be  looked  for  chiefly 
in  standing  waters.  Small  shallow  pools  that  do  not  dry  up  in 
summer,  especially  in  open,  exposed  situations,  such  as  boggy 
moors,  are  most  productive.  The  larger  and  heavier  species 
commonly  lie  at  the  bottom  of  the  pools,  either  spread  out  as  a 

thin  gelatinous  stratum, 
or  collected  into  finger 
like  tufts.  By  gently 
passing  the  fingers  be- 
iieath  these  they  may  be 
caused  to  rise  towards  the 
surface  of  the  water,  and 
may  then  be  lifted  out  by 
a  tin  box  or  scoop.  Other 
species  form  a  slimy 
stratum  floating  on  the 
surface  of  bog- pools,  or  a 
greenish  or  dirty  cloud 
upon  the  stems  and  leaves 
of  other  aquatic  plants ; 
and  these  also  are  best 
detached  by  passing  the 
hand  beneath  them,  and 
'  stripping '  the  plant  be- 
tween the  fingers,  so  as  to 
carry  off  upon  them  what 
adhered  to  it.  If,  on  the 
other  hand,  the  bodies  of 
which  we  are  in  search 
should  be  much  diffused 
through  the  water,  there 
is  no  other  course  than  to 
take  it  up  in  large  quanti- 
FIG.  440.— Binary  subdivision  and  conjugation  of  ties  by  the  box  Or  SCOOp 
Desmidium  cylindricum :  A,  portion  of  filament,  and  to  separate  them  by 
surrounded  by  gelatinous  envelope  :  B,  dividing  et  „«,:„:„„  fVii-nucrli  a  ™'an 
cell;  C,  single  cell  viewed  transversely;  D,  two  stia;inmg  tlllOUgil  a  pie 
cells  in  conjugation ;  E,  formation  of  zygospore.  OI  linen.  At  first,  nothing 

appears  on  the  linen  but 

a  mere  stain  or  a  little  dirt ;  but  by  trie  straining  of  repeated 
quantities  a  considerable  accumulation  may  be  gradually  made. 
This  should  then  be  scraped  off  with  a  knife,  and  transferred 
into  bottles  with  fresh  water.  If  what  has  been  brought  up 
by  hand  be  richly  charged  with  these  forms,  it  should  be  at 
once  deposited  in  a  bottle ;  this  at  first  seems  only  to  contain 
foul  water ;  but  by  allowing  it  to  remain  undisturbed  for  a 
little  time,  the  desmids  will  sink  to  the  bottom,  and  most  of 
the  water  may  then  be  poured  off,  to  be  replaced  by  a  fresh 


DESMIDIACEJE  ;   DIATOMACEJK  587 

supply.  If  the  bottles  be  freely  exposed  to  solar  light,  these  little 
plants  will  flourish,  apparently  as  well  as  in  their  native  pools ;  and 
their  various  phases  of  multiplication  and  reproduction  may  be 
observed  during  successive  months  or  even  years.  If  the  pools  be 
too  deep  for  the  use  of  the  hand  and  the  scoop,  a  collecting- bottle 
attached  to  a  stick  may  be  employed  in  its  stead.  The  ring-net 
may  also  be  advantageously  employed,  especially  if  it  be  so  con- 
structed as  to  allow  of  the  ready  substitution  of  one  piece  of  muslin 
for  another.  For,  by  using  several  pieces  of  previously  wetted 
muslin  in  succession,  a  large  -number  of  these  minute  organisms 
may  be  separated  from  the  water;  the  pieces  of  muslin  may  be 
brought  home  folded  up  in.  wide-mouthed  bottles,  either  separately 
or  several  in  one,  according  as  the  organisms  are  obtained  from  one 
or  from  several  waters  ;  and  they  are  then  to  be  opened  out  in  jars 
of  filtered  river  water  and  exposed  to  the  light,  when  the  desmids 
will  detach  themselves. 

The  Diatomaceae  or  Bacillariaceae,  like  the  Desmidiacea?,  are 
simple  cells,  having  a  firm  external  coating,  within  which  is  included 
an  endochrome  whose  superficial  layer  constitutes  a  '  parietal 
utricle,'  but  their  external  coat  is  consolidated  by  silex,  the  pre- 
sence of  which  is  one  of  the  most  distinctive  characters  of  the 
group,  and  gives  rise  to  the  peculiar  surface-markings  of  its  members. 
It  has  been  thought  by  some  that  the  solidifying  mineral  forms  a 
distinct  layer  exuded  from  the  exterior  of  the  cellulose  wall ;  but 
there  seems  good  reason  for  regarding  that  wall  as  itself  inter- 
penetrated by  the  silex,  since  a  membrane  bearing  the  characteristic 
surface-markings  is  found  to  remain  after  its  removal  by  hydro- 
fluoric acid.  The  endochrome  of  diatoms  consists,  as  in  other 
plants,  of  a  viscid  protoplasm,  in  which  float  the  granules  of 
colouring  matter.  In  the  ordinary  condition  of  the  cell  these 
granules  are  diffused  through  it  with  tolerable  uniformity,  except 
in  the  central  spot,  which  is  occupied  by  a  nucleus  ;  round  this 
nucleus  they  commonly  form  a  ring,  from  which  radiating  lines  of 
granules  may  be  seen  to  diverge  into  the  cell-cavity.  Instead  of 
being  bright  green,  however,  the  endochrome  is  a  yellowish  brown. 
The  principal  colouring  substance  appears  to  be  a  modification  of 
ordinary  chlorophyll ;  it  takes  a  green  or  greenish-blue  tint  with 
sulphuric  acid,  and  often  assumes  this  hue  in  drying ;  but  with  it  is 
combined  in  greater  or  less  proportion  a  yellow  colouring  matter 
termed  diatomin,  which  is  very  unstable  in  the  light  and  fades  in 
drying.  At  certain  times,  oil-globules  are  observable  in  the 
protoplasm ;  these  seem  to  represent  the  starch-granules  of  the 
Desmidiacece  and  the  oil-globules  of  other  protophytes.  A  distinct 
movement  of  the  granular  particles  of  the  endochrome,  closely 
resembling  the  cyclosis  of  the  Desmidiacece,  has  been  noticed  by 
Professor  \V.  Smith  in  some  of  the  larger  species  of  Diatomacece, 
such  as  Stirirella  biseriata,  Xitzschia  scalaris,  and  Campylodiscus 
spiralis,  and  by  Professor  Max  Schultze  in  Coscinodiscus,  Biddulphia, 
and  Rhizosolenia ••  but  this  movement  has  not  the  regularity  so 
remarkable  in  the  preceding  group. 

The  name  of  the  class  is  derived  from  the  ease  with  which  the 


5 88    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — TH ALLOPHYTES 

parts  separate  from  eacli  other.  This  is  well  seen  in  the  genus 
Diatoma,  formed  of  rectangular  individual  fr  list  ales,  where  the 
arrangement  resulting  from  the  principle  of  lateral  union  causes 
them  to  develop  into  filaments  or  zigzag  chains,  the  frustules  remain- 
ing perfectly  distinct,  and  united  only  by  a  small  isthmus  or  cushion 
at  the  angles.  A  similar  cohesion  at  the  angles  is  seen  in  the  allied 
genus  Grammatophora  (fig.  452),  in  Isihmia  (fig.  457),  and  in  inanv 
other  diatoms ;  in  Biddulphia  (fig.  445)  there  even  seems  to  be  a 
special  organ  of  attachment  at  these  points.  In  some  diatoms, 
however,  the  frustules  produced  by  successive  acts  of  binary  subdi- 
vision habitually  remain  coherent  one  to  another,  and  thus  are  pro- 
duced, filaments  or  clusters  of  various  shapes.  Thus  it  is  obvious 
that  when  each  frustule  is  a  short  cylinder,  an  aggregation  of  such 
cylinders,  end  to  end,  must  form  a  rounded  filament,  as  in  Meloxi r« 
(fig.  444)  ;  and,  whatever  may  be  the  form  of  the  sides  of  the 
frustules,  if  they  be  parallel  one  to  the  other  a  straight  filament 
will  be  produced,  as  in  Achnanthes  (fig.  461).  But  if,  instead  of 
being  parallel,  the  sides  be  somewhat  inclined  towards  each  other, 
a  curved  band  will  be  the  result;  this  may  not  continue  entire, 
but  may  so  divide  itself  as  to  form  fan-shaped  expansions,  as  those 
of  Licmophora  flabellata  (fig.  450) ;  or  the  cohesion  may  be  sufficient 
to  occasion  the  band  to  wind  itself  (as  it  were)  round  a  central  axis, 
and  thus  to  form,  not  merely  a  complete  circle,  but  a  spiral  of  several 
turns,  as  in  Meridian  circulare  (fig.  448).  Many  diatoms,  again, 
possess  a  stipe,  or  stalk-like  appendage,  by  which  aggregations  of 
frustules  are  attached  to  other  plants,  or  to  stones,  pieces  of  wood. 
Ac. ;  and  this  may  be  a  simple  foot-like  appendage,  as  in  Achnanthes 
longipes  (fig.  461),  or  it  may  be  a  composite  plant-like  structure,  as 
in  Licmophora  (fig.  450),  Gomphonema  (fig.  462),  and  Mastogloia 
(fig.  465).  Little  is  known  respecting  the  nature  of  this  stipe  ;  it 
is,  however,  quite  flexible,  and  may  be  conceived  to  be  an  extension 
of  the  cellulose  coat,  unconsolidated  by  silex,  analogous  to  the 
prolongations  which  have  been  seen  in  the  Deamidiacece,  and  to  the 
filaments  which  sometimes  connect  the  cells  of  the  PalntettacecR. 
Some  diatoms,  again,  have  a  mucous  or  gelatinous  investment,  which 
may  even  be  so  substantial  that  their  frustules  lie — as  it  were — in  a 
bed  of  it,  as  in  Mastogloia  (figs.  465  B,  466),  or  may  form  a  sort  of 
tubular  sheath  to  them,  as  in  Schizonema  (fig.  464).  In  a  large 
proportion  of  the  group,  however,  the  frustules  are  always  met  with 
entirely  free,  neither  remaining  in  the  least  degree  coherent  one  to 
another  after  the  process  of  binary  subdivision  has  once  been  com- 
pleted, nor  being  in  any  way  connected,  either  by  a  stipe,  or  by  a 
gelatinous  investment.  This  is  the  case,  for  example,  with  Tricera- 
tium  (fig.  442),  Pleurosigma  (Plate  I,  figs.  1,  2),  Actinocyclus, 
Actinoptychus  (fig.  467),  Arachnoidiscus  (Plate  XII),  Campy lodiscu* 
(fig.  454),,SWm^a  (fig.  453),  Coscinodiscus  (Plate  I,  figs.  3,  4, fig.  455), 
Heliopelta,  and  many  others.  The  solitary  discoidal  forms,  however, 
when  obtained  in  their  living  state,  are  commonly  found  cohering 
to  the  surface  of  aquatic  plants. 

We  have  now  to  examine  more  minutely  into  the  curious  struc- 
ture   of  the    silicified  casing  which    encloses  every  diatom-cell  or 


DIATOMACEJE 


589 


frustule  and  the  presence  of  which  imparts  a  peculiar  interest  to 
the  group;  not  merely  011  account  of  the  elaborately  mar  keel  pattern 
which  it  often  exhibits,  but  also  through  the  perpetuation  of  the 
minutest  details  of  that  pattern  in  the  specimens  obtained  from 
fossilised  deposits.  This  silicified  casing  is  usually  formed  of  two 
perfectly  symmetrical  valves  united  to  one  another  by  means  of  two 
embracing  rings  which  constitute  the  connecting  zone  or  girdle,  and 
thus  exactly  represent  a  minute  box  which  serves  for  the  reproduc- 
tion of  the  'species.  This  process  is  known  as  the  encystmeiit,  and 
is  not  uncommon,  especially  amongst  the  Xariculett'.  frustules  being 
frequently  found  amongst  them  o*pen  from  the  separation  of  the  t\vo 
valves,  showing  the  two  rings  covering  each  other,  as  the  lid  of  a 
box  may  cover  a  portion  of  the  box  itself. 

The  following  definitions  of  terms  used  in  describing  the  siliceous 
envelope  of  diatoms  have  been  proposed  by  the  late  eminent  diato- 
mologist,  Mi'.  J.  Deby.  The  radiating  lines  (called  by  some  '  cost* ' 
or  '  caiialiculi ')  starting  from  the  outer  margin  of  the  valve,  and 
converging  towards  the  interior  of  the  disc,  are  rays  or  marginal 
rays.  They  may  be  simple,  which  is  most  usual ;  or  moniliform, 
i.e.  composed  of  a  single  or  double  row  of '  beads  ; '  or  infundibuliform, 
having  the  outline  of  a  funnel  with  a  long  outlet ;  the  upper  broad 
portion  is  the  *  funnel/  the  slender  part  the  '  stem.'  The  central 
portion  of  the  valve  inside  the  internal  termination  of  the  rays  is 
the  area-  it  may  be  smooth  and  hyaline,  or  it  may  be  striate,  or 
simply  punctate  or  dotted,  the  dots  forming  regular  lines  or  else 
being  irregularly  scattered.  If  this  area  becomes  reduced  to  a 
median  linear  blank  space,  or  to  a  simple  elongated  line,  it  is  known 
as  tin-  rft/thfi  or  psetido-raphe. 

Dr.  O.  Miiller  proposes  the  term  epitheca  for  the  overlapping 
half-cell  of  the  diatom,  the  under-lapping  half-cell  being  the  hypo- 
theca  ;  for  the  girdle- bands  he  proposes  the  term  pleura . 

In  describing  diatoms,  the  aspect  in  which  the  girdle  is  turned 
towards  the  observer  is  known  as  the  'front'  or  'girdle'  view ;  that 
in  which  the  surface  of  the  valve  is  turned  towards  the  observer  is 
the  'side  '  or  '  valve '  view. 

It  is  not  correct  to  designate  the  line  shown  in  the  front  view 
of  the  outer  ring  as  the  line  of  '  suture,'  since  the  suture  is  the  line 
of  meeting  bounding  two  surfaces  placed  on  the  same  plane.  The 
form  resulting,  however,  varies  widely  in  different  diatoms  ;  for 
sometimes  each  valve  is  hemispherical,  so  that  the  cavity  is  globular  ; 
sometimes  it  is  a  smaller  segment  of  a  sphere  resembling  a  watch- 
glass,  so  that  the  cavity  is  lenticular  ;  sometimes  the  central  portion 
is  completely  flattened  and  the  sides  abruptly  turned  up,  so  that  the 
valve  resembles  the  cover  of  a  pill-box,  in  which  case  the  cavity  will 
l>e  cylindrical;  and  these  and  other  varieties  may  co-exist  with  any 
modifications  of  the  contour  of  the  valves,  which  may  be  square, 
triangular  (fig.  442),  heart-shaped  (fig.  454,  A)  ,boat-shaped  (fig.  453, 
A),  or  very  much  elongated  (fig.  449),  and  may  be  furnished  (though 
this  is  rare  among  diatoms)  with  projecting  outgrowths  (figs.  458, 
45D).  Hence  the  shape  presented  by  the  frustule  differs  completely 
with  the  aspect  under  which  it  is  seen.  In  all  instances,  the 


5QO    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE — THALLOPHYTES 

frustule  is  considered  to  present  its  '  front '  view  when  its  line  of 
meeting  is  turned  towards  the  eye,  as  in  fig.  453,  B,  C ;  whilst  its 
'  side  '  view  is  seen  when  the  centre  of  either  valve  is  directly 
beneath  the  eye  (A).  Although  the  two  valves  meet  along  the  line 
of  junction  in  those  newly  formed  frustules  which  have  been  just 
produced  by  binary  subdivision  (as  shown  in  fig.  445,  A,  e),  yet,  as 
soon  as  they  begin  to  undergo  any  increase,  the  valves  separate  from 
one  another  ;  and  by  the  silicification  of  the  cell-membrane  thus  left 
exposed  a  pair  of  hoops  is  formed,  each  of  which  is  attached  by  one 
edge  to  the  adjacent  valve,  while  the  other  edge  is  free.1  As  will 
be  presently  explained,  one  of  the  valves  is  always  older  than  the 
other  ;  and  the  hoop  of  the  older  valve  partly  encloses  that  of  the 
younger,  just  as  the  cover  of  a  pill-box  surrounds  the  upper  part  of 
the  box  itself.2  As  the  newly  formed  cell  increases  in  length, 
separating  the  valves  from  one  another,  both  hoops  increase  in 
breadth  by  additions  to  their  free  edges,  and  the  outer  hoop  slides 
off  the  inner  one,  until  there  is  often  but  a  very  small  '  overlap.' 
As  growth  and  binary  division  are  continually  going  on  when  the 
frustules  are  in  a  healthy  vigorous  condition,  it  is  rare  to  find  a 
specimen  in  which  the  valves  are  not  in  some  degree  separated  by 
the  interposition  of  the  hoops. 

The  impermeability  of  the  silicified  casing  seems  to  render  neces- 
sary the  existence  of  special  apertures  through  which  the  surrounding 
water  may  come  into  communication  with  the  contents  of  the  cell. 
Some  have  believed  that  they  have  seen  such  apertures  along  the 
so-called  '  line  of  suture '  of  the  disc-shaped  diatoms,  and  at  the  extre- 
mities only  of  the  elongated  forms.  Ehrenberg,  followed  by  KUtzing, 
has  interpreted  as  apertures  or  ostioles  the  central  and  terminal 
nodules  of  the  Navicalecv,  Cymbellew,  and  similar  forms;  but  this 
view  is  more  generally  regarded  as  incorrect.  We  have,  in  fact,  no 
positive  demonstration  of  the  existence  of  special  apertures  communi- 
cating between  the  outside  and  the  inside  of  the  cell ;  and  we  are  com- 
pelled to  have  recourse,  on  this  point,  to  hypothesis.  It  is,  however, 
certain  that  the  diatom-cell  is  always  composed  of  at  least  two  valves, 
between  which  the  possibility  of  such  a,  communication  must 
necessarily  be  admitted,  or  at  least  the  existence  of  endosinotic  and 
exosmotic  currents  in  the  liquids.  In  the  encysted  forms  we  have 
ascertained  also  the  existence  of  an  interval  between  the  two  rings, 
although  it  may  be  very  minute ;  while  Navicula  has  been  some- 
times seen  with  the  valves  actually  separated. 

1  [This  refers  to  those  diatoms  in  which  the  process  of  binary  subdivision  is 
possible ;  but  this,  as  will  be  seen  presently,  is  not  the  case  in  many  genera.— ED.] 

-  This  was  long  since  pointed  out  by  Dr.  Wallich  in  his  important  memoir  on  the 
'Development  and  Structure  of  the  Diatom- valve '  (Transact,  of  Microsc.  Soc.  n.s. 
vol.  viii.  1860,  p.  129)  ;  but  his  observation  seems  not  to  have  attracted  the  notice  of 
diatomists,  until  in  1877  he  called  attention  to  it  in  a  more  explicit  manner  (Monthly 
Microsc.  Journ.  vol.  xvii.  p.  61).  The  correctness  of  his  statement  has  been  coii- 
iirmed  by  the  distinguished  American  diatomist,  Prof.  W.  Hamilton  Smith  ;  but  as  it 
has  been  called  in  question  by  Mr.  J.  D.  Cox  (American  Journal  of  Microscopy, 
vol.  iii.  1878,  p.  100),  who  asserts  that  in  Isthmia  there  are  three  hoops— two 
attached  to  the  two  valves,  and  the  third  overlapping  them  both  at  their  line  of 
junction — the  Author  has  himself  made  a  very  careful  examination  of  a  large  series 
of  specimens  of  Isthmia  and  Biddu-lphia,  the  result  of  which  has  fully  satisfied  him 
of  the  correctness  of  Dr.  Wallich's  original  description. 


DIATOMACEJ2 


The  nature  of  the  delicate  markings  with  which  almost  every 
diatom  frustule  is  beset  has  been  one  of  the  most  interesting  in- 
quiries of  the  students  of  these  forms  since  the  introduction  of  the 
homogeneous,  and  especially  the  apochromatic,  objectives;  and  it 
cannot  be  doubted  that  certain  peculiarities  of  structure  have  been 
demonstrated  which  were  never  before  seen.  In  the  present  state 
of  the  theory  and  practice  of  microscopy  it  would  be  extremely 
unwise  to  give  absolute  adhesion  to  any  present  interpretation  of 
what  is  now  held  by  some  students  of  diatom  structure  of  no  mean 
repute  and  of  unrivalled  manipulative  skill  to  be  the  absolute  struc- 
ture of  some  of  the  larger  forms!*' 

Thus,  concerning  the  group  Coscinodiscece,  representing  the  most 
beautiful  of  the  discoid  forms  of  the  whole  group  of  Diatomacece,  we 
represent  in  Plate  I,  fig.  3,  a  photo-micrographic  image  of  Costino- 
discus  aster  omphal  us  magnified  110  diameters.  But  in  fig.  441 
the  areolce  of  this  diatom  are 
seen  under  great  magnification 
with  recent  powers.  It  is 
contended  that  the  diatom, 
although  consisting  of  a  single 
siliceous  membrane,  has  a 
double  structure,  viz.  coarse 
and  fine  areolations,  the  latter 
within  the  former  ;  and  there 
appears  little  reason  to  doubt 
this.  The  coarse  areolations 
are  for  the  most  part  circular 
in  outline,  and  the  intervening 
silex  is  thick.  Inside  these 
areolations  is  an  extremely 
delicate  perforated  membrane, 
the  outer  row  of  whose  perfora- 
tions are  larger  than  the  rest. 
From  the  very  delicacy  of  this 
membrane,  and  its  consequent 
easy  fracture,  it  is  often  want- 
ing. In  Plate  I,  fig.  4,  we  present  a  photo-micrograph  of  the  same 
object  magnified  2,000  diameters. 

In  Isthmia  nervosa,  a  side  and  front  view  of  which  are  seen  in 
fig.  457,  a  similar  construction  is  discoverable.  In  this  diatom  the 
coarse  areolations  are  very  large  and  the  silex  correspondingly  thick  ; 
but  the  inner  membrane  is  excessively  thin  and  delicate.  The  per- 
forations are  large  and  irregular  in  shape  around  the  margin,  but 
small  and  circular  in  the  centre.  In  fig.  443  the  form  of  areola- 
tions is  shown,  and  a  broken  membrane  seen,  with  the  fracture 
passing  through  the  perforations.1 

Xot  less  interesting  is  the  beautiful  form  Aidcicodiscus  Kltton'd; 
a  photo-micrograph  of  this  magnified  270  diameters  is  seen  in  Plate 
I,  fig.  5 ;  while  a  small  portion  of  the  centre  of  a  kindred  form, 

1  Note  on  the  finer  structure  of  certain  diatoms,  E.  M.  Nelson  and  G.  C.  Karop, 
Junrn.  Quekett  Club,  vol.  ii.  ser.  ii.  p.  269. 


FIG.  441.  -Magnification,  of  '  ultimate  struc- 
ture '  of  Coscinodiscus  aster  omphalus, 
from  a  drawing  by  Messrs.  Nelson  and 
Karop  (Jour)i.  Quekett  Club,  vol.  ii. 
ser.  ii.  p.  269). 


592    MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

. I.  titurtii,  magnified  2,000  times,  is  shown  in  fig.  6  in.  the   same 
plate. 

The  '  beaded '  appearance  of  diatom-Valves  is  so  universal  in  all 
those  which  have  been  examined,  that  it  must  be  regarded  as 
common  to  all  diatoms,  although  this  is  not  yet  absolutely  proved. 
But,  while  it  is  admitted  that  the  beading  of  the  valves  may  be 
common  to  all  diatoms,  it  cannot  be  regarded  as  proved  that  the 
siliceous  envelope  is  composed  of  globular  particles  of  silex  arranged 


FIG.  442. —Triccratiumfavus:  A,  side  view  ;  B,  front  view. 

in  regular  rows ;  while  the  variety  in  the  size  and  arrangement  of 
these  particles  shows  that  they  are  correlated  with  the  vital  pro- 
cesses of  the  organisms,  and  afford  characters  for  the  discrimination 
of  the  species.  The  nature  of  these  granules,  their  size,  and  the 
mode  in  which  they  are  arranged  have  from  the  earlier  days  of  micro- 
scopy rendered  diatoms  of  special  value  as  *  test- objects.'  This 
appearance  has  led  to  the  use,  in  speaking  of  diatoms,  of  the  incorrect 
terms  '  transverse,'  '  longitudinal,'  or  '  oblique  stri^,'  these  being  in 
truth  simply  the  intervals  which  separate 
the  boundaries  of  the  '  beads/  apertures,  or 
their  equivalents,  whatever  they  may  ulti- 
mately prove  to  be ;  and  this  is  clearly  seen 
when  they  are  observed  with  objectives  of 
sufficient  numerical  aperture  and  propor- 
tional power.  Pleurosigma  angulatum  is 
one  of  the  most  commonly  employed  test 
objects,  and  at  the  same  time  one  of  the  most 
reliable,  its  remarkable  constancy  rendering 
it  especially  valuable  for  this  purpose; 
while,  on  the  contrary,  Amphipleura  pdlucida 
is  extremely  variable,  and  is,  as  it  were,  the 
torment  of  microscope-makers  and  rival 
diatoin-resolvers,  who  do  not  take  into  account  the  variability  of 
this  type,  forgetting,  in  fact,  that  one  A.pellucida  maybe  extremely 
fine,  and  another,  being  in  truth  a  varietal  form,  may  be  nearly  as 
coarse  as  Navicula  rhomboides.  The  new  apochromatic  objectives, 
and  the  compensating  eye-pieces,  both  for  the  eye  and  for  projection, 
constructed  by  Zeiss,  of  Jena,  have  brought  about  such  progress  in 
micrography  that  the  image  of  P.  anyulatum  appears  to  some  minds 


PIG.  443. — Areolations  in 
Isthmia  >iervosa. 


PLATE     X. 


PLEUROSIGMA   ANGULATUM. 

(Magnified  4900  diams. 

From  a  Pnoto-Micrograph  by  Dr.  R.  Zeiss  taken  with  the  2  m/m.  Apochromatic  Objective 
N.  A.  1.30  and  projection  eyepiece  4. 


rollotjpe  Ptg.  Co.,  282  High  Holborn,  W.C. 


DIATOMACE^E  593 

to  leave  no  doubt  as  to  the  details  of  its  structure.  If  we  closely 
examine  the  photographic  image  of  a  portion  of  P.  angulatum,  pro- 
duced under  a  magnification  of  4,300  diameters,  and  shown  in  Plate 
X,  taken  from  a  photograph  by  Dr.  Zeiss,  it  will,  in  the  majority  of 
cases,  leave  perhaps  little  doubt  that  the  valves  are  covered  by  the 
beads  or  apertures  in  a  decussate  arrangement.  We  have,  in  the 
judgment  of  Count  Castracane,  to  do  here  with  '  beads '  and  not  with 
'  cavities.'  But,  from  the  recent  advances  of  our  knowledge,  this  by  no 
means  follows;  they  may  with  high  probability  be  considered  per- 
forations in  the  silex  of  the  frus^ule.  This  is,  indeed,  placed  almost 
in  the  form  of  a  demonstration 'by  the  interesting  fact  that  Mr.  C. 
Haughton  G  ill  succeeded  in  filling  up  the  '  dots '  or  *  pearls  '  of  the 
^Yaviculce  and  the  secondarv  markings  of  the  discoid  and  other  forms, 
so  as  to  give  evidence  that  the  filling  must  be  deposited  in  cavities. 
It  is  done  by  soaking  clean  diatoms  in  a  solution  of  subnitrate  of 
mercury  until  their  markings  are  filled  with  it ;  then  they  are 
immersed  in  sulphide  of  ammonium  ;  a  double  decomposition  takes 
place,  by  which  black  insoluble  sulphide  of  mercury  is  produced,  and 
left  in  the  minute  cavities  in  which  it  certainly  appears  to  be  formed. 
By  observing  the  lines  of  fracture,  which  always  follow  the  interval 
between  two  rows  of  '  beads,'  there  will  be  much  suggestion  given  to 
the  observer  on  this  subject.  Count  Castracane,  referring  to  Plate 
X,  asked,  '  Would  it  have  been  possible  to  have  seen  these  pearl-like 
objects  isolated,  if,  instead  of  beads,  we  had  had  apertures  or  depres- 
sions ? '  We  can  only  reply  that  misinterpretation  on  such  a  subject 
is  so  possible  that  it  is  only  by  employing  all  the  aids  to  interpretation 
which  ingenuity  can  place  within  our  reach,  that  we  can  ever  be  certain 
as  to  our  visual  interpretation  of  these  minute  phenomena.  On  the 
other  hand,  the  areolated  valves  of  Triceratiumfaviis  (fig.  442)  present 
a  line  of  fracture  which  traverses  indifferently  the  hexagonal  areolee 
and  the  lines  in  relief  which  connect  them. 

Dr.  Yan  Heurck  has  been  able  to  employ  the  new  lens  made  by 
Abbe,  having  a  numerical  aperture  of  1'63,  upon  his  special  subject, 
the  Diatomacece.  He  concludes  that  diatom  valves  consist  of  two 
membranes  or  thin  films  and  of  an  intermediate  layer,  the  latter 
being  pierced  with  openings.  The  outer  membrane  is  delicate,  and 
may  be  easily  destroyed  by  acids,  friction,  and  the  several  processes 
of '  cleaning.'  When  the  openings  or  apertures  of  this  interior  portion 
are  arranged  in  alternate  rows  they  assume  the  hexagonal  form  ; 
when  in  straight  rows  then  the  openings  are  square  or  oblong. 

It  is,  however,  due  to  Mr.  T.  F.  Smith,  who  worked  at  this 
subject  for  years,  to  say  that  he  long  maintained  this  view,  and 
has  presented  skilful  photo-micrographs  in  support  of  his  contention. 
[11  Plate  I,  fig.  1,  we  have  a  photograph  of  his,  showing  the  inside 
of  a  valve  of  P.  angulatum  magnified  1,750  diameters,  and  ex- 
hibiting the  ;  postage-stamp '  fracture  ;  while  in  fig.  2,  in  the  same 
plate,  we  have  the  outside  of  P.  angulatum,  showing  a  different 
structure ;  and  Mr.  Smith  has  abundant  evidence  of  the  existence 
of  what  he  has  so  long  maintained. 

By  using  the  new  lens  of  the  great  aperture  of  1*63,  Dr.  Van 
Heurck  has  produced  some  remarkable  photo-micrographs,  which 

Q  Q 


594     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


rather  confirm  these  general  inferences  than  present  any  new  data 
of  knowledge  concerning  the  diatoms.  By  his  great  courtesy  we 
have  been  favoured  with  a  phototype  plate  prepared  by  Dr.  Yan 
Heurck  from  his  own  photo -micrographs,  and  the  reader  will  be 
enabled  to  study  these  in  Plate  XI,  of  which  a  full  description  is  given 
in  the  earlier  part  of  this  treatise,  giving  descriptions  of  the  plates. 
He  has  further  enhanced  the  plate  by  giving  in  fig.  7  a  photo-micro- 
graph of  Robert's  nineteenth  band. 

Diatoms,  like  other  organisms  already  described,  are  reproduced 
by  conjugation,  and  multiply  by  autofission  or  division.     Repro- 
duction is  necessary  to   every  organism,   while    multiplication   by 
fission  belongs  only  to  certain  organic  types.     In  the  early  days  of 
the  study  of  diatoms,  it  would  appear  that  even  that  distinguished 
observer  William  Smith  had  at  least  not  a  clear  idea  of  the  encyst- 
ing   of   the  frustule  or 
individual  diatom,  which 
implies  the  existence  of 
the  two  valves   and   of 
the    double    girdle     or 
zone  or  connecting  ring 
projecting     from     each 
valve  in  a  direction  at 
right  angles  to  its  plane. 
Hence,  instead  of  find- 
ing, as  a  result  of  fission, 
a    progressive    diminu- 
tion of  the  diameter  of 
the  frustules,  Mr.  Smith 
speaks  of  their  increase, 
of  which  he  is  unable  to 
offer    any    explanation. 
The  fact  that  in  Afelosira 
siibflexilis  (fig.  444,  A) 
and     M.    varians     (fig. 
Melosird  varians.  444,  B)  large  and  small 
.  frustules  are  seen  united 
in  rows,  ought  to  be  sufficient  to  show  that  they  are  dependent  not 
only  on  binary  subdivision,  but  also  011  the  special  conditions  of 
evolution   of  the  new    frustule,    by    which    it  is  able    to    increase 
materially  in  size.     This  power  of  diatoms  to  expand  their  siliceous 
coatings  has  therefore  been  denied  by  some,  who  are  induced  to 
maintain  this    necessary   consequence  of  the  division   of  encysted 
frustules,    viz.    the    progressive    decrease    in    size    of    the    young 
frustules,  which  would  thus  reach  the  smallest  possible  dimensions. 
This  has  led  Pfitzer  L  to  imagine  that  when  diatoms  have  reached 
their   smallest    possible    dimensions    by    repeated    binary   division, 
the  process   of  conjugation   takes   place   between   them,  resulting 
in    the    formation    of  an    auxospore,    capable    of  reproducing   two 
sporangial  frustules  of  considerably  larger-  size,  which  would  again 
give    rise,    by  fission,  to    a    new    series   of  diminishing    frustules, 
1  Untersuchungen  iiber  Bau  it..  Entwickelung  der  Bacillarien,  8vo.  Bonn,  1871. 


A  FIG.  444. 

Melosira  siibflexilis. 


PLATE     XI, 


Fig.  1. 


Fig.  3. 


Dr.  H.  Van 


Collotype  Ptg,  Co.,  282  High  Holborn,  VV.C, 

TEST   OBJECTS    FOR   THE    MICROSCOPE. 

Objective  by  C.  Zeiss,  N.A.  roo;  Eyepiscs  12.     Monochromatic:  illumination  by  sunlight. 


DIATOMACE^E 


595 


until  these  again  reach  their  minimum  size.  This  theory  ha.s. 
in  the  judgment  of  Count  Castracane,  deceived  many  botanists, 
from  the  idea  that  it  was  founded  on  actual  observation,  and  has 
at  the  same  time  been  in  harmony  with  the  natural  tendency  to 
generalisation,  in  attributing  to  the  whole  family  of  diatoms  that 
faculty  of  division  which  has  been  regarded  as  the  universal  property 
of  the  vegetable  cell.  The  '  auxospore '  theory  rests  on  the  supposed 
inability  of  the  siliceous  walls  of  diatoms  to  expand  ;  and  implies, 
secondly,  the  idea  that  all  diatoms  are  capable  of  binary  sub- 
division ;  and  thirdly,  that  there,>is  no  mode  of  reproduction  except 
by  auxospores.  That  the  siliceous'walls  of  diatoms  are  capable  of 
distension  seems  to  result  from  the  examples  already  given  of  Melosira 
subflexilis  and  M.  varians,  as  also  from  some  other  species  in  which 
there  may  often  be  observed  a  sudden  variation  in  diameter  in  frus- 
tules  united  together  in  a  row.  But  the  power  of  increase  in  size  of 
the  siliceous  diatom-cell  is  evidently  proved  by  the  sporangial  frus- 
tules  ofOrthosira  Dickiei,1  where,  in  the  chain  of  cylindrical  frus- 
tules  of  the  same  diameter,  the  sporangial  frustule  is  dilated  in  its 
equatorial  axis,  but  much  more  so  in  its  polar  axis,  pushing  back  the 
base  of  the  next  cell  and  forcing  it  to  fold  itself  up  so  as  to  occupy 
the  whole  cell-cavity,  and  sometimes  even  that  of  the  next  frustule. 
The  exactness  and  fidelity  of  the  figure  given  in  Smith's  '  Synopsis,' 
besides  being  guaranteed  by  the  authority  of  the  distinguished  author 
and  by  the  signature  of  the  celebrated  artist  Tuffen  West,  Count 
Castracane  was  able  to  confirm  by  a  magnificent  preparation 
of  these  diatoms  in  which  are  a  number  of  sporangial  frustules. 
The  auxospore  theory  supposes  the  fact  that  all  diatoms  are  capable 
of  binary  subdivision,  since  the  auxospore  is  understood,  according 
to  Pfitzer,  to  provide  for  the  progressive  decrease  in  size  of  the 
frustules,  with  the  production  of  larger  sporangial  frustules,  destined 
to  commence  a  new  descending  series.  But  binary  subdivision  cannot 
take  place  in  genera  with  unequal  valves,  as  it  is  universally 
acknowledged  that  the  two  new  valves  which  are  formed  in  the 
process  of  binary  subdivision  must  stereotype  themselves  on  the  old 
valves  ;  and  for  this  reason  the  process  cannot  take  place  in  those 
genera  in  which  the  axes  cross  one  another,  like  Campylodiscus,  or 
in  those  in  which  the  two  valves,  although  equal,  yet  constantly 
unite  in  such  a  way  that  the  similar  parts  alternate  with  one 
another,  as  may  be  seen  in  Asterolampra.  That  it  is  impossible  for 
binary  subdivision  to  take  place  in  these  three  classes  of  forms,  is 
confirmed  by  the  fact  that,  notwithstanding  that  there  are  recorded 
not  less  than  seventy-five  observations  of  the  process  of  division  in 
them,  not  one  affords  an  exception  to  the  rule  given  above. 

Where  multiplication  by  binary  subdivision  occurs  among  the 
Diatomacece,  it  takes  place  on  the  same  general  plan  as  in  the  Des- 
niidiacece,  but  with  some  modifications  incident  to  peculiarities  of 
the  structure  of  the  former  group.  The  first  stage  consists  in  the 
elongation  of  the  cell,  and  the  formation  of  a  '  hoop '  adherent  to 

1  See  Castracane,  '  The  Theory  of  the  Reproduction  of  Diatoms,'  AUi  deW  Accad. 
Pontif.  dei  Nuovi  Lincei,  May  31,  1874 ;  and  '  New  Arguments  to  prove  that 
Diatoms  are  reproduced  by  means  of  Germs,'  ibid.  March  19,  1876, 


596     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE—  THALLOPHYTES 


each  end-valve,  so  that  the  two  valves  are  separated  by  a  band, 
which  progressively  increases  in  breadth  by  addition  to  the  free 
edges  of  the  hoops,  as  is  well  seen  in  fig.  445,  A..  In  the  newly 
formed  cell  e,  the  two  valves  are  in  immediate  apposition  ;  in  d  a 
band  intervenes  ;  in  a  this  band  has  become  much  wider  ;  and  in  b 
the  increase  has  gone  on  until  the  original  form  of  the  cell  is  com- 
pletely changed.  At  the  same  time  the  endochrome  separates  into 
two  halves  ;  the  nucleus  also  subdivides  in  the  manner  formerly  shown 
(fig.  417,  G,  H,  I)  ;  and  the  parietal  utricle  folds  in,  first  forming 
a  mere  constriction,  then  an  hour-glass  contraction^  and  finally  a 
complete  double  partition,  as  in  other  instances.  From  each  of  its 

adjacent  surfaces  a  new 
siliceous  valve  is  formed,  as 
shown  at  fig.  445,  A,  C,  just 
as  a  new  cellulose  wall  is 
generated  in  the  subdivision 
of  other  cells  ;  and  this  valve 
is  usually  the  exact  counter- 
part of  the  one  to  which  it 
is  opposed,  and  forms  with 
it  a  complete  cell,  so  that 
the  original  frustule  is  re- 
placed by  two  frustules,  each 
of  which  has  one  old  and  one 
new  valve,  just  as  in  Desmi- 
diacece.  Generally  speaking, 
the  new  valves  are  a  little 
smaller  than  their  prede- 
cessors ;  so  that,  after  re- 
peated subdivisions  (as  in 
chains  of  Isthmia),  a  diminu- 
tion of  diameter  becomes 
obvious.1  But  sometimes 
the  new  valves  are  a  little 
PIG.  U5.—Biddulphia  pulchella  :  A,  chain  of  larger  than  their  predeces- 
cells  in  different  states  :  a,  full  size  ;  6,  elon-  gors  ;  SO  that,  in  the  fila- 
gation  preparatory  to  subdivision  ;  c,  forma-  ^p^™,-  <,npm-p«  fhprp  TYI^V 
tioii  of  two  new  cells  ;  d,  e,  young  cells  ;  B,  ment°Us  Species,  tne  >  may 
end  view  ;  C,  side  view  of  a  cell  more  highly  be  an  increase  sufficient  to 
magnified.  occasion  a  gradual  widening 

of    the    filament,    although 

not  perceptible  except  when  two  continuous  frustules  are  com- 
pared ;  whilst,  in  the  free  forms,  frustules  of  different  sizes  may 
be  met  with,  of  which  the  larger  are  more  numerous  than  the 
smaller,  the  increase  in  number  having  taken  place  in  geometrical 
progression,  whilst  that  of  size  was  uniform.  It  is  not  always  clear 
what  becomes  of  the  hoop.  In  Melosira  (fig.  444,  A  and  B),  and 
perhaps  in  the  filamentous  species  generally,  the  hoops  appear  to 
keep  the  new  frustules  united  together  for  some  time.  This  is  at 
first  the  case  also  in  Biddulpkia  and  Isthmia  (fig.  457),  in  which  the 

1  This  could  not  be  explained  on  the  hypothesis  of  the  rigidity  of  the  walls  within 
which  fission  takes  place. 


PLATE   XII 


ARACHNOJDISCUS  JAPONICUS. 


DIATOMACE.E  597 

continued  connection  of  the  two  frustules  by  its  means  gives  rise  to 
an  appearance  of  two  complete  frustules  having  been  developed 
within  the  original  (fig.  445,  A,  C)  ;  subsequently,  however,  the  two 
new  frustules  slip  out  of  the  hoop,  which  then  becomes  completely 
detached.  The  same  thing  happens  with  many  other  diatoms,  so 
that  the  hoops  are  to  be  found  in  large  numbers  in  the  settlings  of 
water  in  which  these  plants  have  long  been  growing. 

But  in  some  other  cases  all  trace  of  the  hoop  is  lost,  so  that  it 
may  be  questioned  whether  it  has  ever  been  properly  silicified,  and 
whether  it  does  not  become  fused  (as  it  were)  into  the  gelatinous 
envelope.  During  the  healthy  life  of  the  diatom  l  the  process  of 
binary  division  is  continually  feeing  repeated  ;  and  a  very  rapid 
multiplication  of  frustules  thus  takes  place,  all  of  which  must  be 
considered  to  be  repetitions  of  one  and  the  same  individual  form. 
Hence  it  may  happen  that  myriads  of  frustules  may  be  found  in  one 
locality,  uniformly  distinguished  by  some  peculiarity  of  form,  size, 
or  marking,  which  may  yet  have  had  the  same  remote  origin  as 
another  collection  of  frustules  found  in  some  different  locality,  and 
alike  distinguished  by  some  peculiarity  of  its  own.  For  there  is 
strong  reason  to  believe  that  such  differences  spring  up  among  the 
progeny  of  any  true  generative  act,  and  that  when  that  progeny  is 
dispersed  by  currents  into  different  localities,  each  will  continue  to 
multiply  its  own  special  type  so  long  as  the  process  of  binary  division 
goes  on. 

We  have  seen  that  division  is  of  the  nature  of  multiplication, 
and  not  of  reproduction  ;  and  that,  where  it  does  take  place,  it  must 
be  regarded  as  the  exception,  and  not  as  the  rule.  As  respects 
reproduction,  Count  Castracane,  who  was  an  observer  during 
thirty  years  devoted  to  the  study  of  diatoms,  had  the  opportunity 
of  noting  in  what  way  the  process  differs  in  particular  cases. 
He  contended  that  he  had  been  able  to  see  in  a  Podosphenia  the 
emission  of  gonids  or  sporules  or  embryonal  forms,  in  the  same  way 
in  which  Rabenhorst  saw  it  in  Melosira  varians,  and  O'Meara  in 
Pleurosigma  Spencerii ;  and  in  another  case  there  were  seen  a 
number  of  oval  cysts  of  a  species  of  Navicula  easily  recognisable. 
The  greater  number  of  these  were  in  a  quiescent  state ;  but  some 
few  were  seen  in  motion  by  means  of  two  flagelliform  cilia ;  so  that 
these  larger  or  smaller  cysts  represented  zygospores,  and  some  of 
them  were  shown  to  be  zoozygospores.  Castracane  had  the  good 
fortune  to  meet  with  a  number  of  large  and  small  oval  cysts 
imbedded  in  a  gelatinous  mass,  all  of  them  having  in  the  centre  two 
similar  corpuscles.  From  the  condition  of  two  greenish  oblong 
indistinct  forms,  these  went  on,  by  an  easy  transition,  to  manifest 
themselves  as  naviculoid  types,  and  at  length  developed  into  full- 
grown  frustules  of  Mastogloia.  All  this  proved,  in  his  judgment, 
how  reproduction  in  diatoms  may  present  itself  in  different  forms 
and  with  different  peculiarities ;  for  which  reason  one  ought  to 
avoid  arguing  from  special  cases  to  general  laws.  The  only  thing 
which  can  be  asserted  of  all  cases  of  reproduction,  is  that  it  must 
be  preceded  by  conjugation,  which  results  in  the  fertilisation  of  the 

1  This  refers  to  those  diatoms  in  which  binary  subdivision  can  take  place. 


598     MICEOSCOPIC  FOEMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

sporules  or  gonids,  which,  after  a  period  of  repose  or  of  incubation 
inclosed  within  a  cyst,  or  within  a  membranous  frond,  or  within  n 
frustule,  attain  a  condition  for  living  an  independent  life  and 
reproducing  in  every  respect  the  adult  type  of  the  mother-cell ;  thus 
the  cyst,  the  membranous  frond,  or  the  frustule,  performs  the 
function  of  a  sporaiige.  Castracane  was  of  opinion  that  these  gonids 
or  embryonal  forms  could  have  no  traces  of  silex  in  their  cell- walls, 
scarcely  yet  formed,  until  a  few  years  ago,1  among  the  diatoms 
of  a  marine  deposit  of  the  Miocene  period,  he  met  with  a  perfect 
frustule  of  Coscinodiscus  punctatus,  which,  between  the  two  planes 
of  the  valves,  and  therefore  within  the  cell,  exhibited  some  round 
marks  which  admitted  of  no  other  interpretation  except  that  of 
impressions  or  traces  of  the  embryonal  forms  surprised  by  death 
while  still  attached  to  the  mother-cell.  More  recently  he  met  with 
other  cases  identical  in  character,  so  that  he  has  no  longer  any 
doubt  as  to  the  presence  of  silex  in  the  cell -walls  of  diatoms  which 
have  not  yet  emerged  to  the  light.  , 

The  formation  of  *  endocysts'  within  the  frustule  of  diatoms  lias 
also  been  observed  by  Comber,  Murray,  and  others. 

No  one  appears  at  present  to  have  given  attention  to  a  circum- 
stance described  by  Castracane  2  in  relation  to  a  specimen  of  Striatella 
unipunctata,  which  has  passed  thousands  of  times  under  the  eyes  of 
all,  without  its  significance  being  recognised.  The  diatoms  which 
we  have  most  frequently  under  our  observation  do  not  ahvjivs 
exhibit  the  same  arrangement  of  their  endochrome.  The  attempt 
has,  indeed,  been  made  to  found  the  classification  of  diatoms  on  the 
arrangement  of  the  endochrome,  according  as  it  is  present  in  the 
form  of  plates  or  of  granules ;  thus  distinguishing  the  placochromatic 
and  the  coccochromatic  forms ;  but  a  difficulty  is  presented  in  the 
way  of  this  classification  by  certain  types  which  sometimes  belong  to 
the  one,  sometimes  to  the  other  class.  And  this  cannot  be  the 
result  of  accident.  Such  variations  might  occur  in  some  diatoms 
as  the  result  of  special  biological  conditions  of  the  individual. 
There  may  frequently  be  seen,  for  example,  a  specimen  of  Melosira 
varians  with  its  cell-cavity  filled  with  endochrome,  not  in  a  condition 
of  unequal  amorphous  masses,  but  of  uniform  rounded  corpuscles ; 
and  this  demands  particular  attention,  or  at  least  gives  good  ground 
for  special  research.  A  diligent  examination  instituted  in  these 
cases  has  demonstrated  the  existence  in  them  of  a  special  organi- 
sation ;  and  the  determination  of  a  narrow  and  well-defined  limit  of 
outline  seems  to  prove  that  these  were  perfectly  distinct  and 
independent  of  one  another.  From  the  perfect  resemblance  of 
these  to  the  gonids  and  embryonal  forms  seen  to  escape  from  the 
mother -cell  by  Rabenhorst,  O'Meara,  and  Castracane,  he  concludes 
that  this  special  arrangement  of  the  endochrome  must  be  interpreted 
as  a  prelude  to  the  process  of  reproduction. 

These  observations  may  possibly  attract  the  attention  of  some 

1  See  '  Observations  on  a  Fossil  Diatom  in  relation  to  the  Process  of  Beproduc- 
tion,'  Atti  deW  Accad.  Pontif.  del  Nuovi  Lincei,  May  17,  1885. 

2  See  'The  Diatoms  of  the  Coasts  of  Istria  and  Dalmatia,'  Atti  dell'  Accad. 
Pontif.  dei  Nuovi  Lincei,  April  27  and  May  25,  1873. 


DIATOMACE^E  599 

who  are  applying  themselves  to  the  study  of  diatoms  to  so  important 
MII  argument,  on  which  may  depend  the  possibility  of  establishing  a 
really  good  classification  of  diatoms  which  will  at  length  satisfy 
diatomists.  At  present  preference  is  generally  accorded  to  the 
classification  proposed  by  H.  L.  Smith,  which  establishes  the  class  of 
Raphidece  from  the  presence  of  a  raphe  in  the  plane  of  the  valves. 
If  there  is,  on  the  valves,  in  place  of  the  raphe,  a  simple  line  of 
division,  the  forms  thus  characterised  are  termed  Pseudoraphidece ; 
while  those  in  which  the  valves  have  neither  raphe  nor  its  equivalent 
are  called  Cryptoraphidece,  or,  better,  Anaraphidece.  While,  there- 
fore, in  the  present  state  of  our>knowledge  of  diatoms,  any  classifica- 
tion can  only  be  regarded  as  provisional,  we  do  not  propose  any 
innovation  on  this  point,  although  we  are  disposed  to  accord  our 
preference  to  that  suggested  by  H.  L.  Smith. 

Conjugation,  so  far  as  is  at  present  known,  takes  place  among, 
the  ordinary  Diatomacece  almost  exactly  as  among  the  De&midiacece, 
except  that  it  sometimes  results  in  the  production  of  two  '  zygo- 
spores '  instead  of  a  single  one.  Thus  in  Surirella  (fig.  453),  the 
valves  of  two  free  and  adjacent  frustules  separate  from  each  other, 
and  the  two  endochromes  (probably  included  in  their  parietal 
utricles)  are  discharged ;  these  coalesce  to  form  a  single  mass, 
which  becomes  enclosed  in  a  gelatinous  envelope,  and  in  due  time 
this  zygospore  shapes  itself  into  a  frustule  resembling  that  of  its 
parent,  but  of  larger  size.  But  in  Epithemia  (fig.  446,  A,  B),  the 
first  diatom  in  which  the  conjugating  process  was  observed  by 
Mr.  Thwaites,1  the  endochrome  of  each  of  the  conjugating  frustules 
(C,  D)  appears  to  divide  at  the  time  of  its  discharge  into  two  halves  ; 
each  half  coalesces  with  half  of  the  other  endochrome  ;  and  thus 
two  zygospores  (E.  F)  are  formed,  which,  as  in  the  preceding  case, 
become  invested  with  a  gelatinous  envelope,  and  gradually  assume 
the  form  and  markings  of  the  parent  frustules,  but  grow  to  a  very 
much  larger  size,  the  sporangial  masses  having  obviously  a  power  of 
self-increase  up  to  the  time  when  their  envelopes  are  consolidated. 
It  seems  to  be  in  this  way  that  the  normal  size  is  recovered,  after 
the  progressive  diminution  which  is  incident  to  repeated  binary 
multiplication.  Of  the  subsequent  history  of  the  zygospores  much 
remains  to  be  learnt ;  and  it  may  not  be  the  same  in  all  cases. 
Appearances  have  been  seen  which  make  it  almost  certain  that  the 
contents  of  each  zygospore  break  up  into  a  brood  of  gonids,  and 
that  it  is  from  these  that  the  new  generation  originates.  These 
gonids,  if  each  be  surrounded  (as  in  many  other  cases)  by  a  distinct 
cyst,  may  remain  undeveloped  for  a  considerable  period ;  and  they 
must  augment  considerably  in  size  before  they  obtain  the  dimensions 
of  the  parent  frustule.  It  is  in  this  stage  of  the  process  that  the 
modifying  influence  of  external  agencies  is  most  likely  to  exert  its 
effects  ;  and  it  may  be  easily  conceived  that  (as  in  higher  plants  and 
animals)  this  influence  may  give  rise  to  various  diversities  among 
the  respective  individuals  of  the  same  brood  ;  which  diversities,  as 
we  have  seen,  will  be  transmitted  to  all  the  repetitions  of  each 

1  See  Annals  of  Natural  History,  vol.  xx.  ser.  i.  1847,  pp.  9,  343  and  vol.  i. 
ser.  ii.  1848,  p.  161. 


600     MICROSCOPIC  FOEMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


that  are  produced  by  the  process  of  binary  division.  Hence  a  very 
considerable  latitude  is  to  be  allowed  to  the  limits  of  species,  when  the 
different  forms  of  Diatomacece  are  compared  ;  and  here,  as  in  many 
other  cases,  a  most  important  question  arises  as  to  what  are  those 
limits — a  question  which  can  only  be  answered  by  such  a  careful 
study  of  the  entire  life-history  of  every  single  type  as  may  advan- 
tageously occupy  the  attention  of  many  a  microscopist  who  is  at 
present  devoting  himself  to  the  resolution  of  the  markings  on 
diatom- valves,  and  to  the  multiplication  of  reputed  species  by  the 
detection  of  minute  differences.1 

This  formation  of  what  are  termed  auxospores — as   serving   to 

augment  the  size  of  the 
cells  which  are  to  give 
origin  to  a  new  genera- 
tion— takes  place  on  a 
very  different  plan  in 
some  of  those  filamentous 
types,  such  as  Melosira 
(fig.  444,  A,  B),  in  which 
a  strange  inequality 
presents  itself  in  the 
diameters  of  the  differ- 
ent cells  of  the  same 
filament,  the  larger  ones 
being  usually  in  various 
-stages  of  binary  sub- 
division, by  which  they 
multiply  themselves 
longitudinally.  Accord- 
ing to  the  observations 
of  Mr.  Thwaites  (loc. 
FIG.  446.— Conjugation  of  Epithemia  turgida :  A,  cit.),  these  also  are  the 
front  view  of  single  frustule ;  B,  side  view  of  the  v  ^f  ^  •  j  f 

same;  C,  two  frustules  with  their  concave  surfaces  Products  Ol  a  kind  of 
in  close  apposition ;  D,  front  view  of  one  of  the  conjugation  between  the 
wing  the  separation  of  its  valves  E,  adjacent  cells  of  the  or- 
dinary diameter,  taking 
place  before  the  comple- 
tion of  their  separation.  He  describes  the  endochrome  of  particular 
frustules,  after  separating  as  if  for  the  formation  of  a  pair  of  new  cells, 
as  moving  back  from  the  extremities  towards  the  centre,  rapidly 
increasing  in  quantity  and  aggregating  into  a  zygospore  (fig.  447, 
No.  2,  a,  5,  c) :  around  this  a  new  envelope  is  developed,  which  may  or 
may  not  resemble  that  of  the  ordinary  frustules,  but  which  remains 
in  continuity  with  them  ;  and  this  zygospore  soon  undergoes  binary 

1  See  on  this  subject  a  valuable  paper  by  Prof.  W.  Smith  '  On  the  Determination 
of  Species  in  the  Diatomacece ','  in  the  Quart.  Journ.  of  Microsc.  Science,  vol.  iii. 
1855,  p.  130 ;  a  memoir  by  Prof.  W.  Gregory  '  On  Shape  of  Outline  as  a  Specific 
Character  of  Diatomacece,'  in  Trans,  of  Microsc.  Soc.  2nd  series,  vol.  iii.  1855, 
p.  10 ;  and  the  Author's  Presidential  Address,  in  the  same  volume,  pp.  44-50 ;  '  On 
Navicula  crassinervis,  Frustulia  saxonica,  and  N.  rhomboides,  as  Test-objects,'  by 
W.  H.  Dallinger,  Monthly  Micro.  Journ.  1876,  vol.  xvii.  p.  1 ;  also  an  Additional 
note  on  the  identity  of  these,  by  the  same  Author,  ibid.  p.  173. 


frustules,  si 

F,  side  and  front  views  after  the  formation  of  the 

zygospores. 


DIATOMACKJ-: 


6oi 


subdivision  (No.  3,  a,  b,  c),  the  cells  of  the  new  series  thus  developed 
presenting  the  character  of  those  of  the  original  filament  (1),  but 
greatly  exceeding  them  in  size.  From  what  has  been  already  stated, 
it  seems  probable  that  a  gradual  reversion  to  the  smaller  form  takes 
place  in  subsequent  subdivisions,  a  further  reduction  being  checked 
by  a  new  formation  of  zygospores.  The  various  modes  of  formation 
of  auxospores  in  the  Diatomacese  are  classified  by  Klebahn  under 
five  different  heads,  viz. : — (1)  Rejuvenescence  of  a  single  cell,  accom- 
panied by  an  increase  in  size ;  this  is  the  simplest  type,  and  one 
of  the  most  common.  (2)  Two  daughter-cells  are  produced  from  the 
protoplasm  of  a  mother-cell,  and  from  these  arise  two  auxospores 
(Achnanthes  longipes,  Rhabdommcf  arcuatum).  (3)  Two  cells  lying 
side  by  side  cast  off  their  old  valves,  and  each  grows  into  an 
auxospore,  without  any  previous  fusion,  or  any  visible  interchange 
of  contents  ;  this  is  the  commonest  type  of  all.  (4)  A  true  conjuga- 
tion takes  place  ;  the  protoplasmic  contents  of  the  two  cells  fuse 


FIG.  447. — Self -conjugation  (?)  of  Melosira  italica  (Aulacosira  crenulata 
Thwaites) :  1,  simple  filament ;  2,  filament  developing  auxospores  ;  a,  &,  c,  succes- 
sive stages  in  the  formation  of  auxospores  ;  auxospore-frustules  in  successive  stages, 
a,  b,  c,  of  multiplication. 

together  into  one,  and  this  mass  grows  into  an  auxospore.  (5)  Before 
conjugation,  the  protoplasm  of  each  of  the  two  cells  divides  before- 
hand into  two  daughter-cells,  and  two  auxospores  are  formed  by 
the  fusion  of  a  daughter-cell  from  each  mother-cell  with  the 
daughter-cell  of  the  other  one  lying  opposite  to  it ;  this  is  the  most 
complicated  process  (Amphora  ovalis,  Epithemia  Argus,  Rhopa- 
lodia  gibba,  &c.). 

The  most  curious  phenomenon  presented  by  diatoms  is  un- 
doubtedly their  power  of  movement,  which  induced  Ehrenberg  and 
the  other  early  observers  of  these  organisms  to  place  them  erro- 
neously in  the  animal  kingdom,  although  it  affords  no  evidence  of 
consciousness.  This  power  of  movement,  if  not  common  to  all 
diatoms,  is  very  evident  in  those  species  which  are  normally  or 
accidentally  free,  and  most  conspicuously  in  oblong  forms,  such  as  the 
species  of  Navicula.  In  those  also  which  are  stalked  it  has  been 
noticed  that  if,  from  any  cause,  a  frustule  becomes  detached,  it  is 


602     MICROSCOPIC  POEMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

endowed  with  a  motion  similar  to  that  of  the  species  which  are 
normally  free.  This  circumstance  has  caused  the  abandonment 
of  Mr.  W.  Smith's  proposal  to  assign  a  generic  value  to  the  condition 
in  which  the  frustule  is  possessed  of  this  property  without  regard  to  its 
form .  Hence  those  genera  are  not  now  generally  recognised  which  differ 
only  in  being  enclosed  in  a  membranous  frond,  or  in  being  stalked, 
especially  since  frustules  contained  in  a  sheath,  for  example  in  Schizo- 
nema,1  have  been  seen  to  escape  from  it,  and  to  be  prevented  from 
returning  again  to  it  in  company  with  the  sister  JVaviculce.  Hence 
the  genera  Schizotiema,  Berkeleya,  and  Dickiea  must  be  reunited  to 
Nawcula ;  Cocconema,  Endonema,  and  Colletonema  to  Cymbella  ;  and 
Hotneocladia  to  Nitzschia.  The  singular  phenomenon  of  movement 
which  may  be  observed  in  many  genera  of  diatoms — among  which 
the  most  singular  is  that  presented  by  Bacillaria parudoxa  (fig.  449), 
in  which  the  rod-like  frustules  are  seen  to  be  continually  gliding  one 
along  another,  in  a  retrograde  direction,  before  they  become  detached 
— is  found  to  be  in  general  a  movement  backwards  and  forwards  in  a 
straight  line  so  far  as  they  meet  with  no  impediment,  while  the 
intervention  of  obstacles  determines  a  passive  change  of  direction. 
The  backward  and  forward  movements  of  the  Naviculce  have  been 
already  described  ;  in  Sicrirella  (fig.  453)  and  Campylodiscus 
(fig.  454)  the  motion  never  proceeds  further  than  a  languid  roll  from 
one  side  to  the  other  ;  and  in  Gomphonema  (fig.  463),  in  which  a 
foramen  fulfilling  the  nutritive  office  is  found  at  the  larger  extremity 
only,  the  movement  (which  is  only  seen  when  the  frustule  is  separated 
from  its  stipe)  is  a  hardly  perceptible  advance  in  intermitted  jerks 
in  the  direction  of  the  narrow  end.  The  cause  of  this  movement  is 
uncertain.  It  has  been  referred  by  different  authors  to  the  action 
of  endosmose  and  exosmose  ;  to  cilia ;  to  the  projection  of  pseudopode- 
like  masses  of  protoplasm  through  orifices  in  the  raphe,  or  of  a  single 
elongated  protoplasmic  thread  ;  but  the  most  probable  interpretation 
attributes  it  to  the  action  of  the  changes  resulting  from  the  nutrition 
of  the  cell,  which  must  necessarily  absorb  food  in  a  liquid  condition. 
Taking  account,  therefore,  of  the  relatively  considerable  quantity  of 
silex  necessary  to  the  organisation  of  the  diatom  cell  in  proportion 
to  its  minute  dimensions,  and  bearing  in  mind,  at  the  same  time, 
the  incalculably  small  traces  of  silex  in  solution  in  the  water,  it 
may  be  understood  how  active  must  be  the  exchange  from  the 
exterior  to  the  interior  of  the  cell,  and  vice  versa,  and  hence  how 
such  an  exchange  must  determine  a  continual  change  of  position 
backwards  and  forwards,  through  the  reaction  exercised  on  the 
delicate  floating  frustules. 

The  principles  upon  which  this  interesting  group  should  be  classi- 
fied cannot  be  properly  determined  until  the  history  of  the  genera- 
tive process — of  which  nothing  whatever  is  yet  known  in  a  large 
proportion  of  diatoms,  and  but  little  in  any  of  them — shall  have 
been  thoroughly  followed  out.  The  observations  of  Focke  2  render  it 

1  See  Castracane,  '  Observations  on  the  Genera  Homeocladia  and  Schizonevia,' 
in  Atti  dell'  Accad.  Pontif.  del  Nuovi  Lincei,  May  23,  1880. 

2  According  to  this  observer  (Ann.  of  Nat.  Hist.  2nd  series,  vol.  xv.  1855,  p.  237) 
Navicula  bifrons  forms,  by  the  spontaneous  fission  of  its  internal  substance,  spherical 
bodies,  which,  like  gemmules,  give  rise  to  Surirella  microcora.     These  by  conjuga- 


DIATOM  A  CE.K 


603 


highly  probable  that  many  of  the  forms  at  present  considered  as  dis- 
tinct from  each  other  would  prove  to  be  but  different  states  of  the 
same  if  their  whole  history  were  ascertained.  On  the  other  hand, 
it  is  by  no  means  impossible  that  some  which  appear  to  be  nearly 
related  in  the  structure  of  their  frustules  and  in  their^mode  of 
growth  may  prove  to  have  quite  different  modes  of  reproduction. 
At  present,  therefore,  any  classification  must  be  merely  provisional ; 
and  in  the  notice  now  to  be  taken  of  some  of  the  most  interesting 
forms  of  the  Diatomacece,  the  method  of  Professor  Kiitzing,  which 
is  based  upon  the  characters  of  the  individual  frustules,  is  followed, 
in  preference  to  that  of  Mr.  W.  ^Smith,  which  was  founded  on 
the  degree  of  connection  remaining  between  the  several  frustules 
after  binary  division.1  In  each  family  the  frustules  may  exist  under 
four  conditions  : — (a)  free,  the  binary  division  being  entire,  so  that  the 
frustules  separate  as  soon  as  the  process  has  been  completed ;  (b) 
stipitate,  the  frustules  being  implanted  upon  a  common  stem  (fig. 


FIG.  44£. — Meridian  circulare. 


FIG.  449. — Bacillaria  paradoxa. 


450),  which  keeps  them  in  mutual  connection  after  they  have  them- 
selves undergone  a  complete  binary  division  ;  (c)  united  in  a  filament, 
which  will  be  continuous  (fig.  445,  A,  B)  if  the  cohesion  extend  to 
the  entire  surfaces  of  the  sides  of  the  frustules,  but  may  be  a  mere 
zigzag  chain  (fig.  451)  if  the  cohesion  be  limited  to  their  angles; 
(d)  aggregated  into  a  frond  (fig.  464),  which  consists  of  numerous 
frustules  more  or  less  regularly  enclosed  in  a  gelatinous  investment. 
Commencing  with  the  last-named  division  (A),  the  first  family 

tion  produce  ^V.  splendida,  which  gives  rise  to  N.  bifrons  by  the  same  process.  He  is 
only  able  to  speak  positively,  however,  as  to  the  production  of  N.  bifrons  from  N. 
splendida;  that  of  Surirella  microcora  from  N.  bifrons,  and  that  of  N.  splendida 
from  Surirella  microcora,  being  matters  of  inference  from  the  phenomena  witnessed 
by  him. 

1  The  method  of  Kiitzing  was  the  one  followed,  with  some  modification,  by  Mr. 
Ralfs  in  his  revision  of  the  group  for  the  fourth  edition  of  Pritchard's  Infusoria ; 
and  to  his  systematic  arrangement  the  Author  would  refer  such  as  desire  more 
detailed  information. 


604     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


is  that  of  Eunotiece,  of  which  we  have  already  seen  a  characteristic 
example  in  Epithemia  turgida  (fig.  446).  The  essential  characters 
of  this  family  consist  in  the  more  or  less  lunate  form  of  the  frustules 
in  the  lateral  view  (fig.  446,  B),  and  in  the  strise  being  continuous 
across  the  valves  without  any  interruption  by  a  longitudinal  line. 
In  the  genus  Eunotia.  the  frustules  are  free  ;  in  Epithemia  they  are 
very  commonly  adherent  by  the  fiat  or  concave  surface  of  the  con- 
necting zone  ;  and  in  Himantidium  they  are  usually  united  into 
ribbon-like  filaments.  In  the  family  Meridiece  we  find  a  similar 
union  of  the  transversely  striated  individual  frustules  ;  but  these  are 
narrower  at  one  end  than  at  the  other,  so  as  to  have  a  cuneate  or 
wedge-like  form,  and  are  regularly  disposed  with  their  corresponding 
extremities  always  pointing  in  the  same  direction,  so  that  the  fila- 
ment is  curved  instead  of 
straight,  as  in  the  beauti- 
ful Meridian  circulare  (fig. 
448).  Although  this  plant, 
when  gathered  and  placed 
under  the  microscope,  pre- 
sents the  appearance  of 
circles  overlying  one  an- 
other, it  really  grows  in  a 
helicoid  (screw  like)  form, 
making  several  continuous 
turns.  This  diatom  abounds 
in  many  localities  in  this 
country  ;  but  there  is  none 
in  which  it  presents  itself 
in  such  rich  luxuriance  as 
in  the  mountain-brooks 
about  West  Point  in  the 
United  States,  the  bottoms 
of  which,  according  to  Pro- 
fessor Bailey,  '  are  literally 
covered  in  the  first  warm 
days  of  spring  with  a  fer- 
ruginous-coloured mucous 
matter,  about  a  quarter  of 
an  inch  thick,  which,  on  examination  by  the  microscope,  proves  to 
be  filled  with  millions  and  millions  of  these  exquisitely  beautiful 
siliceous  bodies.  Every  submerged  stone,  twig,  and  spear  of  grass  is 
enveloped  by  them,  and  the  waving  plume-like  appearance  of  a  fila- 
mentous body  covered  in  this  way  is  often  very  elegant.'  The  frus- 
tules of  Meridian  are  attached  when  young  to  a  gelatinous  cushion  ; 
but  this  disappears  with  the  advance  of  age.  In  the  family  Licmo- 
pharece  also  the  frustules  are  wedge-shaped  ;  in  some  genera  they  have 
transverse  markings,  whilst  in  others  these  are  deficient ;  but  in 
most  instances  there  are  to  be  observed  two  longitudinal  suture-like 
lines  on  each  valve  (which  have  received  the  special  designation  of 
vittee)  connecting  their  two  extremities.  The  newly  formed  part  of 
the  stipe  in  the  genus  Licmophora,  instead  of  itself  becoming  double 


FIG.  450. — Licmophora  flabellata. 


DIATOMACE^] 


605 


with  each  act  of  binary  division  of  the  frustule,  increases  in  breadth, 
while  the  frustules  themselves  remain  coherent,  so  that  a  beautiful 
fan-like  arrangement  is  produced  (fig.  450).  A  splitting  away  of  a 
few  frustules  seems  occasionally  to  take  place,  from  one  side  or  the 
other,  before  the  elongation  of  the  stipe ;  so  that  the  entire  plant 
presents  us  with  a  more  or  less  complete  flabetta  or  fan  upon  the 
summit  of  the  branches,  with  imperfect  flabella?  or  single  frustules 
irregularly  scattered  throughout  the  entire  length  of  the  footstalk. 
This  beautiful  plant  is  marine,  and  is  attached  to  seaweeds  and 
zoophytes. 

In  the  next  family,  that  of  Frdgilariece,  the  frustules  are  of  the 
same  breadth  at  each  end,  so  that  if  they  unite  into  a  filament  they 
form  a  straight  band.  In 
some  genera  they  are 
smooth,  in  others  trans- 
versely striated,  with  a 
central  nodule  ;  when  striae 
are  present,  they  run  across 
the  valves  without  inter- 
ruption. To  this  family 
belongs  the  genus  Diatoma, 
which  gives  its  name  to  the 
entire  group,  that  name 
(which  means  cutting 
through)  being  suggested 
by  the  curious  habit  of  the 
genus,  in  which  the  frus- 
tules, after  division,  sepa- 
rate from  each  other  along 
their  lines  of  junction,  but 
remain  connected  at  their 
angles,  so  as  to  form  zigzag 
chains  (fig.  451).  The 


451.  FIG.  452. 

FIG.  451. — Diatoma  vulgare :  a,  side  view  of 
frustule;  b,  frustule  undergoing  division, 

FIG.  452. — Grammatophoraserpentina:  a,  front 
and  side  views  of  single  frustule ;  6,  b,  front 
and  end  views  of  divided  frustule  ;  c.  frustule 
about  to  undergo  division;  d,  frustule  com- 
pletely divided. 


valves  of  Diatoma,  when 
turned  sideways  (a),  are 
seen  to  be  strongly  marked 
by  transverse  striae,  which 
extend  into  the  front  view. 
The  proportion  between  the 
length  and  the  breadth  of 

each  valve  is  found  to  vary  so  considerably  that,  if  the  extreme  forms 
only  were  compared,  there  would  seem  adequate  ground  for  regarding 
them  as  belonging  to  different  species.  The  genus  inhabits  fresh 
water,  preferring  gently  running  streams,  in  which  it  is  sometimes 
very  abundant.  The  genus  Fragilaria  is  nearly  allied  to  Diatoma,  the 
difference  between  them  consisting  chiefly  in  the  mode  of  adhesion  of 
the  frustules,  which  in  Fragilaria  form  long,  straight  filaments  with 
parallel  sides ;  the  filaments,  however,  as  the  name  of  the  genus 
implies,  very  readily  break  up  into  their  component  frustules,  often 
separating  at  the  slightest  touch.  Its  various  species  are  very 
common  in  pools  and  ditches.  This  family  is  connected  with  the 


606     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


neit  by  the  genus  Nitzschia,  which  is  a  somewhat  aberrant  form,  dis- 
tinguished by  the  presence  of  a  prominent  keel  on  each  valve,  divid- 
ing it  into  two  portions  which  are  usually  unequal,  while  the  entire 
valve  is  sometimes  curved,  as  in  N.  sigmoidea,  which  has  been  used 
as  a  test-object,  but  is  not  suitable  for  that  purpose  on  account  of 
the  extreme  variability  of  its  striation.  Nearly  allied  to  this  is 
the  genus  Bacillaria,  so  named  from  the  elongated  staff-like  form  of 
its  frustules  ;  its  valves  have  a  longitudinal  punctated  keel,  and 
their  transverse  striie  are  interrupted  in  the  median  line.  The 
principal  species  of  this  genus  is  the  B.  paradoxa,  whose  remarkable 
movement  has  been  already  described.  Owing  to  this  displacement 
of  the  frustules,  its  filaments  seldom  present  themselves  with  straight 
parallel  sides,  but  nearly  always  in  forms  more  or  less  oblique,  such 
as  those  represented  in  fig.  449.  This  curious  object  is  an  inhabitant 
of  salt  or  of  brackish  water.  Many  of  the  species  formerly  ranked 
under  this  genus  are  now  referred  to  the  genus  Diatoma.  The 
genera  Nitzschia  and  Bacillaria  have  been  associated  by  Mr.  Kalfs 

with  some  other  genera 

A  which  agree  with  them 

in  the  bacillar  or  staff- 
like  form  of  the  frus- 
tules and  in  the  presence 
of  a  longitudinal  keel, 
in  the  sub-family  A'itz- 
schiece,  which  ranks  as 
a  section  of  the  Suri- 
rellece.  Another  sub- 
family, Synedrece,  con- 
sists of.  the  genus 
Synedra  and  its  allies, 
in  which  the  bacillar 
form  is  retained,  but 
the  keel  is  wanting, 
and  the  valves  are 
but  little  broader  than  the  front  of  the  frustule. 

In  the  Surirellece  proper  the  frustules  are  no  longer  bacillar, 
and  the  breadth  of  the  valves  is  usually  (though  not  always)  greater 
than  the  front  view.  The  distinctive  character  of  the  genus 
Surirella,  in  addition  to  the  presence  of  the  supposed  '  canaliculi,' 
is  derived  from  the  longitudinal  line  down  the  centre  of  each  valve 
(fig.  453,  A)  and  the  prolongation  of  the  margins  into  *  ala?.' 
Numerous  species  are  known,  which  are  mostly  of  a  somewhat  ova  in- 
form, some  being  broader  and  others  narrower  than  AS',  constricta  ; 
the  greater  part  of  them  are  inhabitants  of  fresh  or  brackish  water, 
though  some  few  are  marine  ;  and  several  occur  in  those  infusorial 
earths  which  seem  to  have  been  deposited  at  the  bottoms  of  lakes, 
such  as  that  of  the  Mourne  Mountains  in  Ireland  (fig.  468,  b,  c,  X:). 
In  the  genus  Campylodiscus  (fig.  454)  the  valves  are  so  greatly 
increased  in  breadth  as  to  present  almost  the  form  of  discs  (A),  and 
at  the  same  time  have  more  or  less  of  a  peculiar  twist  or  saddle- 
shaped  curvature  (B).  It  is  in  this  genus  that  the  supposed  ;  cana- 


Fio.  453. — Surirella  constricta  :  A,  side  view 
B,  front  view ;  C,  binary  subdivision. 


DIATOMACEJE 


607 


liculi '  are  most  developed,  and  it  is  consequently  here  that  they  may 
be  best  studied  ;  and  of  there  being  here  really  costce,  or  internally 
projecting  ribs,  no  reasonable  doubt  can  remain  after  examination 
of  them  under  the  binocular  microscope,  especially  with  the  '  black  - 
ground  '  illumination.  The  form  of  the  valves  in  most  of  the  species 
is  circular  or  nearly  so ;  some  are  nearly  fiat,  whilst  in  others  the 
twist  is  greater  than  in  the  species  here  represented.  Some  of  the 
species  are  marine,  whilst  others  occur  in  fresh  water ;  a  very 
beautiful  form,  the  C.  clypeus,  exists  in  such  abundance  in  the 
infusorial  stratum  discovered  by  Ehrenberg  at  Soos,  near  Ezer,  in 
Bohemia,  that  the  earth  seems  almost  entirely  composed  of  it. 

The  next  family,  the  Striatellece,  forms  a  very  distinct  group, 
differentiated  from  every  other  by  having  longitudinal  costae  on  the 
connecting  portions  of  the  frustules,  these  costre  being  formed  by 
the  inward  projection  of  annular  siliceous  plates  (which  do  not, 
however,  reach  to  the  centre),  so  as  to  form  septa  dividing  the  cavity 
of  the  cell  into  imperfectly  separated  chambers.  In  some  instances 
these  annular  septa  are  only  formed  during  the  production  of  the 


FIG.  454. — Campylodiscua  contains  :  A,  front  view;  B,  side  view. 

valves  in  the  act  of  division,  and  on  each  repetition  of  such  produc- 
tion, being  thus  always  definite  in  number  ;  whilst  in  other  cases 
the  formation  of  the  septa  is  continued  after  the  production  of  the 
valves,  and  is  repeated  an  uncertain  number  of  times  before  the 
recurrence  of  a  new  valve -production,  so  that  the  annuli  are  indefinite 
in  number.  In  the  curious  Grammatophora  serpent ina  (fig.  452) 
the  septa  have  several  undulations  and  incurved  ends,  so  as  to  form 
serpentine  curves,  the  number  of  which  seems  to  vary  with  the 
length  of  the  frustule.  The  lateral  surfaces  of  the  valves  in  Gram 
matophora  are  very  finely  striated,  and  some  species,  as  G.  subtilissima 
and  G.  marina,  are  used  as  test-objects.  The  frustules  in  most  of 
the  genera  of  this  family  separate  into  zigzag  chains,  as  in  Diatonia; 
but  in  a  few  instances  they  cohere  into  a  filament,  and  still  more 
rarely  they  are  furnished  with  a  stipe.  The  small  family  Terpsinoect; 
was  separated  by  Mr.  Ralfs  from  the  /Striatellece,  with  which  it  is 
nearly  allied  in  general  characters,  because  its  septa  (which  in  the 
latter  are  longitudinal  and  divide  the  central  portions  into  chambers) 
are  transverse,  and  are  confined  to  the  lateral  portions  of  the 


608     MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

frustules,  which  appear  in  the  front  view  as  in  Biddulphiece.  The 
typical  form  of  this  family  is  the  Terpsinoe  musica,  so  named  from 
the  resemblance  which  the  markings  of  its  costse  bear  to  musical 
notes. 

We  next  come  to  two  families  in  which  the  lateral  surfaces  of 
the  frustules  are  circular  ;  so  that,  according  to  the  flatness  or  con- 
vexity of  the  valves  and  the  breadth  of  the  intervening  hooped  band, 
the  frustules  may  have  the  form  either  of  thin  discs,  short  cylinders, 
biconvex  lenses,  oblate  spheroids,  or  even  of  spheres.  Looking  at 
the  structure  of  the  individual  frustules,  the  line  of  demarcation 
between  these  two  families,  Melosirece  and  Coscinodiscece,  is  by  no 
means  distinct,  the  principal  difference  between  them  being  that 
the  valves  of  the  latter  are  commonly  areolated,  whilst  those  of  the 
former  are  smooth.  Another  important  difference,  however,  lies  in 
this,  that  the  frustules  of  the  Coscinodiscece  are  always  free,  whilst 
those  of  the  Melosirece  remain  coherent  into  filaments,  which  often 
so  strongly  resemble  those  of  the  simple  Confervacece  as  to  be  readily 
distinguishable  only  by  the  effect  of  heat.  Of  these  last  the  most 
important  genus  is  Melosira  (fig.  444).  Some  of  its  species  are 
marine,  others  fresh-water ;  one  of  the  latter,  M.  ochracea,  seems  to 
grow  best  in  boggy  pools  containing  a  ferruginous  impregnation  ; 
and  it  is  stated  by  Professor  Ehrenberg  that  it  takes  up  from  the 
water,  and  incorporates  with  its  own  substance,  a  considerable 
quantity  of  iron.  The  filaments  of  Melosira  very  commonly  fall 
apart  at  the  slightest  touch,  and  in  the  infusorial  earths  in  which 
some  species  abound  the  frustules  are  always  found  detached 
(fig.  468,  a,  a,  d,  d).  The  meaning  of  the  remarkable  difference  in 
the  sizes  and  forms  of  the  frustules  of  the  same  filaments  (fig.  444) 
has  not  yet  been  fully  ascertained.  The  sides  of  the  valves  are 
often  marked  with  radiating  striae  (fig.  468,  d,  d)~  and  in  some 
species  they  have  toothed  or  serrated  margins,  by  which  the  frustules 
lock  together.  To  this  family  belongs  the  genus  Hyalodiscus,  of 
which  H.  subtilis  was  first  brought  into  notice  by  the  late  Professor 
Bailey  as  a  test-object,  its  disc  being  marked,  like  the  engine-turned 
back  of  a  watch,  with  lines  of  exceeding  delicacy,  only  visible  by 
good  objectives  and  careful  illumination. 

The  family  Coscinodiscece  includes  a  large  proportion  of  the  most 
beautiful  of  those  discoidal  diatoms  of  which  the  valves  do  not 
present  any  considerable  convexity,  and  are  connected  by  a  narrow 
zone.  The  genus  Coscinodiscus,  which  is  easily  distinguished  from 
most  of  the  genera  of  this  family  by  not  having  its  disc  divided  into 
compartments,  is  of  great  interest  from  the  vast  abundance  of  its 
valves  in  certain  fossil  deposits  (fig.  467,  a,  a,  a)  especially,  the 
infusorial  earth  of  Richmond  in  Virginia,  of  Bermuda,  and  of  Oran, 
as  also  in  guano.  Each  frustule  is  of  discoidal  shape,  being  com- 
posed of  two  delicately  undulating  valves  united  by  a  hoop  ;  so  that 
if  the  frustules  remain  in  adhesion,  they  would  form  a  filament 
resembling  that  of  Melosira  (fig.  444,  B).  The  regularity  of  the 
hexagonal  areolation  shown  by  its  valves  renders  them  beautiful 
microscopic  objects ;  in  some  species  the  areolse  are  smallest  near 
the  centre,  and  gradually  increase  in  size  towards  the  margin ;  in 


DIATOMACE^E  609 

others  a  few  of  the  central  areolee  are  the  largest,  and  the  rest  are 
of  nearly  uniform  size  ;  while  in  others,  again,  there  are  radiating 
lines  formed  by  areolee  of  a  size  different  from  the  rest.  Most  of 
the  species  are  either  marine  or  are  inhabitants  of  brackish  water  ; 
when  living  they  are  most  commonly  found  adherent  to  seaweeds 
or  zoophytes  ;  but  when  dead  the  valves  fall  as  a  sediment  to  the 
bottom  of  the  water.  In  both  these  conditions  they  were  found  by 
Professor  J.  Quekett  in  connection  with  zoophytes  which  had  been 
brought  home  from  Melville  Island  by  Sir  E.  Parry  ;  and  the  species 
seem  to  be  identical  with  those  of  the  Richmond  earth.  The  in- 
vestigations of  Mr.  J.  W.  StepheAson l  on  Coscinodiscus  oculus  iridis 
si  LOW  that  the  peculiar  *  eye-like  '  appearance  in  the  centre  of  each 
of  its  hexagonal  areolse  arises  from  the  intermingling  of  the  mark- 
ings of  two  distinct  layers,  differing  considerably  in  structure,  the 
markings  of  the  lower  layer  being'  partially  seen  through  those  of 
the  upper.  By  fracturing  these  diatoms  Mr.  Stephenson  succeeded 
in  separating  portions  of  the  two  layers,  so  that  each  could  be 
examined  singly.  He  also  mounted  them  in  bisulphide  of  carbon, 


FIG.  455. — Structure  of  siliceous  valve  of  Coscinodiscus  oculus  iridis :  1,  hexagonal 
areola  of  inner  or  '  eye-spot '  layer ;  2,  areola  of  outer  layer. 

the  refractive  index  of  which  is  high ;  and  also  in  a  solution  of 
phosphorus  in  bisulphide  of  carbon,  which  has  a  still  higher  refrac- 
tive index.  If  we  suppose  a  diatom  to  be  marked  with  convex 
depressions,  they  would  act  as  concave  lenses  in  air,  which  is  less 
refractive  than  their  own  silex ;  but  when  such  lenses  are  immersed 
in  bisulphide  of  carbon,  or  in  the  phosphorus  solution,  they  would 
be  converted  into  convex  lenses  of  the  more  refractive  substance, 
and  have  their  action  in  air  reversed.  Analogous  but  opposite 
changes  must  take  place  when  convex  diatom-lenses  are  viewed  first 
in  air,  and  then  in  the  more  refractive  media.  Applying  these  and 
other  tests  to  Coscinodiscus  oculus  iriclis,  Mr.  Stephenson  considered 
both  layers  to  be  composed  of  hexagons,  represented  in  fig.  455 
from  drawings  by  Mr.  Stewart.  The  upper  layer  is  much  stronger 
and  thicker  than  the  low^er  one,  and  the  framework  of  its  hexagons 
more  readily  exhibits  its  beaded  appearance.  The  lower  layer  is 
nearly  transparent,  and  but  little  conspicuous  when  seen  in  bisulphide 
of  carbon,  except  as  shown  in  the  figure,  when  the  framework  of 

1  Monthly  Microscopical  Journal,  vol.  x.  1873,  p.  1. 

R  B 


6lO  MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

the  hexagons  and  the  rings  in  the  midst  of  them  appear  thickene 
and  more  refractive.  In  both  layers  the  balance  of  observations 
tends  to  the  belief  that  the  hexagons  have  no  floors,  and  are  in  fact 
perforated  by  foramina  like  those  of  minute  polycystina.  The  cells 
formed  by  the  hexagons  of  the  upper  layer  are  of  considerable 
depth  ;  those  of  the  lower  layer  are  shallower.  It  is  very  desirable 
that  living  forms  of  Coscinodisci  should  be  carefully  examined ; 
since,  if  they  really  have  foramina,  some  minute  organs  may  be  pro- 
truded through  them. 

The  genus  Actinocyclus :  closely  resembles  the  preceding  in  form, 
but  differs  in  the  markings  of  its  valvular  discs,  which  are  minutely 
and  densely  punctated  or  areolated,  and  are  divided  radially  by 
single  or  double  dotted  lines,  which,  however,  are  not  continuous 
but  interrupted.  The  discs  are  generally  iridescent ;  and,  when 
mounted  in  balsam,  they  present  various  shades  of  brown,  green, 
blue,  purple,  and  red  ;  blue  or  purple,  however,  being  the  most 
frequent.  An  immense  number  of  species  have  been  erected  by 
Professor  Ehrenberg  on  minute  differences  presented  by  the  rays  as 
to  number  and  distribution  ;  but  since  scarcely  two  specimens  can 
be  found  in  which  there  is  a  perfect  identity  as  to  these  particulars, 
it  is  evident  that  such  minute  differences  between  organisms  other- 
wise similar  are  not  of  sufficient  account  to  serve  for  the  separation 
of  species.  This  form  is  very  common  in  guano  from  Ichaboe.  Allied 
to  the  preceding  are  the  two  genera  A sterolampra  and  Aster om,phalus, 
both  of  which  have  circular  discs  of  which  the  marginal  portion  is 
minutely  areolated,  whilst  the  central  area  is  smooth  and  perfectly 
hyaline  in  appearance,  but  is  divided  by  lines  into  radial  compart- 
ments which  extend  from  the  central  umbilicus  towards  the  periphery. 
The  difference  between  them  simply  consists  in  this,  that  in  Astero- 
lampra  all  the  compartments  are  similar  and  equidistant  and  the  rays 
equal,  whilst  in  Asteromphalus  (PI.  I,  fig.  3)  two  of  the  compartments 
are  closer  .together  than  the  rest,  and  the  enclosed  hyaline  ray  (which 
is  distinguished  as  the  median  or  basal  ray)  differs  in  form  from 
the  others,  and  is  sometimes  specially  continuous  with  the  umbilicus. 
The  eccentricity  thus  produced  in  the  other  rays  has  been  made  the 
basis  of  another  generic  designation,  Spatangidium  ;  but  it  may  be 
doubted  whether  this  is  founded  on  a  valid  distinction.2  These 
beautiful  discs  are  for  the  most  part  obtainable  from  guano,  and 
from  soundings  in  tropical  and  antarctic  seas.  From  these  we  pass 
on  to  the  genus  Actitwptychus  (fig.  456),  of  which  also  the  frustules 
are  discoidal  in  form,  but  in  which  each  valve,  instead  of  being  flat, 
has  an  undulating  surface,  as  is  seen  in  front  view  (B),  giving  to  the 
side  view  (A)  the  appearance  of  being  marked  by  radiating  bands. 
Owing  to  this  peculiarity  of  shape,  the  whole  surface  cannot 
be  brought  into  focus  at  once  except  with  a  low  power  ;  and  the 

1  The  Author  concurs  with  Mr.  Kalfs  in  thinking  it  preferable  to  limit  the  genus 
Actinocyclus  to  the  forms  originally  included  in  it  by  Ehrenberg,  and  to  restore  the 
genus  Actinoptyclius  of  Ehrenberg,  which  had  been  improperly  united  with  Acti>/o- 
cijclus  by  Professors  Kiitzing  and  W.  Smith. 

2  See  Greville  in  Quart.  Journ.  Microsc.  Science,  vol.  vii.  1859,  p.  158;  and  in 
Trans.  Microsc.  Soc.  vol.  viii.  n.s.  1860,  p.  102,  and  vol.  x.  1862,  p.  41 ;  also  Wallich 
in  the  same  Transactions,  vol.  viii.  1860,  p.  44. 


DIATOMACEvE 

difference  of  aspect  which  the  different  radial  divisions  present  in 
fig.  456  is  simply  due  to  the  fact  that  one  set  is  out  of  focus  whilst 
the  other  is  in  it,  since  the  appearances  are  reversed  by  merely 
altering  the  focal  adjustment.  The  number  of  radial  divisions  lias 
been  considered  a  character  of  sufficient  importance  to  serve  for  the 
distinction  of  species ;  but  this  is  probably  subject  to  variation  ; 
since  we  not  unfrequently  meet  with  discs,  of  which  one  has  (say) 
eight,  and  another  ten  such  divisions,  but  which  are  precisely  alike 
in  every  other  particular.  The  valves  of  this  genus  also  are  very 
abundant  in  the  infusorial  earth  o£  Richmond,  Bermuda,  and  Oran 
(fig.  467,  b,  b,  b),  and  many  of  the  same  species  have  been  found  in 
guano  and  in  the  seas  of  various  parts  of  the  world.  The  frustules 
in  their  living  state  appear  to  be  generally  attached  to  seaweeds  or 
zoophytes. 

The  Bermuda  earth  also  contains  the  very  beautiful  form 
which,  though  scarcely  separable  from  Actinoptychus  except  by 
its  marginal  spines,  has  received  from  Professor  Ehrenberg  the  dis- 
tinctive appellation  of  Heliopelta  (sun  shield).  The  object  is  repre- 
sented as  seen  on  its  internal  aspect  by  the  parabolic  illuminator, 
which  brings  into  view  certain  features  that  can  scarcely  be  seen  by 
ordinary  transmitted  light. 
Five  of  the  radial  divisions  are 
seen  to  be  marked  out  into 
circular  arealse ;  but  in  the 
five  which  alternate  with  them 
a  minute  beaded  structure  is 
observable.  This  may  be 
shown,  by  careful  adjustment 

of  the  focus,  to  exist  over  the 

,     ,       .    ,     '.  ,  FIG.  456.— Actinoptychus  undulatus . 

whole   interior  of  the   valve,  A,  side  view ;  B,  front  view, 

even  on  the  divisions  in  which 

the  circular  areolation  is  here  displayed ;  and  it  hence  appeal's  pro- 
bable that  this  marking  belongs  to  the  internal  layer,1  and  that  the 
circular  areolation  exists  in  the  outer  layer  of  the  silicified  valves. 
In  the  alternating  divisions  whose  surface  is  here  displayed,  the 
areolation  of  the  outer  layer,  when  brought  into  view  by  focussing 
down  to  it,  is  seen  to  be  formed  by  equilateral  triangles ;  it  is  not, 
however,  nearly  so  well  marked  as  the  circular  areolation  of  the 
first-mentioned  divisions.  The  dark  spots  seen  at  the  end  of  the 
rays,  like  the  dark  centre,  appear  to  be  solid  areolations  of  silex  not 
traversed  by  markings,  as  in  many  other  diatoms  ;  they  are  appa- 
rently not  orifices,  as  supposed  by  Professor  Ehrenberg.  Of  this  type, 
again,  specimens  are  found  presenting  six,  eight,  ten,  or  twelve  radial 
divisions,  but  in  other  respects  exactly  similar  ;  on  the  other  hand, 
two  specimens  agreeing  in  their  number  of  divisions  may  exhibit 
minute  differences  of  other  kinds  ;  in  fact,  it  is  rare  to  find  two 

1  It  is  stated  by  Mr.  Stodder  (Quart .  Journ.  Microsc.  Science,  vol.  iii.  n.s.  1863, 
p.  21"))  that  not  only  has  he  seen,  in  broken  specimens,  the  inner  granulated  plate 
projecting  beyond  the  outer,  but  that  he  has  found  the  inner  plate  altogether 
separated  from  the  outer.  The  Author  is  indebted  to  this  gentleman  for  pointing 
out  that  his  figure  represents  the  inner  surface  of  the  valve. 

R  R  2 


6l2   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

that  are  precisely  alike.  It  seems  probable,  then,  that  we  must 
allow  a  considerable  latitude  of  variation  in  these  forms  before 
attempting  to  separate  any  of  them  as  distinct  species.  Another 
very  beautiful  discoidal  diatom,  which  occurs  in  guano,  and  is  also 
found  attached  to  seaweeds  from  different  parts  of  the  wrorld 
(especially  to  a  species  employed  by  the  Japanese  in  making  soup), 
is  the  Arachnoidiscus  (Plate  XII),  so  named  from  the  resemblance 
which  the  beautiful  markings  on  its  disc  cause  it  to  bear  to  a 
spider's  web.  According  to  Mr.  Shadbolt,1  who  first  carefully 
examined  its  structure,  each  valve  consists  of  two  layers  ;  the  outer 
one,  a  thin  flexible  horny  membrane,  indestructible  by  boiling 
in  nitric  acid ;  the  inner  one  siliceous.  It  is  the  former  which 
has  upon  it  the  peculiar  spider's- web -like  markings  ;  whilst  it  is 
the  latter  that  forms  the  supporting  framework  which  bears  a  very 
strong  resemblance  to  that  of  a  circular  Gothic  window.  The 
two  can  occasionally  be  separated  entire  by  first  boiling  the  discs 
for  a  considerable  time  in  nitric  acid  and  then  carefully  washing 
them  in  distilled  water.  Even  without  such  separation,  however, 
the  distinctness  of  the  two  layers  can  be  made  out  by  focussing  for 
each  separately  under  a  J-  or  \  -inch  objective  ;  or  by  looking  at  a 
valve  as  an  opaque  object  (either  by  the  parabolic  illuminator, 
or  by  the  Lieberkiihn,  or  by  a  side  light)  with  a  f^-inch  objective, 
first  from  one  side  and  then  from  the  other.  But  it  can  be  seen  to 
very  best  advantage  by  the  use  of  apochromatic  objectives  of 
suitable  power  and  a  suitable  diaphragm  for  dark-ground  illumi- 
nation. 

This  family  is  connected  with  the  succeeding  by  the  small  group 
Eupodiscecv,  the  members  of  which  agree  with  the  Coscinodiscece  in 
the  general  character  of  their  discoid  frustules,  and  with  the  Bid- 
dulphiece  in  having  areolar  processes  on  their  lateral  surfaces.  In 
the  beautiful  Aulacodiscus  these  areolations  are  situated  near  the 
margin,  and  are  connected  with  bands  radiating  from  the  centre  ;  the 
surface  also  is  frequently  inflated  in  a  manner  that  reminds  us  of 
Actinoptychus.  These  forms  are  for  the  most  part  obtained  from 
guano. 

The  members  of  the  next  family,  Jtiddulphiece,  differ  greatly  in 
their  general  form  from  the  preceding,  being  remarkable  for 
the  great  development  of  the  lateral  valves,  which,  instead  of  being 
nearly  flat  or  discoidal,  so  as  only  to  present  a  thin  edge  in  front 
view,  are  so  convex  or  inflated  as  always  to  enter  largely  into  the 
front  view,  causing  the  central  zone  to  appear  like  a  band  between 
them.  This  band  is  very  narrow  when  the  new  frustules  are  first 
produced  by  binary  division,  but  it  increases  gradually  in  breadth, 
until  the  new  frustule  is  fully  formed  and  is  itself  undergoing  the 
same  duplicative  change.  In  Biddulphia  (fig.  445)  the  frustules 
have  a  quadrilateral  form,  and  remain  coherent  by  their  alternate 
angles  (which  are  elongated  into  tooth-like  projections),  so  as  to  form 
a  zigzag  chain.  They  are  marked  externally  by  ribbings  which  seem 
to  be  indicative  of  internal  costce  partially  subdividing  the  cavity. 
Nearly  allied  to  this  is  the  beautiful  genus  Isthmia  (fig.  457),  in 
1  Trans.  Microsc.  Soc.  1st  series,  vol.  iii.  p.  49. 


DIATOMACEJE 


613 


which  the  frustules  have  a  trapezoidal  form  owing  to  the  oblique 
prolongation  of  the  valves ;  the  lower  angle  of  each  frustule  is 
coherent  to  the  middle  of  the  next  one  beneath,  and  from  the  basal 
frustule  proceeds  a  stipe  by  which  the  filament  is  attached.  Like 
the  preceding,  this  genus  is  marine,  and  is  found  attached  to  the 
seaweeds  of  our  own  shores.  The  areolated  structure  of  its  surface 
is  very  conspicuous  both  in  the  valves  and  in  the  connecting  '  hoop  ; ' 
and  this  hoop,  being  silicified,  not  only  connects  the  two  new  frus- 
tules (as  at  b,  fig.  457),  until  they  have  separated  from  each,  other, 
but,  after  such  separation,  remains  for  a  time  round  one  of  the 
frustules,  so  as  to  give  it  a  truncated  appearance  (a,  c). 

The  family  Anguliferce,  distinguished  by  the  angular  form  of  its 
valves  in  their  lateral  aspect,  is  in  many  respects  closely  allied  to 
the  preceding  ;  but  in  the  comparative  flattening  of  their  valves  its 
members  more  resemble  the  Coscinodiscece 
and  Eupodiscece.  Of  this  family  we  have 
a  characteristic  example  in  the  genus 
Triceratium,  of  which  striking  form  a  con- 
siderable number  of  species  are  met  with 
in  the  Bermuda  and  other  infusorial 
earths,  while  others  are  inhabitants  of  the 
existing  ocean  and  of  tidal  rivers.  T.favus 
(fig.  442),  which  is  one  of  the  largest  and 
most  regularly  marked  of  any  of  these, 
occurs  in  the  mud  of  the  Thames  and  in 
various  other  estuaries  on  our  own  coast ; 
it  has  been  found,  also,  on  the  surface  of 
large  sea-shells  from  various  parts  of  the 
world,  such  as  those  of  Hippopus  and 
JIaliotis,  before  they  have  been  cleaned ; 
and  it  presents  itself  likewise  in  the  in- 
fusorial earth  of  Petersburg  (U.S.A.). 
The  projections  at  the  angles  which  are 
shown  in  that  species  are  prolonged  in 
some  other  species  into  '  horns  ; '  whilst 
in  others,  again,  they  are  mere  tubercular 
elevations.  Although  the  triangular  form 
of  the  frustule,  when  looked  at  sideways, 

is  that  which  is  characteristic  of  the  genus,  yet  in  some  of  the  species 
there  seems  a  tendency  to  produce  quadrangular  &nd  even  pentagonal 
forms,  these  being  marked  as  varieties  by  their  exact  correspondence 
in  sculpture,  colour,  &c.,  with  the  normal  triangular  forms.1  This 
departure  is  extremely  remarkable,  since  it  breaks  down  what  seems 
at  first  to  be  the  most  distinctive  character  of  the  genus ;  and  its 
occurrence  is  an  indication  of  the  degree  of  latitude  which  we  ought 
to  allow  in  other  cases.  It  is  difficult,  in  feet,  to  distinguish  the 
square  forms  of  Triceratium  from  those  included  in  the  genus 

1  See  Mr.  Brightwell's  excellent  memoirs  'On  the  genus  Triceratium'  in 
Quart.  Journ.  Microsc.  Science,  vol.  i.  1853,  p.  245  ;  vol.  iv.  1856,  p.  272;  vol.  vi. 
1858,  p.  153 ;  also  Wallich  in  the  same  Journal,  vol.  iv.  1858,  p.  242 ;  and  Greville  in 
Trans.  Microsc.  Soc.  n.s.  vol.  ix.  1861,  pp.  43,  69. 


FIG.  457. — Isthmia  nervosa. 


6 14  MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 


Amphitetras,  which  is  chiefly  characterised  by  the  cubiform  shape  of 
its  frustules.  In  the  latter  the  frustules  cohere  at  their  angles,  so 
as  to  form  zigzag  filaments,  whilst  in  the  former  the  frustules  are 
usually  free,  though  they  have  occasionally  been  found  in  chains. 

Another  group  that  seems  allied  to  the  Biddidpkiece  is  the  curious 
assemblage  of  forms  brought  together  in  the  family  Chcetocerece,  some 
of  the  filamentous  types  of  which  seem  also  allied  to  the  Melosirece. 
The  peculiar  distinction  of  this  group  consists  in  the  presence  of 
tubular  '  awns,'  frequently  proceeding  from  the  connecting  hoop, 
sometimes  spinous  and  serrated,  and  often  of  great  length  (fig.  458) ; 
by  the  interlacing  of  which  the  frustules  are  united  into  filaments 
whose  continuity,  however,  is  easily  broken.  In  the  genus  Bacterias- 
trum  (fig.  459)  there  are  sometimes  as  many  as  twelve  of  these  awns, 
radiating  from  each  frustule  like  the  spokes  of  a  wheel,  and  in  some 

instances  regularly  bifurcating. 
"With  this  group  is  associated  the 
genus  Rhizosolenia,  of  which  several 
species  are  distinguished  by  the  ex- 
traordinary length  of  the  frustule 
(which  may  be  from  six  to  twenty 


J 


FIG.  458; — Chcetoceros  Wighamii:  «,  front 
view,  and  &,  side  view  of  frustule ;  c,  side 
view  of  connecting  hoop  and  awns ; 
d,  entire  filament. 


FIG.  459. — Bacteriastrum 
furcatum 


times  its  breadth),  giving  it  the  aspect  of  a  filament  (fig.  460), 
by  a  transverse  annulatioii  that  imparts  to  this  filament  a  jointed 
appearance,  and  by  the  termination  of  the  frustule  at  each  end  in  a 
cone,  from  the  apex  of  which  a  straight  awn  proceeds.  It  is  not  a 
little  remarkable  that  the  greater  number  of  the  examples  of  this 
curious  family  are  obtained  from  the  stomachs  of  Ascidians,  Salpa?, 
Holothurise,  and  other  marine  animals.1 

The  second  principal  division  (B)  of  the  Diatomacece  consists,  it 
will  be  remembered,  of  those  in  which  the  frustules  have  a  median 
longitudinal  line  and  a  central  nodule.  In  the  first  of  the  families 
which  it  includes,  that  of  Cocconeidece,  the  central  nodule  is  obscure 
or  altogether  wanting  on  one  of  the  valves,  which  is  distinguished  as 

1  See  Brightwell  in  Quart.  Journ.  Microsc.  Science,  vol.  iv.  1856,  p.  105 ;  vol.  vi. 
1858,  p.  93  ;  Wallich  in  Trans.  Microsc.  Soc.  n.s.  vol.  viii.  1860,  p.  48  ;  and  West,  in 
the  same,  p.  151. 


DIATOMACE.E 


6i5 


the  inferior.  This  family  consists  but  of  a  single  genus,  Cocconeis, 
which  includes,  however,  a  great  number  of  species,  some  or  other  of 
them  occurring  in  every  part  of  the  globe.  Their  form  is  usually 
that  of  ellipsoidal  discs,  with  surfaces  more  or  less  exactly  parallel, 
plane,  or  slightly  curved ;  and  they  are  very  commonly  found  ad- 
herent to  each  other.  The  frustules  in  this  genus  are  frequently  in- 
vested by  a  membranous  envelope  which  forms  a  border  to  them  ;  but 
this  seems  to  belong  to  the  immature  state,  subsequently  disappear- 
ing more  or  less  completely. 


FIG.  4BO.—Bhiso- 
solenia  imbri- 
cuta. 


FIG.  l&L.—Achnanthes 
longipes :  a,  b,  c,  d,  e, 
frustules  in  different 
stages  of  binary  divi- 


FIG.  462.  —  Gomphonema  gemi- 
natum :  its  frustules  connected  by 
a  dichotomous  stipe. 


Another  family  in  which  there  is  a  dissimilarity  in  the  two 
lateral  surfaces  is  that  of  the  Achnanthece,  the  frustules  of  which 
are  remarkable  for  the  bend  they  show  in  the  direction  of  their 
length,  often  more  conspicuously  than  in  the  example  here  repre- 
sented. This  family  contains  free,  adherent,  and  stipitate  forms, 
one  of  the  most  common  of  the  latter  being  Achnanthes  longipes 
(fig.  461),  which  is  often  found  growing  on  marine  algae.  The 
difference  between  the  markings  of  the  upper  and  lower  valves  is 
here  distinctly  seen  ;  for,  while  both  are  traversed  by  striae,  which 
are  resolvable  under  a  sufficient  power  into  rows  of  dots,  as  well  as 


6l6  MICKOSCOPIC  FOKMS  OF  VEGETABLE  LIFE- THALLOPH  YTKS 

by  a  longitudinal  line  which  sometimes  has  a  nodule  at  each  end 
(as  in  Navicula),  the  lower  valve  (a)  has  also  a  transverse  line  form- 
ing a  stauros,  or  cross,  which  is  wanting  in  the  upper  valve  (e).  A 
persistence  of  the  connecting  membrane,  so  as  to  form  an  additional 
connection  between  the  cells,  may  sometimes  be  observed  in  this 
genus ;  thus  in  fig.  461  it  not  only  holds  together  the  two  new 
frustules  resulting  from  the  subdivision  of  the  lowest  cell,  a,  which 
are  not  yet  completely  separated  the  one  from  the  other,  but  it  may 
be  observed  to  invest  the  two  frustules  b  and  c,  which  have  not 
merely  separated,  but  are  themselves  beginning  to  undergo  binary 
subdivision  ;  and  it  may  also  be  perceived  to  invest  the  frustule  d, 
from  which  the  frustule  e,  being  the  terminal  one,  has  more  com- 
pletely freed  itself. 

In  the  family  Cymbellece,  on  the  other  hand,  both  valves  possess 
the  longitudinal  line  with  a  nodule  in  the  middle  of  its  length ;  but 
the  valves  have  the  general  form  of  those  of  the  Eunotiece,  and  the 
line  is  so  much  nearer  one  margin  than  the  other  that  the  nodule 

is  sometimes  rather  mar- 

A  B  °  ginal  than  central,  as  we 

see    in    Cocconema    (fig. 
468,/). 

The  Gomj}honemece, 
like  the  Meridiece  and 
Liemophorece,  have  frus- 
tules which  are  cuneate 
or  wedge-shaped  in  their 
front  view  (figs.  462, 463), 
but  are  distinguished 
from  those  forms  by  the 
presence  of  the  longi- 
tudinal line  and  central 
JbiG.  463. — Gomphonema  qeminatum,  more  highly  *,  -,  .  -,,-,  -,  ,-, 

magnified :  A, , side  view  of  frustule ;  B,  front  view ;  nodule.      Although  there 
C,  frustule  in  the  act  of  division.  are   some    free  forms  in 

this  family,  the  greater 

part  of  them,  included  in  the  genus  Gomphonema,  have  their  frus- 
tules either  affixed  at  their  bases  or  attached  to  a  stipe.  This  stipe 
seems  to  be  formed  by  an  exudation  from  the  frustule,  which  is 
secreted  only  during  the  process  of  binary  division ;  hence,  when 
this  process  has  been  completed,  the  extension  of  the  single  filament 
below  the  frustule  ceases  ;  but  when  it  recommences,  a  sort  of  joint 
or  articulation  is  formed,  from  which  a  new  filament  begins  to  sprout 
for  each  of  the  half-frustules  ;  and  when  these  separate,  they  cany 
apart  the  peduncles  which  support  them  as  far  as  their  divergence 
can  take  place.  It  is  in  this  manner  that  the  dichotomous  character 
is  given  to  the  entire  stipe  (fig.  462).  The  species  of  Gomphonema 
are,  with  few  exceptions,  inhabitants  of  fresh  water,  and  are  among 
the  commonest  forms  of  Diatomacece. 

Lastly,  we  come  to  the  large  family  Naviculea,  the  members 
of  which  are  distinguished  by  the  symmetry  of  their  frustules,  as 
well  in  the  lateral  as  in  the  front  view,  and  by  the  presence  of  a 
median  longitudinal  line  and  central  nodule  in  both  valves.  In  the 


DIATOMACE.E  617 

genus  Navicula  and  its  allies  the  frustules  are  free  or  simply 
adherent  to  each  other ;  while  in  another  large  section  they  are 
included  within  a  gelatinous  envelope,  or  are  enclosed  in  a  defi- 
nite tubular  or  gelatinous  frond.  Of  the  genus  Naviwda  an 
immense  number  of  species  have  been  described,  the  grounds  of 
separation  being  often  extremely  trivial.  Those  which  have  a 
lateral  sigmoid  curvature  have  been  separated  by  Mr.  "W.  Smith 
under  the  designation  Pleurosigma,  which  is  now  generally  adopted  ; 
but  his  separation  of  another  set  of  species  under  the  name  Pinnu- 
laria  (which  had  been  previously  applied  by  Ehrenberg  to  designate 
the  striated  species),  on  the  grqund  that  its  stria?  (costse)  are  not 
resolvable  into  dots,  was  not  considered  valid  by  Mr.  Ralfs, 
because  in  many  of  the  more  minute  species  it  is  impossible  to 
distinguish  with  certainty  between  striae  and  costa?.  Mr.  Slack  has 
since  given  an  account  of  the  resolution  of  the  so-called  costse  of 
twelve  species  of  Pinnularice  into  *  beaded '  structures.1  The  beauti- 
ful genus  Stauroneis,  which  belongs  to  the  same  group,  differs  from 
all  the  preceding  forms  in  having  the  central  nodule  of  each  valve 
dilated  laterally  into  a  band  free  from  striae,  which  forms  a  cross 
with  the  longitudinal  band.  The  multitudinous  species  of  the  genus 
Navicida  are  for  the  most  part  inhabitants  of  fresh  water  ;  and  they 
constitute  a  large  part  of  most  of  the  so-called  '  infusorial  earths ' 
which  wrere  deposited  at  the  bottoms  of  lakes.  Among  the  most  re- 
markable of  such  deposits  are  the  substances  largely  used  in  the  arts 
for  the  polishing  of  metals,  under  the  names  of  Tripoli  and  rotten  - 
stone  ;  these  consist  in  great  part  of  the  frustules  of  Namculce,  and 
Pinnidarice.  The  Poliersckiefer,  or  '  polishing  slate,'  of  Bilin  in 
Bohemia,  the  powder  of  which  is  largely  used  in  Germany  for  the 
same  purpose,  and  which  also  furnishes  the  fine  sand  used  for  the  most 
delicate  castings  in  iron,  occurs  in  a  series  of  beds  averaging  fourteen 
feet  in  thickness,  and  these  present  appearances  which  indicate  that 
they  have  been  at  some  time  exposed  to  a  high  temperature.  The 
well-known  '  Turkey-stone,'  so  generally  employed  for  the  sharpening 
of  edge-tools,  seems  to  be  essentially  composed  of  a  similar  aggrega- 
tion of  frustules  of  JVaviculce,  &c.,  which  have  been  consolidated  by 
heat.  The  species  of  Pleurosigma,  on  the  other  hand,  are  for  the  most 
part  either  marine  or  are  inhabitants  of  brackish  water,  and  they 
comparatively  seldom  present  themselves  in  a  fossilised  state.  Of 
Stauroneis  some  species  inhabit  fresh  water,  while  others  are  marine  ; 
and  the  former  present  themselves  frequently  in  certain  '  infusorial 
earths.' 

Of  the  members  of  the  sub-family  Schizonemece,  consisting  of 
those  Navicidece  in  which  the  frustules  are  united  by  a  gelatinous 
envelope,  some  are  remarkable  for  the  great  external  resemblance 
they  bear  to  acknowledged  alga?.  This  is  especially  the  case  with 
the  genus  Schizonema,  in  which  the  gelatinous  envelope  forms  a 
regular  tubular  frond,  more  or  less  branched,  and  of  nearly  equal 
diameter  throughout,  within  which  the  frustules  lie  either  in  single 
file  or  without  any  definite  arrangement  (fig.  464),  all  these  frustules 
having  arisen  from  the  binary  division  of  one  individual.  In  the 
1  Monthly  Microscopical  Journal,  vol.  vi.,  1871,  p.  71. 


6l8   M1CEOSCOPIC  FORMS  OF  VEGETABLE  LIFE--THALLOPHYTES 

genus  Mastogloia,  which  is  specially  distinguished  by  having  the 
annulus  furnished  with  internal  costse  projecting  into  the  cavity  of 
the  frustule,  each  frustule  is  separately  supported  on  a  gelatinous 
cushion  (fig.  465,  B),  which  may  itself  be  either  borne  on  a  branching 
stipe  (A),  or  may  be  aggregated  with  others  into  an  indefinite  mass 
(fig.  466).  The  careful  study  of  these  composite  forms  is  a  matter 
of  great  importance,  since  it  enables  us  to  bring  into  comparison 
with  each  other  great  numbers  of  frustules  which  have  unquestionably 
a  common  descent,  and  which  must  therefore  be  accounted  as  of  the 
same  species,  and  thus  to  obtain  an  idea  of  the  range  of  variation 
prevailing  in  this  group,  without  a  knowledge  of  which  specific  defi- 
nition is  altogether  unsafe.  Of  the  very  strongly  marked  varieties 
which  may  occur  within  the  limits  of  a  single  species,  we  have  an 


FIG.  464. — Schizonema  Grevillii  :  A,  natural  size ;  B,  portion  magnified  five 
diameters ;  C,  filament  magnified  100  diameters  ;  D,  single  frustule. 

example  in  the  valves  C,  D,  E,  F  (fig.  465),  which  would  scarcely 
have  been  supposed  to  belong  to  the  same  specific  type  did  they  not 
occur  upon  the  same  stipe.  The  careful  study  of  these  varieties  in 
every  instance  in  which  any  disposition  to  variation  shows  itself, 
so  as  to  reduce  the  enormous  number  of  species  with  which  our  sys- 
tematic treatises  are  loaded,  is  a  pursuit  of  far  greater  real  value 
than  the  multiplication  of  species  by  the  detection  of  such  minute 
differences  as  may  be  presented  by  forms  discovered  in  newly 
explored  localities ;  such  differences  as  have  already  been  pointed 
out  being,  probably,  in  a  large  proportion  of  cases,  the  result  of  the 
multiplication  of  some  one  form,  which,  under  modifying  influences 
that  we  do  not  yet  understand,  has  departed  from  the  ordinary  type. 
The  more  faithfully  and  comprehensively  this  study  is  carried  out  in 


DIATOMACE^: 


619 


any  department  of  natural  history,  the  more  does  it  prove  that  the 
range  of  variation  is  far  greater  than  had  been  previously  imagined  ; 
and  this  is  especially  likely  to  be  the  case  with  such  humble 
organisms  as  those  we  have  been  considering,  since  they  are  obviously 
more  influenced  than  those  of  higher  types  by  the  conditions  under 
which  they  are  developed  ;  whilst,  from  the  very  wide  geographical 
range  through  which  the  same  forms  are  diffused,  they  are  subject 
to  very  great  diversities  of  such  conditions. 

The  general  habits  of  this   most   interesting   group  cannot  be 
better    stated    than  in    the    words    of    Mr.    W.    Smith  : — '  The 


Fid   4G5. 


FIG.  46G. 


FIG.  465. — Mastogloia    Smitlrii :    A,   entire   stipe ;    B,   frustule  in  its    gelatinous 
envelope;  C-F,  different  forms  of  frustule  as  seen  in  side  view;   G,  front  view 
H,  frustule  undergoing  subdivision. 

FIG.  466. — Mastogloia  lanceolata. 

Diatomacece  inhabit  the  sea  or  fresh  water  ;  but  the  species  peculiar 
to  the  one  are  never  found  in  a  living  state  in  the  other  locality ; 
though  there  are  some  which  prefer  a  medium  of  a  mixed  nature,  and 
are  only  to  be  met  with  in  water  more  or  less  brackish.  The  latter  are 
often  found  in  great  abundance  and  variety  in  districts  occasionally 
subject  to  marine  influences,  such  as  marshes  in  the  neighbourhood 
of  the  sea,  or  the  deltas  of  rivers,  where,  on  the  occurrence  of  high 
tides,  the  freshness  of  the  water  is  affected  by  percolation  from  the 
'adjoining  stream,  or  more  directly  by  the  occasional  overflow^  of  its 
banks.  Other  favourite  habitats  of  the  Diatomacece  are  stones  of 
mountain  streams  or  waterfalls,  and  the  shallow  pools  left  by  the 


620   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

retiring  tide  at  the  mouths  of  our  larger  rivers.  They  are  not,  how- 
ever, confined  to  the  localities  I  have  mentioned — they  are,  in  fact, 
most  ubiquitous,  and  there  is  hardly  a  roadside  ditch,  water- trough, 
or  cistern,  which  will  not  reward  a  search  and  furnish  specimens 
of  the  tribe.'  Such  is  their  abundance  in  some  rivers  and  estuaries 
that  their  multiplication  is  affirmed  by  Professor  Ehrenberg  to  have 
exercised  an  important  influence  in  blocking  up  harbours  and 
diminishing  the  depth  of  channels  !  Of  their  extraordinary  abundance 
in  certain  parts  of  the  ocean  the  best  evidence  is  afforded  by  the 
observations  of  Sir  J.  D.  Hooker  upon  the  Diatomacece  of  the  southern 
seas ;  for  within  the  Antarctic  Circle  they  are  rendered  peculiarly 
conspicuous  by  becoming  enclosed  in  the  newly  formed  ice,  and  by 


FIG.  467. — Fossil  Diatomacese,  &c.,  from  Oran :  a,  a,  a,  Coscinodiscus;  0,  b,  b, 
Actinocyclus ;  c,  Dictyochya  fibula  ;  d,  Lithasteriscug  radiatus  ;  .e,  Spongolithis 
acicularis  ;  /,  /,  Grammatophora  parallela  (side  view) ;  g,  g,  GrammatopJiora 
angulosa  (front  view). 

being  washed  up  in  myriads  by  the  sea  on  to  the  '  pack  '  and  *  bergs,' 
everywhere  staining  the  white  ice  and  snow  a  pale  ochreous  brown. 
A  deposit  of  mud,  chiefly  consisting  of  the  siliceous  valves  of  Diato- 
macece,  not  less  than  400  miles  long  and  120  miles  broad,  was  found 
at  a  depth  of  between  200  and  400  feet  on  the  flanks  of  Victoria  Land 
in  70°  south  latitude.  Of  the  thickness  of  this  deposit  no  conjecture 
could  be  formed  ;  but  that  it  must  be  continually  increasing  is  evi- 
dent, the  silex  of  which  it  is  in  a  great  measure  composed  being 
indestructible.  A  fact  of  peculiar  interest  in  connection  with 
this  deposit  is  its  extension  over  the  submarine  flanks  of  Mount 
Erebus,  an  active  volcano  of  12,400  feet  elevation,  since  a  commu- 
nication between  the  ocean  waters  and  the  bowels  of  a  volcano,  sucli 


DIATOMAC&E  62 1 

as  there  are  other  reasons  for  believing  to  be  occasionally  formed, 
would  account  for  the  presence  of  Uiatomaoew  in  volcanic  ashes 
and  pumice  which  was  discovered  by  Professor  Ehrenberg.  It  is 
remarked  by  Sir  J.  D.  Hooker  that  the  universal  presence  of  this 
microscopic  vegetation  throughout  the  South  Polar  Ocean  is  a  most 
important  feature,  since  there  is  a  marked  deficiency  in  this 
region  of  higher  forms  of  vegetation  ;  and  were  it  not  for  them,  there 
would  neither  be  food  for  aquatic  animals,  nor  (if  it  were  possible 
for  these  to  maintain  themselves  by  preying  on  one  another)  could 
the  ocean  waters  be  purified  of  the  carbonic  acid  which  animal 
respiration  and  decompositionXypuld  be  continually  imparting  to  them. 


FIG.  468.— Fossil  Diatomaceae,  &c.,  from  Mourne  Mountains,  Ireland :  a,  a,  a, 
Gaillonella  (Meh>sira)procera  and  G.  granulata  ;  d,  d,  rf,  G.biseriata  (side  view) ; 
b,  b,  SurireUa  plica t a ;  c,  S.  craticula;  k,  S.  caledonica;  e,  GompJwnema 
gracile;  /,  Cocconema  fiiaidhun  ;  g,  Tabellaria  vulgaris',  li,  Pinnularia 
dactylus  ;  /,  P.  nobilis  ;  /,  Sij)iedra  ulna. 

It  is  interesting  to  observe  that  some  species  of  marine  diatoms  are 
found  through  every  degree  of  latitude  between  Spitzbergen  and 
Victoria  Land,  whilst  others  seem  limited  to  particular  regions.  One 
of  the  most  singular  instances  of  the  preservation  of  diatomaceous 
forms  is  their  existence  in  guano,  into  which  they  must  have  passed 
from  the  intestinal  canals  of  the  birds  of  whose  accumulated  excre- 
ment that  substance  is  composed,  those  birds  having  received  them,  it 
is  probable,  from  shell-fish,  to  which  these  minute  organisms  serve 
as  ordinary  food. 

The  indestructible  nature  of  the  silicified  casings  of  Diatomacece 
has  also  served  to  perpetuate  their  presence  in  numerous  localities 
from  which  their  living  forms  have  long  since  disappeared  ;  for  the 


622   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

accumulation  of  sediment  formed  by  their  successive  production  and 
death,  even  on  the  bed  of  the  ocean  or  on  the  bottoms  of  fresh- 
water lakes,  gives  rise  to  deposits  which  may  attain  considerable 
thickness,  and  which,  by  subsequent  changes  of  level,  may  come  to 
form  part  of  the  dry  land.  Thus  very  extensive  siliceous  strata, 
consisting  almost  entirely  of  marine  J)iatomacece,  are  found  to  alter- 
nate, in  the  neighbourhood  of  the  Mediterranean,  with  calcareous 
strata  chiefly  formed  of  Foraminifera,  the  whole  series  being  the 
representative  of  the  chalk  formation  of  Northern  Europe,  in  which 
the  silex  that  was  probably  deposited  at  first  in  this  form  has  under- 
gone conversion  into  flint,  by  agencies  hereafter  to  be  considered. 
Of  the  diatomaceous  composition  of  these  strata  we  have  a  character- 
istic example  in  fig.  467,  which  represents  the  fossil  Diatomacece  of 
Oran  in  Algeria.  The  so-called  '  infusorial  earth  '  of  Richmond  in 
Virginia,  and  also  that  of  Bermuda,  both  marine  deposits,  are  very 
celebrated  among  microscopists  for  the  number  and  beauty  of  the 
forms  they  have  yielded  ;  the  former  constitutes  a  stratum  of  eighteen 
feet  in  thickness,  underlying  the  whole  city,  and  extending  over  an 
area  whose  limits  are  not  known.  Several  deposits  of  more  limited 
extent,  and  apparently  of  fresh-water  origin,  have  been  found  in  our 
own  islands  ;  as,  for  instance,  at  Dolgelly  in  North  Wales,  at  South 
Mourne  in  Ireland  (fig.  468),  and  in  the  island  of  Mull  in  Scotland. 
Similar  deposits  in  Sweden  and  Norway  are  known  under  the  name 
of  Bergmehl,  or  mountain -flour  ;  and  in  times  of  scarcity  the  inha- 
bitants of  those  countries  are  accustomed  to  mix  these  substances 
with  their  dough  in  making  bread.  This  has  been  supposed  merely 
to  have  the  effect  of  giving  increased  bulk  to  their  loaves,  so  as  to 
render  the  really  nutritive  portion  more  satisfying ;  but  as  the 
Bergmehl  has  been  found  to  lose  from  a  quarter  to  a  third  of  its 
weight  by  exposure  to  a  red  heat,  there  seems  a  strong  probability 
that  it  contains  organic  matter  enough  to  render  it  nutritious  in 
itself.  When  thus  occurring  in  strata  of  a  fossil  or  sub- fossil 
character,  the  diatomaceous  deposits  are  generally  distinguishable  as 
white  or  cream-coloured  powders  of  extreme  fineness. 

For  collecting  fresh  Diatomacece  those  general  methods  are  to  be 
had  recourse  to  which  have  been  already  described.  *  Their  living 
masses,'  says  Mr.  W.  Smith,  '  present  themselves  as  coloured  fringes 
attached  to  larger  plants,  or  forming  a  covering  to  stones  or  rocks 
in  cushion -like  tufts — or  spread  over  their  surface  as  delicate  velvet — 
or  depositing  themselves  as  a  filmy  stratum  on  the  mud,  or  inter- 
mixed with  the  scum  of  living  or  decayed  vegetation  floating  on  the 
surface  of  the  water.  Their  colour  is  usually  a  yellowish-brown  of  a 
greater  or  less  intensity,  varying  from  a  light  chestnut  in  individual 
specimens  to  a  shade  almost  approaching  black  in  the  aggregated 
masses.  Their  presence  may  often  be  detected,  without  the  aid  of  a 
microscope,  by  the  absence,  in  many  species,  of  the  fibrous  tenacity 
which  distinguishes  other  plants  ;  when  removed  from  their  natural 
position,  they  become  distributed  through  the  water,  and  are  held  in 
suspension  by  it,  only  subsiding  after  some  little  time  has  elapsed.' 
Notwithstanding  every  care,  the  collected  specimens  are  liable  to  be 
mixed  with  much  foreign  matter ;  this  may  be  partly  got  rid  of  by 


DIATOMACEyE  623 

repeated  washings  in  pure  water,  and  by  taking  advantage,  at  the 
same  time,  of  the  different  specific  gravities  of  the  diatoms  and  of 
the  intermixed  substances,  to  secure  their  separation.  Sand,  being 
the  heaviest,  will  subside  first ;  fine  particles  of  mud,  on  the  other 
hand,  will  float  after  the  diatoms  have  subsided.  The  tendency  of 
living  diatoms  to  make  their  way  towards  the  light  will  afford  much 
assistance  in  procuring  the  free  forms  in  a  tolerably  clean  state  ;  for 
if  the  gathering  which  contains  them  be  left  undisturbed  for  a  suffi- 
cient length  of  time  in  a  shallow  vessel  exposed  to  the  sunlight,  they 
may  be  skimmed  from  the  surface.  Marine  forms  must  be  looked 
for  upon  seaweeds,  and  in  the^fijae  mud  or  sand  of  soundings  or 
dredgings ;  they  are  frequently  found  also,  in  considerable  numbers, 
in  the  stomachs  of  Holothurise,  Ascidians,  and  Salpae,  in  those  of  the 
oyster,  scallop,  whelk,  and  other  testaceous  molluscs,  in  those  of  the 
crab  and  lobster,  and  other  Crustacea,  and  even  in  those  of  the  sole, 
turbot,  and  other  flat-fish.  In  fact,  the  diatom  collector  will  do 
well  to  examine  the  digestive  cavity  of  any  small  aquatic  animals 
that  may  fall  in  his  way,  rare  and  beautiful  forms  having  been 
obtained  from  the  interior  of  Noctiluca.  The  separation  of  the 
diatoms  from  the  other  contents  of  these  stomachs  must  be  accom- 
plished by  the  same  process  as  that  by  which  they  are  obtained 
from  guano  or  the  calcareous  *  infusorial  earths.'  Of  this  the 
following  are  the  most  essential  particulars  :  The  guano  or  earth  is 
first  to  be  washed  several  times  in  pure  water,  which  should  be  well 
stirred,  and  the  sediment  then  allowed  to  subside  for  some  hours 
before  the  water  is  poured  off;  since,  if  it  be  decanted  too  soon,  it 
may  carry  the  lighter  forms  away  with  it.  Some  kinds  of  earth 
have  so  little  impurity  that  one  washing  suffices ;  but  in  any  case  it 
is  to  be  continued  so  long  as  the  water  remains  coloured.  The 
deposit  is  then  to  be  treated,  in  a  flask  or  test-tube,  with  hydro- 
chloric acid,  and,  after  the  first  effervescence  is  over,  a  gentle 
heat  may  be  applied.  As  soon  as  the  action  has  ceased,  and  time 
has  been  given  for  the  sediment  to  subside,  the  acid  should  be 
poured  off  and  another  portion  added  ;  and  this  should  be  repeated 
as  often  as  any  effect  is  produced.  When  hydrochloric  acid  ceases 
to  act,  strong  nitric  acid  should  be  substituted ;  and  after  the  first 
effervescence  is  over,  a  continued  heat  of  about  200°  F.  should  be 
applied  for  some  hours.  When  sufficient  time  has  been  given  for 
subsidence,  the  acid  may  be  poured  off  and  the  sediment  treated  with 
another  portion  ;  and  this  is  to  be  repeated  until  no  further  action 
takes  place.  The  sediment  is  then  to  be  washed  until  all  trace  of 
the  acid  is  removed  ;  and,  if  there  have  been  no  admixture  of  siliceous 
sand  in  the  earth  or  guano,  this  sediment  will  consist  almost  entirely 
of  Diatomacece,  with  the  addition,  perhaps,  of  sponge-spicules.  The 
separation  of  siliceous  sand  and  the  subdivision  of  the  entire  aggre- 
gate of  diatoms  into  the  larger  and  the  finer  kinds,  may  be  accom- 
plished by  stirring  the  sediment  in  a  tall  jar  of  water,  and  then, 
while  it  is  still  in  motion,  pouring  off  the  supernatant  fluid  as  soon 
as  the  coarser  particles  have  subsided  ;  this  fluid  should  be  set  aside, 
and,  as  soon  as  a  finer  sediment  has  subsided,  it  should  again  be 
poured  off;  and  this  process  may  be  repeated  three  or  four  times  at 


624  MICROSCOPIC  FOKMS  OF  VEGETABLE  LIFE— THALLOPHYTES 

increasing  intervals,  until  no  further  sediment  subsides  after  the 
lapse  of  half  an  hour.  The  first  sediment  will  probably  contain  all 
the  sandy  particles,  with,  perhaps,  some  of  the  largest  diatoms, 
which  may  be  picked  out  from  among  them  ;  and  the  subsequent 
sediments  will  consist  almost  exclusively  of  diatoms,  the  sizes  of 
which  will  be  so  graduated  that  the  earliest  sediments  may  be 
examined  with  the  lower  powers,  the  next  with  medium  powers, 
while  the  latest  will  require  the  highest  powers — a  separation  which 
is  attended  with  great  convenience.1  It  sometimes  happens  that 
fossilised  diatoms  are  so  strongly  united  to  each  other  by  siliceous 
cement  as  not  to  be  separable  by  ordinary  methods ;  in  this  case, 
small  lumps  of  the  deposit  should  be  boiled  for  a  short  time  in  a 
weak  alkaline  solution,  which  will  act  upon  this  cement  more  readily 
than  on  the  siliceous  frustules ;  and  as  soon  as  the  lump  is  softened, 
so  as  to  crumble  to  mud,  this  must  be  immediately  washed  in  a  large 
quantity  of  water,  and  then  treated  in  the  usual  way.  If  a  very 
weak  alkaline  solution  does  not  answer  the  purpose,  a  stronger  one 
may  then  be  tried.  This  method,  devised  by  Professor  Bailey,  has 
been  practised  by  him  with  much  success  in  various  cases.2 

The  mode  of  mounting  specimens  of  Diatomacece  will  depend 
upon  the  purpose  which  they  are  intended  to  serve.  If  they  can  be 
obtained  quite  fresh,  and  if  it  be  desired  that  they  should  exhibit,  as 
closely  as  possible,  the  appearance  presented  by  the  living  plants, 
they  should  be  put  up  in  aqueous  media  within  cement-cells  ;  but  if 
they  are  not  thus  mounted  within  a  short  time  after  they  have  been 
gathered,  about  a  tenth  part  of  alcohol  should  be  added  to  the  water. 
If  it  be  desired  to  exhibit  the  stipitate  forms  in  their  natural  position 
adherent  to  other  aquatic  plants,  the  entire  mass  may  be  mounted 
in  Deane's  medium  or  in  glycerin  jelly,  in  a  deeper  cell ;  and  such  a 
preparation  is  a  very  beautiful  object  for  the  background  illumina- 
tion. If,  on  the  other  hand,  the  minute  structure  of  the  siliceous 
envelopes  is  the  feature  to  be  brought  into  view,  the  fresh  diatoms 
must  be  boiled  in  nitric  or  hydrochloric  acid,  which  must  then  be 
poured  off  (sufficient  time  being  allowed  for  the  deposit  of  the 
residue)  ;  and  the  sediment,  after  being  washed,  should  be  boiled  in 
water  with  a  small  piece  of  soap,  whereby  the  diatoms  will  be  cleansed 
from  the  flocculent  matter  which  they  often  obstinately  retain.3 
After  a  further  washing  in  pure  water,  they  are  to  be  either  mounted 
in  balsam  in  the  ordinary  manner,  or  be  set  up  *  dry '  on  a  very  thin 
slide.  In  order  to  obtain  a  satisfactory  view  of  their  markings, 
objectives  of  very  large  aperture  are  required,  and  all  the  improve- 

1  A  somewhat  more  complicated  method  of  applying  the  same  principle  is  described 
by  Mr.  Okeden  in   the    Quart.    Journ.   Microsc.    Science,   vol.   iii.   1855,   p.    158. 
The  Author  believes,  however,  that  the  method  above  described  will  answer  every 
purpose. 

2  For  other  methods  of  cleaning  and  preparing  diatoms,  see  Quart.  Journ.  of 
Microsc.  Science,  vol.  vii.  1859,  p.  167,  and  vol.  i.  n.s.  1861,  p.  143 ;  and  Trans,  of 
Microsc.  Soc.  vol.  xi.  n.s.  1863,  p.  4.     A  little  book  entitled  Practical  Directions 
for  Collecting,  Preserving,  Transporting,  Preparing,  and  Mounting  Diatoms  (New 
York,  1877),  containing  papers  by  Professors  A.  Mead  Edwards,  Christopher  Johnson, 
and  Hamilton  L.  Smith,  will  be  found  to  contain  much  useful  information. 

5  See  Prof.  H.  L.  Smith  in  Amer.  Journ.  of  Microscopy,  vol.  v.  1880,  p.  257. 
It  is  important  that  the  soap  should  be  free  from  kaolin,  silex,  or  any  other  insoluble 
matter. 


DIATOMACE.E  ;    PH^OSPOREjE  625 

ments  which  have  recently  been  introduced  in  the  construction  and 
mode  of  using  the  sub-stage  condenser  require  to  be  put  into  prac- 
tice. But  to  those  who  have  the  time,  the  will,  and  the  appliances, 
there  is  a  fine  field  now  open  for  working,  to  a  far  higher  point  than 
we  have  touched  at  present,  the  true  structure  of  such  diatoms  as 
can  be  made  amenable  to  the  powers  possessed  by  our  best  recent 
optical  appliances  ;  and  for  the  leisure  of  a  professional  or  commercial 
man  we  know  of  no  more  suitable  and  attractive  employment  for  the 
microscope.  It  will  often  be  convenient  to  mount  certain  particular 
forms  of  Diatomacece  separately  from  the  general  aggregate  ;  but,  on 
account  of  their  minuteness,  they^cannot  be  selected  and  removed 
by  the  usual  means.  The  larger  forms,  which  maybe  readily  distin- 
guished under  a  simple  microscope,  may  be  taken  up  by  a  camel's- 
hfiir  pencil  which  has  been  so  trimmed  as  to  leave  two  or  three  hairs 
projecting  beyond  the  rest.  But  the  smaller  can  only  be  dealt  with 
by  a  single  fine  bristle  or  stout  sable-hair,  which  may  be  inserted 
into  the  cleft  end  of  a  slender  wooden  handle ;  and  if  the  bristle  or 
hair  should  be  split  at  its  extremity  in  a  brush-like  manner  it  will 
be  particularly  useful.  (Such  split  hairs  may  always  be  found  in  a 
shaving-brush  which  has  been  for  some  time  in  use ;  those  should  be 
selected  which  have  their  split  portions  so  closely  in  contact  that 
they  appear  single  until  touched  at  tfteir  ends.)  When  the  split 
extremity  of  such  a  hair  touches  the  glass  slide,  its  parts  separate 
from  each  other  to  an  amount  proportionate  to  the  pressure  ;  and,  on 
being  brought  up  to  the  object,  first  pushed  to  the  edge  of  the  fluid 
on  the  slide,  may  generally  be  made  to  seize  it.  A  very  experienced 
American  diatomist,  Professor  Hamilton  Smith,  strongly  recom- 
mends a  thread  of  glass  drawn  out  to  capillary  fineness  and  flexi- 
bility, by  which  (he  says)  the  most  delicate  diatom  may  be  safely 
taken  up,  and  deposited  upon  a  slide  damped  by  the  breath.  For 
the  selection  and  transference  of  diatoms  under  the  compound 
microscope,  recourse  may  be  had  to  some  of  the  forms  of  '  mechanical 
finger '  which  have  been  devised  by  American  diatomists.1 

Phseosporese. — The  greater  number  of  the  seaweeds  exhibit  a 
higher  type  of  organisation  than  any  that  has  hitherto  been  described. 
The  old  classification  of  seaweeds  into  Melanosporece,  Rhodosporecv, 
and  Chlorosporece,  according  as  their  colouring  matter  is  olive-brown, 
red,  or  green,  cannot  altogether  be  retained.  Under  the  head  of 
PhoeosporecB  are  now  included  a  very  large  number  of  the  brown  and 
olive-brown  seaweeds.  In  ascending  this  series  we  shall  have  to 
notice  a  gradual  differentiation  of  organs,  those  set  apart  for  repro- 
duction being  in  the  first  place  separated  from  those  appropriated 

1  For  a  description  of  those  of  Prof.  Hamilton  Smith  and  Dr.  Kezner,  see  Journ. 
of  Boy.  Microsc.  Soc.  vol.  ii.  1879,  p.  951,  and  that  of  Mr.  Veeder,  vol.  iii.  1880, 
p.  700,  of  the  same  Journal. 

[A  very  large  number  of  observations  have  been  made  during  recent  years  by 
Castracane,  O.  Miiller,  Lauterborn,  Comber,  Murray,  Miquel,  and  others,  on  the 
structure  of  the  diatom-valve,  on  the  various  modes  of  reproduction,  and  on  the 
phenomena  accompanying  their  apparently  spontaneous  powers  of  motion,  and 
several  schemes  of  classification  of  the  genera  have  been  proposed.  On  these,  too 
numerous  to  mention  here,  and  some  of  which  still  require  confirmation,  the  reader 
should  consult  the  successive  volumes  of  the  Journal  of  the  Royal  Microscopical 
Society.—  ED.] 

S  S 


626  MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

to  nutrition  ;  while  the  principal  parts  of  the  nutritive  apparatus, 
which  are  at  first  so  blended  into  a  uniform  expansion  or  thallus 
that  no  real  distinction  exists  between  root,  stem,  and  leaf,  are 
progressively  evolved  on  types  more  and  more  peculiar  to  each 
respectively,  and  have  their  functions  more  and  more  limited  to 
themselves  alone.  Hence  we  find  a  '  differentiation,'  not  merely  in 
the  external  form  of  organs,  but  also  in  their  internal  structure,  its 
degree  bearing  a  close  correspondence  to  the  degree  in  which  their 
functions  are  respectively  specialised  or  limited  to  particular  actions. 
But  this  takes  place  by  very  slow  gradations,  a  change  of  external 
form  often  showing  itself  before  there  is  any  decided  differentiation 
either  in  structure  or  function.  Thus  in  the  simple  Ulvacece,  what- 
ever may  be  the  extent  of  the  thallus,  every  part  has  exactly  the 
same  structure,  and  performs  the  same  actions,  as  every  other  part, 
living  for  and  by  itself  alone.  And  though,  when  we  pass  to  the 
higher  seaweeds,  such  as  the  common  Fucus  and  Laminaria,  we 
observe  a  certain  foreshadowing  of  the  distinction  between  root, 
stem,  and  leaf,  this  distinction  is  very  imperfectly  carried  out,  the 
root-like  and  stem-like  portions  serving  for  little  else  than  the 
mechanical  attachment  of  the  leaf-like  part  of  the  plant.  There  is 
not  yet  any  departure  from  .the  simple  cellular  type  of  structure, 
the  only  modification  being  that  the  several  layers  of  cells,  where 
many  exist,  are  of  different  sizes  and  shapes,  the  texture  being 
usually  closer  on  the  exterior  and  looser  within^  and  that  the  tex- 
ture of  the  stem  and  roots  is  denser  than  that  of  the  leaf-like  expan- 
sions or  fronds.  The  cells  of  the  Phceosporece  contain  a  substance 
closely  resembling  starch,  and  an  olive-brown  pigment,  which  they 
share  with  the  Fucacece,  known  as phyco-phcein  or  fuco-xanthin.  The 
group  of  olive-green  seaweeds  presents  us  with  the  lowest  type  in 
the  family  Ectocarpacece,  which,  notwithstanding,  contains  some  of  the 
most  elegant  structures  that  are  anywhere  to  be  found  in  the  group, 
the  full  beauty  of  which  can  only  be  discerned  by  the  microscope. 
Such  is  the  case,  for  example,  with  Sphacelaria,  a  small  and  delicate 
seaweed,  which  is  very  commonly  found  growing  upon  larger  algre, 
either  near  low-water  mark  or  altogether  submerged,  its  general 
form  being  remarkably  characterised  by  a  symmetry  that  extends  also 
to  the  individual  branches,  the  ends  of  which,  however,  have  a 
decayed  look.  The  apical  cell  of  each  branch  is  uncorticated,  and 
frequently  develops  into  a  hollow  chamber  of  considerable  size, 
termed  a  sphacele,  and  filled,  when  young,  with  a  dark  mucilaginous 
substance  which,  at  a  later  stage,  becomes  watery.  The  Sphacela- 
riacece  are  propagated  in  a  non-sexual  manner  by  peculiar  buds  or 
gemmae  known  as  propayules. 

The  ordinary  mode  of  propagation  of  the  Phceosporece  is  by  non- 
sexual  zoospores ;  and  these  are  of  two  kinds,  produced  respectively  in 
unilocular  and  multilocular  zoosporanges.  The  former  are  compara- 
tively large,  nearly  spherical,  ovoid  or  pear-shaped  cells,  the  contents 
of  which  break  up  into  a  large  number  of  zoospores.  The  multi- 
locular zoosporanges  have  the  appearance  of  jointed  hairs,  and  are 
divided  internally  into  a  number  of  chambers,  each  of  which  gives 
birth  to  a  single  zoospore.  The  zoospores  from  the  unilocular 


PH.ffiOSPOKE.iE;  FUCACEjE 


627 


sporanges  appear  in  all  cases  to  germinate  directly,  while  those  from 
the  multilocular  sporanges  sometimes  coalesce  in  pairs  before  ger- 
minating.    The  different  families   of  Phceosporece   present  a  most 
interesting  gradual  transition  from  the  conjugation  of  swarm-cells 
to  the  impregnation  of  a  female  *  ob'sphere  '  by  male  antherozoids. 
In  Ectocarpus,  Giraudia,  and  Scytosiphon,  conjugation  takes  place 
between  swarm-cells  from  the  multilocular  sporanges  which  appear 
to  be  exactly  alike,  but  a  slight  differentiation  is  exhibited  in  one  of 
them  coming  to  rest  and  partially  losing  its  cilia  before  conjugation 
takes  place  (fig.  469,  II).     Malfe  Asexual  organs  also  occur  in  the 
Sphacelariacece,  but  no  actual  process  of  conjugation  has  as  yet  been 
observed.     In  Cutleria  and  Zaiiar- 
dinia  the    differentiation    is   more 
complete.      The    male   and    female 
swarm-cells  are  produced  either  on 
the  same  or  on  different  individuals  ; 
the  latter  are  much  larger  than  the 
former,  and  come  perfectly  to  rest, 
entirely   losing    their   cilia    before 
being  impregnated  by  the   former. 
In   Dictyota  the   differentiation    is 
( -ariied      still      further,      and     the 
female  reproductive  bodies  are  true 
'  obspheres/    being    from   the    first 
motionless  masses  of  protoplasm  not 
provided  with   cilia,  while  the  an- 
therozoids exhibit  motility  only  for 
a  very  short  time,  and  each  is  pro- 
vided only  with  a  single  cilium  of 
unusual    length.       In    the    family 
Laminar  iacece,    belonging     to    the 
Phceosporece,  are  included  many  of 
the  largest  of  the  seaweeds,  chiefly 
natives  of  southern  seas,  the  frond 
often    attaining    enormous   dimen- 
sions, and    exhibiting  rudimentary 
differentiation     into      rhizoids     or 
organs    of   attachment,    stem,   and 
leaves.     Such   are   Lessonia,  which 
grows  to   a   great   height   and   re- 
sembles a  branching  tree  with  pendent  leaves  two  or  three  feet 
long  ;  Macrocystis,  where  the  stalk  like  base  of  each  branch  of  the 
leaf  is  hollowed  out  into  a  large  pear-shaped  air-bladder  ;  Nereocystis, 
Laminaria,  and  others. 

In  the  Fucaceae  the  generative  apparatus  is  contained  in  the 
globular  '  conceptacles/  which  are  usually  sunk  in  the  tissue  near  the 
extremities  of  the  fronds.  In  some  species,  as  Fucus  platycarpus, 
the  same  conceptacles  contain  both  *  antherids '  and  l  obgones  ; '  in 
others  these  two  sexual  elements  are  disposed  in  different  conceptacles 
on  the  same  plant ;  whilst  in  the  commonest  of  all,  F.  vesiculosus 
(bladder- wrack),  they  are  limited  to  different  individuals.  When  a 

s  s  2 


FIG.  469. — Process  of  conjugation 
in  Ectocarpus  silicnlosus.  (From 
Vines's  '  Physiology.')  I.  a-/,  the 
female  zobspore  coming  to  rest ; 
II.,  the  female  zoospore  at  rest, 
surrounded  by  male  zoospores  ; 
III.  a-e,  fusion  of  male  and  female 
zoospores. 


628   MICROSCOPIC  FORMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

section  is  made  through  one  of  the  flattened  conceptacles  of  F.  platy- 
carpus,  its  interior  is  seen  to  be  a  nearly  globular  cavity  (fig.  470), 
lined  with  hairs,  some  of  which  are  greatly  elongated,  so  as  to 
project  through  the  pore  by  which  the  cavity  opens  on  the  surface. 
Among  these  are  to  be  distinguished,  towards  the  period  of  their 
maturity,  certain  filaments  (fig.  471,  A),  the  antherids,  whose 
granular  contents  acquire  an  orange  hue,  and  gradually  shape 
themselves  into  oval  bodies  (B),  each  with  an  orange -coloured  spot 
and  two  vibratile  cilia  of  unequal  length,  placed  laterally,  which, 
when  discharged  by  the  rupture  of  the  containing  cell,  have  for  a 

time  a  rapid,  undulatory 
motion  whereby  these 
antherozoids  are  diffused 
through  the  surround- 
ing liquid.  Lying  amidst 
the  mass  of  hairs,  near 
the  walls  of  the  cavity, 
are  seen  (fig.  470) 
numerous  dark  pear- 
shaped  bodies,  which  are 
the  oogones,  or  parent- 
cells  of  the  oospheres. 
Each  of  these  oogones 
gives  origin,  by  binary 
subdivision,  to  a  cluster 
of  eight  '  germ-cells  '  or 
oospheres ;  and  these 
are  liberated  from  their 
envelopes  before  the  act 
of  fertilisation  takes 
place.  This  act  consists 
in  the  swarming  of  the 
antherozoids  over  the 
surface  of  the  oospheres, 
to  which  they  communi- 
cate a  rotatory  motion 
by  the  vibration  of  their 
own  cilia.  In  the  herm- 

FIG.  470. — Vertical  section  of  conceptacle  of  Fucus  aphrodite  Fuel  this  takes 
platy  corpus (lined  with  filaments, ,  among  which  lie  lace  wjthin  the  COn- 
the  antheridial  cells  and  the  oogones  containing  * 

oospheres.  ceptacles,    so    that    the 

oospheres  do  not  make 

their  exit  from  the  cavity  until  after  they  have  been  fecundated  ; 
but  in  the  monoecious  and  dioecious  species  each  kind  of  conceptacle 
separately  discharges  its  contents,  which  come  into  contact  on  their 
exterior.  The  antheridial  cells  are  usually  ejected  entire,  but  soon 
rupture  so  as  to  give  exit  to  the  antherozoids ;  and  the  oogones  also 
discharge  their  oospheres,  which,  meeting  with  antherozoids,  are 
fecundated  by  them.  The  fertilised  odspores  soon  acquire  a  new  and 
firm  envelope  ;  and,  under  favourable  circumstances,  they  speedily 
begin  to  develop  themselves  into  new  plants.  The  first  change  is 


FUCACEJE 


629 


the  projection  and  narrowing  of  one  end  into  a  kind  of  foot-stalk, 
by  which  the  oospore  attaches  itself,  its  form  passing  from  the 
globular  to  the  pear-shaped  ;  a  partition  is  speedily  observable  in  its 
interior,  its  single  cell  being  subdivided  into  two  ;  and  by  a  con- 
tinuation of  a  like  process  of  bipartition,  first  a  filament  and  then 
a  frondose  expansion  is  produced,  which  gradually  evolves  itself 
into  the  likeness  of  the  parent  plant. 

The  whole  of  this  process  may  be  watched  without  difficulty  by 
obtaining  specimens  of  F.  vesiciilostis  at  the  period  at  which  the 
fructification  is  shown  to  be  mature  by  the  recent  discharge  of  the 
contents  of  the  conceptacles  in  Irltle  gelatinous  masses  outside  their 
orifices  ;  for  if  some  of  the  oospheres  which  have  been  set  free  from 
the  olive-green  (female)  conceptacles  be  placed  in  a  drop  of  sea- 
water  in  a  very  shallow  cell,  and  a  small  quantity  of  the  mass  of 


PIG.  471.— Antherids  and  antherozoids  of  Fucus  platijcarpus :  A,  branching 
articulated  hairst  detached  from  the  walls  of  the  conceptacle,  bearing  antherids  in 
different  stages  of  development ;  B,  antherozoids,  some  of  them  free,  others  still 
included  in  their  antheridial  cells. 

antherozoids,  set  free  from  the  orange-yellow  (male)  conceptacles, 
be  mingled  with  the  fluid,  they  will  speedily  be  observed,  with  the 
aid  of  a  magnifying  power  of  200  or  250  diameters,  to  go  through 
the  actions  just  described ;  and  the  subsequent  processes  of  germi- 
nation may  be  watched  by  means  of  the  '  growing  slide.' x  The 
winter  months,  from  December  to  March,  are  the  most  favourable 
for  the  observation  of  these  phenomena ;  but  where  Fuel  abound, 
some  individuals  will  usually  be  found  in  fructification  at  almost 
any  period  of  the  year.  This  process  of  fertilisation  usually  takes 
place  on  fronds  exposed  to  the  air  on  the  wet  beach  between  high- 
and  low- water  mark  ;  and,  to  assist  in  it,  the  comparatively  heavy 
fronds  of  many  Fucacece  are  buoyed  up  by  air-cavities,  which  take 
the  form  of  the  well-known  '  bladders  '  of  the  '  bladder- wrack '  and 

1  A  shallow  cell  should  be  used,  so  as  to  keep  the  pressure  of  the  thin  glass  from 
the  minute  bodies  beneath,  whose  movements  it  will  otherwise  impede. 


630  MICKOSCOPIC  FORMS  OF  VEGETABLE  LIFE-THALLOPHYTES 

other  species  of  Fucua,  imbedded  in  the  frond,  and  the  '  berries  '  of 
Sargassum  bacciferum,  the  'gulf- weed'  of  the  Atlantic,  which  are 
elevated  on  pedicels  above  the  surface  of  the  water.     The  whole 
substance    of  the    Fucacece,  including  the  reproductive    organs,    i 
coloured  brown  by  fuco-xanthin,  the  same  pigment  as  that  which  n 
found  in  the  Phceosporece. 

Among  the  Floridese,  or  red  seaweeds,  also,  we  find  various 
simple  but  most  beautiful  forms,  which  connect  this  group  with  the 
lower  algae,  especially  with  the  family  Coleochcetacece  ;  such  delicate 
feathery  or  leaf-like  fronds  belong  for  the  most  part  to  the  family 
Ceramiacece,  some  members  of  which  are  found  upon  every  part  of 


Fig.  472. — Arrangement  of  tetraspores  in  Carpocaulon  mediterraneum  :  A,  entire 
plant;  B,  longitudinal  section  of  spore-bearing  branch.  (N.B. — "Where  only  three 
tetraspores  are  seen,  it  is  merely  because  the  fourth  did  not  happen  to  be  so  placed 
as  to  be  seen  at  the  same  view.) 


our  coasts,  attached  either  to  rocks  or  stones  or  to  larger  algae,  and 
often  themselves  affording  an  attachment  to  zoophytes  and  polyzoa. 
They  chiefly  live  in  deeper  water  than  the  other  seaweeds,  and 
their  richest  tints  are  only  exhibited  when  they  grow  under  the 
shade  of  projecting  rocks  or  of  larger  dark-coloured  algae.  Hence, 
in  growing  them  artificially  in  aquaria,  it  is  requisite  to  protect 
them  from  an  excess  of  light,  since  otherwise  they  become  unhealthy. 
Various  species  of  the  genera  Ceramium,  Griffithsia,  Callithamnion, 
and  Ptilota  are  extremely  beautiful  objects  for  low  powers  when 
mounted  in  glycerin  jelly.  In  many  of  them  the  phenomenon  to 
which  we  have  previously  referred  under  the  name  of  '  continuity  of 
protoplasm '  is  very  beautifully  exhibited.  The  colour  of  the  red 


FLOEIDE.U 


63I 


seaweeds  is  due  to  the  presence  of  a  pigment  known  as  rhodospermin 
or  phyco-erythrin,  soluble  in  fresh  water,  which  may  be  separated  in 
the  form  of  beautiful  regular  crystals. 

The  only  mode  of  propagation  which  was  until  recently  known 
to  exist  in  this  group  of  seaweeds  is  the  production  and  liberation 
of  tetraspores  (fig.  472,  B),  formed  by  two  successive  binary 
subdivisions  of  the  contents  of  special  cells,  which  sometimes 
form  part  of  the  general  substance  of  the  frond,  but  sometimes 
congregate  in  particular  parts  or  are  restricted  to  special  branches. 
If  the  second  binary  division  takes  place  in  the  same  direction  as 
the  first,  the  tetraspores  are  ai¥anged  in  linear  series  ;  but  if  its 


IV 


FIG.  473. — Nemalion  mnltifidum  :  I,  a  branch  with  a  carpogone,  c,  and  pollinoids,  - 
sp  ;  II,  III,  commencement  of  the  formation  of  the  fructification ;  IV,  V,  develop- 
ment of  the  spore-cluster  ;  t  denotes  the  trichogyiie,  c  the  carpogone  and  fructifica- 
tion.    (From  Goebel's  '  Outline  of  Classification.'     The  Clarendon  Press.) 

direction  is  transverse  to  that  of  the  first,  the  four  spores  cluster 
together.  These,  when  separated  by  the  rupture  of  their  envelope, 
do  not  comport  themselves  as  zoospores  ;  but,  being  destitute  of 
propulsive  organs,  are  passively  dispersed  by  the  motion  of  the  sea 
itself.  Their  production,  however,  taking  place  by  simple  cell- 
division,  and  not  being  the  result  of  any  form  of  sexual  conjugation, 
the  tetraspores  of  the  Floridece  must  be  regarded,  like  the 
zoospores  of  the  Ulvacece,  as  gonids,  analogous  rather  to  the  buds 
than  to  the  seeds  of  higher  plants.  It  is  now  known  that  a  true 
sexual  process  takes  place  in  this  group ;  but  the  sexual  organs 
are  not  usually  found  on  the  plants  which  produce  tetraspores. 
Antheridial  cells  are  found,  sometimes  on  the  general  surface  of  the 
frond,  more  commonly  at  the  ends  of  branches,  and  occasionally  in 
special  conceptacles.  Their  contents,  however,  are  not  motile 


632   MICEOSCOPIC  FORMS  OF  VEGETABLE  LIFE—THALLOPHYTES 

antherozoids,  but  minute  rounded  particles,  known  as  pollinoids  or 
*  spermatia,'  having  no  power  of  spontaneous  movement.  Some- 
times on  the  same  individuals  as  the  antherids,  and  sometimes  on 
different  ones,  are  produced  the  female  organs,  which  curiously 
prefigure  the  pistil  in  flowering  plants.  This  organ  is  known  as 
the  procarp,  and  consists,  in  its  simplest  form,  e.g.  in  Porphyra,  the 
'  purple  laver,'  of  a  single  cell  with  a  lateral  hair-like  appendage,  the 
trichogyne.  In  the  higher  forms  it  is  composed  of  one  or  more 
fertile  cells  constituting  the  carpogone,,  and  one  or  more  sterile  cells 
which  make  up  the  trichophore,  and  convey  the  fertilising  substance 
from  the  trichogyne  to  the  carpogone.  Fertilisation  is  effected  by 
the  attachment  of  one  of  the  pollinoids  to  the  trichogyne,  the  walls  of 
which  are  absorbed  at  that  spot,  so  that  the  fertilising  material  passes 
down  its  tube  to  the  trichophore,  and  thence  to  the  carpogone  ;  one 
of  the  cells  of  the  carpogone  contains  the  oosphere,  which,  after 
fertilisation,  breaks  up  into  a  number  of  carpospores  ;  round  these 
is  frequently  formed  a  hard  investment,  and  this  structure  is  then 
known  as  a  cystocarp  ;  from  it  the  carpospores  ultimately  escape, 
and  then  germinate.  In  the  true  Corallines,  which  are  Floridecc 
whose  tissue  is  consolidated  by  calcareous  deposit,  not  only  the 
tetraspores,  but  also  both  kinds  of  sexual  organ,  are  produced  in 
cavities  or  conceptacles,  imbedded  in  the  thallus  or  forming  wart-like 
swellings  ;  the  female  conceptacle  opens  by  a  terminal  orifice  or 
ostiole ;  the  pollinoids  are  furnished  with  wing-like  appendages. 
In  a  considerable  number  of  the  red  seaweeds,  as,  for  example,  in 
Dudresnaya,  the  process  of  fertilisation  is  more  complex  than  this, 
and  consists  of  two  distinct  stages.  First  the  trichogyne  is  impreg- 
nated by  the  pollinoids  ;  and  secondly,  the  fertilising  principle  is 
then  conveyed  from  the  trichophore-cells  at  the  base  of  the  tricho- 
gyne to  the  cells  which  ultimately  produce  the  carpospores,  and  which 
may  be  at  a  considerable  distance  from  the  trichogyne,  even  on  a 
different  branch.  This  transference  is  effected  by  means  of  long 
simple  or  branched  tubes  which  are  known  as  '  fertilising  tubes.' 
The  late  Professor  F.  Schmitz  held  that,  in  the  higher  Floridea?, 
there  are  two  acts  of  fertilisation,  that  of  the  pollinoid  with  the 
trichogyne,  and  that  of  the  fertilising  tube  .with  the  cells  which  produce 
the  carpospores ;  but  this  view  is  not  accepted  by  all  authorities  ; 
and  it  is  doubtful  whether  more  than  one  true  act  of  fertilisation, 
i.e.  the  fusion  of  male  and  female  nuclei,  takes  place.  The  sexual 
mode  of  reproduction  has,  however,  at  present  been  observed  in 
comparatively  few  species  of  seaweed  ;  and,,  considering  the  number 
of  species  of  Floridece  found  on  our  coasts,  there  is  no  branch  of 
microscopical  observation  which  is  more  likely  to  reward  the  young 
investigator  with  new  discoveries. 


633 


CHAPTER   IX 


FUXGI,  as  already  mentioned,  differ  essentially  from  algae  in  the 
absence  of  chlorophyll,  and  therefore  in  the  absence  of  any  power  of 
directly  forming  starch  or  other  similar  substance  by  the  mutual 
decomposition  of  carbonic  acid  and  water,  accompanied  by  evolution 
of  oxygen.  They  must  therefore,  in  all  cases,  be  either  saprophytes 
or  parasites,  deriving  their  nourishment  from  already  organised  food- 
materials,  either,  in  the  former  case,  from  decaying  animal  or  vege- 
table substances,  or,  in  the  latter  case,  from  the  living  tissues  of 
other  plants  or  of  animals.  Fungus-parasites  are  the  cause  of  most 
of  the  diseases  to  which  plants,  and  of  a  large  number  of  those  to 
which  animals,  are  subject. 

The  individual  fungus  always  consists  of  one  or  more  hyph(f, 
slender  filaments  containing  protoplasm  and  a  nucleus  (except  possibly 
in  some  of  the  most  simple  forms),  but  no  chlorophyll  and  rarely  any 
pigment.  The  cell-wall  is  composed  of  a  substance  differing  some- 
what in  its  properties  from  ordinary  cellulose,  since  it  is  not  coloured 
blue  by  iodine  after  treatment  with  sulphuric  acid  ;  it  is  known  as 
fungus-cellulose.  These  hyphse  may  be  quite  distinct  or  very  loosely 
attached  to  one  another  ;  those  which  penetrate  the  soil,  or  the 
tissue  of  the  '  host  '  on  which  the  fungus  is  parasitic,  constitute  the 
mycele.  In  the  larger  fungi,  such  as  the  mushroom,  the  portion 
above  the  soil  is  composed  of  a  dense  mass  of  these  hyphae,  lying 
side  by  side,  constituting  a  so-called  pseudo-parenchyme,  but  never  a 
true  tissue.  In  some  families  the  hyphae  have  a  tendency  to  become 
agglomerated  into  balls  of  great  hardness  called  sclerotes,  which  have 
the  power  of  maintaining  their  vitality  for  very  long  periods.  The 
modes  of  reproduction  of  fungi,  both  sexual  and  non-sexual,  are 
very  various.  Among  the  latter  the  most  common  are  by  non- 
motile  spores  or  gonids,  and  by  zoospores.  The  former  are  very 
minute  bodies,  each  composed  of  a  single  cell,  or  less  often  of  several 
cells,  which  are  either  formed  within  a  spore-case  or  sporange,  or 
are  detached  from  the  extremity  of  hyphae  by  a  process  of  pinching 
off  or  abstriction.  From  their  extreme  lightness  they  are  wafted 
through  the  air  in  enormous  numbers,  and  thus  bring  about  the 
extraordinarily  rapid  spread  of  many  fungi,  such  as  moulds.  The 
zoospores  are,  like  those  of  the  lower  algae,  minute  naked  masses  of 
protoplasm  provided  with  one  or  more  vibratile  cilia,  by  means  of 
which  they  move  very  rapidly  through  water,  and  finally  force  their 
way  into  the  tissue  of  the  host,  where  the  zoospore  loses  its  cilia, 


634  FUNGI 

invests  itself  with  a  cell-wall,  and  proceeds  to  germinate.  This  is 
effected,  both  in  the  case  of  the  zob'spores  and  in  that  of  the  ordinary 
spores,  by  putting  out  a  germinating  filament,  which  ultimately 
develops  into  the  new  fungus  plant.  In  a  large  number  of  fungi 
no  process  of  sexual  reproduction  is  known.  The  various  modes 
which  do  occur  will  be  described  under  the  separate  families. 

Some  families  of  fungi  are  characterised  by  the  remarkable  pheno- 
menon known  as  alternation  of  generations.  Each  species  occurs  in 
two  (or  sometimes  three)  perfectly  distinct  forms,  which  bear  no 
resemblance  to  one  another,  and  were  long  supposed  to  belong  to 
widely  separated  families.  Each  phase  or  'generation'  has  its  own 
mode  of  reproduction,  but  does  not  reproduce  its  own  special  form, 
but  the  other  or  one  of  the  other  forms ;  and  two  or  three  generations 
are  thus  required  to  complete  the  cycle.  Each  member  of  the  cycle 
is,  generally  speaking,  parasitic  on  a  totally  different  plant  from  the 
'  host '  of  the  other  forms. 

The  classification  of  fungi  is  attended  with  very  great  difficulties, 
owing  to  our  still  imperfect  acquaintance  with  the  mode  of  reproduc- 
tion in  several  of  the  groups.  The  following  are  the  more  distinct 
and  remarkable  types  :  ] — 

The  Myxomycetes,  Myxogastres,  or  Mycetozoa,  are  a  group  of  very 
singular  organisms,  on  the  very  confines  of  the  animal  and  vege- 
table kingdoms,  doubtfully  included  among  the  fungi,  and  believed 
by  many  to  have  a  closer  affinity  to  the  rhizopods.  They  appear, 
indeed,  at  one  period  of  their  life-history  to  have  an  animal,  at  another 
period  a  vegetable  mode  of  existence.  Several  species  are  not  un- 
common on  decayed  w^ood,  bark,  heaps  of  decaying  leaves,  &c.  The 
'  plasmode  '  of  jEthalium  septicum,  known  as  '  flowers  of  tan,'  forms 
yellow  flocculent  masses  in  tan-pits.  The  development  of  other 
species  is  represented  in  fig.  474.  Commencing  with  the  germina- 
tion of  the  spores,  each  spore  is  a  spherical  cell  (0)  enclosed  in  a 
delicate  membranous  wall ;  and  when  it  falls  into  water  this  wall 
undergoes  rupture  (D),  and  an  amoeba-like  body  (E)  escapes  from 
it,  consisting  of  a  little  mass  of  protoplasm,  with  a  round  central 
nucleus  enclosing  a  nucleole  and  a  contractile  vesicle,  and  having 
amoeba -like  movements  connected  with  the  protrusion  and  withdrawal 
of  peculiar  processes  or  pseudopodes.  This  soon  elongates  (F),  and 
becomes  pointed  at  one  end,  whence  a  longflagellum  is  put  forth,  the 
lashing  action  of  which  gives  motion  to  the  body,  which  may  now  be 
termed  a  swwm-spore.  After  a  time  the  flagellum  disappears  and  the 
active  movements  of  the  spore  cease ;  but  it  now  begins  again  to  put 
forth  and  to  withdraw  finger-like  pseudopodes,  by  means  of  which  it 
creeps  about  like  an  Amceba,  and  feeds  like  that  rhizopod  upon  solid 
particles  which  it  engulfs  within  its  soft  protoplasm.  These  swarm- 
cells  may  multiply  by  bipartition  to  an  indefinite  extent ;  but  after 
a  time  '  conjugation  '  takes  place  between  two  of  these  myxamcetxK 
(H),  their  substance  undergoing  a  complete  fusion  into  one  body  (I), 

1  [The  classification  of  fungi  here  adopted  is  essentially  that  of  De  Bary  in  his 
Comparative  Morphology  and  Biology  of  the  Fungi,  Mycetozoa,  and  Bacteria. 
Owing  to  the  very  large  recent  additions  to  our  knowledge  of  the  structure  of  fungi, 
it  has  been  found  necessary  entirely  to  rearrange  this  portion  of  Dr.  Carpenter's 
work. — ED.] 


MYXOMYCETES 


635 


from  which  extensions  are  put  forth  { J)  ;  and  by  the  union  of  a 
number  of  these  bodies  are  produced  the  motile  protoplasmic  bodies 
known  as  plasmocles,  the  ordinary  form  in  which  these  singular  bodies 
are  known.  These  continue  to  grow  by  the  ingestion  and  assimila- 
tion of  the  solid  nutriment  which  they  take  into  their  substance  ; 
and,  by  the  ramification  and  inosculation  of  these  extensions,  a 
complete  network  is  formed. 

The  filaments  of  this  network  exhibit  active  undulatory  move- 


FIG.  474. — Development  of  Myxomycetes  :  A,  plasrnode  of  Didymium  serpula  ;  B, 
successive  stages,  a,  a',  6,  of  sporanges  of  Arcyria  flava ;  C,  ripe  spore  of 
Physarum-  album-,  D,  its  contents  escaping;  E,  F,  G,  the  swarm-spore  first 
becoming  flagellated,  and  then  amoeboid  ;  H,  conjugation  of  two  amceboids,  which, 
at  I,  have  fused  together,  and,  at  J,  are  beginning  to  put  out  extensions  and  ingest 
nutriment,  of  which  two  pellets  are  seen  in  its  interior. 

meiits,  which  in  the  larger  ones  are  visible  under  an  ordinary  lens, 
or  even  to  the  naked  eye,  but  which  it  requires  microscopic  power  to 
discern  in  the  smaller.  With  sufficiently  high  amplification,  a  con- 
stant movement  of  granules  may  be  seen  flowing  along  the  threads, 
and  streaming  from  branch  to  branch.  Here  and  there  offshoots  of 
the  protoplasm  are  projected,  and  again  withdrawn,  in  the  manner  of 
the  pseudopodes  of  an  Amoeba  ;  while  the  whole  organism  may  be 
occasionally  seen  to  abandon  the  support  over  which  it  had  grown, 


636  FUNGI 

and  to  creep  over  neighbouring  surfaces,  thus  far  resembling  in  all 
respects  a  colossal  ramified  Amoeba.  The  plasmodes  are  often  found 
to  have  taken  up  into  them  and  enclosed  a  great  variety  of  foreign 
bodies,  such  as  the  spores  of  fungi,  parts  of  plants,  &c.  They  are 
curiously  sensitive  to  light,  and  may  sometimes  be  found  to  have 
retreated  during  the  day  to  the  dark  side  of  the  leaves,  or  into  the 
recesses  of  the  tan  over  which  they  had  been  growing,  and  again  to 
creep  out  on  the  approach  of  night.  Under  certain  conditions  the 
swarm-spores  may  lose  their  power  of  motion  and  become  encysted  ; 
they  are  then  known  as  microcysts,  and  may  remain  in  this  resting 
condition  for  a  considerable  time,  especially  if  desiccated.  If  again 
placed  in  water,  they  return  to  their  motile  swarming  state.  The 
plasmodes  may  also  enter  a  resting  state,  in  which  they  assume  a  wax- 
like  consistence,  and  dry  up  into  a  brittle  horny  mass.  They  are  then 
known  as  sclerotes.  In  a  few  genera  the  spores  are  not  contained  in 
sporanges,  but  are  borne  on  external  supports  or  sporophores.  But 
in  the  great  majority  of  genera  the  plasmode  becomes  ultimately 
transformed  into  sporanges  (B,  a,  a! ,  b) ;  either  each  plasmode 
becomes  a  single  sporange,  or  it  divides  into  a  larger  or  smaller 
number  of  pieces,  each  of  which  undergoes  this  transformation. 
When  mature,  the  cavity  of  the  sporange  is  either  entirely  filled 
with  the  very  numerous  spores,  or  in  most  genera  tubes  or  threads  of 
different  forms  occur  among  the  spores,  and  constitute  the  capillitium. 
These  capillitium-tubes  have  often  a  spiral  appearance,  owing  to 
irregular  thickenings  of  the  cell- wall,  and  are  very  beautiful  objects 
under  the  microscope.  The  growth  of  many  species  of  Myxomycetes 
is  exceedingly  rapid,  going  through  their  whole  cycle  of  development, 
with  its  various  phases,  in  the  course  of  a  few  days. 

The  ChytridiaceSB  are  a  group  of  minute  microscopic  fungi  showing 
an  affinity  in  some  respects  to  the  Myxomycetes,  and  even  to  the 
infusorial  animalcules.  Their  ordinary  mode  of  propagation  is  by 
zoospores  bearing  one  or  two  cilia,  which  either  germinate  directly 
or  conjugate  to  produce  a  resting-spore.  They  are  parasitic  on  fresh- 
water organisms,  both  animal  and  vegetable  ;  and  their  chief  interest 
to  the  microscopist  is  that  their  zoospores  have  apparently  frequently 
been  mistaken  for  antherozoids  of  the  '  host.' 

The  Ustilagineae  are  fungi  parasitic  on  flower  ing  plants,  attacking 
the  stem,  leaves,  and  other  parts,  where  they  form  brown  or  yellow 
spots.  They  are  often  exceedingly  destructive  to  vegetation,  causing 
the  diseases  of  cereal  crops  known  as  bunt,  smut,  &c.  The  course 
of  development  of  these  fungi  is  not  yet  in  all  cases  accurately 
known.  The  mycele,  consisting  of  slender  segmented  hyphse,  spreads 
extensively  within  the  tissues  of  the  host,  and  bears  spores  which 
either  reproduce  the  mycele  again  directly,  or  with  the  intervention 
of  so-called  '  sporids.' 

The  TIredineae  afford  the  most  remarkable  illustration  among 
fungi  of  the  phenomenon  already  mentioned,  that  of  alternation  of 
generations ;  forms  previously  considered  to  belong  to  widely  separated 
groups  being  now  known  to  be  stages  in  the  cycle  of  development  of 
the  same  species.  A  striking  instance  of  this  is  furnished  by  the 
well-known  and  very  destructive  disease  of  wheat  and  other  grasses 


UKEDIXE.E 


637 


known  as  '  mildew,'  produced  by  the  .attacks  of  the  parasitic  fungus 
Puccinia  graminis.  It  was  long  ago  observed  that  wheat  was 
especially  liable  to  this  disease  in  the  vicinity  of  barberry  bushes  ; 
and  it  is  now  known  that  a  fungus  parasitic  on  barberry  leaves,  for- 
merly known  as  ^Ecidium  berberidis,  is  the  '  ascidiospore '  generation 
of  the  same  species  of  which  Puccinia  graminis  is  the  '  teleutospore ' 


Sfl 


FIG.  475. — Puccinia  graminis.  From  De  Bary's  '  Comparative  Morphology  and 
Biology  of  the  Fungi.'  (The  Clarendon  Press.)  A,  portion  of  leaf  of  Herberts 
with  young  aecidium;  I.,  section  through  leaf  containing  secidia ;  sp,  spermo- 
gones;  a,  iecidia  opened  ;  ^,  peridium  ;  II.,  group  of  ripe  teleutospores 
bursting  through  the  epiderm  e  in  leaf  of  Triticum  repens  ;  t,  teleutospores ; 
III.,  teleutospores  t,  and  uredospores  ur ;  I.  slightly  magnified ;  II.  x  190; 
III.  x  390. 

generation.  The  complete  cycle  of  development  of  the  best  known 
Uredinece,  such  as  the  mildew  (fig.  475),  is  this.  The  form  known  as 
Puccinia  graminis  produces  teleutospores,  thick-walled  spores,  borne 
usually  in  pairs,  at  the  extremity  of  elongated  cells  known  as  basids 
or  steriymata.  Each  of  these  teleutospores  gives  rise,  on  germinating 
within  the  tissue  of  the  grass,  to  a  hypha  or  promycele,  the  terminal 
cells  of  which  develop,  on  slender  basids,  each  a  single  spore  or 


638  FUSCH 

sporid.  These  sporids  will  germinate  only  on  the  leaves  of  the  bar- 
berry, where  they  produce,  first  of  all,  a  mass  of  interwoven  hyphse 
within  the  tissue,  and  then  the  peculiar  reproductive  bodies  known 
as  cecidia  (fig.  476).  The  'secidium  '  is  a  cup-shaped  receptacle  of  a 
bright  red  or  yellow  colour,  which  breaks  through  the  epiderm  of 
the  leaf,  and  discharges  a  large  number  of  cecidiospores,  which  are 
produced  in  rows  or  chains  springing  from  basids  at  the  base  of  the 
receptacle.  These  are  accompanied,  often  on  the  other  surface  of  the 
leaf,  by  spermogones,  smaller  spherical  or  flask-shaped  receptacles, 
which  also  eventually  break  through  the  epiderm,  and  are  filled  with 
barren  hyphae  known  as  paraphyses.  Among  these  are  other  shorter 
hyphae  or  '  sterigmata,'  from  the  extremities  of  which  are  abstricted 
narrow  ellipsoidal  cells,  the  spermatia.  The  purpose  of  these  is 
unknown  ;  but  they  may  be  male  elements  which  have  lost  their 
function.  The  secidiospores  will  germinate  only  on  the  leaves  and 
stems  of  grasses,  either  producing  the  teleutospore-form  directly,  or 


•^  V-w  r«**cr^4T^  •CTPWW-*  'i « 

FIG.  476. — JEcidium  tussilaginis :  A,  portion  of  the  plant,  magnified  ;  B,  section 
of  one  of  the  '  secidia  '  with  its  spores. 


with  its  spores. 

giving  rise  to  a  third  *  uredo-form.'  This  consists  of  filiform  basids, 
each  of  which  bears  a  round  oval  spore,  the  uredospore,  which  ger- 
minates very  rapidly,  constantly  reproducing  the  same  form.  The 
same  mycele  which  produces  the  uredo-form  also  gives  rise  subse- 
quently to  the  teleutospore-form.  The  fungus  usually  hibernates 
and  remains  in  a  state  of  rest  in  the  teleutospore-form. 

Of  the  Peronosporeae  (fig.  477)  some  species  grow  on  the  dead 
bodies  of  animals  and  on  dead  plants,  others  are  parasitic  in  the 
living  tissues  of  flowering  plants,  causing  widespread  diseases,  such 
as  the  potato -blight.  On  the  mycele,  consisting  of  a  number  of  dis- 
tinct septated  hyphse,  are  produced  the  sexual  organs,  oogones  and 
antherids.  Fertilisation  is  not  effected  by  means  of  motile  anthero- 
zoids,  as  in  other  classes  of  fungi  and  of  algae,  but  the  antherid  puts 
out  a  cylindrical  or  conical  tube-like  process,  the  fertilisation-tube. 
The  antherids  and  oogones  are  each  single  enlarged  cells  produced  in 
close  proximity  to  one  another ;  the  fertilisation-tube  is  produced 
from  the  part  of  the  antherid  which  is  in  immediate  contact  with 


PERONOSPORE.*: 


639 


the  ob'gone,  and  discharges  into  the  latter  the  contents  of  the 
antherid,  thus  causing  its  protoplasmic  contents  or  *  ob'sphere  '  to 
develop  into  the  impregnated  '  oospore.'  The  further  history  of  the 
oospore  is  singularly  different,  even  in  different  species  of  the  same 
genus.  In  some  it  germinates  directly  into  a  new  mycele  ;  in  others 
it  breaks  up  into  a  number  of  swarm-spores  or  zoospores  ;  each  of  these 
comes  to  rest,  and  after  a  time  germinates  into  a  new  mycele.  In 


K 


FIG.  477. — A-G,  Cystopus  candidus :  H,  Phytophthora  infestans.  A,  branch  of 
mycele  growing  at  the  apex,  t,  with  haustoria,  h,  between  the  cells  of  the  pith  of 
Lepidium  sativum',  B,  branch  of  mycele  bearing  gonids  ;  C,  D,  E,  formation  of 
swarm-spores  from  gonids;  F,  swarm-spores  germinating;  G,  swarm-spores 
germinating  on  a  stomate  and  piercing  the  epiderm  of  the  stem  of  a  potato  at  H. 
After  De  Bary  ;  magnified  about  400  times.  From  '  Outlines  of  Classification  and 
Special  Morphology  of  Plants,'  by  Dr.  K.  Goebel. 

addition  to  the  sexual  organs  of  reproduction,  many  species  of  Perono- 
sporese  also  produce  non-sexual  S2>ores  or  gonids,  which  are  borne  on 
special  branches  springing  erect  from  the  mycele,  the  sporophdres  or 
yonidiophores.  A  similar  difference  is  exhibited  in  the  further 
development  of  these  spores.  Either  they  germinate  directly  in  water 
into  a  new  mycele,  or  the  protoplasmic  contents  break  up  into  a 
number  of  zoospores  which  germinate  in  the  stime  way.  In  those 


640 


FUNGI 


species  which  are  parasitic  on  living  plants,  such  as  Phytophthora 
infestans,  which  produces  the  potato-disease,  and  Cystopus  candidus, 
very  common  on  cress  and  other  cruciferous  plants,  the  rapid  spiv; id 
of  the  disease  is  caused  by  the  great  facility  with  which  the  spores 
are  disseminated  by  the  wind  ;  falling  on  leaves  in  moist  weather, 
they  there  germinate ;  the  germinating  tube  passes  through  a 

stomate,  and  the  mycele  is  developed  with 
great  rapidity  within  the  tissue  of  the 
host.  The  most  favourable  condition  in 
the  case  of  the  potato- disease  is  said  by 
Professor  De  Bary  to  consist  in  an 
undue  thinness  of  the  cuticle,  accompanied 
by  excessive  humidity,  whereby  the  spores 
of  the  fungus  will  germinate  on  the  sur- 
face of  the  plant,  sending  out  processes 
which  penetrate  to  its  interior,  though 
otherwise  germinating  only  on  cut  sur- 
faces. 

The  Saprolegniese  are  saprophytic  or 
parasitic  fungi,  nearly  allied  to  the  Pero- 
nosporece,  and  differing  from  them  chiefly 
in  two  points  :  although  organs  are 
known  in  many  species  closely  resem- 
bling the  antherids  of  the  Peronosporece, 
the  act  of  impregnation  has  not  actually 
been  observed,  the  ob'spore  being,  at  least 
in  many  cases,  apparently  produced  par- 
thenoyenetically,  i.e.  without  impregna- 
tion. In  some  species  a  single  ob'spore  is 
produced  within  each  obgone  ;  but  more 
often  the  contents  of  the  latter  break  up 
into  a  number  of  obspores,  each  of  which 
gives  rise  to  a  mycele,  or  breaks  up  into 
zobspores.  In  some  genera,  e.g.  Achlya 
(fig.  478),  zobspores  are  also  produced  in 
.  very  large  numbers  by  the  breaking-up 

FIG.  478.— Two  zoosporanges  of        .  •£.  f        ..  •   i 

Achlya.  From GoebelV  Out-  of  the  contents  of  zoosporanges,  special 
lines  of  Classification  and  enlarged  cells  of  the  mycele.  The  well- 
Special  Morphology.'  A  still  known  ^}mon  disease  is  caused  bv  the 
closed;  B,  open  to  discharge  ,  „  ,,  ...  Ci  -.  .  •  . 

the  zoospores;    a,  zoospores    attacks  of  the  parasitic  haprolegma  Jerax 
on  the  living  flesh  of  the  animal. 

The  Mucorini  are  filamentous  fungi, 
resembling  the  two  last  orders  in  their 
vegetative  development,  but  differing  in 
their  mode  of  reproduction.  To  this  family  belong  some  of  the  most 
common  moulds  which  make  their  appearance  on  damp  or  decaying 
organic  substances.  The  ordinary  mode  of  non-sexual  reproduction 
is  by  endogenous  spores,  produced  within  a  sporange  (fig.  479,  A). 
The  sporanges  are  borne  at  the  ends  of  sporangiophores,  long,  erect, 
uiiseptated  hyphse,  springing  directly  from  the  mycele  or  from 
the  original  germinating  filament.  Several  other  kinds  of  non- 


ejected,  but  still  resting ;  c, 
zoospores  which  have  left 
their  membrane  at  b  behind 
them.  Magn.  about  300. 


MUCORINI 


641 


.sexual  spores  occur  in  the  family,  including  chlamydospores,  repro- 
ductive cells  formed  within  the  ordinary  cells  of  the  hyphse.  Sexual 
reproduction  takes  place  by  means  of  zyyospores  (C),  but  is  at 
present  known  only  in  a  few  species.  Either  from  ordinary  hyplue 
or  from  sporangiophores  spring  a  pair  of  short  branches,  the 
extremities  of  which  become  firmly  attached  to  one  another.  These 


FIG.  479. — B,  mycele  (three  days  old)  of  Phycomyces  nitens,  grown  in  a  drop  of 
mucilage  with  a  decoction  of  plums ;  the  finest  ramifications  are  omitted ;  g,  the 
conidiophore  of  Mucor  mucedo  in  optical  longitudinal  section ;  C,  a  germinating 
zygospore  of  Mucor  mucedo  ;  the  germ-tube,  k,  puts  out  a  lateral  conidiophore,  g. 
In  D  are  conjugating  branches,  b  &,  the  extremities  of  which,  a  a,  though  they  have 
not  yet  coalesced,  are  already  cut  off  by  transverse  walls;  the  zygospore  is  formed 
from  the  coalescence  of  the  cells  a  a.  A,  C,  D,  after  Brefeld,  greatly  magnified  ; 
B,  from  nature,  slightly  magnified.  From  Goebel's  '  Outlines  of  Classification 
and  Special  Morphology.' 

swell  out  greatly  into  an  obconical  form,  on  account  of  the  passage 
into  them  of  a  large  amount  of  nutrient  material.  A  larger  or 
smaller  piece  is  then  cut  off  from  each  of  them  by  a  transverse 
wall ;  the  median  cell- wall  which  separates  them  disappears,  and  the 
two  terminal  portions  thus  cut  off  coalesce  to  form  the  zygospore, 

T  T 


642  FUNGI 

which  often  swells  to  a  considerable  size,  and  its  outer  coat  be- 
comes frequently  beautifully  covered  with  warts  or  other  protu- 
berances. After  a  period  of  rest  the  zygospore  germinates,  its 
inner  coat  of  cellulose  bursting  through  the  outer  warty  and 
cuticularised  epispore,  and  developing  into  the  first  germinating 
filament. 

Very  nearly  allied  to  the  Mucorini  are  the  Entomophthorese, 
parasitic  fungi,  the  mycele  of  which  develops  within  the  bodies  of 
living  insects,  especially  caterpillars  and  flies,  and  after  death 
spreads  outside  the  body  as  a  flocculent  felt.  An  example  of  this 
family  of  fungi  is  frequently  presented  in  the  destruction  of  the 
common  house-fly  by  Empusa  muscce.  In  its  fully  developed  con- 
dition the  spore-bearing  filaments  of  this  plant  stand  out  from  the 
body  of  the  fly  like  the  '  pile '  of  velvet,  and  the  spores  thrown  off 
from  these  in  all  directions  form  a  white  circle  round  it,  as  it  rests 
motionless  on  a  window-pane.  The  filaments  which  show  them- 
selves externally  are  the  fructification  of  the  fungus  which  occupies 
the  interior  of  the  fly's  body,  and  this  originates  in  the  spores 
which  find  their  way  into  the  circulating  fluid  from  without.  A 
healthy  fly  shut  up  with  a  diseased  one  takes  the  disease  from  it  by 
the  deposit  of  a  spore  on  some  part  of  its  surface  ;  for  this,  beginning 
to  germinate,  sends  out  a  process  which  finds  its  way  into  the 
interior,  either  through  the  breathing-pores  or  between  the  rings 
of  the  body ;  and,  having  reached  the  interior  cavities,  it  gives  oft 
the  germinating  filaments  which  constitute  the  earliest  stage  of  the 
Empusa.  Again,  it  is  not  at  all  uncommon  in  the  West  Indies  to 
see  individuals  of  a  species  of  Polistes  (the  representative  of  the 
wasp  of  our  own  country)  flying  about  with  plants  of  their  own 
length  projecting  from  some  part  of  their  surface,  the  germs  of 
which  have  probably  been  introduced  (as  in  the  preceding  case) 
through  the  breathing-pores  at  their  sides,  and  have  taken  root  in 
their  substance,  so  as  to  produce  a  luxuriant  vegetation.  In  time, 
however,  this  fungus  growth  spreads  through  the  body  and  destroys 
the  life  of  the  insect ;  it  then  seems  to  grow  more  rapidly,  the 
decomposing  tissue  of  the  dead  body  being  still  more  adapted  than 
the  living  structure  to  afford  it  nutriment. 

The  Ascomycetes  include  an  enormous  number  of  species,  most 
of  which  are  parasitic  on  living,  or  saprophytic  on  decaying  leaves, 
many  of  them  microscopic.  The  mycele  always  consists  of  branched 
and  septated  hyphse.  In  only  a  comparatively  few  species  is  a 
.sexual  mode  of  reproduction  known ;  the  special  character  of  the 
group  is  the  non-sexual  reproduction  of  ascospores  within  elongated 
sacs  or  tubes  known  as  asci.  These  are  commonly  collected  together 
in  masses  ;  the  collection  of  hyph?e  which  give  birth  to  the  asci  is 
known  as  the  kymeni/wn,  the  mass  of  tissue  enclosing  or  bearing  the 
hymenia  as  the  receptacle  or  fructification.  Its  form  and  structure 
vary  greatly  in  the  different  sections  of  the  family.  The  ascospores 
are  always  produced  within  the  ascus  by  free-cell  formation,  and 
their  number  is  almost  always  four  or  a  ;  power '  of  four,  most  com- 
monly eight,  occasionally  less  than  four.  The  asci  are  usually 
surrounded  by  enlarged  club-shaped  or  sterile  hyphse,  the  para- 


ASCOMYCETES 


643 


physes.  In  many  Ascomycetes,  in  addition  to  the  ascospores, 
ordinary  exogenous  spores  or  conids  are  produced  at  the  extremity 
of  sporophores  or  conidiophores  (fig.  480,  A).  This  is  the  case  with 
a  large  number  of  moulds  or  mildews,  of  which  the  common  blue 
mould,  Penicillium  glaucum,  may  be  taken  as  a  type.  The  familiar 
form  of  these  moulds  is  that  in  which  they  produce  these  spores  in 
enormous  quantities  ;  but,  under  certain  conditions,  especially  when 
the  supply  of  nutriment  is  limited,  the  sexual  mode  of  reproduction 


FIG.  480. — Development  of  Eurotium  repens:  A,  small  part  of  a  mycele  with  the 
conidiophore,  c,  and  young  ascogones,  as ;  B,  the  spiral  ascogone,  as,  with  the 
antheridial  branch,  p  ;  C,  the  same  with  the  filaments  beginning  to  grow  round  it 
to  form  the  wall  of  the  sporocarp ;  D,  a  sporocarp  seen  from  without ;  E,  F, 
young  sporocarp  in  optical  longitudinal  section ;  w,  parietal  cells ;  /,  the  filling 
tissue  (pseudo-parenchymatous) ;  as,  the  ascogone ;  G,  an  ascus ;  H,  an  ascospore. 
After  De  Bary.  A,  magnified  190,  the  rest  600  times. 

sets  up  (fig.  480,  B-H).  One  of  the  branches  of  the  mycele 
elongates,  and  coils  spirally  upon  itself  into  a  corkscrew-like  body, 
the  carpogone  or  ascogone,  which  constitutes  the  female  organ  \ 
whilst  another  branch  acts  as  the  male  organ  or  antherid,  extending 
itself  over  the  spire  and  impregnating  the  ascogone  by  the  passage  of 
its  protoplasm  into  the  latter  organ.  The  structure  thus  formed 
becomes  enclosed  in  a  mass  of  sterile  tissue,  and  within  this  are 

Kleveloped    the    asci,    each    containing    numerous     spores,    which 
TT2 


644 


FUNGI 


germinate  directly  into  a  new  mycele.  The  enveloping  tissue,  together 
with  the  asci,  is  known  as  the  sporocarp.  In  a  large  number  of 
Ascomycetes  the  asci  are,  however,  formed  without  any  previous 
sexual  process  that  has  yet  been  detected.  According  to  the  struc- 
ture of  the  mature  sporocarp,  the  Ascomycetes  may  be  arranged 
under  three  sections  :  the  Discomycetes,  in  which  the  sporocarp  is 
exposed,  and  is  then  known  as  an  apothece  ;  the  Pyrenomycetes,  in 


FIG.  481. — liotrytis  bassiana  :•  A,  the  fungus  as  it  first  appears  at  the  orifices  of  the 
.   stigmas :  B,  tubular  filaments  bearing  short  branches,  as  seen,  two  days  after- 
wards ;  E,  magnified  view  of  the  same  ;  C,  D,  appearance  of  filaments  on  the  fourth 
and  sixth  days ;  F,  masses  of  mature  spores  falling  off  the  branches,  with  filaments 
proceeding  from  them. 

which  the  perithece  is  enclosed  in  a  flask-shaped  cavity  with  open 
neck  ;  and  a  third  section,  in  which  the  sporocarps  are  completely 
enclosed. 

In  some  Ascomycetes  a  tendency  is  exhibited  to  the  formation  of 
sclerotes,  dense  hardened  masses  of  interwoven  hyphse.  An  example 
of  this  is  furnished  by  the  structure  known  as  *  ergot/  the  sclerote 
of  a  fungus  of  this  kind,  Claviceps  purpurea,  which  attacks  the  ovary 


ASCOMYCETES;   SACCHAROMYCETES  645 

of  rye  and  other  grasses.  Many  species  of  Peziza  have  a  peculiar 
form  known  as  the  botrytis  form,  reproduced  by  conids  only,  and 
long  believed  to  be  altogether  distinct  from  the  Ascomycetes.  Of  this 
nature  is  the  so-called  Botrytis  bassiana  (fig.  481),  a  kind  of  mould,  the 
growth  of  which  is  the  real  source  of  the  disease  termed  muscardine 
which  formerly  carried  off  silkworms  in  large  numbers,  just  when 
they  were  about  to  enter  the  chrysalis  state,  to  the  great  injury  of 
their  breeders.  The  plant  presents  itself  under  a  considerable 
variety  of  forms  (A-F),  all  of  which,  however,  are  of  extremely 
simple  structure,  consisting  of  elongated  or  rounded  cells,  connected 
in  necklace-like  filaments,  very  nearly  as  in  the  ordinary  '  bead- 
moulds.'  The  spores  of  this  fungus,  floating  in  the  air,  enter  the 
breathing-pores  which  open  into  the  tracheal  system  of  the  silk- 
worm ;  they  first  develop  themselves  within  the  air-tubes,  which 
are  soon  blocked  up  by  their  growth ;  and  they  then  extend  them- 
selves through  the  fatty  mass  beneath  the  skin,  occasioning  the 
destruction  of  this  tissue,  which  is  very  important  as  a  reservoir  of 
nutriment  to  the  animal  when  it  is  about  to  pass  into  its  chrysalis 
condition.  The  disease  invariably  occasions  the  death  of  the  grub 
which  it  attacks ;  but  it  seldom  shows  itself  externally  until  after- 
wards, when  it  rapidly  shoots  forth  from  beneath  the  skin,  especially 
at  the  junction  of  the  rings  of  the  body.  Although  it  spontaneously 
attacks  only  the  larva,  yet  it  may  be  communicated  by  inoculation 
to  the  chrysalis  and  the  moth,  as  well  as  to  the  grub  ;  and  it  has 
also  been  observed  to  attack  other  lepidopterous  insects.  A  careful 
investigation  of  the  circumstances  which  favour  the  development  of 
this  disease  was  made  by  Audouin,  who  first  discovered  its  real 
nature  ;  and  he  showed  that  its  spread  was  favoured  by  the  over- 
crowding of  the  worms  in  the  breeding  establishments,  and  parti- 
cularly by  the  practice  of  throwing  the  bodies  of  such  as  died  into  a 
heap  in  the  immediate  neighbourhood  of  a  living  silkworm  ;  for  this 
heap  speedily  became  covered  with  this  kind  of  mould,  which 
found  upon  it  a  most  congenial  soil ;  and  it  kept  up  a  continual 
supply  of  spores,  which,  being  diffused  through  the  atmosphere  of 
the  neighbourhood,  were  drawn  into  the  breathing- pores  of  indi- 
viduals previously  healthy.  The  precautions  obviously  suggested  by 
the  knowledge  of  the  nature  of  the  disease,  thus  afforded  by  the 
microscope,  having  been  duly  put  in  force,  its  extension  was  success- 
fully kept  down.  A  similar  growth  of  different  species  of  the  genu& 
Xphceria  takes  place  in  the  bodies  of  certain  caterpillars,  in  New 
Zealand,  Australia,  and  China  ;  and  being  thus  completely  pervaded 
by  a  dense  substance,  which,  when  dried,  has  almost  the  solidity  of 
wood,  these  caterpillars  coine  to  present  the  appearance  of  tw"igs, 
with  long  slender  stalks  that  are  formed  by  the  growth  of  the  fungus 
itself.  The  Chinese  species  is  valued  as  a  medicinal  drug. 

Some  forms  of  Ascomycetes,  such  as  the  genus  Tuber,  to  which 
the  truffle  belongs,  are  formed  completely  underground. 

The  Saccharomycetes  are  now  generally  regarded  as  a  degraded 
form  of  the  Ascomycetes.  They  resemble  the  Schizomycetes  in  the 
simplicity  of  their  character  and  in  their  '  zymotic '  action.  The  most 
familial'  form  of  this  family  is  the  Saccharomyces  (Torula)  cerevisicn, 


646  FUNGI 

the  presence  of  which  in  yeast  gives  to  it  the  power  of  exciting  the 
alcoholic  fermentation  in  saccharine  liquids.  When  a  small  drop  of 
yeast  is  placed  under  a  magnifying  power  of  400  or  500  diameters, 
it  is  seen  to  consist  of  a  large  number  of  globular  or  ovoid  cells, 
averaging  about  ^^^th  of  an  inch  in  diameter,  for  the  most  part 
isolated,  but  sometimes  connected  in  short  series  ;  and  each  cell 
is  filled  with  a  nearly  colourless  *  endoplasm,'  usually  exhibiting 
one  or  more  vacuoles.  When  placed  in  a  fermentable  fluid  con- 
taining some  form  of  nitrogenous  matter  in  addition  to  sugar,1 
they  vegetate  in  the  manner  represented  in  fig.  482.  Each  cell 
puts  forth  one  or  two  projections,  which  seem  to  be  young  cells 
developed  as  buds  or  offsets  from  their  predecessors ;  these,  in 
the  course  of  a  short  time,  become  complete  cells,  and  again  per- 
form the  same  process ;  and  in  this  manner  the  single  cells  of  yeast 
develop  themselves,  in  the  course  of  a  few  hours,  into  rows  of  four, 
five,  or  six,  which  remain  in  connection  with  each  other  whilst  the 
plant  is  still  growing,  but  which  separate  if  the  fermenting  process 
be  checked,  and  return  to  the  isolated  condition  of  those  which 
originally  constituted  the  yeast.  Thus  it  is  that  the  quantity  of 
yeast  first  introduced  into  the  fermentable  fluid  is  multiplied  six 


FIG.  482. — Saccliaromyces  cerevisice,  or  yeast-plant,  as  developed  during  the  process 
of  fermentation :  a,  b,  c,  d,  successive  stages  of  cell-multiplication. 

times  or  more  during  the  changes  in  which  it  takes  part.  Under 
certain  conditions  not  yet  determined,  the  yeast-cells  multiply  in 
another  mode — namely,  by  the  breaking  up  of  the  endoplasm  into 
segments,  usually  four  in  number,  around  each  of  which  a  new  '  cell- 
wall'  forms  itself;  and  these  endogenous  spores  are  ultimately  set 
free  by  the  dissolution  of  the  wall  of  the  parent  cell,  and  soon  enlarge 
and  comport  themselves  as  ordinary  yeast-cells.  The  process  of  the 
formation  of  these  spores  resembles  in  all  essential  points  the 
formation  of  ascospores ;  and  hence  Torula  is  regarded  as  a  low  or 
degraded  type  of  that  order.  Many  other  fungi  of  like  simplicity 
have  the  power  to  act  as  *  ferments ; '  thus  in  wine -making  the 
fermentation  of  the  juices  of  the  grapes  or  other  fruit  employed  is  set 
going  by  the  development  of  minute  fungi  whose  germs  have  settled 
on  their  skins,  these  germs  not  being  injured  by  desiccation,  and 
being  readily  transported  by  the  atmosphere  in  the  dried-up  state. 

1  It  appears  from  the  researches  of  Pasteur  that,  although  the  presence  of  albu- 
minous matter  (such  as  is  contained  in  a  saccharine  wort,  or  in  the  juices  of  fruits) 
favours  the  growth  and  reproduction  of  yeast,  yet  that  it  can  live  and  multiply  in  a 
solution  of  pure  sugar,  containing  ammonium  tartrate  with  small  quantities  of  mineral 
salts,  the  decomposition  of  the  ammonia  salt  affording  it  the  nitrogen  it  requires  for 
the  production  of  protopiasm,  while  the  sugar  and  water  supply  the  carbon,  oxygen, 
and  hydrogen. 


SACCHAROMYCETES  ;   BASIDIOMYCETES 


647 


There  is  reason  to  believe,  moreover,  that  a  similar  '  zymotic '  action 
may  be  excited  by  fungi  of  a  higher  grade  in  the  earlier  stages  of 
their  growth,  the  alcoholic  fermentation  being  set  up  in  a  suitable 
liquid  (such  as  an  aqueous  solution  of  cane-sugar,  with  a  little  fruit- 
juice)  by  sowing  in  it  the  spores  of  any  one  of  the  ordinary  moulds, 
such  as  Penicillium  glaucum,  Mucor,  or  Aspergillus,  provided  the 
temperature  be  kept  up  to  blood-heat ;  and  this  even  though  the 
solution  has  been  pre- 
viously heated  to  284° 
Fahr.,  a  temperature 
which  must  kill  any 
germs  it  may  itself  con- 
tain. 

The  Basidiomycetes 
are  distinguished  by  the 
entire  absence,  as  far  as 
is  at  present  known,  of 
sexual  organs,  and  by 
the  formation  of  their 
conids  or  spores  at  the 
apex  of  special  enlarged 
cells,  the  basids.  They 
include  the  largest  and 
most  familiar  of  our 
fungi,  such  as  the  genera 
Agaricus,  Boletus,  Poly- 
porus,  Lycoperdon,  Phal- 
lus, &c.  They  are  sapro- 
phytes, obtaining  their 
nourishment  from  the 
decaying  vegetable  mat- 
ter in  the  soil,  stumps 
of  trees,  &c.,  tfcc.,  among 
which  the  mycele  pene- 
trates, consisting  often 
of  a  dense  weft  of  sep- 
tated  hyphae,  the ;  spawn ' 
of  the  mushroom.  The 
aerial  portion,  known  as 
the  receptacle  or  fructifi- 
cation, bears  either  ex- 
ternally, as  in  the  case 
of  the  mushroom  (fig. 
483),  or  internally,  as 
in  the  case  of  the  Lycoperdon,  or  'puff-ball,'  the  fertile  portion 
or  hymenium.  On  this  hymenium  project  the  extremities  of  special 
hyphae,  which  are  swollen  into  basids ;  the  non-sexual  conids  or 
basidiospores  are  formed  at  the  extremity  of  the  basids,  usually  in 
fours,  from  which  they  are  easily  detached,  and,  from  their  small 
size  and  great  lightness,  are  readily  carried  through  the  air  in  great 
quantities.  In  the  Hymeiwmycetes,  of  which  the  common  mushroom 


FIG.  483. — Agaricus  campestris,  formation  of  the 
hymenium  :  A  and  B,  slightly  magnified ;  C,  a  part 
of  B,  magnified  550  times.  The  portion  marked 
with  fine  dots  is  protoplasm.  (From  Goebel's 
'  Classification  and  Morphology  of  Plants.') 


648 


FUNGI 


(Agaricus  campestris)  may  be  taken  as  a  type,  the  receptacle  has  the 
form  of  a  cap-shaped  pileus  (fig.  484),  raised  on  a  stalk  or  stipe, 
the  whole  composed  of  a  pseudo-parenchyme  consisting  of  a  dense 
agglomeration  of  parallel  hyphse,  the  cortical  portion  of  which  is 
slightly  differentiated  into  an  epiderm.  In  the  family  to  which  the 
mushroom  belongs,  the  hymenium  is  borne  at  the  edge  of  narrow 
gill-like  projections  or  lamellc?  radiating  from  the  apex  of  the  stipe 
on  the  under  side  of  the  pileus.  Among  the  basids  are  seen  other 

cells  of  similar  shape  and 
usually  larger  size,  also  the 
extremities  of  special  hyphse, 
called  cystids,  the  function  of 
which  is  obscure.  The  basi- 
diospores  vary  greatly  in 
colour  in  different  genera. 
They  are  always  unicellular, 
and  the  membrane  consists 
of  two  coats,  the  endospore 
and  exospore,  the  former  of 
which  consists  of  fungus- 
cellulose,  while  the  latter  is 
more  or  less  cuticularisecl. 
On  germinating  the  endo- 
spore bursts  through  the 
exospore,  and  grows  into  a 
germinating  filament,  from 
which  is  developed  the  my- 
cele,  and  on  this  ultimately 
the  receptacles. 

Lichens, — The  micro- 
scopic study  of  this  group 
has  acquired  a  new  interest 
for  the  botanist,  from  the" 
remarkable  discovery  an- 
nounced in  its  complete 
form  by  Schwendener  in 
1869  ]  (and  now  accepted 
by  the  highest  authorities), 
that  instead  of  constituting 
a  special  type  of  Thallo- 
phytes,  parallel  toAlgce  (with 
which  they  correspond  in 
their  vegetative  characters)  and  Fungi  (to  which  they  are  more 
allied  in  fructification),  they  are  really  to  be  regarded  as  compo- 
site structures,  having  an  algal  base,  on  which  fungi  have  sown 
themselves  and  live  parasitically.  As,  however,  they  do  not 
furnish  objects  of  interest  to  the  ordinary  microscopist  (the 
peculiar  density  of  their  structure  rendering  a  minute  examina- 
tion of  it  more  than  ordinarily  difficult),  nothing  more  than  a 

1  See  his  memorable  work  Ueber  die  Algentypen  der  Fleclitengonidien  (Basel, 
1869). 


FIG.  484. — Agaricus  campestris,  natural  size. 
(From  Goebel's  '  Classification  and  Morpho- 
logy of  Plants.') 


LICHENS  649 

general  account  of  their  curious  organisation  will  here  be  attempted. 
The  algal  portion  of  a  lichen  belongs  to  one  or  other  of  the  lower 
groups,  and  consists  of  cells  termed  gonids — usually  green,  but 
sometimes  red  or  bluish-green — interspersed  among  long  cellular 
filaments.  The  proportion  between  these  two  components  of  the 
thallus  varies  in  different  examples  of  the  type.  Thus,  in  the 
simplest  wall-lichens  the  palmella-like  pa  rent- cell  gives  origin,  by 
the  ordinary  process  of  cell-division,  to  a  single  layer  of  cells,  which 
spreads  itself  over  the  stony  surface  in  a  more  or  less  circular  form  ; 
and  the  '  thallus,'  which  increase^  in  thickness  by  the  formation  of 
new  layers  upon  its  free  surface,  fras  no  very  defined  limit,  and,  in 
consequence  of  the  slight  adhesion  of  its  components,  is  said  to  be 
'  pulverulent.'  But  in  the  more  complex  forms  of  lichens  the  thallus 
is  mainly  composed  of  long  hyphse,  which  dip  down  into  the  superficial 
layers  of  the  bark  of  the  trees  on  which  they  grow,  and  form  by  their 


FIG.  485. — Leptoffium  scotinum :  Vertical  section  of  the  gelatinous  thallus,  magnified 
550  times.  An  epidermal  layer  clothes  the  inner  tissue,  which  consists  for  the  most 
part  of  formless  and  colourless  jelly,  in  which  the  coiled  strings  of  gonids  lie  ; 
single  larger  cells  of  the  strings  (the  limiting  cells)  are  of  a  higher  colour  ;  between 
thejn  run  the  slender  hyphae.,  (From  Goebel's  'Classification.') 

interweaving  a  hard  crustaceous  '  thallus,'  in  which  the  gonids  are 
imbedded,  sometimes  irregularly,  sometimes  in  definite  layers,  known 
as  the  gonidial  layer  (fig.  485),  covered  by  an  envelope  of  interlacing 
filaments.  It  is  from  this  algal  portion  of  the  structure  that  the 
soredes  of  lichens  are  formed,  little  projections  of  the  surface,  com- 
posed of  single  or  aggregate  gonids,  invested  by  hyphae,  and  falling, 
when  dry,  into  a  powder,  of  which  every  particle  is  capable  of 
reproducing  the  plant  from  which  it  proceeded. 

The  fructification  of  lichens,  on  the  other  hand,  is  the  production 
of  their  fungal  overgrowths,  which  are  nourished  by  the  algal 
vegetation.  The  lichen-forming  fungi,  in  fact,  live  upon  their  algal 
hosts,  like  the  endophytic  fungi  (such  as  the  *  blights '  of  corn), 
which  infest  the  higher  forms  of  vegetation,  each  of  the  former 
choosing  its  own  alga,  just  as  the  latter  mostly  attach  themselves  to 
particular  victims.  The  peculiarity  in  the  parasitism  of  the  lichen- 
fungi  lies  in  the  fact  that  they  are  not  attached  to  their  host  externally 
at  any  one  particular  spot,  and  do  not  penetrate  into  its  cells,  but 


650 


FUNGI 


weave  themselves  round  them,  and  enclose  them  in  their  hyphal 
tissue.  But  not  only  this  :  the  algal  constituent  of  the  lichen 
appears  also  to  derive  benefit  from,  and  to  be  nourished  by,  the 
fungus-hyphre,  affording  an  example  of  the  singular  kind  of  mutual 
dependence  known  as  commensalism  or  symbiosis  (fig.  486.)  The 
formation  of  sexually  produced  'spores '  usually  takes  place  in  asci 
arranged  vertically  in  the  midst  of  straight  elongated  sterile  cells 
termed  paraphyses,  so  as  to  form  a  layer  that  lies  either  on  the  surface 


FIG.  486. — Examples  of  various  algse  which  are  employed  as  the  gonids  of  lichens : 
h  indicates  always  the  hypha  of  the  fungus ;  g'  the  gonid :  A,  germinating 
spore,  s,  of  Physcia  parietina,  the  germ- tube  of  which  adheres  closely  to  Proto- 
coccus  viridis ;  B,  a  filament  of  Scytonema  withhyphae  of  Stereocaulonramulosus 
twined  round  it ;  C,  from  the  thallus  of  the  lichen  Phijsma  clialaganum — a  hyphal 
branch  is  entering  a  cell  of  the  Nostoc  filament  (gonid) ;  D,  from  the  thallus  of 
the  lichen  Synalissa  sympliorea — the  gonids  are  the  alga  Glceocapsa ;  E,  from 
the  thallus  of  the  lichen  Cladonia  furcata ;  the  gonids,  which  are  being  sur- 
rounded by  the  hyphae,  are  the  cells  of  Protococcus.  After  Bornet.  A,  C,  D,  E, 
magnified  950 ;  B,  650  times.  (From  Goebel's  '  Classification  and  Special  Mor- 
phology of  Plants.') 

of  apotheces,  or  is  completely  enclosed  within  perit/ieces.  Each  of  the 
asci  contains  a  definite  number  of  ascospores,  usually  eight,  which 
are  projected  from  the  receptacles  with  some  force ;  and  their 
emission,  which  seems  to  be  due  to  the  different  effects  of  moisture 
upon  the  several  layers  of  the  receptacle,  is  often  kept  up  con- 
tinuously for  some  time.  The  formation  of  these  asci,  as  in  the 
case  of  the  ordinary  Ascomycetes,  is  probably  the  result  of  a  sexual 
union  which  takes  place  between  the  male  pollinoids  or  '  spermatia ' 
and  the  female  trichogyne.  These  pollinoids  are  produced  within 


Plate  XIH. 


'-      . 

".-.- 


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Fu,   2Z 


J?ig    2.9. 


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Fig  24  fig    ZS.  ?*#•  Z£ 


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Si0.Z9.  Tig- 30 


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RACTKRIA  ,  SCHIZOMYCETES,  OR  FISSION  FUNGI. 


LICHENS  ;   BACTERIA  65  I 

antherids  which  are  often  specially  designated  '  spermogones,'  formed 
within  these  cavities,  and,  when  mature,  escaping  in  great  numbers 
from  their  orifices.  Having  no  power  of  spontaneous  movement, 
they  must  probably  be  conveyed  by  the  infiltration  of  rain-water 
to  a  trichogyne  which  lies  imbedded  in  the  tissue  beneath  ;  and 
when  they  have  imparted  their  fertilising  influence  to  the  contents 
of  the  ascogone  at  its  base,  these  develop  themselves  into  a  spore  - 
bearing  apothece,  the  whole  mass  of  spores  wrhich  this  contains 
being  the  product  of  the  cell-division  of  the  originally  fertilised 
'  ob'spore.' 

The  fungus-constituent  of  licliens  belongs,  in  the  great  majority 
of  cases,  to  the  Ascomycetes,  in  a  very  few  to  the  Basidiomycetes. 
The  gonids  have  been  referred  to  a  very  large  number  of  genera  of 
algae,  among  which  may  be  mentioned  Protococcus,  Chrodcoccus, 
Glceocapsa,  Palmella,  Scytonema,  Nostoc,  and  Chroolepus.  , 

The  Bacteria  or  Schizomycetes. — At  the  close  of  this  chapter 
we  place  the  Bacteria,  Schizomycetes,  or  fission-fungi.  These  micro- 
organisms have  been  defined  as  minute  vegetable  cells  destitute  of 
nuclei.  In  spite  of  the  labour  which  has  been  bestowed  upon  this 
group,  and  vast  as  the  literature  is  to  which  it  has  given  rise,  it  is 
impossible  to  assign  an  exact  and  clearly  definable  position  to  what 
is  at  the  same  time  a  remarkable  and  important  group ;  and  we 
therefore,  as  a  matter  of  convenient  arrangement,  place  them  as 
PROTOPHYTES,  at  the  base  of  the  lowest  Fungi,  for  no  other,  and 
therefore  for  the  quite  insufficient  reason  in  the  main,  that  they 
contain  no  chlorophyll  (Plate  XIII). 

There  can  be  no  doubt  that  some  forms  of  the  Bacteria  manifest 
affinity  with  the  chlorophyllaceous  Algae ;  but  the  affinity  is  in  the 
present  state  of  our  knowledge  none  the  less  indefinable,  even  if 
our  knowledge  of  the  Bacteria  as  an  entire  group  were  complete 
enough  to  admit  of  a  generalisation  of  their  relations.  On  the 
other  hand,  according  to  Dallinger,  the  affinities  of  the  Bacteria  as  a 
complete  group  are  closer  with  the  Flagellata  than  is  generally 
admitted  ;  and  whenever  the  saprophytic  Flagellata — which  are  the 
indispensable  agents,  not  in  the  putrefactive  fermentation  by  which 
infusions  and  gelatine  masses  are  broken  up,  but  by  which  great 
masses  of  organic  tissue  are  reduced — and  at  the  same  time  the 
Bacteria,  as  a  whole,  have  been  broadly  and  comprehensively  worked 
out,  it  may  be  found  that  both  their  morphological  and  physiological 
affinities  are  of  the  closest  order.  It  is  impossible  to  take,  for 
example,  such  a  form  as  B.  lineola,  which  has  an  easily  demonstrated 
flagellate  character,  and  reproduces  in  every  fission  a  flagellum, 
common  to  both  dividing  forms,  which  snaps  at  the  moment  of 
complete  division,  leaving  each  form  with  a  flagellum  at  either  end 
— perfect  as  the  primal  form  whence  the  fission  arose — without 
observing  how  completely  this  coincides  with  the  mode  of  fission  in 
half  a  dozen  saprophytic  monads.  But  as  an  instance  Cercomonas 
t i/pica  (named  by  Kent)  may  be  given,1  where  the  process  is 
identical.  True,  the  Cercomonas  has  a  conjugating  and  subsequent 
resting  stage,  after  which  swarms  emerge  from  spores  thus  formed. 
1  Manual  of  the  Infusoria,  i.  259. 


652  FUNGI 

But  a  fuller  knowledge  of  7>.  lineola  is  certainly  wanting  before  we 
can  deny  the  further  analogy. 

No  doubt  if  such  affinity  were  established,  it  would  lead  to  much 
rearrangement  at  the  base  of  the  organic  series. 

Since  there  is  an  apparent  and  highly  suggestive  leaning  of  the 
Bacteria  to  those  forms  of  Algse  which  form  the  group  of  Nostocacece, 
these  also  would  be  brought  nearer  the  Flagellata  ;  while  the  Myce- 
tozoa  will  have  singular  points  of  contact  with  these,  one  of  which 
has  reference  to  the  mode  of  sparing  of  one  at  least  of  the  flagellate 
saprophytes,1  while  it  is  suggestive  that  the  same  grouping,  should 
the  affinity  be  established,  would  involve  a  connection  with  the  Algre 
and  the  Fungi. 

It  is  only  definite  results  leading  to  a  comprehensive  view  of  the 
morphology  of  the  Bacteria  as  a  whole  that  can  render  generalisation 
in  this  matter  safe. 

By  the  word  Bacteria  we  mean,  strictly  speaking,  rod-shaped 
micro-organisms,  but  the  term  is  now  commonly  used  to  indicate 
the  whole  group  of  fission-fungi,  which  includes  not  only  rod-forms 
varying  in  length  but  also  spherical  and  egg-shaped  cells.  Motile 
forms,  whether  longer  or  shorter,  are  possessed,  as  a  rule,  of  fine 
flagella.  The  mode  of  multiplication  commonly  observed  is  by 
fission.  The  products  of  successive  fissions  may  remain  together  in 
a  single  filiform  row  loosely  attached,  or  attached  by  the  unbroken 
filament  of  the  flagella,  or  they  may  at  once  separate  from  the 
primal  cell.  Multiplication  also  occurs  by  processes  which  may  be 
considered  as  representing  fructification. 

Of  the  nature  of  this  simplest  cell  we  have  hitherto  learnt 
comparatively  little  ;  the  protoplasm  is  generally  homogeneous,  but 
in  some  species  contains  starch  granules.  Thus  Clostridium 
butyricum  gives  the  starch  reaction  with  iodine.  Sulphur  granules 
are  present  in  species  of  Beyyiatoa  which  thrive  in  sulphur  springs. 
Others  again  contain  pigment.  The  most  remarkable  of  the 
coloured  forms  uniformly  tinged  red  was  found  and  named  by  Bay 
Lankester ;  2  other  forms,  coloured  green  by  chlorophyll,  have  been 
described  by  Van  Tieghem  and  Yon  Engelmann,  but  it  is  quite 
possible  that  these  may  be  Algre,  and  further  researches  are  required 
before  these  particular  micro-organisms  can  be  included  among 
bacteria. 

Within  the  protoplasm  of  the  Bacteria,  however,  no  nuclei  have 
hitherto  been  discovered,  but  there  is  a  delicate  investing  envelope, 
probably  a  mere  thickening  of  the  outmost  area  of  the  protoplasm, 
which  is  often  also  gelatinous  in  its  outer  portions. 

Many  forms  of  Bacteria  have  the  power  of  entirely  free  move- 
ment. Frequently  this  movement  is  coincident  with  a  revolution  on 
the  longer  axis  of  the  rod,  curved  or  straight,  and  in  the  vast 
majority  of  cases  this  is  directly  correlated  with  a  vortical  action  of 
a  front  flagellum — an  action  which  may  be  seen  with  the  utmost 
ease,  if  the  proper  means  be  employed,  in  the  case  of  Spirillum 

i  J.E.M.S.  vol.  v.  ser.  ii.  pp.  189-90,  fig.  16,  Plate  V. 
-  Quart.  Journ.  Microsc.  Sci.  new  series,  xiii.  408. 


FORMS   OF   BACTERIA  653 

volutans,  and  less  easily,  but  with  almost  equal  certainty,  in  the 
majority  of  other  forms,  not  excluding  B.  termo. 

The  simplest  forms  in  which  Bacteria  are  found  are  as  isolated 
cells  of  round  or  ovate  shape  :  these  are  known  as  Micrococci,  and 
must  be  distinguished  from  immature  and  developing  monads  of  the 
saprophytiq  group.  The  fission  by  which  micrococci  multiply  may 
take  place  in  one  direction  only,  and  if  the  resulting  cells  remain 
attached  they  form  diplococci.  If  fission  again  occurs  in  each  of 
these  cells  and  is  repeated  again  and  again  and  the  resulting  cells 
remain  attached,  they  give  rise>  to  beautiful  chains,  rosaries,  or 
streptococci.  If  fission  occurs  in"  single  cells  in  two  directions 
tetracocci  are  formed,  and  if  in  three  directions  packets  of  eight  are 
formed,  or  sarcina-cocci . 

The  rod-like  forms  are  found  isolated  and  free,  or  in  chains. 
Formerly  short  rods  were  called  Bacteria,  and  long  rods  Bacilli  ; 
but  as  the  term  bacteria  is  applied  to  the  whole  group  of  fission- 
fungi,  it  is  more  usual  now  to  avoid  confusion  by  speaking  of  all  rod 
forms,  independently  of  their  length,  as  bacilli.  Some  which  are 
fusiform  in  appearance  are  known  as  Clostridia. 

The  coiled  rods  or  spiral  forms  are  either  (1)  closely  coiled,  when 
they  are  known  as  Spirillum  and  Spirochcrte  (more  threadlike)  ;  or 
(2)  those  more  openly  coiled  are  known  as  Vibriones. 

There  are  also  very  elonyated  filiform  varieties  known  as  Lepto- 
t/</'i.>;  and  branched  forms  as  streptothrix.  In  Beggiatoa  the  fila- 
ments are  fixed  at  one  extremity  and  stretch  the  other  free  in 
the  surrounding  fluid. 

Colin  classified  bacteria  according  to  their  shape,  but  Ray  Lankester, 
Zopf,  and  others  have  shown  that  several  micro-organisms  in  their 
life -cycle  exhibit  successively  the  shapes  characteristic  of  the  orders 
of  Cohn.  These  pleomorphic  species  may  be  illustrated  by  Beygiatoa 
alba. 

In  the  refuse  waters  discharged  from  factories,  especially  the 
sulphuretted  effluents  of  sewage  works,  is  found  this  form — the 
1  sewage  fungus  '  of  engineers.  It  may  have  a  thickness  of  5//,  and 
it  maybe  as  attenuated  as  to  measure  only  lp.  It  is  attached  in  an 
erect  manner  to  objects  in  the  impure  water  it  affects  (fig.  487) ;  and 
the  filaments  consist  of  rows  of  cells,  and  in  the  protoplasm  of  these 
granules  of  sulphur  are  enclosed.  The  filaments  readily  break  up 
into  cells  about  as  long  as  they  are  broad,  and  become  at  length 
active  but  eventually  attach  themselves  to  some  object  and  come  to 
rest,  when  they  multiply  by  fission  and  accumulate  in  masses  of 
zooylcca.  '  They  may  develop  into  rods,  and  these  again  into  the 
filaments  after  the  rods  have  passed  through  the  swarming  state.' 

Spirally  twisted  forms  also  arise  in  this  species.  These  break 
into  coiled  parts,  possessed  of  flagella,  and  exhibit  extremely  active 
movement.  The  flagella  in  these  are  as  strong  and  easily  seen  as 
in  the  Spirillum  volutans,  and  these  forms  were  known  at  an  earlier 
period  as  Ophidomonas. 

Fig.  487  shows  at  1  a  group  of  the  attached  filaments  of  Beggiatoa 
alba :  2  to  5  show  portions  of  filaments  of  differing  diameters ;  5 
shows  a  filament  in  the  act  of  multipartition.  The  small  dark  circles 


654 


FUNGI 


throughout  represent  the  granules  of  sulphur  ;  6  to  8  show  fragments 
rich  in  sulphur  with  transverse  septation  developed  by  treatment 
with  methyl -violet  solution.  In  8  the  formation  of  cocci  and  spores 


FIG.  i87.—13eggiatoa  alba.    (From  De  Bary's  '  Comparative  Morphology  of  Fungi.') 


is  seen  ;  9  shows  the  result  of  filaments  having  broken  up  into  spores  ; 
10  shows  spores  in  movement.  1  is  magnified  540  diameters,  the 
remainder  900  diameters. 

Figure  488  shows  the  growth  of  the  curved  and  spiral  forms 


BACTEEIA 


C55 


of  the  same  :  A  is  a  group  of  attached  filaments ;  B  to  H  show  por- 
tions of  spiral  filaments  ;  C,  D,  F,  to  H  represent  the  act  of  division 
into  smaller  fragments  but  without  motion ;  in  H  the  separate 
cells  are  distinctly  showTi ;  E  shows  the  separation  of  a  complete 
spirillum  form  possessed  of  flagella  and  capable  of  great  activity. 

Bacteria  may  be  united  by  some  interfusing  gelatinous  material 
in  which  all  action  ceases  or  is  of  the  most  limited  kind ;  and  these 
living  films,  which  appear  on  the  surface  or  suspended  in  the  interior 
of  putrescent  fluids,  are 
known  as  Zooglw. 
They  may  also  be  found 
on  the  surfaces  of  solid 
bodies,  where  the  putre- 
factive ferment  is  in 
action. 

Bacteria  have  been 
divided  into  two  classes, 
distinguished  by  the 
formation  of  endospores 
in  the  one  and  of  arthro- 
spores  in  the  other. 

I.  The  endosporous 
forms  are  those  whose 
multiplication  is  brought 
about  by  the  formation 
within  a  cell  of  a  minute 
globular  or  oval  body, 
which,  while  the  sur- 
rounding protoplasm  of 
the  mother-cell  is  assimi- 
lated, gradually  reaches 
its  mature  condition. 
What  it  is  that  exactly 
determines  the  act  of 
spore-formation  is  not 
known,  but  it  is  probable 
that  free  access  to  oxygen 
constitutes  an  important 
factor. 

A  chosen  illustration 
of  the  endosporous  Bac- 
teria is  Bacillus  mega- 
therium. It  was  first 

observed  on  boiled  cabbage  leaves,  and  is  considered  by  De  Bary  as 
an  'exceedingly  instructive  form.'  It  is  2'5/u  in  short  diameter  and 
about  four  times  as  long  as  this.  It  is  illustrated  in  fig.  489.  a  re- 
presents a  motile  chain  of  the  Bacilli  in  active  vegetation.  This  is 
magnified  250  diameters,  b  two  active  rods  magnified  600  diameters. 
p  shows  the  result  of  treating  a  form  in  the  condition  b  with  an 
alcoholic  solution  of  iodine,  c  is  a  rod  with  five  cells  preparing  to 
form  spores,  d  to /represent  successive  stages  of  a  pair  of  rods  in 


FIG.  488. — Beggiatoa  alba,  curved  and  spiral 
forms.  (From  De  Bary's  '  Comparative  Morpho- 
logy of  Fungi.') 


656 


FUNGI 


the  act  of  forming  spores,  e  an  hour  later  than  d,  and  /"an.  hour 
later  than  e.  The  cells  which  did  not  contain  spores  disappeared  or 
perished,  r  is  a  quadricellular  rod  with  ripe  spores.  </1  is  a  five- 
celled  rod  with  three  ripe  spores  placed  in  a  nutrient  solution  after 

j  several  days'  desiccation.  </2  is  the 
same  an  hour  after ;  g3  is  the  same 
after  another  two  hours  and  a  half. 
hl  is  two  spores  with  the  walls  of 
the  mother-cells  dried  and  placed  in 
a  nutrient  solution  ;  A2  is  the  same 
forty-five  minutes  later  ;  i,  k,  I,  three 
stages  of  germination  of  the  spore. 

Bacillus  anthracis  and  B.  subtHis 
are  very  typical  examples  of  endo- 
sporous  bacteria.  B.  anthracis  has 
been  proved  to  be  the  virus  of 
anthrax  or  splenic  fever.  It  is 
found  in  great  profusion  in  the 
blood  and  tissues  of  animals  attacked 
by  this  disease  in  the  form  of  rods 
and  filaments 


FIG.  489.  —  Bacillus  megatherium. 
(From  De  Bary's  '  Comparative 
Morphology  of  Fungi.') 

5/uf  to  20/u  in 

length  and  I/*  to  1'25/z  in  width  (fig.  490). 
Fig.  491  shows  two  filaments  grown  on  a 
microscopic  slide  (De  Bary)  in  a  solution  of 
meat  extract,  partly  in  an  advanced  state  of 


jj 
' 


FIG.  490. — Bacillus  anthracis,  x  1,200.  Blood 
corpuscles  and  bacilli  unstained  ;  from  an  inocu- 
lated mouse.  (Frimkel  and  Pfeiffer.) 


FIG.  491.— A,  Bacillus 
anthracis ;  B,  B.  sub- 
tilis.  (From  De  Bary's 
1  Fungi.') 


spore-formation.  At  the  upper  part  of  the  figure  two  ripe  spores 
have  escaped.  These  spores  on  germination  elongate  and  give  rise 
to  new  groups  of  rods  and  filaments. 


BACTERIA 


657 


B.  subtilis,  which  is  the  Bacillus  common  to  decomposing  hay 
infusion,  has  a  life-history  extremely  similar  to  B.  anthracis.  It 
spores  in  precisely  the  same  manner.  The  outer  wall  of  the  spore 
is  comparatively  thick,  and  the  protoplasm  elongates  in  the  direction 
of  the  longer  axis  of  the  spore  and  of  the  mother-cell  with  which 
this  coincided.  B,  fig.  491 ,  represents  the  development  of  B.  subtilis  : 
1  shows  fragment  of  filaments  with  ripe  spores ;  at  2  the  spore  is 
beginning  to  germinate  ;  3,  the  young  rod  is  projecting  from  the  wall 
of  the  spore  ;  4  represents  germ -rods  curved  in  a  horseshoe  shape 
and  with  the  extremities  connected,  gne  of  them  having  one  extremity 
subsequently  released ;  5,  germ-tubes  with  the  two  extremities 


FIG.  492. — Leuconostoc  mesenteroides :  I,  Spores ;  2,  Spores  after  germination, 
showing  gelatinous  envelope  ;  3,  4,  5,  6,  Increase  by  division  ;  7,  Glomerular  form 
of  zooglcea ;  8,  Section  of  an  old  mass  of  zoogloea ;  9,  Cocci  chains  with  arthro- 
spores  (Tieghem  and  Cienkowski). 

remaining  connected  and  already  greatly  increased  in  size.  The  whole 
represents  a  magnification  of  600  diameters. 

II.  Arthrosporous  forms  are  reproduced  by  the  separation  of 
single  members  from  their  connection  with  a  group,  which  then 
give  origin  to  new  generations.  These  cells,  apparently  not  differing 
from  the  rest,  become  larger,  with  tougher  walls  and  more  refrac- 
tive, and  while  the  rest  of  the  group  die  they,  having  acquired  the 
properties  of  spores,  can  produce  a  new  growth  in  any  fresh 
nourishing  soil. 

A  sufficiently  detailed  illustration  of  the  arthrosporous  Bacteria 
may  be  seen  in  Leuconostoc  mesenteroides  (fig.  492).  This  micro- 
organism occurs  occasionally  in  beetroot  juice  and  the  molasses  of 
sugar-makers,  forming  large  gelatinous  masses  resembling  frog  spawn. 


658 


FUNGI 


Chains  of  cocci  are  found  by  microscopical  examination,  and  some  of 
the  cells  in  a  chain  are  enlarged  without  changing  their  form  and 
develop  into  typical  arthrospores. 

The  Bacteria  behave  very  variously  under  the  same  conditions 
of  supply  or  exclusion  of  oxygen.  The  aerobic  require  free  oxygen 
in  quantity,  as  e.g.  B.  subtilis;  while  in  the  anaerobic  the  vital 
activities  are  promoted  by  its  exclusion.  But  there  can  be  no  doubt 
that  gradual  modification  of  either  condition  will  bring  about  adapta- 
tions. Naegeli  has  shown  that  there  are  forms  which  usually  depend 
on  oxygen  which  continue  to  vegetate  when  free  oxygen  ceases. 


FIG.  493. — A,  Bacterium  termo,  each  cell  furnished  with  a  single  flagellum.  Magni- 
fied 4,000  diameters.  B,  C,  D,  Bacterium  lineola,  each  cell  when  separated 
having  a  flagellum  at  either  end.  Magnified  3,000  diameters.  (Dallinger.) 

Their  nutrition  is  carried  on  like  that  of  other  vegetative  forms 
devoid  of  chlorophyll.  The  actual  typical  group  are  without  doubt 
the  saprophytic  Bacteria.  The  relation  of  the  parasitic  or  pathogenic 
forms  to  these  is  one  of  the  most  interesting  problems  in  microscopic 
biology.  That  they  are  physiological  modifications  of  the  saprophytic 
forms  appears  per  se  a  possibility ;  but  in  the  light  thrown  upon 
biological  change  and  survival  by  the  hypothesis  of  the  origin  of 
species,  the  suggestion  incites  to  practical  inquiry  and  research.  If 
the  parasitic  Bacteria  are  physiological  modifications  of  the  saprophytic 
forms,  to  know  the  path  by  which  they  biologically  became  such  in.tv 


FIG.  494, — Four  individuals  of  Vibrio  ntgula,  each  showing  flagellum  at  one  or 
both  ends ;  two  other  individuals,  a  and  b,  separating  from  each  other,  and  draw- 
ing out  a  protoplasmic  filament  to  form  their  second  flagella.  Magnified  2,000 
diameters.  (Dallinger.) 

be  to  put  more  into  the  hands  of  medicine  than  could  be  accomplished 
by  any  other  means. 

Bacterium  termo  is  the  most  universally  present  and  abundant  of 
the  saprophytic  species.  It  is  Ip  to  I'Qp.  long, and  0'5  to  0'7/i  broad, 
usually  of  dumbbell  form.  These  Bacteria  are  usually  seen  in  '  vacil- 
lating '  movement  in  their  free  state  ;  each  cell  bears  a  flagellum  at 
each  end,  as  B,  D  (fig.  493),  whilst  the  double  cells  bear  a  flagellum 
at  each  extremity.  The  formation  of  the  second  flagellum  takes  place 
by  the  drawing  out  of  a  filament  of  protoplasm  between  two  cells 
that  are  separating  from  each  other  (as  in  fig.  494.  «,  b),  the  rupture 


SPIEILLA 


659 


of  which  gives  a  new  flagellum  to  each.  Their  flagella  are  so  minute 
as  to  be  among  the  most  '  difficult '  of  all  microscopic  objects,  their 
diameter  being  calculated  from  200  measurements  by  Dallinger 
at  no  more  than  ^Tfo\uro*n  °f  an  inch.1  Although  this  species  does 
not  ordinarily  multiply  in  any  other  way  than  by  transverse  sub- 
division, yet,  under  '  cultivation '  at  a  temperature  of  86°  Fahr.,  its 
cells  have  been  seen  to  elongate  themselves  into  motionless  rods, 
resembling  those  of  Bacilli,  whose  endoplasm  breaks  up  into  separate 
particles  that  are  set  free  as  smajl  bright  almost  spherical  spores, 
which  sometimes  congregate  so  as  to  form  a  zooglcea-£\m.  These 
germinate  into  short  slender  rods,  which  are  at  first  motionless,  but 
soon  undergo  transverse  fission,  and  then  acquire  flagella.2 

The  Vibriones  may  be  represented  by  V.  rugula,  seen  in  fig.  494. 
They  are  slightly  curved  rods  and  threads,  from  6/n  to  1 6/u  long,  and 
varying  in  thickness  from  0'5/u  to  2^u.  They  have  well-marked  flagella, 
one  at  each  end.  They  appeal*  in  vegetable  infusions,  causing  fer- 
mentation of  cellulose. 

The  Spirilla  are  the  largest  forms  in  the  group,  characterised  b} 


FIG.  495. — A,  Spirillum  unduhi,  showing  flagellum  at  each  end.     Magnified  3,000 
diameters.     B,  Spirillum  volutans.     Magnified  2,000  diameters.     (Dallinger.) 

their  spirally  formed  cells  and  their  graceful  spiral  motion.  They 
are  fairly  represented  in  fig.  495  by  Spirillum  undula  (A)  and 
Spirillum  volutans  (B).  The  threads  of  the  former  are  from  1'lw  to 
1'4/u  in  thickness,  and  from  9/z  to  12/*  in  length.  They  are  intensely 
active,  and  possess  a  flagellum  at  either  end.  They  are  found  iii 
varying  decomposing  infusions. 

Spirillum  volutans  was  known  to  and  named  by  Ehrenberg.  It 
is  from  l'5/i  to  2'3/z  in  thickness,  and  varies  from  25/i  to  30/z  or 
more  in  length.  It  has  distinctly  granular  contents,  and  a  very 
easily  demonstrable  flagellum  at  each  end  of  the  spiral ;  a  fla- 
gellum was  distinctly  suggested  by  Ehrenberg  on  account  of  the  vor- 
tical action  visible  in  the  fluid  before  this  spirillum  as  it  advanced. 

With  the  beautifully  corrected  6mm.  power  of  Zeiss  (apochromatic 
dry  N.  A.  0'95),  all  but  the  most  difficult  of  these  can  be  seen  in  fresh 
specimens  with  relative  ease  on  a  dark  ground  with  a  12  or  18  eye- 
piece, provided  they  be  examined  alive  with  the  flagella  in  motion. 


1  Journ.  of  Roy.  Micrmc.  Soc.  vol.  i.  (1878),  p.  175, 


2  Ewart,  loc.  cit. 
u  u  2 


66o 


FUNGI 


For  the  more  difficult  ones  (B.  termo  and  E.  lineola)  more  careful 
arrangements  are  required.  In  dried  specimens  the  flagella  can  be 
readily  demonstrated,  and  easily  photographed,  by  staining  them  by 
a  special  method  introduced  by  Loffler  (fig.  496). 

The  germinating  power  of  the  spores  of  Bacteria  may  be  brought 
into  operation  at  once  on  their  reaching  ripeness,  or  they  may  be 
desiccated  for  an  indefinite  time,  and  again,  on  reaching  suitable 
surroundings,  will  germinate  as  before.  This  power  is  held  in  vari- 
ous degrees  by  difterent  forms,  but  the  whole  subject  needs  more 
uniform  and  exhaustive  inquiry.  The  spores  of  B.  subtilis  retain 
their  vitality  for  years  if  kept  in  a  dry  air,  while  those  of 
B.  anthracis  are  stated  by  Pasteur  to  remain  alive  in  absolute 
alcohol ;  l  and  Brefeld  found  their  power  to  germinate  uninjured 
after  the  lapse  of  three  ye*ars  in  a  dry  atmosphere.  He  also  found 
them  proof  against  the  boiling-point  of  water,  and  even  a  higher 

temperature,  but  he 
found  that  fewer  and 
fewer  survived  in  boil- 
ing nutrient  fluid  until 
the  end  of  the  third 
hour,  when  all  wrere 
destroyed.  So  Buchner 
found  that  the  same 
spores  were  wholly 
killed  only  after  three 
or  four  hours'  boiling ; 2 
wThile  Pasteur  states 
that  groups  of  un- 
certain spores  can 
withstand  a  tempera- 
ture of  130°  C.  There 
is,  however,  uncer- 
tainty, because  a  want 
of  uniformity,  in  the 
1,000,  stained  results  from  various 
sources  ;  20°  to  25°  C. 
may  be  taken  as  the 

average  degree  of  temperature  at  which  these  organisms  will  freely 
germinate  ;  but  J3.  termo,  for  example,  has  been  known  to  germinate 
from  5-5°  C.  to  40°  C. 

Nothing  like  '  conjugation,'  or  any  other  form  of  sexual  genera- 
tion,  has  yet  been  witnessed  in  any  Bacteria  ;  and  until  such  shall 
have  been  discovered,  no  confidence  can  be  felt  that  we  know  the 
entire  life-history  of  any  one  type.3  When  these  facts  are  allowed 


.  496.— Flagella  of  Typhoid  Bacilli,  x  1,000,  sta 
by  Loffler's  method.     (Friinkel  and  Pfeiffer.) 


FIG.  496.— 


1  '  Charbon  et  Septicemie,'  Compt.  Rend.  Ixxxv.  p.  99. 
-  Naegeli,  Unters.  uber  niedere  Pilze,  1882,  p.  220. 


•"  As  it  seems  unquestionable  that  among  the  higher  Fungi  '  conjugation '  often 
takes  place  at  a  very  early  stage  of  growth,  it  seems  a  not  very  improbable  surmise 
that  the  '  granular  spheres  '  observed  by  Ewart  in  Bacillus  and  Spirillum,  which 
seem  to  correspond  with  the  '  microplasts '  observed  by  Ray  Lankester  in  his 
Bacterium  rubescens,  may  be  a  product  of  conjugation  in  the  micrococcus  stage  of 
these  organisms. 


BACILLUS   ANTHRACIS 


66 1 


their  due  weight,  no  difficulty  can  be  felt  in  admitting  the  action  of 

Bacteria,   &c.,  in  producing  decomposition  under  conditions  which 

might  at  first  view  be  fairly  supposed  to  preclude  the  possibility  of 

their  presence.     This  action  is  altogether  analogous  to  that  of  the 

yeast-plant  in  producing  saccharine  fermentation ;  and  the  careful 

and  exact  experiments  of  Pasteur,  repeated 

and  verified  in  a  great  variety  of  modes  by 

Lister,  Tyndall,  and  others,  leave  no  doubt 

on  these 'two  points — (1)  that  putrefactive 

fermentation  does  not  take  place,*  even    in 

liquids  which  are  peculiarly  disposed  to  pass 

into  it,  except  in  the  presence  of  Bacteria ; 

and   (2)  that  neither  these  germs  nor  any 

others  arise  in  such  liquids  de  novo,  but  that 

they  are  all  conveyed  into  them  by  the  air 

when  not  otherwise  introduced.     It  is  thus 

also  with  the  parasitic  or  pathogenic  forms 

of   Bacteria    in    setting   up   disease.      Thus 


FIG.  497.  —  Spore-bearing  threads  of  Bacillus 
anthracis,  double-stained  withfuchsine  and 
methyleneblue,  x  1,200.  (Crookshank.) 


FIG.  498.  —  Photograph  of  a 
pure-cultivation  of  Ba- 
cillusanthracis.  (  Crook  - 
shank.) 


1  splenic  fever  '  is  producible  by  the  inoculation  of  Bacillus  anthracis 

(figs.  497  and  498)  ;  and  tetanus  or  'lock-jaw  '  by  inoculation  with 

another  species  of  Bacillus,  the  microbes  having  been  in  both  cases 

'cultivated,'   so  as  to  be  free   from   other  contaminating  matter. 

Similar  observations  have  been  made 

upon  tuberculosis  (figs.  499  and  500), 

actinomycosis,   glanders,   so   that   an 

animal  suffering  under  any  of  these 

diseases  may  be  a  focus  of  infection 

to    others,    for    precisely   the    same 

reason  that  a  tub  of  fermenting  beer 

is  capable  of  propagating  its  fermen- 

tation to  fresh  wort.    A  most  notable 

instance     of     such     propagation     is 

afforded  by  the  spread  of  the  disease   FIG. 

termed    '  pebrine  '   among   the    silk- 

worms of  the  south  of  France,  which, 

according  to  Pasteur,  is  caused  by  a 

minute  organism  named  Nosema  Bombycis,  the  mortality  caused  by 

it  being  estimated  to  produce  a  money  loss  of  from  three  to  four 

millions  sterling  annually  for  several  years  following  1853,  when  it 


499. — Bacilli  of  tubercle  in 
sputum,  x  2,500  (from  photo- 
graphs), stained  with  carbolised 
fuchsine.  (Crookshank.) 


662 


FUNGI 


first  oroke  out  with  violence.  It  has  been  shown  by  microscopic 
investigation  that  in  silkworms  strongly  affected  with  this  disease, 
every  tissue  and  organ  in  the  body  is  swarming  with  these  minute 
cylindrical  corpuscles  about  4'2^u  long,  and  that  these  even  pass  into 
the  undeveloped  eggs  of  the  female  moth,  so  that  the  disease  is 
hereditarily  transmitted.  And  it  has  been  further  ascertained  by 
the  researches  of  Pasteur  that  these  corpuscles  are  the  active  agents 
in  the  production  of  the  disease,  which  is  engendered  in  healthy 
silkworms  by  their  reception  into  their  bodies ;  whilst,  if  due  pre- 
cautions be  taken  against  their  transmission,  the  malady  may  be 
completely  exterminated. 


Fl<J.  500. — Pure-cultivations  on  glycerine- agar  from  human  tubercular  sputum : 
a,  after  six  months'  growth  (fifth  sub-culture) ;  6,  c,  after  ten  months'  growth 
(fourth  sub-culture).  (Crookshank.) 

Bacteriology  is  now  so  distinctly  a  branch  of  biological  science 
that  it  would  be  out  of  place  here  to  present  even  a  summary  of  its 
voluminous  details  and  methods  of  research.  The  microscope  in  its 
most  perfect  form  is  an  indispensable  adjunct  to  the  rapidly  progres- 
sive work  of  this  department  of  biological  research,  and  the  most 
delicate  and  refined  employment  of  the  microscope  and  all  its 
adjuncts  is  in  the  last  degree  important.  Only  a  skilled  microscopist 
can  be  a  successful  bacteriologist.  But  for  the  methods  of  the 
bacteriological  laboratory  we  must  refer  the  reader  to  treatises  on 
this  branch  of  science,1  it  being  enough  here  to  remark  that  the 

1  The  English  student  will  find  an  admirable  aid  in  the  Text-book  of  Bacterio- 
logy and  Infective  Diseases  (4th  ed.),  by  Professor  E.  Crookshank. 


CULTIVATION  AND  COLONIES 


663 


employment  of  nutrient   gelatine,  nutrient    agar-agar,  aiul    other 
similar  media  on  glass  plates,  and  in  test-tubes  (fig.  501),  so  as  1>\ 


FIG.  501. — Pure-cultivations  of  Streptococcus pyogenes  :  a,  on  the  surface  of 
nutrient  gelatine  ;  &,  in  the  depth  of  nutrient  gelatine ;  e,  011  the  surface  of 
nutrient  agar.  (Crookshank.) 


FIG.  502.— Colonies  of  Bacillus  anthracis,  x  80  :  a,  after  24  hours  ; 
6,  after  48  hours.     (Fliigge.) 

inoculation  to  obtain  cultures  of  specific  and  isolated  forms  with 
their    characteristic    appearances,  is  one  of  the  essential    methods 


664  FUNGI 

(Plate  XIV).  The  inoculated  bacteria,  instead  of  moving  freely,  as 
they  would  in  a  liquid  medium,  are  fixed  to  one  spot,  where  they 
develop  '  colonies '  in  a  characteristic  manner,  showing  their  own 
morphological  features  (fig.  502).  Cleanliness  and  care,  as  well  as 
practice  in  manipulation,  are  essential.  In  the  same  way  we  can  only 
allude  to  the  investigation  of  the  chemical  products  of  bacteria,  such 
as  toxins,  and  to  those  antidotal  substances  or  antitoxins  which 
develop  in  the  blood  of  suitable  animals  inoculated  with  gradually 
increasing  doses  of  toxins.  Antitoxins  and  vaccines  are  now  largely 
used  in  the  treatment  of  tetanus,  diphtheria,  typhoid  fever,  plague, 
cholera,  and  septic  diseases  in  the  human  subject. 

The  pathological  and  therapeutic  value  of  these  researches  is 
far  beyond  our  present  ability  to  estimate,  and  must  have  an 
apparently  increasing  value.  But  it  is  a  science  with  which  a  work 
of  this  sort  may  not  deal  further  than  to  show  the  light  use  of  the 
microscope  and  its  appliances,  by  which  the  work  of  pathological 
bacteriology  can  alone  be  successfully  done. 


!>!;-»{<•  XIV. 


CO 

•00 


. 


665 


CHAPTER   X 

r> 

MICROSCOPIC  STRUCTURE   OF' THE  HIGHER   CRYPTOGAMS 

Hepaticae. — Quitting  now  the  algal  and  fungoid  types,  and 
entering  the  series  of  terrestrial  cryptogams,  we  have  first  to  notic 
the  little  group  of  Hepaticce,  or  liverworts.  This  group  presents 
numerous  objects  of  great  interest  to  the  microscopist ;  and  no 
species  is  richer  in  these  than  the  very  common  Marchantia  poly- 
morpha,  which  may  often  be  found  growing  between  the  paving- 
stones  of  damp  courtyards,  but 
which  particularly  luxuriates  in 
the  neighbourhood  of  springs  or 
waterfalls,  where  its  lobed  fronds 
are  found  covering  extensive  sur- 
faces of  moist  rock  or  soil,  adher- 
ing by  the  radical  filaments  (rhi- 
zoids)  which  arise  from  their  lower 
surface.  At  the  period  of  fructi- 
fication these  fronds  send  up 
stalks,  which  carry  at  their  sum- 
mits either  round  shield-like  discs, 
or  radiating  bodies  that  bear  some  FIG.  503.— Frond  of  Marchantia  poly- 
resemblance  to  a  wheel  without  worpha,  with  gemmiparous  concep- 
its  tire  (fig.  503).  The  former  JS^^  lobed  recePtacles  bearins 
carry  the  male  organs  or  an- 

therids ;  while  the  latter  in  the  first  instance  bear  the  female 
organs  or  archegones,  which  afterwards  give  place  to  the  sporanges, 
or  spore-cases.1 

The  green  surface  of  the  frond  of  Marchantia  is  seen,  under  a  low 
magnifying  power,  to  be  divided  into  minute  diamond -shaped  spaces 
(fig.  504,  A,  a,  #),  bounded  by  raised  bands  (c,  c) ;  every  one  of  these 
spaces  has  in  its  centre  a  curious  brownish-coloured  body  (5,  5),  with 
an  opening  in  its  middle,  which  allows  a  few  small  green  cells  to  be 
seen  through  it.  When  a  thin  vertical  section  is  made  of  the  frond 
(B),  it  is  seen  that  each  of  the  lozenge-shaped  divisions  of  its  surface 
corresponds  with  an  air-chamber  in  its  interior,  which  is  bounded 
below  by  a  floor  (a,  a)  of  closely  set  cells,  from  whose  under  surface 
the  rhizoids  arise ;  at  the  sides  by  walls  (c,  c)  of  similar  solid 

1  In  some  species  the  same  shields  bear  both  sets  of  organs  ;  and  in  Marchantia 
(tndrogyna  we  find  the  upper  surface  of  one  half  of  the  shield  developing  antherids, 
whilst  the  under  surface  of  the  other  half  bears  archegones. 


666      MICKOSCOPIC    STKUCTURE   OF  HIGHER   CRYPTOGAMS 


parenchyma,  the  projection  of  whose  summits  forms  the  raised 
bands  on  the  surface  ;  and  above  by  an  epiderm  (b,  b)  formed  of  a 
single  layer  of  cells;  whilst  its  interior  is  occupied  by  a  loosely 
arranged  parenchyme  composed  of  branching  rows  of  cells  (/,  /)  that 
seem  to  spring  from  the  floor,  these  cells  being  what  are  seen  from 
above  when  the  observer  looks  down  through  the  central  aperture 

just  mentioned.  If  the  vertical 
section  should  happen  to  traverse 
one  of  the  peculiar  bodies  which 
occupy  the  centres  of  the  divi- 
sions, it  will  bring  into  view  a 
structure  of  remarkable  com- 
plexity. Each  of  these  stomates 
(as  they  are  termed,  from  the 
Greek  oro/io,  mouth)  forms  a  sort 
of  shaft  (y),  composed  of  four  or 
'five  rings  (like  the  '  courses '  of 
bricks  in  a  chimney)  placed  one 
upon  the  other  (A),  every  ring 
being  made  up  of  four  or  five 
cells;  and  the  lowest  of  these 
rings  (i)  appears  to  regulate  the 
aperture  by  the  contraction  or 
expansion  of  the  cells  which 
compose  it,  and  is  hence  termed 
FIG.  504.-StructureoffrondofMarc^a»-  the  '  obturator-ring/  In  this 
tia  polymorpha  :  A,  portion  seen  from  manner  each  of  the  air-chambers 
above;  a  a,  lozenge-shaped  divisions;  of  the  frond  is  brought  into  COin- 
b,b,  stomates  in  the  centre  of  the  lozenges;  .  <-  , 

c,    c,   greenish    bands    separating    the    munication    With     the     external 
lozenges.  B,  vertical  section  of  the  frond,  atmosphere,  the  degree  of  that 

showing -a,  a,  the .dense  layer of cellular    communicatiOn    being    regulated 

tissue    forming   the    floor    of    the    air-    .  ...... 

chamber,  d,  d,  the  epidermal  layer,  6,  6,    by  the  limitation  of  the  aperture. 

forming  its  roof ;  c,  c,  its  walls ;  /,/,  loose   We  shall  hereafter  find  that  the 

cells .in  its  interior;  g,  stomate  divided  per-    j  f  tl       higher  plants   CO11- 

pendicularly;  Brings  of  cells  forming  its         .        . 

wall ;  i,  cells,  forming  the  obturator-ring,    tain    intercellular   spaces,  which 

also  communicate  with  the  ex- 
terior by  stomates,  but  that  the  structure  of  these  organs  is  far  less 
complex  in  them  than  in  this  humble  liverwort. 

The  frond  of  Marchantia  usually  bears  upon  its  surface,  as  shown 
in  fig.  503,  a  number  of  little  open  basket-shaped  gemmiparous  con- 
ceptacles  (fig.  505),  which  may  often  be  found  in  all  stages  of  develop- 
ment, and  are  structures  of  singular  beauty.  They  contain  when 
mature  a  number  of  little  green  round  or  oblong  discoidal  gewimt'. 
each  composed  of  two  or  more  layers  of  cells  ;  and  their  wall  is  sur- 
mounted by  a  glistening  fringe  of  *  teeth,'  whose  edges  are  themselves 
regularly  fringed  with  minute  outgrowths.  This  fringe  is  at  first 
formed  by  the  splitting  up  of  the  epiderm,  as  seen  at  B,  at  the 
time  when  the  conceptacle  and  its  contents  are  first  making  their 
way  above  the  surface.  The  little  gemmae  are  at  first  evolved  as 
single  globular  cells,  supported  upon  other  cells  which  form  their 
footstalks ;  these  single  cells,  undergoing  binary  subdivision,  evolve 


STRUCTURE    OF  MARCHANTIA 


667 


themselves  into  the  gemmae  ;  and  these  gemma?,  when  mature, 
spontaneously  detach  themselves  from  their  footstalks,  and  lie  free 
within  the  cavity  of  the  conceptacle. 
Most  commonly  they  are  at  last 
washed  out  by  rain,  and  are  thus 
carried  to  different  parts  of  the 
neighbouring  soil,  on  which  they 
grow  very  rapidly  when  well  sup- 
plied with  moisture ;  sometimes, 
however,  they  may  be  found  grow- 
ing whilst  still  contained  withfn 
the  conceptacles,  forming  natural 
grafts  (so  to  speak)  upon  the  stock 
from  which  they  have  been  de- 
veloped or  detached  ;  and  many  of 
the  irregular  lobes  which  the  frond 
of  Marchantia  puts  forth  seem 
to  have  this  origin.  The  very 
curious  observation  was  long  ago 
made  by  Mirbel,  who  carefully 
watched  the  development  of  these 
gemmae,  that  stomates  are  formed 
on  the  side  which  happens  to  be 
exposed  to  the  light,  and  that 
rhizoids  are  put  forth  from  the 
lower  side,  it  being  apparently  a 
matter  of  indifference  which  side  of 
the  little  gemma  is  at  first  turned 
upwards,  since  each  has  the  power 
of  developing  either  stomates  or 
rhizoids  according  to  the  influence 
it  receives.  After  the  tendency  to 
the  formation  of  these  organs  has 
once  been  given,  however,  by  the 
sufficiently  prolonged  influence  of 
light  upon  one  side  and  of  darkness  and  moisture  on  the  other,  any 
attempt  to  alter  it  is  found  to  be  vain ;  for  if  the  surfaces  of  the 
young  fronds  be  then  inverted,  a  twisting  growth  soon  restores  them 
to  their  original  aspect. 

When  Marchantia  vegetates  in  damp  shady  situations  which 
are  favourable  to  the  nutritive  processes,  it  does  not  readily  produce 
the  true  fructification,  which  is  to  be  looked  for  rather  in  plants 
growing  in  more  exposed  places.  Each  of  the  stalked  peltate 
(shield-like)  discs  contains  a  number  of  flask-shaped  cavities  opening 
upon  its  upper  surface,  which  are  brought  into  view  by  a  vertical 
section ;  and  in  each  of  these  cavities  is  lodged  an  antherid  which 
is  composed  of  a  mass  of  '  sperm-cells,'  within  which  are  developed 
antherozoids  like  those  of  Chara  ;  the  whole  being  surmounted  by  a 
long  neck  that  projects  through  the  mouth  of  the  flask-shaped  cavity. 
The  wheel-like  receptacles  (fig.  503),  on  the  other  hand,  bear  on  their 
under  surface,  at  an  early  stage,  concealed  between  membranes  that 


FIG.  505. — Gemmiparous  conceptacles 
ofMarchantiapolymorpha :  A,  con- 
ceptacle fully  expanded,  rising  from 
the  surface  of  the  frond,  a,  a,  and 
containing  gonidial  gemmae  already 
detached.  B,  first  appearance  of 
conceptacle  on  the  surface  of  the 
frond,  showing  the  formation  of  its 
fringe  by  the  splitting  of  the  epiderm. 


668       MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMS 


connect  the  origins  of  the  lobes  with  one  another,  a  set  of  archegones, 
shaped  like  flasks  with  elongated  necks  (fig.  507) ;  each  of  these  has 
in  its  interior  an  '  oosphere '  or  '  germ-cell,'  to  which  a  canal  leads 
down  from  the  extremity  of  the  neck,  and  which  is 
fertilised  by  the  penetration  of  the  antherozoids 
through  this  canal  until  they  reach  it.  Instead, 
however,  of  at  once  evolving  itself  into  a  new  plant 
resembling  its  parent,  the  fertilised  oosphere  or 
1  embryo-cell '  develops  itself  into  a  mass  of  cells  en- 
closed within  a  capsule,  which  is  termed  a  sporange  ; 
and  thus  the  mature  receptacle,  in  place  of  arche- 
gones,  bears  capsules  or  sporanges,  each  of  them  filled 
with  an  aggregation  of  cells  that  constitute  the  im- 
mediate progeny  of  the  fertilised  germ-cell.  These 
cells,  discharged  by  the  bursting  of  the  sporange, 
are  of  two  kinds :  namely,  spores,  each  enclosed  in 
a  double  spore-membrane ;  and  elaters,  which  are 
very  elongated  cells,  each  containing  a  double  spiral 
fibre  coiled  up  in  its  interior.  This  fibre  is  so  elastic 
that  when  the  surrounding  pressure  is  withdrawn 
by  the  bursting  of  the  sporange,  the  elaters  ex- 
tend themselves  (fig. 
506),  tearing  apart  the 
cell  -  membrane  ;  and 
they  do  this  so  suddenly 
as  to  jerk  forth  the 
spores  which  may  be 
adherent  to  their  coils, 
and  thus  assist  in 
their  dispersion.  The 
spores,  when  subjected 
to  moisture,  with  a 
moderate  amount  of 
light  and  warmth,  de- 
velop themselves  into 
little  collections  of  cells, 
which  gradually  assume  the  form  of  flattened  fronds ;  and  thus  the 
species  is  very  extensively  multiplied,  every  one  of  the  aggregate  of 
spores  which  is  the  product  of  a  single  germ-cell  being  capable  of 
giving  origin  to  an  independent  individual. 

Marchantia  is  the  type  of  the  section  known  as  the  thalloid 
Hepaticse.  Another  section,  the  foliose  Hepaticse,  is  represented 
by  the  genus  Jungermannia,  exceedingly  common  plants,  of  a  moss- 
like  habit,  growing  on  moist  banks  and  similar  situations.  While 
the  structure  of  the  sexual  organs,  and  of  the  sporanges,  resem- 
bles in  its  main  features  that  of  Marchantia,  the  vegetative 
organs  are  very  different,  consisting  of  a  slender  creeping  stem 
with  small  semi-transparent  leaves.  This  distinct  differentiation  of 
stem  and  leaves  indicates  a  decided  advance  in  organisation,  and 
marks  the  passage  from  the  thallo2)hytic  to  the  cormophytic  type  of 
structure. 


FIG.  506.— Elater  FIG.  507.— Arcliegone  of  Mar- 
aud spores  of  c7iantiapolymorplta,msucces- 
Marckantia.  sive  stages  of  development. 


STRUCTURE   OF  MOSSES 


669 


Musci. — There  is  not  one  of  the  tribe  of  Mosses  whose  external 
organs  do  not  serve  as  beautiful  objects  when  viewed  with  low  powers 
of  the  microscope  ;  while  their  more  concealed  wonders  are  ad- 
mirably fitted  for  the  detailed  scrutiny  of  the  practised  observer. 
Mosses  always  possess  a  distinct  axis  of  growth,  commonly  more  or 
less  erect,  on  which  the  minute  and  delicately  formed  leaves  are 
arranged  with  great  regularity.  The  stem  shows  some  indication  of 
the  separation  of  a  cortical  or  external  portion  from  the  medullary 
or  central,  by  the  intervention  of  a  circle  of  bundles  of  elongated 
cells,  which  seem  to  prefigure  the  woody  portion  of  the  stem  of 


PIG.  508. — Structure  of  mosses :  A,  plant  of  Funaria  lujgroinntrina,  showing,  /  the 
leaves,  u  the  sporanges  supported  upon  the  setae  or  footstalks  s,  closed  by  the  opercule 
o,  and  covered  by  the  calypter  c.  B,  sporanges  of  Encalyptra  vulga  ris,  one  of  them 
closed  and  covered  with  the  calypter,  the  other  open  ;  u,  u,  the  sporanges;  o,  o,  the 
opercules  ;  c,  calypter  ;  p,  peristome  ;  s,  s,  setae.  C,  longitudinal  section  of  very 
young  sporange  of  Splaclmum  ;  a,  solid  tissue  forming  the  lower  part  of  the  capsule  ; 
c,  columel ;  /,  space  around  it  for  the  development  of  the  spores ;  e,  epidermal 
layer  of  cells,  thickened  at  the  top  to  form  the  opercule  o ;  p,  two  intermediate 
layers,  from  which  the  peristome  will  be  formed ;  s,  inner  layer  of  cells  forming 
the  wall  of  the  cavity. 


higher  plants,  and  from  which  prolongations  pass  into  the  leaves,  so 
as  to  afford  them  a  sort  of  midrib.  The  leaf  usually  consists  of  either 
a  single  or  a  double  layer  of  cells,  having  flattened  sides  by  which 
they  adhere  one  to  another ;  they  rarely  present  any  distinct 
epidermal  layer ;  but  such  a  layer,  perforated  by  stomates  of  sipmle 
structure,  is  commonly  found  on  the  seta  or  bristle-like  footstalk 
bearing  the  fructification,  and  sometimes  on  the  midribs  of  the  leaves. 
The  rhizoids  of  mosses,  like  those  of  Marchantia,  consist  of  long 
tubular  cells  of  extreme  transparency,  within  which  the  protoplasm 
may  frequently  be  seen  to  circulate,  as  in  the  elongated  cells  of 
Chora. 


6/0       MICROSCOPIC  STRUCTURE  OF  HIGHER  CRYPTOGAMS 


The  *  urn,'  or  '  capsule,'  of  mosses,  filled  with  spores,  and  borne 
at  the  top  of  a  long  footstalk  that  springs  from  the  centre  of  a  cluster 
of  leaves  (fig.  508,  A),  is  the  ultimate  result  of  an  act  of  fertili- 
sation ;  for  mosses,  like  liverworts,  possess  both  antherids  and 
archegones.  These  organs  are  sometimes  found  in  the  same  envelope, 


FIG.  509. — Antherids  and  antherozoids  of  Polytnchum  commune:  A,  group  of 
antherids,  mingled  with  hairs  and  sterile  filaments  (paraphyses).  Of  the  three 
antherids,  the  central  one  is  in  the  act  of  discharging  its  contents  ;  that  on  the 
left  is  not  yet  mature ;  while  that  on  the  right  has  already  emptied  itself,  so  that 
the  cellular  structure  of  its  walls  becomes  apparent  B,  cellular  contents  of  an 
antherid,  previously  to  the  development  of  the  antherozoids  ;  C,  the  same, 
showing  the  first  appearance  of  the  antherozoids ;  D,  the  same,  mature  and 
discharging  the  antherozoids. 

or  perigone,  sometimes  on  different  parts  of  the  same  plant,  some 
times  only  on  different  individuals ;  but   in    either   case   they  are 
usually  situated  close  to  the  axis,  among  the  bases  of  the  leaves. 
The  antherids  are  globular,  oval,  or  elongated  bodies  (fig.  509,  A), 
composed  of  aggregations  of  cells,  of  which  the  interior  are  '  sperm- 


FKUCTIFICATION  OF  MOSSES  671 

cells,'  each  of  which,  as  it  comes  to  maturity,  develops  within  itself 
an  antherozoid  (B,  C,  D)  ;  and  the  antherozoids,  set  free  by  the 
rupture  of  the  cells  within  which  they  are  formed,  make  their 
escape  by  a  passage  that  opens  for  them  at  the  summit  of  the  antherid. 
The  antherids  are  generally  surrounded  by  a  cluster  of  hairlike 
filaments  (fig.  509,  A),  which  are  called  paraphyses ;  these  seem  to 
be  '  sterile  '  or  undeveloped  antherids.  In  the  '  hair-moss,'  Poly- 
trichum  commune,  one  of  the  largest  of  our  mosses,  common  on  dry 
heaths,  these  antherids  are  collected  into  conspicuous  starlike 
clusters  at  the  extremities  of  tlie  branches  of  the  *  male '  plants. 
These  are  to  be  seen  about  April,  and  at  the  same  time  the  arc-he  - 
gones  may  be  detected  concealed  among  the  leaves  on  the  '  female  ' 
plant ;  while  the  capsules,  or  sporanges,  in  this  and  most  other 
mosses,  make  their  appearance  late  in  the  summer,  and  remain 
through  the  winter.  The  archegones  bear  a  general  resemblance  to 
those  of  Marchantia  (fig.  507),  and  the  fertilisation  of  their  con- 
tained oospheres,  or  germ-cells,  is  accomplished  in  the  manner 
already  described.  The  fertilised  embryo-cell  becomes  gradually 
developed  by  cell-division  into  a  conical  body  elevated  upon  a  stalk  ; 
and  this  at  length  tears  across  the  walls  of  the  flask-shaped  arche- 
gone  by  a  circular  fissure,  carrying  the  higher  part  upwards  on  its 
summit  as  a  calypter  or  hood  (fig.  508,  B,  c),  while  the  lower  part 
remains  to  form  a  kind  of  collar  round  the  base  of  the  stalk,  known 
as  the  vayine. 

The  urn,  theca,  or  sporange,  which  is  the  immediate  product  of 
the  generative  act,  is  closed  at  its  summit  by  an  opercule,  or  lid 
(fig.  508,  B,  o,  0),  which  falls  off  when  the  contents  of  the  sporange 
are  mature,  so  as  to  give  them  free  exit  ;  and  the  mouth  thus  laid 
open  is  surrounded,  in  many  mosses,  by  a  beautiful  toothed  fringe, 
which  is  termed  the  jteristonie.  This  fringe,  as  seen  in  its  original 


FIG.  510. — Mouth  of  sporange  of  Funaritt,  FIG.  511. — Double  peristome 

showing  the  peristome  in  situ.  of  Fontinalis  antipyretira. 

undisturbed  position  (fig.  510),  is  a  beautiful  object  for  the  binocular 
microscope  ;  it  is  very  '  hygrometric/  executing,  when  breathed  on, 
a  curious  movement  which  is  probably  concerned  in  the  dispersion 
of  the  spores.  In  figs.  511-513  are  shown  three  different  forms  of 
peristome,  spread  out  and  detached,  illustrating  the  varieties  which 


672     MICROSCOPIC   STRUCTURE   OF  HIGHER   CRYPTOGAMS 

it  exhibits  in  different  genera  of  mosses — varieties  whose  existence 
and  readiness  of  recognition  render  them  characters  of  extreme  value 
to  the  systematic  botanist,  whilst  they  furnish  objects  of  great 
interest  and  beauty  for  the  microscopist.  The  peristome  seems 
always  to  be  originally  double,  one  layer  springing  from  the  outer, 
and  the  other  from  the  inner,  of  two  layers  of  cells  which  may  be 
always  distinguished  in  the  immature  sporange ;  but  one  or  other  of 
these  is  frequently  wanting  at  the  time  of  maturity,  and  sometimes 
both  are  obliterated,  so  that  there  is  no  peristome  at  all.  The 
number  of  the  teeth  is  always  a  « power '  of  four,  varying  from 
four  to  sixty-four  ;  sometimes  they  are  prolonged  into  straight  or 
twisted  hairs.  The  spores,  or  gonidial  cells,  are  contained  in  the 
upper  part  of  the  sporange,  where  they  are  clustered  round  a  central 
pillar  which  is  termed  the  columel.  In  the  young  sporange  the 
whole  mass  is  nearly  solid  (fig.  508,  C),  the  space  (I)  in  which  the 


FIG.  512.— Double  peristome  of 
Brynm  inter medi inn . 


FIG.  518. — Double  peristome 
of  CinclidiiDii  nrcticunt. 


spores  are  developed  being  very  small ;  but  this  gradually  augments, 
the  walls  becoming  more  condensed,  and  at  the  time  of  maturity 
the  interior  of  the  sporange  is  almost  entirely  occupied  by  the  spores. 
These  are  formed  in  groups  of  four  by  the  binary  subdivision  of  the 
mother-cells  which  first  differentiate  themselves  from  those  forming 
the  capsule  itself.  The  capsule  and  seta  of  mosses  together  consti- 
tute the  organ  known  as  the  sporogone. 

The  development  of  the  spore  into  a  new  plant  commences  with 
the  rupture  of  its  firm  yellowish-brown  outer  coat  or  exospore,  and 
the  protrusion  of  its  cell-wall  proper,  or  endospore,  from  the 
projecting  extremity  of  which  new  cells  are  put  forth  by  a  process 
of  outgrowth,  forming  a  sort  of  confervoid  filament  known  as  the 
protoneme.  At  certain  points  of  this  filament  its  component  cells 
multiply  by  subdivision,  so  as  to  form  rounded  clusters  or  buds,  from 
every  one  of  which  an  independent  plant  may  arise.  The  Musci, 
therefore,  present  an  example  of  the  phenomenon  known  as  alter- 
nation of  generations.  The  life-history  of  each  individual  may  be 
divided  into  two  '  generations  : '  the  sexual  generation  or  '  oophyte,' 


SPORANGE   OF  MOSSES 


673 


which  consists  of  the  leafy  plant  bearing  the  male  and  female  organs  ; 
and  the  non-sexual  generation  or  '  sporophyte,'  composed  of  the 
sporogone  with  its  spores,  these  two  generations  alternating  with  one 
another  in  the  complete  cycle  of  development. 

The  tribe  of  Sphagnacecv,  or  *  bog-mosses/  is  now  separated  by 
muscologists  from  true  mosses  on  account  of  the  marked  differences 
by  which  they  are  distinguished,  the  three  groups,  Hepaticce, 
hryacecK  (or  ordinary  mosses),  and  Sphaynacece,  being  ranked  as  to- 
gether forming  the  group  of  Musc^neKK, 
The  stem  of  Sphaynacece  is  more  dis-  b  b  b 

tinctly  differentiated  than  that  of 
Bryacece  into  the  central  or  medullary, 
the  outer  or  cortical,  and  the  inter- 
mediate or  woody  portions  ;  and  a  very 
rapid  passage  of  fluid  takes  place 
through  its  elongated  cells,  especially 
in  the  medullary  and  cortical  layers,  so 
that  if  one  of  the  plants  be  placed  dry 
in  a  flask  of  water,  with  its  rosette 
of  leaves  bent  downwards,  the  water 
will  speedily  drop  from  this  until  the 
flask  is  emptied.  The  leaf-cells  of  the 
Sphagnacecv  exhibit  a  very  curious  de- 
parture from  the  ordinary  type  ;  for 
instead  of  being  small  and  polygonal, 
they  are  large  and  elongated  (fig.  514)  ; 
they  contain  no  chlorophyll,  but  have 
spiral  fibres  loosely  coiled  in  their  in- 
terior ;  and  their  membranous  walls 
have  large  rounded  apertures,  by  which 
their  cavities  freely  communicate  with 
one  another,  as  is  sometimes  curiously 
evidenced  by  the  passage  of  wheel- 
animalcules  that  make  their  habitation  in  these  chambers.  Between 
these  coarsely  spiral  cells  are  some  thick-walled  narrow  elongated 
cells  containing  chlorophyll ;  these,  which  give  to  the  leaf  its  firm- 
ness, do  not,  in  the  very  young  leaf,  differ  much  in  appearance  from 
the  others,  the  peculiarities  of  both  being  evolved  by  a  gradual  pro- 
cess of  differentiation.  The  antherids,  or  male  organs,  of  SpJmynacece 
resemble  those  of  liverworts,  rather  than  those  of  mosses,  in  their 
form  and  arrangement ;  they  are  grouped  in  ;  catkins '  at  the  tips 
of  lateral  branches,  each  of  the  imbricated  perigonal  leaves  enclosing 
a  single  globose  antherid  on  a  slender  footstalk,  and  they  are  sur- 
rounded by  very  long  branched  paraphyses  of  cobweb-like  tenuity. 
The  female  organs,  or  archegoiies,  which  do  not  differ  in  structure 
from  those  of  mosses,  are  grouped  together  in  a  sheath  of  deep  green 
leaves  at  the  end  of  one  of  the  short  lateral  branchlets  at  the  side  of 
the  rosette  or  terminal  crown  of  leaves.  The  two  sets  of  organs 
are  always  distributed  on  different  branches,  and  in  some  instances 
on  different  plants.  The  '  sporange  '  which  is  formed  as  the  product 
of  the  impregnation  of  the  germ-cell  is  very  uniform  in  all  the 

x  x 


FIG.  514.— Portion  of  the  leaf  of 
Sphagnum,  showing  the  large 
empty  cells,  «,  a,  a,  with  spiral 
fibres,  and  communicating  aper- 
tures ;  and  the  intervening 
bands,  b,  b,  b,  composed  of 
small  elongated  chlorophyllous 
cells. 


674    MICBOSCOPIC   STBUCTUBE   OF  HIGHER   CRYPTOGAMS 

species,  being  almost  spherical,  with  a  slightly  convex  lid,  without 
beak  or  point,  and  showing  no  trace  of  a  peristome  ;  and  the  spores 
it  contains  are  produced  in  groups  of  four  (as  in  mosses)  around  a 
hemispherical  'columel.'  Besides  the  ordinary  spores,  however, 
the  Sphagnacece  sometimes  develop  a  smaller  kind,  the  *  microspores,' 
formed  by  a  further  division  of  the  mother-cells ;  the  significance  of 
these  is  unknown.1  The  ordinary  spores,  when  germinating,  do  not 
produce  the  branched  confervoid  filament  of  true  mosses,  but  if 
growing  on  wet  peat  evolve  themselves  into  a  lobed  foliaceous  '  pro- 
thallium,'  resembling  the  frond  of  liverworts;  whilst  if  they 
develop  in  water  a  single  long  filament  is  formed,  of  which  the 
lower  end  gives  off  rhizoids,  while  the  upper  enlarges  into  a  bud, 
from  which  the  young  plant  is  evolved.  In  either  case  the  pro- 
thallium  and  its  temporary  roots  wither  away  as  soon  as  the  young 
plant  begins  to  branch.  From  their  extraordinary  power  of  imbibing 
and  holding  water,  the  Sphagnacece,  are  of  great  importance  in  the 
economy  of  Nature,  clothing  with  vegetation  many  areas  which 
would  otherwise  be  sterile,  and  serving  as  reservoirs  for  storing  up 
moisture  for  the  use  of  higher  forms  of  vegetation. 

Filices. — In  the  general  structure  of  Ferns  we  find  a  much  nearer 
approximation  to  flowering  plants ;  but  this  does  not  extend  to 
their  reproductive  apparatus,  which  is  formed  upon  a  type  essentially 
the  same  as  that  of  mosses,  though  evolved  at  a  very  different  period 
of  life.  As  the  tissues  of  which  their  fabrics  are  composed  are 

essentially  the  same  as  those  to  be  de- 
scribed in  the  next  chapter,  it  will  not 
be  requisite  here  to  dwell  upon  them. 
The  stem  (where  it  exists)  is  for  the 
most  part  made  up  of  cellular  par- 
enchyme,  which  is  separated  into  a 
coi'tical  and  a  medullary  portion  by  the 
interposition  of  a  circular  series  of 
fibro-vascular  bundles  containing  true 
woody  tissue  and  ducts.  These  bundles 
form  a  kind  of  irregular  network,  from 
which  prolongations  are  given  oft'  that 
pass  into  the  leaf-stalks,  and  thence 
into  the  midrib  and  its  lateral  branches  ; 
and  it  is  their  peculiar  arrangement  hi 

Fsfcilk5'o70  fern"6  ktf°  sho^ng    ^  leaf-s*alk  of  th^  common  brake  which 
bundle  of  scalariform  ducts.         gives  to  the  transverse  section  the  mark- 
ing commonly  known  as  *  King  Charles 

in  the  oak.'  A  thin  section,  especially  if  somewhat  oblique  (fig.  515), 
displays  extremely  well  the  peculiar  character  of  the  ducts  of  the 
fern,  which  are  termed  scalariform  from  the  resemblance  of  the 
regular  markings  on  their  walls  to  the  rungs  of  a  ladder.  These 
bundles  of  scalariform  ducts  or  '  trachei'ds  '  are  usually  surrounded  by 
sheaths  of  sclerenchyme,  tissue  composed  of  cells  the  walls  of  which 

1  These  so-called  '  microspores '  are  now  believed  to  be   spores   of  a  parasitic 
fungus. — ED. 


STRUCTUKE    OF    FEBNS 


67S 


have  become  very  hard  and  of  a  deep  brown  colour.  These  scleren- 
chymatous  sheaths  are  a  very  conspicuous  feature  in  a  transverse 
section  of  the  stem  or  rhizome  of  most  ferns,  and  are  the  principal 
agent  in  giving  it  strength  and  solidity. 

What  is  usually  termed  the  fructification  of  the  fern  affords 
a  most  beautiful  and  readily  prepared  class  of  opaque  objects  for  the 
lower  powers  of  the  microscope  ;  nothing  more  being  necessary  than 
to  lay  a  fragment  of  the  frond  that  bears  it  upon  the  glass  stage  - 
plate  or  to  hold  it  in  the  stage-forceps,  and  to  throw  an  adequate  light 
upon  it  by  the  side-condenser.  It  usually  presents  itself  in  the  form 
of  isolated  spots  on  the  under  surface  of  the  frond  termed  sori, 
as  in  the  common  Polypodium  (fig.  516),  and  in  Aspidium  (fig. 
518);  but  sometimes  these  sori  are  elongated  into  bands,  as  in 


FIG.  516.— Leaflet  of  Poly- 
podium,  with  sori. 


FIG.  517.— Portion  of  frond  of  Hcemionitis, 
with  sori. 


the  common  Scolopendrium  (hart's  tongue) ;  and  these  may  coalesce 
with  each  other,  so  as  almost  to  cover  the  surface  of  the  frond  with 
a  network,  as  in  Hcemionitis  (fig.  517) ;  or  they  may  form  merely  a 
single  band  along  its  borders,  as  in  the  common  Pteris  (brake-fern). 
The  sori  are  sometimes  *  naked '  on  the  under  surface  of  the  fronds  ; 
but  they  are  frequently  covered  with  a  delicate  membrane  termed 
the  indusium,  which  may  either  form  a  sort  of  cap  upon  the  summit 
of  each  sorus,  as  in  Aspidium  (fig.  518),  or  a  long  fold,  as  in  Scolo- 
pendrium and  Pteris,  or  a  sort  of  cup,  as  in  Deparia  (fig.  519), 
Each  of  these  sori,  when  sufficiently  magnified,  is  found  to  be  made 
up  of  a  multitude  of  sporanges,  or  spore-capsules  (figs.  518,  519). 
which  are  sometimes  closely  attached  to  the  surface  of  the  frond, 
but  more  commonly  spring  from  it  by  a  pedicel  or  footstalk.  The 

x  x  2 


6/6     MICROSCOPIC    STRUCTURE   OF  HIGHER   CRYPTOGAMS 

wall  of  the  sporange  is  composed  of  flattened  cells,  applied  to  each 
other  by  their  edges  ;  but  there  is  generally  one  row  of  these  thicker 
and  larger  than  the  rest  which  springs  from  the  pedicel,  and  is 
continued  over  the  summit  of  the  sporange,  so  as  to  form  a  projecting 
ring,  which  is  known  as  the  annidus  (fig.  519).  This  ring  has  an 
elasticity  superior  to  that  of  all  the  rest  of  the  wall  of  the  capsule, 
causing  it  to  split  across  when  mature,  so  that  the  contained  spores 
may  escape  ;  and  in  many  instances  the  two  halves  of  the  sporange  are 
carried  widely  apart  from  each  other,  the  fissure  extending  to  such  a 
depth  as  to  separate  them  completely.  In  Osmiinda  (the  so-called 
'  flowering  fern  '  or  '  royal  fern  ' )  and  Ophioglossum  (adder's  tongue) 
the  sporanges  have  no  annulus,  or  one  greatly  modified.  It  will 
frequently  happen  that  specimens  of  fern-fructification  gathered  for 
the  microscope  will  be  found  to  have  all  the  sporanges  burst  and 
the  spores  dispersed,  whilst  in  others  less  advanced  the  sporanges 
may  all  be  closed  ;  others,  however,  may  often  be  met  with  in  which 
some  of  the  sporanges  are  closed  and  others  are  open  ;  and  if  these 
be  watched  with  sufficient  attention  the  rupture  of  some  of  the 


FIG.  518. — Sorus  and  indusium  of  FIG.  519. — Sorus  and  cup-shaped 

Aspidium.  indusium  of  Deparia  prolifera. 

sporanges  and  the  dispersion  of  the  spores  may  be  observed  to  take 
place  while  the  specimen  is  under  observation  in  the  field  of  the 
microscope.  In  sori  whose  sporanges  have  all  burst,  the  an  mil  i 
connecting  their  twro  halves  are  the  most  conspicuous  objects,  look- 
ing, when  a  strong  light  is  thrown  upon  them,  like  strongly  banded 
worms  of  a  bright  brown  hue.  This  is  particularly  the  case  in 
Scolopendrium,  whose  elongated  sori  are  remarkably  beautiful 
objects  for  the  microscope  in  all  their  stages  ;  until  quite  mature, 
.however,  they  need  to  be  brought  into  view  by  turning  back  the 
two  indusial  folds  that  cover  them.  The  commonest  ferns,  indeed, 
which  are  found  in  almost  every  hedge,  furnish  objects  of  no  less 
beauty  than  those  yielded  by  the  rarest  exotics  ;  and  it  is  in  every 
respect  a  most  valuable  training  to  the  young  to  teach  them  how 
much  may  be  found  to  interest,  when  looked  for  with  intelligent 
eyes,  even  in  the  most  familiar,  and  therefore  disregarded,  specimens 
of  Nature's  handiwork. 

The  '  spores '  (fig.  520,  A)  set  free  by  the  bursting  of  the  spo- 
ranges, usuallv  have  a  somewhat  angular  form,  and  are  invested  by  a 


FRUCTIFICATION   OF  FERNS 


677 


yellowish  or  brownish  outer  coat,  the  exospore,  which  is  marked 
\*>ry  much  in  the  manner  of  pollen-grains  (fig.  565)  with  points, 
streaks,  ridges,  or  reticulations.  When  placed  upon  a  damp 
surface,  and  exposed  to  a  sufficiency  of  light  and  warmth,  the. 
spore  begins  to  germinate,  the  first  indication  of  its  vegetative 
activity  being  a  slight  enlargement,  which  is  manifested  in  the 
rounding  off  of  its  angles.  This  is  followed  by  the  putting  forth 
of  a  tubular  prolongation  (fig.  520,  B,  a)  of  the  internal  cell- wall  or 
endospore  through  an  aperture  in  the  outer  spore-coat ;  and  mois- 
ture being  absorbed  through  this,r>the  cell  becomes  so  distended  as 
to  burst  the  external  unyielding  integument,  and  soon  begins  to 
elongate  itself  in  a  direction  opposite  to  that  of  the  first  rhizoid.  A 
production  of  new  cells  by  subdivision  then  takes  place  from  its  grow- 


FIG.  520.— Development  of  prothallium  of  Pteris  serrulata  :  A,  spore  set  free  from 
the  sporange ;  B,  spore  beginning  to  germinate,  putting  forth  the  tubular  pro- 
longation a,  from  the  principal  cell  b  ;  C,  first-formed  linear  series  of  cells  ;  D,  pro- 
thallium  taking  the  form  of  a  leaf-like  expansion  ;  a,  first,  and  6,  second  rhizoid  ; 
c,  d,  the  two  lobes,  and  e,  the  indentation  between  them ;  /,  /,  first-formed  part  of 
the  prothallium ;  g,  external  coat  of  the  original  spore  ;  k,  h\  antherids. 

ing  extremity  ;  this  at  first  proceeds  in  a  single  series,  so  as  to  form 
a  kind  of  confervoid  filament  (C) ;  but  the  cell-division  soon  takes 
place  transversely  as  well  as  longitudinally,  so  that  a  flattened  leaf- 
like  expansion  (D)  is  produced,  so  closely  resembling  that  of  a  young 
Mtircharttia  as  to  be  readily  mistaken  for  it.  This  expansion,  which 
is  termed  the  prothallium,  varies  in  its  configuration  in  different 
species,  but  its  essential  structure  always  remains  the  same.  From 
its  under  surface  are  developed,  not  merely  the  rhizoids  (a,  b),  which 
serve  at  the  same  time  to  fix  it  in  the  soil  and  to  supply  it  with 
moisture,  but  also  the  antherids  and  archegones,  which  constitute 
the  true  representatives  of  the  essential  parts  of  the  flower  of  higher 
plants.  Some  of  the  .former  may  be  distinguished  at  an  early  period 
of  the  development  of  the  prothallium  (h,  h)  ;  and  at  the  time  of  its 
complete  evolution  these  bodies  are  seen  in  considerable  numbers, 


678       MICROSCOPIC   STKUCTUEE   OF  HIGHER   CRYPTOGAMS 


especially  in  the  neighbourhood  of  the  rhizoids.  Each  has  its  origin 
in  a  peculiar  protrusion  that  takes  place  from  one  of  the  cells  of  the 
prothallium  (fig.  521,  A,  a);  this  is  at  first  entirely  filled  with 
chlorophyll-granules,  but  soon  cell-division  sets  up  in  it.  A  central 
cell  b  becomes  distinguished  from  all  the  rest  by  its  much  larger  size 

and  is  surrounded  by  one 
or  two  layers  of  much 
smaller  cells  known  as 
the  tapetal  or  mantle- 
cells.  These  take  no 
part  in  the  formation  of 
the  antherozoids  ;  but 
the  protoplasmic  con- 
tents of  the  large  central 
cell  divide  by  free-cell- 

FIG.  521.—  Development  of  the  antherids  and  anthe-  formation  into  a  large 
rozoids  of  Pteris  serrulata:  A,  projection  of  one  number  of  cells  known  as 
of  the  cells  of  the  prothallium,  showing  the  anthe-  f^e  anther  ozoid-wwther- 
ridial  cell  b,  with  its  sperm-cells  e,  within  the  cavity  //  /  \  i  f  -i 

of  the  original  cell  a.  B,  antherid  completely  ^  (c)  ;  each  ot  these 
developed  ;  a,  wall  of  antheridial  cell  ;  e,  sperm-  again  breaks  up  into 
cells,  each  enclosing  an  antherozpid.  Canthero-  four  cellg  not  at  first 
zoid  more  highly  magnified,  showing  its  l*rge  ex-  .  ,  ,  '.,  ,  ,,  ,, 

tremity  a,  its  small  extremity  b,  and  its  cilia  d,  d.      provided  with  cell-walls, 

the  sperm-cells.  Each 
of  the  sperm-cells  (B,  e) 
is  seen,  as  it  approaches 
maturity,  to  contain  a 
spirally  coiled  filament  ; 
and  when  set  free  by 
the  bursting  of  the 
antherid  the  sperm- 
cells  themselves  burst, 
and  give  exit  to  their 
antherozoids  (C),  which 
execute  rapid  move- 

ments   of    rotation    on 
FIG.  522.—  Archegone    of    Pteris    serrulata  :     A,    as    ,.  ,        , 

seen  from  above;  a,  a,  a,  cells  surrounding  the  their  axes,  partly  de- 
base of  the  cavity  ;  b,  c,  d,  successive  layers  of  pendent  on  the  long- 
cells,  the  highest  enclosing  a  quadrangular  orifice.  c^ja  wfth  which  they 
B,  side  view,  showing  A,  A,  cavity  containing  the  „  '11 

germ-cell,  a  ;    B,   B,   walls  of  the  archegone,  made  are 

up  of  the  four  layers  of  cells,  b,  c,  d,  e,  and  having  The     archegones    are 

an  opening,/,  on  the  summit;  c,  c,  antherozoids  fewer  in  number,  and 
within  the  cavity;  g,  large  extremity;  h,  vibratile  „  ,  -,.,. 

cilia;  i,  small  extremity  in  contact  with  the  germ-  are  found  upon  a  dlt- 
cell,  and  dilated.  ferent  part  of  the  pro- 

thallium  .     Each  of  them 

originates  in  a  single  cell  of  its  superficial  layer,  which  undergoes 
subdivision  by  a  horizontal  partition.  Of  the  two  cells  thus  produced 
the  upper  gives  origin,  by  successive  subdivisions,  to  the  *  neck  '  of 
the  archegone,  which,  when  fully  developed  (fig.  522),  is  composed 
of  twelve  or  more  cells,  built  up  in  layers  of  four  cells  each,  one  upon 
another,  so  as  to  form  a  kind  of  chimney  or  shaft.  The  lower  of  the 
two  first-  formed  cells  becomes  the  central  cell  of  the  archegone  ; 


SEXUAL  GENEEATION  OF  FERNS  679 

and  this  again  undergoing  horizontal  subdivision,  the  lower  half  be- 
comes the  oosphere  or  germ-cell,  whilst  the  upper  extends  itself  into 
the  neck.  By  the  conversion  into  mucilage  of  a  central  row,  an  open 
passage  or  canal  is  formed,  through  which  the  antherozoids  make 
their  way  to  the  oosphere  lying  at  its  bottom  (fig.  522,  B,  a).  The 
oosphere,  when  fertilised  by  the  penetration  of  the  antherozoids, 
becomes  the  '  embryo-cell '  of  a  new  plant,  the  development  of  which 
speedily  commences.1  In  the  aberrant  group  of  Ophioglossacece 
(adder's-tongue  ferns),  the  development  of  the  prothallium  takes 
place  underground*  in  the  form  clt-a  small  roundish  tuber,  composed 
of  parenchymatous  tissue  containing  no  chlorophyll,  and  producing 
antherids  and  archegones  on  its  upper  surface. 

The  early  development  of  the  embryo-cell  takes  place  according 
to  the  usual  method  of  repeated  subdivision,  producing  a  homo- 
geneous globular  mass  of  cells.  Soon,  however,  rudiments  of  special 
organs  begin  to  make  their  appearance  ;  the  embryo  grows  at  the 
expense  of  the  nutriment  prepared  for  it  by  the  prothallium,  and  it 
bursts  forth  from  the  cavity  of  the  archegone,  which  organ  in  the 
meantime  is  becoming  atrophied.  In  the  very  beginning  of  its 
development  the  tendency  is  seen  in  the  cells  of  one  extremity  to 
grow  upward  so  as  to  evolve  the  stem  and  leaves,  and  in  those  of  the 
other  extremity  to  grow  downward  to  form  the  root ;  and  when 
these  organs  have  been  sufficiently  developed  to  absorb  and  prepare 
the  nutriment  which  the  young  fern  requires,  the  prothallium  decays 
away.  Thus,  then,  the  *  spore '  of  the  fern  must  be  considered  as  a 
generative  'gonid '  or  detached  flower-bud  capable  of  developing 
itself  into  a  prothallium  that  may  be  likened  to  a  receptacle  bearing 
the  sexual  apparatus.  But  this  prothallium  serves  the  further  pur- 
pose of  '  nursing '  the  embryos  originated  by  the  generative  act ; 
which  embryos  finally  develop  themselves,  not,  as  in  mosses,  into 
mere  sporogones,  but,  as  in  Phanerogams,  into  entire  plants,  com- 

1  The  study  of  the  development  of  the  spores  of  ferns,  and  of  the  act  of  fertilisa- 
tion and  of  its  products,  may  be  conveniently  prosecuted  as  follows : — Let  a  frond  of 
a  fern  whose  fructification  is  mature  be  laid  upon  a  piece  of  fine  paper,  with  its 
spore-bearing  surface  downwards  ;  in  the  course  of  a  day  or  two  this  paper  will  be 
found  to  be  covered  with  a  very  fine  brownish  dust,  which  consists  of  the  discharged 
spores.  This  must  be  carefully  collected,  and  should  be  spread  upon  the  surface  of 
a  smoothed  fragment  of  porous  sandstone,  the  stone  being  placed  in  a  saucer,  the 
bottom  of  which  is  covered  with  water  ;  and  a  glass  tumbler  being  inverted  over  it, 
the  requisite  supply  of  moisture  is  ensured,  and  the  spores  will  germinate  luxuriantly. 
Some  of  the  prothallia  soon  advance  beyond  the  rest ;  and  at  the  time  when  the 
advanced  ones  have  long  ceased  to  produce  antherids,  and  bear  abundance  of 
archegones,  those  which  have  remained  behind  in  their  growth  are  beginning  to  be 
covered  with  antherids.  If  the  crop  be  now  kept  with  little  moisture  for  several 
weeks,  and  then  suddenly  watered,  a  large  number  of  antherids  and  archegones 
simultaneously  open ;  and  in  a  few  hours  afterwards  the  surface  of  the  larger  pro- 
thallia will  be  found  almost  covered  with  moving  antherozoids.  Such  prothallia  as 
exhibit  freshly  opened  archegones  are  now  to  be  held  by  one  lobe  between  the  forefinger 
and  thumb  of  the  left  hand,  so  that  the  upper  surface  of  the  prothallium  lies  upon  the 
thumb  ;  and  the  thinnest  possible  sections  are  then  to  be  made  with  a  thin  narrow- 
bladed  knife,  perpendicularly  to  its  surface.  Of  these  sections,  which,  after  much 
practice,  may  be  made  no  more  than  one- fifteenth  of  a  line  in  thickness,  some  will 
probably  lay  open  the  canals  of  the  archegones ;  and  within  these,  when  examined 
with  a  power  of  200  or  300  diameters,  antherozoids  may  be  occasionally  dis- 
tinguished. The  prothallium  of  the  common  Osmunda  regalis  will  be  found  to 
afford  peculiar  facilities  for  observation  of  the  development  of  the  antherids,  which 
are  produced  at  its  margin. 


680       MICROSCOPIC   STRUCTURE   OF  HIGHER   CRYPTOGAMS 

plete  in  everything  but  the  true  generative  organs,  which  evolve 
themselves  from  the  detached  spores.  Here  we  have,  therefore,  an 
example  of  alternation  of  generations  differing  in  one  important 
respect  from  that  in  mosses.  In  ferns  the  '  sexual  generation  '  or 
'  oophyte '  which  results  from  the  germination  of  the  spore  consists 
of  the  prothallium  only  with  its  archegones  and  antherids,  the  leafy 
plant  which  bears  the  sporanges  constituting  the  '  sporophyte '  or 
'  non-sexual  generation,'  the  product  of  the  fertilisation  of  the  arche- 
gone  by  an  antherozoid.  In  mosses,  on  the  other  hand,  the  leafy 
plant  belongs  to  the  sexual  generation. 

The  singular  discovery  has  recently  been  made  by  the  researches 
of  De  Bary,  Farlow,  and  others,  that  the  ordinary  alternation  of 
generations  in  ferns  may  be  interrupted  by  the  suppression  either  of 
the  sporophyte,  the  non-sexual  or  spore-bearing  generation,  or  of  the 
oophyte  or  sexual  generation  which  bears  the  true  reproductive 
organs.  These  phenomena  are  called  respectively  apospory  and 
apogamy.  The  former  has  been  observed  especially  in  varieties  of 
Athyrium  Filix-fcemina  and  Polystichum  angular e,  and  is  shown  by 
the  production  of  prothalloid  structures  bearing  antherids  and 
archegones  on  the  fronds  in  the  place  of  ordinary  sori.  The  latter 
occurs  not  unfrequently  in  Pteris  serrulata,  the  sporophytic  genera- 
tion springing  directly  from  the  prothallium  without  the  interven- 
tion of  archegones  and  antherids. 

The  little  group  of  Equisetaceae  (horse-tails),  which  seem  nearly 
allied  to  the  ferns  in  the  type  of  their  generative  apparatus,  though 
that  of  their  vegetative  portion  is  very  different,  affords  certain 
objects  of  considerable  interest  to  the  microscopist.  The  whole  of 
their  structure  is  penetrated  to  such  an  extraordinary  degree 
by  sileXy  that  even  when  its  organic  portion  has  been  destroyed  by 
prolonged  maceration  in  dilute  nitric  acid,  a  consistent  skeleton  still 
remains.  This  mineral,  in  fact,  constitutes  in  some  species  not  less 
than  13  per  cent,  of  the  whole  solid  matter,  and  50  per  cent,  of  the 
inorganic  ash  ;  and  it  especially  abounds  in  the  epiderm,  which  is 
used  by  cabinet-makers  for  smoothing  the  surface  of  wood.  Some  of 
the  siliceous  particles  are  distributed  in  two  lines,  parallel  to  the 
axis ;  others,  however,  are  grouped  into  oval  forms,  connected  with 
each  other,  like  the  jewels  of  a  necklace,  by  a  chain  of  particles 
forming  a  sort  of  curvilinear  quadrangle ;  and  these  (which  are,  in 
fact,  the  particles  occupying  the  guard-cells  of  the  stomates)  are 
arranged  in  pairs.  Their  form  and  arrangement  are  peculiarly  well 
seen  under  polarised  light,  for  which  the  prepared  epiderm  is  an 
extremely  beautiful  object ;  and  it  is  asserted  by  Sir  D.  Brewster 
(whose  authority  upon  this  point  has  been  generally  followed)  that 
each  siliceous  particle  has  a  regular  axis  of  double  refraction.  What 
is  usually  designated  as  the  fructification  of  the  Equisetaceee  forms  a 
cone  or  spike  at  the  extremity  of  certain  of  the  stem-like  branches 
(the  real  stem  being  a  horizontal  rhizome)  and  consists  of  a  cluster 
of  shield-like  discs,  each  of  which  carries  a  circle  of  sporanges  or 
spore-capsules,  that  open  by  longitudinal  slits  to  set  free  the  spores. 
In  addition  to  the  spores  each  sporange  contains  a  number  of  elastic 
filaments  (fig.  523),  called  elaters.  These  are  at  first  coiled  up  around 


EQUISETACEA:  ;  RHIZOCARPE/E  ;  LYCOPODIACE^:      68 1 

the  spore,  in  the  manner  represented  at  A,  though  more  closely 
applied  to  the  surface ;  but,  on  the  liberation  of  the  spore,  they  ex- 
tend themselves  in  the  manner  shown  at  B,  the  slightest  application 
of  moisture,  however,  serving  to  make  them  close  together  (the 
assistance  which  they  afford  in  the  dispersion  of  the  spores  being  no 
longer  required)  when  the  spores  have  alighted  on  a  damp  surface. 
If  a  number  of  these  spores  be  spread  out  on  a  slip  of  glass  under  the 
field  of  view,  and,  whilst  the  observer  watches  them,  a  bystander 
breathes  gently  upon  the  glass,  all  the  filaments  will  be  instanta- 
neously put  in  motion,  thus  presenting  an  extremely  curious  spec- 
tacle, and  will  almost  as  suddenly  return  to  their  previous  condition 
when  the  effect  of  the  moisture  has  passed  off.  If  one  of  the  sporanges 
which  has  opened,  but  has  not  discharged  its  spores,  be  mounted 
in  a  cell  with  a  movable  cover,  this  curious  action  may  be  exhibited 
over  and  over  again.  These  spores,  like  those  of  ferns,  develop  into  a 
prothallium  ;  and  this  bears  antherids  and  archegones,  the  former  at 
the  extremities  of  the  lobes,  and  the  latter  in  the  angles  between  them. 
Nearly  allied  to  Ferns,  also,  is  a  curious  little  group  of  small 
aquatic  plants,  the  Rhizocarpeae  (or  Pepper- worts),  which  either 
float  on  the  surface  or  creep  along  shallow  bottoms.  These  differ 


FIG.  523. — Spores  of  Equisetum,  with  their  elaters. 

from  Ferns  and  Horse-tails  in  having  two  kinds  of  spore,  produced 
in  separate  sporanges  ;  the  larger,  or  *  megaspores,'  giving  origin  to 
prothallia  which  produce  archegones  only;  and  the  smaller,  or 
*  microspores,'  undergoing  progressive  subdivision,  usually  without 
the  formation  of  a  distinct  prothallium,  each  of  the  cells  thus  formed 
giving  origin  to  an  antherozoid.  In  this,  as  we  shall  presently  see, 
there  is  a  distinct  foreshadowing  of  the  mode  in  which  the  genera- 
tive process  is  performed  in  flowering  plants,  the  *  microspore '  cor- 
responding to  the  pollen-grain,  while  the  '  megaspore  '  may  be  con- 
sidered to  represent  the  primitive  cell  of  the  ovule. 

Another  alliance  of  Ferns  is  to  the  Lycopodiacese  (Club-mosses), 
a  group  which  at  the  present  time  attains  a  great  development  in 
warm  climates,  and  which,  it  would  seem,  constituted  a  large  part 
of  the  arborescent  vegetation  of  the  Carboniferous  epoch.  In  the 
LycopodieoK  proper  the  sporanges  are  all  of  one  kind,  and  all  the 
spores  are  of  the  same  size,  each,  as  in  Ophioglossum,  giving  origin 
to  a  subterraneous  prothallium  that  develops  both  antherids  and 
archegones.  The  plant  which  originates  from  the  fertilised  '  germ- 
cell  '  of  the  archegone  attains  in  colder  climates  only  a  moss-like 


682     MICKOSCOPIC   STRUCTURE    OF  HIGHER   CRYPTOGAMS 

growth,  with  a  creeping  stein  usually  branching  dichotomously,  and 
imbricated  leaves ;  but  is  distinguished  from  the  true  mosses,  not 
only  by  its  higher  general  organisation  (which  is  on  a  level  with  that 
of  ferns),  but  by  the  character  of  its  fructification,  which  is  a  club- 
shaped  *  spike,'  bearing  small  imbricated  leaves,  in  the  axils  of  which 
lie  the  sporanges.  The  spores  developed  within  these  are  remarkable 
for  the  large  quantity  of  oily  matter  they  contain,  giving  them  an 
inflammability  that  causes  their  being  used  in  theatres  to  produce 
'  artificial  lightning.'  But  in  the  allied  groups  of  SelagineUece,  and 
Isoetece  there  are  (as  in  the  Rhizocarpece)  two  kinds  of  spore  pro- 
duced in  separate  sporanges  ;  one  set  producing  '  megaspores,'  from 
which  archegone-bearing  prothallia  are  developed,  and  the  other 
producing  '  microspores,'  which,  by  repeated  subdivision,  give  origin 
to  antherozoids  without  the  formation  of  prothallia.  It  is  a  very 
interesting  indication  of  a  tendency  towards  the  phanerogamic  type 
of  sexual  generation,  that  the  prothallium  in  this  group  is  chiefly 
developed  ivithin  the  sporange,  forming  a  kind  of  '  endosperm,'  only 
the  small  part  which  projects  from  the  ruptured  apex  of  the  spore 
producing  one  or  more  archegones.  The  arborescent  Lepidodendra 
and  SigiUarice  of  the  Coal-measures  seem  to  have  formed  connecting 
links  between  the  Vascular  Cryptogams  and  the  Phanerogams,  alike 
in  the  structure  of  their  stems  and  in  their  fructification.  For  the 
Lepidostrobi  or  cone-like  *  fruit '  of  these  trees  represent  the  clul  >- 
shaped  spikes  of  the  Lycopodiacece  ;  and  seem  to  have  borne  '  mega- 
spores '  in  the  sporanges  of  their  basal  portion,  and  '  microspores  ' 
in  those  of  their  upper  part.  Some  of  the  best  seams  of  coal  appear 
to  have  been  chiefly  formed  by  the  accumulation  of  these  '  mega- 
spores.' 


Thus,  in  our  ascent  from  the  lower  to  the  higher  Cryptogams,  we 
have  seen  a  gradual  change  in  the  general  plan  of  structure,  bring- 
ing their  superior  types  into  a  close  approximation  to  the  flowering 
plant,  which  is  undoubtedly  the  highest  form  of  vegetation.  But 
we  have  everywhere  encountered  a  mode  of  generation  which, 
whilst  essentially  the  same  throughout  the  series,  is  no  less  essen- 
tially distinct  from  that  of  the  Phanerogam,  the  fertilising  material 
of  the  '  sperm-cells  '  being  embodied,  as  it  were,  in  self-moving  fila- 
ments, the  antherozoids,  which  find  their  way  to  the  '  germ-cells '  by 
their  own  independent  movements,  and  the  '  embryo-cell '  being 
destitute  of  that  store  of  prepared  nutriment  which  surrounds  it  in 
the  true  seed,  and  supplies  the  material  for  its  early  development. 
In  the  lower  Cryptogams  we  have  seen  that  the  fertilised  ob'spore 
is  thrown  at  once  upon  the  world,  so  to  speak,  to  get  its  own  living ; 
but  in  ferns  and  their  allies  the  '  embryo-cell '  is  nurtured  for  a 
while  by  the  prothallium  of  the  parent  plant.  While  the  true 
reproduction  of  the  species  is  effected  by  the  proper  generative  act, 
the  multiplication  of  the  individual  is  accomplished  by  the  production 
and  dispersion  of  '  gonidial '  spores  ;  and  this  production,  as  we  have 
seen,  takes  place  at  very  different  periods  of  existence  in  the  several 


ALTERNATION    OF    GENERATIONS  683 

groups,  dividing  the  life  of  each  into  two  separate  epochs,  in  which 
it  presents  itself  under  two  very  distinct  phases  that  contrast 
remarkably  with  each  other.  Thus,  the  frond  of  Marchantia, 
evolved  from  the  spore  and  bearing  the  antherids  and  archegones, 
is  that  which  seems  naturally  to  constitute  the  plant ;  but  that  which 
represents  this  phase  in  the  ferns  is  the  minute  Marckantia-like 
prothallium.  In  ferns,  on  the  other  hand,  the  product  into  which 
the  fertilised  '  embryo-cell '  evolves  itself  is  that  which  is  commonly 
regarded  as  the  plant ;  and  this. is  represented  in  the  liverworts  and 
mosses  by  the  sporogone  alone.1*  We  shall  encounter  a  similar 
diversity  (which  has  received  the  inappropriate  designation  of  *  alter- 
nation of  generations '  )  in  some  of  the  lower  forms  of  the  animal 
kingdom. 

1  For  more  detailed  information  on  the  structure  and  classification  of  the  Crypto- 
gams generally  the  reader  is  referred  to  Goebel's  Outlines  of  Classification  and 
Special  Morphology  and  De  Bary's  Comparative  Anatomy  of  the  Phanerogams 
and  Ferns,  translations  of  both  of  which  have  been  published  by  the  Clarendon 
Press  ;  and  especially  to  Bennett  and  Murray's  Handbook  of  Cryptogamic  Botany, 
published  by  Longmans  (London,  1889). 


684  MICROSCOPIC   STRUCTURE   OF  PHANEROGAMIC    PLANTS 


CHAPTER   XI 

OF  THE  MICROSCOPIC  STRUCTURE  OF  PHANEROGAMIC  PLANTS 

BETWEEN  the  two  great  divisions  of  the  Vegetable  Kingdom  which 
are  known  as  Cryptogamia  and  Phanerogamia  the  separation  is  by 
no  means  so  abrupt  as  it  formerly  seemed  to  be.  For,  as  has  been 
already  shown,  though  the  Cryptogamia  were  formerly  regarded  as 
altogether  non-sexual,  a  true  generative  process,  requiring  the 
concurrence  of  male  and  female  elements,  is  traceable  almost  through- 
out the  series.  And  in  the  higher  types  of  that  series  we  have  seen 
a  foreshadowing  of  those  provisions  for  the  nurture  of  the  fertilised 
embryo  which  constitute  the  distinctive  characters  of  the  Phanero- 
gamia. On  the  other  hand,  although  we  are  accustomed  to  speak  of  • 
Phanerogamia  as  '  flowering  plants,'  yet  not  only  are  the  conspicuous 
parts  of  the  flower  often  wanting,  but  in  the  important  group  of 
Gymnosperms  (including  the  Conifercv  and  Cycadece)  the  essential 
parts  of  the  generative  apparatus  are  reduced  to  a  condition  closely 
approximating  to  that  of  the  higher  Cryptogams.  There  are,  how- 
ever, certain  fundamental  differences  between  the  modes  in  which 
the  act  of  fertilisation  is  performed  in  the  two  groups.  For  (1) 
whilst  in  all  the  higher  Cryptogams  it  is  in  the  condition  of  free- 
moving  *  antherozoids '  that  the  contents  of  the  sperm-cell  find  their 
wTay  to  the  germ-cell,  these  are  conveyed  to  it,  throughout  the 
phanerogamic  series,  by  an  extension  of  the  lining  membrane  of  the 
sperm-cell  or  pollen-grain  into  a  tube,  which  penetrates  to  the  germ- 
cell,  contained  in  the  interior  of  the  body  called  the  ovule.1  Again 
(2),  while  the  '  germ-cell '  or  oosphere  in  the  higher  Cryptogams  is 
contained  in  a  structure  that  originated  in  a  spore  detached  from  the 
parent  plant,  it  is  not  only  formed  and  fertilised  in  all  Phanerogams 
whilst  still  borne  on  the  parent  fabric,  but  continues  for  some  time 
to  draw  from  it  the  nutriment  it  requires  for  its  development  into  the 
embryo.  And  at  the  time  of  its  detachment  from  the  parent  the 

1  A  very  remarkable  and  interesting  discovery,  for  which  we  are  largely  indebted 
to  the  brilliant  observations  of  two  Japanese  botanists,  Professors  Ikeno  and  Hirase, 
has  recently  thrown  great  light  on  the  approximation  referred  to  by  Dr.  Carpenter 
between  the  higher  Cryptogamia  and  the  lower  Phanerogamia.  It  is  now  known 
that  in  both  the  larger  groups  of  Gymnosperms,  the  Coniferae  and  the  Cycadeas,  there 
are  species  in  which  the  fertilising  body  is  a  motile  antherozoid  formed  within  a 
pollen-tube,  thus  combining  the  distinctive  modes  of  fertilisation  characteristic  of 
the  two  great  sections  of  the  vegetable  kingdom.  As  Dr.  Carpenter  does  not  include  in 
his  account  of  the  '  Microscopic  Structure  of  Phanerogamic  Plants  '  a  full  description 
of  the  mode  of  impregnation  in  flowering  plants,  the  reader  is  referred,  for  further 
details,  to  the  most  recent  Text-books  of  Botany,  or  to  the  Summary  of  Current  Re- 
searches in  Botany  in  the  Journal  of  the  R.  Microscopical  Society. — EDITOB.] 


STRUCTURE    OF   PHANEROGAMIA  685 

matured  *  seed '  contains,  not  merely  an  embryo  already  advanced 
a  considerable  stage,  but  a  store  of  nutriment  to  serve  for  its  further 
development  during  germination.  As  there  is  nothing  parallel  to 
this  among  Cryptogams,  it  may  be  said  that  reproduction  by  seeds, 
not  the  possession  of  flowers,  is  the  distinctive  character  of  Phanero- 
gams. The  ovules,  which  when  fertilised  and  matured  become  seeds, 
are  developed  from  specially  modified  leaves,  which  remain  open  in 
Gymnosperms,  but  which  in  all  other  Phanerogams  fold  together  so 
as  to  enclose  the  o wiles  within  an  ovary.  Each  ovule  consists  of  a 
nucellus  surrounded  by  integuments  which  remain  unclosed  at  the 
apex,  leaving  open  a  short  canal  termed  the  micropyle  or  '  foramen.' 
One  cell  of  the  nucellus  undergoes  great  enlargement,  and  becomes 
the  embryo-sac,  whose  cavity  is  filled,  in  the  first  instance,  with  a 
mucilaginous  fluid  containing  protoplasm.  At  the  end  of  the 
embryo-sac  which  lies  nearest  the  micropyle  a  germ -cell  or  odsphere 
is  developed  ;  in  Angiosperms  by  free-cell-formation,  but  in 
Gymnosperms  indirectly  after  the  formation  of  a  '  corpuscle,'  which 
represents  the  archegone  of  Selaginella.  By  a  further  process  of 
free-cell-formation  the  remainder  of  the  embryo-sac  comes  to  be 
filled  with  cells  constituting  what  is  termed  the  endosperm;  and 
this  serves,  like  the  prothallium  of  ferns,  to  imbibe  and  prepare 
nutriment  which  is  afterwards  appropriated  by  the  embryo.  In 
many  seeds  (as  those  of  the  Leguminosce)  the  whole  nutritive  material 
of  the  endosperm  has  been  absorbed  into  the  cotyledons  (or  seed- 
leaves)  of  the  embryo  by  the  time  that  the  seed  is  fully  matured  and 
independent  of  the  parent ;  but  in  other  cases  it  remains  as  a  '  sepa- 
rate endosperm.'  In  either  case  it  is  taken  into  the  substance  of  the 
embryo  during  its  germination. 

Elementary  Tissues. — No  marked  change  shows  itself  in  general 
organisation  as  we  pass  from  the  cryptogamic  to  the  phanerogamic 
series  of  plants.,  A  large  proportion  of  the  fabric  of  even  the 
most  elaborately  formed  tree  (including  the  parts  most  actively  con- 
cerned in  living  action)  is  made  up  of  components  of  the  very  same 
kind  as  those  which  constitute  the  entire  organisms  of  the  simplest 
cryptogams.  For,  although  the  stems,  branches,  and  roots  of  trees 
and  shrubs  are  principally  composed  of  woody  tissue,  such  as  we  do 
not  meet  with  in  any  but  the  highest  Cryptogams,  yet  the  special 
office  of  this  is  to  afford  mechanical  support ;  when  it  is  once  formed, 
it  takes  no  further  share  in  the  vital  economy  than  to  serve  for  the 
conveyance  of  fluid  from  the  roots  upwards  through  the  stem  and 
branches  to  the  leaves  ;  and  even  in  these  organs  (in  Exogens  or 
Dicotyledons),  not  only  the  pith  and  the  cortex,  with  the  '  medullary 
rays,'  which  serve  to  connect  them,  but  the  *  cambium  layer  '  inter- 
vening between  the  bark  and  the  wood  in  which  the  periodical 
formation  of  the  new  layers  both  of  bark  and  wood  takes  place,  are 
composed  of  cellular  substance.  This  tissue  is  found,  in  fact, 
wherever  growth  is  taking  place ;  as,  for  example,  in  the  growing 
points  of  the  root-fibres,  in  the  leaf-buds  and  leaves,  and  in  the 
flower-buds  and  sexual  parts  of  the  flower  ;  it  is  only  when  these 
organs  attain  an  advanced  stage  of  development  that  woody  structure 
is  found  in  them  ;  its  function  (as  in  the  stem)  being  merely  to  give 


686    MICROSCOPIC   STRUCTURE    OF  PHANEROGAMIC   PLANTS 


support  to  their  softer  textures  ;  and  the  small  proportion  of  their 
substance  which  it  forms  is  at  once  seen  in  those  beautiful '  skeletons ' 
which,  by  a  little  skill  and  perseverance,  may  be  made  of  leaves, 
flowers,  and  certain  fruits.  All  the  softer  and  more  pulpy  tissue 

of  these  organs  is  com- 
posed of  cells,  more  or  less 
compactly  aggregated  to- 
gether, and  having  forms 
that  approximate  more  or 
less  closely  to  the  globu- 
lar or  ovoidal,  which  may 
be  considered  as  their 
original  type. 

As  a  general  rule,  the 
rounded  shape  is  pre- 
served only  when  the  cells 
are  but  loosely  aggre- 
gated, as  in  the  parenchy- 
matous  (or  pulpy)  sub- 
stance of  leaves,  which 
often  forms  a  distinct 
layer  known  as  the 
'  spongy  parenchyme  ' 
immediately  beneath  the 
epiderm  of  the  upper  sur- 
face (fig.  524)  ;  and  it  is  then  only  that  the  distinctness  of  their 
walls  becomes  evident.  When  the  tissue  becomes  more  solid,  the 
sides  of  the  vesicles  are  pressed  against  each  other,  so  as  to  flatten 


FIG.  524 — Section  of  leaf  of  Agave,  treated  with 
dilute  nitric  acid,  showing  the  protoplasmic  con- 
tents contracted  in  the  interior  of  the  cells ;  a, 
epidermal  cells  b,  guard-cells  of  the  stomate ; 
c,  cells  of  parenchyme;  d,  their  protoplasmic 
contents. 


FIG.  525. — Sections  of  cellular  parenchyme  of  Aralia,  or  rice-paper  plant 
A,  transversely  to  the  axis  of  the  stem ;  B,  in  the  direction  of  the  axis. 

them  and  to  bring  them  into  close  apposition,  and  then  the  cavities 
of  adjacent  cells  are  separated  by  a  single  partition  wall.  Fre- 
quently it  happens  that  the  pressure  is  exerted  more  in  one  direction 
than  in  another,  so  that  the  form  presented  by  the  outline  of  the  cell 


STRUCTURE   OF  THE   CELL 


687 


varies  according  to  the  direction  in  which  the  section  is  made.  This 
is  well  shown  in  the  pith  of  the  young  shoots  of  elder,  lilac,  or  other 
rapidly  growing  trees,  the  cells  of  which,  when  cut  transversely,  gene- 
rally exhibit  circular  outlines  ;  whilst,  when  the  section  is  made  verti- 
cally, their  borders  are  straight,  so  as  to  make  them  appear  like 
cubes  or  elongated  prisms,  as  in  fig.  524.  A  very  good  example  of 
such  a  cellular  pareiichyme  is  to  be  found  in  the  substance  known  as 
'  rice-paper,'  which  is  made  by  cutting  the  herbaceous  stem  of  a 
Chinese  plant  termed  Aralia  pavyrifera,  vertically  round  and  round 
with  a  long  sharp  knife,  so  that  its  tissues  may  be  (as  it  were)  unrolled 
in  a  sheet.  The  shape  of  its  cells  when  thus  prepared  is  irregularly 
prismatic,  as  shown  in  fig.  525,  B  ;  but  if  the  stem  be  cut  transversely, 
their  outlines  are  seen  to  be  circular  or  nearly  so  (A).  When,  as 
often  happens,  the  cells  have  a  very  elongated  form,  this  elongation 
is  in  the  direction  of  their  growrth,  which  is  that,  of  course,  wherein 
there  is  least  resistance.  Hence  their  greatest  length  is  nearly 
always  in  the  direction  of  the  axis  ;  but  there  is  one  remarkable 
exception,  that,  namely,  which  is  afforded  by  the  '  medullary  rays ' 
of  exogenous  stems,  whose  cells  are  greatly  elongated  in  the  horizontal 
direction  (fig.  547,  «),  their  growth  being  from  the  centre  of  the  stem 
towards  its  circumference.  It  is  obvious  that  fluids  will  be  more 
readily  transmitted  in  the  direction  of  greatest  elongation,  being  that 
in  wThich  they  will  have  to  pass  through  the  least  number  of  parti- 
tions ;  and  whilst  their  ordinary  course  is  in  the  direction  of  the  length 
of  the  roots,  stems,  or  branches,  they  will  be  enabled  by  means  of  the 
medullary  rays  to  find  their  way  in  the  transverse  direction.  One 
of  the  most  curious  varieties  of 
form  which  vegetable  cells  pre- 
sent is  the  stellate  cell,  repre- 
sented in  fig.  526,  forming  the 
spongy  parenchymatous  substance 
in  the  stems  of  many  aquatic 
plants,  of  the  rush  for  example, 
which  are  furnished  with  air- 
spaces. In  other  instances  these 
air-spaces  are  large  cavities  which 
are  altogether  left  void  of  tissue : 
such  is  the  case  in  NupTiar  Itttfft 
(the  yellow  water-lily),  the  foot- 
stalks of  whose  leaves  contain  large  air-chambers,  the?  walls  of 
which  are  built  up  of  very  regular  cubical  cells,  whilst  some  curiously 
formed  large  stellate  cells  project  into  the  cavity  which  they  bound 
(fig.  527).  The  dimensions  of  the  component  vesicles  of  cellular  tissue 
are  extremely  variable  ;  for  although  their  diameter  is  very  com- 
monly between  ^^th  and  ^th  of  an  inch,  they  occasionally  mea- 
sure as  much  as  -j^th  of  an  inch  across,  whilst  in  other  instances 
they  are  not  more  than  ^^th, 

The  cells  of  a  growing  tissue  are  always  formed,  as  we  have  seen, 
by  cell-division,  that  is,  by  the  formation  of  cellulose  walls  across 
cells  previously  in  existence.  The  original  cell-wall  must  therefore 
always  be  single.  It  is  only  in  older  thick-walled  cells  that  a  line  of 


FIG.  526.— Section  of  stellate 
parenchyme  of  rush. 


688     MICKOSCOP1C    STRUCTURE   OF  PHANEROGAMIC   PLANTS 


demarcation  becomes  obvious  in  the  form  of  an  intermediate  lamella , 
at  one  time  called  '  intercellular  substance,'  and  supposed  to  be  a 
distinct  structure,  but  now  shown  to  be  the  result  merely  of  a  differ- 
ence in  density  or  molecular  structure  of  the  cell-walls  during  their 
thickening.  This  layer  very  frequently  ultimately  assumes  a  muci- 
laginous character.  Where  cells  have  a  rounded  outline,  it  is 
obvious  that  intercellular  spaces  must  exist  between  them  ;  and  as 
the  tissue  develops,  these  spaces  often  increase  greatly  in  size.  They 
are  called  schizoyenous  if  formed  simply  by  the  parting  of  cells  from 
one  another ;  lysigenous  if  resulting  from  the  disappearance  or 
absorption  of  ceils.  Recent  observations  have  shown  that  the  wall 
of  intercellular  spaces  is  frequently  clothed  with  a  lining  of  proto- 
plasm. There  are  many  forms  of  fully  developed  cellular  paren- 
chyme,  in  which,  in  consequence  of  the  loose  aggregation  of  their 
component  cells,  these  may  be  readily  isolated,  so  as  to  be  prepared 

for  separate  examination  without 
the  use  of  reagents  which  alter 
their  condition ;  this  is  the  case 
with  the  pulp  of  ripe  fruits, 
such  as  the  strawberry  or  currant 
(the  snowberry  is  a  particularly 
favourable  subject  for  this  kind 
of  examination),  and  with  the 
parenchyme  of  many  fleshy  leaves, 
such  as  those  of  the  carnation 
(Dianthus  caryophyttus)  or  the 
London  pride  (Saxifraga  wm- 
brosa).  Such  cells  usually  con- 
tain evident  nuclei  which  are 
turned  brownish-yellow  by  iodine, 
whilst  their  membrane  is  only 
turned  pale  yellow,  and  in  this 
way  the  nucleus  may  be  brought 
into  view  when,  as  often  happens, 
it  is  not  previously  distinguish- 
able. If  a  drop  of  the  iodised 

.solution  of  chloride  of  zinc  be  subsequently  added,  the  cell-membrane 
becomes  of  a  beautiful  blue  colour,  whilst  the  nucleus  and  the  granu- 
lar protoplasm  that  surrounds  it  retain  their  brownish-yellow  tint. 
The  use  of  dilute  nitric  or  sulphuric  acid,  of  alcohol,  of  syrup,  or  of 
several  other  reagents,  serves  to  bring  into  view  the  '  primordial '  or 
parietal  utricle,  its  contents  being  made  to  coagulate  and  shrink,  so 
that  it  detaches  itself  from  the  cellulose  wall  with  which  it  is  ordi- 
narily in  contact,  and  shrivels  up  within  its  cavity,  as  shown  in 
fig.  524.  It  would  be  a  mistake,  however,  to  regard  this  as  a  distinct 
membrane  ;  for  it  is  nothing  else  than  the  peripheral  layer  of  proto- 
plasm, naturally  somewhat  more  dense  than  that  which  it  includes, 
but  passing  into  it  by  insensible  gradations. 

It  is  probable  that  all  cells,  at  some  stage  or  other  of  their 
growth,  exhibit,  in  a  greater  or  less  degree  of  intensity,  that  curious 
movement  of  cijclosis  which  has  been  already  described  as  occurring 


FIG.    527.  —  Cubical    parenchyme, 
stellate  cells,  from  petiole  of 
lutea. 


with 


CYCLOSIS   OF   PROTOPLASM  689 

in  the  Characew  (see  p.  564),  and  which  consists  in  the  steady  flow 
of  one  or  of  several  currents  of  protoplasm  over  the  inner  wall  of 
the  cell,  this  being  rendered  apparent  by  the  movement  of  the 
particles  which  the  current  carries  along  with  it.  The  best  exam- 
ples of  it  are  found  among  submerged  plants,  in  the  cells  of  wrhich  it 
continues  for  a  much  longer  period  than  it  usually  does  elsewhere  ; 
and  among  these  are  two,  Vallisneria  spiralis  and  Anacharis  alsi- 
nastrum  (or  Elodea  canadensis),  which  are  peculiarly  fitted  for  the 
exhibition  of  this  interesting  phenomenon.  Vallisneria  is  an  aquatic 
plant  that  grows  abundantly  in  the  rivers  of  the  south  of  Europe, 
but  is  not  a  native  of  this  country  ;  it  may,  however,  be  readily 
grown  in  a  tall  glass  jar  having  at  the  bottom  a  couple  of  inches  of 
mould,  which,  after  the  roots  have  been  inserted  into  it,  should  be 
closely  pressed  down,  the  jar  being  then  filled  with  water,  of  which 
a  portion  should  be  occasionally  changed.1  The  jar  should  be  freely 
exposed  to  light,  and  should  be  kept  in  as  warm  but  equable  a  tem- 
perature as  possible.  The  long  grass-like  leaves  of  this  plant  are  too 
thick  to  allow  the  transmission  of  sufficient  light  through  them  for 
the  purpose  of  this  observation,  and  it  is  requisite  to  make  a  thin 
slice  or  shaving  wTith  a  sharp  knife.  If  this  be  taken  from  the 
surface,  so  that  the  section  chiefly  consists  of  the  superficial  layer  of 
cells,  these  will  be  found  to  be  small,  and  the  particles  of  chlorophyll, 
though  in  great  abundance,  wrill  rarely  be  seen  in  motion.  This 
layer  should  therefore  be  sliced  off  (or  perhaps  still  better,  scraped 
away)  so  as  to  bring  into  view  the  deeper  layer,  which  consists  of 
larger  cells,  some  of  them  greatly  elongated,  wTith  particles  of  chloro 
phyll  in  smaller  number,  but  carried  along  in  active  rotation  by  the 
current  of  protoplasm ;  and  it  will  often  be  noticed  that  the  direc- 
tions of  the  rotation  in  contiguous  cells  are  opposite.  If  the  move- 
ment (as  is  generally  the  case)  be  checked  by  the  shock  of  the 
operation,  it  will  be  revived  again  by  gentle  warmth ;  and  it  may 
continue  under  favourable  circumstances,  in  the  separated  fragment, 
for  a  period  of  weeks,  or  even  of  months.  Hence,  when  it  is  desired 
to  exhibit  the  phenomenon,  the  preferable  method  is  to  prepare  the 
sections  a  little  time  before  they  are  likely  to  be  wanted,  and  to 
c.irry  them  in  a  small  vial  of  water  in  the  waistcoat  pocket,  so  that 
they  may  receive  the  gentle  and  continuous  warmth  of  the  body. 
In  summer,  when  the  plant  is  in  its  most  vigorous  state  of  growth, 
the  section  may  be  taken  from  any  one  of  the  leaves ;  but  in  winter 
it  is  preferable  to  select  those  wliich  are  a  little  yellow.  An  objec- 
tive of  J-inch  focus  will  serve  for  the  observation  of  this  interesting 
phenomenon,  and  very  little  more  can  be  seen  with  a  ^-inch ;  but 
the  J^-mdi  constructed  by  Messrs.  Powell  and  Lealand  enables  the 
borders  of  the  protoplasmic  current,  which  carries  along  the 
particles  of  chlorophyll,  to  be  distinctly  defined  ;  and  this  beautiful 

1  Mr.  Quekett  found  it  the  most  convenient  method  of  changing  the  water  in  the 
jars  in  which  Chara,  Vallisneria,  &c.,  are  growing,  to  place  them  occasionally  under 
a  water-tap,  and  allow  a  very  gentle  stream  to  fall  into  them  for  some  hours  ;  for  by 
the  prolonged  overflow  thus  occasioned  all  the  impure  water,  with  the  Conferva  that 
is  apt  to  grow  on  the  sides  of  the  vessel,  may  be  readily  got  rid  of. 

Y  Y 


690     MICROSCOPIC    STRUCTURE   OF   PHANEROGAMIC   PLANTS 

phenomenon  may  be  most  luxuriously  watched   under  their  patent 
binocular. 

Anacharis  alsinastrum  is  a  water- weed  which,  having  been  acci- 
dentally introduced  into  this  country  many  years  ago,  has  since 
spread  itself  with  such  rapidity  through  our  canals  and  rivers  as  in 
many  instances  seriously  to  impede  their  navigation.  It  does  not 
require  to  root  itself  in  the  bottom,  but  floats  in  any  part  of  the  water 
it  inhabits ;  and  it  is  so  tenacious  of  life  that  even  small  fragments 
are  sufficient  for  the  origination  of  new  plants.  The  leaves  have  no 
distinct  epiderm,  but  are  for  the  most  part  composed  of  two  layers  of 
cells,  and  these  are  elongated  and  colourless  in  the  centre,  forming  a 
kind  of  midrib  ;  towards  the  margins  of  the  leaves,  however,  there  is 
but  a  single  layer.  Hence  no  preparation  whatever  is  required  for  the 
exhibition  of  this  interesting  phenomenon,  all  that  is  necessary  being 
to  take  a  leaf  from  the  stem  (one  of  the  older  yellowish  leaves  being 
preferable),  and  to  place  it,  with  a  drop  of  water,  either  in  the  aqua- 
tic box  or  on  a  slip  of  glass  beneath  a  thin  glass  cover.  A  higher- 
magnifying  power  is  required,  however,  than  that  which  suffices  for 
the  examination  of  the  cyclosis  in  Chara  or  in  Vallisneria,  the  ^-iiich 
object-glass  being  here  preferable  to  the  ^-inch,  and  the  assist- 
ance of  the  achromatic  condenser  being  desirable.  With  this  ampli- 
fication the  phenomenon  may  be  best  studied  in  the  single  layer  of 
marginal  cells,  although,  when  a  lower  power  is  used,  it  is  most  evi- 
dent in  the  elongated  cells  forming  the  central  portion  of  the  leaf. 
The  number  of  chlorophyll-granules  in  each  cell  varies  from  three  or 
four  to  upwards  of  fifty  ;  they  are  somewhat  irregular  in  shape,  some 
being  nearly  circular  flattened  discs,  whilst  others  are  oval ;  and 
they  are  usually  from  y^-^th  to  r^^th  of  an  inch  in  diameter. 
When  the  rotation  is  active  the  greater  number  of  these  granules 
travel  round  the  margin  of  the  cells,  a  few,  however,  remaining  fixed 
in  the  centre  ;  their  rate  of  movement,  though  only  ^th  of  an  inch 
per  minute,  being  sufficient  to  carry  them  several  times  round  the 
cell  within  that  period.  As  in  the  case  of  Vallisneria,  the  motion 
may  frequently  be  observed  to  take  place  in  opposite  directions 
in  contiguous  cells.  The  thickness  of  the  layer  of  protoplasm  in 
which  the  granules  are  carried  round  is  estimated  by  Mr.  Wenham 
at  no  more  than  ^0^00th  of  an  inch.  When  high  powers  and 
careful  illumination  are  employed,  delicate  ripples  may  be  seen  in  the 
protoplasmic  currents.1 

Cyclosis,  however,  is  by  no  means  restricted  to  submerged  plants  ; 
for  it  has  been  witnessed  by  numerous  observers  in  so  great  a  variety 
of  other  species  that  it  may  fairly  be  presumed  to  be  universal.  It  is 
especially  observable  in  the  hairs  of  the  epidermal  surface.  Such 
hairs  are  furnished  by  various  parts  of  plants ;  and  what  is  chiefly 
necessary  is  that  the  part  from  which  the  hair  is  gathered  should  be 
in  a  state  of  vigorous  growth.  The  hairs  should  be  detached  by 
tearing  off  with  a  pair  of  fine  pointed  forceps  the  portion  of  the 
epiderm  from  which  they  spring,  care  being  taken  not  to  grasp  the 
hair  itself,  whereby  such  an  injury  would  be  done  to  it  as  to  check 
the  movement  within  it.  The  apochromatic  hair  should  then  be 
1  Quart.  Journ.  of  Microsc.  Science,  vol.  iii.  (1855^,  p.  277. 


CYCLOSIS   OF  PROTOPLASM 


69  c 


placed  with  a  drop  of  water  under  thin  glass  ;  and  it  will  generally 
be  found  advantageous  to  use  a  ^-inch  with  the  12  or  the  18  eye- 
piece objective  with  an  achromatic  condenser.  The  nature  of 
the  movement  in  the  hairs  of  different  species  is  far  from  being 
uniform.  In  some  instances,  the  currents  pass  in  single  lines 
along  the  entire  length  of  the  cells,  as  in  the  hairs  from  the  filaments 
of  Tradescantia  virginicd,  or  Virginian  spiderwort  (fig.  528,  A) ;  in 
others  there  are  several  such  cur-^ 
rents  which  retain  their  distinct- 
ness, as  in  the  jointed  hairs  of  the 
calyx  of  the  same  plant  (B)  ;  in 
others,  again,  the  streams  coalesce 
into  a  network,  the  reticulations 
of  which  change  their  position  at 
short  intervals,  as  in  the  hairs  of 
Glaucium  luteum ;  whilst  there 
are  cases  in  wrhich  the  current 
flows  in  a  sluggish  uniformly 
moving  sheet  or  layer.  Where 
several  distinct  currents  exist  in 
one  cell,  they  are  all  found  to 
have  one  common  point  of  depar- 
ture and  return,  namely,  the 
nucleus  (B,  a),  from  which  it 
seems  fairly  to  be  inferred  that 
this  body  is  the  centre  of  the 
vital  activity  of  the  cell.  In  all 
cases  in  which  the  cyclosis  is 
seen  in  the  hairs  of  a  plant,  the 
cells  of  the  epiderm  also  display 
it,  provided  that  their  walls  are 
not  so  opaque  or  so  strongly 
marked  as  to  prevent  the  move- 
ment from  being  distinguished. 
The  epiderm  may  be  most  readily 
torn  off  from  the  stalk  or  the 
midrib  of  the  leaf,  and  must 
then  be  examined  as  speedily  as 
possible,  since  it  loses  its  vitality 
when  thus  detached  much  sooner 
than  do  the  hairs.  Even  when 
no  obvious  movement  of  particles 


FIG.  528.— Rotation  of  fluid  in  hairs  of 
Tradescantia  virginica:  A,  portion  of 
epiderm  with  hair  attached  ;  a,  b,  c, 
successive  cells  of  the  hair ;  d,  cells  of 
the  epiderm  ;  e,  stomate.  B,  joints  of  a 
beaded  hair  showing  several  currents ; 
a,  nucleus. 


is  to  be  seen,  the  existence  of 
a  cyclosis  may  be  concluded  from  the  peculiar  arrangement  of  the 
molecules  of  the  protoplasm,  which  are  remarkable  for  their  high 
refractive  power,  and  which,  when  arranged  in  a  '  moving  train,' 
appeal-  as  bright  lines  across  the  cell ;  and  these  lines,  on  being 
carefully  watched,  are  seen  to  alter  their  relative  positions.  The 
leaf  of  the  common  Plantago  (plantain)  furnishes  an  excellent  example 
of  cyclosis,  the  movement  being  distinguishable  at  the  same  time 
both  in  the  cells  and  in  the  hairs  of  the  epiderm  torn  from  its  stalk 

Y  y  2 


692     MICROSCOPIC    STRUCTURE   OF   PHANEROGAMIC   PLANTS 


or  midrib.  It  is  a  curious  circumstance  that  when  a  plant  which 
exhibits  the  cyclosis  is  kept  in  a  cold  dark  place  for  one  or  two 
days,  not  only  is  the  movement  suspended,  but  the  moving  particles 
collect  together  in  little  heaps,  which  are  broken  up  again  by  the 
separate  motion  of  their  particles  when  the  stimulus  of  light  ami 
warmth  occasions  a  renewal  of  the  activity.  It  is  well  to  collect  the 
A  (>  specimens  about  midday,  that 

being  the  time  when  the  rotation 
is  most  active,  and  the  move- 
ment is  usually  quickened  In- 
artificial warmth,  which,  indeed, 
is  a  necessary  condition  in  some 
instances  to  its  being  seen  at  all. 
The  most  convenient  method  of 
applying  this  warmth,  while  the 
object  is  on  the  stage  of  the 
microscope,  is  to  blow  a  stream 
of  air  upon  the  thin  glass  cover 

through  a  glass  or  metal  tube 
FIG.  529.— Tissue  of  the  testa  or  seed-coat 

of   star-anise :    A,   as   seen   in   section ; 
B,  as  seen  on  the  surface. 


in 


spirit- 


previously  -heated 

lamp. 

The    walls   of    the   cells    of 

plants  are  frequently  thickened  by  deposits,  which  are  first  formed 
on  the  inner  surface,  and  which  may  present  very  different  appear- 
ances according  to  the  manner  in  which  they  are  arranged.  In 


FIG.  530.— Section  of  cherry-stone, 
cutting  the  cells  transversely. 


FIG.  531.— Section  of  coquilla 
nut,  in  the  direction  of  the 
long  diameter  of  the  cells. 


its  simplest  condition  such  a  deposit  forms  a  thin  uniform  layer 
over  the  whole  internal  surface  of  the  cellulose  wall,  scarcely  detract- 
ing at  all  from  its  transparency,  and  chiefly  distinguishable  by  the 
*  dotted  '  appearance  which  the  membrane  then  presents  (fig.  ~)-2Z.  A). 
These  dots,  however,  are  not  pores,  as  their  aspect  might  naturally 
.suggest,  but  are  merely  points  at  which  the  deposit  is  wanting,  so 


TISSUES   OF   PHANEROGAMIA  693 

that  the  original  cell- wall  there  remains  imthickened.  A  more 
complete  consolidation  of  cellular  tissue  is  effected  by  deposits 
of  sclerogen  (a  substance  which,  when  separated  from  the  resinous 
and  other  matters  that  are  commonly  associated  with  it,  is  found 
to  be  allied  in  chemical  composition  to  cellulose)  in  successive 
layers,  one  within  another  (fig.  529,  A),  wrhich  present  them- 
selves MS  concentric  rings  when  the  cells  containing  them  are  cut 
throng] i ;  and  these  layers  are  sometimes  so  thick  and  numerous 
us  almost  to  obliterate  the  original  cavity  of  the  cell.  Such  a  tissue 
is  known  as  sclerenchyme  or  sclerenchymatous  tissue.  By  a  con- 
tinuance of  the  same  arrangement  as  that  which  shows  itself  in  the 
single  layer  of  the  dotted  cell — each  deposit  being  deficient  at  certain 
points,  and  these  points  corresponding  with  each  other  in  the  succes- 
sive layers — a  series  of  passages  is  left,  by  which  the  cavity  of  the 
cell  is  extended  at  some  points  to  its  membranous  wall ;  and  it 
commonly  happens  that  the  points  at  which  the  deposit  is  wanting 
011  the  walls  of  the  contiguous  cells  are  coincident,  so  that  the 
membranous  partition  is  the  only  obstacle  to  the  communication 
between  their  cavities  (figs.  529-531).  It  is  of  such  tissue  that 
the  '  stones '  of  stone-fruit,  the  gritty  substance  which  surrounds  the 
seeds  and  forms  little  hard  points  in  the  fleshy  substance  of  the  pear, 
the  shell  of  the  cocoa-nut,  and  the  endosperm  of  the  seed  of  Phyt- 
elephas  (known  as  'vegetable  ivory')  are  made  up;  and  we  see  the 
use  of  this  very  curious  arrangement  in  permitting  the  cells,  even 
after  they  have  attained  a  considerable  degree  of  consolidation, 
still  to  remain  permeable  to  the  fluid  required  for  the  nutrition  of 
the  parts  which  such  tissue  encloses  and  protects. 

The  deposit  sometimes  assumes,  however,  the  form  of  definite 
fibres,  which  lie  coiled  up  on  the  inner 
surface  of  the  cells,  so  as  to  form  a  single, 
a  double,  or  even  a  triple  or  quadruple 
spire  (fig.  532).  Such  spiral  cells  are  found 
abundantly  in  the  leaves  of  certain  orchi- 
daceous plants,  immediately  beneath  the 
epiderm,  where  they  are  brought  into 
view  by  vertical  sections;  and  they  may 
be  obtained  in  an  isolated  state  by  mace- 
rating the  leaf  and  peeling  off  the  epiderm 
so  as  to  expose  the  layer  beneath,  which  is 
1 1  ieii  easily  separated  into  its  components.  FIG.  532.— Spiral  cells  of  leaf 
In  an  orchidaceous  plant  named  Saccola-  of  Onddium. 

biutn  guttatutn  the  spiral  cells  are  unusu 

ally  long,  and  have  spires  winding  in  opposite  directions,  so  that  by 
their  mutual  intersection  a  series  of  diamond-shaped  markings  is  pro- 
diu-cd.  Spiral  cells  are  often  found  upon  the  surface  of  the  testa  or 
outer  coat  of  seeds  ;  arid  in  Collomia  grandiflora,  Salvia  verbenaca 
(wild  clary),  and  some  other  plants,  the  membrane  of  these 
cells  is  so  weak,  and  the  elasticity  of  their  fibres  so  great,  that 
when  the  membrane  is  softened  by  the  action  of  water  the  fibres 
suddenly  uncoil  and  elongate  themselves  (fig.  533),  springing  out, 
as  it  were,  from  the  surface  of  the  seed,  to  which  they  give  a 


694     MICROSCOPIC   STRUCTURE   OF  PHANEROGAMIC   PLANTS 


peculiar  flocculent  appearance.  This  very  curious  phenomenon  may 
be  best  observed  in  the  following  manner : — A  very  thin  trans- 
verse slice  of  the  seed  should  first  be  cut,  and  laid  upon  the  lower 

glass  of  the  aquatic  box  ;  the  cover 
should  then  be  pressed  down, 
and  the  box  placed  upon  the 
stage,  so  that  the  microscope  may 
be  exactly  focussed  to  the  object, 
the  power  employed  being  the 
1-inch,  §-inch,  or  J-inch.  The 
cover  of  the  aquatic  box  h(»in# 
then  removed,  a  small  drop  of 
water  should  be  placed  on  that 
part  of  its  internal  surface  with 
which  the  slice  of  the  seed  had  been 
in  contact ;  and  the  cover  being 
replaced,  the  object  should  be  im- 
mediately looked  at.  It  is  im- 
portant that  the  slice  of  the  seed 
should  be  very  thin,  for  two 
reasons :  first,  that  the  view  of 
the  spirals  may  not  be  confused 
by  their  aggregation  in  too  great  numbers  ;  and  second,  that  the 
drop  of  water  should  be  held  in  its  place  by  capillary  attraction, 
instead  of  running  down  and  leaving  the  object,  as  it  will  do  if  the 
glasses  be  too  widely  separated. 

In  some  part  or  other  of  most  plants  we  meet  with  cells  contain- 
ing granules  of  starch,  which  specially  abound  in  the  tubers  of  the 
potato  and  in  the  seeds  of  cereals.  Starch-grains  are  originally 
formed  in  the  interior  of  chlorophyll-corpuscles,  and  therefore  within 
the  protoplasm-layer  of  the  cell ;  but  as  they  increase  in  size,  the 
protoplasm-layer  thins  itself  out  as  a  mere  covering  film,  and  at  last 
almost  entirely  disappears.  So  long  as  the  starch-grains  remain 
imbedded  in  the  protoplasm-layer,  they  continue  to  grow  ;  but  when 
hey  accumulate  so  as  to  occupy  the  cell-cavity,  their  growth  stops. 


FIG.  533. — Spiral  fibres  of  seed-coat  of 
Collomia. 


FIG.  534.— Cells  of  peony  filled 
with  starch. 


FIG.  535. — Granules  of  starch  as 
seen  under  polarised  light. 


They  are  sometimes   minute   and   very   numerous,  and    so   closely 
packed  as  to  fill  the  cell-cavity  (fig.  534);  in  other  instances  they 
are  of  much  larger  dimensions,  so  that  only  a  comparatively  smal 
number   of  them    are    included    in    any    one  cell ;     while  in  other 


STARCH-GEAINS  695 

cases,  again,  they  are  both  few  and  minute,  so  that  they  form  but 
a  small  proportion  of  the  cell-contents.  Their  nature  is  at  once 
detected  by  the  addition  of  a  solution  of  iodine,  which  gives  them  a 
beautiful  blue  colour.  Each  granule  when  highly  magnified  exhibits 
a  peculiar  spot,  termed  the  hilumi,  round  which  are  seen  a  set  of 
circular  lines  that  are  for  the  most  part  concentric  (or  nearly  so) 
with  it.  When  viewed  by  polarised  light  each  grain  exhibits  a  dark 
cross,  the  point  of  intersection  being  at  the  hilum  (fig.  535)  ;  and 
when  a  selenite  plate  is  interposed  the  cross  becomes  beautifully 
coloured.  Opinions  have  been>very  much  divided  regarding  the 
internal  structure  of  the  starch-grain,  but  the  doctrine  of  Nageli 
that  it  is  composed  of  successive  layers  which  increase  by  '  intus- 
susception,' that  is,  by  the  intercalation  of  fresh  molecules  of  starch 
between  those  already  in  existence,  is  favoured  by  many  authorities, 
though  the  alternative  theory  of  formation  by  the  '  apposition '  of 
successive  layers  also  has  many  advocates.  These  layers  differ  in 
their  proportion  of  water,  the  outermost  layer,  which  is  the  most 
solid,  having  within  it  a  watery  layer,  this,  again,  being  succeeded 
by  a  firm  layer,  which  is  followed  by  a  watery  layer,  and  so  on,  the 
proportion  of  water  increasing  towards  the  centre  in  both  kinds  of 
layer,  and  attaining  its  maximum  in  the  innermost  part  of  the 
grain,  where  the  formation  of  new  layers  takes  place,  causing  the 
distension  of  the  older  ones.  Although  the  dimensions  of  the 
starch-grains  produced  by  any  one  species  of  plant  are  by  no  means 
constant,  yet  there  is  a  certain  average  for  each,  from  which  none 
of  them  depart  very  widely  ;  and  by  reference  to  this  average  the 
starch-grains  of  different  plants  that  yield  this  product  in  abundance 
may  be  microscopically  distinguished  from  one  another — a  circum- 
stance of  considerable  importance  in  commerce.  The  largest  starch- 
grains  in  common  use  are  those  of  the  plant  (a  species  of  Canna) 
known  as  '  tous-les-mois.'  The  average  diameter  of  those  of  the 
potato  is  about  the  same  as  the  diameter  of  the  smallest  of  the 
'  tous-les-mois,'  and  the  size  of  the  ordinary  starch-grains  of  wheat 
and  of  sago  is  about  the  same  as  that  of  the  smallest  grains  of 
potato-starch  ;  whilst  the  granules  of  rice-starch  are  so  very  minute 
as  to  be  at  once  distinguishable  from  any  of  the  preceding. 

In  certain  plants,  especially  those  belonging  to  particular  natural 
orders,  the  stem,  leaves,  and  other  parts  are  permeated  by  long- 
branched  tubes,  constituting  the  laticiferous  tissue.  The  elements 
of  this  tissue  may  be  either  greatly  enlarged  prosenchymatous  cells 
or  true  vessels.  In  either  case  they  contain  a  copious  milky-white 
or  coloured  juice,  the  latex,  wThich  exudes  freely  when  the  part  con- 
taining it  is  wounded,  and  dries  rapidly  on  exposure.  The  chemical 
composition  of  the  latex  varies ;  it  may  contain  in  solution  powerful 
alkaloids,  as  in  the  case  of  the  opium-poppy,  or  gum-resins.  Caou- 
tchouc and  gutta-percha  are  the  dried  latex  of  tropical  trees  and 
shrubs  belonging  to  several  natural  orders.  Good  examples  of  lati- 
ciferous tissue  are  furnished  by  the  Papaveracese,  of  which  our 
common  field-poppy  is  an  example,  many  Composite  such  as  the 
dandelion  and  lettuce,  Convolvulacese,  Euphorbiacese  or  spurges, 
Apocynacese,  Moraceae  including  the  mulberry  &c. 


696   MICROSCOPIC    STRUCTURE   OF  PHANEROGAMIC   PLANTS 

Deposits  of  mineral  matter  in  a  crystalline  condition,  known  as 
raphides,  are  not  unfrequently  found  in  vegetable  cells,  where  they 
are  at  once  brought  into  view  by  the  use  of  polarised  light.  Their 
designation  (derived  from  pact's,  a  needle)  is  very  appropriate  to  one 
of  the  most  common  states  in  which  these  bodies  present  themselves, 
that,  namely,  of  bundles  of  needle-like  crystals,  lying  side  by  side  in 
the  cavity  of  the  cells  ;  such  bundles  are  well  seen  in  the  cells  lying 
immediately  beneath  the  epiderm  of  the  bulb  of  the  medicinal 
squill.  It  does  not  apply,  however,  to  other  forms  which  are 
scarcely  less  abundant ;  thus,  instead  of  bundles  of  minute  needles, 
single  large  crystals,  octahedral  or  prismatic,  are  frequently  met 
with,  and  the  prismatic  crystals  are  often  aggregated  in  beautiful 
stellate  groups.  The  most  common  material  of  these  crystals  is 
oxalate  of  lime,  which  is  generally  found  in  the  stellate  form  ;  and  no 
plant  yields  these  stellate  raphides  so  abundantly  as  the  common 
rhubarb,  the  best  specimens  of  the  dry  medicinal  root  containing  as 
much  as  35  per  cent,  of  them.  In  the  epiderm  of  the  bulb  of  the 
onion  the  same  material  occurs  in  the  octahedral  or  the  prismatic 
form.  In  other  instances,  the  calcareous  base  is  combined  with 
tartaric,  citric,  or  malic  acid  ;  the  acicular  raphides  consist  almost 
invariably  of  oxalate  of  lime.  Some  raphides  are  as  long  as  ^Vth  of 
an  inch,  while  others  measure  no  more  than  y-^th.  They  occur  in  all 
parts  of  plants — the  wood,  pith,  bark,  root,  leaves,  stipules,  sepals, 
petals,  fruit,  and  even  in  the  pollen.  They  are  always  situated  in 
cells,  and  not  in  the  intercellular  passages  ;  the  cell-membrane,  how- 
ever, is  often  so  much  thinned  away  as  to  be  scarcely  distinguish- 
able. Certain  plants  of  the  Cactus  tribe,  when  aged,  have  their 
tissues  so  loaded  with  raphides  as  to  become  quite  brittle,  so  that 
when  some  large  specimens  of  C.  senilis,  said  to  be  a  thousand  years 
old,  were  sent  to  Kew  Gardens  from  South  America,  some  years 
since,  it  was  found  necessary  for  their  preservation  during  transport 
to  pack  them  in  cotton  like  jewellery.  Raphides  are  probably  to  be 
considered  as  non-essential  results  of  the  vegetative  processes,  being 
for  the  most  part  produced  by  the  union  of  organic  acids  generated 
in  the  plant  with  mineral  bases  imbibed  by  it  from  the  soil.  The 
late  Mr.  E.  Quekett  succeeded  in  artificially  producing  raphides 
within  the  cells  of  rice-paper,  by  first  filling  these  with  lime-water 
by  means  of  the  air-pump,  and  then  placing  the  paper  in  weak 
solutions  of  phosphoric  and  oxalic  acids.  The  artificial  raphides  of 
phosphate  of  lime  were  rhombohedral ;  while  those  of  oxalate  of 
lime  were  stellate,  exactly  resembling  the  natural  raphides  of  the 
rhubarb.  Besides  the  structures  already  mentioned  as  affording  good 
illustrations  of  different  kinds  of  raphides,  may  be  mentioned  the 
parenchyme  of  the  leaf  of  Agave,  Aloe,  Cycas,  Encephalartos,  &c. ; 
the  epiderm  of  the  bulb  of  the  hyacinth,  tulip,  and  garlic  ;  the  bark  of 
the  apple,  Cascarilla,  Cinchona,  lime,  locust,  and  many  other  trees ;  the 
pith  of  Eltvagnus,  and  the  testa  of  the  seeds  of  Anagallis  and  the  elm. 

A  large  proportion  of  the  denser  parts  of  the  fabric  of  the  higher 
plants  is  made  up  of  the  substance  which  is  known  as  woody  fibre  or 
prosenchymatous  tissue.  This,  however,  can  only  be  regarded  as  a 
variety  of  cellular  tissue  ;  for  it  is  composed  of  peculiarly  elongated 


TISSUES   OF  PHANEROGAMIA 


697 


cells  (fig.  551),  usually  pointed  at  their  two  extremities  so  as  to 
become  spindle-shaped,  whose  walls  have  a  special  tendency  to 
undergo  consolidation  by  the  internal  deposit  of  sclerogen.  It  is 
obvious  that  a  tissue  consisting  of  elongated  cells,  adherent  together 
by  their  entire  length,  and  strengthened  by  internal  deposit,  must 
possess  much  greater  tenacity  than  any  tissue  in  which  the  cells 
depart  but  little  from  the  primitive  spherical  form  ;  and  we  accord- 
ingly find  woody  fibre  present  wherever  it  is  requisite  that  the  fabric 
should  possess  not  merely  density,  but  the  power  of  resistance  to 
tension.  In  the  higher  classes  of  the  vegetable  kingdom  it  consti- 
tutes the  chief  part  of  the  stem  and  branches,  where  these  have  a 
firm  and  durable  character  ;  and  even  in  more  temporary  structures, 
such  as  the  herbaceous  stems  of  annual  plants,  and  the  leaves  and 
flowers  of  almost  every  tribe,  this  tissue  forms  a  more  or  less  import- 
ant constituent,  being  especially  found  in  the  neighbourhood  of  the 
spiral  vessels  and  ducts,  to  which  it  affords  protection  and  support. 
Hence  the  bundles  of  fasciculi  composed  of  these  elements,  which 
form  the  '  veins  '  of  leaves,  and  which  give  '  stringiness  '  to  various 
esculent  vegetable  substances,  are  commonlv  known  under  the 
name  of  fbro-vascular  tissue.  In  their  young  and  unconsolidated 
state  the  woody  cells  seem  to  conduct  fluids  with  great  facility  in 
the  direction  of  their  length  ;  and  in  the  Coni/trce,  whose  stems  and 
branches  are  destitute  of  ducts,  they  afford  the  sole  channel  for  the 
ascent  of  the  sap.  The  fbro-vascular  bundles,  which  are  the  chief 
strengthening  elements  of  such  organs  as  the  stem,  branches,  leaf- 
stalks, flower-stalks,  <fec.,  are,  in  the  higher 
plants,  structures  of  considerable  com- 
plexity ;  in  Exogens  they  consist  of  three 
distinct  portions,  the  &-v/^w- portion  com- 
posed chiefly  of  the  different  kinds  of 
vessels  hereafter  to  be  described,  &phloem- 
portion  composed  of  prosenchymatous 
tissue  and  '  sieve-tubes,'  and  a  formative 
cambi  urn-portion . 

A  peculiar  set  of  markings  seen  on 
the  woody  fibres  of  the  Coniferce,  and  of 
some  other  tribes,  is  represented  in  fig. 
536  ;  in  each  of  these  spots  the  inner 
circle  appears  to  mark  a  deficiency  of 
the  lining  deposit,  as  in  the  pitted  cells 
of  other  plants  ;  whilst  the  outer  circle 
indicates  the  boundary  of  a  lenticular- 
cavity  which  intervenes  between  the  ad- 
jacent cells  at  this  point.  There  are 
varieties  in  this  arrangement  so  charac- 
teristic of  different  tribes  that  it  is 
sometimes  possible  to  determine,  by  the 

microscopic  inspection  of  a  minute  fragment,  even  of  a  fossil  wood, 
the  tribe  to  which  it  belonged.  Markings  of  this  kind,  very 
characteristic  of  the  wood  of  Conifers,  though  not  peculiar  to  that 


FIG.  586.— Section  of  coniferous 
wood  in  the  direction  of  the 
trachei'ds,  showing  their 
'  bordered  pits ;  '  a,  a,  a,  me- 
dullary rays  crossing  the 
fibres. 


698     MICROSCOPIC    STRUCTURE    OF   PHANEROGAMIC   PLANTS 

order,  are  known  as  bordered  pits,  and  the  elongated  cells  in  which 
they  occur  as  tradte'ids. 

All  the  more  perfect  forms  of  Phanerogams  contain,  in  some 
part  of  their  fabric,  the  peculiar  structures  which  are  known  as 
spiral  vessels.1  These  have  the  elongated  shape  of  fibre-cells  ;  but 
the  internal  deposit,  as  in  the  spiral  cells,  takes  the  form  of  a  spiral 
fibre  winding  from  end  to  end,  and  retaining  its  elasticity  ;  this 
fibre  may  be  single,  double,  or  even  quadruple,  this  last  character  pre- 
senting itself  in  the  very  large  elongated  fibre-cells  of  Nepenthes 
(pitcher-plant).  Such  vessels  are  especially  found  in  the  delicate 
membrane  (medullary  sheath)  surrounding  the  pith  of  Exogens, 
and  in  the  '  xylem-portion '  of  the  woody  bundles  of  Exogens  and 
Endogens  ;  thence  they,  proceed  to  the  leaf -stalks,  through  which 
they  are  distributed  to  the  leaves.  By  careful  dissection  under  the 
microscope  these  fibro- vascular  bundles  may  be  separated  entire  ; 
but  their  structure  may  be  more  easily  displayed  by  cutting  round, 
but  not  through,  the  leaf-stalk  of  the  strawberry,  geranium,  &c.,  and 
then  drawing  the  parts  asunder.  The  membrane  composing  the 
tubes  of  the  vessels  will  thus  be  broken  across  ;  but  the  fibres  within, 
being  elastic,  will  be  drawn  out  and  unrolled.  Spiral  vessels  are 
sometimes  found  to  convey  fluid,  whilst  in  other  cases  they  contain 
air  only. 

Although  fluid  generally  finds  its  way  with  tolerable  facility 
through  the  various  forms  of  cellular  tissue,  especially  in  the  direction 
of  the  greatest  length  of  the  cells,  a  more  direct  means  of  connection 
between  distant  parts  is  required  for  its  active  transmission.  This  is 
afforded  by  the  peculiar  kind  of  vessels  known  as  ducts,  which  consist 
of  cells  laid  end  to  end,  the  partitions  between  them  being  more  or 
less  obliterated.  The  origin  of  these  ducts  is  occasionally  very  evi- 
dent, both  in  the  contraction  of  their  diameter  at  regular  intervals, 
and  in  the  persistence  of  remains  of  their  partitions  (fig.  551, 
b,  b) ;  but  in  most  cases  it  can  only  be  ascertained  by  studying  the 
history  of  their  development,  neither  of  these  indications  being  trace- 
able. Some  of  these  ducts  (fig.  537,  2)  are  indistinguishable  from 
the  spiral  vessels  already  described,  save  in  the  want  of  elasticity  in 
their  spiral  fibre,  which  causes  it  to  break  when  the  attempt  is  made 
to  draw  it  out.  This  rupture  would  seem  to  have  taken  place,  in 
some  instances,  from  the  natural  elongation  of  the  cells  by  growth, 
the  fibre  being  broken  up  into  rings,  which  lie  sometimes  close 
together,  but  more  commonly  at  considerable  intervals  ;  such  a  duct 
is  said  to  be  annular  (fig.  537,  i).  Intermediate  forms  between  the 
spiral*  and  annular  ducts,  which  show  the  derivation  of  the  latter 
from  the  former,  are  very  frequently  to  be  met  with.  The  spirals  are 
sometimes  broken  up  still  more  completely,  and  the  fragments  of  the 
fibre  extend  in  various  directions,  so  as  to  meet  and  form  an  irregular 
network  lining  the  duct,  which  is  then  said  to  be  reticulated.  The 
continuance  of  the  deposit,  however,  gradually  contracts  the  meshes, 

1  So  long,  however,  as  they  retain  their  original  cellular  character,  and  do  not 
coalesce  with  each  other,  these  fusiform  spiral  cells  cannot  be  regarded  as  having 
any  more  claim  to  the  designation  of  vessels,  than  have  the  elongated  cells  of  the 
woody  tissue. 


TISSUES   OF  PHANEROGA3IIA 


699 


leaving  the  walls  of  the  duct  marked  only  by  pores  like  those  of 
porous  cells ;  and  such  canals,  designated  as  pitted  ducts,  are 
especially  met  with  in  parts  of  most  solid  structure  and  least  rapid 
growth  (fig.  537,  3).  The  scalariform  ducts  of  ferns  may  be  re- 
garded as  a  modification  of  the  spiral ;  but  spiral  ducts  are  fre- 
quently to  be  met  with  also  in  the  rapidly  growdng  leaf-stalks  of 
flowering  plants,  such  as  the  rhubarb.  Not  unfrequently,  however, 
we  find  all  forms  of  ducts  in  the  same  bundle,  as  seen  in  fig.  537. 
The  size  of  these  ducts  is  occasionally  so  great  as  to  enable  their 
openings  to  be  distinguished  by  fche  unaided  eye  ;  they  are  usually 
largest  in  stems  whose  size  is  small  in  proportion  to  the  surface  of 
leaves  which  they  support,  such  as  the  common  cane  or  the  vine  ; 
and,  generally  speaking, 
they  are  larger  in  woods 
of  dense  texture,  such  as 
oak  and  mahogany,  than 
in  those  of  which  the 
fibres,  remaining  uncoil  - 
solidated,  can  serve  for  the 
conveyance  of  fluid.  They 
are  entirely  absent  in  the 
Coniferce. 

The  vegetable  tissues 
whose  principal  forms 
have  been  now  described, 
but  among  which  an  im  • 
mense  variety  of  detail  is 
found,  may  be  either 
.studied  as  they  present 
themselves  in  thin  sec- 
tions of  the  various  parts 
of  the  plant  under  exami- 
nation, or  in  the  isolated 
conditions  in  which  they 

are  obtained  by  dissection.    FIG.  537.— Longitudinal  section  of  stem  of  Italian 
The  former  process  is  the       reed  :    a,   cells   of    the    pith ;    &,    fibre- vascular 
most    easy,   and   yields    a       bu^le,   containing   1,   annular  ducts;    2,    spiral 
«  r7  j.  ducts ;  3,  pitted  ducts  with  woody  fibre  ;  c,  cells 

large  amount  of  mforma-      Of  the  epiderm. 
tion ;   but  still  it  cannot 

be  considered  that  the  characters  of  any  tissue  have  been  properly 
determined  until  it  has  been  dissected  out.  Sections  of  some  of  the 
hardest  vegetable  substances,  such  as  '  vegetable  ivory/  the  '  stones  ' 
of  fruit,  the  '  shell '  of  the  cocoa-nut,  &c.,  can  scarcely  be  obtained 
except  by  slicing  and  grinding ;  and  these  may  be  mounted  either  in 
Canada  balasm  or  in  glycerin  jelly.  In  cases,  however,  in  which  the 
tissues  are  of  only  moderate  firmness,  the  section  may  be  most  readily 
and  effectually  made  with  the  '  microtome  ; '  and  there  are  few  parts 
of  the  vegetable  fabric  which  may  not  be  advantageously  examined 
by  this  means,  any  very  soft  or  thin  portions  being  placed  in  it 
between  two  pieces  of  cork,  elder-pith,  or  carrot.  In  certain  cases, 
however,  in  which  even  this  compression  would  be  injurious,  the 


7CO     MICKOSCOPIC   STRUCTURE    OF   PHANEROGAMIC   PLANTS 

sections  must  be  made  with  a  shnrp  knife,  the  substance  being  laid  on 
the  nail  or  on  a  slip  of  glass.  In  dissecting  the  vegetable  tissues, 
scarcely  any  other  instrument  will  be  found  really  necessary  than 
a  pair  of  needles  (in  handles),  one  of  them  ground  to  a  cutting  edge. 
The  adhesion  between  the  component  cells,  fibres,  &c.,  is  often 
sufficiently  weakened  by  a  few  hours'  maceration  to  allow  of  their 
readily  coming  apart,  when  they  are  torn  asunder  by  the  needle- 
points beneath  the  simple  lens  of  a  dissecting  microscope.  But  if 
this  should  not  prove  to  be  the  case,  it  is  desirable  to  employ  some 
other  method  for  the  sake  of  facilitating  their  isolation.  None  is  so 
effectual  as  the  boiling  of  a  thin  slice  of  the  substance  under  exami- 
nation either  in  dilute  nitric  acid  or  in  a  mixture  of  nitric  acid  and 
chlorate  of  potassa.  This  last  method  (which  was  devised  by 
Schultz)  is  the  most  rapid  and  effectual,  requiring  only  a  few 
minutes  for  its  performance ;  but  as  oxygen  is  liberated  with  such 
freedom  as  to  give  an  almost  explosive  character  to  the  mixture,  it 
should  be  put  in  practice  with  extreme  caution.  After  being  thus 
treated,  the  tissue  should  be  boiled  in  alcohol,  and  then  in  water  ; 
arid  it  will  then  be  found  very  easy  to  tear  apart  the  individual  cells, 
ducts,  &c.  of  wrhich  it  may  be  composed.  These  may  be  preserved 
by  mounting  in  weak  spirit. 

Stem  and  Boot. — It  is  in  the.  stems  and  roots  that  we  find  the 
greatest  variety  of  tissues  in  combination,  and  the  most  regular 
plans  of  structure  ;  and  sections  of  these  viewed  under  a  low  mag- 
nifying power  are  objects  of  peculiar  beauty,  independently  of  the 
scientific  information  which  they  afford.  The  axis  (under  which 
term  are  included  the  stem  with  its  branches,  and  the  root  with  its 
ramifications)  always  has  for  the  basis  of  its  structure  a  dense  cellular 
parenchyme ;  though  in  an  advanced  stage  of  development  this 
may  constitute  but  a  small  portion  of  it.  In  the  midst  of  the 
parenchyme  we  generally  find  fibro-vascular  bundles,  consisting  of 
wroody  fibre,  with  ducts  of  various  kinds,  and  (almost  always)  spiral 
vessels.  It  is  in  the  mode  of  arrangement  of  these  bundles  that  the 
fundamental  difference  exists  between  the  stems  which  are  commonly 
designated  as  endogenous  (growing  from  within),  and  those  which 
are  more  correctly  termed  exogenous  (growing  on  the  outside)  ;  for 
in  the  former  the  bundles  are  dispersed  throughout  the  whole 
diameter  of  the  axis  without  any  peculiar  plan,  the  intervals  between 
them  being  filled  up  by  cellular  parenchyme  ;  whilst  in  the  latter 
they  are  arranged  side  by  side  in  such  a  manner  as  to  form  a  cylinder 
of  wood,  which  includes  within  it  the  portion  of  the  cellular  substance 
known  as  pith,  whilst  it  is  itself  enclosed  in  an  envelope  of  the  same 
substance  that  forms  the  bark.  These  two  plans  of  axis-formation 
respectively  characteristic  of  those  two  great  groups  into  which 
Phanerogams  are  subdivided — namely,  the  Monocotyledons  and  the 
Dicotyledons — will  now  be  more  particularly  described. 

When  a  transverse  section  (fig.  538)  of  a  monocotyledonous  stem 
is  examined  microscopically,  it  is  found  to  exhibit  a  number  of  fibro- 
vascular  bundles,  disposed  without  any  regularity  in  the  midst  of 
the  mass  of  cellular  tissue,  which  forms  (as  it  were)  the  matrix  or 
basis  of  the  fabric.  Each  bundle  contains  two,  three,  or  more  large 


STRUCTURE    OF   STEMS 


701 


ducts,  which  are  at  once  distinguished  by  the  size  of  their  openings ; 
and  these  are  surrounded  by  woody  fibre  and  spiral  vessels,  the 
transverse  diameter  of  which  is  so  extremely  small  that  the  portion 
of  the  bundles  which  they  form  is  at  once  distinguished  in  transverse 


FIG.  538. — Transverse  section  of  stem  of  young  palm. 

section  by  the  closeness  of  its  texture  (fig.  539).  The  bundles  are 
least  numerous  in  the  centre  of  the  stem,  and  become  gradually  more 
crowded  towards  its  circumference ;  but  it  frequently  happens  that  the 
portion  of  the  area  in  which  they  are 
most  compactly  arranged  is  not  abso- 
lutely at  its  exterior,  this  portion  being 
itself  surrounded  by  an  investment 
composed  of  cellular  tissue  only  ;  and 
sometimes  we  find  the  central  portion 
also  completely  destitute  of  fibro-vas- 
cular  bundles  ;  so  that  a  sort  of  indica- 
tion of  the  distinction  between  pith, 
wood,  and  bark  is  here  presented. 
This  distinction,  however,  is  very  im- 
perfect ;  for  we  do  not  find  either  the 
central  or  the  peripheral  portions  ever 
separable,  like  pith  and  bark,  from 
the  intermediate  woody  layer.  In  its 
young  state  the  centre  of  the  stem  is 
always  filled  up  with  cells  ;  but  these 
not  un frequently  disappear  after  a 
time,  except  at  the  nodes,  leaving 
the  stem  hollow,  as  we  see  in  the 

whole  tribe  of  grasses.  When  a  vertical  section  is  made  of  a  woody 
stem  (as  that  of  a  palm)  of  sufficient  length  to  trace  the  whole  extent 
of  the  fibro-vascular  bundles,  it  is  found  that,  whilst  they  pass  at 
their  upper  extremity  into  the  leaves,  they  pass  at  the  lower  end 


FIG.  539. — Portion  of  transverse 
section  of  stem  of  Wanghie  cane. 


702     MICROSCOPIC    STRUCTURE    OF   PHANEROGAMIC   PLANTS 


towards  the  surface  of  the  stem,  and  assist,  by  their  interlacement 
with  the  outer  bundles,  in  forming  that  extremely  tough  investment 
which  the  lower  ends  of  these  stems  present.  New  fibro- vascular 
bundles  are  being  continually  formed  in  the  upper  part  of  the  stem, 
in  continuity  with  the  leaves  which  are  successively  put  forth  at  its 
summit ;  but  while  these  take  part  in  the  elongation  of  the  stem, 

they  contribute  but  little  to  the  increase 
of  its  diameter.  For  those  which  are 
most  recently  formed  only  pass  into  the 
centre  of  the  stem  during  the  higher 
part  of  their  course,  and  usually  make 
their  way  again  to  its  exterior  at  no 
great  distance  below  ;  and,  when  once 
formed,  they  receive  no  further  additions. 
It  was  from  the  idea  formerly  enter- 
tained that  these  successively  formed 
bundles  descend  in  the  interior  of  the 
stem  through  its  entire  length  until  they 
reach  the  roots,  and  that  the  stem  is  thus 
continually  receiving  additions  to  its 
interior,  that  the  term  endogenous  was 
given  to  this  type  of  stem-structure ; 
but,  from  the  fact  just  stated  regarding 
the  course  of  the  nbro-vascular  bundles, 
it  is  obvious  that  such  a  doctrine  cannot  be  any  longer  admitted. 

In  the  stems  of  dicotyledonous  phanerogams,  on  the  other  hand, 
we  find  a  method  of  arrangement  of  the  several  parts  which  must 
be  regarded  as  the  highest  form  of  the  development  of  the  axis, 
being  that  in  which  the  greatest  differentiation  exists.  A  distinct 
division  is  always  seen  in  a  transverse  section  (fig.  540)  between  three 
concentric  areas — the  pith,  the  wood,  and  the  bark — the  first  (a)  being 


FIG.  540.  —  Diagram  of  the  first 
formation  of  an  exogenous 
stem  :  a,  pith  ;  b,  b,  bark ;  c,  c, 
plates  of  cellular  tissue  (me- 
dullary rays)  left  between  the 
woody  bundles  d  d. 


FIG.  541. — Transverse  section  of  stem  of  Clematis:  a,  pith;  b,  b,  b,  woody  bundles 
c,  c,  c,  medullary  rays. 

central,  the  last  (b)  peripheral,  and  these  having  the  wood  interposed 
between  them,  its  circle  being  made  up  of  wedge-shaped  bundles 
(d  d),  kept  apart  by  the  medullary  rays  composed  of  unchanged  cel- 
lular tissue  (c,  c)  that  pass  between  the  pith  and  the  bark.  The  pith 
(fig.  541,  a)  is  almost  invariably  composed  of  cellular  tissue  only, 
which  usually  presents  (in  transverse  section)  an  hexagonal  areolation. 
When  newly  formed  it  has  a  greenish  hue,  and  its  cells  are  filled  with 


STRUCT UEE   OF   STEMS  703 

fluid  ;  but  it  gradually  dries  up  and  loses  its  colour ;  and  not  un- 
frequently  its  component  cells  are  torn  apart  by  the  rapid  growth 
of  their  envelope,  so  that  irregular  cavities  are  found  in  it ;  or  if 
the  stem  should  increase  with  extreme  rapidity  it  becomes  hollow, 
the  pith  being  reduced  to  fragments,  which  are  found  adhering  to 
its  interior  wall.  The  pith  is  immediately  surrounded  by  a  delicate 
membrane,  consisting  almost  entirely  of  spiral  vessels,  which  is 
termed  the  medullary  sheath. 

The  woody  portion  of  the  sten*  (fig.  541,  b,  b)  is  made  up  of  woody 
fibres,  usually  witli  the  addition  of  ducts  of  various  kinds  ;  these, 
however,  are  absent  in  one  large  group,  the  Coniferce  or  fir-tribe 
with  its  allies  (figs.  545-548),  in  which  the  prosenchymatous  cells 
or  trache'ids  are  of  unusually  large  diameter,  and  are  marked  by 
the  bordered  pits  already  described.  In  any  stem  or  branch  of  more 
than  one  year's  growth  the  woody  structure  presents  a  more  or  less 
distinct  appearance  of  division  into  concentric  rings,  the  number  of 


FIG.  542. — Transverse  section  of  stem  FIG.  548. — Portion  of  the 

of  RJiamnus  (buckthorn),  showing  same      more      highly 

concentric  layers  of  wood.  magnified. 

which  varies  with  the  age  of  the  tree  (fig.  542).  The  composition  of 
the  several  rings,  which  are  the  sections  of  so  many  cylindrical 
layers,  is  uniformly  the  same,  however  different  their  thickness  ;  but 
the  arrangement  of  the  two  principal  elements — namely,  the  cellular 
and  the  vascular  tissue — varies  in  different  species,  the  vessels  being 
sometimes  almost  uniformly  diffused  through  the  whole  layer,  but  in 
other  instances  being  confined  to  its  inner  part ;  while  in  other 
cases,  again,  they  are  dispersed  with  a  certain  regular  irregularity 
(if  such  an  expression  may  be  allowed),  so  as  to  give  a  curiously 
figured  appearance  to  the  transverse  section  (figs.  542,  543).  The 
general  fact,  however,  is  that  the  vessels  predominate  towards  the 
inner  side  of  the  ring  (which  is  the  part  of  it  first  formed),  and  that 
the  outer  portion  of  each  layer  is  almost  exclusively  composed  of 
cellular  tissue.  Such  an  arrangement  is  shown  in  fig.  541.  This 
alternation  of  vascular  and  cellular  tissue  frequently  serves  to  mark 
the  succession  of  layers  when,  as  is  not  uncommon,  there  is  no  very 
distinct  line  of  separation  between  them. 


704    MICROSCOPIC   STRUCTURE    OF  PHANEROGAMIC    PLANTS 

The  number  of  layers  is  usually  considered  to  correspond  with 
that  of  the  years  during  which  the  stem  or  branch  has  been  growing  ; 
and  this  is,  no  doubt,  generally  true  in  regard  to  the  trees  of 
temperate  climates,  which  thus  ordinarily  increase  by  'annual  layers.' 
There  can  be  no  doubt,  however,  that  such  is  not  the  universal  rule  ; 
and  that  we  should  be  more  correct  in  stating  that  each  layer  indi- 
cates an  '  epoch  of  vegetation,'  which,  in  temperate  climates,  is  usually 
(but  riot  invariably)  a  year,  but  which  is  commonly  much  less  in  the 
case  of  trees  nourishing  in  tropical  regions.  Thus  among  the  latter 
it  is  very  common  to  find  the  leaves  regularly  shed  and  replaced 
twice  or  even  thrice  in  a  year,  or  five  times  in  two  years  ;  and  for 
every  crop  of  leaves  there  will  be  a  corresponding  layer  of  wood. 
It  sometimes  happens,  even  in  temperate  climates,  that  trees  shed 
their  leaves  prematurely  in  consequence  of  continued  drought,  and 
that,  if  rain  then  follow,  a  fresh  crop  of  leaves  appears  in  the  same 
season  ;  and  it  cannot  be  doubted  that  in  such  a  year  there  would 
be  two  rings  of  wood  produced,  which  would  probably  not  together 
exceed  the  ordinary  single  layer  in  thickness.  That  such  a  division 
may  even  occur  as  a  consequence  of  an  interruption  to  the  processes 
of  vegetation  produced  by  seasonal  changes — as  by  heat  and  drought 


FIG.  544. — Portion  of  transverse  section  of  stem  of  hazel,  showing,  in  the  portion 
a,  b,  c,  six  narrow  layers  of  wood. 

in  a  tree  that  flourishes  best  in  a  cold,  clamp  atmosphere,  or  by  a  fall 
of  temperature  in  a  tree  that  requires  heat— would  appeal-  from  the 
frequency  with  which  a  double  or  even  a  multiple  succession  of  rings 
is  found  in  transverse  sections  of  wood  to  occupy  the  place  of  a 
single  one.  Thus  in  a  section  of  hazel  stem  (in  the  Author's  posses- 
sion), of  which  a  portion  is  represented  in  fig.  544,  between  two 
layers  of  the  ordinary  thickness  there  intervenes  a  band  whose 
breadth  is  altogether  less  than  that  of  either  of  them,  and  which  is 
yet  composed  of  no  fewer  than  six  layers,  four  of  them  (c)  being  very 
narrow,  and  each  of  the  other  two  (a,  b)  being  about  as  wide  as 
these  four  together.  The  inner  rings  of  wood,  being  not  only  the 
oldest,  but  the  most  solidified  by  resinous  matters  deposited  within 
their  component  cells  and  vessels,  are  spoken  of  collectively  under 
the  designation  duramen  or  '  heart-wood.'  On  the  other  hand,  it  is 
through  the  cells  and  ducts  of  the  outer  and  newer  layers  that  the 
sap  rises  from  the  roots  towards  the  leaves  ;  and  these  are  conse- 
quently designated  as  alburnum  or  'sap-wood.'  The  line  of  demar- 
cation between  the  two  is  sometimes  very  distinct,  as  in  lignum  vitse 
and  cocos-wood  ;  and  as  a  new  ring  is  added  every  year  to  the  ex- 
terior of  the  alburnum,  an  additional  ring  of  the  innermost  part  of 
the  alburnum  is  every  year  consolidated  by  internal  deposit,  arid  is 


STRUCTURE   OF   STEMS 


705 


thus  added  to  the  exterior  of  the  duramen.  More  generally,  how- 
ever, this  consolidation  is  gradually  effected,  and  the  alburnum  and 
duramen  are  not  separated  by  any  abrupt  line  of  division. 

The  medullary  rays  which  cross  the  successive  rings  of  wood 
connecting  the  cellular  substance  of  the  pith  with  that  of  the  bark, 
and  dividing  each  ring  of  wood  into  wedge-shaped  segments,  are  thin 


FIG.  545. — Portion  of  transverse  section  of  the  stem  of  cedar :  a,  pith  ; 
b,  b,  b,  woody  layers  ;  c,  bark. 

plates  of  cellular  tissue  (fig.  541,  c,  c),  not  usually  extending  to  any 
great  depth  in  the  vertical  direction.  It  is  not  often,  however,  that 
their  character  can  be  so  clearly  seen  in  a  transverse  section  as  in 
the  diagram  just  referred  to  ;  for  they  are  usually  compressed  so 
closely  as  to  appear  darker  than  the  wedges  of  woody  tissue  between 
which  they  intervene  (figs.  543,  545),  and  their  real  nature  is  best 
understood  by  a  comparison  of  longitudinal  sections  made  in  two 
different  directions  —  namely, 
radial  and  tangential — with  the 
transverse.  Three  such  sec- 
tions of  a  fossil  coniferous  wood 
in  the  Author's  possession  are 
shown  in  figs.  546-548.  The 
stem  was  of  such  large  size  that, 
in  so  small  a  part  of  the  area  of 
its  transverse  section  as  is  re- 
presented in  fig  546,  the  medul- 
lary rays  seem  to  run  parallel  to 
each  other,  i  listen  d  of  radiating 
from  a  common  centre.  They  are  FIG.  546.— Portion  of  transverse  section  of 
very  narrow  ;  but  are  SO  closelv  large  stem  of  coniferous  wood  (fossil), 
set  together  that  only  two  oV  tt&*X£ftl&ff£ 
three  rows  of  trachei'ds  (no  numerous  medullary  rays. 

ducts  being  here  present)  in- 
tervene between  any  pair  of  them.  In  the  longitudinal  section 
taken  in  a  radial  direction  (fig.  547),  and  consequently  passing  in  the 
same  course  with  the  medullary  rays,  these  are  seen  as  thin  plates 
(a,  a,  a)  made  up  of  superposed  cells  very  much  elongated,  and 
crossing  in  a  horizontal  direction  the  trachei'ds  which  lie  parallel  to 
one  another  vertically.  And  in  the  tangential  section  (fig.  548), 

z  z 


706  MICROSCOPIC   STRUCTURE   OF  PHANEROGAMIC   PLANTS 


which  is  taken  in  a  direction  at  right  angles  to  that  of  the  medul- 
lary rays,  and  therefore  cuts  them  across,  we  see  that  each  of  the 


FIG.  547. — Portion  of  vertical  section  of  the 
same  wood,  taken  in  a  radial  direction, 
showing  the  trachei'ds  with  '  bordered 
pits,'  without  ducts,  crossed  by  the  medul- 
lary rays,  a,  a. 


FIG.  548. — Portion  of  vertical 
section  of  the  same  wood, 
taken  in  a  tangential  direc- 
tion, so  as  to  cut  across  the 
medullary  rays. 


plates  thus  formed  has  a  very  limited  depth  from  above  downwards, 
and  is  composed  of  no  more  than  one  thickness  of  cells  in  the 
horizontal  direction.  A  section  of  the  stem  of  mahogany  taken  in 
the  same  direction  as  the  last  (fig.  549) 
gives  a  very  good  view  of  the  cut  ends  of 
the  medullary  rays  as  they  pass  between 
the  prosenchymatous  cells ;  and  they  are 
seen  to  be  here  of  somewhat  greater  thick- 
ness, being  composed  of  twro  or  three  rows 
of  cells,  arranged  side  by  side. 

In  another  fossil  wood,  whose  transverse 
section  is  shown  in  fig.  550,  and  its  tan- 
gential section  in  fig.  551,  the  medullary 
rays  are  seen  to  occupy  a  much  larger  part 
of  the  substance  of  the  stem,  being  shown 
in  the  transverse  section  as  broad  bands 
(a  a,  a  a)  intervening  between  the  closely 
set  prosenchymatous  cells,  among  which 
some  large  ducts  are  scattered  ;  whilst  in 
the  tangential  section  they  are  observed 
to  be  not  only  deeper  than  the  preceding 
urnvi  v  M^V'MU^  i.  fr°m  above  downwards,  but  also  to  have 

,.  a  much  greater  thickness.     This  section 
FIG.  549. — Vertical  section   of     .          .      &  „   .,         , 

mahogany.  also  gives  an  excellent  vie\v  ot  the  ducts, 

b  6,  b  5,  which  are  here  plainly  seen  to  be 

formed  by  the  coalescence  of  large  cylindrical  cells  lying  end  to  end. 
In  another  fossil  wood  in  the  Author's  possession  the  medullary  rays 


STRUCTURE   OF   STEMS 


707 


constitute  a  still  larger  proportion  of  the  stem ;  for  in  the  transverse 
section  (fig.  552)  they  are  seen  as  very  broad  bands  (b,  6),  alternating 


FIG.  550. — Transverse  section  of  a 
fossil  wood,  showing  the  medullary 
rays,  a,  a,  a,  a,  a,  a,  running  nearly 
parallel  to  each  other,  and  the 
openings  of  large  ducts  in  the  midst 
of  the  prosenchymatous  tissue. 


Fi3.  551.— Vertical  (tangential)  sec- 
tion of  the  same  wood,  showing  the 
prosenchymatous  cells  separated 
by  the  medullary  rays,  and  by  the 
large  ducts,  b  &,  6  b. 


with  plates  of  woody  structure  (a  a),  whose  thickness  is  often  less 
than  their  own  ;  whilst  in  the  tangential  section  (fig.  553)  the  cut 


FIGS.  552  and  553. — Transverse  and  vertical  sections  of  a  fossil  wood, 
showing  the  separation  of  the  woody  plates,  a  a,  a  a,  by  the  very 
large  medullary  rays,  b  b,  b  b. 

extremities  of  the  medullary  rays  occupy  a  very  large  part  of 
the  area,  having  apparently  determined  the  sinuous  course  of  the 
prosenchymatous  cells,  instead  of  looking  (as  in  fig  548)  as  if  they 

zz  2 


708     MICROSCOPIC   STRUCTURE    OF   PHANEROGAMIC    PLANTS 

had  forced  their  way  between  these  cells,  which  there  hold  a  nearly 
straight  and  parallel  course  on  either  side  of  them.  The  medullary 
rays  maintain  a  connection  between  the  external  and  the  internal 
parts  of  the  cellular  tissue  or  fundamental  parenchyma  (also  called 
'ground-tissue')  of  the  stem,  which  have  been  separated  by  the 
interposition  of  the  wood. 

The  bark  is  usually  found  to  consist  of  three  principal  layers : 
the  external  or  epiphloeum,  which  includes  the  suberous  (or  corky) 
layer ;  the  middle,  or  mesophlceum,  also  termed  the  *  cellular  envelope  ; r 
and  the  internal,  or  endophlceum,  which  is  more  commonly  known 
as  the  liber.1  The  two  outer  layers  are  entirely  cellular,  and  are 
chiefly  distinguished  by  the  form,  size,  and  direction  of  their  cells. 
The  epiphlceum  is  generally  composed  of  one  or  more  layers  of  colour- 
less or  brownish  cells,  which  usually  present  a  cubical  or  tabular 
form,  and  are  arranged  with  their  long  diameters  in  the  horizontal 
direction  ;  it  is  this  which,  when  developed  to  an  unusual  thickness, 
forms  cork,  a  substance  which  is  by  no  means  the  product  of  one 
kind  of  tree  exclusively,  but  exists  in  greater  or  less  abundance  in 
the  bark  of  every  exogenous  stem.  The  mesophlceum  consists  of 
cells,  usually  containing  more  or  less  chlorophyll,  prismatic  in  their 
form,  and  disposed  with  their  long  diameters  parallel  to  the  axis  ;  it 
is  more  loosely  arranged  than  the  preceding,  and  contains  inter- 
cellular passages,  which  often  form  a  network  of  canals  which  have 
the  character  of  laticiferous  vessels ;  and,  although  usually  less 
developed  than  the  suberous  layers,  it  sometimes  constitutes  the 
chief  thickness  of  the  bark.  The  liber  or  '  inner  bark,'  on  the  other 
hand,  usually  contains  woody  fibre  in  addition  to  the  cellular  tissue 
and  laticiferous  canals  of  the  preceding  ;  and  thus  approaches  more 
nearly  in  its  character  to  the  woody  layers,  with  which  it  is  in  close 
proximity  on  its  inner  surface.  The  liber  may  generally  be  found  to 
be  made  up  of  a  succession  of  thin  layers,  equalling  in  number  those 
of  the  wood,  the  innermost  being  the  last  formed ;  but  110  such 
succession  can  be  distinctly  traced  either  in  the  cellular  envelope  or 
in  the  suberous  layer,  although  it  is  certain  that  they,  too,  augment 
in  thickness  by  additions  to  their  interior,  whilst  their  external  por- 
tions are  frequently  thrown  off  in  the  form  of  thickish  plates,  or 
detach  themselves  in  smaller  and  thinner  lamina?.  The  bark  is 
always  separated  from  the  wood  by  the  cambium  layer,  which  is  the 
part  wherein  all  new  growth  takes  place.  This  layer  seems  to  con- 
sist of  mucilaginous  semi-fluid  matter ;  but  it  is  really  made  up  of 
cells  of  a  very  delicate  texture,  which  gradually  undergo  transfor- 
mation, whereby  they  are  for  the  most  part  converted  into  tracheids, 
ducts,  spiral  vessels,  &c.  These  materials  are  so  arranged  as  to 
augment  the  fibro-vascular  bundles  of  the  wood  on  their  external 
surface,  thus  forming  a  new  layer  of  alburnum,  which  encloses  all 
those  that  preceded  it ;  whilst  they  also  form  a  new  layer  of  liber 
011  the  interior  of  all  those  which  preceded  it.  They  also  extend  the 
medullary  rays,  which  still  maintain  a  continuous  connection  between 
the  pith  and  the  bark  ;  and  a  portion  remains  unconverted,  so  as 

1  [The  term  'liber*  is  also  sometimes  applied  to  the  'phloem-portion'  of  a  fibro- 
vascular  bundle. — Eo.J 


STRUCTURE   OF   STEMS  709 

always  to  keep  apart  the  liber  and  the  alburnum.  This  type  of 
stem-structure  is  termed  exogenous ;  a  designation  which  applies 
very  correctly  to  the  mode  of  increase  of  the  woody  layers,  although 
(as  just  shown)  the  liber  is  formed  upon  a  truly  endogenous  plan. 

.Numerous  departures  from  the  normal  type  are  found  in  particu- 
lar tribes  of  dicotyledons.  Thus  in  some  the  wood  is  not  marked  by 
concentric  circles,  their  growth  not  being  interrupted  by  any  seasonal 
change.  In  other  cases,  again,  each  woody  zone  is  separated  from 
the  next  by  the  interposition  of  a  thick  layer  of  cellular  substance. 
Sometimes  \vood  is  formed  in  the  bark  (as  in  Calycanthus),  so  that 
several  woody  columns  are  produced,  which  are  quite  independent  of 
the  principal  woody  axis,  and  cluster  around  it.  Occasionally  the 
woody  stem  is  divided  into  distinct  segments  by  the  peculiar  thick- 
ness of  certain  of  the  medullary  rays,  and  in  the  stem,  of  which 
fig.  554  represents  a  transverse  section,  these  cellular  plates  form 


FIG.  554. — Transverse  section  of  the  FIG.  555. — Portion  of  transverse 

stem  of  a  climbing   plant   (Aristo-  section  of  Arctium  (burdock), 

locJiia  ?)  from  New  Zealand.  showing  one  of  the  fibro- vascu- 

lar bundles    that  lie  beneath 
the  cellular  epiderra. 

four  large  segments  disposed  in  the  manner  of  a  Maltese  cross,  and 
alternating  with  the  four  woody  segments,  which  they  equal  in  size. 
The  exogenous  stem,  like  the  (so-called)  endogenous,  consists,  in 
its  first -developed  state,  of  cellular  tissue  only ;  but  after  the  leaves 
have  been  actiyely  performing  their  function  for  a  short  time,  we 
find  a  circle  of  fibro-vascular  bundles,  as  represented  in  fig.  540, 
interposed  between  the  central  (or  medullary)  and  the  peripheral 
(or  cortical)  portions  of  the  fundamental  tissue,  these  fibro-vascular 
bundles  being  themselves  separated  from  each  other  by  plates  of 
cellular  tissue,  which  still  remain  to  connect  the  central  and  the 
peripheral  portions  of  that  tissue.  This  first  stage  in  the  formation 
of  the  exogenous  axis,  in  which  its  principal  parts — the  pith,  wood, 
bark,  and  medullary  rays — are  marked  out,  is  seen  even  in  the 
stems  of  herbaceous  plants,  which  are  destined  to  die  down  at  the 
end  of  the  season  (fig.  555) ;  and  sections  of  these,  which  are  very 


710   MICROSCOPIC   STRUCTURE    OF  PHANEROGAMIC   PLANTS 

easily  prepared,  are  most  interesting  microscopic  objects.  In  such 
stems  the  difference  between  the  endogenous  and  the  exogenous 
types  is  manifested  in  little  else  than  the  disposition  of  the  fibro- 
vascular  layers  which  are  scattered  through  nearly  the  whole  of 
the  fundamental  tissue  (although  more  abundant  towards  its 
exterior)  in  the  former  case,  but  are  limited  to  a  circle  within  the 
peripheral  portion  of  the  cellular  tissue  in  the  latter.  It  is  in  the 
further  development  which  takes  place  during  succeeding  years  in 
the  woody  stems  of  perennial  exogens  that  those  characters  are 
displayed  which  separate  them  most  completely  from  the  ferns  and 
their  allies,  whose  stems  contain  a  cylindrical  layer  of  fibro-- vascular 
bundles,  as  well  as  from  (so-called)  endogens.  For  whilst  the  fibro- 
vascular  layers  of  the  latter,  when  once  formed,  undergo  no  further 
increase,  those  of  exogenous  stems  are  progressively  augmented  on 
their  outer  side  by  the  metamorphosis  of  the  cambium  layer  ;  so 
that  each  of  the  bundles  which  once  lay  as  a  mere  series  of  parallel 
cords  beneath  the  cellular  epiderm  of  a  first-year's  stem,  may  become 
in  time  the  small  end  of  a  wedge-shaped  mass  of  wood  extending 
continuously  from  the  centre  to  the  exterior  of  a  trunk  of  several 
feet  in  diameter,  and  becoming  progressively  thicker  as  it  passes 
upwards.  The  fibro- vascular  bundles  of  exogens  are  therefore 
spoken  of  as  '  indefinite '  or  open,  whilst  those  of  endogens  and 
vascular  cryptogams  (ferns,  &c.)  are  said  to  be  '  definite '  or  closed. 
The  open  nbro-vascular  bundles  of  exogens  and  of  gymnosperms 
may  be  stated  to  consist  of  three  distinct  parts :  the  xylem  portion, 
which  consists  chiefly  of  ducts,  of  the  nature  of  spiral,  annular,  or 
pitted  vessels,  and  which  is  the  portion  of  the  bundle  nearest  to  the 
centre  of  the  organ ;  the  phloem  or  '  bast '  portion,  which  consists 
largely  of  prosenchymatous  cells,  among  which  are  almost  always 
sieve-tubes  with  their  sieve  plates,  and  which  is  the  peripheral 
portion  of  the  bundle ;  while  between  them  is  the  formative  cam- 
bium, from  which  fresh  xylem  is  constantly  being  formed  on  one 
side,  fresh  phloem  on  the  other  side.  The  closed  bundles  of 
endogens  and  of  vascular  cryptogams  consist  of  xylem  and  phloem 
only.  When  the  xylem  and  phloem  portions  of  fibro-vascular 
bundle  lie  side  by  side,  as  is  usually  the  case,  the  bundle  is  said  to 
be  collateral ;  when  either  portion  encloses  the  other  like  a  cylinder, 
it  is  concentric. 

The  structure  of  the  roots  of  endogens  and  exogens  is  essentially 
the  same  in  plan  as  that  of  their  respective  stems.  Generally 
speaking,  however,  the  roots  of  exogens  have  no  pith,  although  they 
have  medullary  rays ;  and  the  succession  of  distinct  rings  is  less 
apparent  in  them  than  it  is  in  the  stems  from  which  they  diverge. 
In  the  delicate  branches  which  proceed  from  the  larger  root-fibres 
a  central  bundle  of  vessels  will  be  seen  enveloped  in  a  sheath  of 
cellular  substance ;  and  this  investment  also  covers  in  the  end  of 
the  branch,  which  is  usually  somewhat  dilated,  and  is  furnished  at 
its  extremity  with  one  or  more  layers  of  cells,  which  are  constantly 
being  thrown  off,  known  as  the  pileorhiza  or  root-cap.  The  structure 
of  the  branches  of  the  root  may  be  well  studied  in  the  common 
buckweed,  every  floating  leaf  of  which  has  a  single  root  hanging  down 


STEUCTUEE   OF   STEMS   AND   EOOTS  711 

from  its  lower  surface.  The  central  fibre-vascular  cylinder,  which  is 
characteristic  of  the  finer  roots  of  exogens,  as  well  as  of  endogens,  is 
surrounded  by  a  single  layer  of  cells  very  clearly  differentiated  from 
the  surrounding  fundamental  tissue,  known  as  the  bundle-sheath. 
We  have  already  seen  the  peculiar  form  assumed  by  the  bundle- 
sheath  in  the  stem  of  ferns  and  other  vascular  cryptogams. 

The  structure  of  stems  and  roots  cannot  be  thoroughly  examined 
in  any  other  way  than  by  making  sections  in  different  directions 
with  the  microtome.  The  general  instructions  already  given  leave 
little  to  be  added  respecting  this*  special  class  of  objects,  the  chief 
points  to  be  attended  to  being  the  preparation  of  the  stems,  &c.  for 
slicing,  the  sharpness  of  the  knife,  and  the  dexterity  with  which  it 
is  handled,  and  the  method  of  mounting  the  sections  when  made. 
The  wood,  if  green,  should  first  be  soaked  in  strong  alcohol  for  a 
few  days,  to  get  rid  of  the  resinous  matter ;  and  it  should  then  be 
macerated  in  water  for  some  days  longer  for  the  removal  of  ite 
gum,  before  being  submitted  to  the  cutting  process.  If  the  wood 
be  dry,  it  should  first  be  softened  by  soaking  for  a  sufficient  length 
of  time  in  water,  and  then  treated  with  spirit,  and  afterwards  with 
water,  like  green  wood.  Some  woods  are  so  little  affected  even  by 
prolonged  maceration  that  boiling  in  water  is  necessary  to  bring 
them  to  the  degree  of  softness  requisite  for  making  sections.  No 
wood  that  has  once  been  dry,  however,  yields  such  good  sections  as 
that  which  is  cut  fresh.  When  a  piece  of  appropriate  length 
lias  been  placed  in  the  grasp  of  the  section  instrument  (wedges  of 
deal  or  other  soft  wood  being  forced  in  with  it,  if  necessary  for  its 
firm  fixation),  a  few  thick  slices  should  first  be  taken,  to  reduce  its 
surface  to  an  exact  level ;  the  surface  should  then  be  wetted  with 
spirit,  the  micrometer-screw  moved  through  a  small  part  of  a  revo- 
lution, and  the  slice  taken  off  with  the  razor,  the  motion  given  to 
which  should  partake  both  of  drawing  and  pushing.  A  little  prac- 
tice will  soon  enable  the  operator  to  discover  in  each  case  how  thin 
he  may  venture  to  cut  his  sections  without  a  breach  of  continuity, 
and  the  micrometer-screw  should  be  turned  so  as  to  give  the  required 
elevation.  If  the  surface  of  the  wood  has  been  sufficiently  wetted, 
the  section  will  not  curl  up  in  cutting,  but  will  adhere  to  the  sur- 
face of  the  razor,  from  which  it  is  best  detached  by  dipping  the 
razor  in  water  so  as  to  float  away  the  slice  of  wood,  a  camel-hair 
pencil  being  used  to  push  it  off  if  necessary.  All  the  sections  that 
may  be  found  sufficiently  thin  and  perfect  should  be  put  aside  in  a 
bottle  of  weak  spirit  until  they  be  mounted.  For  the  minute  exami- 
nation of  their  structure,  they  may  be  mounted  either  in  weak 
spirit  or  in  glycerin-jelly.  Where  a  mere  general  view  only  is  needed, 
dry  mounting  answers  the  purpose  sufficiently  well ;  and  there  are 
many  stems,  such  as  that  of  Clematis,  of  which  transverse  sections 
rather  thicker  than  ordinary  make  very  beautiful  opaque  objects 
when  mounted  dry  on  a  black  ground.  Canada  balsam  should  not 
be  had  recourse  to,  except  in  the  case  of  very  opaque  sections,  as  it 
usually  makes  the  structure  too  transparent.  Transverse  sections, 
however,  when  slightly  charred  by  heating  between  two  plates  of 
glass  until  they  turn  brown,  may  be  mounted  with  advantage  in 


712     MICKOSCOPIC   STRUCTURE   OF  PHANEROGAMIC   PLANTS 


Canada  balsam,  and  are  then  very  showy  specimens  for  the  gas- 
microscope.  The  number  of  beautiful  and  interesting  objects  which 
may  be  thus  obtained  from  even  the  commonest  trees,  shrubs,  and 
herbaceous  plants  at  the  cost  of  a  very  small  amount  of  trouble 
can  scarcely  be  conceived  save  by  those  who  have  specially  attended 
to  these  wonderful  structures ;  and  a  careful  study  of  sections 
made  in  different  parts  of  the  stem,  especially  in  the  neighbourhood 
of  the  'growing  point,'  will  reveal  to  the  eye  of  the  physiologist 
some  of  the  most  important  phenomena  of  vegetation.  The  judi- 
cious use  of  the  staining  process  not  only  improves  the  appearance  of 
such  sections,  but  adds  greatly  to  their  scientific  value.  Fossil 
woods,  when  well  preserved,  are  generally  silici/ied,  and  can  only 
be  cut  and  polished  by  a  lapidary's  wheel.  Should  the  microscopist 
be  fortunate  enough  to  meet  with  a  portion  of  a  calcified  stem  in 
which  the  organic  structure  is  preserved,  he  should  proceed  with  it 


FIG.  556. — Epiderm  of  leaf  of 
Yucca,  showing  stomates. 


FIG.  557. — Epiderm  of  leaf  of  Indian 
corn  (Zea  Mais),  showing  stomates. 


after  the  manner  of  other  hard  substances  -which  need  to  be  reduced 
by  grinding. 

Epiderm  of  Leaves. — On  all  the  softer  parts  of  the  higher  plants, 
save  such  as  grow  under  water,  we  find  a  surface  layer  differing  in 
its  texture  from  the  parenchyme  beneath,  and  constituting  a  dis- 
tinct membrane,  known  as  the  epiderm.  This  membrane  is  composed 
of  cells,  the  walls  of  which  are  flattened  above  and  below,  whilst 
they  adhere  closely  to  each  other  laterally,  so  as  to  form  a  continuous 
stratum  (figs.  560,  562,  a,  a).  The  shape  of  these  cells  is  different  in 
almost  every  tribe  of  plants ;  thus  in  the  epiderm  of  the  Yucca  (fig. 
556),  Indian  corn  (fig.  557),  Iris  (fig.  561),  and  most  other  mono- 
cotyledons, they  are  elongated,  and  present  an  approach  to  a 
rectangular  contour,  their  margins  being  straight  in  the  Yucca 
and  Iris,  but  minutely  sinuous  or  crenated  in  the  Indian  corn. 
In  most  dicotyledons,  on  the  other  hand,  the  cells  of  the  epiderm 
depart  less  from  the  rounded  form,  but  their  margins  usually 
exhibit  large  irregular  sinuosities,  so  that  they  seem  to  fit  together 


STRUCTURE   OF   LEAVES 


713 


like  the  pieces  of  a  dissected  map,  as  is  seen  in  the  epiderm  of  the  apple 
(fig.  558,  b,  b).  Even  here,  however,  the  cells  of  that  portion  of  the 
epiderm  («,  a)  which  overlies  the  *  veins  '  of  the  leaf  have  an  elongated 
form,  approaching  that  of  the  wood-cells  of  which  these  veins  are 
chiefly  composed ;  and  it  seems  likely,  therefore,  that  the  elongation 
of  the  ordinary  epiderm  cells  of  monocotyledons  has  reference  to 


FIG.  558. — Portion  of  epiderm  of  lower  surface  of  leaf  of  apple, 
with  layer  of  parenchyme  in  immediate  contact  with  it : 
a,  a,  elongated  cells  overlying  the  veins  of  the  leaf;  &,  6, 
ordinary  epiderm-cells,  overlying  the  parenchyme;  c,  c, 
stomates ;  d,  d,  green  cells  of  the  spongy  parenchyme, 
forming  a  very  open  network  near  the  lower  surface  of  the 
leaf. 

that  parallel  arrangement  of  the  veins  which  their  leaves  almost 
constantly  exhibit. 

The  cells  of  the  epiderm  are  colourless,  or  nearly  so,  having  no  or 
but  little  chlorophyll  in  their  interior  ;  and  their  walls  are  generally 
A  B 


FIG.  559. — Portion  of  epiderm  of  upper  surface  of  leaf  of 
Rochea  falcata,  as  seen  at  A  from  its  inner  side,  and  at  B 
from  its  outer  side :  «,  a,  small  cells  forming  inner  layer ; 
&,  b,  large  prominent  cells  of  outer  layer ;  c,  c,  stomates  dis- 
posed between  the  latter. 

thickened  by  secondary  deposit,  especially  on  the  side  nearest  the 
atmosphere.  This  outermost  hardened  continuous  w^all  of  the 
epidermal  layer  of  cells  is  known  as  the  cuticle.  The  deposit  (cutin) 
is  of  a  nature  to  render  the  membrane  very  impermeable  to  fluids, 
so  as  to  protect  the  soft  tissue  of  the  leaf  from  drying  up.  In  most 


714     MICROSCOPIC    STRUCTURE    OF   PHANEROGAMIC   PLANTS 

European  plants  the  epiderm  consists  of  but  a  single  row  of  cells, 
\vhich,  moreover,  are  usually  thin-walled  ;  whilst  in  the  generality 
of  tropical  species  there  exist  two,  three,  or  even  four  layers  of 
thick-walled  cells,  this  last  number  being  seen  in  the  oleander,  the 
epiderm  of  which,  when  separated,  has  an  almost  leathery  firmness. 
This  difference  in  conformation  is  obviously  adapted  to  the  conditions 
of  growth  under  which  these  plants  respectively  exist ;  since  the 
epiderm  of  a  plant  indigenous  to  temperate  climates  would  not  afford 
a  sufficient  protection  to  the  interior  structure  against  the  rays  of  a 
tropical  sun  ;  whilst  the  less  powerful  heat  of  this  country  would 
scarcely  overcome  the  resistance  presented  by  the  dense  and  non- 
conducting integument  of  a  species  formed  to  exist  in  tropical 
climates. 

A  very  curious  modification  of  the  epiderm  is  presented  by 
Kochea  falcata,  which  has  the  surface  of  its  ordinary  epiderm  (figs. 
559,  560,  #,  a)  nearly  covered  with  a  layer  of  large  prominent 
isolated  cells,  b,  b.  A  somewhat  similar  structure  is  found  in 
Mesembryanthemumcrystallinum,  commonly  known  as  the  'ice-plant,' 
a  designation  it  owes  to  the  peculiar  appearance  of  its  surface, 
which  looks  as  if  it  were  covered  with  frozen  dewdrops.  In  other 
instances  the  epiderm  is  partially  invested  by  a.  layer  of  scales, 
which  are  nothing  else  than  flattened  hairs,  often  having  a  very 
peculiar  form  ;  the  '  peltate  scales  '  of  Elceagnus  and  other  shrubs 
and  herbs  are  very  beautiful  objects  under  the  microscope.  In 

numerous  other  cases,  again, 
we  find  the  surface  beset  with 
true  hairs,  which  occasionally 
consist  of  single  elongated 
cells,  but  are  more  commonly 
made  up  of  a  linear  series, 
attached  end  to  end.  Some- 
times these  hairs  bear  little 
glandular  bodies  at  their  ex- 
PIG.  560.— Portion  of  vertical  section  of  leaf  tremities,  by  the  secretion  of 
of  Rochea,  showing  the  small  cells,  a,  a,  which  a  peculiar  viscidity  is 
ce.StroVtKutLr^otoftf:  pv*nto  the  aur&ce  of  the  leaf, 
stomates ;  d,  d,  cells  of  the  parenchyme  ;  stem,  Or  flower-stalk,  as  in 
L  cavity  between  the  parenchymatous  many  kinds  of  rose,  geranium, 
cells  into  which  the  stomate  opens.  ^  ^  ^^  [nsi°nce^  the 

hair  has  a  glandular   body  at 

its  base,  containing  a  peculiar  secretion  ;  when  this  secretion  is  of 
an  irritating  quality,  as  in  the  nettle,  it  constitutes  a  *  sting/  A 
great  variety  of  such  organs  may  be  found  by  a  microscopic 
examination  of  the  surface  of  the  leaves  of  plants  having  any 
kind  of  superficial  investment  to  the  epiderm.  Many  connecting 
links  present  themselves  between  hairs  and  scales,  such  as  the 
stellate  hairs  of  Deutzia  scabra,  which  a  good  deal  resemble  those 
within  the  air  chambers  of  the  yellow  water-lily  (fig.  527).  The  so- 
called  '  glands '  or  '  tentacles '  of  the  sundew  (Drosera)  are  not 
really  hairs,  but  outgrowths  of  the  internal  tissue  of  the  leaf,  each 
being  penetrated  by  a  fibro-vascular  bundle. 


STRUCTURE   OF  LEAVES 


The  epiderm  in  many  plants,  especially  those  belonging  to  the 
grass  tribe,  has  its .  cell- walls  impregnated  with  silex,  like  that  of 
Equisetum  ;  so  that,  when  the  organic  matter  seems  to  have  been 
got  rid  of  by  heat  or  by  acids,  the  forms  of  the  epidermal  cells,  hairs, 
stomates,  &c.,  are  still  marked  out  in.  silex,  and  (unless  the  dissipa- 
tion of  the  organic  matter  has  been  most  perfectly  accomplished) 
are  most  beautifully  displayed  by  polarised  light.  Such  silicified 
epiderms  are  found  in  the  husks  of  the  grains  yielded  by  these  plants  ; 
and  there  is  none  in  which  a  larger  proportion  of  mineral  matter 
exists  than  that  of  rice,  which  contains  some  curious  elongated  cells 
with  toothed  margins.  The  hairs  with  which  thepalew  (chaff-scales) 
of  most  grasses  are  furnished  are  strengthened  by  the  like  siliceous 
deposit ;  and  in  Festuca  pratensis,  one  of  the  common  meadow- 
grasses,  the'paleaB  are  also  beset  with  longitudinal  rows  of  little  cup- 
like  bodies  formed  of  silex.  The  epiderm  and  scaly  hairs  of  Deutzia 
scabra  also  contain  a  large  quantity  of  silex,  and  are  remarkably 
beautiful  objects  for  the  polariscope. 

In  nearly  all  plants  which  possess  a  distinct  epiderm,  this  is 
perforated  by  the  minute  openings  termed  stomates  (figs.  557,  561), 
which  are  bordered  by  cells  of  a  peculiar  form,  the  guard-cells, 
differing  from  those  of  the  epiderm,  and  more  resembling  in  character 
those  of  the  tissue  beneath. 
They  are  further  distinguished 
by  containing  a  larger  number 
of  chlorophyll-grains  than  the 
ordinary  cells  of  the  epiderm. 
These  guard-cells  are  usually 
somewhat  kidney-shaped,  and 
lie  in  pairs  (fig.  561,  b),  with 
an  oval  opening  between  them  ; 
but  by  an  alteration  in  their 
form,  the  opening  may  be  con- 
tracted or  nearly  closed.  In 
the  epiderm  of  Yucca,  however, 
the  opening  is  bounded  by  two 
pairs  of  cells,  and  is  somewhat 
quadrangular  (fig.  556) ;  and  a 
like  doubling  of  the  guard- 
cells,  with  a  narrower  slit  be- 
tween them,  is  seen  in  the  epi- 
derm of  the  Indian  corn  (fig. 
557).  In  the  stomates  of  no 
phanerogam,  however,  do  we 


germanica  torn  from  its  surface, 
carrying  away  with  it  a  portion  of  the 
parenchymatous  layer  in  immediate  con- 
tact with  it :  a,  a,  elongated  cells  of  the 
epiderm ;  b,  6,  cells  of  the  stomates  ;  c,  c, 
cells  of  the  pai'eiichyme  ;  d,  d,  impressions 
on  the  epidermal  cells  formed  by  their 
contact ;  e,  cavity  in  the  parenchyme,  cor- 
responding to  the  stomate. 


meet  with  any  conformation  at  all  to  be  compared  in  complexity 
with  that  which  has  been  described  in  the  humble  Marchantia. 
Stomates  are  usually  found  most  abundantly  (and  sometimes  exclu- 
sively) in  the  epiderm  of  the  lower  surface  of  leaves,  where  they  open 
into  the  air-chambers  that  are  left  in  the  parenchyme  which  lies 
next  the  inferior  epiderm  ; '  in  leaves  which  float  on  the  surface  of 
water,  however,  they  are  found  in  the  epiderm  of  the  upper  surface 
only  ;  whilst  in  leaves  that  habitually  live  entirely  submerged,  as 


716   MICROSCOPIC   STRUCTURE   OF   PHANEROGAMIC   PLANTS 

there  is  no  distinct  epiderm,  so  there  are  no  stomates.  In  the  erect 
leaves  of  grasses,  the  Iris  tribe,  &c.,  they  are  found  equally  (or  nearly 
so)  on  both  surfaces.  As  a  general  fact,  they  are  least  numerous  in 
succulent  plants,  whose  moisture,  obtained  in  a  scanty  supply,  is 
destined  to  be  retained  in  the  system  ;  whilst  they  abound  most  in 
those  which  exhale  fluid  most  readily,  and  therefore  absorb  it  most 
quickly.  It  has  been  estimated  that  no  fewer  than  160,000  are  con- 
tained in  every  square  inch  of  the  under  surface  of  the  leaves  of 
Hydrangea  and  of  several  other  plants,  the  greatest  number  seem- 
ing always  to  be  present  where  the '  upper  surface  of  the  leaves  is 
entirely  destitute  of  these  organs.  In  Iris  germanica  each  surface 
has  nearly.  12, 000  stomates  in  every  square  inch  ;  and  in  Yucca  each 
surface  has  40,000.  In  the  oleander,  Banksia,  and  some  other  plants, 
the  stomates  do  not  open  directly  upon  the  lower  surface  of  the 
epiderm,  but  lie  in  the  deepest  part  of  little  pits  or  depressions, 
which  are  excavated  in  it  and  lined  with  hairs  ;  the  mouths  of  these 
pits,  with  the  hairs  that  line  them,  are  well  brought  into  view  by 
taking  a  thin  slice  from  the  surface  of  the  epiderm  with  a  sharp 
knife  ;  but  the  form  of  the  cavities  and  the  position  of  the  stomates 
can  only  be  well  made  out  in  vertical  sections  of  the  leaves. 

The  internal  structure  of  Leaves  is  best  brought  into  view  by 
making  vertical  sections,  traversing  the  two  layers  of  epiderm  and 
the  intermediate  cellular  parenchyme  ;  portions  of  such  sections  are 
shown  in  figs.  560,  562,  and  563.  In  close  apposition  with  the  cells 

of  the  upper  epiderm  (fig. 
562,  a,  a),  which  may  or  may 
not  be  perforated  with  the 
stomates  (c,  c,  d,  d),  we  find  a 
layer  of  soft,  thin- walled  cells, 
with  their  longest  diameter 
at  right  angles  to  the  surface 
of  the  leaf,  and  containing 
a  "large  quantity  of  chloro- 
phyll ;  these  generally  pi-ess 

FIG.  562. — Vertical  section  of  epiderm  and  of     Qr.    plnQplv    r^o    o0-mn«d-    -m 
portion  of  subjacent  parenchyme  of  leaf  of     b°    plOSely         i     •    g    • 
Iris  germanica  taken  in  a  transverse  direc-      other    that    their    Sides     be- 
tion  :  a,  a,  cells  of  epiderm;  6,  b,  cells  at  the     come  mutually  flattened,  and 
sides  of  the  stomates ;  c,  c,  guard-cells ;  d,d,  eg  are  left    saye     h 

openings  of  the  stomates;  e,  e,  cavities  in  the  "  .  .      '. 

parenchyme  into  which  the  stomates  open ;     there  is  a denmte air-chamber 
f,  f,  cells  of  the  parenchyme.  into  which  the  stomate  opens 

(fig.  562,  e)  ;  and  the  com- 
pactness of  this  superficial  layer  is  well  seen  when,  as  often  happens,  it 
adheres  so  closely  to  the  epiderm  as  to  be  carried  away  with  this  when 
it  is  torn  off  (fig.  561,  c,  c).  This  layer,  usually  peculiar  to  the  upper 
surface  of  leaves,  is  known  as  the  palisade-par enchyme.  Beneath 
this  first  layer  of  leaf-cells  there  are  usually  several  others  rather 
less  compactly  arranged  ;  and  the  tissue  gradually  becomes  more 
and  more  lax,  its  cells  not  being  in  close  apposition,  and  large  inter- 
cellular passages  being  left  amongst  them,  until  we  reach  the  lower 
epiderm,  which  the  parenchyme  only  touches  at  certain  points,  its 
lowest  layer  forming  a  sort  of  network,  the  so-called  spongy  paren- 


STRUCTURE   OF  LEAVES  717 

chyme  (fig.  558,  d,  d),  with  large  interspaces,  into  which  the  stomates 
open.  It  is  to  this  arrangement  that  the  darker  shade  of  green 
almost  invariably  presented  by  the  upper  surface  of  leaves  is  prin- 
cipally due,  the  colour  of  the  component  cells  of  the  parenchyme 
not  being  deeper  in  one  part  of  the  leaf  than  in  another.  In  those 
plants,  however,  whose  leaves  are  erect  instead  of  being  horizontal, 
so  that  their  two  surfaces  are  equally  exposed  to  light,  the  paren- 
chyme is  arranged  on  both  sides  in  the  same  manner,  and  their 
epiderms  are  furnished  with  an^qual  number  of  stomates.  This  is 
the  case,  for  example,  with  the  Cleaves  of  the  common  garden  Iris 
(fig.  563),  in  which,  moreover,  we  find  a  central  portion  (d,  d) 
formed  by  thick-walled  colourless  tissue,  very  different  either  from 
ordinary  leaf-cells  or  from  woody  fibre.  The  explanation  of  its 
presence  is  to  be  found  in  the  peculiar  conformation  of  the  leaves  ; 
for  if  we  pull  one  of  them  from  its  origin,  we  shall  find  that  what 
appears  to  be  the  flat  expanded  blade  really  exposes  but  half  its 
surface,  the  blade  being  doubled  together  longitudinally,  so  that 
what  may  be  considered  its  under  surface  is  entirely  concealed. 


FIG.  563.— Portion  of  vertical  longitudinal  section  of  leaf 
of  Iris,  extending  from  one  of  its  flattened  sides  to  the 
other  :  a,  a,  elongated  cells  of  epiderm  ;  b,  b,  stomata  cut 
through  longitudinally ;  c,  c,  green  cells  of  parenchyme ; 
d,  d,  colourless  tissue,  occupying  interior  of  leaf. 

The  two  halves  are  adherent  together  at  their  upper  part ;  but  at 
their  lower  they  are  commonly  separated  by  a  new  leaf  which  comes 
up  between  them  ;  and  it  is  from  this  arrangement,  which  resembles 
the  position  of  the  legs  of  a  man  on  horseback,  that  the  leaves  of 
the  Iris  tribe  are  said  to  be  equitant.  Now  by  tracing  the  middle 
layer  of  colourless  cells,  d,  d,  down  to  that  lower  portion  of  the  leaf 
where  its  two  halves  diverge  from  one  another,  we  find  that  it  there 
becomes  continuous  with  the  epiderm,  to  the  cells  of  which  (fig.  563, 
a)  these  bear  a  strong  resemblance  in  every  respect,  save  the  greater 
proportion  of  their  breadth  to  their  length.  Another  interesting 
variety  in  leaf-structure  is  presented  by  the  water-lily  and  other 
plants  whose  leaves  float  on  the  surface ;  for  here  the  usual  arrange- 
ment is  entirely  reversed,  the  closely  set  layers  of  green  leaf-cells 
being  found  in  contact  with  the  lower  surface,  whilst  all  the  upper 
part  of  the  leaf  is  occupied  by  a  loose  spongy  parenchyme,  containing 
a  very  large  number  of  air-spaces  that  give  buoyancy  to  the  leaf ; 
and  these  spaces  communicate  with  the  external  air  through  the 


7l8   MICROSCOPIC    STRUCTURE   OF   PHANEROGAMIC   PLANTS 

numerous  stomates,  which,  contrary  to  the  general  rule,  are  here 
found  in  the  upper  epiderm  alone. 

The  examination  of  the  foregoing  structures  is  attended  with 
very  little  difficulty.  Many  epiderms  may  be  torn  off,  by  the  exer- 
cise of  a  little  dexterity,  from  the  surfaces  of  the  leaves  they 
invest  without  any  preparation ;  this  is  especially  the  case  with 
monocotyledons  generally,  the  veins  of  whose  leaves  run  parallel, 
and  with  such  dicotyledons  as  have  very  little  woody  structure  in 
their  leaves.  In  those,  on  the  other  hand,  whose  leaves  are  furnished 
with  reticulated  veins  to  which  the  epiderm  adheres  (as  is  the  case  in 
by  far  the  larger  proportion),  this  can  only  be  detached  by  first 
macerating  the  leaf  for  a  few  days  in  water ;  and  if  their  texture 
is  particularly  firm,  the  addition  of  a  few  drops  of  nitric  acid  to 
the  water  will  render  their  epiderms  more  easily  separable.  Epi- 
derms may  be  advantageously  mounted  either  in  weak  spirit  or  in 
glycerin-jelly.  Very  good  sections  of  most  leaves  may  be  made  by 
a  sharp  knife,  handled  by  a  careful  manipulator  ;  but  it  is  generally 
preferable  to  use  the  microtome,  placing  the  leaf  between  two  pieces 
either  of  very  soft  cork  or  of  elder- pith  or  carrot,  or  imbedding  it  in 
paraffin.  In  order  to  study  the  structure  of  leaves  with  the  fulness 
that  is  needed  for  scientific  research,  numerous  sections  should  be 
made  in  different  directions,  and  slices  taken  parallel  to  the  surfaces 
at  different  distances  from  them  should  also  be  examined.  There  is 
no  known  medium  in  which  such  sections  can  be  preserved  altogether 
without  change ;  but  some  one  of  the  methods  formerly  described 
will  generally  be  found  to  answer  sufficiently  well. 

Flowers. — Many  small  flowers,  when  looked  at  entire  with  a  low 
magnifying  power,  are  very  striking  microscopic  objects  ;  and  the 
interest  of  the  young  in  such  observations  can  scarcely  be  better 

excited  than  by  directing  their 
attention  to  the  new  view  they 
thus  acquire  of  the  '  composite ' 
nature  of  the  humble  down- 
trodden daisy,  or  to  the  beauty 
of  the  minute  blossoms  of  many 
of  those  umbelliferous  plants 
which  are  commonly  regarded 
only  as  rank  weeds.  The 
scientific  microscopist,  how- 
ever, looks  more  to  the  organi- 
sation of  the  separate  parts  of 
FIG.  564. — Cells  from  petal  of  the  flower  ;  and  among  these 

Pelargonium.  }ie  nncis    abundant    sources    of 

gratification,  not  merely  to  his 

love  of  knowledge,  but  also  to  his  taste  for  the  beautiful.  The  general 
structure  of  the  sepals  and  petals,  which  constitute  the  perianth,  or 
floral  envelope,  closely  corresponds  to  that  of  leaves.  The  petals 
seldom  contain  unchanged  chlorophyll  ;  but  usually  either  the 
chlorophyll  in  the  petals  (and  sometimes  also  in  the  sepals)  is 
changed  into  a  solid  yellow  pigment  (carotin?) ;  or  the  chlorophyll 
lias  entirely  disappeared,  and  is  replaced  by  a  pigment,  blue,  red, 


STRUCTURE    OF   FLOWERS  719 

purple,  or  some  other  bright  colour,  anthocyan,  erytlirophyll,  tfec., 
dissolved  in  the  cell-sap.  There  are  some  petals  whose  cells  exhibit 
very  interesting  peculiarities,  either  of  form  or  marking,  in  addition 
to  their  distinctive  coloration;  such  are  those  of  the  Pelargonium, 
of  which  a  small  portion  is  represented  in  fig.  564.  The  different 
portions  of  this  petal — when  it  has  been  dried  after  stripping  it  of 
its  epiderra,  immersed  for  an  hour  or  two  in  oil  of  turpentine,  and 
then  mounted  in  Canada  balsam — exhibit  a  most  beautiful  variety 
of  vivid  coloration,  which  is  seen  to  exist  chiefly  in  the  thickened 
partitions  of  the  cells  ;  whilst  the  surface  of  each  cell  presents  a 
very  curious  opaque  spot  with  numerous  diverging  prolongations. 
This  method  of  preparation,  however,  does  not  give  a  true  idea  of 
the  structure  of  the  cells ;  for  each  of  them  has  a  peculiar  mammil- 
lary  protuberance,  the  base  of  which  is  surrounded  by  hairs  ;  and 
this  it  is  which  gives  the  velvety  appearance  to  the  surface  of  the 
petal,  and  which,  when  altered  by  drying  and  compression,  occa- 
sions the  peculiar  spots  represented  in  fig.  564.  Their  real  character 
may  be  brought  into  view  by  Dr.  Inman's  method,  which  consists 
in  drying  the  petal  (when  stripped  of  its  epiderm)  on  a  slip  of  glass, 
to  which  it  adheres,  and  then  placing  on  it  a  little  Canada  balsam 
diluted  with  turpentine,  which  is  to  be  boiled  for  an  instant  over 
the  spirit  lamp,  after  which  it  is  to  be  covered  with  a  thin  glass. 
The  boiling  '  blisters '  it,  but  does  not  remove  the  colour ;  and  on 
examination  many  of  the  cells  will  be  found  showing  the  mammilla 
very  distinctly,  with  a  score  of  hairs  surrounding  its  base,  each  of 
these  slightly  curved,  and  pointing  towards  the  apex  of  the  mammilla. 
The  petal  of  the  common  scarlet  pimpernel  (Anagallis  arvensis), 
that  of  the  common  chick  weed  (Stellaria  media) ,  together  with  many 
others  of  a  small  and  delicate  character,  are  also  very  beautiful 
microscopic  objects;  and  the  two  just  named  are  peculiarly  favour- 
able subjects  for  the  examination  of  the  spiral  vessels  in  their  natural 
position.  For  the  *  veins  '  which  traverse  these  petals  are  entirely 
made  up  of  spiral  vessels,  none  of  which  individually  attain  any 
great  length,  but  one  follows  or  takes  the  place  of  another,  the 
conical  commencement  of  each  somewhat  overlapping  the  like  termi- 
nation of  its  predecessor ;  and  where  the  '  veins '  seem  to  branch, 
this  does  not  happen  by  the  bifurcation  of  a  spiral  vessel  but  by 
the  '  splicing  on '  (so  to  speak)  of  one  to  the  side  of  another,  or  of 
two  new  vessels  diverging  from  each  other  to  the  end  of  that  which 
formed  the  principal  vein. 

The  Anthers  and  Pollen-grains  also  present  numerous  objects  of 
great  interest,  both  to  the  scientific  botanist  and  to  the  amateur 
microscopist.  In  the  first  place,  they  afford  a  good  opportunity  of 
studying  that  form  of  *  free-cell-formation  '  which  seems  peculiar  to 
the  parts  concerned  in  the  reproductive  process,  and  which  consists 
in  the  development  of  new  cell -walls  round  a  number  of  isolated 
masses  of  protoplasm  forming  parts  of  the  contents  of  a  parent 
cell,  so  that  the  new  cells  lie  free  within  its  cavity,  instead  of  being 
formed  by  its  subdivision,  as  in  the  ordinary  method  of  multiplica- 
tion. If  the  anther  be  examined  by  thin  sections  at  an  early  stage 
of  its  development  within  the  young  flower-bud,  it  will  be  found  to 


720  MICROSCOPIC   STRUCTURE   OF  PHANEROGAMIC   PLANTS 

be  made  up  of  ordinary  cellular  parenchyme  in  which  no  peculiarity 
anywhere  shows  itself;  but  a  gradual  differentiation  speedily  takes 
place,  consisting  in  the  development  of  a  set  of  very  large  cells  in 
two  vertical  rows,  which  occupy  the  place  of  the  loctdi  or  '  pollen- 
chambers  '  that  afterwards  present  themselves  ;  and  these  cells  give 
origin  to  the  pollen-grains,  whilst  the  ordinary  parenchyme  remains 
to  form  the  walls  of  the  pollen-chambers.  The  pollen-grains  are 
formed  within  '  mother-cells,'  the  eiidoplasm  of  each  breaking  up 
into  four  segments.  These  become  invested  by  a  double  envelope,  a 
firm  extine,  and  a  thin  intine  ;  and  they  are  set  free,  when  mature, 
by  the  bursting  of  the  pollen-chambers.  It  is  not  a  little  curious 
that  the  layer  of  cells  which  lines  the  pollen-chambers  should  exhibit, 
in  a  considerable  proportion  of  plants,  a  strong  resemblance  in  struc- 
ture, though  not  in  form,  to  the  elaters  of  Marchantia  (fig.  506). 
For  they  have  in  their  interior  a  fibrous  deposit,  which  sometimes 
forms  a  continuous  spiral  (like  that  in  fig.  532),  as  in  Narcissus  and 
Hyoscyamus ;  but  it  is  often  broken  up,  as  it  were,  into  rings,  as  in 
the  Iris  and  hyacinth  ;  in  many  instances  it  forms  an  irregular 
network,  as  in  the  violet  and  saxifrage ;  in  other  cases  again,  a 
set  of  interrupted  arches,  the  fibres  being  deficient  on  one  side,  as  in 
the  yellow  water-lily,  bryoiiy,  primrose,  &c.  ;  whilst  a  very  peculiar 
stellate  aspect  is  often  given  to  these  cells  by  the  convergence  of  the 
interrupted  fibres  towards  one  point  of  the  cell-wall,  as  in  the  cactus, 
geranium,  madder,  and  many  other  well-known,  plants.  Various 
intermediate  modifications  exist ;  and  the  particular  form  presented 
often  varies  in  different  parts  of  the  wall  of  one  and  the  same  anther. 
It  seems  probable  that,  as  in  Hepaticse,  the  elasticity  of  these  spiral 
cells  may  have  some  share  in  the  opening  of  the  pollen-chambers  and 
in  the  dispersion  of  the  pollen-grains. 

The  form  of  the  pollen-grains  seems  to  depend  in  part  upon  the 
mode  of  division  of  the  cavity  of  the  parent  cell  into  quarters ; 
generally  speaking,  it  approaches  the  spheroidal,  but  it  is  very  often 
elliptical,  and  sometimes  tetrahedral.  It  varies  more,  however, 
when  the  pollen  is  dry  than  when  it  is  moist ;  for  the  effect  of  the 
imbibition  of  fluid,  which  usually  takes  place  when  the  pollen  is 
placed  in  contact  with  it,  is  to  soften  down  angularities,  and  to 
bring  the  cell  nearer  to  the  typical  sphere.  The  extine,  or  outer 
coat  of  the  pollen-grain,  often  exhibits  very  curious  markings,  which 
seem  due  to  an  increased  thickening  at  some  points  and  a  thimiiiig 
away  at  others.  Sometimes  these  markings  give  to  the  surface  layer 
so  close  a  resemblance  to  a  stratum  of  cells  (fig.  565,  B,  C,  D)  that 
only  a  very  careful  examination  can  detect  the  difference.  The 
roughening  of  the  surface  by  spines  or  knobby  protuberances,  as 
shown  at  A,  is  a  very  common  feature  ;  ajid  this  seems  to  enable 
the  pollen-grains  more  readily  to  hold  to  the  surface  whereon  they 
may  be  cast.  Besides  these  and  other  inequalities  of  the  surface, 
most  pollen-grains  have  what  appear  to  be  pores  or  slits  in  their 
extine  (varying  in  number  in  different  species),  through  which  the 
intine  protrudes  itself  as  a  tube,  when  the  bulk  of  its  contents  has 
been  increased  by  imbibition.  It  seems  probable,  however,  that  the 
extine  is  not  absolutely  deficient  at  these  points,  but  is  only  thinned 


POLLEN-GKAINS 


721 


away.  Sometimes  the  pores  are  covered  by  little  disc-like  pieces  or 
lids,  which  fall  off  when  the  pollen-tube  is  protruded.  This  action 
takes  place  naturally  when  the  pollen-grains  fall  upon  the  surface  of 
the  stigma,  which  is  moistened  with  a  viscid  secretion  ;  and  the 
pollen-tubes,  at  first  mere  protrusions  of  the  inner  coat  of  their  cell, 
insinuating  themselves  between  the  loosely  packed  cells  of  the  stigma, 
grow  downwards  through  the  style,  sometimes  even  to  the  length  of 
several  inches,  until  they  reach  the  ovary.  The  first  change,  namely 
the  protrusion  of  the  inner  mem  bi^ine  through  the  pores  of  the  exterior, 
may  be  made  to  take  place  artificially  by  moistening  the  pollen 
with  water,  thin  syrup,  or  dilute  acids  (different  kinds  of  pollen- 
grains  requiring  different  modes  of  treatment)  ;  but  the  subsequent 
extension  by  growth  will  only  take  place  under  the  natural  con- 
ditions. By  treating  some  pollen -grains,  as  those  of  Lilium 
japonicum,  L.  rubrum,  or  L.  auratum,  with  the  viscid  liquid  abun- 
dantly secreted  by  the  stigma, 
not  only  may  the  extrusion 
and  lengthening  of  the  pollen - 
tubes  be  watched,  but  the 
grains  with  their  extruded 
tubes  may  be  preserved  almost 
unchanged  by  mounting  in 
this  liquid. 

The  darker  kinds  of  pollen 
may  be  generally  rendered 
transparent  by  mounting  in 
Canada  balsam ;  or,  if  it  be 
desired  to  avoid  the  use  of 
heat,  in  the  benzol  solution 
of  Canada  balsam,  setting- 
aside  the  slide  for  a  time  in 
a  warm  place.  For  the  less 
opaque  pollens  the  dammar 
solution  is  preferable.  The 
more  delicate  pollens,  how- 
ever, become  too  transparent 

in  either  of  these  media  ;  and  it  is  consequently  preferable  to  mount 
them  either  dry,  or  (if  they  will  bear  it  without  rupturing)  in  fluid. 
The  most  interesting  forms  are  found,  for  the  most  part,  in  plants 
of  the  orders  Amaranthacew,  Gichoriacece,  Cucurbitacece,  Malvacece, 
and  PassiflwecK ;  others  are  furnished  also  by  Convolvulus,  Cam- 
panula, CEnothera,  Pelargonium  (geranium),  Polygonum,  Sedum,  and 
many  other  plants.  It  is  frequently  preferable  to  lay  down  the 
entire  anther,  with  its  adherent  pollen-grains  (where  these  are  of 
a  kind  that  hold  to  it),  as  an  opaque  object ;  this  may  be  done  with 
great  advantage  in  the  case  of  the  common  mallow  (Malva  syl- 
vestris)  or  of  the  hollyhock  (Althcm  rosea),  the  anthers  being  picked 
soon  after  they  have  opened,  whilst  a  large  proportion  of  their  pollen 
is  yet  undischarged,  and  laid  down  as  flat  as  possible,  before 
they  have  begun  to  wither,  between  two  pieces  of  smooth  blotting- 
paper,  then  subjected  to  moderate  pressure,  and  finally  mounted 


FIG.  565. — Pollen-grains  of — A,  Altltcea  rosea 
(hollyhock) ;  B,  Cobcea  scandens  ;  C,  Passi- 
flora  ccerulea;  D,  Ipomcea  puryurea. 


722    MICROSCOPIC    STRUCTURE    OF  PHANEROGAMIC    PLANTS 

upon  a  black  surface.  They  are  then,  when  properly  illuminated, 
most  beautiful  objects  for  objectives  of  §-,  1-,  1^-,  or  2-in.  focus, 
especially  with  the  binocular  microscope.1 

There  are,  in  fact,  few  more  interesting  objects  for  the  young 
microscopist  than  pollen-grains,  both  from  the  ease  with  which  they 
can  always  be  procured,  and  the  almost  infinite  variety  and  beauty 
in  their  forms.  Some  of  the  commonest  weeds,  such  as  the  dandelion 
and  groundsel,  are  distinguished  by  the  beauty  of  their  pollen-grains. 
The  grains  are  sometimes  nearly  or  quite  spherical,  as  in  the  hazel, 
birch,  or  poplar ;  or  of  very  irregular  outline,  as  in  many  grasses. 
But  the  most  common  form  is  elliptical,  with  three  or  five  longi- 
tudinal furrows,  as  in  the  wallflower,  hyacinth,  and  crocus,  the 
surface  being  sometimes  covered  with  warts,  as  in  the  snowdrop. 
In  the  fuchsia  they  are  triangular.  In  addition  to  the  rnallowT  and 
hollyhock,  spiny  pollen-grains  occur  in  the  groundsel,  dandelion, 
Cineraria,  and  many  other  plants.  Sometimes  the  grains  are  united 
together  by  delicate  threads,  as  in  the  Rhododendron  and  Fuchsia  ; 
and  this  union  is  much  more  complete  in  the  Orchidece  and  Ascle- 
piadece,  where  the  whole  of  the  pollen  in  each  anther- lobe  is  glued 
together  by  a  viscid  substance  into  a  club-shaped  pollinium,  or  pollen- 
mass.  In  what  are  called  anemophilous  flowers,  in  which  the  pollen 
is  carried  through  the  air  by  the  agency  of  the  wind,  the  grains  are 
small,  light,  dry,  and  usually  spherical ;  while  in  entomophilous 
flowers,  the  pollen  of  which  is  carried  from  flower  to  flower  by 
insects  in  search  of  honey,  the  various  forms  above  described,  and 
many  others,  are  adapted  to  cause  the  grains  to  adhere  to  the  hairy 
under  side  of  the  body  of  the  insect,  and  thus  promote  their  dis- 
persion. The  various  species  of  EpUobium  (willow-herb)  and 
(Enothera  (evening  primrose)  are  very  favourable  objects  for  ob- 
serving the  emission  of  pollen-tubes  and  their  entrance  into  the 
stigma. 

The  structure  and  development  of  the  ovules  that  are  produced 
within  the  ovary  at  the  base  of  the  pistil,  and  the  operation  in  which 
their  fertilisation  essentially  consists,  are  subjects  of  investigation 
which  have  a  peculiar  interest  for  scientific  botanists,  but  which,  in 
consequence  of  the  special  difficulties  that  attend  the  inquiry,  are 
not  commonly  regarded  as  within  the  province  of  ordinary  micro- 
scopists.  Some  general  instructions,  however,  may  prove  useful  to 
such  as  would  like  to  inform  themselves  as  to  the  mode  in  which  the 
generative  function  is  performed  in  phanerogams.  In  tracing  the 
origin  and  early  history  of  the  ovule,  very  thin  sections  should  be  made 
through  the  flower-bud,  both  vertically  and  transversely ;  but  when 
the  ovule  is  large  and  distinct  enough  to  be  separately  examined,  it 
should  be  placed  on  the  thumb-nail  of  the  left  hand,  and  very  thin 

1  It  sometimes  happens  that  when  the  pollen  of  pines  or  firs  is  set  free,  large 
quantities  of  it  are  carried  by  the  wind  to  a  great  distance  from  the  woods  and 
plantations  in  which  it  has  been  produced,  and  are  deposited  as  a  fine  yellow  dust, 
so  strongly  resembling  sulphur  as  to  be  easily  mistaken  for  it.  This  (supposed) 
general  diffusion  of  sulphur  (such  as  occurred  in  the  neighbourhood  of  Windsor 
in  1879)  has  frightened  ignorant  rustics  into  the  belief  that  the  '  end  of  the  world ' 
was  at  hand.  Its  true  nature  is  at  once  revealed  by  placing  a  few  grains  of  it  under 
the  microscope. 


FEKTILiSATION   OF   THE   OVULE 


723 


sections  made  with  a  sharp  razor  ;  the  ovule  should  not  be  allowed 
to  dry  up,  and  the  section  should  be  removed  from  the  blade  of  the 
razor  by  a  wetted  camel-hair  pencil.  The  tracing  downwards  of  the 
pollen-tubes  through  the  tissue  of  the  style  may  be  accomplished  by 
sections  (which,  however,  will  seldom  follow  one  tube  continuously 
for  any  great  part  of  its  length),  or,  in  some  instances,  by  careful 
dissection  with  needles.  Plants  of  the  Orchis  tribe  are  the  most 
favourable  subjects  for  this  kind  of  investigation,  which  is  best 
carried  on  by  artificially  applying  the  pollen  to  the  stigma  of  several 
flowers,  and  then  examining  one"  or  more  of  the  styles  daily.  <  If 
the  style  of  a  flower  of  Epipactis,'  says  Schacht,  '  to  which  the  pollen 
has  been  applied  about  eight  days  previously,  be  examined  in  the 
manner  above  mentioned,  the  observer  wTill  be  surprised  at  the 
extraordinary  number  of  pollen-tubes,  and  he  will  easily  be  able  to 
trace  them  in  large  strings,  even  as  far  as  the  ovules.  Viola  tricolor 
(heartsease)  and  Ribes  nigrum  and  rubrum  (black  and  red  currant) 
are  also  good  plants  for  the  purpose  ;  in  the  case  of  the  former  plant 
withered  flowers  may  be  taken  and  branched  pollen-tubes  will  not 
unfrequently  be  met  with.'  The  entrance  of  the  pollen-tube  into 
the  micropyle  may  be  most  easily  observed  in  orchidaceous  plants 
and  in  Euphrasia,  it  being  only  necessary  to  tear  open  with  a  needle 
the  ovary  of  a  flower  which  is  just  withering,  and  to  detach  from  the 
placenta  the  ovules,  almost  every  one  of  which  will  be  found  to  have 
a  pollen-tube  sticking  in  its  micropyle.  These  ovules,  however,  ai'3 
too  small  to  allow  of 
sections  being  made, 
whereby  the  origin  of 
the  embryo  may  be  dis- 
cerned ;  and  for  this  pur- 
pose, (Enothera  (evening 
primrose)  has  been  had  re- 
course to  by  Hofmeister, 
whilst  Schacht  recom- 
mends Lathrcea  sqaam- 
<iria,  Pedicidaris  palus- 
tris,  and  particularly 
Pedicular  is  sylvatica. 

We  have  now,  in 
the  last  place,  to  notice 
the  chief  points  of  inter- 
est to  the  microscopist 
which  are  furnished  by 

7         T\/r  ^f     *IG-  56°. — Seeds  as  seen  under  a  low  magnifying 

mature  seeds.  Many  of  power.  A?  poppy;  B  Amaranth™  (prince's 
the  smaller  kinds  of  feather);  0,  Antirrhinum  majus  (snapdragon); 
these  bodies  are  very  D>  Dianthus  (clove-pink) ;  E,  Bignonia. 
curious,  and  some  are  very  beautiful  objects  when  looked  at  in  their 
natural  state  under  a  low  magnifying  power.  Thus  the  seed  of  the 
poppy  (fig.  566,  A)  presents  a  regular  reticulation  upon  its  surface, 
pits,  for  the  most  part  hexagonal,  being  left  between  projecting  walls  ; 
that  of  the  pink  (1))  is  regularly  covered  with  curiously  jagged  divisions, 
every  one  of  which  has  a  small  bright  black  hemispherical  knob  in  its 

3A2 


724   MICROSCOPIC    STRUCTURE   OF   PHANEROGAMIC   PLANTS 

middle ;  that  of  Amaranthus  hypochondriacus  has  its  surface  traced 
with    extremely    delicate    markings    (B) ;   that    of  Antirrhinum   is 
strangely  irregular  in  shape  (C),  and  looks  almost  like  a  piece  of 
furnace-slag  ;  and  those  of  many  Bignoniacece  are  remarkable  for  the 
beautiful  radiated  structure  of  the    translucent  membrane  which 
surrounds  them  (E).      This   structure   is   extremely  well   seen  in 
the   seed    of    Eccremocarpus  scaber,   a    half-hardy   climbing   plant 
common   in   our   gardens ;    and    when    its    membranous    '  wing '  is 
examined  under  a  sufficient   magnifying  power,  it  is  found  to  be 
formed  by  an  extraordinary  elongation  of  the  cells  of  the  seed-coat 
at  the  margin  of  the  seed ;  the   side-walls   of  which   cells   (those, 
namely,  which  lie  in  contact  with  one  another)  being  thickened  so  as 
to  form  radiating  ribs  for  the  support  of  the  wing,  whilst  the  front 
and  back  walls  (which  constitute  its  membranous  surface)  retain  their 
original  transparence,  and  are  marked  only  with  an  indication  of 
spiral  deposit  in  their  interior.     In  the  seed  of  Dictyoloma  peruviana, 
besides  the  principal  ;  wing '  prolonged  from  the  edge  of  the  seed- 
coat,  there  is  a  series  of  successively  smaller  wings,  whose  margins 
form  concentric  rings  over  either  surface  of  the  seed  ;  and  all  these 
wings  are  formed  of  radiating  fibres  only,  composed,  as  in  the  pre- 
ceding case,  of  the  thickened  walls  of  adjacent  cells,  the  intervening 
membrane,  originally  formed  by  the  front  and  back  walls  of  these 
cells,  having  disappeared,  apparently  in  consequence  of  being  un- 
supported by  any  secondary  deposit.     Several  other  seeds,  as  those 
of  Sphenogyne  speciosa  and  Lophospermum  erubescent,  possess  wing- 
like  appendages :  bnt  the  most  remarkable  development   of  these 
organs  is  said  by  Mr.  Quekett  to  exist  in   a  seed  of  Calosanthes 
indica,  an  East  Indian  plant,  in  which  the  wing  extends  more  than 
an  inch  on  either  side  of  the  seed.     Some  seeds  are  distinguished  by 
a  peculiarity  of  form  which,  although   readily  discernible   by   the 
naked  eye,  becomes  much  more  striking  when  they  are  viewed  under 
a  very  low  magnifying  power.     This  is  the  case,  for  example,  with 
the  seeds  of  the  carrot,  whose  long  radiating  processes  make  it  bear, 
under  the  microscope,  no  trifling  resemblance  to  some  kinds  of  star- 
fish ;  and  with  those  of  Cyanthus  minor,  which  bear  about  the  same 
degree  of  resemblance  to  shaving-brushes.     In  addition  to  the  pre- 
ceding,  the   following   may   be   mentioned   as   seeds   easily   to   be 
obtained  and  as  worth  mounting  for  opaque  objects   : — Anagallis, 
Anethum  graveolens,   Begonia,    Carum    carui,    Coreopsis    tinctoria, 
Datura,   Delphinium,  Digitalis,  Palatine,  Erica,  Gentiana,  Gesnera, 
Hyoscyamus,  Hypericum,  Lepidium,  Limnocharis,  Linaria,  Lychnis, 
Mesembryanthemum,  Nicotiana,  Origanum  onites,  Orobanche,  Petunia, 
Reseda,    Saxifraga,    Scrophidaria,    Sedum,    Sempervivum,     Silene, 
Stellaria,    Symphytum   asperrimum,   and    Verbena.      The  following 
may   be    mounted     as     transparent     objects    in    Canada    balsam : 
Drosera,  Hydrangea,  Monotropa,  Orchis,  Parnassia,  Pyrola,    Saxi- 
fraga.1     The  seeds  of  umbelliferous  plants  generally  are  remarkable 
for  the  peculiar  vittce,  or  receptacles  for  essential   oil,    which   are 
found  in  the  closely  applied  pericarp  or  seed-vessel  which  encloses 

1  A  part  of  these  lists  have  been  derived  from  the  Micrograpliic  Dictionary. 


STRUCTURE   OF   SEEDS  725 

them.  Various  points  of  interest  respecting  the  structure  of  the 
testa  or  envelope  of  seeds,  such  as  the  fibre-cells  of  Cobcea  and 
Collomia,  the  stellate  cells  of  the  star-anise,  and  the  densely  con- 
solidated tissue  of  the  'shells'  of  the  coquilla-nut,  cocoa-nut,  &c. 
having  been  already  noticed,  we  cannot  here  stop  to  do  more  than 
advert  to  the  peculiarity  of  the  constitution  of  the  husk  of  corn- 
grains.  In  these,  as  in  other  grasses,  the  ovary  itself  continues  to 
envelop  the  seed,  giving  a  covering  to  it  that  surrounds  the  testa, 
and  closely  adheres  to  it.  The.-*  bran '  detached  in  grinding  consists 
not  only  of  these  two  coats,  but  also  (as  the  microscope  reveals)  of 
an  outer  layer  of  the  grain  itself,  formed  of  hexagonal  cells  disposed 
with  great  regularity.  As  these  are  filled  with  gluten,  the  removal 
of  this  layer  takes  away  one  of  the  most  nutritious  parts  of  the 
grain ;  and  it  is  most  desirable,  therefore,  that  only  the  two  outer 
indigestible  coats  should  be  detached  by  the  *  decorticating  '  process 
devised  for  the  purpose.  The  hexagonal  cell-layer  is  so  little  altered 
by  a  high  temperature  as  still  to  be  readily  distinguishable  when 
the  grain  has  been  ground  after  roasting,  thus  enabling  the 
microscopist  to  detect  even  a  small  admixture  of  roasted  corn  with 
coffee  or  chicory  without  the  least  difficulty.1 

1  In  a  case  in  which  the  Author  was  called  upon  to  make  such  an  investigation, 
he  found  as  many  as  thirty  distinctly  recognisable  fragments  of  this  cellular  enve- 
lope in  a  single  grain  of  a  mixture  consisting  of  chicory  with  only  5  per  cent,  of 
roasted  corn. 


726 


CHAPTER  XII 

MICROSCOPIC  FORMS  OF  ANIMAL   LIFE— PROTOZOA 

PASSING  on,  now,  to  the  Animal  Kingdom,  we  begin  by  directing 
our  attention  to  those  minute  and  simple  forms  which  correspond  in 
the  animal  series  with  the  Protophyta  in  the  vegetable  (Chap.  VIII.)  ;. 
and  this  is  the  more  desirable  since  the  formation  of  a  distinct  group 
to  which  the  name  of  PROTOZOA  (first  proposed  in  this  sense  by 
Siebold)  may  be  appropriately  given  is  one  of  the  most  interesting 
results  of  microscopic  inquiry.  This  group,  which  must  be  placed  at 
the  very  base  of  the  animal  scale,  is  characterised  by  the  apparent 
simplicity  that  prevails  in  the  structure  of  the  beings  that  compose 
it,  the  lowest  of  them  being  single  protoplasmic  particles  or  *  jelly- 
specks,'  whilst  even  among  the  highest,  however  numerous  their 
units  may  be,  these  are  (as  among  protophytes)  mere  repetitions  of  one 
another,  each  capable  of  maintaining  an  independent  existence.  In 
this  there  is  a  very  curious  and  significant  parallelism  to  the  earliest 
embryonic  stage  of  higher  animals ;  for  the  fertilised  germ  of  any  one 
of  these  first  shapes  itself  as  a  single  cell,  and  then,  by  repeated  binary 
subdivisions,  develops  itself  into  a  morula  or  '  mulberry-mass '  of 
cells,  corresponding  to  the  *  multicellular '  organisms  met  with 
among  the  higher  Protozoa.  There  is,  so  far,  in  neither  case  any 
sign  of  that  *  differentiation '  of  organs  which  is  characteristic  of  the 
higher  animals  ;  but  whilst,  in  the  Protozoon,  each  cell  is  not  merely 
similar  to  its  fellows,  but  is  independent  of  them,  the  morula,  in 
such  as  go  on  to  a  higher  stage,  becomes  the  subject  of  a  series  of 
developmental  changes  tending  to  the  production  of  a  single  whole, 
whose  parts  are  mutually  dependent.  The  first  of  these  changes  is 
its  conversion  into  a  gastrula  or  primitive  stomach,  whose  wall  is 
formed  of  a  double  membrane,  the  outer  lamella,  or  ectoderm,1 
being  derived  directly  from  the  external  cell-layer  of  the  morula 
whilst  the  inner,  or  endoderm,  is  formed  by  the  '  invagination '  of 
that  layer  into  the  space  left  void  by  the  dissolution  of  the  central 
cells  of  the  '  morula.'  This  gastrula-stage?  as  we  shall  see  hereafter, 
remains  permanent  in  the  great  group  of  Ccelentera,  though  the 
endoderm  and  ectoderm  are  separated  from  each  other  in  its  higher 
forms  by  the  development  of  generative  and  other  organs  between 

1  The  terms  epiblast  and  hypoblast  are  generally  used  by  English  embryologists 
in  place  of  the  '  ectoderm '  and  '  endoderm '  used  here. 

2  The  gastrula- stage  is  in  a  number  of  cases  brought  about  by  a  concentric  split- 
ting of  the  walls  of  the  morula  into  two  layers,  and  by  the  appearance  at  one  point 
of  an  orifice  which  leads  into  the  central  cavity  ;  this  cavity  is  the  original  segmenta- 
tion cavity  of  the  morula,  and  not  a  fresh  cavity,  as  in  '  invaginate  gastrulse.' 


PROTOZOA— PROTOMYXA  727 

them.  But  4n  all  classes  above  the  coelenterates  the  primitive 
stomach  forms  a  part,  and  often  only  an  insignificant  part,  of  the 
whole  digestive  tract.  Thus  the  whole  animal  kingdom  may  be 
divided,  in  the  first  place,  into  the  PROTOZOA,  which  are  either  single 
cells  or  aggregates  of  similar  cells  corresponding  to  the  morula- 
stage  of  higher  types  ;  and  the  METAZOA,  in  which  the  morula  takes 
on  the  condition  of  an  individualised  organism,  the  life  of  every  part 
of  which  contributes  to  the  general  life  of  the  whole.  Putting  this 
important  truth  into  other  word*,  we  may  say  of  the  Protozoa  that 
they  are  either  unicellular  or  unicellular  aggregates,  while  the 
Metazoa  are  multicellular,  and  their  constituent  cells  have  different 
functions. 

The  lowest  of  the  Protozoa,  however,  like  the  simplest  proto- 
phytes,  do  not  even  attain  the  rank  of  a  true  cell,  understanding 
by  that  designation  a  definite  protoplasmic  unit  (plastid),  which  is 
limited  by  a  cell- wall,  and  contains  a  '  nucleus.'  For  they  consist 
of  particles  of  protoplasm,  termed  '  cytodes,'  of  indefinite  extent, 
wrhich  have  neither  cell-wall  nor  nucleus,  but  which  yet  take  in  and 
digest  food,  convert  it  into  the  material  of  their  own  bodies,  cast  out 
the  indigestible  portions,  and  reproduce  their  kind,  with  the  regu- 
larity and  completeness  that  we  have  been  accustomed  to  regard 
as  characteristic  of  higher  animals.  With  regard,  however,  to  this 
apparent  absence  of  a  nucleus  we  have  to  bear  in  mind  that  the 
progress  of  research  is  continually  diminishing  the  number  of  forms 
devoid  of  a  nucleus,  or,  at  any  rate,  of  a  nuclear  material  scattered 
throughout  the  substance  of  the  plastid  ;  in  retaining,  therefore,  the 
group  of  non-nucleated  Protozoa  we  are  acting  on  the  principle  of 
not  going  beyond  our  evidence,  and  by  no  means  reflecting  on  the 
later  systematists  who  have  merged  the  various  types  (whether 
nucleated  or  non-nucleated)  among  other  divisions  of  the  Protozoa. 
Between  some  of  these  Monerozoa  (as  they  have  been  designated 
by  Professor  Haeckel,  who  first  drew  attention  to  them)  and  the 
Myxomycetes  or  the  Ghlamyclomyxa  already  described,  no  definite 
line  of  division  can  be  drawn,  the  only  justification  for  the  separa- 
tion here  adopted  being  that  the  affinities  of  the  former  seem  to 
be  rather  with  the  lowest  forms  of  vegetation,  whilst  the  whole 
life-history  of  the  types  now  to  be  described,  and  the  connected 
graduation  by  which  they  pass  into  undoubted  rhizopods,  leave  no 
doubt  of  their  claim  to  a  place  in  the  animal  kingdom. 


MONEROZOA. 

A  characteristic  example  of  this  lowest  protozoic  type  is  presented 
by  the  Protomyxa  aurantiaca  (fig.  567),  a  marine  '  moner '  of  an 
orange-red  colour,  found  by  Professor  Haeckel  upon  the  dead  shells  of 
Spirula  which  are  so  abundant  on  the  shores  of  the  Canary  Islands. 
In  its  active  state  it  has  the  stellar  form  shown  at  F,  its  arborescent 
extensions  dividing  and  inosculating  so  as  to  form  a  constantly 
changing  network  of  protoplasmic  threads,  along  which  stream  in  all 
directions  orange-red  granules,  obviously  belonging  to  the  body 


728      MICROSCOPIC   FORMS   OF  ANIMAL  LIFE— PROTOZOA 

itself,  together  with  foreign  organisms  (ft,  c) — such  as  marine  diatoms, 
radiolarians,  and  infusoria — which,  having  been  entrapped  in  the 
pseudopodial  network,  are  carried  by  the  protoplasmic  stream  into 
the  central  mass,  where  the  nutrient  matter  of  their  bodies  is  extracted, 
the  hard  skeletons  being  cast  out.  Neither  nucleus  nor  contractile 
vesicle  is  to  be  discerned,  but  numerous  floating  and  inconstant  vacu- 
oles  (a)  are  dispersed  through  the  substance  of  the  body.  After  a  time 
the  currents  become  slower  ;  the  ramified  extensions  are  gradually 


FIG.  567. — Protonujxa  aurantiaca:  A,  encysted  statospore;  B,  inci- 
pient formation  of  swarm-spores,  shown  at  C  escaping  from  the  cyst, 
at  D  swimming  freely  by  their  flagellate  appendages,  and  at  E  creep- 
ing in  the  amoeboid  condition;  F,  fully  developed  reticulate  organism, 
showing  numerous  vacuoles,  a,  and  captured  prey,  b,  c. 

drawn  inwards  ;  and,  after  ejecting  any  indigestible  particles  it  m;iy 
still  include,  the  body  takes  the  form  of  an  orange-red  sphere  round 
which  a  cyst  soon  forms  itself,  as  shown  in  A.  After  a  period  of 
quiescence  the  protoplasmic  substance  retreats  from  the  interior  of 
the  cyst,  and  breaks  up  into  a  number  of  small  spheres  (.B),  which,  at 
first  inactive,  soon  begin  to  move  within  the  cyst,  and  change  their 
shape  to  that  of  a  pear  with  the  small  end  drawn  out  to  a  point. 
The  cyst  then  bursts,  and  the  red  pear-shaped  bodies  issue  forth 
into  the  water  (C),  moving  freely  about  by  the  vibrations  otflagetta 


PEOTOZOA — PEOTOMYXA 


729 


formed  by  the  drawing  out  of  their  small  ends,  just  as  do  the 
flagellated  zoospores  of  protophytes.  These  bodies,  being  without 
trace  of  either  nucleus,  contractile  vesicle,  or  cell-wall,  are  to  be 
regarded  as  particles  of  simple  homogeneous  protoplasm,  to  which 
the  designation  plastidules  has  been  appropriately  given.  After 
about  a  day  the  motions  cease  ;  the  flagella  are  drawn  in,  and  the 
plastidules  take  the  form  and  lead  the  life  of  Amcebce,  putting  forth 
inconstant  pseudopodial  processes,  and  engulfing  nutrient  particles 
in  their  substance  (D).  Two  or  more  of  these  amcebiform  bodies 
unite  to  form  a  '  plasmodium,'  a*s.,in  the  Myxomycetes ;  its  pseudo- 
podial extensions  send  out  branches  which  inosculate  to  form  a  net- 


FIG.  5(58. — Vampyrella  tpirogyrfB^  as  seen  at  A,  sucking  out  contents 
of  Spirogtji-a-cell;  at  B  in  encysted  condition,  the  cyst  a  enclosing 
granular  protoplasm  b ;  at  C,  division  of  contents  of  cyst  into 
tetraspores,  of  which  one  is  escaping  in  the  amoeboid  condition 
to  develop  itself  into  the  adult  form  shown  at  D. 

work  ;  and  the  body  grows,  by  the  ingestion  of  nutriment,  to  the 
size  of  the  original.  In  this  cycle  of  change  there  seems  no  interven- 
tion of  a  generative  act,  the  coalescence  of  the  amoebiform  plastidules 
having  none  of  the  characters  of  a  true  '  conjugation.'  But  it  is  by 
no  means  improbable  that  after  a  long  course  of  multiplication  by 
successive  subdivisions  some  kind  of  conjugation  may  intervene. 

Another  very  interesting  *  moneric '  type  is  the  Vampyrella, 
of  which  one  form  (fig.  568)  has  long  been  known  in  its  encysted 
condition  as  a  minute  brick-red  sphere  attached  to  the  filaments  of 
the  conjugate  Spirogyra ;  whilst  another  (fig.  569)  similarly 
attaches  itself  to  the  branches  of  Gomphonema.  The  walls  of  the 


730       MICROSCOPIC   FORMS   OF   ANIMAL  LIFE— PROTOZOA 

cysts  are  composed  of  two  membranes,  of  which  the  interior  gives 
the  characteristic  reaction  of  cellulose,  whilst  the  softer  external 
layer  is  nitrogenous.  After  remaining  some  time  in  the  quiescent 
condition  the  encysted  protoplasm  breaks  up  into  two  or  four 
'  tetraspores '  (fig.  569,  6,  d) ;  these  escape  by  openings  in  the  cyst 
(fig.  568,  C),  and  soon  take  the  spherical  form,  emitting  very  slender 
pseudopodial  filaments  (figs.  568,  D,  569,  e)  like  those  of  an  Actino- 
phrys,  but  possessing  neither  nucleus  nor  contractile  vesicle.  In  this 
condition  they  show  great  activity,  moving  about  in  search  of  the 
special  nutriment  they  require,  drawing  themselves  out  in  strings 
and  fine  filaments  which  tear  asunder  and  again  unite  to  send  off 
branches  and  form  fine  fan-like  expansions,  and  these  occasionally 
contracting  again  into  minute  spheres.  When  the  V.  spirogyrce  is 
watched  in  water  containing  some  filaments  of  Spirogyra,  it  may  be 
seen  to  wander  until  it  meets  one  of  these  filaments,  to  which,  if  it 
be  healthy  and  loaded  with  chlorophyll,  it  attaches  itself.  It  soon 
begins  to  perforate  the  wall  of  the  filament ;  and  when  the  interior 
of  this  has  been  reached,  its  endoplasm,  carrying  with  it  the  chloro- 
phyll-granules it  includes,  passes  slowly  into  the  body  of  the  Vam- 
pyrella.  In  this  manner  cell  after  cell  is  emptied  of  its  contents ; 
and  the  plunderer,  satiated  with  food,  resumes  its  quiescent  spherical 
form  to  digest  it.  The  chlorophyll-granules  wrhich  it  has  ingested 
become  diffused  through  the  body,  but  gradually  cease  to  be  distin- 
guishable, the  protoplasmic  mass  assuming  a  brick-red  colour.  The 
first  layer  it  exudes  to  form  its  cyst  is  the  outer  or  nitrogenous  invest- 
ment, within  which  the  cellulose  layer  is  afterwards  formed.  Jhe  V, 
yomphoneniatis  in  like  manner  creeps  over  the  stems  and  branches  of 
the  Gomphonema  (fig.  569,  e),  adapting  itself  to  the  form  of  its  sup- 
port ;  and  as  soon  as  it  has  reached  one  of  the  terminal  siliceous  cells 
of  the  diatom,  it  extends  itself  over  it  so  as  completely  to  envelop 
the  cell  in  a  thin  layer  of  protoplasm.  From  the  surface  of  this  a 
number  of  fine  pseudopodia  radiate  into  the  surrounding  water  (f) ; 
whilst  another  portion  of  the  protoplasm  finds  its  way  between  the 
two  siliceous  valves  into  the  interior,  and  appropriates  its  contents. 
The  valves,  when  emptied,  break  off  from  their  support,  and  are  cast 
out  of  the  body  of  the  Vampyrella,  which  soon  proceeds  to  another 
6fompkonema-ce\\  and  plunders  it  in  the  same  manner.  After  thus 
ingesting  the  nutriment  furnished  by  several  cells,  and  acquiring  its 
full  size,  it  passes,  like  V.  spirogyrce,  into  the  encysted  condition, 
to  recommence — after  a  period  of  quiescence — the  same  cycle  of 
change.  Mr.  Bolton  discovered  near  Birmingham,  and  Professor  Ray 
Lankester  described,  a  form  allied  to  Vampyrella — Archerina  Boltoni 
— which  is  remarkable  for  being  chlorophyllogenous ;  this  species 
presents  another  interesting  peculiarity  : — '  Groups  of  ghost-like 
outlines  corresponding  to  chlorophyll -corpuscles  and  their  radiant 
filamentous  pseudopodia,  entirely  devoid  of  any  substance,'  were 
observed,  and  were  compared  to  the  numerous  cellulose  chambers 
which  are  secreted  and  abandoned  by  the  protoplasm  of  Chlamy- 
(lomyxa. 

Intermediate  between  the  foregoing  and  the  ;  reticularian  '  rhizo- 
pods  to  be  presently  described,  is    another   simple    protozoon    dis- 


LIEBERKUEHNIA 


731 


covered  in  ponds  in  Germany  by  MM.  Claparede  and  Lachmaimr 
and  named  by  them  Lieberkuehnia  Wayeneri.1  The  whole  sub- 
stance of  the  body  of  this  animal  and  its  pseudopodial  extensions 
(fig.  570)  is  composed  of  a  homogeneous,  semi-fluid,  granular  proto- 
plasm, the  particles  of  which,  when  the  animal  is  in  a  state  of 
activity,  are  continually  performing  a  circulatory  movement,  which 
may  be  likened  to  the  rotation  of  the  particles  in  the  protoplasmic 


FIG.  569. —  Vampyrella  gompJitmematis:  A,  colony  of 
Gomphonema  attacked  by  Va mpyrellce  ;  a,  encysted  state; 
b,  b,  cysts  with  contents  breaking  up  into  tetraspores,  d,  d, 
seen  escaping  at  e  ;  at/  is  shown  a  Vampyrella  sucking  out 
contents  of  Gomphonema-cells,  the  emptied  frustules  of 
which,  g,  h,  are  cast  forth.  B,  isolated  Vampyrella  creeping 
about  by  its  extended  pseudopodia. 

network  within  the  cell  of  a  Tradescantia.  It  is  a  marked  peculiarity 
of  the  pseudopodial  extension  of  this  type  that  it  does  not  take  place 
by  radiation  from  all  parts  of  the  body  indifferently,  but  that  it 

1  Etudes surles  Infusoires  et  les  Bhizopodes,  Geneva,  1858-1861.  The  beautiful 
figure  of  Lieberkuehnia,  given  by  M.  Claparede,  has  been  reproduced  by  the  Author 
in  Plate  I.  of  his  Introduction  to  the  Study  of  the  Foraminifera. 


732      MICROSCOPIC  FORMS   OF  ANIMAL  LIFE— PROTOZOA 


proceeds  entirely  from  a  sort  of  trunk  that  soon  divides  into  branches 
which  again  speedily  multiply  by  further  subdivision,  until  at  last 
a  multitude  of  finer  and  yet  finer  threads  are  spun  out  by  whose 
continual  inosculations  a  complicated  network  is  produced,  which 
may  be  likened  to  an  animated  spider's  web.  The  protoplasm  is 
invested  in  a  very  delicate  and  closely  applied  envelope.  Any  small 
alimentary  particles  that  may  come  into  contact  with  the  glutinous 
surface  of  the  pseudopodia  a.re  retained  in  adhesion  by  it,  and 
speedily  partake  of  the  general  movement  going  on  in  their  sub- 
stance. This  movement  takes  place  in  two  principal  directions — 
from  the  body  towards  the  extremities  of  the  pseudopodia,  and  from 

these  extremities  back  to  the 
body  again.  In  the  larger 
branches  a  double  current  may 
be  seen,  two  streams  passing 
at  the  same  time  in  opposite 
directions ;  but  in  the  finest 
filaments  the  current  is  single 
and  a  granule  may  be  seen  to 
move  in  one  of  them  to  its  very 
extremity,  and  then  to  return, 
perhaps  meeting  and  carrying 
back  with  it  a  granule  that 
was  seen  advancing  in  the 
opposite  direction.  Even  in 
the  broader  processes  granules 
are  sometimes  observed  to  come 
to  a  stand,  to  oscillate  for  a 

J   I          V"""  VY  \//Vk  time,  and  then  to  take  a  retro- 

r.\  /          \\     \      \  grade   course,    as    if  they  had 

been  entangled  in  the  opposing 
current,  just  as  is  often  to  be 
seen  in  Chara.  When  a  granule 
arrives  at  a  point  where  a  fila- 
ment bifurcates,  it  is  often  arrested  for  a  time,  until  drawn  into  one 
or  the  other  current ;  and  when  carried  across  one  of  the  bridge- 
like  connections  into  a  different  band,  it  not  unfrequently  meets  a 
current  proceeding  in  the  opposite  direction,  and  is  thus  carried  back 
to  the  body  without  having  proceeded  very  far  from  it.  The 
pseudopodial  network  along  which  this  '  cyclosis '  takes  place  is  con- 
tinually undergoing  changes  in  its  own  arrangement,  new  filaments 
being  put  forth  in  different  directions,  sometimes  from  its  margin, 
sometimes  from  the  midst  of  its  ramifications,  whilst  others  are 
retracted.  Not  unfrequently  it  happens  that  to  a  spot  where  two  or 
more  filaments  have  met,  there  is  an  afflux  of  the  protoplasmic  sub- 
stance that  causes  it  to  accumulate  there  as  a  sort  of  secondary  centre, 
from  which  a  new  radiation  of  filamentous  processes  takes  place. 
Occasionally  the  pseudopodia  are  entirely  retracted,  and  all  activity 
ceases ;  so  that  the  body  presents  the  appearance  of  an  inert  lump. 
But  if  watched  sufficiently  long  its  activity  is  resumed,  so  that  it 
may  be  presumed  to  have  been  previously  satiated  with  food,  which 


JV? 


FIG.  570. — Lieberkuehnia  Wageneri. 


RHIZOPODA  733 

is  undergoing  digestion  during  its  stationary  period.  No  encysting 
process  has  been  noticed  in  Lieberkuehnia  ;  but  Cienkowsky  has  dis- 
covered that  in  L.  paludosa  reproduction  is  effected  by  a  process  of 
fission,  which  commences  with  the  formation  of  a  new  pseudopodial 
stalk  at  the  base  of  the  animal,  the  envelope  being  perforated  at  this 
point.  As  the  marine  type  of  it  occurs  on  our  own  coasts,  the  fresh- 
water type  may  very  likely  be  found  in  our  ponds,  and  either  may 
be  recommended  as  a  most  worthy  object  of  careful  study. 

> 
RHIZOPODA. 

We  now  arrive  at  the  group  of  rhizopods,  or  '  root-footed '  animals, 
first  established  by  Dujardin  for  the  reception  of  the  Amoeba  and  its 
allies,  which  had  been  included  by  Professor  Ehrenberg  among  his 
iiifusory  animalcules,  but  which  Dujardin  separated  from  them  as 
being  mere  particles  of  sarcode  (protoplasm),  having  neither  the  defi- 
nite body- wall  nor  the  special  mouth  of  the  true  Infusoria,  but  put- 
ting forth  extensions  of  their  sarcodic  substance,  which  he  termed 
pseudopodia  (or  false  feet),  serving  alike  as  instruments  of  locomotion 
and  as  prehensile  organs  for  obtaining  food.  According  to  Dujardin's 
definition  of  this  group,  the  Monerozoa,  already  described,  would  be 
included  in  it ;  but  it  seems  on  various  grounds  desirable  to  limit  the 
term  Rhizopoda  to  those  Protozoa  in  which  the  presence  of  a  nucleus, 
the  differentiation  of  an  ectosarc  (or  firmer  superficial  layer  of  proto- 
plasm) from  the  semi-fluid  endosarc,  together  with  the  more  definite 
form  and  restricted  size,  indicate  a  distinct  approach  to  the  condition 
of  true  cells.  Many  different  schemes  for  the  classification  of  the 
rhizopods  have  been  proposed,  but  none  of  them  can  be  regarded 
as  entirely  satisfactory,  our  knowledge  of  the  reproductive  processes, 
and  of  other  important  parts  of  the  life-history  of  these  creatures, 
being  still  extremely  imperfect ;  and  as  some  parts  of  the  scheme 
proposed  by  the  Author  many  years  ago,1  based  on  the  characters  of 
the  pseudopodial  extensions,  have  been  accepted  by  more  recent 
systematists,  it  seems  best  still  to  adhere  to  it. 

I.  In  the  first  division,  Reticularia,  the  pseudopodia  freely 
ramify  and  inosculate,  so  as  to  form  a  network,  exactly  as  in  Lieber- 
kuehnia, from  which  they  are  distinguished  by  the  possession  of  a 
nucleus  and  by  the  investment  of  their  sarcodic  bodies  in  a  firm 
envelope.  This  is  most  commonly  either  a  calcareous  shell  of  very 
definite  shape,  or  a  test  built  up  of  sand- grains  or  other  minute 
particles  more  or  less  firmly  united  by  a  calcareous  cement  exuded 
from  the  sarcodic  body.  These  testaceous  forms,  which  are  exclu- 
sively marine,  constitute  the  group  of  Foraminifera,  whose  special 
interest  to  the  microscopist  entitles  it  to  separate  consideration ; 
and  it  is  only  for  convenience  that  two  Reticularia  which  in- 
habit fresh  water  also,  and  the  envelopes  of  whose  bodies  are 
usually  membranous,  are  here  separated  from  the  Foraminifera  (to 
which  they  properly  belong)  for  description  as  types  of  the  group. 
The  Reticularia  have  little  locomotive  power,  and  only  seem  to 

1  Natural  History  Review,  1861,  p.  456;  and  Introduction  to  the  Study  of  the 
Foraminifera,  1862,  chap.  ii. 


734      MICEOSCOPIC   FORMS   OF  ANIMAL  LIFE -PROTOZOA 

exercise  it  to  find  a  suitable  situation  for  their  attachment,  the 
capture  of  their  food  being  effected  by  their  pseudopodial  net- 
work. 

II.  The  second  division,  Heliozoa,  consists  of  the  rhizopods  whose 
pseudopodia   extend  themselves  as  straight  radiating  rods,  having 
little  or  no  tendency  to  subdivide  or  ramify,  though  they  are  still 
sufficiently  soft  and  homogeneous  (at  least  in  the  lower  types)  to 
coalesce  when  they  come  into  contact  with  each  other.     These  have 
usually  (probably  always)  a  contractile  vesicle  as  well  as  a  nucleus ; 
and  the  higher  forms  of  them  are  characterised  by  the  enclosure  of 
symbiotic  yellow  corpuscles  (zoochlorellce)  in  the  substance  of  their 
endosarc.     By  far  the  larger  number  of  this  group  also  have  skeletons 
of  mineral  matter,  which  are  always  siliceous ;  and  these  are  some- 
times   perforated   casings   of  great    regularity   of  form,  as  in  the 
marine    Polycystina,  sometimes  internal  frameworks  of  marvellous 
symmetry,  as  in  the  marine  Rqdiolaria.     These  two  groups,  also, 
will  be   reserved   for   special   notice,   the   simple   ffeliozoa,   which 
are   among   the    commonest    inhabitants    of  fresh    water,    furnish- 
ing  the  best  illustrations  of  the  essential  characters  of  the  type. 
They  seem,  for  the  most  part,  to  have  but  little  locomotive  power, 
capturing  their  prey  by  their  extended  pseudopodia.     The  tendency 
of  modern  writer  sis  to  separate  the  Heliozoa,  as  here  understood,  into 
the  two  groups  of  Heliozoa  (sens,  strict.)  and  Radiolaria,  the  latter 
being  distinguished  by  the  presence  of  a  central  capsule  or  mass  of 
protoplasm  surrounded  by  a  special  envelope,  the  better  develop- 
ment of  the  skeleton,  the  greater  tendency  of  the  pseudopodia  to 
coalesce  with  one  another,  and  the  not  imfrequent  presence  of '  yellow 
bodies.' 

III.  The  third  group,  Lobosa,  contains  the  rhizopods  which  most 
nearly  approach  the  condition  of  true  cells,  in  the  differentiation  of 
their  almost  membranous  ectosarc  and  their  almost  liquid  endosarc. 
and  in  the  non-coalescence  of  their  pseudopodial  extensions,  which, 
instead  of  being  either  thread-like  or  rod-like,  are  lobate,  that  is, 
irregular  projections  of  the  body,  including  both  ectosarc  and  endo- 
sarc,   which  are  continually  undergoing  -change  both    in  form  and 
number.     The  Lobosa  are  comparatively  active  in  their  habits,  moving 
freely  about  in  search  of  food,  which  is  still  received  into  the  sub- 
stance of  their  bodies  through  any  part  of  their  surface — unless  this 
is  enclosed  in  envelopes  such  as  are  formed  by  many  of  them,  either 
by  exudation  from    the  surface    of  their    bodies    of  some  material 
(probably  chitinous)  which  hardens  into  a  membrane,  or  by  aggre- 
gating and  uniting  grains  of  sand  or  other  small  solid  particles, 
which  they  build  up  into  '  tests.'     A  large  proportion  of  them  are 
inhabitants  of  fresh  water,    and    some   are    even    found    in    damp 
earth. 

Reticularia. — This  type  is  very  characteristically  represented  by 
the  genus  Gromia  (fig.  571),  some  of  whose  species  are  marine,  and 
are  found,  like  ordinary  Foramini/era,  among  tufts  of  corallines, 
algae,  &c.  ;  whilst  others  inhabit  fresh  water,  adhering  to  Confervse 
and  other  plants  of  running  streams.  It  was  in  this  type  that  the 
presence  of  a  nucleus,  formerly  supposed  to  be  wanting  in  Reticularia 


GROMIA 


735 


generally,  was  first  established  by  Dr.  Wallich.  The  sarcode:body 
of  this  animal  is  encased  in  an  egg-shaped,  brownish-yellow,  chitiiious 
envelope,  which  may  attain  a  diameter  of  from  ^th  to  j^th  of  an 
inch,  looking  to  the  naked  eye  so  like  the  egg  of  a  zoophyte  or  the 
seed  of  an  aquatic  plant,  that  its  real  nature  would  not  be  suspected 
so  long  as  it  remained  quiescent.  The  'test'  has  a  single  round 
orifice,  from  which,  when 
the  animal  is  in  a  state 
of  activity,  the  sarcodic 
substance  streams  forth, 
speedily  giving  off  ramify- 
ing extensions,  which,  by 
further  ramification  and 
inosculation,  form  a  net- 
work like  that  of  Lieber- 
kuehiiia.  But  the  sarcode 
also  extends  itself  so  as 
to  form  a  continuous 
layer  over  the  whole  ex- 
terior of  the  'test,'  and 
from  any  part  of  this 
layer  fresh  pseudopodia 
may  be  given  off.  By 
the  alternate  extension 
and  contraction  of  these, 
minute  protophytes  and 
protozoa  are  entrapped 
and  drawn  into  the  in- 
terior of  the  test,  where 
their  nutritive  material 
is  extracted  and  assimi- 
lated ;  and  if  the  '  test 
(as  happens  in  some 
species)  be  sufficiently 
transparent,  the  indi- 
gestible hard  parts  (such 
as  the  siliceous  valves  of , 
diatoms,  shown  in  fig 
571)  may  be  distinguished 
in  the  midst  of  the  sar- 
codic substance.  By  the 
same  agency  the  Gromia 
sometimes  creeps  up  the  sides  of  a  glass  vessel.  In  the  intervals  of 
quiescence,  on  the  other  hand,  the  whole  sarcodic  body,  except  a 
film  that  serves  for  the  attachment  of  the  test,  is  withdrawn  into  its 
interior. 

Another  example  of  the  reticularian  group  is  afforded  by  the 
curious  little  Microgromia  socialis  (fig.  572),  first  discovered  by  Mr. 
Archer,  and  further  investigated  writh  great  care  by  Hertwig,1  which 


FIG.  571. — Gromia  oviform  is,  with  its 
pseudopodia  extended. 


'\jeber  Microgroinia,'  in  Arcliiv  fur  Mikr.  Anat.  bd.  x.  Supplement. 


736      MICROSCOPIC   FORMS   OF   ANIMAL   LIFE— PROTOZOA 

has  the  curious  habit  of  uniting  with  neighbouring  individuals  by  the 
fusion  of  the  pseudopodia,  into  a  common  ;  colony,'  the  individuals 
sometimes  remaining  at  a  distance  from  one  another  as  at  A,  but 
sometimes  aggregating  themselves  into  compact  masses  as  at  B.  The 
nearly  globular  thin  calcareous  shell  is  prolonged  into  a  short  neck 
having  a  circular  orifice,  from  which  the  sarcode-body  extends  itself, 


FIG.  572. — Microgromia  socialis  :  A,  colony  of  individuals  in  extended  state, 
some  of  them  undergoing  transverse  fission ;  B,  colony  of  individuals 
(some  of  them  separated  from  the  principal  mass)  in  compact  state  ;  C,  D, 
formation  and  escape  of  swarm-spore,  seen  free  at  E. 

giving  off  very  slender  pseudopodia  which  radiate  in  all  directions. 
A  distinct  nucleus  can  be  seen  in  the  deepest  part  of  the  cavity ; 
while  a  contractile  vesicle  lies  imbedded  in  the  sarcodic  substance 
nearer  the  mouth.  Multiplication  by  duplicative  subdivision  has 
been  distinctly  observed  in  this  type ;  but  with  a  peculiar  departure 


HEL10ZOA  73; 

from  the  usual  method.  A  transverse  constriction  divides  the  body 
into  two  halves— as  shown  in  two  individuals  of  colony  A — each  half 
possessing  its  own  nucleus  and  contractile  vesicle  ;  the  posterior  seg- 
ment, which  at  first  lies  free  at  the  bottom  of  the  cell,  then  pi-esses 
forwards  towards  its  orifice,  as  shown  at  C,  and  finally,  by  amoeboid 
movements,  escapes  from  it,  sometimes  stretching  itself  out  like  a 
worm  (as  seen  at  D),  sometimes  contracting  itself  into  a  globe,  and 
sometimes  spreading  itself  out  irregularly  over  the  pseudopodia  of 
the  colony.  But  it  finally  gather.^  itself  together  and  takes  an  oval 
form  ;  and  either  develops  a  pair  of  iiagella,  and  forsakes  the  colony 
as  a  free-swimming  monad,  or  assumes  the  form  of  an  Actinophrys, 
moving  about  by  three  or  four  pointed  pseudopodia — probably  in 
each  case  coming  after  a  time  to  rest,  excreting  a  shell,  and  laying 
the  foundation  of  a  new  colony.  There  is  reason  to  think  that  a 
multiplication  by  longitudinal  fission  also  takes  place,  in  which  the 
escaping  segment  and  the  one  left  behind  in.  the  old  shell  remain 
attached  by  their  pseudopodia,  and  the  former  develops  a  new  shell 
without  undergoing  any  change  of  condition. 

Heliozoa.1 — The  Actinophrya  sol,  sometimes  termed  the  -  sun- 
animalcule  '  (fig.  573),  is  one  of  the  commonest  examples  of  this  group, 
being  often  met  with  in  lake>,  ponds,  and  streams,  amongst  Conferva3 
and  other  aquatic  plants,  as  a  whitish -grey  spherical  particle  dis- 
tinguishable by  the  naked  eye,  from  which  (when  it  is  brought  under 
sufficient  magnifying  power)  a  number  of  very  pellucid,  slender, 
pointed  rods  are  seen  to  radiate.  The  central  portion  of  the  body  is 
composed  of  homogeneous  sarcode,  inclosing  a  distinct  nucleus ;  but 
the  peripheral  part  has  a  'vesicular'  aspect,  as  in  the  type  next 
to  be  described  (fig.  574).  This  appearance  is  due  to  the  number 
of  l  vacuoles '  filled  with  a  watery  fluid,  which  are  included  in 
the  sarcodic  substance,  and  which  may  be  artificially  made  either 
to  coalesce  into  larger  ones  or  to  subdivide  into  smaller.  A  '  con- 
tractile vesicle.'  pulsating  rhythmically  with  considerable  regu- 
larity, is  always  to  be  distinguished,  either  in  the  midst  of  the 
sarcode  body,  or  (more  commonly)  at  or  near  its  surface ;  and 
it  sometimes  projects  considerably  from  this,  in  the  form  of  a 
sacculus  with  a  delicate  membranous  wall,  as  shown  at  fig.  573, 
A,  cv.  The  cavity  of  this  sacculus  is  not  closed  externally,  but 
communicates  with  the  surrounding  medium — not,  however,  by  any 
distinct  and  permanent  orifice,  the  membraniform  wall  giving  way 
when  the  vesicle  contracts,  and  then  closing  over  again.  This  alter- 
nating action  seems  to  serve  a  respiratory  purpose,  the  water  thus 
taken  in  and  expelled  being  distributed  through  a  system  of  channels 
and  vacuoles  excavated  in  the  substance  of  the  body,  some  of  the 
vacuoles  which  are  nearest  the  surface  being  observed  to  undergo 
distension  when  the  vesicle  contracts,  and  to  empty  themselves 
gradually  as  it  refills.  The  body  of  this  animal  is  nearly  motionless,2 

1  A    systematic    account    of   this    group  is  to  be  found  in    Dr.   F.  Schaudiim's 
'Heliozoa,'  the  first  part    of   the   comprehensive  Das    T/iterreicJi,   edited    by    the 
German  Zoological  Society,  Berlin,  189(5.     M.  Peiiard's  memoir, '  Etudes  sur  quelques 
Heliozoaires  d'Eau  Douce,'  in  vol.  ix.  of  the  Arch,  de  Biol.,  should  be  consulted. 

2  A  swimming  Heliozoiin  has  lately  been  described  by  M.  E.  Pennrd,  who  calls  it 
Myrioplinjs  paradn.ru . 

3  B 


738        MICROSCOPIC   FORMS   OF   ANIMAL   LIFE— PROTO/OA 

but  it  is  supplied  with  nourishment  by  tJie  instrumentality  of  its 
pseudopodia,  its  food  being  derived  not  merely  from  vegetable  par- 
ticles, but  from  various  small  animals,  some  of  which  (as  the  young  of 
Entomostraca)  possess  great  activity  as  well  as  a  comparatively  high 
organisation.  When  one  of  these  happens  to  come  into  contact  with 
one  of  the  pseudopodia  (which  have  firm  axis-filaments  (ax)  clothed 
with  a  granular  sarcode),  this  usually  retains  it  by  adhesion  ;  but  the 
mode  in  which  the  particle  thus  taken  captive  is  introduced  into  the 
body  differs  according  to  circumstances.  If  the  prey  is  large  and 
vigorous  enough  to  struggle  to  escape  from  its  entanglement,  it  may 
usually  be  observed  that  the  neighbouring  pseudopodia  bend  over  and 


FIG.  573. — Actinophrys  sol :  A,  figure  showing  the  wide  vacuolated  cortical 
layer  or  ectosarc  (B)  and  the  fine  granulated  endosarc  (M)  ;  n,  central 
nucleus,  ax,  axial  filaments  of  pseudopodia  ;  cv,  contractile  vacuole  ;  N,  food- 
mass  inclosed  in  a  large  food-vacuole.  B,  a  colony  of  four  individuals,  after 
treatment  with  acetic  acid  ;  K,  M,  and  N,  as  before  ;  v,  v,  vacuoles.  C,  a  cyst ; 
z,  r,  outer  and  inner  envelopes.  D,  a  burst  .cyst  from  which  the  young  is 
escaping,  though  still  inclosed  by  the  inner  envelope.  (From  Biitschli, 
after  Grenadier,  Stein,  and  Cienkowsky.) 

apply  themselves  to  it,  so  as  to  assist  in  holding  it  captive,  and  that  it 
is  slowly  drawn  by  their  joint  retraction  towards  the  body  of  its 
captor.  Any  small  particle  not  capable  of  offering  active  resistance, 
011  the  other  hand,  may  be  seen  after  a  little  time  to  glide  towards 
the  central  body  along  the  edge  of  the  pseudopodium,  without  any 
visible  movement  of  the  latter,  much  in  the  same  manner  as  in  Gromia. 
"When  in  either  of  these  modes  the  food  has  been  brought  to  the 
surface  of  the  body,  this  sends  over  it  on  either  side  a  prolongation  of 


HELIOZOA 


739 


its  own  Barcode-substance;  and  thus  a  marked  prominence  is  formed 
(fig.  573,  A,  N),  which  gradually  subsides  as  the  food  is  drawn  more 
completely  into  the  interior.  The  struggles  of  the  larger  animals, 
and  the  ciliary  action  of  Infusoria  and  Rotifera,  may  sometimes  be 
observed  to  continue  even  after  they  have  been  thus  received  into 
the  body  ;  but  these  movements  at  last  cease,  and  the  process  of 
digestion  begins.  The  alimentary  substance  is  received  into  one 
of  the  vacuoles,  where  it  lies  in  the  first  instance  surrounded 
by  liquid ;  and  its  nutritive  portion  is  gradually  converted  into 
an  indistinguishable  gelatinous  mass,  which  becomes  incorporated 
with  the  material  of  the  Barcode-body,  as  may  be  seen  by  the 
general  diffusion  of  any  colouring  particles  it  may  contain.  Several 


FIG.  574. — Actinosphcerium  Eichornii  :  m,  endosarc  ;  r,  ectosarc  ; 
c,  c,  contractile  vacuoles. 


vacuoles  may  be  thus  occupied  at  one  time  by  alimentary  particles  ; 
frequently  four  to  eight  are  thus  distinguishable,  and  occasionally 
ten  or  twelve  ;  Ehrenberg,  in  one  instance,  counted  as  many  as 
sixteen,  which  he  described  as  multiple  stomachs.  Whilst  the 
digestive  process,  which  usually  occupies  some  hours,  is  going  on, 
a  kind  of  slow  circulation  takes  place  in  the  entire  mass  of  the  endo- 
sarc with  its  included  vacuoles.  If,  as  often  happens,  the  body 
taken  in  as  food  possesses  some  hard  indigestible  portion  (as  the  shell 
of  an  entomostracaii  or  rotifer),  this,  after  the  digestion  of  the  soft 
parts,  is  gradually  pushed  towards  the  surface,  and  is  thence  extruded 
by  a  process  exactly  the  converse  of  that  by  which  it  was  drawn  in. 
If  the  particle  be  large,  it  usually  escapes  at  once  by  an  opening  which 

3  B  2 


740        MICROSCOPIC   FORMS   OF   ANIMAL   LIFE — PROTOZOA 

extemporises  itself  for  the  occasion  ;  but  if  small  it  sometimes  glides 
along  a  pseudopodium  from  its  base  to  its  point,  and  escapes  from 
its  extremity. 

The  ordinary  mode  of  reproduction  in  Actinophrys  seems  to  be 
by  binary  subdivision,  its  spherical  body  showing  an  annular  con- 
striction, which  gradually  deepens  so  as  to  separate  its  two  halves  by 
a  sort  of  hour-glass  constriction,  and  the  .connecting  band  becoming 
more  and  more  slender,  until  the  two  halves  are  completely  separated. 
The  segments  thus  divided  are  not  always  equal,  and  sometimes  their 
difference  in  size  is  very  considerable.  A  junction  of  two  individuals, 
on  the  other  hand,  has  been  seen  to  take  place  in  Actinophrys,  and 
has  been  supposed  to  correspond  to  the  '  conjugation  '  of  protophy tes  ; 
it  is  very  doubtful,  however,  whether  this  junction  really  involves  a 
complete  fusion  of  the  substance  of  the  bodies  which  take  part  in  it, 


FIG.  575. — Marginal  portion  of  Actinospli(erinm  Eichomii 
as  seen  in  optical  section  under  a  higher  magnifying 
power :  m,  endosarc  ;  r,  ectosarc ;  a,  a,  a,  pseudopodia ; 
M,  H,  nuclei  with  nucleoli ;  /',  ingested  food-mass. 

and  there  is  not  sufficient  evidence  that  it  has  any  true  generative 
character.  Under  these  circumstances  we  must  hope  that  Dr.  F. 
Schaudimi's  preliminary  notes  of  his  observations1  may  soon  be 
followed  by  a  more  detailed  account.  This  author  claims  to  have 
demonstrated  the  fusion  of  the  nuclei  of  A.  sol,  and  the  resemblance 
of  the  course  of  events  to  the  maturation  of  the  ova  of  higher 
animals  is  very  striking.  Certain  it  is  that  such  a  junction  or 
'  zygosis '  may  take  place,  not  between  two  only,  but  even  several 
individuals  at  once,  their  number  being  recognised  by  that  of  their 
contractile  vesicles  ;  and  that,  after  remaining  thus  united  for  several 
i  SB.  Akad.,  Berlin,  1896,  p.  49. 


HELK»/OA 


741 


hours    as    a    colony,     they    may     separate    again    without     having 
undergone  any  discoverable  change. 

Under  the  generic  name  Actinophrys  was  formerly  ranked  the 
larger  but  less  common  Heliozoon.  now  distinguished  as  Actino- 
sphcrrium  Eichornii  (fig.  574)  ;  the  pseudopodia  are  longer  and 
more  numerous  ;  there  are  generally  a  number  instead  of  one  con- 
tractile vacuole,  and  there  is  more  than  one  nucleus.  The  axis  of  the 
pseudopodia  may  be  seen  to  be  clothed  with  a  layer  of  soft  sarcode 
derived  from  the  super- 
ficial or  cortical  zone  of 
the  body.  Severn!  nuclei 
(v?,  n)  are  usually  to  be 
seen  imbedded  in  the 
protoplasmic  mass.  The 
general  life-history  of 
this  type  corresponds 
with  that  of  the  pre- 
ceding, but  its  mode  of 
reproduction  presents 
some  marked  peculiari- 
ties. In  many  if  not  in 
all  cases  it  commences. 
as  first  observed  by 
Kolliker,  with  the  con- 
jugation of  two  separate 
individuals.  The  binary 
segmentation  is  pre- 
ceded by  a  withdrawal 
of  the  pseudopodia.  even 
their  clearly  defined 
axis  becoming  indistinct 
and  finally  disappear- 
ing ;  the  body  becomes 
enveloped  by  a  clear 
gelatinous  exudation, 
which  forms  a  kind  of 
cyst  ;  and  within  this 
the  process  of  binary 

subdivision  is  repeatedly 

f  i  ,-i       .  i  " 

performed,       until      the 

original    Single    mass   is 
replaced    by   a    sort    of 

morula,  each  spherule  of  which  shows  the  distinction  between  the 
central  and  cortical  regions,  the  former  including  a  single  nucleus, 
whilst  the  latter  is  strengthened  by  siliceous  deposit  into  a  firm 
investment.  After  remaining  in  this  state  during  the  winter  the 
young  ActinoepJuxria  come  forth  in  the  spring  without  this  siliceous 
investment,  and  gradually  grow  into  the  likeness  of  their  parent.1 

1  On  the  results  of  the  artificial  division  of  Actinospharium  see  K.  Brandt,  Ueber 
Actinospkcerium  Eirhornii,  Halle  a/S.,  1877;  Gruber,  Bericltte  d.  Naturf.  Ges.  zu 
Freiburg  i/B.,  18s»>  ;  Nussbaum,  Arcli.f.  Mikr.  A)iat.  xxvi. 


FIG.  576.  —  Claihruhna  elcqans:  A,  complete 
organism;  B,  swarm-spore  showing  nucleus,  n, 
and  two  contractile  vesicles  near  its  opposite  end. 


742        MICROSCOPIC   FORMS   OF  ANIMAL  LIFE — PROTOZOA 


A  large  number  of  new  arid  curious  fresh-water  forms  of  this 
type  are  being  frequently  brought  under  notice,  of  which  the  Clatlrn- 
llna  elegans  (fig.  576)  may  be  specially  mentioned  as  presenting  an 
obvious  transition  to  the  Polycystine  type.  This  has  been  found 
in  various  parts  of  the  -Continent,  and  also  (by  Mr.  Archer1)  in 
Wales  and  Ireland,  occurring  chiefly  in  dark  ponds  shaded  by 
trees  and  containing  decaying  leaves.  Its  soft  sarcode-body,  which 
is  not  differentiated  into  ectosarc  and  endosarc,  is  encased  by  a 
siliceous  capsule  of  spherical  form,  regularly  perforated  with  oval 
apertures,  and  supported  on  a  long  silicified  peduncle.  The  body 
itself  and  the  pseudopodia  which  it  puts  forth  through  the  aper- 
tures of  the  capsule  seem  closely  to  correspond  with  those  of 
Actinophrys.  Reproduction  here  takes  place  not  only  by  binary 
fission,  but  by  the  formation  of  '  swarm-spores.'  In  the  first  mode, 
one  of  the  two  segments  remains  in  possession  of  the  siliceous  cap- 
sule, whilst  the  other  finds  its  way  out  through  one  of  the  apertures. 
lives  for  some  hours  in  a  free  condition  as  an  Actinophrys,  and 
ultimately  produces  the  capsule  and  stem  characteristic  of  its  type. 
In  the  second  mode  numerous  small  rounded  sarcode  masses,  cadi 
possessing  a  nucleus,  are  produced  within  the  capsule,  in  what 
manner  cannot  be  clearly  made  out  ;  and  every  one  of  these  is 

enveloped  in  a  firm  en- 
velope, set  round  with 
short  spines,  probably 
siliceous.  These  cysts 
remain  for  months  with- 
in the  common  capsule  ; 
siiid  when  the  time  arrives 
for  their  further  develoj  >- 
meiit  the  sarcode-cdr- 
puscles  slip  out  of  their 
cysts,  and  escape  through 
the  orifices  of  the  capsule 
as  flagellated  monads  of 
oval  form  (fig.  576,  B), 

FIG.  577.-Diagrammatic  representation  of  \  Amoeba    each    having    a    nucleus. 
proteus:  hi  (J,  ectosarc;  Jii  JN,  endosarc;  C  V,  con-  ,-,  °  ,  ,,     , 

tractile   vesicle;    N,   nucleus;   P,    pseudopodia; 
VIL,  villous  tuft. 


VIL 


, 

tile 


,-,       , 

near  tne    lmse 
flagella,     and    two     con- 
tractile  vesicles  near  its 

opposite  end.  After  swarming  for  some  hours  in  this  condition, 
they  change  to  the  free  Actinophrys  form,  and  finally  acquire  the 
siliceous  capsule  and  stem  of  the  Clathruliiia. 

Lobosa.  —  No  example  of  the  rhizopod  type  is  more  common  in 
streams  and  ponds,  vegetable  infusions,  &c.,  than  the  Anweba 
(fig.  577);  a  creature  which  cannot  be  described  by  its  form,  for 
this  is  as  changeable  as  that  of  the  fabled  Proteus,  but  may  yet  be 
definitely  characterised  by  peculiarities  that  separate  it  from  the 
two  groups  already  described.  The  distinction  between  *  ectosarc  ' 
and  *  endosarc  '  is  here  clearly  marked,  so  that  the  body  approaches 

1  See  his  memoir  on  Fresh-water  Radiolaria  in  Quart.  Journ.  of  Microsc.  Sri. 
n.s.  vol.  ix.  1869,  p.  250. 


LOBOSA  743 

much  more  closely  in  its  characters  to  an  ordinary  '  cell '  composed 
of  cell-wall  and  cell-contents.  It  is  through  the  '  endosarc '  alone, 
E  X,  that  those  coloured  and  granular  particles  are  diffused,  on 
which  the  hue  and  opacity  of  the  body  depend  ;  its  central  portion 
seems  to  have  an  almost  watery  consistence,  the  granular  particles 
being  seen  to  move  quite  freely  upon  one  another  with  every  change 
in  the  shape  of  the  body  ;  but  its  superficial  portion  is  more  viscid, 
and  graduates  insensibly  into  the  firmer  substance  of  the  '  ectosarc.' 
The  ectosarc,  E  C,  which  is  perfectly  pellucid,  forms  an  almost 
membranous  investment  to  the  endosarc  ;  still  it  is  not  possessed  of 
such  tenacity  as  to  oppose  a  solution  of  its  continuity  at  any  point,  for 
the  introduction  of  alimentary  particles,  or  for  the  extrusion  of  effete 
matter  ;  l  and  thus  there  is  no  evidence,  in  Amceba  and  its  immediate 
allies,  of  the  existence  of  anymore  definite  orifice,  either  oral  or  anal, 
than  exists  in  other  rhizopods.  The  more  advanced  differentia- 
tion of  the  ectosarc  from  the  endosarc  of  Amceba  is  made  evident 
by  the  effects  of  reagents.  If  an  Amceba  radiosa  be  treated  with 
a  dilute  alkaline  solution,  the  granular  and  molecular  endosarc 
sin-inks  together  and  retreats  towards  the  centre,  leaving  the  radia- 
ting extensions  of  the  ectosarc  in  the  condition  of  cfiecal  tubes,  of 
w^hich  the  walls  are  not  soluble  at  the  ordinaiy  temperature  either 
in  acetic  or  mineral  acids,  or  in  dilute  alkaline  solutions,  thus 
agreeing  with  the  envelope  noticed  by  Cohn  as  possessed  by  Para- ' 
'niecium  and  other  ciliated  Infusoria,  and  with  the  containing  mem- 
brane of  ordinary  animal  cells.  A  'nucleus,'  N,  is  always  distinctly 
visible  in  Amceba,  adherent  to  the  inner  portion  of  the  ectosarc,  and 
projecting  from  this  into  the  cavity  occupied  by  the  endosarc ;  when 
most  perfectly  seen  it  presents  the  aspect  of  a  clear  flattened  vesicle 
surrounding  a  solid  and  usually  spherical  nucleolus ;  it  is  readily 
soluble  in  alkalies,  and  first  expands  and  then  dissolves  when  treated 
with  acetic  or  sulphuric  acid  of  moderate  strength ;  but  when 
treated  with  dilute  acid  it  is  rendered  darker  and  more  distinct,  in 
consequence  of  the  precipitation  of  a  finely  granular  substance  in 
the  clear  vesicular  space  that  surrounds  the  nucleolus.  A  i  contrac- 
tile vesicle,'  C  V,  seems  also  to  be  uniformly  present,  though  it 
does  not  usually  make  itself  so  conspicuous  by  its  external  prominence 
as  it  does  in  Actinoplirys ;  and  the  neighbouring  part  of  the  body 
is  often  prolonged  into  a  set  of  villous  processes,  V  I  L,  the  presence 
of  which  has  been  thought  by  some  to  mark  a  specific  distinction, 
but  which  seems  too  variable  and  transitory  to  be  so  regarded. 

The  pseudopodia,  which  are  not  appendages,  but  lobate  exten- 
sions of  the  body  itself,  are  few  in  number,  short,  broad,  and  rounded  ; 
and  their  outlines  present  a  sharpness  which  indicates  that  the 
substance  of  which  their  exterior  is  composed  possesses  considerable 
tenacity.  No  movement  of  granules  can  be  seen  to  take  place  along 
the  surface  of  the  pseudopodia  ;  and  when  two  of  these  organs  come 

1  This  remarkable  character  has  been  stated  by  Professor  Huxley  in  the  following 
admirable  sentence :  '  Physically  the  ectosarc  might  be  compared  to  the  wall  of  a 
soap-bubble,  which,  though  fluid,  has  a  certain  viscosity,  which  not  only  enables  its 
particles  to  hold  together  and  form  a  continuous  sheet,  but  permits  a  rod  to  be  passed 
into  or  through  the  bubble  without  bursting  it,  the  walls  closing  together,  and  re- 
covering their  continuity  as  soon  as  the  rod  is  drawn  away.' 


744        MICROSCOPIC    FORMS   OF   ANIMAL   LIFE — PROTOZOA 

into  contact  they  scarcely  sho\v  any  disposition  even  to  mutual 
cohesion,  still  less  to  fusion  of  their  substance.  Sometimes  the 
protrusion  seems  to  be  formed  by  the  ectosarc  alone,  but  more 
commonly  endosarc  also  extends  into  it.  and  an  active  current  of 
granules  may  be  seen  to  pass  from  what  was  previously  the  centre 
of  the  body  into  the  protruded  portion,  when  the  latter  is  undergoing 
rapid  elongation  ;  whilst  a  like  current  may  set  towards  the  centre 
of  the  body  from  some  other  protrusion  which  is  being  withdrawn 
into  it.  It  is  in  this  manner  that  an  Amoeba  moves  from  place  to 
place,  a  protrusion  like  the  finger  of  a  glove  being  first  formed,  into 
which  the  substance  of  the  body  itself  is  gradually  transferred,  and 
another  protrusion  being  put  forth,  either  in  the  same  or  in  some 
different  direction,  so  soon  as  this  transference  has  been  accom- 
plished, or  even  before  it  is  complete.  The  kind  of  progression  thus 
executed  by  an  Amoeba  is  described  by  most  observers  as  a  '  rolling  ' 
movement,  this  being  certainly-  the  aspect  which  it  commonly 
seems  to  present ;  but  it  is  maintained  by  MM.  Olaparede  and 
Lachmann  that  the  appearance  of  rolling  is  an  optical  illusion, 
since  the  nucleus  and  contractile  vesicle  always  maintain  the  same 
position  relatively  to  the  rest  of  the  body,  and  that  '  creeping '  would 
be  a  truer  description  of  the  mode  of  progression.  It  is  in  the 
course  of  this  movement  from  place  to  place  that  the  Amoeba  en- 
counters particles  which  are  fitted  to  afford  it  nourishment ;  and  it 
appeal's  to  receive  such  particles  into  its  interior  through  any  part 
of  the  ectosarc,  whether  of  the  body  itself  or  of  any  of  its  lobose 
expansions,  insoluble  particles  which  resist  the  digestive  process 
being  got  rid  of  in  the  like  primitive  fashion. 

It  may  often  be  seen  that  portions  of  the  sarcode-body  of  an 
Amoeba,  detached  from  the  rest,  can  maintain  an  independent  exist- 
ence ;  and  it  is  probable  that  such  separation  of  fragments  is  a,ii 
ordinary  mode  of  increase  in  this  group.  When  a  pseudopodial  lobe 
lias  been  put  forth  to  a  considerable  length,  and  has  become  en- 
larged and  fixed  at  its  extremity,  the  subsequent  contraction  of  the 
connecting  portion,  instead  of  either  drawing  the  body  towards  the 
fixed  point,  or  retracting  the  lobe  into  the  body,  causes  the  connect- 
ing band  to  thin  away  until  it  separates  ;  and  the  detached  portion 
speedily  shoots  out  pseudopodial  processes  of  its  own,  and  comports 
itself  in  all  respects  as  an  independent  Amosba.  Multiplication 
also  takes  place  by  regular  binary  subdivision.  Various  observer's 
have  seen  phenomena  which  they  have  supposed  to  be  evidence  of  the 
formation  of  '  swarm-spores '  *  or  of  the  development  of  cysts,  but  it 
must  be  borne  in  mind  that  a  large  number  of  protozoa  pass  during 
the  course  of  their  life  through  amoebiform  stages,  some  of  which 
may  have  been  taken  as  true  species  of  Amoeba.  No  sexual  act  has 
been  certainly  recognised  as  part  of  the  life-history  of  Amoeba,  the 
union  of  two  or  more  individuals,  which  may  be  occasionally  wit- 
nessed, having  more  the  character  of  the  '  zygosis  '  of  Actinophiys. 
A  sarcodic  organism  discovered  by  (ireef,  and  named  by  him 
Pel&myxa  palustris  (fig.  578),  which  spreads  over  the  bottom  of 
stagnant  ponds  in  the  condition  of  sliniy  masses  of  indefinite  form, 
1  Prof.  A.  M.  Edwards  (U.S.A.)  in  Monthly  Microbe.  Joi/ni.  vol.  viii.1872,  p.  29. 


LOBOSA  745 

exhibits  a  further  advance  upon  the  Amoeban  type.  The  substance 
of  its  body,  which  may  be  of  the  size  of  two  millimetres,  exhibits  a 
very  clear  differentiation  between  the  homogeneous  hyaline  ectosarc 
(B.  a,  d)  and  the  contained  eiidosarc,  which  contains  such  a  multi- 
tude of  spherical  vacuoles,  6,  as  to  have  a  'vesicular'  or  frothy 
aspect.  When  it  feeds  upon  the  decomposing  vegetable  matter  at 
the  bottom  of  the  pool  it  inhabits,  its  body  acquires  a  blackish  hue, 
but  in  other  situations  it  may  be  colourless.  Besides  the  vacuoles 
there  are  seen  in  the  endosarc  a  ^reat  number  of  nucleus-like  bodies, 


FIG.  5!S.—  PeJomyxa  palu&tris:  A,  as  it  appears  when  in  amoeboid 
motion  ;  B,  portion  more  highly  magnified,  showing  «,  «,  the  hyaline 
ectosarc ;  fo,  one  of  the  vacuoles  of  the  endosarc  ;  r,  rod-like  bodies  (pro- 
bably Bacteria]  scattered  through  the  endosarc ;  d,  protruded  exten- 
sion of  ectosarc  with  endosarc  passing  into  it;  e,e,  nuclei ;  /,/,  globular 
hyaline  bodies.  •- 

e,  e,  and  also  many  hyaline  globular  brilliant  bodies,  f,f,  which  are 
regarded  by  Greef  as  germs  or  swarm-spores  developed  from  nucleoli 
set  free  within  the  general  cavity  of  the  body  by  the  bursting  of  the 
nuclei.  This  creature  during  the  active  period  of  its  life  moves  like 
an  amoeba,  either  by  general  undulations  of  its  surface,  or  by  special 
pseudopodial  extensions,  d.  After  a  time,  however,  its  movements 
cease,  and  it  looks  as  if  dead  ;  but  by  the  giving  way  of  its  ecto- 
sarc. a  multitude  of  minute  amoebiform  bodies  break  forth,  each 
1  laving  its  nucleus  and  contractile  vesicle.  These  at  first  live  as 
Ain<x,lw\  but  afterwards  pass  into  a  resting  state,  assuming  a  spherical 


746        MICROSCOPIC   FORMS   OF   ANIMAL  LIFE  -  PROTOZOA 

or  oval  shape,  and  then  put  forth  flagella,  by  which  they  swim 
actively  for  a  time ;  later  on,  they  probably  settle  down  to  develop 
themselves  into  the  parental  form. 

The  Amoeban  like  the  Actinophryaii  type  shows  itself  in  the 
testaceous  as  well  as  in  the  naked  form,  the  commonest  examples 
of  this  being  known  under  the  names  Arcella  and  Difflugia.  The 
body  of  the  former  is  inclosed  in  a  '  test '  composed  of  a  horny 
membrane,  apparently  resembling  in  constitution  the  chitln  which 
gives  solidity  to  the  integuments  of  insects ;  it  is  usually  discoidal 
(fig.  579,  C,  D)  \\dth  one  face  flat  and  the  other  arched,  the  aperture 
being  in  the  centre  of  the  Hat  side  ;  and  its  surface  is  often  marked 
with  a  minute  and  regular  pattern.  The  test  of  Difflugia,  on  the 
other  hand,  is  more  or  less  pitcher-shaped  (A,  B),  and  is  chiefly 
made  up  of  minute  particles  of  gravel,  shell,  &c.  cemented  together. 
In  each  of  these  genera  the  sarcode-body  resembles  that  of  Amoeba 
in  every  essential  particular,  the  contrast  being  very  marked  be- 
tween its  large,  distinct,  lobose  extensions,  and  the  ramifying  and 
inosculating  pseudopodia  of  Groniia  (fig.  571).  In  each  case  a  de- 
tached portion  of  the  sarcodic-  body  will  put  forth  pseudopodia  of 


FIG.  579. — Testaceous  forms  of  Amoeban  rhizopods :  A,  Difflngia 
proteiformis  ;  B,  Difflngia  oblonga ;  C,  Arcella  acuminata ;  D, 
Arcella  dentata. 

its  own  type  ;  and  the  separation  of  a  bud  or  gemmule  put  forth 
from  the  mouth  of  the  test  seems  to  be  an  ordinary  mode  of  propa- 
gation among  the  amcebans  thus  inclosed.  In  Arcella  it  has  been 
observed  that  the  pseudopodia  of  two  or  more  individuals  unite  by 
bridges  of  protoplasm,  and  afterwards  separate  ;  and  it  seems  to  be 
almost  certain  that  this  is  a  true  'conjugation,'  and  not  a  mere 
'  zygosis.'  A  remarkable  method  of  reproduction  has  been  observed 
by  Gruber  in  Euglypha  alveolata  ;  in  an  active  form  highly  refractive 
bodies,  which,  seen  from  the  surface,  look  like  discs,  are  to  be  found 
beside  the  nucleus.  Reproduction  commences  with  the  protrusion 
of  protoplasm  from  the  orifice  of  the  test,  and,  later  011,  the  just- 
mentioned  bodies  pass  out  also,  and  form  a  covering  for  the  extruded 
protoplasm ;  in  about  an  hour  the  process  is  complete,  but  the  new 
or  daughter-cell  is  still  without  a  nucleus.  This  is  derived  from  that 
of  the  mother,  which  increases  in  size,  elongates  greatly,  and  then 
becomes  constricted ;  the  anterior  portion  passes  into  the  daughter?* 
cell.  Here  we  have  the  remarkable  phenomena  of  the  formation  of 
the  test  by  the  parent-cell  and  the  rare  case  of  division  of  the 
protoplasmic  body  preceding  that  of  its  nucleus. 


COCCOLITH8   AND   COCCOSPHERES 


747 


Many  testaceous  amaibans  have  been  recently  discovered,  whicli 
form  tests  of  remarkable  regularity  and  sometimes  of  singular 
beauty ;  and  it  is  difficult  to  determine,  in  many  cases,  whether  the 
minute  plates  of  which  they  are  composed  have  been  formed  by 
exudation  from  their  own  bodies  or  have  been  picked  up  from  the 
surface  over  which  the  animals  crawl.  There  can  be  no  doubt  of 
this  kind,  however,  in  regard  to  the  Qiiadrula  symmetrica  repre- 
sented in  fig.  580  ;  the  sarcode-body  is  here  encased  in  a  pear-shaped 
test,  of  glassy  transparence,  made  up  of  a  great  number  of  square 
plates  which  touch  each  other  by  their  edges.  The  sarcode-body 
does  not  usually  fill  the  test,  the  intervening  space  being  occupied 
by  a  clear  liquid,  and  traversed  by  bands  of  protoplasm.  In  the 
posterior  part  of  the  body  is  seen  a  large  clear  spherical  nucleus, 
with  a  distinct  dark  iiucleo- 
lus  ;  and  in  front  of  this  are 
contractile  vesicles,  usually 
two  in  number.1 

CoccolitJis  and  Cocco- 
spheres. — This  would  seem 
the  most  appropriate  place 
for  the  description  of  certain 
peculiar  little  bodies  found 
very  extensively  diffused  over 
the  deep-sea  bottom,  espe- 
cially abounding  in  the  Olo- 
bigerina-mud,  which  may  be 
considered  as  chalk  in  process 
of  formation.  It  was  in  the 
specimens  of  this  mud 
brought  up  by  the  '  Cyclops ' 
soundings  in  1857  that  Pro- 
fessor Huxley  first  found 
the  Coccoliths  (fig.  581,  l,  2) 
which  Dr.  Wallich  in  1860 
found  aggregated  in  the 
spherical  masses  which  he 
designated  as  *  coccospheres  ' 
(3).  Regarding  the  gelati- 
nous matrix  in  which  they 
were  imbedded  as  a  new  type  of  the  Monerozoa  described  by  Haeckel, 
having  the  condition  of  an  indefinitely  extended  plasmodium,  Pro- 
fessor Huxley  proposed  to  designate  it  by  the  name  Bathybiiis, 
indicative  of  its  habitat  in  the  depths  of  the  sea ;  and  this  idea  was 
accepted  by  Haeckel,  whose  representation  of  a  living  specimen  of 
Bamybiux,  with  imbedded  coccoliths,  is  given  in  fig.  581,  3.  The 
observations  made  in  the  *  Challenger '  Expedition,  however,  have 
not  confirmed  this  view  ;  the  supposed  Bathybius  being  a  gelatinous 

1  See  especially  the  admirable  work  of  Professor  Leidy  on  the  fresh-water 
rhizopods  of  the  United  States,  1880.  It  is  to  be  regretted  that  its  able  author's  time 
and  opportunities  did  not  permit  him  to  follow  out  the  life-histories  of  the  many 
interesting  forms  whicli  he  has  described  and  figured. 


FIG.  580. — Qnadnda  symmetrica,  with 
extended  pseudopodia. 


748        MICROSCOPIC    FORMS    OF   ANIMAL   LIFE— PROTOZOA 


precipitate,  consisting  of  sulphate  of  lime,  slowly  deposited  in  water 
to  which  strong  spirit  has  been  added.  Whatever  be  their  nature,1 
coccoliths  and  coccospheres  are  bodies  of  great  interest ;  since  their 
occurrence  in  chalk  and  in  very  early  limestones  is  an  additional  link 
in  the  evidence  of  the  similarity  of  the  conditions  under  which  they 
were  formed  to  those  at  present  prevailing  on  the  sea-bed  of  the 
Atlantic  and  other  oceans.  Two  distinct  types  are  recognisable  among 
the  coccoliths,  which  Professor  Huxley  has  designated  respectively 
discoliths  and  cyatholiths.  The  former  are  round  or  oval  discs,  having 
a  thick  strongly  refracting  rim  and  a  thinner  internal  portion,  the 
greater  part  of  w^hich  is  occupied  by  a  slightly  opaque,  cloud-like 
patch  lying  round  a  central  corpuscle  (fig.  518,  5).  In  general,  the 
'  discoliths '  are  slightly  convex  011  one  side,  slightly  concave  011  the 
other,  and  the  rim  is  raised  into  a  prominent  ridge  011  the  more 


FIG.  581.  —  Coccoliths  and  Coccospheres  :  1,  2,  7,  cyatholiths  seen 
obliquely  ;  8,  coccosphere  with  imbedded  cyatholiths  ;  4,  coccoliths  im- 
bedded in  supposed  protoplasmic  expansion  ;  5,  discolith  seen  in  front 
view;  6,  cyatholith  seen  in  front  view,  showing  (1)  central  corpuscle,  (2) 
granular  zone,  (8)  transparent  outer  zone  ;  8,  9,  discoliths  seen  edgewise. 

convex  side  ;  so  that  when  viewed  edgewise  they  present  the  appear- 
ances shown  in  figs.  <9,  9.     Their  length  is  ordinarily  between 


of  an  inch  ;  but  it  ranges  from  ^Vo-th  to 


T  t 


Th 


largest  are  commonly  free,  but  the  smallest  are  generally  found  im- 
bedded among  heaps  of  granular  particles,  of  which  some  are  probably 
discoliths  in  an  early  stage  of  development.  The  '  cyatholiths,'  also, 
which  have  the  general  appearance  of  a  cup  and  saucer,  have,  when 
full  grown,  an  oval  contour,  though  they  are  oftrtl  circular  when 
immature.  They  are  convex  011  one  face  and  flat  or  concave  on  the 
other  ;  and  when  left  to  themselves  they  lie  on  one  or  other  of  these 
two  faces.  In  either  of  these  aspects  they  seem  to  be  composed  of 
two  concentric  zones  (fig.  6,  2,  3)  surrounding  an  oval  thick-  walled 
central  corpuscle  (^),  in  the  centre  of  which  is  a  clear  space  some- 

1  Messrs.  Murray  and  Blackmail  have,  in  a  preliminary  notice  (Pror.  Hoy.  Soc. 
London,  Ixiii.  1898,  p.  269),  suggested  that  the  Coccospheraceae  are  unicellular  Algtv. 


SPOKOZOA  749 

times  divided  into  two.  The  zone  (2)  immediately  surrounding 
the  central  corpuscle  is  usually  more  or  less  distinctly  granular. 
and  sometimes  has  an  almost  bead-like  margin.  The  narrower 
outer  zone  (3)  is  generally  clear,  transparent,  and  structureless. 
but  sometimes  shows  radiating  stria?.  When  viewed  sidewise  or 
obliquely,  however,  the  '  cyatholiths  '  are  found  to  have  a  form 
somewhat  resembling  that  of  a  shirt-stud  (figs.  1,  2,  7).  Each  con- 
sists of  a  lower  plate,  shaped  like  a  deep  saucer  or  watch-glass  ;  of 
a  smaller  upper  plate,  which  is  sometimes  flat,  sometimes  more  or 
less  concavo-convex  ;  of  the  oval,  thick-walled,  flattened  corpuscle, 
which  connects  these  two  plates  together  at  their  centres  ;  and  of 
an  intermediate  granular  substance  which  more  or  less  completely 
fills  up  the  interval  between  the  two  plates.  The  length  of  these 
cyatholiths  ranges  from  about  yeViyth  to  wL-Qth  of  an  inch,  those  of 


IT  oVo  °f  an  incn  and  under  being  always  circular.  It  appears 
from  the  action  of  dilute  acids  upon  the  coccoliths  that  they  must 
mainly  consist  of  calcareous  matter,  as  they  readily  dissolve,  leaving 
scarcely  a  trace  behind.  When  the  cyatholiths  are  treated  with 
very  weak  acetic  acid,  the  central  corpuscle  rapidly  loses  its  strongly 
refracting  character  ;  and  there  remains  an  extremely  delicate, 
finely  granular  membranous  framework.  When  treated  with  iodine 
they  are  stained,  but  not  very  strongly,  the  intermediate  sub- 
stance being  the  most  affected.  Both  discoliths  and  cyatholiths  are 
completely  destroyed  by  strong  hot  solutions  of  caustic  potass  or 
soda.  The  coccospheres  (fig.  3)  are  made  up  by  the  aggregation  of 
bodies  resembling  '  cyatholiths  '  of  the  largest  size  in  all  but  the 
absence  of  the  granular  zone  ;  they  sometimes  attain  a  diameter  of 
T(joth  of  an  inch.  What  is  their  relation  to  the  coccoliths,  and 
under  what  conditions  these  bodies  are  formed,  are  questions  whereon 
no  positive  judgment  can  be  at  present  given. 

SPOROZOA. 

The  term  Sporozoa  was  applied  by  Leuckart  to  a  group  of 
protozoic  animals  of  which  the  well-known  Gregarinida,  the  Coccidi- 
idea,  the  Ha?mosporidia,  the  Myxosporidia,  and  the  Sarcosporidia  !  are 
the  chief  divisions.  They  are  especially  characterised  by  the  peculi- 
arities of  their  mode  of  reproduction,  in  which  a  period  of  encystation 
(which  may  or  may  not  be  preceded  by  conjugation)  is  succeeded  by 
the  breaking  up  of  the  contained  protoplasm  into  a  large  number  of 
small  '  spores,'  the  products  of  which  become  intracellular  parasites. 

The  Gregarinida  lead  a  parasitic  life,  and  may  often  be  met  with 
in  the  intestinal  canal  or  other  cavities  of  earthworm,  insects,  &c., 
and  sometimes  in  that  of  higher  animals.  An  individual  Greyarina 
essentially  consists  of  a  large  single  cell,  usually  more  or  less  ovate 
in  form,  and  sometimes  attaining  the  extraordinary  length  of  two- 
thirds  of  an  inch.'2  A  sort  of  beak  or  proboscis  frequently  projects 
from  one  extremity  ;  and  in  some  instances  this  is  furnished  with  a 

1  Consult  the  memoir  by  Dr.  K.  Blanchard  in  Sail.  Soc.  ZooL  France,  x.  p.  '244. 

-  See  Prof.  Ed.  Van  Beneden  on  Oregarina  gigantea  (found  in  the  intestinal 
canal  of  the  lobster)  in  Quart.  Journ.  Microsc.  Sci.  n.s.  vol.  x.  1870,  p.  51,  and  vol. 
xi.  p.  242. 


750       MICROSCOPIC   FORMS   OF  ANIMAL  LIFE — PROTOZOA 

circular  row  of  booklets,  closely  resembling  that  which  is  seen  on 
the  head  of  Teenia.  There  is  here  a  much  more  complete  differentia- 
tion between  the  cell-membrane  and  its  contents  than  exists  either 
in  Actinophrys  or  in  Amoeba  ;  and  in  this  respect  we  must  look  upon 
(r'regarina  as  representing  a  decided  advance  in  organisation.  Being 
nourished  upon  the  juices  already  prepared  for  it  by  the  digestive 
operations  of  the  animal  which  it  infests,  it  has  no  need  of  any  such 
apparatus  for  the  introduction  of  solid  particles  into  the  interior  of 
its  body,  as  is  provided  in  the  *  pseudopodia '  of  the  rhizopods  and 
in  the  oral  cilia  of  the  Infusoria.  Within  the  cavity  of  the  cell, 
whose  contents  are  usually  milk-white  and  minutely  granular,  there 
is  generally  seen  a  pellucid  nucleus ;  and  when,  as  often  happens. 


FIG.  582. — Cyst  of  Monocystis  agilis,  the  Gregarinid  of  the  earthworm 
(750  diams.),  showing  ripe  chlamydospores  and  complete  absence  of 
any  residual  protoplasm  in  the  cyst.  (After  Professor  Ray  Laukester.) 


the  cell  undergoes  duplicative  subdivision,  the  process  commences  in 
a  constriction  and  cleavage  of  this  nucleus.  The  membrane  and  its 
contents,  except  the  nucleus,  are  soluble  in  acetic  acid.  The  move- 
ments of  the  body  are  of  very  various  kinds ;  there  is  a  forward 
movement  which  may  be  due,  as  suggested  by  Lankester,  to  the 
undulations  of  the  body.  The  cell  itself  may  undergo  contraction,  and 
consequent  change  in  form,  which  may,  or  may  not.  be  accompanied 
by  locomotion ;  circular  constrictions  may  extend  along  the  body ; 
or  the  cell  may  bend  on  itself  and  again  straighten  out.  By  Van 
Beneden  the  contractility  of  the  cell  is  localised  in  a  layer  of  the 


SPOKOZOA 


751 


ectoplasm,  the  so-called  '  myocyte  '  which  he  has  found  to  consist  of 
a  layer  of  contractile  fibrils.  When  the  process  of  encystation  com- 
mences we  find  that,  whatever  the  original  form  of  the  body  may  be, 
it  becomes  globular,  ceases  to  move,  and  becomes  invested  by  a 
structureless  '  cyst,'  within  which  the  substance  of  the  body  under- 
goes a  singular  change.  The  nucleus  disappears,  and  the  sarcodic 
mass  breaks  up  into  a  series  of  globular  particles,  which  gradually 
resolve  themselves  (as  shown  at  ft,  c,  d,  e,  fig.  583)  into  forms  very 
like  those  of  Namculw,  and  a  .cyst  more  advanced,  and  greatly 
magnified,  is  shown  in  fig.  582.  These  '  pseudo-navicella? '  or 
'  spores.'  as  it  is  better  to  call  them,  are  set  free  in  time  by  the 
bursting  of  the  capsule  that  incloses  them ;  and  they  develop  them- 
selves into  a  new  generation  of  Gregariiue,  first  passing  through  an 


FIG.  583. —  Gregarina  Scenuridis,  from  testis  of  Tubifex  rivulorum, 
two  adults  uniting :  a,  succeeding  stage ;  b,  encystation  stage ;  in  c 
and  d  the  contents  are  seen  breaking  up;  in  e  the  characteristic 
nseudo-navicellar  form  has  been  acquired  by  the  spores.  (After 
Kolliker.) 


amoeba-like  stage.  A  sort  of  '  conjugation'  has  been  seen  to  take 
place  between  two  individuals  whose  bodies,  coming  into  contact 
with  each  other  by  corresponding  points,  first  became  more  globular 
in  shape,  and  are  then  encysted  by  the  formation  of  a  capsule  around 
them  both ;  the  partition-walls  between  their  cavities  disappear ; 
and  the  substance  of  the  two  bodies  becomes  completely  fused 
together.  But  as  the  products  of  this  *  zygosis '  are  the  same  as  that 
of  the  ordinary  encysting  process,  there  seems  no  sufficient  reason 


752        MICROSCOPIC    FORMS   OF   ANIMAL  LIFE — PROTOZOA 

for  regarding  it.  like   the   '  conjugation  '   of  protophytes,   as  a   true 
generative  act. 

The  Coccidia  (fig.  584)  are  Sporozoa  which  look  like  minute  ova, 
and  which  are  found  resting  within  the  cells  of  their  hosts  ;  the  young, 
developed  from  spores,  are  falciform  in  shape,  and,  moving  about 
actively,  are  able  to  penetrate  fresh  cells.  They  have  been  found  in 
the  epithelium  of  the  intestine  of  various  forms,  and  in  the  liver  of 
vertebrates.  Some  parasites  found  in  the  blood  (Haemamcebidse), 
such  as  Drepanidium  ranarum.  Lankester,  are  allied  to  the  Coccidia, 
but  are  distinguished  by  having  naked  spores.  Their  chief  interest 
lies  perhaps  in  their  relation  to  various  forms  of  malaria.1  Among 


FIG.  584.  —Coccidi.li iii  ovifonne  (Leuckart)  from  the  liver  of  the  rabbit : 
a,  cyst  just  formed;  &,  condensed  contents,  the  outer  envelope  has 
disappeared  ;  c,  contents  divided  into  four  sporoblasts  ;  d,  the  sporo- 
blasts  have  become  rounded  and  clearer  internally;  e  and  /,  formation 
of  the  falciform  germ;  g  and  ft,  spores  more  highly  magnified — ffh'om 
the  side,  //  from  in  front. 

the  Myxosporidia  is  Gluyea,  the  cause  of  the  silkworm  disease.  The 
Sarcosporidia  are  only  known  from  the  striped  muscular  tissue  of 
some  vertebrates. 

Of  the  imperfectly  known  Myxosporidia  it  may  be  said  that 
their  spores  are  the  bodies  which  are  known  as  '  psorosperms  ;  ' 
while  the  bodies  observed  by  Raiiiey  and  others,  and  wrongly 
regarded  as  the  cause  of  the  cattle  plague,  are  sarcocystids  which 
live  in  the  muscular  fibre  of  mammals. 


1  More  and  more  interest  is  being  taken  in  this  subject,  and  some  of  the  result  s  of 
recent  researches  are  of  great  interest  and  importance.  Malaria  appears  to  be  due  to  a 
Hsemamoebid  which  develops  m  gnats  of  the  genus  Anopheles  ;  when  they  arrive  in  the 
human  subject  they  appear  as  minute  amcebulse  which  live  in  or  on  the  ivd  Mood 
corpuscles  ;  they  give  rise  to  sporocytes  which  multiply  indefinitely,  or  to  sexual 
gametocytes  which  undertake  their  sexual  functions  as  soon  as  they  enter  the 
stomach  of  gnats.  See  Ross  and  Fielding  Ould,  Quart.  Jonni.  Micr.  ,SV/.  xliii. 
(1900)  p.  571,  and  a  very  interesting  '  Note  on  the  Morphological  Significance  of  the 
Various  Phases  of  Hiemamoebidte,'  by  E.  Ray  Lankester,  torn.  cit.  p.  581.  The 
student  should  also  consult  M.  A.  Labbe's  '  Recherches  Zoologiques,  Cytologiques 
et  Biologiques  sur  les  Coccidies,'  in  Arch.  Zool.  Exper.  1896,  p.  517  et  *>'</.,  and  Dr. 
Wasielewski's  Sporozoenkunde,  Jena,  1896.  A  detailed  bibliography  will  be  found  in 
Prof.  G.  Sclmeidem Hill's  Die  Protozoen  als  Krankheitaerreger,  Leipzig,  1898.  The 
various  Memoirs  of.Grassi,  Lave  ran,  and  Leger  may  be  profitably  studied. 


753 


CHAPTER   XIII 

ANIMALCULES— INFUSORIA   AND  BOTIFEEA 

NOTHING  can  be  more  vague  or  scientifically  inappropriate  than  the 
title  Animalcules;  since  it  only  expresses  the  small  dimensions  of 
the  beings  to  which  it  is  applied,  and  does  not  indicate  any  of  their 
characteristic  peculiarities.  In  the  infancy  of  microscopic  know- 
ledge, it  was  natural  to  associate  together  all  those  creatures  which 
could  only  be  discerned  at  all  under  a  high  magnifying  power,  and 
whose  internal  structure  could  not  be  clearly  made  out  with  the 
instruments  then  in  use ;  and  thus  the  most  heterogeneous  assem- 
blage of  plants,  zoophytes,  minute  crustaceans,  larvae  of  wrorms, 
molluscs,  Ac.,  came  to  be  aggregated  with  the  true  animalcules 
under  this  head.  The  class  was  being  gradually  limited  by  the 
removal  of  all  such  forms  as  could  be  referred  to  others  ;  but  still 
very  little  was  known  of  the  real  nature  of  those  that  remained  in 
it  until  the  study  was  taken  up  by  Professor  Ehrenberg,  with  the 
advantage  of  instruments  which  had  derived  new  and  vastly  im- 
proved capabilities  from  the  application  of  the  principle  of  achro- 
matism. One  of  the  first  and  most  important  results  of  his  study, 
and  that  which  has  most  firmly  maintained  its  ground,  notwith- 
standing the  overthrow  of  Professor  Ehrenberg's  doctrines  on  other 
points,  was  the  separation  of  the  entire  assemblage  into  two  distinct 
groups,  having  scarcely  any  feature  in  common  except  their  minute 
size,  one  being  of  very  low,  and  the  other  of  comparatively  high 
organisation-  On  the  lower  group  he  conferred  the  designation  of 
Poly  gastr  lea  (many-stomached),  in  consequence  of  having  been  led 
to  form  an  idea  of  their  organisation  which  the  united  voice  of  the 
most  trustworthy  observers  now  pronounces  to  be  erroneous  ;  and 
as  the  retention  of  this  term  must  tend  to  perpetuate  the  error,  it 
is  well  to  fall  back  on  the  name  Infusoria,  or  infusory  animalcules, 
which  simply  expresses  their  almost  universal  prevalence  in  infusions 
of  organic  matter.  To  the  higher  group  Professor  Ehrenberg's 
name  Rotlfara  or  Rotator-la  is,  on  the  whole,  very  appropriate,  as 
significant  of  that  peculiar  arrangement  of  their  cilia  upon  the 
anterior  parts  of  their  bodies,  which,  in  some  of  their  most  common 
forms,  gives  the  appearance  (when  the  cilia  are  in  action)  of  wheels 
in  revolution  ;  the  group,  however,  includes  many  members  in  which 
the  ciliated  lobes  are  so  formed  as  not  to  bear  the  least  resemblance 
to  wheels.  In  their  general  organisation  these  '  wheel -animalcules ' 
stand  at  a  much  higher  level  than  the  unicellular  Infusoria,  but  it 

3  c 


754  MICROSCOPIC   FOKMS   OF  ANIMAL   LIFE 

is  difficult  to  decide  what  is  their  relationship  to  other  groups  of 
animals.  Notwithstanding  the  wide  zoological  separation  between 
these  two  kinds  of  animalcules,  it  seems  most  suitable  to  the  plan 
of  the  present  work  to  treat  of  them  in  connection  with  one  another  ; 
since  the  microscopist  continually  finds  them  associated  together, 
and  studies  them  under  similar  conditions. 

SECTION  I. — INFUSORIA. 

This  term,  as  now  limited  by  the  separation  of  the  Rhizopoda 
on  the  one  hand,  and  of  the  Rotifer  a  on  the  other,  is  applied  to  a 
far  smaller  range  of  forms  than  was  included  by  Professor  Ehren- 
berg  under  the  name  of  '  polygastric '  animalcules.  For  a  large 
section  of  these,  including  the  JJesmidiacew,  Diatomace<ij,  Volvocinece, 
and  many  other  protophytes,  have  been  transferred  by  general 
(though  not  universal)  consent  to  the  vegetable  kingdom.  And 
it  is  not  impossible  that  many  of  the  reputed  Infusoria  may  be  but 
larval  forms  of  higher  organisms,  instead  of  being  themselves  com- 
plete animals.  Still  an  extensive  group  remains,  of  which  no  other 
account  can  at  present  be  given  than  that  the  beings  of  which  it  is 
composed  go  through  the  whole  of  their  lives,  so  far  as  we  are  ac- 
quainted writh  them,  in  a  grade  of  existence  which  is  essentially 
protozoic,  each  individual  apparently  consisting  of  but  a  single  cell, 
though  its  parts  are  often  so  highly  differentiated  as  to  represent 
(only,  however,  by  way  of  analogy}  the  'organs'  of  the  higher 
animals  after  which  they  are  usually  named. 

Among  the  ciliate  Infusoria,  which  form  not  only  by  far  the 
largest,  but  also  the  most  characteristic  division  of  the  group,  there 
is  probably  none  save  such  as  are  degraded  by  parasitic  habits 
which  has  not  a  mouth,  or  permanent  orifice  for  the  introduction 
of  food,  which  is  driven  towards  it  by  ciliary  currents ;  while  a 
distinct  anal  orifice,  for  the  ejection,  of  the  indigestible  residue,  is 
not  infrequently  present.  The  mouth  is  often  furnished  with  a 
dental  armature,  and  leads  to  an  wsopkageal  canal,  down  which 
the  food  passes  into  the  digestive  cavity.  This  cavity  is  still 
occupied,  however,  as  in  rhizopods,  by  the  endosarc  of  the  cell ;  but 
instead  of  lying  in  mere  vacuoles  formed  in  the  midst  of  this,  the 
food-particles  are  usually  aggregated,  during  their  passage  down 
the  oesophagus,  into  minute  pellets,  each  of  which  receives  a  special 
investment  of  firm  protoplasm,  constituting  it  a  digestive  vesicle 
(fig.  589)  ;  and  these  go  through  a  sort  of  circulation  within  the 
cell- cavity. 

The  '  contractile  vesicles,'  again,  attain  a  much  higher  develop- 
ment in  this  group,  and  are  sometimes  in  connection  with  a  network 
of  canals  channelled  out  in  the  '  ectosarc  ; '  while  their  rhythmical 
action  resembles  that  of  the  circulatory  and  respiratory  apparatus 
of  higher  animals.  There  is  ample  evidence,  also,  of  the  presence 
of  a  specially  contractile  modification  of  the  protoplasmic  substance, 
having  the  action  (though  not  the  structure)  of  muscular  fibre ; 
and  the  manner  in  which  the  movements  of  the  active  free-swimming 
Infusoria  are  directed  so  as  to  avoid  obstacles  and  find  out  passages 


INFUSORIA  755 

seems  to  indicate  that  another  portion  of  their  protoplasmic  sub- 
stance  must  have  to  a  certain  degree  the  special  endowments  which 
characterise  the  .'le/'rottx  systems  of  higher  animals.  Altogether,  it 
may  be  said  that  in  the  ciliate  Infusoria  the  life  of  the  single  cell 
finds  its  highest  expression.1 

Before  proceeding  to  the  description  of  the  ciliate  Infusoria, 
however,  it  will  be  of  advantage  to  notice  two  smaller  groups — the 
flayellate  and  the  suctorial — which,  on  account  of  the  peculiarities 
of  their  structure  and  actions,  aYe  now  ranked  as  distinct,  and  of 
whose  '  unicellular '  character  there  can  be  no  reasonable  doubt, 
since  they  are,  for  the  most  part,  *  closed '  cells,  scarcely  distinguish- 
able morphologically  from  those  of  protophytes. 

Flagellata. — Our  knowledge  of  this  tribe  has  been  greatly  aug- 
mented in  recent  years,  not  only  by  the  discovery  of  a  great  variety 
of  new  forms,  but  still  more  by  the  careful  study  of  the  life-history 
of  several  among  them.  The  monads,  properly  so  called,2  which  are 
amongst  the  smallest  living  things  at  present  known,  are  its  simplest 
representatives  ;  but  it  also  includes  organisms  of  much  greater 
complexity  ;  and  some  of  its  composite  forms  seem  to  have  a  very 
remarkable  relation  to  sponges.  The  Monas  lens,  long  familial' 
to  microscopists  as  occurring  in  stagnant  waters  and  infusions  of 
decomposing  organic  matter,  is  a  spheroidal  particle  of  protoplasm, 
from  or/out-h  to  5oV>oth  of  an  inch  in  diameter,  enclosed  in  a  delicate 
hyaline  investment  or  '  ectosarc,'  and  moving  freely  through  the 
water  by  the  lashing  action  of  its  slender  flayellum,  whose  length 
is  from  three  to  five  times  the  diameter  of  the  body.  Within  the 
body  may  be  seen  a  variable  number  of  vacuoles ;  and  these  are 
occasionally  occupied  by  particles  distinguishable  by  their  colour, 
which  have  been  introduced  as  food.  These  seem  to  enter  the  body, 
not  by  any  definite  mouth  (or  permanent  opening  in  the  ectosarc), 
but  through  an  aperture  that  forms  itself  in  some  part  of  the  oral 
region  near  the  base  of  the  flagellum.  In  some  true  Monadinw 
neither  nucleus  nor  contractile  vesicle  is  distinguishable,  but  in 
the  majority  a  nucleus  can  be  clearly  seen.  The  life-history  of 
several  simple  Monadince  presenting  themselves  in  infusions  of 
decaying  animal  matter  (a  cod's  head  being  found  the  most  pro- 
ductive material)  has  been  studied  with  admirable  perseverance 

1  The  doctrine  of  the  unicellular  nature  of  the  Infusoria  has  been  a  subject  of 
keen  controversy  amongst  zoologists  from  the  time  when  it  was  first  definitely  put 
forward  by  Von  Siebold  (Lehrbuch  der  vergleicJi.  Anat.  Berlin,  1845)  in  opposition 
to  the  then  paramount  doctrine  of  Ehreiiberg  as  to  the  complexity  of  their  organisa- 
tion, which  had  as  yet  been  called  in  question  only  by  Dujardin  (Hist.  Nat.  des 
Infusoires,  Paris,  1841).  Of  late,  however,  there  has  been  a  decided  convergence  of 
opinion  in  the  direction  above  indicated ;  which  has  been  brought  about  in  great 
degree  by  the  contrast  between  the  protozoic  simplicity  of  the  reproductive  and  de- 
velopmental processes  in  Infusoria,  as,  for  example,  shown  by  Dallinger  and  Drysdale, 
and  by  the  former  alone  in  the  life-histories  of  the  Saprophytes,  and  the  complexity 
of  the  like  processes  as  seen  in  even  the  lowest  of  the  Metazoa,  which  has  been 
specially  and  forcibly  insisted  on  by  Haeckel  ('  Zur  Morphologic  der  Infusorien,' 
Jenaische  Zeitschr.  Bd.  vii.  1878).  An  excellent  summary  of  the  whole  discussion 
was  given  by  Professor  Allman  in  his  Presidential  Address  to  the  Linnean  Society  in 
1875. 

-  The  family  Mottadina  of  Ehreiiberg  and  Dujardin  consists  of  an  aggregate  of 
forms  now  known  to  be  of  very  dissimilar  nature,  many  of  them  belonging  to  the 
vegetable  kingdom 

3c2 


756  MICROSCOPIC   FOEMS   OF  ANIMAL  LIFE 

and  thoroughness  by  Messrs.  Dallinger  and  Diysdale,  of  whose  im- 
portant observations  a  general  summary  will  now  be  given.1 

The  present  Editor  adopts  the  lead  of  Dr.  Carpenter,  in 
arranging  the  saprophytic  monad  forms  in  this  place  in  the  organic 
series.  They  possess  features  that  ally  them,  as  has  been  already 
suggested,  to  the  vegetable  series,  and  indicate  affinities  with 
certain  NostocaceaB  and  the  Bacteria. 

There  are  some  reasons  for  looking  at  the  saprophytic  monad 
forms  as  a  possibly  degraded  but  still  specialised  group.  In  common 
with  saprophytic  Bacteria ,  they  are  specifically  related  to  the  setting 
up  and  carrying  on  of  decomposition  in  dead  organic  tissues.  In 
organic  infusions  and  films  of  gelatine,  or  tubes  of  agar-agar,  the 
bacterial  forms  are,  as  a  rule,  enough  to  set  up  and  carry  on  the 
destructive  ferment.  But  where  great  masses  of  tissue  are  decom- 
posing, the  presence  of  the  larger  monad  forms  is  certain  and  in- 
evitable ;  and  by  them,  accompanied  by  the  Bacteria,  the  processes 
of  fermentative  rotting  are  carried  to  the  end. 

It  is  their  morphology  which  points  to  the  Flagellata,  and  we 
should  incline  to  consider  them  a  degenerate,  and  by  degeneration 
specialised  form  of  the  Flagellata  if  they — about  eight  or  nine  dis- 
tinct forms  in  this  latitude — belong  properly  to  the  Flagellata  at 
all. 

The  simplest  of  these  organisms  is  represented  in  fig.  l,  Plate 
XV,  A.  It  has  been  named  by  Saville  Kent  Nonas  D  ailing  eri, 
and  has  by  comparison  a  simple  life-history.  As  it  is  with  the 
entire  group,  all  is  subservient  to  rapidity  of  multiplication  ;  and 
there  are  two  methods  in  which  this  is  effected.  The  first  and  com- 
monest is  by  fission  ;  fig.  l,  A,  represents  the  normal  form  of  the 
organism.  It  has  a  long  diameter  of  about  the  jnnrath  °f  an  inch, 
and  has  great  ease  and  grace,  and  relative  power  of  movement. 
In  a  certain  stage  of  its  history  as  it  swims  freely  there  suddenly 
appears  a  constriction  across  its  body,  as  in  fig.  2.  This  is  at  once 
accompanied  by  an  apparent  effort  of  the  opposite  flagella  to  pull 
against  each  other  ;  the  consequence  is  a  very  rapid  stretching  of 
a  neck  of  sarcode  between  two  halves  of  the  body,  as  at  fig.  3.  This 
becomes  longer,  as  at  4,  and  attains  the  length  of  two  flagella  as  at 
5,  when  the  two  dividing  halves  approach  and  mutually  dart  from 
each  other,  snapping  the  connecting  fibre  of  sarcode  in  the  middle, 
so  that  two  perfect  forms  are  set  free,  as  in  <>  and  7. 

This,  in  the  course  of  from  two  to  three  minutes,  is  once  more 
begun  and  carried  on  in  each  half  successively,  so  that  there  is  an 
increase  of  the  form  by  this  means  in  rapid  geometric  ratio. 

But  this  is  an  exhaustive  process  vitally,  for  after  a  period  vary- 
ing from  eight  to  ten  days  there  always  appear  in  the  unaltered 
and  unchanged  field  of  observation  normal  forms,  with  a  remarkable 
diffluent  or  amoeba-like  envelope,  as  seen  in  figs.  8  and  9,  A.  These 

1  See  their  successive  papers  in  the  Monthly  Microsc.  Jo  urn.  vol.  x.  1873, 
pp.  53,  245 ;  vol.  xi.  1874,  pp.  7,  69,  97  ;  vol.  xii.  1874,  p.  261 ;  and  vol.  xiii.  1875, 
p.  185  ;  and  Proceed.  Hoy.  Soc.  vol.  xxvii.  1878,  p.  332.  But  especially  for  the  latest 
results  with  recent  objectives,  Jour')}.  Roy.  Micro.  Soc.  vol.  v.  1885,  p.  177;  vol.  vi. 
p.  193;  vol.  vii.  p.  185;  vol.  viii.  p.  177. 


Plats 


-o- 


\      16          •       \        o 

'         •'    '    Vc'     • 

.       "      o    o        0 


o  c     / 


W.H.DaIlk£er  del  adna,t. 


A.  S.Huth,  JitK  London. 


LIFE     HISTORIES      OF     SAPROPHYTES. 


MONAS  757 

sometimes  swim  and  sometimes  creep,  amoeba-like,  by  pseudopodia  ; 
but  directly  the  diffluent  sarcode  of  one  touches  that  of  another 
they  at  once  melt  together,  as  in  fig.  10,  A.  This  leads  to  the  rapid 
approach  of  the  oval  bodies  of  the  two  organisms,  as  in  fig.  11,  B, 
resulting  in  their  fusion,  as  in  figs.  12,  13,  14,  and  a  still  condition 
of  the  sac  (fig.  14)  for  a  period  of  not  less  than  six  hours  ;  when  it 
bursts,  as  seen  in  fig.  15,  pouring  out  an  immense  host  of  exquisitely 
minute  spores,  as  shown  in  fig.  15.  These  are  opaque  or  semi-opaque, 
but  by  observation  upon  them  at  a  temperature  of  65°  to  70°  Fahr., 
they  in  the  course  of  thirty  minutes  become  transparent,  elongate, 
as  in  figs.  16  and  17,  and,  continuing  to  grow,  assume  the  conditions 
and  sizes  represented  in  figs.  1 8  and  1 9 ;  and  we  were  able  to  trace 
them  through  all  their  changes  of  growth  from  the  spore  into  the 
adult  condition,  as  at  fig.  20,  until  they  entered  upon  and  passed 
through  the  self-division  into  two  described  and  figured  in  A. 

The  next  form,  though  even  more  simple  in  appearance,  has  a 
much  more  complex  morphological  history.  It  is  seen  in  its  normal 
form  in  fig.  1,  C.  It  has  but  one  flagellum,  and,  as  we  believe,  on 
that  account  has  a  much  more  restricted  power  of  movement.  It 
is  from  the  .^^-th  to  the  yJ^th  of  an  inch  in  long  diameter.  In 
its  motion  at  one  stage  of  its  life  its  oval  body  becomes  uncertain  in 
form,  as  seen  in  2,  3,  4,  C;  but  when  this  has  continued  for  not 
more  than  a  minute,  the  flagellum  falls  in  upon  the  body,  as  in  4, 
and  the  organism  becomes  perfectly  still.  In  this  condition,  after  a 
space  ranging  from  ten  to  twenty  minutes,  two  white  bars  at  right 
angles  suddenly  appear,  as  in  fig.  5  ;  this  is  almost  immediately 
followed  by  another  and  a  similar  one  at  right  angles  to  the  first,  as 
in  fig.  6.  Then  the  circumference  of  the  flattened  sphere  twists, 
leaving  the  centre  unaffected,  so  that  the  body  assumes  a  turbined 
appearance  as  seen  in  fig.  7.  After  this  the  interior  substance  breaks 
up,  and  becomes  a  knot  of  slightly  moving  but  compact  forms,  as  in 
fig.  8 ;  which  remains  in  this  state  for  from  fifteen  to  twenty 
minutes,  and  then  becomes  dissociated,  as  in  fig.  9  ;  so  that  we  have 
here  a  complex  form  of  multiple  partition,  giving  rise  to  enormous 
numbers,  because,  although  much  smaller  than  the  form  in  which 
they  arose,  they  consume  and  assimilate  food  all  over,  and  are 
simply  swimming  in  their  pabulum,  and  so  rapidly  reach  the 
normal  size,  when  they  each  enter  upon  and  pass  through  a  similar 
process. 

But  here  also  at  certain  periods  there  appeared  forms  that  in- 
augurate distinctly  genetic  processes.  A  form  like  fig.  10,  C,  appears, 
larger  than  the  normal  form,  and  always  mottled  in  the  part  near- 
est the  flagellum.  These  forms  rapidly  attached  themselves  to  the 
normal  forms,  as  seen  in  fig.  11,  which  resulted  in  a  blending  of  the 
two  as  they  swam  together,  until  '  either  was  melted  into  other  ; ' 
and  a  still  sac,  shown  in  fig.  12,  resulted. 

This  remained  from  thirty  to  thirty-six  hours  absolutely  inert ; 
but  at  the  expiration  of  that  time  it  burst,  as  seen  in  fig.  13,  D,  and 
poured  out  an  enormously  -diffusive  fluid,  which  as  it  flowed  into  the 
surrounding  water  appeared  like  a  denser  fluid,  diffusing  itself  through 
one  of  less  density  ;  but  no  spores  were  at  this  stage  at  all  apparent.  It 


758  MICROSCOPIC    FORMS   OF  ANIMAL  LIFE 

was  only  after  much  effort  that  we  at  last,  by  keeping  the  finest  of  our 
lenses  near  the  mouth  of  the  empty  sac,  were  able  to  discover,  where 
before  nothing  was  visible,  the  appearance  of  minute  specks,  which 
became  larger  and  larger,  growing  as  seen  in  14,  15,  16,  17,  until 
the  adult  size  was  reached,  as  at  is,  and  by  the  act  of  multipartitioii 
on  the  part  of  one  of  these,  watched  from  its  first  disclosure  by  the 
microscope,  we  were  able  to  re-enter  the  cycle  of  its  life-history. 

The  third  form,  which  we  may  here  consider  fully,  so  as  to  present 
a  good  group  of  histories  typical  in  their  presentation  of  the  morpho- 
logy of  the  whole  of  the  monad-saprophytes  as  we  at  present  know 
them,  is  given  in  E  and  F,  Plate  XV. 

The  monad  has  been  named  by  S.  Kent  Dallingeria  Drysdall. 
The  form  more  recently  and  completely  studied  by  Mr.  Dallinger — 
with  all  the  advantages  derived  from  trained  experience,  and  under 
objectives  of  the  highest  quality  and  greatest  magnifying  power — is 
seen  in  its  normal  shape  in  fig.  l,,is  a  long  oval,  slightly  constricted 
in  the  middle,  and  having  a  kind  of  pointed  neck  («),  from  which 
proceeds  a  nagellum  about  half  as  long  again  as  the  body.  From 
the  shoulder-like  projections  behind  this  (b,  c,)  arise  two  other  long 
and  fine  flagella,  which  are  directed  backwards.  The  sarcode-body 
is  clear,  and  apparently  structureless,  with  minute  vacuoles  dis- 
tributed through  it ;  and  in  its  hinder  part  a  nucleus  (d)  is  dis- 
tinguishable. The  extreme  length  of  the  body  is  seldom  more  than 
the  ¥oVoth  of  an  inch,  and  is  often  the  ^^th.  This  monad  swims 
with  great  rapidity,  its  movements,  which  are  graceful  and  varied, 
being  produced  by  the  action  of  the  fiagella,  which  can  not  only 
impel  it  in  any  direction,  but  can  suddenly  reverse  its  course  or  check 
it  altogether.  But  besides  this  free-swimming  movement,  a  very 
curious  '  springing '  action  is  performed  by  this  monad  when  the  de- 
composing organic  matter  of  the  infusion  is  breaking  up,  the  process 
of  disintegration  being  apparently  assisted  by  it.  The  two  posterior 
flagella  anchor  themselves  and  coil  into  a  spiral,  and  the  body  then 
darts  forwards  and  upwards,  until  the  anchored  flagella  straighten 
out  again,  when  the  body  falls  forward  to  its  horizontal  position,  to 
be  again  drawn  back  by  the  spiral  coiling  of  the  anchored  flagella, 
This  monad  multiplies  by  longitudinal  fission,  the  first  stage  of 
which  is  the  splitting  of  the  anterior  flagellum  into  two  (fig.  -J,  a,  5), 
and  a  movement  of  the  nucleus  (c)  towards  the  centre.  In  the  course 
of  from  thirty  to  sixty  seconds  the  fission  extends  down  the  neck  (fig. 
3,  a)  ;  a  line  of  division  is  also  seen  at  the  posterior  end  (c),  and  the 
nucleus  (b)  shows  an  incipient  cleavage.  In  a  few  seconds  the 
cleavage-line  runs  through  the  whole  length  of  the  body,  the  separa- 
tion being  widest  posteriorly  (fig.  4,  a)  ;  and  in  from  one  to  four 
minutes  the  cleavage  becomes  almost  complete  (fig.  5),  the  posterior 
part  of  the  body,  with  the  two  halves  (a  and  b)  of  the  original  nucleus, 
being  now  quite  disconnected,  though  the  anterior  parts  are  still 
held  together  by  a  transverse  band  of  sarcode,  as  seen  in  fig.  6,  which 
continues  to  rapidly  elongate,  as  in  fig.  7,  and  becomes  the  length  of 
two  side  flagella,  as  in  fig.  8.  The  forms  then  approach  and  rapidly 
recede  from  each  other,  snapping  the  cord,  as  in  figs.  9  and  10.  In 
this  way  two  forms  exist  instead  of  one  ;  and  each  of  these  almost  im- 


MONADS  759 

mediately  enters  upon  and  passes  through  the  same  process  of  fission, 
which  from  first  to  last  is  completed  in  from  four  to  seven  minutes  ; 
and  being  repeated  at  intervals  of  a  few  minutes,  this  mode  of  multi- 
plication produces  a  rapid  increase  in  the  number  of  the  monads. 

Such  fission  does  not,  however,  continue  indefinitely,  for  after 
a  successive  series  of  fissions,  followed  in  one  of  the  divided  bodies 
for  eight  or  nine  hours,  certain  individuals  do  not  again  enter  upon 
the  process  of  fission,  but  undergo  a  peculiar  change,  which  shows 
itself  first  in  the  absorption  of  the  two  lateral  flagella  and  the  great 
development  of  the  nucleus,  and  Afterwards  in  the  formation  of  a 
transverse  granular  band  across  the  middle  of  the  body  (fig.  11,  E). 
One  of  these  altered  forms,  swimming  into  a  group  in  the  '  springing  ' 
state,  within  a  few  seconds  firmly  attaches  itself  to  one  of  them,  which 
at  once  unanchors  itself,  and  the  two  swim  freely  and  vigorously  about, 
shown  in  fig.  12,  generally  for  from  thirty-five  to  forty-five  minutes. 
Gradually,  .however,  a  '  fusion '  of  the  two  bodies  and  of  their  re- 
spective nuclei  takes  place,  the  two  trailing  flagella  of  the  '  springing  ' 
form  being  drawn  in  (fig.  13,  F)  ;  arid  in  a  short  time  longer  the  two 
anterior  flagella  also  disappear,  and  all  trace  of  the  separate  bodies  is 
lost,  the  nuclei  vanish,  and  the  resultant  is  an  irregular  amoeboid  mass 
(fig.  14),  which  gradually  acquires  the  smooth,  distended,  arid  '  still ' 
condition  represented  in  fig.  14,  a.  This  is  a  cyst  filled  with  repro- 
ductive particles  of  such  extraordinary  minuteness  that,  when 
emitted  from  the  ends  of  the  cyst  (fig.  15,  a)  after  the  lapse  of  four 
or  five  hours,  they  can  only  be  distinguished  under  an  amplification 
of  5,000  diameters,  with  perfect  central  illumination,  i.e.  the  full 
cone  of  a  large-angled  condenser.  Yet  these  particles,  when  con- 
tinuously watched,  are  soon  observed  to  enlarge  arid  to  undergo 
elongation  (figs.  16.  17.  18,  19,  20),  and  within  two  hours  after  their 
emission  from  the  sac  the  anterior  flagellum,  and  afterwards  the  two 
lateral  flagella  (fig.  19),  can  be  distinguished.  Slight  movements  then 
commence,  the  neck-like  protrusion  shows  itself,  and  in  about 
half  an  hour  more  the  regular  swimming  action  begins.  About 
four  hours  after1  the  escape  of  its  germ  from  the  sac,  the  monad 
acquires  its  characteristic  form  (fig.  21),  though  still  only  one-half  the 
length  of  its  parent  :  but  this  it  attains  in  another  hour,  and  the 
process  of  multiplication  by  fission,  as  already  described,  commences 
very  soon  afterwards.  There  can  be  no  reasonable  doubt  that  the 
'  conjugation '  of  two  individuals,  followed  by  the  transformation  of 
their  fused  bodies  into  a  sac  filled  with  reproductive  germs,  is  to  be 
regarded  (as  in  protophytes)  in  the  light  of  a  true  generative  process  ; 
and  it  is  interesting  to  observe  the  indication  of  sexual  distinction 
here  marked  by  the  different  states  of  the  two  conjugating  individuals. 
There  is  every  reason  to  believe  that  the  entire  Ufa-cycle  of  this  monad 
lias  thus  been  elucidated  ;  and  it  will  now  be  sufficient  to  notice  the 
principal  diversities  observed  by  Messrs.  Dallinger  and  Drysdale  in 
the  life-cycles  of  the  other  inonadine  forms  which  they  have  studied. 

The  bi-jlckgettcde  or*  '  acorn '  monad  of  the  same  observers  (identi- 
fied by  Kent  with  the  Polytoma  uvella  of  Ehrenberg)  presents  some 
remarkable  peculiarities  in  its  mode  of  reproduction.  Its  binary 
fission  extends  only  to  the  protoplasmic  substance  of  its  body,  leaving1 


760  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

its  envelope  entire  ;  and  by  a  repetition  of  the  process,  as  many  as 
sixteen  segments,  each  attaining  the  likeness  of  the  parent,  are  seen 
thus  inclosed,  their  flagella  protruding  through  the  general  invest- 
ment. This  compound  state  being  supposed  by  Ehrenberg  to  be  the 
normal  one,  he  named  it  accordingly.  But  the  parent-cyst  soon 
bursts,  and  sets  free  the  contained  '  macro-spores,'  which  swim  about 
freely,  and  soon  attain  the  size  of  the  parent.  Again,  the  posterior 
part  of  the  body  of  certain  individuals  shows  an  accumulation  of 
granular  protoplasm,  giving  to  that  region  a  roughened  acorn -cup- 
like  aspect ;  the  bursting  of  the  projection,  while  the  creature  is 
actively  swimming  through  the  water,  sets  free  a  multitude  of 
indefinitely  shaped  granular  fragments,  within  each  of  which  a 
minute  bacterium-like  corpuscle  is  developed ;  and  this,  on  its 
release,  acquires  in  a  few  hours  the  size  and  form  of  the  original 
monad.  This  process  seems  analogous  to  the  development  of '  micro- 
spores  '  among  protophytes  by  the  direct  breaking  up  of  the  proto 
plasm.  It  is,  like  the  previous  process,  non-sexual  or  yoii-idial,  the 
true  generative  process  consisting  here,  as  in  the  preceding  cases,  in 
the  'conjugation'  of  two  individuals,  with  the  usual  results. 

The  hooked  monad  (Heteromita  uncinata,  Kent)  is  another  bi- 
flagellate  form,  usually  ovate  \vith  one  end  pointed,  and  from  3-(j\M,th 
to  ¥^o  oth  of  an  inch  in  length,  being  distinguished  from  the  pre- 
ceding by  the  peculiar  character  of  its  flagella,  of  which  the  one  that 
projects  forwards  is  not  more  than  half  the  length  of  the  body,  and 
is  permanently  hooked,  while  the  other,  whose  length  is  about  twice 
that  of  the  body,  is  directed  backwards,  flowing  in  graceful  curves. 
Its  motion  consists  of  a  succession  of  springs  or  jerks  rapidly  follow- 
ing each  other,  which  seems  produced  by  the  action  of  the  hooked 
flagellum.  Multiplication  takes  place  by  transverse  fission,  and  con- 
tinues uninterruptedly  for  several  days.  A  difference  then  becomes 
perceptible  between  larger  and  smaller  individuals,  the  former  being 
further  distinguished  by  the  presence  of  what  seems  to  be  a  con- 
tractile vesicle  in  the  anterior  part  of  the  body.  Conjugation  occurs 
between  one  of  the  larger  and  one  of  the  smaller  forms,  the  latter 
being,  as  it  were,  absorbed  into  the  body  of  the  larger ;  and  the 
resulting  product  is  a  spherical  cyst,  which  soon  begins  to  exhibit 
a  cleavage -process  in  its  interior.  This  continues  until  the  whole 
of  its  sarcodic  substance  is  subdivided  into  minute  oval  particles, 
which  are  set  free  by  the  rupture  of  the  cyst,  and  of  which  each  is 
usually  furnished  w^ith  a  single  flagellum,  by  whose  lashing  move- 
ment it  swims  freely.  These  germs  speedily  attain  the  size  and  form 
of  the  parent,  and  then  begin  to  multiply  by  transverse  fission,  thus 
completing  the  '  genetic '  cycle. 

The  calycine  monad  of  the  same  observers  (Tetramitits  rostratus, 
Perty)  has  a  length  of  from  9^th  to  y^^th  of  an  inch,  and  a  com- 
pressed body  tapering  backwards  to  a  point.  Its  four  flagella  (which 
constitute  its  generic  distinction)  arise  nearly  together  from  the 
flattened  front  of  the  body,  and  its  swimming  movement  is  a  grace- 
ful gliding.  Near  the  base  of  the  flagella  are  a  pair  of  contractile 
vesicles,  and  further  behind  is  a  large  nucleus.  Multiplication  takes 
place  by  longitudinal  fission,  which  is  preceded  by  a  change  to  a  semi- 


MONADS  761 

amoeboid  state.  This  gives  place  to  a  more  regular  pear-like  form, 
the  four  flagella  issuing  from  the  large  end  ;  and  the  fission  commences 
at  their  base,  two  pairs  being  separated  by  the  cleavage-plane.  The 
nucleus  also  undergoes  cleavage,  and  its  two  halves  are  carried  apart 
by  the  backward  extension  of  the  cleavage.  The  two  half-bodies 
at  last  remain  connected  only  by  their  hinder  prolongations,  which 
speedily  give  way,  and  set  them  free.  Each,  however,  has,  as  yet, 
only  two  flagella  ;  but  these  speedily  fix  themselves  by  their  free 
extremities,  undergo  a  rapid  vibratory  movement,  and  in  the  course 
of  about  two  minutes  split  themselves  from  end  to  end.  A  still 
more  complete  change  into  the  amo3boid  condition,  in  which  the 
creature  not  only  moves,  but  also  feeds,  like  an  Amveba  (devouring  all 
the  living  and  dead  Bacteria  in  its  neighbourhood),  occurs  previously 
to  '  conjugation  ; '  and  this  takes  place  between  two  of  the  amoeboid 
forms,  which  begin  to  blend  into  each  other  almost  immediately 
upon  coming  into  contact.  The  conjugated  bodies,  however,  swim 
freely  about  for  a  time,  the  two  sets  of  flagella  apparently  acting  in 
concert.  But  by  the  end  of  about  eighteen  hours  the  fusion  of 
the  bodies  and  nuclei  is  complete,  the  flagella  are  lost,  and  a 
spherical  distended  sac  is  then  formed,  which,  in  a  fewr  hours  more 
without  any  violent  splitting  or  breaking  up,  sets  free  innumerable 
masses  of  reproductive  particles.  These  under  a  magnifying  power 
of  2,500  diameters  can  be  just  recognised  as  oval  granules,  which 
rapidly  develop  themselves  into  the  likeness  of  their  parents,  and 
in  their  turn  multiply  by  duplicative  fission,  thus  completing  the 
'  genetic '  cycle. 

One  of  the  most  important  researches  thus  ably  prosecuted  by 
Messrs.  Dallinger  and  Drysdale  has  reference  to  the  temperatures 
respectively  endurable  by  the  adult  or  developed  forms  of  these 
monads,  and  by  their  reproductive  germs.  A  large  number  of  experi- 
ments upon  the  several  forms  now  described  indubitably  led  to  the 
conclusion  that  all  the  adult  forms,  as  well  as  all  those  which  had 
reached  a  stage  of  development  in  which  they  can  be  distinguished 
from  the  reproductive  granules,  are  utterly  destroyed  by  a  tempera- 
ture of  150°  Fahr.  But,  on  the  other  hand,  the  reproductive  granules 
emitted  from  the  cysts  that  originate  in  'conjugation'  were  found 
capable  of  sustaining  a  fluid  heat  of  220°,  and  a  dry  heat  of  about 
30°  more,  those  of  the  Cercomonad  surviving  exposure  to  a  dry  heat  of 
300°  Fahr.  This  is  a  fact  of  the  highest  interest  in  its  bearing  on  the 
question  of  *  spontaneous  generation,'  or  abiogenesis  ;  since  it  shows 
that  germs  capable  of  surviving  desiccation  may  be  everywhere  diffused 
through  the  air,  and  may,  on  account  of  their  extreme  minuteness 
(as  they  certainly  do  not  exceed  ^ ooVo^th  of  an  inch  in  diameter), 
altogether  escape  the  most  careful  scrutiny  and  the  most  thorough 
cleansing  processes  ;  while  (2)  their  extraordinary  power  of  resisting 
heat  will  prevent  these  germs  from  being  killed,  either  by  boiling,  or 
by  dry-heating  up  to  even  300°  Fahr.1 

Beyond  these  facts  others  of  some  importance,  as  well  as  a  new 

1  Descriptions  of  the  special  apparatus  used  by  Messrs.  Dallinger  and  Drysdale 
in  their  researches  will  be  found  in  Monthly  Micros.  Jon  rn.  vol.  xi.  1874,  p.  97  ;  ibid. 
vol.  xv.  1876,  p.  165 ;  and  Proceed.  Boy.  Soc.  vol.  xxvii.  1878,  p.  343. 


762  MICKOSCOPIC   FORMS   OF   ANIMAL   LIFE 

saprophytic  organism  l  of  special  character,  have  been  discovered 
during  a  recent  period.  But  it  will  be  of  more  moment  here  to  note 
to  what  an  extent  in  this  series  of  observations  the  neiv  homo</<'ii<'<>n* 
objectives,  espe-cialli/  in  tin1  if  apochromatic  form,  have  been  success- 
fully employed  in  enlarging  the  area  of  knowledge. 

The  present  Editor  has  gone  carefully  over  the  greater  part  of  the 
work,  revising  all  the  critical  points  with  the  best  apochromatic  ob- 
jectives, and  the  homogeneous  forms  of  achroinatics  with  an  aperture 
of  1'50  and  with  a  clear  demonstration  of  the  immensely  greater  ease 
with  which  the  work  could  have  been  done  had  these  lenses  been  used 
in  the  original  investigation. 

But  the  easily  accessible  proof  of  this  is  given  in  the  work  done  by 
Dr.  Dallinger  upon  the  nucleus  of  the  nucleated  forms  of  these  monads. 

Briefly  to  present  the  facts,  we  may  recall  the  part  taken  in  the 
act  of  fission  in  the  form  last  described  (Dallingeria  Drysdali).  It 
will  be  seen  by  reference  that  it  appeared  to  us  that  the  nt'-clrtis  fol- 
lowed the  processes  inaugurated  !>;/  the  somatic  sarcode.  That  in  fact 
it  was  a  passive  participator  in  the  act  of  fission.  This  is  all  that 
can  be  made  out  to-day  by  the  very  lenses  originally  employed. 

But  by  the  employment  of  a  ^Vth  inch  and  Troth  inch  homo- 
geneous of  N.A.  1-50  by  Powell  and  Lealaiid,  and  an  apochromatic 
of  TVth  inch  N.A.  1'40  by  the  same  firm  ;  and  also  by  the  use  of 
the  beautiful  3  mm.  and  2  mm.  N.A.  T40  of  Zeiss  (apochromatic), 
it  can  be  seen  with  comparative  ease  that  it  is  in  the  nucleus  that 
all  the  activities  of  the  body  are  originated. 

This  may  be  followed  from  a  study  of  Plate  XVI.  Fig.  1,  A, 
represents  the  nucleus  of  the  form  drawn  at  fig.  i,  E,  Plate 
XV.  In  long  diameter  it  is  of  an  average  length  of  siro-ooth  °f  :m 
inch  ;  but  instead  of  being  a  darkly  refractive  object,  as  seen  with 
the  objectives  used  twelve  years  ago,  it  is  with  the  present  lenses, 
freed  from  chromatic  and  spherical  aberration,  a  body  in  the  monad 
undergoing  no  process  of  change,  an  oval  globule  with  a  complicated 
plexus-like  involution  throughout  its  substance,  as  seen  in  fig.  (>.  A. 
Plate  XVI.  But  directly  the  process  of  fission  is  to  be  inaugurated, 
we  need  not  wait  to  see  its  first  action  in  the  splitting  of  the 
fiagellnm,  as  in  fig.  -2,  E,  Plate  XV  ;  for  by  observing  the  nucleus 
we  discover,  before  any  change  has  begun  in  the  body-substance, 
that  the  plexus  in  the  nucleus  has  condensed  itself  011  either  side  of 
the  nucleus,  as  in  fig.  1,  b,  A,  Plate  XVI.  A  clear  space  is  left  at  c, 
and  no  change  has  taken  place  in  the  bodv-sarcode,  a,  a,  a.  But 
shortly  an  incision  takes  place  in  the  nucleus,  as  at  d,  fig.  2,  and 
this  is  immediately  followed  by  the  incision  f  in  the  body-sarcode, 
and  the  process  goes  011  simultaneously  in  nucleus  and  body,  as  in 
fig.  3,  until  the  division  of  the  nucleus  is  completely  effected,  and  the 
total  severance  of  the  body  follows. 

But  as  soon  as  the  nucleus  is  divided,  the  plexus,  which  has  been 
during  division,  as  in  fig.  3,  condensed  over  part  of  each  dividing 
half,  at  once  distributes  itself  evenly  again,  as  in  fig.  6,  A,  and  re- 
mains so  until  another  change  is  inaugurated  in  the  form  to  which 
the  nucleus  belongs. 

1  Jo  urn.  of  lioi/al  Micros.  Soc.  vol.  v. 


Plate  XVf 


A 


a 

0 


I 


B 


•••.Uiu&c-f  clul.  ad  na,t.     X-x 


utti1'  London. 


STUDIES     OFTHE    NUCLEUS    IN    5APROPHYT1C     ORGANISMS. 


SAPROPHYTIC    LIFE-HISTORIE*  763 

Not  less  remarkable  is  this  in  the  conjugation  of  the  same  form 
With  the  old  lenses  we  could  only  disc-over  that  the  end  of  a 
series  of  fissions  had  been  reached  by  the  change  which  came  over 
tlie  entire  body  of  the  terminal  form  seen  in  fig.  11,  E,  Plate  XV. 
But  now,  before  the  amoeboid  state  preceding  the  assumption  of 
the  condition  shown  in  fig.  11  takes  place,  it  can  be  seen  that  the 
nucleus  undergoes  remarkable  change,  for  it  passes  from  a  highly 
refractive  plexus-like  condition  into  a  large  milky  structureless  state, 
and  in  this  condition  blends  with  one  of  the  ordinary  forms  whose 
nucleus  is  of  the  ordinary  type.  rPl^e  first  result  of  fusion  is  seen  in. 
fig.  4.  A.  Plate  XVI,  showing  only  the  greatly  magnified  blended 
nuclei,  and  where  the  blending  between  them  is  seen  to  be  nearly 
complete  at  ft.  and  a  nucleus  or  nucleolus  is  manifest  ;  while 
when  the  blending  is  more  perfect  there  is  a  diffusion  of  this 
central  or  nucleolar  body  through  the  substance  of  the  whole,  as  in 
fig.  5.  A. 

In  B,  Plate  XVI,  the  nucleus  only,  separate  from  the  body  of 
the  organism  known  as  Tetramitus  restrains,  is  shown  as  we  can 
reveal  it  with  recent  (Jermaii  and  English  apochromatic  objectives. 
Tin's  entire  organism  is  relatively  large,  and  its  nucleus  will  average 
in  long  diameter  the  To7r(nyth  °f  an  inch. 

Hence  it  a  fiords  a  still  better  means  of  studv.  Now  this 
organism  divides  by  fission  for  a  very  considerable  time,  but  at  length 
many  forms  become  amoeboid — acting  precisely  ;is  an  amoeba,  but 
retaining  traces  of  their  primal  form.  In  this  state  two  of  them 
blend,  and  as  a  result  a  sac  of  spore  is  formed  from  which,  a  new 
generation  arises. 

We  could  with  the  old  objectives  determine  nothing  more  than 
the  fact  that  the  amoeboid  form  had  supervened  ;  but  now  it  is  easy 
to  show  that  the  nucleus  in  the  body  of  a  form  not  yet  amoeboid  is 
undergoing  change  upon  which  the  amu-boid  state  is  certain  to 
supervene. 

This  is  even  more  striking  in  the  growth  of  the  germ.  It  attains 
a  certain  size  in  growth,  and  then  there  is  an  arrest  of  all  enlarge- 
ment. This  we  had  long  observed  in  the  earlier  observations.  But 
now  with  apochromatic  object-glasses  it  has  been  demonstrated  that 
this  arrest  of  outward  growth  is  only  the  signal  for  an  internal  de- 
velopment. Fig.  1 ,  B,  Plate  XVI,  shows  the  condition  of  the  nucleus 
when  there  is  an  apparent  pause  in  its  growth.  Fig.  2  shows  the 
same  nucleus  after  about  forty  minutes  of  external  inaction,  a  plexus- 
like  formation  having  filled  its  substance. 

The  nucleus  remains  thus  in  the  mature  body  of  the  monad 
until  Jission  is  to  he  iiunnjm-iited,  when  the  change  seen  in  fig.  3, 
followed  by  the  changes  and  deeper  division  seen  in  figs.  4,  5,  (5,  7, 
and  s,  ensue,  and  after  the  state  of  the  nucleus  seen  in  fig.  4  has 
been  reached,  the  division  of  the  entire  body  begins. 

It  thus  appeai-s  that  a  form  of  karyokinesis  takes  place  in  the 
nucleus  of  even  such  lowly  forms  as  these,  and  that  it  is  the  nucleus 
that  is  the  seat  of  their  intensest  vitality. 

A  large  series  of  more  complex  forms  of  flagellate  Infusoria. 
has  been  brought  to  our  knowledge  by  the  researches  of  the  late 


764  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

Professor  James -Clark  (U.S.A.),1  followed  by  those  of  Stein,  Saville 
Kent,2  and  Bergh.  In  some  of  these  a  sort  of  collar-like  extension  of 
what  appeal's  to  be  the  protoplasmic  ectosarc  proceeds  from  the  anterior 
extremity  of  the  body  (fig.  585,  cl),  forming  a  kind  of  funnel,  from  the 
bottom  of  which  the  nagellum  arises ;  and  by  its  vibrations  a  cur- 
rent is  produced  within  the  funnel,  which  brings  down  food-particles 
to  the  'oral  disc  '  that  surrounds  its  origin  while  the  ectosarc  seems 
softer  than  that  which  envelops  the  rest  of  the  body.  Towards  the 
base  of  the  collar  a  nucleus  (n)  is  seen ;  while  near  the  posterior 
termination  of  the  body  is  a  single  or  double  contractile  vesicle  (cr). 
The  body  is  attached  by  a  pedicel  proceeding  from  its  posterior 
extremity,  which  also  seems  to  be  a  prolongation  of  the  ectosave. 
These  animalcules  multiply  by  longitudinal  fission ;  and  this,  in 
some  cases  (as  in  the  genus  Monosiga),  proceeds  to  the  extent  of  a 
complete  separation  of  the  two  bodies,  which  henceforth,  as  in  the 

ordinary  Monad  //<". 
live  quite  independ- 
ently of  each  other. 
But  in  other  forms,  as 
Codosiya,  the  fission 
does  not  extend  throng]  i 
the  pedicel,  and  the 
twin  bodies  being  thus 
held  together  at  their 
bases,  and  themselves 
undergoing  duplicative 
fission,  clusters  are  pro- 
duced which  spring 
from  common  pedicels 
(fig.  586)  ;  and  by 
the  extension  of  the 
division  down  tin- 
pedicels  themselves, 
composite  arborescent 
fabrics,  like  those  of 

FIG.  585.— Single  zooid  of  Codosiga  umbellate  :  cl,    ^x^l^+oc         -IVP        nvn 
collar  ;  n,  nucleus  ;  CD,  double  contractile  vesicle.       «><>pliytes,       aie       P1( 

duced. 

In  another  group  a  structureless  and  very  transparent  horny 
calyx,  closely  resembling  in  miniature  the  polype-cell  of  a  Campanu- 
laria,  forms  itself  round  the  body  of  the  monad,  which  can  retract 
itself  into  the  bottom  of  it ;  and  in  the  genus  Salpingonca  both 
calyx  and  collar  are  present.  In  some  forms  of  this  group  multi- 
plication seems  to  take  place,  not  by  fission,  but  by  gemmation ; 
and,  as  among  hydroid  polypes,  the  gemma*  may  either  detach 
themselves  and  live  independently,  or  may  remain  in  connection 
with  their  parent-stocks,  forming  composite  fabrics,  in  some  of  which 
the  calyces  follow  one  another  in  linear  series,  while  in  others  they 

1  See  his  memoirs  in  Ann.  Nat.  Hist.  ser.  3,  vol.  xviii.  186(5;  oj).  cit.  ser.  4,  vol.  i. 
1868  ;  vol.  vii.  1871 ;  and  vol.  ix.  1872. 

2  See  his  Manual  of  tin;  Itifnnuria,  1880-82,  2  vols.  and  1  vol.  of  plates. 


FLAGELLATA  765 

take  011  a  ramifying  arrangement.     While  some   of  these  composite 
organisms  are  sedentary,  others,  as  Dinobryon,  are  free-swimming. 

Two  solitary  flagellate  forms,  Anthophyaa  and  Anisonema,  may 
he  specially  noticed  as  presenting  several  interesting  points  of 
resemblance  to  the  peculiar  type  next  to  be  described,  the  most 
noticeable  being  the  presence  of  a  distinct  mouth  and  the  possession 
of  two  different  motor  organs — one  a  comparatively  stout  and  stiff 
bristle,  of  uniform  diameter  throughout,  which  moves  by  occasional 
jerks,  and  the  other  a  very  deljcate  tapering  flagellum,  which  is 
in  constant  vibratory  motion.  If,  as  appears  from  the  observa- 
tions of  Biitschli.  the  well-known  Astasia — of  which  one  species  has 
a  blood-red  colour,  and  sometimes  multiplies  to  such  an  extent  as 
to  tinge  the  water  of  the  ponds  it  inhabits — has  a  true  mouth  for  the 


FIG.  586. — Codosiga  nmlMata:  Colony-stock,  springing  from  single 
pedicel  tripartitely  branched. 

reception  of  its  food,  it  must  be  regarded  as  an  animal,  and  sepa- 
rated from  the  Euylena  (with  which  it  has  been  generally  associated), 
the  latter  being  pretty  certainly  a  plant  belonging  to  the  same 
group  as  Volvos.1 

There  can  be  no  longer  any  doubt  that  the  well-known  Noctilnca 
miliaria — to  which  is  attributable  the  diffused  luminosity  that  fre- 
quently presents  itself  in  British  seas — is  to  be  regarded  as  a  gigantic 
type  of  the  '  unicellular  '  Flagellata.  This  animal,  which  is  of  sphe- 
roidal form,  and  has  an  average  diameter  of  about  g^th  of  an  inch, 
is  just  large  enough  to  be  discerned  by  the  naked  eye  when  the  water 
in  which  it  may  be  swimming  is  contained  in  a  glass  jar  held  up  to 

1  See  the  memoir  by  Prof.  Biitschli  in  Zeitschrift  f.  Wissensrli.  Zool.  Bd.  xxx., 
of  which  an  abridgment  (with  plate »  i-  given  in  Quart.  Jo  urn.  Micros.  Sci.  vol.  xix. 
1H79,  p.  63. 


;66 


MICROSCOPIC   FORMS   OF   ANIMAL   LIFE 


the  light  ;  and  its  tail-like  appendage,  whose  length  about  equals 
its  own  diameter,  and  which  serves  as  an  instrument  of  locomotion, 
may  be  discerned  with  a  hand-magnifier.  The  form  of  Noctiluco  is 
nearly  that  of  a  sphere,  so  compressed  that  while  on  one  aspect  (fig. 
587,  A)  its  outline  when  projected  on  a  plane  is  nearly  circular,  it 
is  irregularly  oval  in  the  aspect  (B)  at  right  angles  to  this.  Along 
one  side  of  this  body  is  a  meridional  groove,  resembling  that  of  a 
peach  ;  and  this  leads  at  one  end  into  a  deep  depression  of  the  sur- 
face ft.  termed  the  atrium,  from  the  shallower  commencement  of 
which  the  tentacle,  d.1  originates  ;  whilst  it  deepens  down  at  the  base 
of  the  tentacle  to  the  mouth,  e.  Along  the  opposite  meridian  there 
extends  a  slightly  elevated  ridge,  c,  which  commences  with  tin- 
appearance  of  a  bifurcation  at  the  end  of  the  atrium  farthest  from 


FIG.  587. — Noctilaca  inilit<rin  as  seen  at  A  on  the  aboral  side,  and  at 
B  on  a  plane  at  right  angles  to  it :  a,  entrance  to  atrium  ;  fe,  atrium  ; 
c,  superficial  ridge  ;  d,  tentacle ;  e,  mouth  leading  to  oesophagus, 
within  which  are  seen  the  flagellum  springing  from  its  base,  and  the 
tooth-like  process  projecting  into  it  from  above  ;/,  broad  process  from 
the  central  protoplasmic  mass  proceeding  to  superficial  ridge ;  g, 
duplicature  of  wall ;  //,  nucleus.  (Magnified  about  90  diameters.) 

the  tentacle  :  this  is  of  firmer  consistence  than  the  rest  of  the  body, 
and  has  somewhat  the  appearance  of  a  rod  imbedded  in  its  walls. 
The  mouth  opens  into  a  short  oesophagus,  which  leads  directly  down 
to  the  great  central  protoplasmic  mass;  on  the  side  of  this  canal, 
farthest  from  the  tentacle,  is  a  firm  ridge  that  forms  a  tooth-like 
projection  into  its  cavity  ;  whilst  from  its  floor  there  arises  a  long 

1  The  organ  here  termed  '  tentacle  '  is  commonly  designated  flagellum  ;  while 
what  is  here  termed  the  fltujcUiun  is  spoken  of  by  most  of  those  who  have  recognised 
it  as  a  ciliuni.  The  Author  agrees  with  M.  Robin  in  considering  the  former  organ, 
which  has  a  remarkable  resemblance  to  a  single  fibrilla  of  striated  muscle,  as 
one  peculiar  to  Noctilucn,  and  the  latter  as  the  true  homologue  of  the  flagellum  of 
the  ordinary  Flagellata.  It  is  curious  that  several  observers  have  been  unable  to  dis- 
cover the  so-called  cilium,  which  was  first  noticed  by  Krohn.  Professor  Huxley  sought, 
for  it  in  at  least  fifty  individuals  without  success  ;  and  out  of  the  great  number  which 
he  afterwards  examined  he  did  not  get  a  clear  view  of  it  in  more  than  half  a  dozen. 


NOCTILUCA 


767 


flctgellum,  which  vibrates  freely  in  its  interior.  The  central  proto- 
plasmic mass  sends  off  in  all  directions  branching  prolongations  of 
its  substance,  whose  ramifications  inosculate  ;  these  become  thinner 
and  thinner  as  they  approach  the  periphery,  and  their  ultimate 
filaments,  coming  into  contact  with  the  delicate  membranous  body- 
wall,  extend  themselves  over  its  interior,  forming  a  protoplasmic 
network  of  extreme  tenuity  (fig.  588).  Besides  these  branching 
prolongations,  there  is  sent  oft' from  the  central  protoplasmic  mass  a 
broad,  thin,  irregularly  quadrangular  extension  (fig.  587,  B,/),  which 
extends  to  the  superficial  rod -like  ridge,  and  seems  to  coalesce  with 
it ;  its  lower  free  edge  has  a  thickened  border  ;  whilst  its  upper 
edge  becomes  continuous  with  a  plate-like  striated  structure,  </,  which 
stM-ms  to  be  formed  by  a  peculiar  duplicature  of  the  body-wall.  At 
one  side  of  the  protoplasmic  mass  is  seen  a  spherical  vesiple,  h,  of 


^^'^^Pvv^ 

X  NP^JXQoC 

-^O*  Cl. 


FIG.  588. — Portion  of  superficial  protoplasmic  reticulation  formed 
by  ramification  of  an  extension  a  of  central  mass.  (Magnified 
1,000  diameters.) 

about  ^^jyths  of  an  inch  in  diameter,  having  clear  colourless 
contents,  among  which  transparent  oval  corpuscles  nifty  usually  be 
detected.  This,  from  the  changes  it  undergoes  in  connection  with 
the  reproductive  process,  must  be  regarded  as  a  nucleus. 

The  particles  of  food  drawn  into  the  mouth  (probably  by  the 
vibrations  of  the  flagellum)  seem  to  be  received  into  the  protoplas- 
mic mass  at  the  bottom  of  the  oesophagus  by  extensions  of  its  sub- 
stance, which  inclose  them  in  filmy  envelopes  that  maintain  them- 
selves as  distinct  from  the  surrounding  protoplasm,  and  thus  consti- 
tute extemporised  digestive  vesicles.  These  vesicles  soon  find  their 
way  into  the  radiating  extensions  of  the  central  mass  (as  shown  in 
fig.  587,  B),  and  are  ensheathed  by  the  protoplasmic  substance  which 
goes  on  to  form  the  peripheral  network  (fig.  589).  Their  number 
and  position  are  alike  variable  ;  sometimes  only  one  or  two  are  to 
be  distinguished  ;  more  commonly  from  four  to  eight  can  be  seen; 


768  MICKOSCOPIC   FOKMS   OP   ANIMAL  LIFE 

and  even  twelve  or  more  are  occasionally  discernible*  The  place  of 
each  in  the  body  is  constantly  being  changed  by  the  contractions  of 
the  protoplasmic  substance,  these  in  the  first  place  carrying  it  from 
the  centre  towards  the  periphery  of  the  body,  and  then  carrying  it 
back  to  the  central  mass,  into  whose  substance  it  seems  to  be 
fused  as  soon  as  it  has  discharged  any  indigestible  material  it  may 
have  contained,  which  is  got  rid  of  through  the  mouth.  Every  part 
of  the  protoplasmic  reticulation  is  in  a  state  of  incessant  change. 
which  serves  to  distribute  the  nutrient  material  that  finds  its  way 
into  it  through  the  walls  of  the  digestive  vesicles;  but  no  regular 
cyclosis  (like  that  of  plants)  can  be  observed  in  it.  Besides  the 
'digestive  vesicles,'  vacuoles  filled  with  clear  fluid  may  be  distin- 
guished, alike  in  the  central  protoplasmic  mass,  and  in  its  extensions 
as  is  shown  in  the  centre  of  fig.  587.  There  is  no  contractile  vesicle. 
The  peculiar  'tentacle  '  of  Noctlluca  is  a  flattened  whip-like  fila- 
ment, gradually  tapering  from  its  base  to  its  extremity,  the  two 
flattened  faces  being  directed  respectively  towards  and  away  from 
the  oral  aperture.  When  either  of  its  flattened  faces  is  examined,  it 


FIG.  589.  -Pair  of  digestive  vesicles  of  NoctUnca  lying  in  course  of  exten- 
sion of  central  protoplasmic  mass,  a,  to  form  peripheral  reticulation, 
6,  and  containing  remains  of  Algte.  (Magnified  480  diameters.) 

shows  an  alternation  of  light  and  dark  spaces,  in  every  respect 
resembling  those  of  striated  muscular  fibre,  except  that  the  clear 
spaces  are  not  subdivided.  But  when  looked  at  in  profile,  it  is  seen 
that  between  the  striated  band  and  the  aboral  surface  is  a  layer  of 
granular  protoplasm.  The  tentacle  slowly  bends  over  towards  the 
mouth  about  five  times  in  a  minute,  and  straightens  itself  still  more 
slowly,  the  middle  portion  rising  first,  while  the  point  approaches 
the  base,  so  as  to  form  a  sort  of  loop,  which  presently  straightens. 
It  seems  probable  that  the  contraction  of  the  substance  forming  the 
dark  bands  produces  the  bending  of  the  filament ;  whilst,  when 
this  relaxes,  the  filament  is  straightened  again  by  the  elasticity  of 
the  granular  layer. 

The  extreme  transparence  of  Noctiluca  renders  it  a  particularly 
favourable  subject  for  the  study  of  the  phenomena  of  phosphorescence. 
When  the  surface  of  the  sea  is  rendered  luminous  by  the  general 
diffusion  of  Noctilucce,  they  may  be  obtained  by  the  tow-net  in  un- 
limited quantities  ;  and  when  transferred  into  a  jar  of  sea-water, 
they  soon  rise  to  the  surface,  where  they  forma  thick  stratum.  The 
slightest  agitation  of  the  jar  in  the  dark  causes  an  instant  emission  of 


NOCTILUCA  769 

their  light,  which  is  of  a  beautiful  greenish  tint,  and  is  vivid  enough 
to  be  perceptible  by  ordinary  lamp-light.  This  luminosity  is  but  of 
an  instant's  duration,  and  a  short  rest  is  required  for  its  renewal.  A 
brilliant  but  short-lived  display  of  luminosity,  to  be  followed  by  its 
total  cessation,  may  be  produced  by  electric  or  chemical  stimulation. 
Professor  Allmaii  found  the  addition  of  a  drop  of  alcohol  to  the  water 
containing  specimens  of  Noctiluca,  on  the  stage  of  the  microscope, 
produced  a  luminosity  strong  enough  to  be  visible  under  a  half- inch 
objective,  lasting  with  full  intensity  for  several  seconds,  and  then 
gradually  disappearing.  He  was  thus  able  to  satisfy  himself  that 
the  special  seat  of  the  phosphorescence  is  the  peripheral  protoplasmic 
reticulation  which  lines  the  external  structureless  membrane. 

The  reproduction  in  this  interesting  type  is  effected  in  various 
ways.  According  to  Cienkowsky,  even  a  small  portion  of  the  proto- 
plasm of  a  mutilated  Noctlluca  will  (as  among  rhizopods)  reproduce 
the  entire  animal.  Multiplication  by  fission  or  binary  subdivision, 
beginning  in  the  enlargement,  const riction,  and  separation  of  the  two 
halves  of  the  nucleus,  has  been  frequently  observed.  Another  form 
of  non-sexual  reproduction,  which  seems  parallel  to  the  '  swarming  ' 
of  many  protophytes,  commences  by  a  kind  of  encysting  process. 
The  tentacle  and  flagellum  disappear,  and  the  mouth  gradually 
narrows,  and  at  last  closes  up  ;  the  meridional  groove  also  disappears, 
so  that  the  animal  becomes  a  closed  hollow  sphere.  The  nucleus 
elongates,  and  becomes  transversely  constricted,  and  its  two  halves 
separate,  each  remaining  connected  with  a  portion  of  the  protoplasmic 
network.  This  duplicative  subdivision  is  repeated  over  and  over 
jigaiii,  until  as  many  as  512  'gemmules'  are  formed,  each  consisting 
of  a  nuclear  particle  enveloped  by  a  protoplasmic  layer,  and  each 
having  its  flagellum.  The  entire  aggregate  forms  a  disc-like  mass 
projecting  from  the  surface  of  the  sphere ;  and  this  mass  sometimes 
detaches  itself  as  a  whole,  subsequently  breaking  up  into  individuals  ; 
whilst,  more  commonly,  the  gemmules  detach  themselves  one  by  one, 
the  separation  beginning  at  the  margin  of  the  disc,  arid  proceeding 
towards  its  centre.  The  gemmules  are  at  first  closed  monadiform 
spheres,  each  having  a  nucleus,  contractile  vesicle,  and  flagellum ; 
the  mouth  is  subsequently  formed,  and  the  tentacle  and  permanent 
flagellum  afterwards  make  their  appearance.  A  process  of  '  conjuga- 
tion '  has  also  been  observed,  alike  in  ordinary  Noctilucce  and  in  their 
closed  or  encysted  forms,  which  seems  to  be  sexual  in  its  nature. 
Two  individuals,  applying  their  oral  surfaces  to  each  other,  adhere 
closely  together,  and  their  nuclei  become  connected  by  a  bridge  of 
protoplasmic  substance.  The  tentacles  are  thrown  off,  the  two  bodies 
gradually  coalesce,  and  the  two  nuclei  fuse  into  one.  The  whole 
process  occupies  about  five  or  six  hours,  but  its  results  have  not  been 
followed  out.1 

1  Noctiluca  has  been  the  subject  of  numerous  memoirs,  of  which  the  following 
are  the  most  recent :  Cienkowsky,  Arch  f.  micros.  Anat.  Bd.  vii.  1871,  p.  131,  and 
Bd.  ix.  1873,  p.  47;  Allman,  Quart.  Journ.  Microsc.  Sci.  n.s.  vol.  xii.  1872,  p.  327; 
Eobin,  Jo  urn.  de  I' Anat.  et  de  Physiol.  torn.  xiv.  1878,  p.  586;  Vignal,  Arch,  de 
Physiol.  ser.  ii.  torn.  v.  1878,  p.  415;  Stein,  Der  Organismus  der  Infusionsthiere, 
iii.  2,  1883;  and  Biitschli,  Morphol,  Jahrbiich.  x.  1885,  p.  529.  For  the  group  of 
which  it  and  the  Mediterranean  genus  Leptodiscus  (Hertwig)  are  the  representatives, 
Haeckel  has  suggested  the  name  Cystoflagellata. 

3  D 


770  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

The  name  CUio-flagellata,  and  the  definition  of  the  group  must 
both  be  altered,  now  that  Klebs  and  Biitschli  have  shown  that  what 
was  regarded  as  cilia  in  the  transverse  grooves  of  their  bodies  is 
really  a  flagellum  ;  the  name  to  be  used  is  Dinoflagellata,.1  Al- 
though this  group  does  not  contain  any  great  diversity  of  forms,  yet  it 
is  specially  worthy  of  notice,  not  only  on  account  of  the  occasional 
appearance  of  some  of  them  in  extraordinary  multitudes,  but  also  for 
their  power  of  forming  cellulose— a  property  which  is  often  thought 
to  be  particularly  characteristic  of  plants.  The  Peridiniuin  observed 
by  Professor  Allmaii  in  1854  w:i>  present  in  such  quantities  that 
it  imparted  a  brown  colour  to  the  water  of  some  of  the  large  ponds 
in  Phoenix  Park,  Dublin,  this  colour  being  sometimes  uniformly 
diffused,  and  sometimes  showing  itself  more  deeply  in  dense  clouds, 
varying  in  extent  from  a  few  square  yards  to  upwards  of  a  hundred. 
The  animal  (fig.  590,  A,  B)  has  a  form  approaching  the  spherical, 
with  a  diameter  of  from  i^Voth  to  -010(lth  of  an  inch,  and  is 
partially  divided  into  two  hemispheres  by  a  deep  equatorial  furrow, 
«,  whilst  the  flagellum- bearing  hemisphere,  A,  has  a  deep  meridional 
groove  on  one  side,  5,  extending  from  the  equatorial  groove  to  the 
pole,  the  flagellum  taking  its  origin  from  the  bottom  of  this  vertical 


FIG.  590. — Peridiniuni   uberrimum:  A,  B,  front  and  back  views ; 
C,  encysted  stage ;  D,  duplicative  subdivision. 

groove,  near  its  junction  with  the  equatorial.  The  members  of  this 
group  vary  considerably  in  their  mode  of  taking  food ;  from  the 
researches  of  Bergh  it  would  appeal-  that  those  which  are  provided 
with  chromatophores  have  a  plant-like  mode  of  obtaining  food,  while 
those  which  are  without  chromatophores  are  truly  animal  in  their 
method  of  alimentation.  A  'contractile  vesicle'  has  been  rarely 
observed  ;  but  a  large  nucleus,  sometimes  oval  and  sometimes  horse- 
shoe-shaped, seems  always  present.  The  Peridinia  multiply  by 
transverse  fission  (fig.  590,  D),  which  commences  in  the  subdivision 
of  the  nucleus,  and  then  shows  itself  externally  in  a  constriction  of 
the  ungrooved  hemisphere,  parallel  to  the  equatorial  furrow.  They 
pass  into  a  quiescent  condition,  subsiding  towards  the  bottom  of  the 
water,  and  the  loricated  forms  appear  to  throw  off  their  envelopes. 
There  is  reason  to  believe  that  conjugation  obtains  in  certain  cases  : 
Glenodiniutn  cinctum  has  been  observed  by  Professor  Askenasy  to 
copulate,  but  the  development  of  the  zygote,  as  the  product  of  copu- 
lation may  be  called,  has  not  yet  been  worked  out.  Some  of  the 
Peridinia  are  found  in  sea- water,2  but  the  most  remarkable  marine 

1  Or,  more  correctly,  Dinomastigopliora, 

•  See  F.  Schiitt, '  Die  Peridincen  der  Plankton  Expedition,'  Err/cbn.  Plankton 
Exped.  1895.     170  pp.  and  27  pis. 


CERATIUM  771 

forms  of  the  cilio- flagellate  group  belong  to  the  genus  Ceratium  (fig. 
f)91),  in  which  the  cuirass  extends  itself  into  long  horny  appendages. 
In  the  Ceratium  tripos  (I)  there  are  three  of  these  appendages  ;  two 
of  them  curved,  proceeding  from  the  anterior  portion  of  the  cuirass, 
and  the  third,  which  is  straight  or  nearly  so,  from  its  posterior 
portion.  They  are  all  more  or  less  jagged  or  spiiious.  In  Ceratium 
furca  (2)  the  two  anterior  horns  are  prolonged  straight  forwards, 
one  of  them  being  always  longer  than  the  other  ;  whilst  the  posterior 
is  prolonged  straight  backwards.  >The  anterior  and  posterior  halves 
of  the  cuirass  are  separated  by  a  ciliated  furrow,  from  one  point  of 
which  the  flagellum  arises  ;  and  at  the  origin  of  this  is  a  deep 


FIG.  591. — 1,  Ceratium  tripos;  2,  Ceratium furca. 

depression  into  which  the  flagellum  may  be  completely  and  suddenly 
withdrawn.  The  Author  has  found  the  Ceratium  tripos  extremely 
abundant  in  Lamlash  Bay,  Arran,  where  it  constitutes  a  principal 
article  of  the  food  of  the  Antedons  that  inhabit  its  bottom.1 

Ciliata. — As  it  is  in  this  tribe  of  animalcules  that  the  action  of 
the  organs  termed  cilia  has  the  most  important  connection  with 
the  vital  functions,  it  seems  desirable  here  to  introduce  a  more 
particular  notice  of  them.  They  are  always  found  in  connection 
with  cells,  of  whose  protoplasmic  substance  they  may  be  considered 
as  extensions,  endowed  in  a  special  degree  with  its  characteristic 
contractility.  The  form  of  the  filaments  is  usually  a  little  flattened, 

1  See  Allman  in  Quart.  Microsc.  Journ.  vol.  iii.  1855,  p.  24  ;  H.  James-Clark  in 
Ann.  Nat.  Hist.  ser.  iii.  vol.  xviii.  1866,  p.  429  ;  Bergh,  Morphol.  Jahrbuch.  vii.  1881. 
p.  177,  and  Vanhoffen,  Zool.  Anzeig.  xix.  1896,  pp.  188-4. 

3c  2 


772 


MICROSCOPIC  FORMS   OF  ANIMAL  LIFE 


tapering  gradually  from  the  base  to  the  point.  Their  size  is  ex- 
tremely variable,  the  largest  that  have  been  observed  being  about 
-Jinth  of  an  inch  in  length,  and  the  smallest  about  i3^oth.  When 
in  motion  each  filament  appears  to  bend  from  its  root  to  its  point, 
returning  again  to  its  original  state,  like  the  stalks  of  corn  when 
depressed  by  the  wind  ;  and  when  a  number  are  affected  in 
succession  with  this  motion,  the  appearance  of  progressive  waves 
following  one  another  is  produced,  as  when  a  cornfield  is  agitated 
by  successive  gusts.  When  the  ciliary  action  is  in  full  activity, 
however,  little  can  be  distinguished  save  the  whirl  of  particles  in 
the  surrounding  fluid  ;  but  the  back  stroke  may  often  be  perceived, 
when  the  forward  stroke  is  made  too  quickly  to  be  seen,  and  the 
real  direction  of  the  movement  is  then  opposite  to  the  apparent.  In 
this  back  stroke,  when  made  slowly  enough,  a  sort  of  'feathering' 
action  may  be  observed,  the  thin  edge  being  made  to  cleave  the 


FIG.  592. — A,  Kerona  stlnrus:  a,  contractile  vesicle;  6, 
mouth ;  r,  c,  animalcules  swallowed  by  the  Kerona,  after 
having  themselves  ingested  particles  of  indigo.  B, 
Paramecium  caudatum:  a,  a,  contractile  vesicles; 
b,  mouth.  The  dotted  lines  indicate  currents. 

liquid  which  has  been  struck  by  the  broad  surface  in  the  opposite 
direction.  It  is  only  when  the  rate  of  movement  has  considerably 
slackened  that  the  shape  and  size  of  the  cilia,  and  the  manner  in 
which  their  stroke  is  made,  can  be  clearly  seen.  Their  action  has 
been  observed  to  continue  for  many  hours,  or  even  days,  after  the 
death  of  the  body  at  large.  As  cilia  are  not  confined  to  animal- 
cules and  zoophytes,  but  give  motion  to  the  zb'ospores  of  many 
protophytes,  and  also  clothe  the  free  internal  surfaces  of  the  respi- 
ratory and  other  passages  in  all  the  higher  animals,  including  man 
(our  own  experience  thus  assuring  us  that  their  action  takes  place, 
not  only  without  any  exercise  of  will,  but  even  without  conscious- 
ness), it  is  clear  that  to  regard  animalcules  as  possessing  a  '  voluntary  ' 
control  over  the  action  of  their  cilia  is  altogether  unscientific. 


CILIATA  773 

In  the  ciliated  Infusoria,  the  differentiation  of  the  sareodic  sub- 
stance into  '  ectosarc  '  or  cell-wall,  and  '  endosarc  '  or  cell-contents, 
becomes  very  complete,  the  ectosarc  possessing  a  membranous 
firmness  which  prevents  it  from  readily  yielding  to  pressure,  and 
having  a  definite  internal  limit,  instead  of  graduating  insensibly 
(as  in  rhizopods)  into  the  protoplasmic  layer  which  lines  it.  A 
'•  nucleus '  seems  always  present,  being  sometimes  '  parietal '  (or 
adherent  to  the  interior  of  the  ectosarc),  in  other  cases  lying  in  the 
midst  of  the  endosarc.  In  many  Ciliata  a  distinct  '  cuticle '  or 
exudation-layer  may  be  recognisecl  on  the  surface  of  the  ectosarc: 
and  this  cuticle,  which  is  studded  with  regularly  arranged  markings 
like  those  of  Diatomacese,  seems  to  be  the  representative  of  the 
carapace  of  Arcella  £c.  as  of  the  cellulose  coat  of  protophytes. 
It  is  sometimes  hardened,  so  as  to  form  a  '  shield  '  that  protects 
the  body  on  one  side  only,  or  a  '  lorica '  that  completely  invests 
it ;  and  there  are  other  cases  in  which  it  is  so  prolonged  and 
doubled  upon  itself  as  to  form  a  sheath  resembling  the  '  cell '  of  a 
zoophyte,  within  which  the  body  of  the  animalcule  lies  loosely,  being 
attached  only  by  a  stalk  at  the  bottom  of  the  case,  and  being  able 
either  to  project  itself  from  the  outlet  or  to  retract  itself  into  the 
interior.  In  the  marine  forms  known  as  Dictocysta,  and  in  Codonella, 
described  by  Haeckel,  the  body  is  enclosed  in  a  silicious  lattice-work 
shell,  usually  bell-shaped  or  helmet-shaped,  which  bears  so  strong 
a  resemblance  to  the  shells  of  many  Radiolaria  as  to  be  easily  mis- 
taken for  them.  The  form  of  the  body  is  usually  much  more 
definite  than  that  of  the  naked  rhizopods,  each  species  having  its 
characteristic  shape,  which  is  only  departed  from,  for  the  most  part, 
when  the  animalcule  is  subjected  to  preSvSure  from  without,  or  when 
its  cavity  has  been  distended  by  the  ingestion  of  any  substance 
alxwe  the  ordinary  size.  The  cilia  and  other  mobile  appendages  of 
the  body  are  extensions  of  the  outer  layer  of  the  '  ectosarc '  proper ; 
and  this  layer,  which  retains  a  high  degree  of  vital  activity,  is  some- 
times designated  the  '  cilia-layer.'  Beneath  this  is  a  layer  in  which 
(or  in  certain  bands  of  which)  regular,  parallel,  fine  strife  may  be 
distinguished,  and  as  this  striation  is  also  distinguishable  in  the 
eminently  contractile  foot-stalk  of  Vorticella1  (fig.  593,  B)  there  seems 
good  reason  to  regard  it  as  indicating  a  special  modification  of  proto- 
plasmic substance,  which  resembles  muscle  in  its  endowments. 
Hence  this  is  termed  the  *  myophan-layer.'  Beneath  this,  in  cer- 
tain species  of  Infusoria,  there  is  found  a  thin  stratum  of  condensed 
protoplasm,  including  minute  'trichocysts,'  which  resemble  in 
miniature  the  '  thread-cells '  of  zoophytes ;  and  this,  where  it 
exists,  is  known  as  the  '  trichocyst-layer.'  The  hair-like  pro- 
cesses of  protoplasm  may  be  caused  to  protrude  from  the  cell 
by  such  irritation  as  is  effected  by  the  addition  of  a  little  iodine  to 
the  water  in  which  the  animalcule  is  living. 

The  vibration  of  ciliary  filaments,  which  are  either  disposed 
along  the  entire  margin  of  the  body,  as  well  as  around  the  oral 

1  On  the  morphology  of  the  Vorticellinse  see  Biitschli,  MorphoJ.  Jahrb.  xi. 
p.  553. 


774 


MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 


aperture  (fig.  593,  A,  B),  or  are  limited  to  some  one  part  of  it, 
which  is  always  in  the  immediate  vicinity  of  the  mouth,  sup- 
plies the  means  in  this  group  of  Infusoria  both  for  progres- 
sion through  the  water  and  for  drawing  alimentary  particles  into 
the  interior  of  their  bodies.  In  some  their  vibration  is  constant, 
whilst  in  others  it  is  only  occasional.  The  modes  of  movement 
which  infusory  animalcules  execute  by  means  of  these  instru- 
ments are  extremely  varied  and  remarkable.  Some  propel  them- 
selves directly  forwards,  with  a  velocity  which  appears,  when  highly 
magnified,  like  that  of  an  arrow,  so  that  the  eye  can  scarcely  follow 

them ;  whilst  others  drag  their 
bodies  slowly  along  like  a  leech. 
Some  attach  themselves  by  one 
of  their  long  filaments  to  a  fixed 
point,  and  revolve  around  it  with 
great  rapidity,  whilst  others 
move  by  undulations,  leaps,  or 
successive  gyrations :  in  short, 
there  is  scarcely  any  kind  of 
animal  movement  which  they 
do  not  exhibit.  But  there  are 
cases  in  which  the  locomotive 
filaments  have  a  bristle-like  firm- 
ness, and,  instead  of  keeping 
themselves  in  rapid  vibration,  are 
moved  (like  the  spines  of  Echini) 
by  the  contraction  of  the  integu- 
ment from  which  they  arise,  in 
such  a  manner  that  the  animal- 
cule crawls  by  their  means  over 
a  solid  surface,  as  we  see  espe- 
cially in  Trichoda  lynceus  (fig. 
FIG.  593.— Group  of  Vorticella  nebuhfera  ^07  -p  c\\  T™  mi^T™ 

showing,  A,  the  ordinary  form  ;  B,  the  D*7>  *>  **)\  in  bMtOOfm  and 
same  with  the  stalk  contracted  ;  C,  the  AflUftMa,  again,  the  mouth  IS  pro- 
same  with  the  bell  closed  ;  D,  E,F,  sue-  videc[  wjth  a  circlet  of  plications 
cessive  stages  of  fissiparous  mulbplica-  Qr  foldgj  looking  Ufc/fcfc^ 

which,  when  imperfectly  seen,  re- 
ceived the  designation  of ;  teeth  ; ' 

their  function,  however,  is  rather  that  of  laying  hold  of  alimen- 
tary particles  by  their  expansion  and  subsequent  drawing  together 
(somewhat  after  the  fashion  of  the  teiitacula  of  zoophytes)  than  of 
reducing  them  by  any  kind  of  masticatory  process.  Some,  like 
Opalina,  are  entoparasitic,  and  have  no  mouth  ;  a  form  allied  to 
Opalina  (Anoplophrya  circulans)  lives  in  the  blood  of  Asellus 
aquations;  other  entoparasites,  such  as  TncJtonympha  in  the  '  white 
ant,'  still  possess  their  mouth.  The  curious  contraction  of  the  foot- 
stalk of  the  Vorticella  (fig.  593),  again,  is  a  movement  of  a  different 
nature,  being  due  to  the  contractility  of  the  tissue  that  occupies 
the  interior  of  the  tubular  pedicle.  This  stalk  serves  to  attach  the 
bell-shaped  body  of  the  animalcule  to  some  fixed  object,  such  as  a 
leaf  or  stem  of  duck-weed and  when  the  animal  is  in  search  of 


CILIATA 


775 


food,  with  its  cilia  in  active  vibration,  the  stalk  is  fully  extended. 
If,  however,  the  animalcule  should  have  drawn  to  its  mouth  any 
particles  too  large  to  be  received  within  it,  or  should  be  touched  by 
any  other  that  happens  to  be  swimming  near  it,  or  should  be 
'  jarred  '  by  a  smart  tap  on  the  stage  of  the  microscope,  the  stalk 
suddenly  contracts  into  a  spiral,  from  which  it  shortly  afterwards 
extends  itself  again  into  its  previous  condition.  The  central  cord, 
to  whose  contractility  this  action  is  due,  has  been  described  as 
muscular,  though  not  possessing  the  characteristic  structure  of  either 
kind  of  muscular  fibre  ;  it  possessed,,  however,  the  special  irritability 
of  muscle,  being  instantly  called  into  contraction  (according  to  the 
observations  of  Kiihiie)  by  electrical  excitation.  The  only  special 
*  impressionable '  organs  l  for  the  direction  of  their  actions  with  the 
possession  of  which  Infusoria  can  be  credited  are  the  delicate 
bristle -like  bodies  which  project  in  some  of  them  from  the  neighbour- 
hood of  the  mouth,  and  in  titentor  from  various  parts  of  the  surface. 
The  red  spots  seen  in  many  Infusoria,  which  have  been  designated 
as  eyes  by  Professor  Ehrenberg,  from  their  supposed  correspondence 
with  the  eye-spots  of  Rotifera,  really  bear  a  much  greater  re- 
semblance to  the  red  spots  which  are  so  frequently  seen  among 
protophytes.  R.  Hertwig,  who  seems  to  have  successfully  defended 
himself  against  the  strictures  of  Professor  Vogt,  has  described  a 
vorticellid — Erythropsis  agilis — as  having  a  pigment-spot  which 
cannot  but  be  regarded  as  a  rudimentary  eye ;  Metschnikoff,  who 
thinks  that  Eryihropsis  is  an  Acinetan,  found  a  similar  form  with  a 
similar  eye  near  Madeira  ;  and  Harker  observed  that  if  light  be 
allowed  to  fall  on  a  part  only  of  a  colony  of  Ophridium  versatile  all 
the  members  soon  congregate  to  the  illuminated  portion.2 

The  interior  of  the  body  does  not  always  seem  to  consist  of  a 
simple  undivided  cavity  occupied  by  soft  protoplasm ;  for  the  tegu- 
mentary  layer  appears  in  many  instances  to  send  prolongations 
across  it  in  different  directions,  so  as  to  divide  it  into  chambers  of 
irregular  shape,  freely  communicating  with  each  other,  which  may 
be  occupied  either  by  protoplasm,  or  by  particles  introduced  from  with- 
out. The  alimentary  particles  which  can  be  distinguished  in  the 
interior  of  the  transparent  bodies  of  Infusoria  are  visually  proto- 
phytes of  various  kinds,  either  entire  or  in  a  fragmentary  state. 
The  Diatomacere  seem  to  be  the  ordinary  food  of  many ;  and  the 
insolubility  of  their  loricce  enables  the  observer  to  recognise  them 
unmistakably.  Sometimes  entire  Infusoria  are  observed  within  the 
bodies  of  others  not  much  exceeding  them  in  size  (fig.  597,  B)  ;  but 
this  is  only  when  they  have  been  recently  swallowed,  since  the  prey 
speedily  undergoes  digestion.  It  would  seem  as  if  these  creatures 
do  not  feed  by  any  means  indiscriminately,  since  particular  kinds  of 
them  are  attracted  by  particular  kinds  of  aliment ;  the  crushed 
bodies  and  eggs  of  Entomostraca,  for  example,  are  so  voraciously 

1  The  term  '  organs  of  sense  '  implies  a  consciousness  of  impressions,  with  which 
it  is  difficult  to  conceive  that  unicellular  Infusoria  can  be  endowed.     The  component 
cells  of  the  human  body  do  their  work  without  themselves  knowing  it. 

2  These  results  are  confirmed  by  the  observations   of  R.  Franze ;  see  Zeitschr. 
wiss.  ZooL  Ivi.  1893,  pp.  138-64. 


776  MICEOSCOPIC   FORMS   OF  ANIMAL  LIFE 

consumed  by  the  Coleps  that  its  body  is  .sometimes  quite  altered  in 
shape  by  the  distension.  This  circumstance,  however,  by  110  means 
proves  that  such  creatures  possess  a  sense  of  taste  and  a  power  of 
determinate  selection  ;  for  many  instances  might  be  cited  in  which 
actions  of  the  like  apparently  conscious  nature  are  performed  with- 
out any  such  guidance.  The  ordinary  process  of  feeding,  as  well  as 
the  nature  and  direction  of  the  ciliary  currents,  may  be  best  studied 
by  diffusing  through  the  water  containing  the  animalcules  a  few 
particles  of  indigo  or  carmine.  These  may  be  seen  to  be  carried  by 
the  ciliary  vortex  into  the  mouth,  and  their  passage  may  be  traced 
for  a  little  distance  down  a  short  (usually  ciliated)  oesophagus. 
There  they  commonly  become  aggregated  together,  so  as  to  form  a 
little  pellet  of  nearly  globular  form  ;  and  this,  when  it  has  attained 
the  size  of  the  hollow  within  which  it  is  moulded,  seems  to  receive 
an  investment  of  firm  sarcodic  substance,  resembling  the  '  digestive 
vesicles'  of  Noctiluca,  and  to  be  then  projected  into  the  softer 
endosarc  of  the  interior  of  the  cell,  its  place  in  the  oesophagus  being 
occupied  by  other  particles  subsequently  ingested.  (This  '  moulding/ 
however,  is  by  no  means  universal,  the  aggregations  of  coloured 
particles  in  the  bodies  of  Infusoria  being  often  destitute  of  any 
regularity  of  form.)  A  succession  of  such  pellets  being  thus  intro- 
duced into  the  cell-cavity,  a  kind  of  circulation  is  seen  to  take  place 
in  its  interior,  those  that  first  entered  making  their  way  out  after 
a  time  (first  yielding  up  their  nutritive  materials),  generally  by  a 
distinct  anal  orifice,  but  sometimes  by  the  mouth.  When  the 
pellets  are  thus  moving  round  the  body  of  the  animalcule,  two  of 
them  sometimes  appear  to  become  fused  together,  so  that  they 
obviously  cannot  have  been  separated  by  any  firm  membranous  in- 
vestment. The  mode  of  formation  of  food  vacuoles  has  been  carefully 
studied  by  Miss  Greenwood  1  in  Carchesium  polypinum,  which  may 
be  recommended  for  the  study  of  the  processes  of  protozoan 
digestion.  When  the  animalcule  has  not  taken  food  for  some  time, 
'vacuoles,'  or  clear  spaces,  extremely  variable  both  in  size  and 
number,  filled  only  with  a  very  transparent  fluid,  are  often  seen  in 
its  protoplasm  ;  and  their  fluid  sometimes  shows  a  tinge  of  colour. 
which  seems  to  be  due  to  the  solution  of  some  of  the  vegetable 
chlorophyll  upon  which  the  animalcule  may  have  fed  last. 

Contractile  vesicles  (fig.  592,  a,  a),  usually  about  the  size  of  the 
'  vacuoles,'  are  found,  either  singly  or  to  the  number  of  from  two  to 
sixteen,  in  the  bodies  of  most  ciliated  animalcules  ;  and  may  be  seen 
to  execute  rhythmical  movements  of  contraction  and  dilatation  at 
tolerably  regular  intervals,  being  so  completely  obliterated,  when 
emptied  of  their  contents,  as  to  be  quite  midistinguishable,  and 
coming  into  view  again  as  they  are  refilled.  These  vesicles  do  not 
change  their  position  in  the  individual,  and  they  are  pretty 
constant,  both  as  to  size  and  place,  in  different  individuals  of  the 
same  species ;  hence  they  are  obviously  quite  different  in  character 
from  the  *  vacuoles.'  In  Paramecium  there  are  always  to  be  observed 
two  globular  vesicles  (fig.  592,  B,  a.  a),  each  of  them  surrounded  by 

1  PML  Trans.  1894,  B.  pp.  855-83. 


CILIATA  777 

several  elongated  cavities,  arranged  in  a  radiating  manner,  so 
as  to  give  to  the  whole  somewhat  of  a  star-like  aspect,  and 
the  liquid  contents  are  seen  to  be  propelled  from  the  former  into 
the  latter,  and  vice  versa.  Further,  in  Stentor,  a  complicated  net- 
work of  canals,  apparently  in  connection  with  the  contractile 
vesicles,  has  been  detected  in  the  substance  of  the  '  ectosarc,'  and 
traces  of  this  may  be  observed  in  other  Infusoria.  In  some  of  the 
larger  animalcules  it  may  be  distinctly  seen  that  the  contractile 
vesicles  have  permanent  valvular  orifices  opening  outwards,  and  that 
an  expulsion  of  fluid  from  the>body  into  the  water  around  it  is 
effected  by  their  contraction  ;  in  some  vorticellids  the  contractile 
vesicle  is  connected  by  a  canal  with  the  '  vestibule '  which  lies  beneath 
the  mouth  opening,  and  when  the  vesicle  contracts  the  water  is  driven 
into  the  mouth,  and  so  to  the  exterior.  Hence  it  appears  likely  that 
their  function  is  of  a  respiratory  and  depuratory  nature  ;  and  that  they 
serve,  like  the  gill-openings  of  fishes,  for  the  expulsion  of  water 
which  has  been  taken  in  by  the  mouth,  and  which  has  traversed  the 
interior  of  the  body. 

Of  the  reproduction  of  the  ciliated  Infusoria  our  knowledge 
though  imperfect  has  advanced.  As  has  been  well  said  by  Mr. 
Adam  Sedgwick,1  'the  more  decent  work  of  Biitschli  and  Maupas 
[has]  shown  that  in  their  reproduction  these  animals  resemble  other 
Protozoa ;  that  is  to  say,  that  the  whole  body  participates  in  the 
reproductive  fission,  that  the  parent  disappears  in  the  offspring,  and 
that  special  conjugating  cells  of  the  nature  of  ova  and  spermatozoa  are 
not  formed.  Maupas2  especially,  by  following  the  history  of  the 
individual  resulting  from  conjugation,  has  definitely  established  the 
fundamental  distinction  between  conjugation  and  reproduction,  and 
has  thrown  a  flood  of  light  upon  the  meaning  of  the  whole  phenomenon 
of  conjugation.'  The  best  evidence  is  that  of  Gruber,  which  will  be 
mentioned  directly.  Binary  subdivision  would  seem  to  be  universal 
among  them,  and  has  in  many  instances  been  observed  (as  elsewhere) 
to  commence  in  the  nucleus.  The  division  takes  place  in  some  species 
longitudinally,  that  is,  in  the  direction  of  the  greatest  length  of  the  body 
(fig.  593,  D,  E,  F),  in  other  species  transversely  (fig.  597,  C,  D) ;  while 
in  some,  as  in  Chilodon  cucullulus  (fig.  595),  it  has  been  supposed  to 
occur  in  either  direction  indifferently.  But  it  may  fairly  be  questioned 
whether,  in  this  last  case,  one  set  of  the  apparent  '  fissions '  is  not 
really  '  conjugation '  of  two  individuals.  This  duplication  is  per- 
formed with  such  rapidity,  under  favourable  circumstances,  that, 
according  to  the  calculation  of  Professor  Ehreiiberg,  no  fewer  than 
268  millions  might  be  produced  in  a  month  by  the  repeated  sub- 
divisions of  a  single  Paramecium.  When  this  fission  occurs  in 
Vorticella  (fig.  593),  it  extends  down  the  stalk,  which  thus  becomes 
double  for  a  greater  or  less  part  of  its  length ;  and  thus  a  whole 
bunch  of  these  animalcules  may  spring  (by  a  repetition  of  the  same 
process)  from  one  base.  In  some  members  of  the  same  family 
arborescent  structures  are  produced  resembling  that  of  Codosiga 

1  Student's  Textbook  of  Zoology,  1898,  p.  26. 

-  See  particularly  his  memoirs,  in  vols.  vi.  and  vii.  of  the  second  series  of  the  Arcli* 
Zool.  Exper.  1888-i). 


MICROSCOPIC    FORMS   OF  ANIMAL  LIFE 


(fig.  586)  by  the  like  process  of  continuous  subdivision.  Another 
curious  result  of  this  mode  of  multiplication  presents  itself  in. 
the  family  Ophrydina,  masses  of  individuals  which  separately  resemble 
certain  Vorticellina  being  found  imbedded  in  a  gelatinous  substance  of 
a  greenish  colour,  sometimes  adherent  and  sometimes  free.  These 
masses,  which  may  attain  the  diameter  of  four  or  five  inches,  present 
such  a  strong  general  resemblance  to  a  mass  of  Xostoc,  or  even  of  frog's 
spawn,  as  to  have  been  mistaken  for  such  ;  but  they  simply  result  from 
the  fact  that  the  multitude  of  individuals  produced  by  a  repetition  of 
the  process  of  self-division  remain  connected  with  each  other  for  a 
time  by  a  gelatinous  exudation  from  the  surface  of  their  bodies, 
instead  of  at  once  becoming  completely  isolated.  From  a  comparison  of 
the  dimensions  of  the  individual  Ophryda,  each  of  which  is  about  i4^th 
of  an  inch  in  length,  with  those  of  the  composite  masses,  some  estimate 

3  may  be  formed  of  the 

number  included  in 
the  latter;  for  a 
cubic  inch  would  con- 
tain nearly  eight  'mil- 
lions of  them  if  closely 
packed ;  arid  many 
times  that  number 
must  exist  in  the 
larger  masses,  even 
making  allowance  for 
the  fact  that  the 
bodies  of  the  animal- 
cules are  separated 
from  each  other 
by  their  gelatinous 
cushion,  and  that 
the  masses  have  their  central  portions  occupied  by  water  only. 
Hence  we  have,  in  such  clusters,  a  distinct  proof  of  the  extra- 
ordinary extent  to  which  multiplication  by  duplicative  subdivision 
may  proceed  without  the  interposition  .of  aiiv  other  operation. 
These  animalcules,  however,  free  themselves  at  times  from  their 
gelatinous  bed,  and  have  been  observed  to  undergo  an  'encysting 
process '  corresponding  with  that  of  the  Vorticellina.  The  chemical 
composition  of  this  jelly  or  zoocytium  has  been  investigated  by 
Halliburton,  who  finds  that  it  resembles  vegetable  cellulose  in  its 
general  properties,  but  differs  from  it  and  agrees  writh  the  form  of 
cellulose  manufactured  by  the  Tunicata  in  being  less  easily  converted 
into  sugar. 

Many,  perhaps  all,  ciliated  Infusoria  at  certain  times  undergo  an 
encysting  process,  resembling  the  passage  of  protophytes  into  the  '  still ' 
condition,  and  apparently  serving  like  it  as  a  provision  for  their  pre- 
servation under  circumstances  which  do  not  permit  the  continuance 
of  their  ordinary  vital  activity.  Previously  to  the  formation  of  the 
cyst,  the  movements  of  the  animalcule  dimmish  in  vigour,  and 
gradually  cease  altogether ;  its  form  becomes  more  rounded ;  its 
oral  aperture  closes ;  and  its  cilia  or  other  filamentous  prolonga- 


FIG.  594.— Reproduction  of  Infusoria. 


CILIATA 


779 


tions  are  either  lost  or  retracted,  as  is  well  seen  in  Vorticella 
(fig.  596,  A).  A  new  wreath  of  cilia,  however,  is  developed  near 
the  base,  and  in  this  condition  the  animal  detaches  itself  from  its 


FIG.  595. — Fissiparous  multiplication  of  Chilodon  cucitUiilus:  A, 
B,  C,  successive  stages  of  longitudinal  fission  (?) ;  D,  E,  F,  succes- 
sive stages  of  transverse  fission. 


stein  and  swims  freely  for  a  short  time,  soon  passing,  however,  into 
the  '  still '  condition.  The  surface  of  the  body  then  exudes  a  gela- 
tinous excretion  that  hardens  around  it  so  as  to  form  a  complete 
coffin-like  case,  within  which  little  of  the  original  structure  of  the 
animal  can  be  distinguished.  Even  after  the  completion  of  the  cyst, 
however,  the  contained 
animalcule  may  often 
be  observed  to  move 
freely  within  it,  and 
may  sometimes  be  c 
caused  to  come  forth 
from  its  prison  by  the 
mere  application  of 
warmth  and  moisture. 
In  the  simplest  form  of 
the  'encysting  process,' 
indeed,  the  animalcule 
seems  to  remain  alto- 
gether quiescent  through 
the  whole  period  of  its 
torpidity  :  SO  that,  how-  FlG-  596.— Encysting  process  in  Vorticella  micro- 
i  il  .-i  stoma:  A,  full-grown  individual  in  its  encysted 

ever   long    may    be  the      state .  a>  r'etracte6d  oval  circlet  Of  cilia ;  b,  nucleus ; 
duration  of  its  imprison-       c,  contractile  vesicle  ;  B,  a  cyst  separated  from  its 

stalk ;  C,  the  same  more  advanced,  the  nucleus 
broken  up  into  spore-like  globules ;  D,  the  same 
more  developed,  the  original  body  of  the  Vorticella, 
d,  having  become  sacculated,  and  containing  many 
clear  spaces  ;  at  E,  one  of  the  sacculations  having 
burst  through  the  enveloping  cyst,  a  gelatinous 
mass,  e,  containing  the  gemmules  is  discharged. 


mentj  it  emerges  with- 
out any  essential  change 
in  its  form  or  condition. 
But  in  other  cases  this 
process  seems  to  be  sub- 


servient either  to  multi- 
plication or  to  metamorphosis.  For  in  Vorticella  the  substance 
of  the  encysted  body  (B)  appears  to  break  up  (C,  D)  into  eight 
or  nine  segments,  which,  when  set  free  by  the  bursting  of  the 
cyst,  come  forth  as  spontaneously  moving  spherules.  Each  of  these 
soon  increases  in  size,  develops  a  ciliary  wreath  within  which  a  mouth 


780  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

makes  its  appearance,  and  gradually  assumes  the  form  of  the  Tricho- 
dina  grandinella  of  Ehrenberg.     It  then  develops  a  posterior  wreath 
of  cilia  and  multiplies  by  transverse  fission  ;  each  half  fixes  itself  by 
the  end  on  which  the  mouth  is  situated,  a  short  stem  becomes  de- 
veloped, and  the  cilia-wreath  disappears.     A   new  mouth  and  cilia- 
wreath  then  form  at  the  free  extremity,  and  the  growth  of  the  stem 
completes   the   development   into   the   true   vorticellan  form.1     In 
Trichoda  lynceus,  again,  the  '  encysting  process '  appears  subservient 
to  a  like  kind  of  metamorphosis,  the  form  which  emerges  from  the 
cyst  differing  in  many  respects  from  that  of  the  animalcule  which 
became  encysted.     According  to  M.  Jules   Haime,  by  whom  this 
history  was  very  carefully  studied,'2  the  form  to  be  considered  as  the 
larval  one  is  that  shown  in  fig.  597,  A,  E,  which  has  been  described 
by  Professor  Ehrenberg  under  the  name  of  Oxytricha.     This  possesses 
a  long,  narrow,  flattened  body,  furnished  with  cilia  along  the  greater 
part  of  both  margins,  and  having  also  at  its  two  extremities  a  set  of 
larger  and  stronger  hair-like  filaments  ;  and  its  mouth,  which  is  an 
oblique  slit  on  the  right-hand  side  of  its  fore-part,  has  a  fringe  of 
minute  cilia  on  each  lip.     Through  this  mouth  large  particles  are  not 
unfrequently  swallowed,  which  are  seen  lying  in  the  midst  of  the 
endosarc  without  any  surrounding  vesicle  ;  and  sometimes  even  an 
animalcule  of  the  same  species,  but  in  a  different  stage  of  its  life,  is 
seen  in  the  interior  of  one  of  these  voracious  little  devourers  (B).    In 
this  phase  of  its  existence  the  Trichoda  undergoes  multiplication  by 
transverse  fission,  after  the  ordinary  mode  (C,  D) ;  and  it  is  usually 
one  of  the  short-bodied  'doubles'  (E)  thus  produced    that  passes 
into  the  next  phase.     This  phase  consists  in  the  assumption  of  the 
globular  form  and  the  almost  entire  loss  of  the  locomotive  append- 
ages (F) ;  in  the  escape  of  successive  portions  of  the  granular  proto- 
plasm, so  that  '  vacuoles '  make  their  appearance  (G) ;   and  in  the 
formation   of  a   gelatinous   envelope  or  cyst,  which,  at  first  soft, 
afterwards  acquires  increased  firmness  (H).      After  remaining  for 
some  time  in  this  condition,  the  contents  of  the  cyst  become  clearly 
separated  from  their  envelope ;  and  a  space  appears  on  one  side,  in 
which    ciliary   movement    can   be    distinguished    (I).       This    space 
gradually  extends  all  round,  and  a  further  discharge  of  granular 
matter  takes  place  from  the  cyst,  by  which  its  form  becomes  altered 
(K) ;  and  the  distinction  between  the  newly  formed  body  to  which 
the  cilia  belong  and  the  effete  residue  of  the  old  becomes  more  and 
more  apparent  (L).     The  former  increases  in  size,  whilst  the  latter 
diminishes ;    and  at  last  the  former  makes  its  escape  through   an 
aperture  in  the  wall  of  the  cyst,  a  part  of  the  latter  still  remaining 
within  its  cavity  (M).     The  body  thus  discharged  (N)  does  not  differ 
much  in  appearance  from  that  of  the  Oxytricha  before  its  encyst- 
ment  (F),  though  of  only  about  two-thirds  its  diameter ;  but  it  soon 
develops  itself  (0,  P,   Q)  into  an  animalcule  very  different  from 
that  in  which  it  originated.     First  it  becomes  still  smaller  by  the 
discharge  of  a  portion  of  its  substance  ;  numerous  very  stiff  bristle- 

1  Everts,  Untersnclmngen  an  Vorticella  nebul/fera,  quoted  by  Professor  Allman, 
toe.  cit. 

2  Annales  des  Set.  Nat.  ser.  iii.  tome  xix.  1853,  p.  109. 


CILIATA 


78! 


like  organs  are  developed,  on  which  the  animalcule  creeps,  as  bv 
legs,  over  solid  surfaces ;  the  external  integument  becomes  more 
consolidated  on  its  upper  surface,  so  as  to  become  a  kind  of  cara- 
pace ;  and  a  mouth  is  formed  by  the  opening  of  a  slit  on  one  side, 
in  front  of  which  is  a  single  hair-like  flagellum,  which  turns  round 
and  round  with  great  rapidity,  so  as  to  describe  a  sort  of  inverted 
cone  whereby  a  current  is  brought  towards  the  mouth.  This  latter 
form  had  been  described  by  Professor  Ehrenberg  under  the  name  of 
Aspidisca.  It  is  very  much  smaller  than  the  larva,  the  difference 
being,  in  fact,  twice  as  great  as>that  which  exists  between  A  and 


FIG.  597. — Metamorphoses  of  Trichoda  lynceus  :  A,  larva  (Ojcijtriclia)\  B,  a 
similar  larva  after  swallowing  the  animalcule  represented  at  M ;  C,  a  very 
large  individual  on  the  point  of  undergoing  fission ;  D,  another  in  which 
the  process  has  advanced  further ;  E,  one  of  the  products  of  such  fission ; 
F,  the  same  body  become  spherical  and  motionless  ;  G,  aspect  of  this 
sphere  fifteen  days  afterwards  ;  H,  later  condition  of  the  same,  showing  the 
formation  of  the  cyst ;  I,  incipient  separation  between  living  substance 
and  exuvial  matter  ;  K,  partial  discharge  of  the  latter,  with  flattening  of 
the  sphere  ;  L,  more  distinct  formation  of  the  confined  animal ;  M,  its 
escape  from  the  cyst ;  N,  its  appearance  some  days  afterwards  ;  O,  more 
advanced  stage  of  the  same;  P,  Q,  perfect  Aspidiscce,  one  as  seen  side- 
ways, moving  on  its  bristles,  the  other  as  seen  from  below  (magnified 
twice  as  much  as  the  preceding  figures). 

P,  Q  (fig.  597),  since  the  last  two  figures  are  drawn  under  a  magni- 
fying power  double  that  employed  for  the  preceding.  How  the 
Aspidisca-fona.  in  its  turn  gives  origin  to  the  Oxytricha-form 
has  not  yet  been  made  out.  A  similar  l  encysting  process '  has 
been  observed  to  take  place  among  several  other  forms  of  ciliated 
Infusoria ;  so  that,  considering  the  strong  general  resemblance 
in  kind  and  degree  of  organisation  which  prevails  throughout  the 
group,  it  does  not  seem  unlikely  that  it  may  occur  at  some  stage  of 
the  life  of  nearly  all  these  animalcules.  And  it  is  not  improbably 
in  the  '  encysted  '  condition  that  their  dispersion  chiefly  takes  place, 
since  they  have  been  found  to  endure  desiccation  in  this  state, 
although  in  their  ordinary  condition  of  activity  they  cannot  be  dried 


782  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

up  without  loss  of  life.  When  this  circumstance  is  taken  into 
account,  in  conjunction  with  the  extraordinary  rapidity  of  multipli- 
cation of  these  animalcules,  there  seems  no  difficulty  in  accounting 
for  the  universality  of  their  diffusion.  It  may  be  stated  as  a  general 
fact  that  wherever  decaying  organic  matter  exists  in  a  liquid  state, 
and  is  exposed  to  air  and  warmth,  it  speedily  becomes  peopled  with 
some  or  other  of  these  minute  inhabitants ;  and  it  may  be  fairly 
presumed  that,  as  in  the  case  of  the  Fungi,  the  dried  cysts  or  germs 
of  Infusoria  are  everywhere  floating  about  in  the  air,  ready  to  de- 
velop themselves  wherever  the  appropriate  conditions  are  presented  ; 
but  we  must  remember  that  but  few  definite  observations  have 
been  made  as  to  the  length  of  time  these  cysts  will  survive  desiccation ; 
at  present,  the  observations  of  Nussbauni  and  Maupas  make  the 
limit  less  than  two  years. 

Gruber  has  recently  reinvestigated  the  process  of  conjugation  in 
the  Infusoria  :  he  finds  that  the  nucleolus  of  each  becomes  a  striated 
spindle,  and  approaches  the  nucleolus  of  the  other  cell;  the  two 
touch  and  finally  fuse,  thereby  effecting  an  intermixture  of  the 
different  germ-plasmas.  If  this  be  the  correct  manner  of  interpret- 
ing the  phenomenon,  it  is  clearly  comparable  to  the  sexual  reproduc- 
tion of  multicellular  animals. 

There  can  be  no  doubt  as  to  the  occurrence  of  'conjugation' 
among  ciliated  Infusoria ;  and  this  not  only  in  the  free-swimming, 
but  also  in  the  attached  forms,  as  Stentor  (fig.  594,  3).  Iri 
Vorticella,  according  to  several  recent  observers,  what,  has  been 
regarded  as  gemmiparous  multiplication — the  putting  forth  of  a  bud 
from  the  base  of  the  body — is  really  the  conjugation  of  a  small 
individual  in  the  free-swimming  stage  with  a  fully  developed  fixed 
individual  (microgamete)  with  whose  body  its  own  becomes  fused. 
But  it  is  doubtful  whether  such  conjugation  has  any  reference  to  the 
encysting  process.  According  to  Biitschli  and  Engelmaim,  the  con- 
jugating process  results  in  the  breaking  up  of  the  nucleus  and  (so- 
called)  nucleolus  of  the  conjugating  individuals ;  these  individuals 
separate  again,  and  after  the  expulsion  of  the  broken-up  nuclear 
structures  the  characteristic  nucleus  and  nucleolus  are  re-formed. 
There  is  still  much  uncertainty  in  regard  to  the  embryonic  forms  of 
ciliate  Infusoria,  some  eminent  observers  asserting  that  the 
'  gemmule  '  in  the  first  instance,  besides  forming  a  cilia-wreath,  puts 
forth  suctorial  appendages  (fig.  594,  1,  A,  B,  C),  by  means  of  which 
it  imbibes  nourishment  until  the  formation  of  its  mouth  permits  it  to 
obtain  its  supplies  in  the  ordinary  way ;  whilst  others  maintain  these 
acinetiform  bodies  to  be  parasites,  which  even  imbed  themselves  in 
the  substance  of  the  Infusoria  they  infest.1 

It  is  obvious  that  no  classification  of  Infusoria  can  be  of  any 
permanent  value  until  it  shall  have  been  ascertained  by  the  study 
of  their  entire  life-history  what  are  to  be  accounted  really  distinct 

1  There  can  be  no  doubt  that  Stein  was  wrong  in  his  original  doctrine  that  the 
fully  developed  Acinetina  are  only  transition  stages  in  the  development  of  Vorti- 
cellina  and  other  ciliated  Infusoria.  But  the  balance  of  evidence  seems  to  the  writer 
to  be  in  favour  of  his  later  statement,  that  the  bodies  figured  in  fig.  594,  i,  are 
really  infusorian  embryos,  and  not  parasitic  Acinetee. 


SUCTORIA  783 

forms.  And  the  differences  between  them,  consisting  chiefly  in  the 
shape  of  their  bodies,  the  disposition  of  their  cilia,  the  possession  of 
other  locomotive  appendages,  the  position  of  the  mouth,  the  presence 
of  a  distinct  anal  orifice,  and  the  like,  are  matters  of  such  trivial 
importance  as  compared  with  those  leading  features  of  their  structure 
and  physiology  on  which  we  have  been  dwelling  that  it  does  not 
seem  desirable  to  attempt  in  this  place  to  give  any  detailed  account 
of  them.  The  life-history  of  the  ciliate  Infusoria  is  a  subject 
pre-eminently  worthy  of  the  attention  of  microscopists,  who  can 
scarcely  be  better  employed  than' in  tracing  out  the  sequence  of  its 
phenomena  with  similar  care  and  assiduity  to  that  displayed  by 
Messrs.  Dalliiiger  and  Drysdale  in  the  study  of  the  3fonadina.  '  In 
pursuing  our  researches/  say  these  excellent  observers,  *  we  have 
become  practically  convinced  of  what  we  have  theoretically  assumed 
— the  absolute  necessity  for  prolonged  and  patient  observation  of 
the  same  forms.  Competent  optical  means,  careful  interpreta- 
tion, close  observation,  and  time  are  alone  capable  of  solving  the 
problem/ 

Suctoria. — The  suctorial  Infusoria  constitute  a  well-marked 
group,  all  belonging  to  one  family,  Acinetina,  the  nature  of  which 
has  been  until  recently  much  misunderstood,  chiefly  on  account  of 
the  parasitism  of  their  habit.  They  may  be  regarded  as  a  sub-class 
of  the  Infusoria,  and  be  known  as  the  Acinetaria,  Like  the 
typical  Jfonadina,  they  are  closed  cells,  each  having  its  nucleus  and 
contractile  vesicle  ;  but  instead  of  freely  swimming  through  the  water, 
they  attach  themselves  by  flexible  peduncles,  sometimes  to  the  stems  of 
Voj  ticellince,  but  also  to  filamentous  Algse,  stems  of  zoophytes,  or  to  the 
bodies  of  larger  a  nimals.  Their  nutriment  is  obtained  through  delicate 
tubular  extensions  of  the  ectosarc,  which  act  as  suctorial  tentacles 
(fig  598),  the  free  extremity  of  each  being  dilated  into  a  little 
knob,  which  flattens  out  into  a  button-like  disc  when  it  is  applied 
to  a  food -par  tide.  Free -swimming  Infusoria  are  captured  by  these 
organs,  of  which  several  quickly  bend  over  towards  the  one  which 
was  at  first  touched,  so  as  firmly  to  secure  the  prey ;  and  when 
several  have  thus  attached  themselves,  the  movements  of  the 
imprisoned  animal  become  feebler,  and  at  last  cease  altogether,  its 
lx)dy  being  drawn  nearer  to  that  of  its  captor.  Instead,  however, 
of  being  received  into  its  interior  like  the  prey  of  Actinophrys,  the 
captured  animalcule  remains  on  the  outside,  but  yields  up  its  soft 
substance  to  the  suctorial  power  of  its  victor.  As  soon  as  the  suck- 
ing disc  has  worked  its  way  through  the  envelope  of  the  body  to 
which  it  has  attached  itself,  a  very  rapid  stream,  indicated  by  the 
granules  it  carries,  sets  along  the  tube,  and  pours  itself  into  the 
interior  of  the  Aciiieta-body.  Solid  particles  are  not  received  through 
these  suctorial  tentacles,  so  that  the  Acinetina  cannot  be  fed  with 
indigo  or  carmine  ;  but,  so  far  as  can  be  ascertained  by  observation 
of  what  goes  011  within  their  bodies,  there  is  a  general  protoplasmic 
cyclosis  without  the  formation  of  any  special  '  digestive  vesicles.' 
The  better  known  forms  of  this  group  are  ranked  under  the  two  genera 
Acineta  and  Podophrya^  which  are  chiefly  distinguished  by  the 
presence  of  a  firm  envelope  or  lorica  in  the  former,  while  the  body 


784  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

of  the  latter  is  naked.  In  one  curious  form,  the  Opfoybdehdron,  the 
suckers  are  borne  in  a  brush-like  expansion  on  a  long  retractile 
proboscis- like  organ;  and  the  rare  Dendrosoma,  whose  size  is  com- 
paratively gigantic,  forms  by  continuous  gemmation  an  arborescent 
*  colony,'  of  which  the  individual  members  remain  in  intimate 
connection  with  one  another. 

Multiplication  in  this  group  seems  occasionally  to  take  place  by 
transverse  fission,  but  this  is  rare  in  the  adult  state.  Some- 
times external  gemmce  are  developed  by  a  sort  of  pinching  off  of  a 
part  of  the  free  end  of  the  body,  which  includes  a  portion  of  the 
nucleus ;  the  tentacula  of  this  bud  disappear,  but  its  surface  be- 


FIG.  598. — Suctorial  Infusoria:  1,   Conjugation  of 

quadripartita ;  2,  formation  of  embryos  by  enlargement  and  sub- 
division of  the  nucleus ;  3,  ordinary  form  of  the  same ;  4,  Podo- 
pliryci  elongdta. 

comes  clothed  with  cilia ;  and,  after  a  short  time,  it  detaches  itself 
and  swims  away — comporting  itself  subsequently  like  the  internal 
embryos,  whose  production  seems  the  more  ordinary  method  of 
propagation  in  this  type.  These  originate  in  the  breaking  up  of 
the  nucleus  into  several  segments,  each  of  which  incloses  itself  in 
a  protoplasmic  envelope ;  and  this  becomes  clothed  with  cilia,  by 
the  vibrations  of  which  the  embryos  are  put  in  motion  within  the 
body  of  the  parent  (fig.  598,  2),  from  which  they  afterwards  escape 
by  its  rupture.  In  this  condition  (a)  they  swim  about  freely,  and 
seem  identical  with  what  has  been  described  by  Ehrenberg  as  a 

1  Now  called,  after  Biitschli,  Tokophrya,on  account  of  its  mode  of  reproduction  ; 
see  his  Protozoa,  p.  1928. 


REPRODUCTION  OF  INFUSORIA 


785 


distinct  generic  form,  Meyatricha.  And,  according  to  the  observa- 
tions of  Mr.  Badcock,1  these  Megatricha-fonna  multiply  freely  by 
self-division.  After  a  short  time,  however,  they  settle  down  upon 
filamentous  Algae  or  other  supports,  lose  their  cilia,  put  forth  suctorial 
tentacles  (which  seem  to  shoot  out  suddenly  in  the  first  instance 
but  are  afterwards  slowly  retracted  and  protruded  with  a  kind  of 
.spiral  movement),  and  assume  a  variety  of  anioebiform  shapes  (fig. 
599,  1,  2,  3),  some  of  them  corresponding  to  that  of  the  genus 
Trichophrya.  In  this  stage  they,  become  quiescent  at  the  approach 
of  winter,  the  suctorial  tentacles  and  the  contractile  vesicles  dis- 
appearing; they  do  not,  however,  seem  to  acquire  any  special 
envelope,  remaining  as  clear,  motionless,  protoplasmic  particles. 
But  with  the  return  of  warmth  their  development  recommences,  a 


FIG.  599. — Immature  forms  of  Podoplirya  quadripartita  :  1,  Amoe- 
boid state  (Trichophrya  of  Claparede  and  Lachmann) ;  2,  the 
same  more  advanced  ;  3,  incipient  division  into  lobes. 


footstalk  is  formed,  and  they  gradually  assume  the  characteristic 
form  of  Podopkrya  qaadrlpartita.  A  regular  '  conjugation '  has  been 
observed  in  this  type,  the  body  of  one  individual  bending  down  so 
as  to  apply  its  free  surface  to  the  corresponding  part  of  another, 
with  which  it  becomes  fused  (fig.  598,  l)  ;  but  whether  this  always 
precedes  the  production  of  internal  embryos,  or  is  any  way  prepara- 
tory to  propagation,  has  not  yet  been  ascertained.2 

1  Journ.  of  Roy.  Microsc.  Soc.  vol.  iii.  1880,  p.  563. 

-  The  Acinetina  were  described  both  by  Ehrenberg  and  Dujardiii ;  but  the  first 
full  account  of  their  peculiar  organisation  was  given  by  Stein  in  his  Organismus  dcr 
I/ifitsionsthierchen.  Misled, however,  by  their  parasitic  habits,  Stein  originally  sub- 
posed  them  not  to  be  independent  types,  but  to  be  merely  transitional  stages  in  the 
development  of  Vorticellince  and  other  ciliate  Infusoria;  this  doctrine  he  long 
since  abandoned.  Much  information  as  to  this  group  will  also  be  found  in  the 
beautiful  Etudes  sitr  les  Infusoires  et  Ics  Bliizopodes  of  MM.  Claparede  and  Lach- 
mann, Geneva,  1858-61. 


786 


MICROSCOPIC   FORMS   OF   ANIMAL   LIFE 


SECTION  II. — ROTIFERA,  OR  WHEEL-ANIMALCULES. 

We  now  come  to  that  higher  group  of  animalcules  which,  in 
point  of  complexity  of  organisation,  is  as  far  removed  from  the  pre- 
ceding as  mosses  are  from  the  simplest  protophytes,  the  only  point 
of  real  resemblance  between  the  two  groups,  in  fact,  being  the 
minuteness  of  size  which  is  common  to  both.  A  few  species  of  the 
wheel-animalcules  are  marine,  or  the  inhabitants  of  brackish  pools 
near  the  seashore.  Dr.  E.  v.  Daday,  who  has  made  a  study  of  the 


/M\ 


PIG.  600. — Rotifer  vulgaris,  as  seen  at  B,  with  the  wheels  drawn  in,  and 
at  A  with  the  wheels  expanded  :  6,  eye-spots  ;  c,  wheels  ;  d,  antenna ; 
",  jaws  and  teeth  ;  /,  alimentary  canal ;  </,  cellular  mass  inclosing  it ; 


7/,  longitudinal  muscles ;   /,  /,  "tubes   of  water-vascular 
young  animal ;  I,  cloaca. 


system ;    Jc, 


Rotifera  of  the  Bay  of  Naples,  stated  that  in  1891,  50  species  were 
known  from  the  Baltic,  13  from  the  Mediterranean,  8  from 
elsewhere,  but  32  of  these  occur  also  in  fresh  water.  The  vast 
majority  known  to  us  belong,  therefore,  to  fresh  water,  *and  are  to  be 
found  in  ditches,  ponds,  reservoirs,  lakes,  and  slowly  running  streams 
— sometimes  attached  to  the  leaves  and  stems  of  water-plants,  some- 
times creeping  on  Algfe,  on  which  some  are  parasitic, l  sometimes 


1  Compare  particularly  the  interesting  observations  of  Prof.  W.  Rothert  in  vol,  ix. 
1896,  of  the  Zooloy.  Jahrbilcher  (Abth.  Systemat.),  pp.  (>72-71i!. 


KOT1FEBA  787 

swimming  freely  through  the  water.  They  are  met  with  also  in 
gutters  on  the  house-top,  in  water-butts,  on  wet  moss,  grass,  and 
liver- worts,  in  the  interior  of  Volvox  ylobator  and  Vancheria,  in  vege- 
table infusions,  on  the  backs  of  Entomostraca,  in  the  viscera  of  slugs, 
earth-worms,  and  Naiades,  and  in  the  body-cavities  of  Synaptce — 
in  fact,  in  almost  every  place  where  there  are  moisture  and  food. 
The  wheel-like  organs  from  which  the  class  derives  its  designation 
are  most  characteristically  seen  in  the  common  Rotifer  (fig.  600), 
where  they  consist  of  two  disc-like  lobes  or  projections  of  the  body 
whose  margins  are  fringed  with  long  cilia ;  and  it  is  the  uninterrupted 
succession  of  strokes  given  by  these  cilia,  each  row  of  which  nearly 
returns  (as  it  were)  into  itself,  that  gives  rise  by  an  optical  illusion 
to  the  notion  of  '  wheels.'  The  disposition  of  the  cilia  varies  much 
in  the  different  genera,  but  it  may  be  said  broadly  that  they  are  ar- 
ranged so  as  to  fulfil  three  different  purposes,  viz.  to  bring  food  to  the 
mouth,  to  conduct  it  through  the  alimentary  canal,  and  to  enable  the 
animal  to  swim. 

The  great  transparence  of  the  Rotifera  permits  their  general 
structure  to  be  easily  recognised.  They  have  usually  an  elongated 
form,  similar  on  the  two  sides  ;  but  this  rarely  exhibits  any  traces  of 
segments!  division.  The  body  is  covered  with  an  envelope  of  two 
layers.  The  inner  of  these  is  a  soft  lining  to  the  outer,  which  may 
be  soft  and  flexible,  or  membranous  and  of  very  varying  degrees  of 
stiffness,  or  even  of  an  inflexible  substance  capable  of  resisting  the 
action  of  caustic  potash.  In  this  latter  condition  it  is  called  a  lorica. 
The  greater  number  of  the  Rotifera  have  an  organ  of  attachment 
at  the  posterior  extremity  of  the  body,  which  is  usually  prolonged 
into  a  tail  or  false  foot,  by  which  they  can  affix  themselves  to  any 
solid  object ;  and  this  is  their  ordinary  position  when  keeping  their 
'  wheels '  in  action  for  a  supply  of  food  or  of  water ;  they  have  no 
difficulty,  however,  in  letting  go  their  hold  and  moving  through  the 
water  in  search  of  a  new  attachment,  and  may  therefore  be  con- 
sidered as  perfectly  free.  The  sessile  species,  in  their  adult  stage, 
on  the  other  hand,  remain  attached  by  the  posterior  extremity  to  the 
spot  on  which  they  have  at  first  fixed  themselves,  and  their  cilia  are 
consequently  employed  for  no  other  purpose  than  that  of  creating 
currents  in  the  surrounding  water.  In  considering  the  internal  struc- 
ture of  Rotifera  we  shall  take  as  its  type  the  arrangement  which 
it  presents  in  Brachionus  rubens  (fig.  601),  a  common  large  and 
handsome  animal,  and  one  that  bears  the  temporary  captivity  of 
a  compressorium  remarkably  well. 

Its  vase-shaped  lorica  is  hard  and  transparent ;  open  in  front  to 
allow  the  protrusion  of  the  head,  and  closed  behind,  except  where  a 
small  aperture  permits  the  passage  of  the  foot.  The  anterior 
dorsal  edge  bears  six  sharp  spines,  and  the  ventral  edge  has  a 
wavy  outline.  The  head  is  shaped  like  a  truncated  cone,  with  the 
larger  end  forward,  is  rounded  at  each  side,  and  carries  on  its  front 
surface  three  protuberances  (sp).  covered  with  stout  vibrating  hairs 
called  styles.  All  round  the  rim  of  the  head  runs  a  row  of  cilia  which 
on  the  ventral  surface  dips  down  into  either  side  of  a  ciliated  buccal 
funnel.  At  the  bottom  of  the  buccal  funnel  is  the  mastax  (mx),  a 

3E2 


;88 


MICKOSCOPIC    FORMS   OF   ANIMAL   LIFE 


muscular  bulb  containing  the  jaws  or  trophi  (ti).  These  latter  are 
hard,  glassy  bodies  consisting  of  two  hammer-like  pieces  called 
mallei  (fig,  602)  and  a  third  anvil-piece  called  an  incus.  Each 
malleus  (ms)  is  in  two  parts — the  manubrium  (mm),  or  handle, 
and  the  uncus  (us),  of  five  finger-like  processes,  which  unite  to 


FIG.  601. — Brackionus  rubens :  sp,  styligerous  prominences  cw,  coronal 
wreath ;  ts,  tactile  styles  |  «,  dorsal  antenna  ;  a',  a',  lateral  antennae ;  lai, 
longitudinal  muscles;  «?,  oesophagus;  oy,  ovary;  om,  ovum;  g,  germ; 
vt,  vibratile  tags  ;  it  intestine  ;  /,  foot ;  £,  toes  ;  gn,  brain  ;  e,  eye  ;  mx, 
mastax ;  ti,  trophi ;  gg,  gastric  glands ;  s,  stomach ;  lc,  longitudinal 
canals;  cv,  contractile  vesicle ;  cl,  cloaca;  fg,  foot-gland.  (After  Dr.  Hudson.) 

form  the  hammer's  head.  The  incus  (is),  or  anvil,  is  formed  of  two 
prism-shaped  bodies,  or  ra/mi  (rs),  pointed  at  their  free  ends,  and 
attached  at  their  broad  ends  to  a  thin  plate  called  the  fulcrum  (fm), 
which,  seen  ventrally  or  dorsally,  looks  like  a  rod.  These  various 
parts  are  connected  by  muscular  fibres,  and  so  acted  on  by  muscles 


ROTIFERA  789 

attached  to  themselves,  and  to  the  interior  of  the  mastax.  that  the 
imci  rise  and  fall  at  the  same  time  that  the  rami  open  and  shut. 
The  food  is  torn  by  the  unci,  crushed  by  the  rami,  and  then  passes 
between  the  latter  down  a  short  oesophagus  (ce)  into  the  stomach  (s). 
This  has  thick  cellular  walls,  and  is  lined  with  cilia,  especially  at  its 
lower  third,  which  is  often  divided  by  a  constriction  from  the  upper 
part,  and  is  often  so  different  in  its  shape  and  contents  as  to  merit 
the  name  of  an  intestine  (i).  The  lower  end  of  the  intestine  gene- 
rally expands  into  a  cloaca  (cl),  into  which  open  the  ducts  of  the 
ovary  (oy\  and  contractile  vesicle  (cv).  Just  above  the  mastax, 
and  sometimes  just  below  it,  on  the  oesophagus,  are  what  are  sup- 
posed to  be  salivary  glands ;  while  ttached  to  the  upper  end  of 
the  stomach  are  two  gastric  glands^ 
(gg),  often  possessing  visible  ducts. 
There  are  two  further  glands  (fg) 
in  the  foot,  which  is  itself  a  prolon- 
gation of  the  ventral  portion  of  the 
trunk  below  the  aperture  of  the  cloaca. 
These  foot-glands  secrete  a  viscid  sub- 
stance which  is  discharged  by  ducts 
passing  to  the  tips  of  the  two  toes  (t) 
and  which  serves  to  attach  the  animal 
to  one  spot  when  it  is  using  its  frontal 
cilia  to  procure  food.  FIG.  602.— Malleate  type  of  jaw. 

Longitudinal  muscles  (Im)  for  with-  <  Us,  uncus. 

drawing  the  head  and  foot  within  the  3 «  r>nm->  manubrium. 

lorica  can  be  readily  seen,  and  these  is,  incus  |  ^  r^^m 

parts   are   driven    out   again   by  the 
pressure  of  transverse  muscular  fibres  acting  on  the  fluids  of  the 

On  either  side  of  the  body  is  a  tortuous  tube  commencing  in  a 
plexus  in  the  head  and  running  down  to  open  on  the  contractile 
vesicle  (cv).  These  tubes  bear  little  tags  (vt),  each  of  which  appears 
to  contain  a  vibrating  cilium.  The  real  structure  of  these  bodies  is 
uncertain,  and  the  use  of  the  whole  apparatus  is  much  disputed  ; 
but  the  tags  are  possibly  very  minutely  ciliated  funnels,  their  free 
ends  open  to  the  body-cavity ;  and  it  seems  probable  that  the  fluids 
of  the  body-cavity  are  conducted  through  them,  along  the  tortuous 
tubes,  into  the  contractile  vesicle,  and  are  by  it  discharged  into  the 
cloaca.  The  apparatus  would  therefore  be  mainly  an  excretory  one.1 

There  is  a  bilobed  nervous  ganglion  (gii)  between  the  buccal 
funnel  and  the  dorsal  surface.  Above  it  is  the  eye  (e) — a  refracting 
sphere  on  a  mass  of  crimson  pigment.  From  the  ganglion  pass 
nerve-threads  to  a  dorsal  antenna  (a)  and  to  two  lateral  antennce  (af) 
on  either  side  of  the  dorsal  surface.  These  latter  organs  are  rocket- 
headed  terminations  of  the  nervous  threads,  and  have  each  a  bundle 
of  fine  hairs  passing  through  a  hole  in  the  lorica.  The  dorsal 

1  But  see  Dr.  Hudson's  Presidential  Address,  Journ.  of  the  Boy.  Microsc.  Soc. 
Feb.  1891,  p.  13,  in  which  reasons  are  given  for  suspecting  that  the  contractile  vesicle 
may  also  have  a  respiratory  function,  and  the  vibratile  tags  and  longitudinal  canals 
an  excretory  one. 


790 


MICROSCOPIC   FORMS    OF   ANIMAL  LIFE 


antenna  has  a  similar  bundle  and  lies  sheathed  in  a  tube  (fig.  GO")) 
which  has  its  base  just  above  the  nervous  ganglion,  and  passes 
thence  between  the  two  central  anterior  spines  of  the  lorica.  It  is 
furnished  with  a  muscle,  by  means  of  which  the  bunch  of  seta>  at 
the  free  extremity  can,  by  imagination,  be  drawn  within  the  tube. 

The  ovary  is  large  and  its  germs  are  conspicuous.  The  animal  is 
oviparous  and  the  huge  egg  is  easily  discharged  through  the  oviduct 
and  cloaca  owing  to  the  very  fluid  condition  of  its  contents.  It  is 
retained  by  a  thread  till  hatched  at  the  bottom  of  the  lorica.  There 
are  three  kinds  of  eggs  :  the  common  soft-shelled  eggs,  which  arc 
large,  oval,  and  produce  females;  similar  soft  eggs,  which  an- 
smaller,  more  spherical,  and  produce  males ;  and  ephippial  eggs  (fig. 
603),  with  thick  cellular  coverings,  often  ornamented  with  spines. 
These  latter  can  be  dried  completely  without  losing  their  vitality, 
and  so,  lying  buried  in  the  mud  of  dried-up  ponds,  preserve  the 
species  for  next  yaar. 


FIG.  603. 
Ephippial  egg. 


FIG.  604.— Male  :  e,  eye  ;  Zc,  longi- 
tudinal canals  ;  i'f,  vibratile  tag; 
cv,  contractile  vesicle ;  ss,  sperm- 
sac  ;  2>,  penis  ;  /,  foot ;  fg,  foot- 
gland. 


The  male  (fig.  604)  is  but  a  third  of  the  length  of  the  female, 
and  is  unlike  it  in  shape.  It  has  a  cylindrical  trunk,  small  foot,  and 
flat  round  head,  surrounded  by  a  simple  ring  of  long  cilia.  It  has 
no  lorica  nor  any  alimentary  tract  of  any  kind,  but  it  has  a 
nervous  system  similar  to  that  of  the  female,  a  red  eye,  and  anteniue. 
Its  excretory  and  muscular  systems  are  also  of  the  female  pattern. 
The  only  other  internal  organ  is  a  large  sperm-sac  (ss)  ending  at  its 
lower  extremity  in  a  protrusile,  ciliated,  hollow  penis  (p),  whose 
outlet  holds  the  position  of  the  anus  in  the  female ;  that  is,  on  the 
dorsal  surface,  at  the  base  of  the  foot. 

The  Rotifera  have  been  divided  by  Dr.  Hudson  anil  Mr.  P.  H.  Gossc ! 
into  four  orders,  according  to  their  powers  of  locomotion.     These  a  n  •  : 
1.  RHIZOTA  (the  rooted).     Fixed  when  adult. 

1  The  Rotifera,  or  Wheel-animalcules.  Longmans,  1889.  It  should  be  added 
that  Dr.  Plate,  in  1890  (Zeitschr.  f.  wiss.  Zool.  xlxi.),  has  suggested  a  division 
according  to  the  paired  or  unpaired  character  of  the  gonads. 


ORDERS   OF   ROTIFERA  791 

2.  BDELLOIDA   (the   leech-like).      That    swim   with   their   ciliary 
\vivath,  and  creep  like  a  leech. 

3.  PLOIMA  (the  sea-worthy).     That  only  swim  with  their  ciliary 
wreath. 

4.  SCIRTOPODA  (the  skippers).    That  swim  with  their  ciliary  wreath 
and  skip  with  arthropodous  limbs. 

The  order  Rhizota  contains  two  families,  chiefly  differing  from 
each  other  in  the  position  of  the  mouth,  which  in  the  FloaculariidcK 
(figs,  l  and  2,  Plate  XVII)  is  central,  lying  in  the  body's  longer  axis, 
but  in  the  Melicertidce  (fig.  3,  Plat^e  XVII)  is  lateral.  Almost  all  the 
species  of  both  families  live  in  gelatinous  tubes  secreted  by  themselves, 
and  often  fortified  in  various  ways  :  by  debris  gathered  from  the 
water  by  the  action  of  their  ciliary  wreaths  and  showered  down  at 
random  ;  by  pellets  formed  in  a  ciliated  cup  near  the  anterior  end 
of  the  body,  and  deposited  in  regular  order  on  the  gela- 
tinous tube ;  or  by  large  ftecal  pellets  also  regularly 
deposited. 

The  second  order,  JBdeUoida  (fig.  7,  Plate  XVII),  while 
having  many  points  in  common  with  the  Melicertidce,,  have 
a  foot  peculiarly  their  own.  It  has  several  false  joints  FIG-  6°5- 
that  can  be  drawn  one  within  the  other  like  those  of  a  jmJ/nna 
telescope.  The  corona  consists  of  two  nearly  circular  discs,  in  tube, 
••a ch  surrounded  with  a  double  row  of  cilia,  and  both  of 
these  can  be  withdrawn  into  an  infolding  of  the  ventral  surface  at  the 
anterior  end  of  the  body,  leaving  the  animal  with  a  long  pointed 
conical  head.  When  the  discs  are  so  furled  the  animal  fixes  the  toes  of 
its  foot,  elongates  the  foot  and  body,  catches  hold  with  the  furthest 
point  of  the  conical  head,  releases  the  foot,  and  then,  contracting  the 
body  and  foot  while  the  head  remains  fixed,  draws  forward  the  toes 
and  refixes  them,  and  so  da,  capo.  It  can  swim,  however,  in  the  usual 
fashion,  with  its  ciliary  wreath.  All  the  species  of  this  order  can, 
under  proper  circumstances,  be  dried  up  into  balls,  which  will  retain 
their  vitality  for  even  years,  though  in  a  state  of  utter  dustiness. 
This  is  due  to  their  secreting  round  their  bodies  (after  having  drawn  in 
both  head  and  foot)  a  gelatinous  covering  which  retains  the  body-fluids 
safe  from  evaporation.1  This  process  takes  some  time,  so  that  if 
MII  attempt  is  made  to  dry  them  on  an  ordinary  glass-slip  they  simply 
disintegrate.  In  a  house  gutter  or  in  wet  moss  or  sand,  where  the 
drying  up  of  the  water,  in  which  the  Rotifera  are,  is  slowly  accom- 
plished, the  animals  have  time  to  complete  their  gelatinous  coverings 
before  the  water  fails  them.  In  this  order  the  males  have  not  as  yet 
been  discovered. 

The  third  order,  Plo'ima,  is  divided  into  a  loricate  and  an  illoricate 
group,  which  are  not,  however,  very  sharply  separated ;  as  in  some 
cases  the  outer  layer  of  the  skirf  is,  though  horny,  yet  thin  and 
flexible.  Brachionus  rabetis  (fig.  601),  which  has  already  been  fully 
described,  is  a  good  type  of  the  Loricata  find  Copeus  cerberus  (fig.  6, 
Plate  XVII)  of  the  Illoricata.  Most  of  the  species  of  this  order  have 

1  See  Davis  in  Monthly  Microsc.  Journ.  vol.  ix.  1863,  p.  207;  Slack,  at  p.  241  of 
same  volume ;  and  the  report  of  a  discussion  on  the  subject  at  the  Royal  Microsco- 
pical Society,  Jo  urn.  of  Itoi/al  Microsc.  Soc.  1887,  p.  179. 


792  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

a  forked  jointed  foot,  the  fork  being  formed  of  two  toes  varying 
greatly  in  size  and  shape,  but  all  secreting  the  viscous  fluid  already 
mentioned.  The  great  majority  of  the  Rotifera  belong  to  the 
Plo'ima. 

The  fourth  order,  Scirtopoda,  contains  but  one  family,  Pedalionidce, 
and  has  only  two  genera,  Pedalion  and  Hexarthra,  and  the  latter  of 
these  has  but  one  known  species,  the  former  only  two.  Pedalion 
(figs.  4,  5,  8,  Plate  XVII)  is  an  extraordinary  creature.  Its  internal 
organs  are  on  the  usual  rotiferous  plan,  but  its  body  bears  110  fewer 
than  six  hollow  limbs,  ending  in  plumes  like  those  of  the  Arthropodti, 
and  worked  by  pairs  of  opposing  muscles  which  traverse  their  entire 
length.  These  limbs  are  arranged  round  the  body,  some-  on  the 
dorsal,  some  on  the  ventral  surface,  and  all  tunning  parallel  to  the 
body's  longer  axis.  In  Hexarthra,  on  the  contrary,  all  the  limbs 
are  on  the  ventral  surface,  and  are  arranged  radiatingly.  There  is 
no  foot  in  either  Rotifer;  but  in  Pedalion  there  are  two  ciliated 
club-like  processes  at  the  posterior  extremity,  rising  above  the 
dorsal  surface  and  secreting  a  similar  viscous  fluid  to  that  secreted 
in  the  toes  of  other  Rotifera. 

This  strange  creature  was  discovered  by  Dr.  C.  T.  Hudson  in  a  pond 
near  Clifton  in  1871  ;  Hexarthra  was  discovered  by  Dr.  Schmarda 
in  a  brackish  ditch  near  the  Nile  in  1853  ;  their  arthropodous  limbs 
give  them  a  strong  resemblance  to  a  Nauplius  larva,  and  make  it 
probable  that  the  nearest  relations  of  the  Rotifera  are  the  ARTHRO- 
PODA  ; L  at  any  rate,  there  is  more  probability  in  this  suggestion  than 
in  that  of  Professor  Hartog  that  they  are  allied  to  the  Pilidium- 
larva  of  Nemertine  worms.2 

1  The  following  treatises  and  memoirs  (in  addition  to  those  already  referred  to) 
contain  valuable  information  in  regard  to  the  life-history  of  animalcules  and  their 
principal   forms: — Ehrenberg,    Die  Infusionsthierchen,    Berlin,    1838  ;    Dujardin, 
Histoire  naturelle  des  Zoophytes  infusoires,  Paris,  1841;  Pritchard,  History  of 
Infusoria,    4th   ed.    London,   1861    (a   comprehensive   repertory   of    information)  ; 
Stein,  Der  Organismus  der  Infusionsthiere,  Leipzig :  Erste  Abtheilung,  1859 ;  Zweite 
Abtheilung,  1867  ;  Dritte  Abtheilung,  Hiilfte  I.  1878.     Saville  Kent's  Manual  of  the 
Infusoria,  1880-1 ;  and  Professor  Biitschli's  Protozoa  (1880-1)  in  the  new  edition  of 
Bronn's   Thierreichs.     For   the   Rhizopoda  and   Infusoria  specially  see  Claparede 
and  Lachmann,  Etudes    sur  les  Infusoires  et  les  *Rhizopodes,  Geneva,  1858-61 ; 
Cohn,  in   Siebold  ^lnd  Kolliker's   Zeitschrift,   1851-4  and   1857;  Lieberkiihn,  in 
Mutter's  Archiv,  1856,  and  Ann.  of  Nat.  Hist.  2nd  ser.  vol.  xviii.  1856;  Engelmann, 
Zur  Naturgeschichte  der  Infusionsthiere,  1862  ;  and  Professor  Biitschli's  Studien 
iiber  die  Conjugation  der  Infusorien  &c.,  1876.     For  the  Eotifera  specially  see 
Leydig,  in  Siebold  und  Kolliker's  Zeitschrift,  Bd.  vi.  1854 ;  Gosse   on  Melicerta 
ringens,  in  Quart.  Journ.  of  Microsc.  Sci.  vol.  i.  1858,  p.  1 ;  on  the  Manducatory 
Organs  of  Eotifera,  Phil.  Trans.  1856;  Huxley  on  Lacinularia  socialis  in  Trans. 
Microsc.  Soc.  ser.  ii.  vol.  i.  1853,  p.  1 ;  Cohn,  in  Siebold  und  Kolliker's  Zeitschrift, 
Bde.  vii.  ix.  1856,  1858 ;  Dr.  Moxon,  Trans.  Linn.  Soc.  1864 ;  Karl  Eckstein,  Siebold 
und  Kolliker's  Zeitschrift,  1883;  Bourne,  Rotifera,  in  the  9th  edition  of  thei///r//- 
clopcedia  Britannica  ;  Joliet, '  Monographic  des  Melicertes,'  Archiv.  zool.  exper.  ser. 
ii.  torn.  i.  p.  131 ;  and  Plate,  Jenaische  Zeitschr.  xix.  p.  1.     The  Rotifera,  or  Wheel- 
animalcules,  by  Hudson  and  Gosse,  Longmans,  1889.     This  has  been  usefully  sup- 
plemented by  Mr.  C.  F.  Eousselet  in  two  papers  entitled  '  List  of  New  Eotifers  since 
1889,'  in  Journ.  R.  Microsc.  Soc.  1893,  pp.  450-8,  and  '  Second  List,'  &c.  in  the 
same  journal  for  1897,  pp.  10-15.     The  bibliographical  lists  appended  by  Mr.  Rousse- 
let  will  be  found  of  much  service,  as  since  the  publication  of  the  work  of  Messrs. 
Hudson  and  Gosse  there  has  been  a  great  revival  among  the  students  of  this  group. 
Mr.  Slack's  Marvels  of  Pond  Life,  2nd  edit.  (London,  1871),  contains  many  interest- 
ing observations  on  the  habits  of  Infusoria  and  Rotifera. 

2  See  his  remarks  on  the  relation  of  the  Rotifera  to  the  Trochophore,  in  Rep.  Brit* 


Typical   Rotifers 


793 


APPENDIX   TO   CHAPTER   XIII 

THE  preparation  and  preservation  of  Rotifers  well  extended  as  in  life  to 
serve  as  type  specimens  is  now  possible,  and  the  following  is  an  outline  of 
Mr.  C.  F.  Rousselet's  method,  which  consists  of  three  stages:  narcotising, 
killing  and  fixing,  and  preserving.  The  whole  operation  is  necessarily 
performed  under  a  dissecting  microscope. 

The  first  step  in  the  preparation  of  Rotifers  is  to  isolate  the  animals 
by  transferring  as  many  as  may  be  available  by  means  of  a  very 
fine  pipette  to  a  fresh  watchglass  frill  of  perfectly  clean  water  until  all 
particles  of  foreign  matter  have  been  eliminated.  This  is  necessary 
because  when  the  animals  are  dead  these  particles  adhere  to  the  cilia  of 
the  Rotifers,  from  whence  it  is  very  difficult  to  remove  them.  In  the  case 
of  fixed  Rotifers,  such  as  Melicerta,  Limnias,  Stephanoceros,  &c.,  it  is 
necessary  to  cut  off  and  trim  a  very  small  piece  of  the  plant  to  which 
they  are  attached  ready  for  mounting,  so  as  not  to  have  to  do  this  when 
the  animals  are  killed  and  prepared.  It  is  also  necessary  to  separate  the 
different  species,  as  most  of  them  require  a  little  different,  more  or  less 
prolonged,  treatment  under  the  narcotic.  The  great  difficulty  with 
Rotifers  has  always  been  to  kill  and  fix  them  whilst  fully  extended  as  in 
life.  The  most  rapid  killing  agents  are  too  slow  to  prevent  complete  re- 
traction ;  recourse,  therefore,  has  been  had  to  narcotising,  and  after  many 
experiments  a  satisfactory  narcotic  has  been  found  in  the  following 
mixture : 

2  per  cent,  solution  of  hydrochlorate  of  cocaine  .         .         .         .3     parts 
Methylated  spirit          .         .         .         .         •         .         .         .         .1 
Water 6         „ 

The  Rotifers  then,  separated  as  to  species,  and  in  a  watchglass  full  of 
perfectly  clean  water,  are  ready  for  narcotising.  One  or  two  drops  of  the 
above  solution  are  added  to  the  water  and  mixed.  The  effect  of  the 
narcotic  is  most  varied  in  different  species.  Some  will  not  mind  it  at 
all  and  continue  to  swim  about,  others  will  contract  at  once  but  soon 
come  out  again  and  swim  about  at  a  diminishing  race  until  they  finally 
sink  to  the  bottom  with  the  cilia  beating  but  feebly.  Then  is  the  right 
time  for  killing  and  fixing.  In  the  case  of  more  vigorous  species,  after 
three  or  four  minutes  another  dose  of  two  or  three  drops  of  the  narcotic 
is  added,  and  then  repeated  again  if  necessary  until  it  is  seen  that  the 
animals  can  move  but  very  slowly.  At  this  moment  the  animals  are 
killed  quickly  and  suddenly  by  adding  one  drop  of  very  weak  (£  to  £  per 
cent.)  solution  of  osmic  acid. 

The  different  species  of  Rotifers  vary  so  much  in  their  behaviour  under 
the -narcotic  that  it  is  by  no  means  easy  to  always  hit  the  exact  moment 
for-  killing  the  animals  fully  extended;  repeated  failures  and  practice 
alone  can  guide  one  in  this  respect.  It  is  very  essential  that  the  animals 
be  still  living  when  the  osmic  acid  is  added,  as  when  a  Rotifer  is  quite 
dead  various  post-mortem  changes  begin  immediately  to  take  place  in 
the  tissues,  whilst  it  is  desired  to  fix  and  preserve  the  tissues  as  in  life. 
The  word  '  fixing '  implies  rapid  killing  and  at  the  same  time  hardening 
of  the  tissues  to  such  an  extent  as  to  prevent  their  undergoing  any 
further  change  by  subsequent  treatment  with  preserving  fluids.  The 
action  of  osrnic  acid  is  very  rapid,  half  a  minute  being  quite  enough ;  if 

Ass.  1896,  p.  836,  and  compare  with  them  the  suggestion  of  Dr.  Plate  in  Zeitschr.  f. 
wiss.  ZooL  xlix.  (1889),  pp.  1-41. 


794  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

left  much  [longer  in  this  fluid  the  animals  will  become  more  or  less 
blackened,  and  it  is  therefore  necessary  to  remove  the  Eotifers  as  soon  as 
possible,  by  means  of  the  fine  pipette,  in  three  or  four  changes  of  clean 
water,  so  as  to  get  rid  of  every  trace  of  the  acid.  Finally  the  animals 
are  transferred  into  the  preservative  fluid,  which  is  a  solution  of  2^  per  cent. 
formaldehyde  (the  commercial  formalin  is  a  40  per  cent,  solution  of 
formaldehyde) . 

In  this  preservative  the  Rotifers  are  mounted  in  ringed  or  excavated 
cells  on  micro-slides  in  the  usual  way.1 

1  More  detailed  particulars  in  the  treatment  of  the  various  species  and  in 
mounting  in  cells  will  be  found  in  Mr.  Rousselet's  papers  on  the  subject,  particularly 
those  of  March  1895  and  November  1898,  in  the  Joarn.  of  the  (Jiickett  Mici:  Club, 
vol.  vi.  pp.  5-18,  and  vol.  vii.  pp.  93-97. 


795 


CHAPTER   XIV 

FOBAMINIFEBA  AND  BADIOLABIA 

RETURNING-  now  to  the  lowest  or  rkizopod  type  of  animal  life 
(Chapter  XII),  we  have  to  direct  our  attention  to  two  very  remarkable 
series  of  forms,  almost  exclusively  marine,  under  which  that  type 
manifests  itself,  all  of  them  distinguished  by  skeletons  so  consolidated 
by  mineral  deposit  as  to  retain  their  form  and  intimate  structure 
long  after  the  animals  to  which  they  belonged  have  ceased  to  live, 
even  for  those  undefined  periods  in  which  they  have  been  imbedded 
as  fossils  in  strata  of  various  geological  ages.  In  the  first  of  these 
groups,  the  Foraminifera,  the  skeleton  usually  consists  of  a  calcareous 
many-chambered  shell,  which  closely  invests  the  sarcode-body,  and 
which,  in  a  large  proportion  of  the  group,  is  perforated  with  numerous 
minute  apertures ;  this  shell,  however,  is  sometimes  replaced  by  a 
1  test,'  formed  of  minute  grains  of  sand  cemented  together  ;  and 
there  are  a  few  cases  in  which  the  animal  has  no  other  protection 
than  a  membranous  envelope.  In  the  second  group,  the  Radiolaria, 
the  skeleton  is  always  silicious  and  may  either  be  composed  of  dis- 
connected spicules,  or  may  consist  of  a  symmetrical  open  framework, 
or  may  have  the  form  of  a  shell  perforated  by  numerous  apertures, 
which  more  or  less  completely  incloses  the  body.  The  Foraminifera 
probably  take,  and  always  have  taken,  the  largest  share  of  any  animal 
group  in  the  maintenance  of  the  solid  calcareous  portion  of  the  earth's 
crust  by  separating  from  its  solution  in  ocean-water  the  carbonate 
of  lime  continually  brought  down  by  rivers  from  the  land.  The 
Radiolaria  do  the  same,  though  in  far  less  measure,  for  the  silex. 
And  both  extract  from  sea- water  the  organic  matter  universally  dif- 
fused through  it,  converting  it  into  a  form  that  serves  for  the  nutri- 
tion of  higher  marine  animals. 

SECTION  I. — FORAMINIFERA. ] 

The  animals  of  this  group  belong  to  that  reticularian  form  of 
the  rhizopod  type  in  which — with  a  differentiation  between  the 
containing  and  the  contained  protoplasm  which  is  involved 
in  the  formation  of  a  definite  investment — a  distinct  nucleus 
(sometimes  single,  in  other  cases  multiple)  is  probably  always 

1  For  the  earlier  literature  qonsult  Mr.  C.  D.  Sherborn's  '  Bibliography  of  the 
)  recent  and  fossil,  from  1565  to  1888,'  London,  1888. 


796  MICEOSCOPIC   FORMS   OF  ANIMAL  LIFE 

present.1  The  shells  of  Foraminifera  are,  for  the  most  part,  poly- 
thalamous,  or  many-chambered  (Plates  XVIII  and  XIX),  often  so 
strongly  resembling  those  of  Nautilus,  Spirula,  and  other  cephalopod 
molluscs,  that  it  is  not  surprising  that  the  older  naturalists,  to  whom 
the  structure  of  these  animals  was  entirely  unknown,  ranked  them 
under  that  class.  But  independently  of  the  entire  difference  in  the 
character  of  the  animal  bodies  by  which  the  two  kinds  of  shells  are 
formed,  there  is  a  most  important  distinction  between  them  in  regard 
to  the  relation  of  the  animal  to  the  shell.  For  whilst  in  the 
chambered  shells  of  the  Nautilus  and  other  cephalopods  the  animal 
is  a  single  individual  tenanting  only  the  last  formed  chamber,  and 
withdrawing  itself  from  each  chamber  in  succession,  as  it  adds  to  this 
another  and  larger  one,  the  animal  of  a  nautiloid  foraminifer  has  a 
composite  body  consisting  of  a  number  (sometimes  very  large)  of 
'  segments,'  each  repeating  the  rest,  which  continues  to  increase  by 
gemmation  or  budding  from  the  last-formed  segment.  And  thus  each 
of  the  chambers,  however  numerous  they  may  be,  is  not  only  formed, 
but  continues  to  be  occupied  by  its  own  segment,  which  is  connected 
with  the  segments  of  earlier  and  later  formation  by  a  continuous 
'stolon'  (or. creeping  stem),  that  passes  through  apertures  in  the 
septa  or  partitions  dividing  the  chambers.  From  what  we  know  of 
the  semi-fluid  condition  of  the  Barcode-body  in  the  reticularian  type, 
there  can  be  little  doubt  that  there  is  an  incessant  circulatory  change 
in  the  actual  substance  of  each  segment  ;  so  that  the  material  taken 
in  as  food  by  the  segments  nearest  the  surface  or  margin  is  speedily 
diffused  through  the  entire  mass.  The  relation  between  these  '  poly- 
thalamous'  forms,  therefore,  and  the  monothalamous  or  single- 
chambered,  of  which  we  have  already  had  an  example  in  Groin  if. 
and  of  which  others  will  be  presently  described,  is  simply  that, 
whereas  any  buds  produced  by  the  latter  detach  themselves  to  form 
separate  individuals,  those  put  forth  by  the  former  remain  in  con- 
tinuity with  the  parent  stock  and  with  each  other,  so  as  to  form  :i 
'  composite '  animal  and  a  '  polythalamous '  shell. 

According  to  the  plan  on  which  the  gemmation  takes  place  will 
be  the  configuration  of  the  shelly  structure  produced  by  the  seg- 
mented body.  Thus,  if  the  bud  should  be  put  forth  from  the 
aperture  of  a  Lagena  (Plate  XIX,  fig.  12)  in  the  direction  of  the 
axis  of  its  body,  and  a  second  shell  should  be  formed  around  this 
bud  in  continuity  with  the  first,  and  this  process  should  be  succes- 
sionally  repeated,  a  straight  rod-like  shell  would  be  produced,  whose 
multiple  chambers  communicate  with  each  other  by  the  openings  that 
originally  constituted  their  mouths,  the  mouth  of  the  last-formed 
chamber  being  the  only  aperture  through  which  the  protoplasmic- 
body,  thus  composed  of  a  number  of  segments  connected  by  a 
peduncle  or  '  stolon  '  of  the  same  material,  could  now  project  itself 
or  draw  in  its  food.  The  successive  segments  may  be  all  of  the 
same  size,  or  nearly  so,  in  which  case  the  entire  rod  will  approach 
the  cylindrical  form,  or  will  resemble  a  line  of  beads ;  but  it  often 
happens  that  each  segment  is  somewhat  larger  than  the  preceding 

1  Dr.  Scliaudinn  (Zeitsclir.  f.  wiss.  Zool.  lix.  1895,  p.  191)  has  traced  the 
details  of  nuclear  division  in  Calcituba  polymorpha* 


AT.  Hollick  lith.         .  Edwin  Wilson, Cambridge 

A  TYPICAL  GROUP  OF  FORAMINIFERA  1  to  11 . 


FORAMINIFERA 


797 


(fig.  16),  so  that  the  composite  shell  has  a  conical  form,  the  apex  of 
the  cone  being  the  original  segment,  and  its  base  the  last  one  formed. 
The  method  of  growth  now  described  is  common  to  a  large  number 
of  Foraminifera,  chiefly  belonging  to  the  sub-family  Nodosarince  ; 
but  even  in  that  group  we  have  every  gradation  between  the  recti- 
lineal (fig.  16)  and  the  spiral  mode  of  growth  (fig.  22) ;  whilst  in 
the  genus  P&neroplis  it  is  not  at  all  uncommon  for  shells  which  com- 
mence in  a  spiral  to  exchange  this  in  a  more  advanced  stage  for  the 
rectilineal  habit.  When  the  successive  segments  are  added  in  a 
spiral  direction,  the  character  of  4fhe  spire  will  depend  in  great  degree 
upon  the  enlargement  or  non-enlargement  of  the  successively  formed 
chambers;  for  sometimes  it  opens  out  very  rapidly,  every  whorl 
being  considerably  broader  than  that  which  it  surrounds,  in  con- 
sequence of  the  great  excess  of  the  size  of  each  segment  over  that  of 
its  predecessor,  as  in  Peneroplis,  fig.  606  ;  but  more  commonly  there  is 
so  little  difference  between  the  successive  segments,  after  the  spire 
has  made  two  or  three  turns,  that  the  breadth  of  each  whorl  scarcely 
exceeds  that  of  its  predecessor,  as  is  well  seen  in  the  section  of  the 
Rotalia  represented  in  fig.  624.  An  intermediate  condition  is 


FIG.  606. — Foraminifera  : — Peneroplis  and  Orbiculina. 

presented  by  Rotalia,  which  may  be  taken  as  a  characteristic  type  of 
a  very  large  and  important  group  of  Foraminifera,  whose  general 
features  will  be  presently  described.  Again,  a  spiral  may  be 
either  '  nautiloid '  or  '  turbinoid,'  the  former  designation  being- 
applied  to  that  form  in  which  the  successive  convolutions  all 
lie  in  one  plane  (as  they  do  in  the  Nautilus),  so  that  the  shell 
is  *  equilateral '  or  similar  on  its  two  sides ;  whilst  the  latter  is 
used  to  mark  that  form  in  which  the  spire  passes  obliquely  round 
an  axis,  so  that  the  shell  becomes  '  inequilateral,'  having  a  more 
or  less  conical  form,  like  that  of  a  snail  or  a  periwinkle,  the  first- 
formed  chamber  being  at  the  apex.  Of  the  former  we  have  charac- 
teristic examples  in  Polystomdla  (Plate  XIX,  fig.  23)  and  Nonionina  ; 
whilst  of  the  latter  \ve  find  a  typical  representative  in  Rotalia 
Beccarii  (fig.  22).  Further,  we  find  among  the  shells  whose  increase 
takes  place  upon  the  spiral  plan  a  very  marked  difference  as  to  the 
degree  in  which  the  earlier  convolutions  are  invested  and  concealed 
by  the  latter.  In  the  great  rotaline  group,  whose  characteristic 
form  is  a  turbinoid  spiral,  all  the  convolutions  are  usually  visible, 
at  least  on  one  side  (fig.  1?) ;  but  among  the  nautiloid  tribes  it  more 
frequently  happens  that  the  last-formed  whorl  encloses  the  preceding 


798  MICROSCOPIC   FORMS   OF  ANIMAL   LIFE 

to  such  an  extent  that  they  are  scarcely,  or  not  at  all,  visible 
externally,  as  is  the  case  in  Cristellaria  (fig.  17),  Polystomella  (fig.  23), 
and  Nonwnina.  The  turbinoid  spire  may  coil  so  rapidly  round  an 
elongated  axis  that  the  number  of  chambers  in  each  turn  is  very 
small;  thus  in  Globigerina  (figs.  20,  •  21,  Plate  XIX)  there  are 
usually  only  four;  and  in  Valvulina  the  regular  number  is  only 
three.  Thus  we  are  led  to  the  ftiaeriaZ  arrangement  of  the  chambers, 
which  is  characteristic  of  the  textularian  group  (fig.  8,  a,  6,  and  9. 
Plate  XVIII),  in  which  we  find  the  chambers  arranged  in  two  rows, 
each  chamber  communicating  with  that  above  and  that  below  ii 
on  the  opposite  side,  without  any  direct  communication  with  the 
chamber  of  its  own  side,  as  will  be  understood  by  reference  to  fig 


FIG.  607. — Discorbina  globularis  (RosaU)ia  varians,  Schultze),   - 
with  its  pseudopodia  extended. 

622,  A,  which  shows  a  '  cast '  of  the  sarcode-body  of  the  animal.  On 
the  other  hand,  we  find  in  the  nautiloid  spire  a  tendency  to  pass 
(by  a  curious  transitional  form  to  be  presently  described)  into  the 
cyclical  mode  of  growth  ;  in  which  the  original  segment,  instead  of 
budding  forth  on  one  side  only,  developes  gemmae  all  round,  so  that 
a  ling  of  small  chambers  (or  chamberlets)  is  formed  around  the 
primordial  chamber,  and  this  in  its  turn  surrounds  itself  after  the 
like  fashion  with  another  ring ;  and  by  successive  repetitions  of  the 
same  process  the  shell  comes  to  have  the  form  of  a  disc  made  up  of 
a  great  number  of  concentric  rings,  as  we  see  in  Orbitolites  (fig.  609) 
and  in  Uydodypeus  (fig.  627). 

These  and  other  differences  in  the  plan  of  growth  were  made  by 


18 


\ 


A.T.Hollick   Hth. 


Edwin  Wilson  , Cambridge 


A  TYPICAL  GROUP  OF  FOHAMINIFERA 12  bo  26. 


EOEAMINIFERA  799 

* 

M.  d'Orbigny  the  foundation  of  his  classification  of  this  group, 
which,  though  at  one  time  generally  accepted,  has  now  been  aban- 
doned by  most  of  those  who  have  occupied  themselves  in  the  study 
of  the  Foraminifera.  For  it  has  come  to  be  generally  admitted  that 
'  plan  of  growth '  is  a  character  of  very  subordinate  importance 
among  the  Foraminifera.  so  that  any  classification  which  is  primarily 
based  upon  it  must  necessarily  be  altogether  unnatural,  those 
characters  being  of  primary  importance  which  have  an  immediate 
and  direct  relation  to  the  physiological  condition  of  the  animal, 
and  are  thus  indicative  of  the  rei*l  affinities  of  the  several  groups 
which  they  serve  to  distinguish.  The  most  important  of  these 
characters  will  now  be  noticed.1 

Two  very  distinct  types  of  shell  structure  prevail  among  ordinary 
Foraminifera — namely,  the  porcellanous  and  the  hyaline  or  vitreous. 
The  shell  of  the  former,  when  viewed  by  reflected  light,  presents  an 
opaque- white  aspect  which  bears  a  strong  resemblance  to  porcelain  ; 
but  when  thin  natural  or  artificial  laminae  of  it  are  viewed  by  trans- 
mitted light  the  opacity  gives  place  to  a  rich  brown  or  amber 
colour,  which  in  a  few  instances  is  tinged  with  crimson.  No 
structure  of  any  description  can  be  detected  in  this  kind  of  shell  sub- 
stance, which  is  apparently  homogeneous  throughout.  Although 
the  shells  of  this  'porcellanous'  type  often  present  the  appearance 
of  being  perforated  with  foramina,  yet  this  appearance  is  illusnrv. 
being  due  to  a  mere  'pitting'  of  the  external  surface,  which,  though 
oft  (jn  very  deep,  never  extends  through  the  whole  thickness  of  the 
shell.  Some  kind  of  inequality  of  that  surface,  indeed,  is  extremely 
common  in  the  shells  of  the  '  porcellanous'  Foraminifera,  one  of  the 
most  frequent  forms  of  it  being  a  regular  alternation  of  ridges  and 
furrows,  such  as  is  occasionally  seen  in  Miliola,  but  which  is  an 
almost  constant  characteristic  of  Peneroplis  (fig.  606).  But  no 
difference  of  texture  accompanies  either  this  or  any  other  kind  of 
inequality  of  surface,  the  raised  and  depressed  portions  being  alike 
homogeneous.  In  the  shells  of  the  vitreous  or  hyaline  type,  on  the 
other  hand,  the  proper  shell  substance  has  an  almost  glassy  trans- 
parence.  which  is  shown  by  it  alike  in  thin  natural  lamellae  and  in 
artificially  prepared  specimens  of  such  as  are  thicker  and  older.  It 
is  usually  colourless,  even  when  (as  in  the  case  with  many  RotaUimi') 
the  substance  of  the  animal  is  deeply  coloured ;  but  in  some  few- 
species,  such  as  Globig&rina  rubra,  Truncatulina  rosea,  and  Polytreina 
Hthnaceum,  the  shell  is  commonly,  like  the  animal  body,  of  a  more 
or  less  deep  crimson  hue.  All  the  shells  of  the  hyaline  type  are 
beset  more  or  less  closely  \\ti\\tulidarperforatioTts.  which  pass  directly, 
and  (in  general)  without  any  subdivision,  from  one  surface  to  the 
other.  These  tubuli  are  in  some  instances  sufficiently  coarse  for 
their  orifices  to  be  distinguished  with  a  low  magnifying  power  as 
•  punctations '  on  the  surface  of  the  shell,  as  is  shown  in  fig.  607  ; 
whilst  in  other  cases  they  are  so  minute  as  only  to  be  discernible  in 
thin  sections  seen  by  transmitted  light  under  a  higher  magnifying 

1  This  subject  will  be  found  amply  discussed  iu  the  Author's  Introduction  to  the 
Sfudi/  of  tli  e  Fora /» in  if  era,  published  by  the  Ray  Society,  to  which  work  he  would 
refer  such  of  his  readers  as  may  desire  more  detailed  information  in  regard  to  it. 


80O  MICROSCOPIC  FORMS   OF  ANIMAL  LIFE 

power,  as  is  shown  in  figs.  632,  633.  When  they  are  very  numerous 
and  closely  set,  the  shell  derives  from  their  presence  that  kind  of 
opacity  which  is  characteristic  of  all  minutely  tubular  textures 
whose  tubuli  are  occupied  either  by  air  or  by  any  substance  having 
a  refractive  power  different  from  that  of  the  intertubular  substance, 
however  perfect  may  be  the  transparence  of  the  latter.  The  straight- 
ness,  parallelism,  and  isolation  of  these  tubuli  are  well  seen  in  verti- 
cal sections  of  the  thick  shells  of  the  largest  examples  of  the  group, 
such  as  Nummulites  (fig.  631).  It  often  happens,  however,  that 
certain  parts  of  the  shell  are  left  imchannelled  by  these  tubuli ;  and 
such  are  readily  distinguished,  even  under  a  low  magnifying  power, 
by  the  readiness  with  which  they  allow  transmitted  light  to  pass 
through  them,  and  by  the  peculiar  vitreous  lustre  they  exhibit  when 
light  is  thrown  obliquely  on  their  surface.  In  shells  formed  upon 
this  type  we  frequently  find  that  the  surface  presents  either  bands 
or  spots  which  are  so  distinguished,  the  non-tubular  bands  usually 
marking  the  position  of  the  septa,  and  being  sometimes  raised  into 
ridges,  though  in  other  instances  they  are  either  level  or  somewhat 
depressed  ;  whilst  the  non-tubular  spots  may  occur  on  any  part  of 
the  surface,  and  are  most  commonly  raised  into  tubercles,  which 
sometimes  attain  a  size  and  number  that  give  a  very  distinctive 
aspect  to  the  shells  that  bear  them. 

Between  the  comparatively  coarse  perforations  which  are  common 
in  the  rotaline  type,  and  the  minute  tubuli  which  are  characteristic 
of  the  nummuline,  there  is  such  a  continuous  gradation  as  indicates 
that  their  mode  of  formation,  and  probably  their  uses,  are  essen- 
tially the  same.  In  the  former,  it  has  been  demonstrated  by  actual 
observation  that  they  allow  the  passage  of  pseudopodial  extensions 
of  the  sarcode-body  through  every  part  of  the  external  wall  of  the 
chambers  occupied  by  it  (fig.  607) ;  and  there  is  nothing  to  oppose 
the  idea  that  they  answer  the  same  purpose  in  the  latter,  since, 
minute  as  they  are,  their  diameter  is  not  too  small  to  enable  them 
to  be  traversed  by  the  finest  of  the  threads  into  which  the  branching 
pseudopodia  of  Foraminifera  are  known  to  subdivide  themselves. 
Moreover  the  close  approximation  of  the  tubuli  in  the  most  finely 
perforated  nummulines  makes  their  collective  area  fully  equal  to 
that  of  the  larger  but  more  scattered  pores  of  the  most  coarsely  per- 
forated rotalines.  Hence  it  is  obvious  that  the  tubulation  or  non- 
tubulation  of  foraminiferal  shells  is  the  key  to  a  very  important 
physiological  difference  between  the  animal  inhabitants  of  the  two 
kinds  respectively  ;  for  whilst  every  segment  of  the  sarcode-body  in 
the  former  case  gives  off  pseudopodia,  which  pass  at  once  into  the 
surrounding  medium,  and  contribute  by  their  action  to  the  nutrition 
of  the  segment  from  which  they  proceed,  these  pseudopodia  are 
limited  in  the  latter  case  to  the  final  segment,  issuing  forth  only 
through  the  aperture  of  the  last  chamber,  so  that  all  the  nutrient 
material  which  they  draw  in  must  be  first  received  into  the  last  seg- 
ment, and  be  transmitted  thence  from  one  segment  to  another  until 
it  reaches  the  earliest.  With  this  difference  in  the  physiological  con- 
dition of  the  animal  of  these  two  types  is  usually  associated  a  further 
very  important  difference  in  the  conformation  of  the  shell — viz. 


FOKAMINIFERA  8oi 

that  whilst  the  aperture  of  communication  between  the  chambers 
and  between  the  last  chamber  and  the  exterior  is  usually  very  small 
in  the  '  vitreous '  shells,  serving  merely  to  give  passage  to  a  slender 
stolon  or  thread  of  sarcode  from  which  the  succeeding  segment  may 
be  budded  off,  it  is  much  wider  in  the  '  porcellanous'  shells,  so  as  to 
give  passage  to  a  '  stolon  '  that  may  not  only  bud  off  new  segments, 
but  may  serve  as  the  medium  for  transmitting  nutrient  material 
from  the  outer  to  the  inner  chambers. 

Between  the  highest  types  of  the  porcellanous  and  the  vitreous 
series  respectively,  which  frequently  bear  a  close  resemblance  to 
each  other  in  /orm,  there  are  certain  other  well-marked  differences 
in  structure,  which  clearly  indicate  their  essential  dissimilarity. 
Thus,  for  example,  if  we  compare  Orbitolites  (fig.  609)  with  Cyclo- 
clypeus  we  recognise  the  same  plan  of  growth  in  each,  the  chamber- 
lets  being  arranged  in  concentric  rings  around  the  primordial 
chamber ;  and  to  a  superficial  observer  there  would  appear  little 
difference  between  them.  But  a  minuter  examination  shows  that 
not  only  is  the  texture  of  the  shell  '  porcellanous '  and  non-tubular 
in  Orbitolites,  whilst  it  is  '  vitreous  '  and  minutely  tubular  in  Cyclo- 
dypeus,  but  that  the  partitions  between  the  chamberlets  are  single 
in  the  former,  whilst  they  are  double  in  the  latter,  each  segment  of 
the  sarcode-body  having  its  own  proper  shelly  investment.  More- 
over, betwen  these  double  partitions  an  additional  deposit  of  cal- 
careous substance  is  very  commonly  found,  constituting  what  may 
be  termed  the  intermediate  skeleton ;  and  this  is  traversed  by  a 
peculiar  system  of  inosculating  canals,  which  pass  around  the 
chamberlets  in  interspaces  left  between  the  two  laminae  of  their  par- 
titions, and  which  seem  to  convey  through  its  substance  extensions 
of  the  sarcode-body  whose  segments  occupy  the  chamberlets.  We 
occasionally  find  this  *  intermediate  skeleton '  extending  itself  into 
peculiar  outgrowths,  which  have  no  direct  relation  to  the  chambered 
shell.  Of  this  we  have  a  very  curious  example  in  Calcarina  ;  and  it 
is  in  these  that  we  find  the  'canal  system'  attaining  its  greatest 
development.  Its  most  regular  distribution,  however,  is  seen  in 
Polystomella  and  in  Operculina  ;  and  an  account  of  it  will  be  given 
in  the  description  of  those  types. 

Porcellanea. — Commencing,  now,  with  the  porcellanous  series, 
we  shall  briefly  notice  some  of  its  most  important  forms,  which  are 
so  related  to  each  other  as  to  constitute  but  the  one  family  Miliolida. 
Its  simplest  type  is  presented  by  the  Cornuspira  of  our  own  coasts, 
found  attached  to  seaweeds  and  zoophytes ;  this  is  a  minute  spiral 
shell,  of  which  the  interior  forms  a  continuous  tube  not  divided  into 
chambers  ;  the  latter  portion  of  the  spire  is  often  very  much  flattened 
out,  as  in  Peneroplis,  so  that  the  form  of  the  mouth  is  changed  from 
a  circle  to  a  long  narrow  slit.  Among  the  commonest  of  the  Fora- 
minifera,  and  abounding  near  the  shores  of  almost  every  sea,  are 
some  forms  of  the  milioline  type,  so  named  from  the  resemblance  of 
some  of  their  minute  fossilised  forms  (of  which  enormous  beds  of 
limestone  in  the  neighbourhood  of  Paris  are  almost  entirely  com- 
posed) to  millet-seeds.  The  peculiar  mode  of  growth  by  which  these 
are  characterised  will  be  best  understood  by  examining,  in  the  first 

SF 


802  MICROSCOPIC   FORMS   OF   ANIMAL  LIFE 

instance,  the  form  which  has  been  designated  as  Spiroloculina.  This 
shell  is  a  spiral,  elongated  in  the  direction  of  one  of  its  diameters, 
and  having  at  each  turn  a  contraction  at  either  end  of  that  diameter 
which  partially  divides  each  convolution  into  two  chambers  ;  the 
separation  between  the  consecutive  chambers  is  often  made  more 
complete  by  a  peculiar  projection  from  the  inner  side  of  the  cavity, 
known  as  the  '  tongue  '  or  '  valve,'  which  may  be  considered  as  an 
imperfect  septum.  Now  it  is  a  very  common  habit  in  the  milioline 
type  for  the  chambers  of  the  later  convolutions  to  extend  themselves 
over  those  of  the  earlier,  so  as  to  conceal  them  more  or  less  com- 
pletely ;  and  this  they  very  commonly  do  somewhat  unequally,  so  that 
more  of  the  earlier  chambers  are  visible  on  one  side  than  on  the 
other.  Miliolce  thus  modified  (fig.  l ,  PI.  XVIII)  have  received  the 
names  of  Quinqueloculina  and  Triloculina  according  to  the  number  of 
chambers  visible  externally ;  but  the  extreme  inconstancy  which  is 
found  to  mark  such  distinctions,  when  the  comparison  of  specimens 
has  been  sufficiently  extended,  entirely  destroys  their  value  as  differ- 
ential characters,  and  the  term  Miliolina  is  now  more  frequently 
applied  to  them  collectively.  Sometimes,  on  the  other  hand,  the 
earlier  convolutions  are  so  completely  concealed  by  the  later  that  only 
the  two  chambers  of  the  last  turn  are  visible  externally  ;  and  in  this 
type,  which  has  been  designated  Biloculina,  there  is  often  such  an 
increase  in  the  breadth  of  the  chambers  as  altogether  changes  the 
usual  proportions  of  the  shell,  which  has  almost  the  shape  of  an  egg 
when  so  placed  that  either  the  last  or  the  penultimate  chamber  faces 
the  observer.  It  is  very  common  in  milioline  shells  for  the  external 
surface  to  present  a  '  pitting,'  more  or  less  deep,  a  ridge-and-furrow 
arrangement  (fig.  3),  or  a  honeycomb  division ;  and  these  diversities 
have  been  used  for  the  characterisation  of  species.  Not  only,  how 
ever,  may  every  intermediate  gradation  be  met  with  between  the 
most  strongly  marked  forms,  but  it  is  not  at  all  uncommon  to  find 
the  surface  smooth  on  some  parts,  whilst  other  parts  of  the  surface 
in  the  same  shell  are  deeply  pitted  or  strongly  ribbed  or  honey- 
combed ;  so  that  here,  again,  the  inconstancy  of  these  differences 
deprives  them  of  much  of  their  value  as  distinctive  characters. 

An  interesting  illustration  of  the  tendency  to  dimorphism 
amongst  the  Foraminifera  has  been  observed  by  MM.  Munier 
Chalmas  and  Schlumberger l  in  the  structure  of  the  shells  of  this 
group.  They  find  that  while  two  forms,  which  they  distinguish  as 
form  A  and  form  B,  are  similar  externally  they  differ  in  internal 
structure,  form  B  having  its  initial  chamber  much  smaller  than  that 
of  form  A,  and  this  '  microsphere  '  is  followed  by  a  larger  number  of 
chambers  than  is  the  '  megasphere  '  of  form  A.  What  this  difference 
signifies  it  is  at  present  impossible  to  say,  but  it  has  been  suggested 
that  it  may  be  one  of  sexual  character,  or,  better,  of  a  series  in  a  cycle 
of  generations.  The  observations  of  the  French  naturalists  referred 
to  open  out  a  new  field  of  inquiry,  and  one  which  is  enjoying  the 
attention  of  several  workers  in  this  department  of  research.2 

1  Bulletin  Soc.  Geol.  ser.  iii.  vol.  xiii.  p.  273. 

-J  Gf.  J.  J.  Lister  in  Phil  Trans.  136  B  (1895),  p.  401,  and  P.  Schaudinn,  '  Ueber 
den  Dimorphismus  der  Foraminiferen,'  S.B.  Ges.  Natitrf.  Berlin,  1895,  p.  87. 


PENEROPLIS;   ORBICULINA  803 

Reverting  again  to  the  primitive  type  presented  in  the  simple 
spiral  of  Cornuspira,  we  find  the  most  complete  development  of 
it  in  Peneroplis  (fig.  606),  a  very  beautiful  form,  which,  although 
not  to  be  found  on  our  own  coasts,  is  one  of  the  commonest  of  all 
Foraminifera  in  the  shore-sands  and  shallow-water  dredgings  of 
warmer  regions.  This  is  normally  a  nautiloid  shell,  of  which  the 
spire  flattens  itself  out  as  it  advances  in  growth.  It  is  marked 
externally  by  a  series  of  transverse  bands,  which  indicate  the  posi- 
tion of  the  internal  septa  that  divide  the  cavity  into  chambers  ;  and 
these  chambers  communicate  with  each  other  by  numerous  minute 
pores  traversing  each  of  the  septa,  and  giving  passage  to  threads 
of  sarcode  that  connect  the  segments  of  the  body.  At  a  is  shown 
the  *  septal  plane '  closing  in  the  last-formed  chamber,  with  its  single 
row  of  pores  through  which  the  pseudopodial  filaments  extend  them- 
selves into  the  surrounding  medium.  The  surface  of  the  shell, 
which  has  a  peculiarly  '  porcellanous '  aspect,  is  marked  by  closely 
set  atrice  that  cross  the  spaces  between  the  successive  septal  bands ; 
these  markings,  however,  do  not  indicate  internal  divisions,  and  are 
due  to  a  surface-furrowing  of  the  shelly  walls  of  the  chambers.  This 
type  passes  into  two  very  curious  modifications,  one  having  a  spire 
which,  instead  of  flattening  itself  out,  remains  turgid,  like  that  of  a 
Nautilus,  having  only  a  single  aperture,  which  sends  out  fissured 
extensions  that  subdivide  like  the  branches  of  a  tree,  suggesting  the 
name  of  Dendritina,  the  other  having  its  spire  continued  in  a  rec- 
tilineal direction,  so  that  the  shell  takes  the  form  of  a  crosier,  this 
being  distinguished  by  the  name  of  Spirolina.  A  careful  examinar 
tion  of  intermediate  forms,  however,  has  made  it  evident  that  these 
modifications,  though  ranked  as  of  generic  value  by  M.  d'Orbigny, 
are  merely  varietal,  a  continuous  gradation  being  found  to  exist 
from  the  elongated  septal  plane  of  the  typical  Peneroplis,  with  its 
single  row  of  isolated  pores,  to  the  arrow-shaped  septal  plane  of 
Dendritina,  with  all  its  pores  fused  together  (so  to  speak)  into  one 
dendritic  aperture,  and  a  like  gradation  being  presented  between 
the  ordinary  forms  and  the  '  spiroline  '  varieties,  with  oval  or  even 
circular  septal  plane,  into  which  both  Peneroplis  and  Dendritina 
tend  to  elongate  themselves. 

From  the  ordinary  nautiloid  multilocular  spiral  we  now  pass  to 
a  more  complex  and  highly  developed  form,  which  is  restricted  to 
tropical  and  subtropical  regions,  but  is  there  very  abundant — that, 
namely,  which  has  received  the  designation  Orbiculina  (fig.  606). 
The  relation  of  this  to  the  preceding  type  will  be  best  understood 
by  an  examination  of  its  earlier  stage  of  growth  ;  for  here  we 
see  that  the  shell  resembles  that  of  Peneroplis  in  its  general  form, 
but  that  its  principal  chambers  are  divided  by  '  secondary  septa ' 
passing  at  right  angles  to  the  primary  into  '  chamberlets  '  occupied 
by  sub-segments  of  the  sarcode-body.  Each  of  these  secondary 
septa  is  perforated  by  an  aperture,  so  that  a  continuous  gallery  is 
formed,  through  which  (as  in  fig.  609)  there  passes  a  stolon  that 
unites  together  all  the  sub-segments  of  each  row.  The  chamberlets 
of  successive  rows  alternate  with  one  another  in  position  ;  and  the 
pores  of  the  principal  septa  are  so  disposed  that  each  chamber-let  of 

3  r2 


804  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

any  row  normally  communicates  with  two  chamberlets  in  each  of  the 
adjacent  rows.  The  later  turns  of  the  spire  very  commonly  grow  com- 
pletely over  the  earlier,  and  thus  the  central  portion  or  *  umbilicus  ' 
comes  to  be  protuberant,  whilst  the  growing  edge  is  thin.  The  spire 
also  opens  out  at  its  growing  margin,  which  tends  to  encircle  the  first- 
formed  portion,  and  thus  gives  rise  to  the  peculiar  shape  represented 
in  fig.  606,  in  the  illustration  on  the  extreme  right,  which  is  the 
common  aduncal  type  of  this  organism.  But  sometimes  even  at  an 
early  age  the  growing  margin  extends  so  far  round  on  each  side 
that  its  two  extremities  meet  on  the  opposite  side  of  the  original 
spire,  which  is  thus  completely  inclosed  by  it ;  and  its  subsequent 
growth  is  no  longer  spiral  but  cyclical,  a  succession  of  concentric 
rings  being  added,  one  around  the  other,  as  shown  in  the  middle 
illustration  in  the  same  figure.  This  change  is  extremely  curious,  as 
demonstrating  the  intimate  relationship  between  the  spiral  and  the 
cyclical  plans  of  growth,  which  at  first  sight  appear  essentially  distinct. 
In  all  but  the  youngest  examples  of  Orbiculina  the  septal  plane  pre- 
sents more  than  a  single  row  of  pores,  the  number  of  rows  increasing 
in  the  thickest  specimens  to  six  or  eight.  This  increase  is  associated 
with  a  change  in  the  form  of  the  sub-segments  of  sarcode  from  little 
blocks  to  columns,  .and  with  a  greater  complexity  in  the  gencrnl 
arrangement,  such  as  will  be  more  fully  described  hereafter  in 
Orbitolites.  The  largest  existing  examples  of  this  type  are  far  sur- 
passed in  size  by  those  which  make  up  a  considerable  part  of  a 
Tertiary  limestone  on  the  Malabar  coast  of  India,  whose  diameter 
reaches  seven  or  eight  lines. 

A  very  curious  modification  of  the  same  general  plan  is  shown  in 
Alveolina,  a  genus  of  which  the  largest  existing  forms  (fig.  608)  are 
commonly  about  one-third  of  an  inch  long,  while  far  larger  speci- 
mens are  found  in  the  Tertiary  limestones  of  Scinde.  Here  the 
spire  turns  round  a  very  elongate  axis,  so  that  the  shell  has  almost 
the  form  of  a  cylinder  drawn  to  a  point  at  each  extremity.  Its 
surface  shows  a  series  of  longitudinal  lines  which  mark  the  principal 
septa  ;  and  the  bands  that  intervene  between  these  are  marked  trans- 
versely by  lines  which  show  the  subdivision  of  the  principal  chambers 
into  *  chamber-lets.'  The  chamberlets  of  each  row  are  connected  with 
each  other,  as  in  the  preceding  type,  by  a  continuous  gallery ;  and 
they  communicate  with  those  of  the  next  row  by  a  series  of  multiple 
pores  in  the  principal  septa,  such  as  constitute  the  external  orifices  of 
the  last-formed  series  seen  on  its  septal  plane  at  a,  a. 

The  highest  development  of  the  cyclical  plan  of  growth  which 
we  have  seen  to  be  sometimes  taken  on  by  Orbiculina  is  found  in 
Orbitolites ;  a  type  which,  long  known  as  a  very  abundant  fossil  in 
the  earlier  Tertiaries  of  the  Paris  basin,  has  lately  proved  to  be 
scarcely  less  abundant  in  certain  parts  of  the  existing  ocean.  The 
largest  recent  specimens  of  it,  sometimes  attaining  the  size  of  a 
shilling,  have  hitherto  been  obtained  only  from  the  coast  of  New 
Holland,  the  Fijian  reefs,  and  various  other  parts  of  the  Polynesian 
Archipelago  ;  but  discs  of  comparatively  minute  size  and  simpler 
organisation  are  to  be  found  in  almost  all  foraminiferal  sands  and 
dredgings  from  the  shores  of  the  warmer  regions  of  the  globe,  being 


ALVEOLINA 


80S 


especially  [abundant  in  those  of  some  of  the  Philippine  Islands,  of  the 
Red  Sea,  of  the  Mediterranean,  and  especially  of  the  JEgean.  When 
such  discs  are  subjected  to  microscopic  examination,  they  are  found 
(if  uninjured  by  abrasion)  to  present  the  structure  represented  in 
fig.  609,  where  we  see  on  the  surface  (by  incident  light)  a  number 


s, 


a 

i 
| 

J 

s" 

i 


of  rounded  elevations,  arranged  in  concentric  zones  around  a  sort  of 
nucleus  (which  has  been  laid  open  in  the  figure  to  show  its  internal 
structure) ;  whilst  at  the  margin  we  observe  a  row  of  rounded  pro- 
jections with  a  single  aperture  or  pore  in  each  of  the  intervening 
depressions.  In  very  thin  discs  the  structure  may  often  be  brought 
into  view  by  mounting  them  in  Canada  balsam  and  transmitting 


8o6  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

light  through  them  ;  but  in  those  which  are  too  opaque  to  be  thus 
seen  through,  it  is  sufficient  to  rub  down  one  of  the  surfaces  upon  a 
stone,  and  then  to  mount  the  specimen  in  balsam.  Each  of  the 
superficial  elevations  will  then  be  found  to  be  the  roof  or  cover  of  an 
ovate  cavity  or  *  chamberlet,'  which  communicates  by  means  of  a 
lateral  passage  with  the  chamberlet  on  either  side  of  it  in  the  same 
ring ;  so  that  each  circular  zone  of  chamberlets  might  be  described 
as  a  continuous  annular  passage  dilated  into  cavities  at  intervals. 
On  the  other  hand,  each  zone  communicates  with  the  zones  that  are 
internal  and  external  to  it  by  means  of  passages  in  a  radiating 
direction;  these  passages  run,  however,  not  from  the  chamberlets  of 
the  inner  zone  to  those  of  the  outer,  but  from  the-  connecting  pas- 
sages of  the  former  to  the  chamberlets  of  the  latter  ;  so  that  the 
chamberlets  of  each  zone  alternate  in  position  with  those  of  the  zones 


FIG.  609. — Orbitolites.     Ideal  representation  of  a  disc  of  complex  type. 

internal  and  external  to  it.  The  radial  passages  from  the  outermost 
annulus  make  their  way  at  once  to  the  margin,  where  they  termi- 
nate, forming  the  '  pores  '  which  (as  already  mentioned)  are  to  be  seen 
on  its  exterior.  The  central  nucleus,  when  rendered  sufficiently 
transparent  by  the  means  just  adverted  to,  is  found  to  consist  of  a 
'  primordial  chamber '  (a),  usually  somewhat  pear-shaped,  that  com- 
municates by  a  narrow  passage  with  a  much  larger  '  circumambient 
chamber '  (b),  which  nearly  surrounds  it,  and  which  sends  off  a  vari- 
able number  of  radiating  passages  towards  the  chamberlets  of  the  first 
zone,  which  forms  a  complete  ring  round  the  circumambient  chamber.1 

1  Although  the  above  may  be  considered  the  typical  form  of  the  Orbitolite,  yet, 
in  a  very  large  proportion  of  specimens,  the  first  few  zones  are  not  complete  circles, 
the  early  growth  having  taken  place  from  one  side  only;  and  there  is  a  very  beautiful 
variety  in  which  this  one-sidedness  of  increase  imparts  a  distinctly  spiral  character 
to  the  early  growth,  which  soon,  however,  gives  place  to  the  cyclical.  In  the  Orbiio- 
litesitalica  (fig.  611),  brought  up  from  depths  of  1,500  fathoms  or  more,  the '  nucleus  ' 


OKBITOLITES 


807 


The  idea  of  the  nature  of  the  living  occupant  of  these  cavities 
which  might  be  suggested  by  the  foregoing  account  of  their  arrange- 
ment, is  fully  borne  out  by  the  results  of  the  examination  of 
the  sarcode-body,  which  may  be  obtained  by  the  maceration  in 
dilute  acid  (so  as  to  remove  the  shelly  investment)  of  specimens  of 
Orbitolites  that  have  been  gathered  fresh  and  preserved  in  spirit. 
For  this  body  is  found  to  be  composed  (fig.  610)  of  a  multitude  of 
segments  of  sarcode,  presenting  not  the  least  trace  of  higher  organi- 
sation in  any  part,  and  connected  together  by  ;  stolons '  of  the  like 
substance.  The  *  primordial '  pear-shaped  segment,  a,  is  seen  to 
have  budded  off  its  '  circumambient '  segment,  6,  by  a  narrow  foot- 
stalk or  stolon  ;  and  this  circumambient  segment,  after  passing  almost 
entirely  round  the  primordial,  has  budded  off  three  stolons,  which 
swell  into  new  sub- 
segments  from  which 
the  first  ring  is  formed. 
Scarcely  any  two  speci- 
mens are  precisely  alike 
as  to  the  mode  in 
which  the  first  ring 
originates  from  the 
'  circumambient  seg- 
ment ; '  for  sometimes 
a  score  or  more  of 
radial  passages  extend 
themselves  from  every 
part  of  the  margin  of 
the  latter  (and  this,  as 
corresponding  with  the 
plan  of  growth  after- 
wards followed,  is 
probably  the  typical 
arrangement)  ;  whilst  FlG-  eiO.-Composite  animal  of  simple  type  of  Orlito- 
in  other  cases  (as  in  lites  complanata: — a,  central  mass  of  sarcode; 
the  example  before  us)  ^>  circumambient  segment,  giving  off  peduncles,  in 
•  .v  •  which  originate  the  concentric  zones  of  sub-segments 

the    number     ot     these         connected  by  annular  bands. 

primary  offsets  is  ex- 
tremely small.  Each  zone  is  seen  to  consist  of  an  assemblage  of 
ovate  sub-segments,  whose  height  (which  could  not  be  shown  in 
the  figure)  corresponds  with  the  thickness  of  the  disc  ;  these  sub- 
segments,  which  are  all  exactly  similar  and  equal  to  one  another, 
are  connected  by  annular  stolons  ;  and  each  zone  is  connected  with 
that  on  its  exterior  by  radial  extensions  of  those  stolons  passing  off 
between  the  sub-segments. 

The  radial  extensions  of  the  outermost  zone  issue  forth  as 
pseudopodia  from  the  marginal  pores,  searching  for  and  drawing  in 
alimentary  materials  in  the  manner  formerly  described  ;  the  whole 
of  the  soft  body,  which  has  no  communication  whatever  with 

is  formed  by  three  or  four  turns  of  a  spiral  closely  resembling  that  of  a  Comuspira 
with  an  interruption  at  every  half-turn,  as  in  Spiroloculina,  the  growth  after- 
wards becoming  purely  concentric. 


808  MICROSCOPIC   FORMS   OF  ANIMAL   LIFE 

the  exterior,  save  through  these  marginal  pores,  being  nourished 
by  the  transmission  of  the  products  of  digestion  from  zone  to  zone 
through  similar  bands  of  protoplasmic  substance.  In  all  cases  in 
which  the  growth  of  the  disc  takes  place  with  normal  regularity  it 
is  probable  that  a  complete  circular  zone  is  added  at  once.  Thus 
we  find  this  simple  type  of  organisation  giving  origin  to  fabrics  of 
by  no  means  microscopic  dimensions,  in  which,  however,  there  is  no 
other  differentiation  of  parts  than  that  concerned  in  the  formation 
of  the  shell,  every  segment  and  every  stolon  (with  the  exception  of 
the  two  forming  the  '  nucleus ')  being,  so  far  as  can  be  ascertained, 
a  precise  repetition  of  every  other,  and  the  segments  of  the  nucleus 
differing  from  the  rest  in  nothing  else  than  their  form.  The  equality 
of  the  endowments  of  the  segments  is  shown  by  the  fact —  of  which 
accident  has  repeatedly  furnished  proof — that  a  small  portion  of  a 
disc,  entirely  separated  from  the  remainder,  will  not  only  continue 


FIG.  611. — Disc  of  Orbitolites  italica,  Costa,  sp.  (O.  tenuissima,  Carp.), 
formed  round  fragment  of  previous  disc. 

to  live,  but  will  so  increase  as  to  form  a  new  disc  (fig.  611),  the  want 
of  the  '  nucleus '  not  appearing  to  be  of  the  slightest  consequence, 
from  the  time  that  active  life  is  established  in  the  outer  zones. 

One  of  the  most  curious  features  in  the  history  of  this  type  is  its 
capacity  for  developing  itself  into  a  form  which,  whilst  funda- 
mentally the  same  as  that  previously  described,  is  very  much  more 
complex.  Tn  all  the  larger  specimens  of  Orbitolites  we  observe  that 
the  marginal  pores,  instead  of  constituting  but  a  single  row,  form 
many  rows  one  above  another ;  and,  besides  this,  the  chamberlets 
of  the  two  surfaces,  instead  of  being  rounded  or  ovate  in  form,  are 
usually  oblong  and  straight-sided,  their  long  diameters  lying  in  a 
radial  direction,  like  those  of  the  cyclical  type  of  Orbiculina.  When 
a  vertical  section  is  made  through  such  a  disc,  it  is  found  that  these 
oblong  chambers  constitute  two  superficial  layers,  between  which 


ORBITOLITES 


809 


are  interposed  columnar  chambers  of  a  rounded  form  ;  and  these 
last  are  connected  together  by  a  complex  series  of  passages,  the 
nrrangement  of  which  will  be  best  understood  from  the  examination 
of  a  part  of  the  sarcocle-body  that  occupies  them  (fig.  612).  For  the 
oblong  superficial  chambers  are  occupied  by  sub-segments  of  sarcode, 
c  c,  d  d,  lying  side  by  side,  so  as  to  form  part  of  an  annulus,  but 
each  of  them  disconnected  from  its  neighbours,  and  communicating 
only  by  a  double  footstalk  with  the  two  annular  '  stolons,'  a  a! ,  b  br, 
which  obviously  correspond  with^he  single  stolon  of  '  simple '  types 
(fig.  610).  These  indirectly  connect  together  not  merely  all  the 
superficial  chamberlets  of  each  zone,  but  also  the  columnar  sub- 
segments  of  the  intermediate  layer  ;  for  these  columns  (e  e,  e'  e'} 
terminate  above  and  below  in  the  annular  stolons,  sometimes  passing 
directly  from  one  to  the  other,  but  sometimes  going  out  of  their 
direct  course  to  coalesce  with 
another  column.  The  columns 
of  the  successive  zones  (two  sets 
of  which  are  shown  in  the 
figure)  communicate  with  each 
other  by  threads  of  sarcode  in 
such  a  manner  that  (as  in  the 
simple  type)  each  column  is 
thus  brought  into  connection 
with  two  columns  of  the  zone 
next  interior,  to  which  it  alter- 
nates in  position.  Similar 
threads,  passing  off  from  the 
outermost  zone  through  the 
multiple  ranges  of  marginal 
pores,  would  doubtless  act  as 
pseudopodia. 

Now  this  plan  of  growth  is 
so  different  from  that  previously 
described  that  there  would  at 
first  seem  ample  ground  for 
separating  the  simple  and  the 
complex  types  as  distinct  species. 
But  the  test  furnished  by  the 
examination  of  a  large  number 
of  specimens,  which  ought  never  to  be  passed  by  when  it  can  possibly 
be  appealed  to,  furnishes  these  very  singular  results :  1st,  that  the 
two  forms  must  be  considered  as  specifically  identical ;  since  there  is 
not  only  a  gradational  passage  from  one  to  the  other,  but  they  are 
often  combined  in  the  same  individual,  the  inner  and  first-formed 
portion  of  a  large  disc  frequently  presenting  the  simple  type,  whilst 
the  outer  and  later-formed  part  has  developed  itself  upon  the  complex  ; 
2nd,  that  although  the  last-mentioned  circumstance  would  naturally 
suggest  that  the  change  from  the  one  plan  to  another  may  be  simply 
a  feature  of  advancing  age,  yet  this  cannot  be  the  case :  since, 
although  the  complex  sometimes  evolves  itself  even  from  the  very 
first  (the  '  nucleus,'  though  resembling  that  of  the  simple  form,  sending 


FIG.  612. — Portion  of  animal  of  complex 
type  of  Orbitolites  complanata: 
a  a',  b  b',  the  upper  and  lower  rings  of 
two  concentric  zones;  c  c,  the  upper 
layer  of  superficial  sub-segments,  and 
d  d,  the  lower  layer,  connected  with  the 
annular  bands  of  both  zones ;  e  e  and 
e'  e',  vertical  sub-segments  of  the  two 
zones. 


8 10  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

out  two  or  more  tiers  of  radiating  threads),  more  frequently  the 
simple  prevails  for  an  indefinite  number  of  zones,  and  then  changes 
itself  in  the  course  of  a  few  zones  into  the  complex.  No  depart- 
ment of  natural  history  could  furnish  more  striking  instances  than 
are  afforded  by  the  different  forms  presented  by  the  foraminiferal 
types  now  described,  of  the  wide  range  of  variation  that  may  occur 
within  the  limits  of  one  and  the  same  species ;  and  the  microscopist 
needs  to  be  specially  put  on  his  guard  as  to  this  point  in  respect  to 
the  lower  types  of  animal  as  to  those  of  vegetable  life,  since  the 
determination  of  form  seems  to  be  far  less  precise  among  such  than 
it  is  in  the  higher  types.1 

In  what  manner  the  reproduction  of  Orbitolites  is  accomplished, 
we  can  as  yet  do  little  more  than  guess ;  but  from  appearances 
sometimes  presented  by  the  sarcode-body,  it  seems  reasonable  to 
infer  that  gemmules,  corresponding  with  the  zoospores  of  proto- 
phytes,  are  occasionally  formed  by  the  breaking  up  of  the  sarcode 
into  globular  masses,  and  that  these,  escaping  through  the  marginal 
pores,  are  sent  forth  to  develop  themselves  into  new  fabrics. 

Arenacea. — In  certain  forms  of  the  preceding  family,  and  espe- 
cially in  the  genus  Mitiola,  we  not  unfrequently  find  the  shells  en- 
crusted with  particles  of  sand,  which  are  imbedded  in  the  proper 
shell  substance.  This  incrustation,  however,  must  be  looked  on  as 
(so  to  speak)  accidental,  since  we  find  shells  that  are  in  every 
other  respect  of  the  same  type  altogether  free  from  it.  A  similar- 
accidental  incrustation  presents  itself  among  certain  '  vitreous  '  and 
perforate  shells ;  but  there,  too,  it  is  usually  on  a  basis  of  true  shell, 
and  the  sandy  incrustation  is  often  entirely  absent.  There  is,  how- 
ever, a  group  of  Foraminifera  in  which  the  true  shell  is  constantly 
and  entirely  replaced  by  a  sandy  envelope,  which  is  distinguished  as 
a  *  test,'  the  arenaceous  particles  being  held  together  only  by  a 
cement  exuded  by  the  animal.  It  is  not  a  little  curious  that  the 
forms  of  these  arenaceous  *  tests '  should  represent  those  of  many 
different  types  among  both  the  '  porcellanous '  and  the  *  vitreous  ' 
series  ;  whilst  yet  they  graduate  into  one  another  in  such  a  manner 
as  to  indicate  that  all  the  members  of  this  '  arenaceous '  group  are 
closely  related  to  each  other,  so  as  to  form  a  series  of  their  own. 
And  it  is  further-  remarkable  that  while  the  deep-sea  dredgings 
recently  carried  down  to  depths  of  from  1,000  to  2,500  fathoms 
have  brought  up  few  forms  of  either  '  porcellanous '  or  '  vitreous ' 
Foraminifera  that  were  not  previously  known,  they  have  added 
greatly  to  our  knowledge  of  the  '  arenaceous  '  types,  the  number  and 
variety  of  which  far  exceed  all  previous  conception.  These  have 
been  systematically  described  by  Mr.  H.  B.  Brady,  F.R.S.,2  whose 
researches  have  led  him  to  believe  that  the  long- established  division 

1  For  further  information  on  the  subject  of  Orbitolites  see  the  Author's  account 
of  the  genus  in  the  reports  of  H.M.S.  Challenger.    Mr.  H.  B.  Brady  in  his '  Challenger ' 
Report  (p.  224)  describes   a   remarkable   allied   type   from  the    Southern  Ocean — 
Keramosphcera  Murrayi — in  which  the  test  is  spherical,  and  the  chambers  are 
arranged  in  concentric  layers. 

2  See  his  important  report  on  the  Foraminifera  dredged  by  H.M.S.  Challenger 
(1884),  illustrated  by  116  plates.     A  large  number  of  deep-sea  forms  has  lately  been 
described  by  Dr.  A.  Goes,  from  the  dredgings  of  the  Albatross;  see  Bull.Mus.  Comp. 
Zool.  xxix.  (1896). 


GLOBIGERINA  8 1 1 

of  the  Foramiiiifera  into  the  arenaceous  and  calcareous  groups  does 
not  correspond  to  any  natural  arrangement ;  for,  although  the  rule 
is  tolerably  constant  in  many  groups,  there  are  others,  notably  certain 
sub-families  of  Textulariidce,  in  which  it  is  by  no  means  uniform. 

In  the  midst  of  the  sandy  mud  which  formed  the  bottom  where 
the  warm  area  of  the  '  Globigerina  mud '  abutted  on  that  over  which 
a  glacial  stream  flowed,  there  were  found  a  number  of  little  pellets, 
varying  in  size  from  a  large  pin's  head  to  a  large  pea,  formed 
of  an  aggregation  of  sand-grains,  minute  foraminifers,  &c.,  held 
together  by  a  tenacious  protoplasmic  substance.  On  tearing  these 
open  the  whole  interior  was  found  to  have  the  same  composition, 
and  no  trace  of  any  structural  arrangement  could  be  discovered  in 
their  mass.  Hence  they  might  be  supposed  to  be  mere  accidental 
agglomerations  were  it  not  for  their  conformity  to  the  '  monerozoic  ' 
type  previously  described  ;  for,  just  as  a  simple  '  moner,'  by  a  differen- 
tiation of  its  homogeneous  sarcode,  becomes  an  Amoeba,  so  would 
one  of  these  uniform  blendings  of  sand  and  sarcode  by  a  separation 
of  its  two  components — the  sand  forming  the  investing  '  test '  and 
the  sarcode  occupying  its  interior — become  the  arenaceous  Astro- 
rhiza.  This  type,  which  abounds  on  the  sea-bed  in  certain  localities 
presents  remarkable  variations  of  form,  being  sometimes  globular, 
sometimes  stellate,  sometimes  cervicorn.  But  the  same  general 
arrangement  prevails  throughout,  the  cavity  being  occupied  by  a 
dark-green  sarcode,  while  the  '  test '  is  composed  of  loosely  aggregated 
sand-grains  not  held  together  by  any  recognisable  cement,  and  has 
no  definite  orifice,  so  that  the  pseudopodia  must  issue  from  inter- 
stices between  the  sand -grains,  which  spaces  are  probably  occupied 
during  life  with  living  protoplasm  that  continues  to  hold  together 
the  sand-grains  after  death.  These  are  by  no  means  microscopic 
forms,  the  '  stellate '  varieties  ranging  to  0*3  or  even  0*4  inch  in 
diameter,  and  the  '  cervicorn  '  to  nearly  O5  inch  in  length.1  A  much 
larger  form  was  found  by  Mr.  Brady  among  the  dredgings  made 
in  the  Faroe  Channel  (see  his  '  "  Challenger "  Report,'  p.  242)  ; 
Syringammina  appears,  when  complete,  to  have  been  a  sphere  about 
an  inch  and  a  half  in  diameter  ;  owing  to  its  large  size  the  almost  com- 
plete absence  of  cement  becomes  very  noticeable,  for  the  fragile  form 
can  scarcely  support  its  own  weight  when  taken  out  of  the  water. 

Later  on  another  large  and  interesting  type  belonging  to 
the  same  group  was  obtained  by  Mr.  Wood-Mason,  late  of  the 
Indian  Museum,  from  the  Bay  of  Bengal.2  This  has  received  the 
generic  name  Masonella.  The  test  consists  of  a  thin  sandy  disc, 
nearly  half  an  inch  in  diameter,  either  flat  or  saucer-shape,  with 
a  central  chamber  and  simple  or  branched  radiating  tubuli  open 
at  the  periphery. 

The  purely  arenaceous  Foraminifera  are  ranged  by  Mr.  H.  B. 
Brady3  (by  whom  they  have  been  especially  studied)  under  two 

1  See  the  description  and  figures  of  this  type  given  by  the  Author  in  Quart. 
Journ.  Microsc.  Sci.  vol.  xvi.  1876,  p.  221. 

2  Ann.  and  Mag.  Nat.  Hist.  1889,  ser.  vi.  vol.  iii.  p.  293,  woodcuts. 

5  See  his  '  Notes '  in  Quart.  Journ.  of  Microcs.  Sci.  n.s.  vol.  xix.  1879,  p  20,  and 
vol.  xxi.  1881,  p.  31. 


812 


MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 


families,  the  first  of  which,  Astrorhizida,  includes  with  the  preceding 
a  number  of  coarse  sandy  forms,  usually  of  considerable  size,  and 
essentially  monothalamous,  though  sometimes  imperfectly  chambered 
by  constrictions  at  intervals.  Some  of  the  more  interesting  examples 
of  this  family  will  now  be  noticed,  beginning  with  the  Saccammina  l 
(Sars),  which  is  a  remarkably  regular  type,  composed  of  coarse  sand- 
grains  firmly  cemented  together  in  a  globular  form,  so  as  to  constitute 
a  wall  nearly  smooth  on  the  outer,  though  rough  on  the  inner  surface, 
with  a  projecting  neck  surrounding  a  circular  mouth  (fig.  613,  a,  6, 
c).  This  type,  which  occurs  in  extraordinary  abundance  in  certain 
localities  (as  the  entrance  of  the  Christiania  fjord,  and  still  further 
north  on  the  shores  of  Franz  Josef  Land),  is  of  peculiar  interest 
from  the  fact  that  a  closely  allied  species  (Saccammina  Carter i)  is, 


FIG.  613. — Arenaceous  Foraminifera  :  a,  Saccammina  sphcerica ;  b,  the  same 
laid  open ;  c,  portion  of  the  test,  enlarged  to  show  its  component  sand- 
grains  ;  d,  Pilulina  Jeffreysii ;  e,  portion  of  the  test  enlarged,  showing 
the  arrangement  of  the  sponge-spicules. 

as  Mr.  H.  B.  Brady  has  shown,  one  of  the  chief  constituents  of 
certain  beds  of  the  Lower  Carboniferous  limestone  of  the  north  of 
England  and  elsewhere.  In  striking  contrast  to  the  preceding  is 
another  single-chambered  type,  distinguished  by  the  whiteness  of 
its  '  test,'  to  which  the  Author  has  given  the  name  of  Pilulina,  from 
its  resemblance  to  a  homoeopathic  'globule'  (fig.  613,  d,e).  The 
form  of  this  is  a  very  regular  sphere  ;  and  its  orifice,  instead  of 
being  circular  and  surrounded  by  a  neck,  is  a  slit  or  fissure  with 
slightly  raised  lips,  and  having  a  somewhat  S-shaped  curvature.  It 
is  by  the  structure  of  its  '  test,'  however,  that  it  is  especially  dis- 
tinguished ;  for  this  is  composed  of  the  finest  ends  of  sponge-spicules, 
very  regularly  '  laid  '  so  as  to  form  a  kind  of  felt,  through  the  sub- 

1  For  a  detailed  account  of  S.  sphcerica  consult  L.  Khumbler,  in  vol.  Ivii.  of  Zeitschr. 
/.  wiss.  Zool. 


ARENACEOUS  FOEAMINIFEBA 


813 


stance  of  which  very  fine  sand-grains  are  dispersed.  This  '  felt '  is 
somewhat  flexible,  and  its  components  do  not  seem  to  be  united  by 
any  kind  of  cement,  as  it  is  not  affected  by  being  boiled  in  strong 
nitric  acid ;  its  tendency,  therefore,  seems  entirely  due  to  the 
wonderful  manner  in  which  the  separate  silicious  fibres  are  *  laid.' 
It  is  not  a  little  curious  that  these  two  forms  should  present  them- 
selves in  the  same  dredging,  and  that  there  should  be  no  perceptible 
difference  in  the  character  of  their  sarcode  bodies,  which,  as  in  the 
preceding  case,  have  a  dark-green  hue.  The  Marsipella  elongata 
(fig.  614,  d),  on  the  other  hand,  fcs  somewhat  fusiform  in  shape,  and 
has  its  two  extremities  elongated  into  tubes,  with  a  circular  orifice 
at  the  end  of  each.  The  materials  of  the  *  tests '  differ  remarkably 
according  to  the  nature  of  the  bottom  whereon  they  live.  When 


FIG.  614. — Arenaceous  Foraminifera :  «,  b,  upper  and  lower  aspects  of  Haplo- 
phragmium  globigeriniforme ;  c,  Hormosina  globulifera  ;  d,  Marsipella 
elongata ;  e,  terminal  portion,  and  /,  middle  portion  of  the  same,  enlarged  ; 
g,  Thurammina  papillata ;  h,  portion  of  its  inner  surface,  enlarged. 

they  come  up  with  '  Globigerina  mud,'  in  which  sponge-spicules 
abound,  whilst  sand-grains  are  scarce,  they  are  almost  entirely 
made  up  of  the  former,  which  are  '  laid  '  in  a  sort  of  lattice-work, 
the  interspaces  of  which  are  filled  up  by  fine  sand-grains ;  but  when 
they  are  brought  up  from  a  bottom  on  which  sand  predominates, 
the  larger  part  of  the  '  test '  is  made  up  of  sand -grains  and  minute 
Foraminifera,  with  here,  and  there  a  sponge-spicule  (fig.  614,  d,f). 
In  each  case,  however,  the  tubular  extensions  (one  of  which  some- 
times forms  a  sort  of  proboscis,  e,  nearly  equalling  the  body  itself 
in  length)  are  entirely  made  up  of  sponge-spicules  laid  side  by  side 
with  extraordinary  regularity.  The  genus  Rhabdammina  (Sars) 
resembles  Saccammina  in  the  structure  of  its  '  test,'  which  is  com- 
posed of  sand-grains  very  firmly  cemented  together  ;  but  the  grains 


8 14  MICROSCOPIC   FORMS   OF  ANIMAL   LIFE 

are  of  smaller  size,  and  they  are  so  disposed  as  to  present  a  smooth 
surface  internally,  though  the  exterior  is  rough.  What  is  most 
remarkable  about  this  is  the  geometrical  regularity  of  its  form, 
which  is  typically  triradiate  (fig.  615,  c),  the  rays  diverging  at  equal 
angles  from  the  central  cavity,  and  each  being  a  tube  (d)  with  an 
orifice  at  its  extremity.  Not  urifrequeiitly,  however,  it  is  quadri- 
radiate,  the  rays  diverging  at  right  angles ;  and  occasionally  a  fifth 
ray  presents  itself,  its  radiation,  however,  being  generally  in  a 
different  plane.  The  three  rays  are  normally  of  equal  length  ;  but 
one  of  them  is  sometimes  shorter  than  the  other  two ;  and  when 
this  is  the  case  the  angle  between  the  long  rays  increases  at  the 
expense  of  the  other  two,  so  that  the  long  rays  lie  more  nearly  in  a 
straight  line.  Sometimes  the  place  of  the  third  ray  is  indicated 
only  by  a  little  knob  ;  and  then  the  two  long  rays  have  very  nearly 
the  same  direction.  We  are  thus  led  to  forms  in  which  there  is  no 
vestige  of  a  third  ray,  but  merely  a  single  straight  tube,  with  an 
orifice  at  each  end ;  and  the  length  of  this,  which  often  exceeds 
half  an  inch,  taken  in  connection  with  the  abundance  in  whicli  it 
presents  itself  in  dredgings  in  which  the  triradiate  forms  are  vaiv. 
seems  to  preclude  the  idea  that  these  long  single  rods  are  broken 
rays  of  the  latter.  It  is  undoubtedly  in  this  group  that  we  are  to 
place  the  genus  ffaliphysema,  which,  from  constructing  its  *  test ' 
entirely  of  sponge-spicules,  and  even  including  these  in  its  pseudo- 
podial  expansions,  has  been  ranked  as  a  sponge,  although  observation 
of  it  in  its  living  state  leaves  no  doubt  whatever  of  its  rhizopodaJ 
character.1 

Lituolida. — The  type  of  this  family,  which  is  named  after  it,  is 
a  large  sandy  many- chambered  fossil  form  occurring  in  the  chalk, 
to  which  the  name  Lituola  was  given  by  Lamarck,  from  its  resem- 
blance in  shape 'to  a  crosier.  A  great  variety  of  recent  forms,  mostly 
obtained  by  deep-sea  dredging,  are  now  included  in  it,  as  bearing 
a  more  or  less  close  resemblance  to  it  and  to  each  other  in  their 
chambered  structure,  and  in  the  arrangement  of  the  Baud-grains  of 
which  their  tests  are  formed.  These  grains  are,  for  the  most  part, 
finer  than  those  of  which  the  tests  of  the-  preceding  family  are  con- 
structed, and  are  set  (so  to  speak)  more  artistically,  and  a  con- 
siderable quantity  of  a  cement  exuded  by  the  animal  is  employed 
in  uniting  them.  This  is  often  mixed  up  with  sandy  particles  of 
extreme  fineness  to  form  a  sort  of  *  plaster- '  with  which  the  exterior 
of  the  test  is  smoothed  off,  so  as  to  present  quite  a  polished  surface. 
It  is  remarkable  that  the  cement  contains  a  considerable  quantity 
of  oxide  of  iron,  which  imparts  a  ferruginous  hue  to  the  '  tests'  in 
which  it  is  largely  employed.  The  forms  of  the  Lituoline  '  tests ' 
often  simulate  in  a  very  curious  way  those  of  the  simpler  types  of 
the  vitreous  series.  Thus,  the  long  spirally  coiled  undivided  sandy 
tube  of  A  mmodiscus  is  the  isomorph  of  Spirillina,  In  the  genus  Haplo- 
phragmium  (fig.  614,^,6,  and  Plate  XVIII,  fig.  6)  we  have  singular 
imitations  of  the  Globigerine,  Rotaline,  and  Nonionine  types  ;  and  in 

1  See  Mr.  Saville  Kent  in  Ann.  of  Nat.  Hist.  ser.  v.  vol.  ii.  1878 ;  Professor  Ray 
Lankester  in  Quart.  Journ.  Microsc.  Sci.  vol.  xix.  1878,  p.  476  ;  and  Professor  Mb'bius's 
Foraminifera  von  Mauritius,  1880. 


ARENACEOUS  FORAMINIFERA 


8I5 


Thurammina  papillata  (fig.  614,  g)  a  not  less  remarkable  imitation  of 
the  Orbuline.  This  last  is  specially  noteworthy  for  the  admirable 
manner  in  which  its  component  sand -grains  are  set  together,  these 
being  small  and  very  uniform  in  size,  and  being  disposed  in  such  a 
manner  as  to  present  a  smooth  surface  both  inside  and  out  (fig.  614,  h), 
whilst  there  are  at  intervals  nipple-shaped  protuberances,  in  every  one 
of  which  there  is  a  rounded  orifice.  A  like  perfection  of  finish  is  seen 
in  the  test  of  Hormosina  globulifera  (fig.  614,  c),  which  is  composed 
of  a  succession  of  globular  chambers  rapidly  increasing  in  size,  each 
having  a  narrow  tubular  neck  with  a  rounded  orifice,  which  is 
received  into  the  next  segment.  In  other  species  of  the  same  genus 
there  is  a  nearer  approach  to  the  ordinary  Nodosarine  type,  their 
tests  being  sometimes  constructed  with  the  regularity  characteristic 
of  the  shells  of  the  true  Nodosaria,  Plate  XIX,  16,  whilst  in  other 


FIG.  615. — Arenaceous  Foraminifera :  «,  b,  exterior  and  sectional  views  of 
Rheophax  sabulosa ;  c,  Rhabdammina  abyssorum ;  d,  cross  section  of  one 
of  its  arms ;  e,  Rheophax  scorpiurus ;  /,  Hormosina  Carpenteri. 

cases  the  chambers  are  less  regularly  disposed  (fig.  615,  /),  having 
rather  the  character  of  bead-like  enlargements  of  a  tube,  whilst  their 
walls  show  a  less  exact  selection  of  material,  sponge-spicules  being 
worked  in  with  the  sand-grains,  so  as  to  give  them  a  hirsute  aspect. 
A  greater  rudeness  of  structure  shows  itself  in  the  Nodosarine  forms 
of  the  genus  Rheophax,  in  which  not  only  are  the  sand-grains  of  the 
test  very  coarse,  but  small  Foraminifera  are  often  worked  up  with 
them  (fig.  615,  e).  A  straight,  many-chambered  form  of  the  same 
genus  (fig.  615,  a,  b)  is  remarkable  for  the  peculiar  finish  of  the  neck 
of  each  segment ;  for  whilst  the  test  generally  is  composed  of  sand- 
grains,  as  loosely  aggregated  as  those  of  which  the  test  of  Astrorhiza 
is  made  up,  the  grains  that  form  the  neck  are  firmly  united  by  fer- 
ruginous cement,  forming  a  very  smooth  wall  to  the  tubular  orifice. 

The  highest  development  of  the  '  arenaceous '  type  at  the  present 
time  is  found  in  the  forms  that  imitate  the  very  regular  nautiloid 


8i6 


MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 


shells,  both  of  the  *  porcellanous '  and  the  '  vitreous '  series  ;  and  the 
most  remarkable  of  these  is  the  Cyclammina  cancellata  (fig.  616), 
which  has  been  brought  up  in  considerable  abundance  from  depths 
ranging  downwards  to  1,900  fathoms,  the  largest  examples  being 
found  within  700  fathoms.  The  test  (fig.  616,  a)  is  composed  of 
aggregated  sand -grains  firmly  cemented  together  and  smoothed  over 
externally  with  '  plaster,'  in  which  large  glistening  sand-grains  are 
sometimes  set  at  regular  intervals,  as  if  for  ornament.  On  laying 
open  the  spire  it  is  found  to  be  very  regularly  divided  into  chambers 
by  partitions  formed  of  cemented  sand-grains  (6),  a  communication 
between  these  chambers  being  left  by  a  fissure  at  the  inner  margin 
of  the  spire,  as  in  Operculina  (fig.  628).  One  of  the  most  curious 
features  in  the  structure  of  this  type  is  the  extension  of  the  cavity 
of  each  chamber  into  passages  excavated  in  its  thick  external  wall, 


FIG.  616. — Cyclammina  cancellata,  showing  at  a,  its  external  aspect ; 
&,  its  internal  structure  ;  c,  a  portion  of  its  outer  wall  in  section,  more 
highly  magnified,  showing  the  sand-grains  of  which  it  is  built  up  and 
the  passages  excavated  in  its  substance. 


each  passage  being  surrounded  by  a  very  regular  arrangement  of 
sand-grains,  as  shown  at  c.  It  not  unfrequently  happens  that  the 
outer  layer  of  the  test  is  worn  away,  and  the  ends  of  the  passages 
then  show  themselves  as  pores  upon  its  surface  ;  this  appearance, 
however,  is  abnormal,  the  passages  simply  running  from  the  chamber- 
cavity  into  the  thickness  of  its  wall,  and  having  (so  long  as  this  is 
complete)  no  external  opening.  This  *  labyrinthic '  structure  is  of 
great  interest,  from  its  relation  not  only  to  the  similar  structure 
of  the  large  fossil  examples  of  the  same  type,  but  also  to  that  which 
is  presented  in  other  gigantic  fossil  arenaceous  forms  to  be  presently 
described. 

Although  some  of  the  nautiloid  Lituolce  are  among  the  largest 
of  existing  Foraminifera,  having  a  diameter  of  0'3  inch,  they  are 
mere  dwarfs  in  comparison  with  two  gigantic  fossil  forms,  whose 


PARKERIA  817 

structure  has  been  elucidated  by  Mr.  H.  B.  Brady  and  the  Author.1 
Geologists  who  have  worked  over  the  Greensand  of  Cambridgeshire 
have  long  been  familiar  with  solid  spherical  bodies  which  there 
present  themselves  not  unfrequently,  varying  in  size  from  that  of  a 
pistol-bullet  to  that  of  a  small  cricket-ball ;  and  whilst  some  regarded 
them  as  mineral  concretions  others  were  led  by  certain  appearances 
presented  by  their  surfaces  to  suppose  them  to  be  fossilised  sponges. 
A  specimen  having  been  fortunately  discovered,  however,  in  which 
the  original  structure  had  remained  unconsolidated  by  mineral  in- 


FIG.  617. — General  view  of  the  internal  structure  of  Parkeria  :  in  the  hori- 
zontal section,  I1,  I'-,  I5,  I4  mark  the  four  thick  layers;  in  the  vertical 
sections  A  marks  the  internal  surface  of  a  layer  separated  by  concentric 
fracture ;  B,  the  appearance  presented  by  a  similar  fracture  passing  through 
the  radiating  processes;  C,  the  result  of  a  tangential  section  passing 
through  the  cancellated  substance  of  a  lamella ;  D,  the  appearance  pre- 
sented by  the  external  surface  of  a  lamella  separated  by  a  concentric 
fracture  which  has  passed  through  the  radial  processes ;  E,  the  aspect  of  a 
section  taken  in  a  radial  direction,  so  as  to  cross  the  solid  lamellae  and  their 
intervening  spaces;  c1,  c?,  r",  c4,  successive  chambers  of  nucleus. 

filtration,  it  was  submitted  by  Professor  Morris  to  the  Author,  who 
was  at  once  led  by  his  examination  of  it  to  recognise  it  as  a  member 
of  the  arenaceous  group  of  Foraminifera,  to  which  he  gave  the  de- 
signation Parkeria,  in  compliment  to  his  valued  friend  and  coadjutor, 
Mr.  W  K.  Parker.  A  section  of  the  sphere  taken  through  its 
centre  (fig.  617)  presents  an  aspect  very  much  resembling  that  of  an 
Orbitolite,  a  series  of  chamberlets  being  concentrically  arranged 
round  a  '  nucleus ; '  and  as  the  same  appearance  is  presented,  what- 
ever be  the  direction  of  the  section,  it  becomes  apparent  that  these 

1  See  their  '  Description  of  Parkeria  and  Loftusia '  in  Philosophical  Trans- 
actions, 1869,  p.  721.  Though  it  appears  convenient  to  allow  this  description  of 
Parkeria  to  remain,  it  .must  be  noted  that  some  of  those  most  competent  to  judge 
are  of  opinion  that  Parkeria,  is  one  of  the  Stromatoporoids,  an  obscure  and  difficult 
group  of  fossil  Hydroida  (see  the  memoir  by  Professor  Alleyne  Nicholson,  published 
in  1886  by  the  Palseoiitographical  Society). 

3G 


8l8  MICROSCOPIC   FORMS   OF   ANIMAL  LIFE 

chamberlets,  instead  of  being  arranged  in  successive  rings  on  a  single 
plane,  so  as  to  form  a  disc,  are  grouped  in  concentric  spheres,  each 
completely  investing  that  which  preceded  it  in  date  of  formation. 
The  outer  wall  of  each  chamberlet  is  itself  penetrated  by  extensions 
of  the  cavity  into  its  substance,  as  in  the  Cyclamminal&si  described; 
and  these  passages  are  separated  by  partitions  very  regularly  built 
up  of  sand-grains,  which  also  close  in  their  extremities,  as  is  shown 
in  fig.  618.  The  concentric  spheres  are  occasionally  separated  by 
walls  of  more  than  ordinary  thickness,  and  such  a  wall  is  seen  in 
fig.  617  to  close  in  the  last-formed  series  of  chamberlets.  But  these 

walls  have  the  same  '  labyrinthic ' 
structure  as  the  thinner  ones, 
and  an  examination  of  numerous 
specimens  shows  that  they  are 
not  formed  at  any  regular  inter- 
vals. The  '  nucleus  '  is  always 
composed  of  a  single  series  of 
chambers  arranged  end  to  end, 
sometimes  in  a  straight  line,  as 
in  fig.  617,  c1,  c2,  c3,  c4,  sometimes 
forming  a  spiral,  and  in  one  in- 
stance returning  upon  itself. 
FIG  618.-Portion  of  one  of  the  lamella  fi  t  th  outermost  chamber  en- 
of  Parkena,  showing  the  sand-grams  of  ,  ..  .,  . ,  ..  _ 

which  it  is  built  up,  and  the  passages  larges,  and  extends  itself  over  the 
extending  into  its  substance.  whole  '  nucleus,'  very  much  as  the 

'  circumambient '  chamber  of  the 

Orbitolite  extends  itself  round  the  primordial  chamber  ;  and  radial 
prolongations  given  off  from  this  in  every  direction  form  the  first 
investing  sphere,  round  which  the  entire  series  of  concentric 
spheres  are  successively  formed.  Of  the  sand  of  which  this  remark 
able  fabric  is  constructed  about  60  per  cent,  consists  of  phosphate  of 
lime,  and  nearly  the  whole  remainder  of  carbonate  of  lime.  Another 
large  fossil  arenaceous  type,  constructed  upon  the  same  general  plan, 
but  growing  spirally  round  an  elongated  axis,  after  the  manner  of 
Alveolina  (fig.  608),  and  attaining  a  length  of  three  inches,  has  been 
described  by  Mr.  H.  B.  Brady  (loc.  cit.)  under  the  name  Loftusia,  after 
its  discoverer,  the  late  Mr.  W.  K.  Loftus,  who  brought  it  from  the 
Turko-Persian  frontier,  where  specimens  were  found  in  considerable 
numbers  imbedded  in  '  a  blue  marly  limestone,'  probably  of  early 
Tertiary  age. 

There  is  nothing,  it  seeing  to  the  Author,  more  wonderful  in 
Nature  than  the  building  up  of  these  elaborate  and  symmetrical 
structures  by  mere  'jelly-specks,'  presenting  no  trace  whatever  of 
that  definite  '  organisation  '  which  we  are  accustomed  to  regard  as 
necessary  to  the  manifestations  of  conscious  life.  Suppose  a  human 
mason  to  be  put  down  by  the  side  of  a  pile  of  stones  of  various  shapes 
and  sizes,  and  to  be  told  to  build  a  dome  of  these,  smooth  on  both 
surfaces,  without  using  more  than  the  least  possible  quantity  of  a 
very  tenacious  but  very  costly  cement  in  holding  the  stones  together. 
If  he  accomplished  this  well,  he  would  receive  credit  for  great  in- 
telligence and  skill.  Yet  this  is  exactly  what  these  little  '  jelly-specks  ' 


VITREOUS   FORAMINIFEKA  819 

do  on  a  most  minute  scale,  the  '  tests  '  they  construct,  when  highly 
magnified,  bearing  comparison  with  the  most  skilful  masonry  of  man. 
From  the  same,  sandy  bottom  one  species  picks  up  the  coarser  quartz- 
grains,  unites  them  together  with  a  ferruginous  cement  secreted  from 
its  own  substance,  and  thus  constructs  a  flask-shaped  '  test,'  having 
a  short  neck  and  a  single  large  orifice.  Another  picks  up  the  finer 
grains  and  puts  them  together  with  the  same  cement  into  perfectly 
spherical  '  tests  '  of  the  most  extraordinary  finish,  perforated  with 
numerous  small  pores  disposed  at  pretty  regular  intervals.  Another 
selects  the  minutest  sand-grains  and  the  terminal  portions  of  sponge- 
spicules  and  works  these  up  together — apparently  with  no  cement 
at  all,  but  by  the  mere  '  laying '  of  the  spicules — into  perfect  white 
spheres,  like  homoeopathic  globules,  each  having  a  single  fissured 
orifice.  And  another,  which  makes  a  straight  many-chambered  'test,' 
the  conical  mouth  of  each  chamber  projecting  into  the  cavity  of  the 
next,  wiiile  forming  the  walls  of  its  chambers  of  ordinary  sand-grains 
rather  loosely  held  together,  shapes  the  conical  mouths  of  the  suc- 
cessive chambers  by  firmly  cementing  to  each  other  the  quartz -grains 
which  border  it.  To  give  these  actions  the  vague  designation  '  in- 
stinctive '  does  not  in  the  least  help  us  to  account  for  them ;  since 
what  we  want  is  to  discover  the  mechanism  by  which  they  are  worked 
out ;  and  it  is  most  difficult  to  conceive  how  so  artificial  a  selection 
can  be  made  by  creatures  so  simple. 

Vitrea. — Returning  now  to  the  Foraminifera  which  form  true 
shells  by  the  calcification  of  the  superficial  layer  of  their  sarcode- 
bodies,  we  shall  take  a  similar  general  survey  of  the  vitreous  series, 
whose  shells  are  perforated  by  multitudes  of  minute  foramina  (fig. 
607).  Thus,  SpiriUina  has  a  minute,  spirally  convoluted,  undivided 
tube,  resembling  that  of  Cornuspira,  but  having  its  wall  somewhat 
coarsely  perforated  by  numerous  apertures  for  the  emission  of  pseudo- 
podia.  The  '  monothalamous  '  forms  of  this  growth  mostly  belong  to 
the  family  Lagenida,  which  also  contains  a  series  of  transition  forms 
leading  up  gradationally  to  the  *  polythalamous  '  nautiloid  type.  In 
Lagena  (Plate  XIX,  figs.  12,  13,  U,  15)  the  mouth  is  narrowed  and 
prolonged  into  a  tubular  neck,  giving  to  the  shell  the  form  of  a  micro- 
scopic flask  ;  this  neck  terminates  in  an  everted  lip,  which  is  marked 
with  radiating  furrows.  A  mouth  of  this  kind  is  a  distinctive 
character  of  a  large  group  of  many-chambered  shells,  of  which  each 
single  chamber  bears  a  more  or  less  close  resemblance  to  the  simple 
Lagena,  and  of  which,  like  it,  the  external  surface  generally  presents 
some  kind  of  ornamentation,  which  may  have  the  form  either  of 
longitudinal  ribs  or  of  pointed  tubercles.  Thus  the  shell  of  Nodo- 
saria  (Plate  XIX,  fig.  16)  is  obviously  made  up  of  a  succession 
of  lageniform  chambers,  the  neck  of  each  being  received  into  the 
cavity  of  that  which  succeeds  it;  whilst  in  Cristellaria  (fig.  17) 
we  have  a  similar  succession  of  chambers,  presenting  the  characteristic 
radiate  aperture,  and  often  longitudinally  ribbed,  disposed  in  a 
nautiloid  spiral.  Between  Nodosaria  and  Cristellaria,  moreover, 
there  is  such  a  gradational  series  of  connecting  forms  as  shows  that 
no  essential  difference  exists  between  these  two  types,  and  it  is  a  fact 
of  no  little  interest  that  some  of  the  simpler  of  these  varietal  forms, 

So  2 


82O  MICKOSCOPIC   FOEMS   OF  ANIMAL   LIFE 

of  which  many  are  to  be  met  with  on  our  own  shores,  but  which  are 
more  abundant  on  those  of  the  Mediterranean  and  especially  of  the 
Adriatic,  can  be  traced  backwards  in  geological  time  at  least  as  far 
as  the  Permian  epoch.  In  another  genus,  Polymorphina,  we  find 
the  shell  to  be  made  up  of  lageniform  chambers  arranged  in  a  double 
series,  alternating  with  each  other  on  the  two  or  more  sides  of  a 
rectilinear  axis  ;  here,  again,  the  forms  of  the  individual  chambers, 
and  the  mode  in  which  they  are  set  one  upon  another,  vary  in  such 
a  manner  as  to  give  rise  to  very  marked  differences  in  the  general 
configuration  of  the  shell,  which  are  indicated  by  the  name  it  bears. 
Globigerinida, — Returning  once  again  to  the  simple  '  monothala- 
mous '  condition,  we  have  in  Orbulina — a  minute  spherical  shell  that 
presents  itself  in  greater  or  less  abundance  in  deep-sea  dredgings, 
from  almost  every  region  of  the  world — a  globular  chamber  with 
porous  walls,  but  destitute  of  any  general  aperture,  the  office  of  which 
is  served  by  a  series  of  larger  pores  scattered  throughout  the  wall  of 
the  sphere.  It  has  been  maintained  by  some  that  Orbulina  is  really 
a  detached  generative  segment  of  Globigerina,  with  which  it  is 
generally  found  associated.  The  shell  of  Globlgerina  consists  of  an 
assemblage  of  nearly  spherical  chambers  (fig.  619),  having  coarsely 


PIG.  619. — Globigerina  bulloides  as  seen  in  three  positions. 

porous  walls,  and  cohering  externally  into  a  more  or  less  regular 
turbinoid  spire,  each  turn  of  which  consists  of  four  chambers  pro- 
gressively increasing  in  size.  These  chambers,  whose  total  number 
seldom  exceeds  sixteen,  may  not  communicate  directly  with  each 
other,  but  open  separately  into  a  common  '  vestibule  '  which  occupies 
the  centre  of  the  under  side  of  the  spire.  This  type  has  attracted 
great  attention,  from  the  extraordinary  abundance  in  which  it  occurs 
at  great  depths  over  large  areas  of  the  ocean  bottom.  Thus  its  minute 
shells  have  been  found  to  constitute  no  less  than  97  per  cent,  of 
the  'ooze '  brought  up  from  depths  of  from  1,260  to  2,000  fathoms 
in  the  middle  of  the  northern  parts  of  the  Atlantic  Ocean.  The 
younger  shells,  consisting  of  from  eight  to  twelve  chambers,  are 
thin  and  smooth,  but  the  older  shells  are  thicker,  their  surface  is 
raised  into  ridges  that  form  an  hexagonal  areolation  round  the  pores 
(fig.  620)  ;  and  this  thickening  is  shown  by  examination  of  thin 
sections  of  the  shell  to  be  produced  by  an  exogenous  deposit  around 
the  original  chamber  wall  (corresponding  with  the  '  intermediate 
skeleton  '  of  the  more  complex  types),  which  sometimes  contains 
little  flask-shaped  cavities  filled  with  sarcode — as  was  first  pointed 
out  by  the  late  Dr.  Wallich.  But  the  sweeping  of  the  upper  waters 


GLOBIGERINA 


821 


of  the  ocean  by  the  '  tow  net,'  which  was  systematically  carried  on 
during  the  voyage  of  the  '  Challenger/  brought  into  prominence  the 
fact  that  these  waters  in  all  but  the  coldest  seas  are  inhabited  by 
floating  Globigerince,  whose  shells  are  beset  with  multitudes  of  de- 
licate calcareous  spines,  which  extend  themselves  radially  from  the 
angles  at  which  the  ridges  meet  to  a  length  equal  to  four  or  five 
times  the  diameter  of  the  shell  (fig.  621).  Among  the  bases  of  these 
spines  the  sarcodic  substance  of  the  body  exudes  through  the  pores 
of  the  shell,  forming  a  flocculent  fringe  around  it ;  and  this  extends 


FIG.  620. — Globigerina  conglobata  (Brady) :  a,  6,  c,  bottom  specimens  ; 
d,  section  of  shell. 

itself  on  each  of  the  spines,  creeping  up  one  side  to  its  extremity, 
and  passing  down  the  other  with  the  peculiar  flowing  movement 
already  described.  The  whole  of  this  sarcodic  extension  is  at  once 
retracted  if  the  cell  which  holds  the  Globigerina  receives  a  sudden 
shock,  or  a  drop  of  any  irritating  fluid  is  added  to  the  water  it  con- 
tains. It  was  maintained  by  Sir  Wyville  Thomson  that  the  bottom 
deposit  is  formed  by  the  continual '  raining  down '  of  the  Globigerinse 
of  the  upper  waters,  which  (he  affirmed)  only  live  at  or  near  the  sur- 
face, and  which,  when  they  die,  lose  their  spines  and  subside.  The 


822 


MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 


Author,  however,  from  his  own  examination  of  the  Globigerina  ooze, 
is  of  opinion  that  the  shells  forming  its  surface-layer  must  live  on  the 
bottom,  being  incapable  of  floating  in  consequence  of  their  weight ; 
and  that  if  they  have  passed  the  earlier  part  of  their  lives  in  the 
upper  waters  they  drop  down  as  soon  as  the  calcareous  deposit  con- 
tinually exuding  from  the  body  of  each  animal,  instead  of  being  em- 
ployed in  the  formation  of  new  chambers,  is  applied  to  the  thicken- 
ing of  those  previously  formed.  That  many  types  of  Foraminifera 

pass  their  whole  lives  at 
depths  of  at  least  2,000 
fathoms  is  proved,  in  regard 
to  those  forming  calcareous 
shells,  by  their  attachment 
to  stones,  corals,  &c.  ;  and 
in  the  case  of  the  arena- 
ceous types  by  the  fact  that 
they  can  only  procure  on  th<' 
bottom  the  sand  of  which 
their  '  tests  '  are  made  up. 

A  very  remarkable  type 
has  recently  been  discovered 
adherent  to  shells  and  corals 
brought  from  tropical  seas, 
to  which  the  name  Carpen- 
teria  has  been  given.  This 
may  be  regarded  as  a  highly 
developed  form  of  Globi- 
gerina, its  first  formed  por- 

FIG.  621.-GloMgerina,  as  captured  by  tow-net    tion  having  all  the  essential 
floating  at  or  near  surface.  characters    of     that     genus. 

It   grows  attached   by   the 

apex  of  its  spire,  and  its  later  chambers  increase  rapidly  in  size, 
and  are  piled  on  the  earlier  in  such  a  manner  as  to  form  a  depressed 
cone  with  an  irregular  spreading  base,  The  essential  character  of 
Globigerina — the  separate  orifice  of  each  of  its  chambers — is  here  re- 
tained with  a  curious  modification ;  for  the  central  vestibule  into 
which  they  all  open  forms  a  sort  of  vent  whose  orifice  is  at  the  apex 
of  the  cone,  and  is  sometimes  prolonged  into  a  tube  that  proceeds 
from  it ;  and  the  external  wall  of  this  cone  is  so  marked  out  by 
septal  bands  that  it  comes  to  bear  a  strong  resemblance  to  a  minute 
Balanus  (acorn-shell),  for  which  this  type  was  at  first  mistaken.  The 
principal  chambers  are  partly  divided  into  chamberlets  by  incomplete 
partitions,  as  we  s,hall  find  them  to  be  in  Eozoon.  The  presence  of 
sponge-spicules  in  large  quantity  in  the  chambers  of  many  of  the 
best  preserved  examples  of  this  type  was  for  some  time  a  source  of 
perplexity ;  but  this  was  explained  by  the  late  Professor  Max 
Schultze,1  who  showed  how  the  pseudopodia  of  this  rhizopod  have 
the  habit,  like  those  of  Haliphysema,  of  taking  into  themselves  sponge- 
spicules,  which  they  draw  into  the  chambers,  so  that  they  become 
incorporated  with  the  sarcode-body.  It  should  be  added  that  Pro- 
1  Archivf.  Naturgesch.  xxix.  1863,  p.  81. 


TEXTULARIA  823 

fessor  Schultze,  with  whom  Mr.  H.  J.  Carter,1  Mr.  H.  B.  Brady,2  and 
Dr.  Goes  3  are  in  agreement,  regard  Carpentaria  as  allied  to  Polytrema. 
Some  interesting  observations  have  been  made  by  Professor  Mb'bius 4 
011  a  large  branching  and  spreading  form  of  Carpenteria  which  he 
recently  met  with  on  a  reef  near  Mauritius,  and  to  which  he  has  given 
the  name  of  C.  rhapkidodendron. 

A  less  aberrant  modification  of  the  Globigerine  type,  however,  is 
presented  in  the  two  great  series  which  may  be  designated  (after  the 
leading  forms  of  each)  as  the  Te&tularian  and  the  Rotalian.  For, 
notwithstanding  the  marked  difference  in  their  respective  plans  of 
growth,  the  characters  of  the  individual  chambers  are  the  same, 
their  walls  being  coarsely  porous,  and  their  apertures  being  oval, 
semi-oval,  or  crescent-shaped,  sometimes  merely  fissured.  In  Textu- 
laria  (Plate  XVIII,  fig.  9)  the  chambers  are  arranged  biserially 
along  a  straight  axis,  the  position  of  those  on  the  two  sides  of  it  being 
alternate,  and  each  chamber  opening  into  those  above  and  below  it 
on  the  opposite  side  bv  a  narrow  fissure,  as  is  well  shown  in  such 

A  B 


FIG.  622. — Internal  silicious  casts  representing  the  forms  of  the  segments  of 
the  animals  of,  A,  Textularia;  B,  Botalia. 

1  internal  casts '  (fig.  622,  A)  as  exhibit  the  forms  and  connections  of 
the  segments  of  sarcode  by  which  the  chambers  were  occupied  during 
life.  In  the  genus  Bulimina  the  chambers  are  so  arranged  as  to  form 
a  spire  like  that  of  a  Bulimus,  and  the  aperture  is  a  curved  fissure 
whose  direction  is  nearly  transverse  to  that  of  the  fissure  of  Textu- 
laria  ;  but  in  this,  as  in  the  preceding  type,  there  is  an  extraordinary 
variety  in  the  disposition  of  the  chambers.  In  both,  moreover,  the 
shell  is  often  covered  by  a  sandy  incrustation,  so  that  its  perforations 
are  completely  hidden,  and  can  only  be  made  visible  by  the  removal  of 
the  adherent  crust.  And  so  many  cases  are  now  known  in  which 
the  shell  of  Textularinice  is  entirely  replaced  by  a  sandy  test,  that 
some  systematists  prefer  to  range  this  group  among  the  Arenacea. 

In  the  Rotalian  series  the  chambers  are  disposed  in  a  turbinoid 
spire,  opening  one  into  another  by  an  aperture  situated  on  the  lower 

1  Annals  and  Mag.  Nat.  Hist.  ser.  iv.  vols.  xvii.  xix.  xx. 

2  '  Challenger'  Heport. 

5  K.  Svenska  Vet.  Handlingar,  xix.  No.  4,  p.  94. 

4  See  his  Foraminifera  von  Mauritius,  1880,  plates  v.  vi. 


824  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

and  inner  side  of  the  spire,  as  shown  in  Plate  XIX,  fig.  22,  the  forms 
and  connections  of  the  segments  of  their  sarcode -bodies  being  shown 
in  such  '  internal  casts '  as  are  represented  in  fig.  622,  B.  One  of  the 
lowest  and  simplest  forms  of  this  type  is  that  very  common  one  now 
distinguished  as  Discorbina.  The  early  form  of  Planorbulina  is  a 
Rotaline  spire,  very  much  resembling  that  of  Discorbina ;  but  this 
afterwards  gives  place  to  a  cyclical  plan  of  growth,  and  in  those 
most  developed  forms  of  this  type  which  occur  in  warmer  seas  the 
earlier  chambers  are  completely  overgrown  by  the  latter,  which  are 
often  piled  up  in  an  irregular  '  acervuline '  manner,  spreading  over 
the  surfaces  of  shells  or  clustering  round  the  stems  of  zoophytes. 

In  the  genus  Tinoporus  there  is  a 
more  regular  growth  of  this  kind,  the 
chambers  being  piled  successively  on 
the  two  sides  of  the  original  median 
plane,  and  those  of  adjacent  piles  com- 
municating with  each  other  obliquely 
(like  those  of  Textularia)  by  large 
apertures,  whilst  they  communicate 
with  those  directly  above  and  below 
by  the  ordinary  pores  of  the  shell. 
The  simple  or  smooth  varieties  of  this 
genus  forming  the  sub-genus  Gypsina 
present  great  diversities  of  shape, with 
FIG.  628.— Tinoporus  baculatus.  great  constancy  in  their  internal  struc- 
ture, being  sometimes  spherical,  some- 
times resembling  a  minute  sugar-loaf,  and  sometimes  being  irregu- 
larly flattened  out.  The  typical  form  (fig.  623),  in  which  the  walls 
of  the  piles  are  thickened  at  their  meeting  angles  into  solid  columns 
that  appeal-  on  the  surface  as  tubercles,  and  are  sometimes  pro- 
longed into  spinous  outgrowths  that  radiate  from  the  central  mass, 
is  of  very  common  occurrence  in  shore-sands  and  shallow-water 
dredgings  on  some  parts  of  the  Australian  coasts  and  among  the 
Polynesian  islands.  To  the  simple  form  of  this  genus  we  are 
probably  to  refer  many  of  the  fossils  of  the  Cretaceous  and 
early  Tertiary  period  that  have  been  described  under  the  name 
Orbitolina,  some  of  which  attain  a  very  large  size.  Globular  Orbito- 
lince,  which  appear  to  have  been  artificially  perforated  and  strung 
as  beads,  are  not  unfrequently  found  associated  with  the  '  flint-imple- 
ments '  of  gravel -beds.  Another  very  curious  modification  of  the 
Rotaline  type  is  presented  by  Polytrema,  which  so  much  resembles 
a  zoophyte  as  to  have  been  taken  for  a  minute  millepore,  but  which 
is  made  up  of  an  aggregation  of  '  Globigerine  '  chambers  communi- 
cating with  each  other  like  those  of  Tinoporus,  and  differs  from  that 
genus  primarily  in  its  erect  and  usually  branching  manner  of  growth 
and  the  freer  communication  between  its  chambers.  This,  again,  is 
of  special  interest  in  relation  to  Eozoon,  showing  that  an  indefinite 
zoophytic  mode  of  growth  is  perfectly  compatible  with  truly  fora- 
miniferal  structure. 

In  Rotalia,  properly  so  called,  we  find  a  marked  advance  towards 
the    highest   type    of  foraminiferal    structure,    the  partitions  that 


KOTALIA 


825 


divide  the  chambers  being  in  the  best  developed  examples  composed 
of  two  laminae,  and  spaces  being  left  between  them  which  give 
passage  to  a  system  of  canals  whose  general  distribution  is  shown 
in  fig.  624.  The  proper  walls  of  the  chambers,  moreover,  are 
thickened  by  an  extraneous  deposit  or  '  intermediate  skeleton,'  which 
sometimes  forms  radiating  outgrowths.  This  peculiarity  of  conforma- 
tion, however,  is  carried  much  further  in  the  genus  Calcarina,  which 
has  been  so  designated 
from  its  resemblance  to  a 
spur-rowel  (fig.  629).  The 
solid  club-shaped  append- 
ages with  which  this  shell 
is  provided  entirely  be- 
long to  the  '  intermediate 
skeleton '  £>,  which  is  quite 
independent  of  the  cham- 
bered structure  a ;  and  this 
is  nourished  by  a  set  of 
canals  containing  prolonga- 
tions of  the  sarcode-body 
which  not  only  furrow  the 
surface  of  these  appendages, 
but  are  seen  to  traverse 
their  interior  when  this  is 
laid  open  by  section,  as 
shown  at  c.  In  no  other 
recent  foraminifer  does  the 
'  canal  system '  attain  a  like 
development ;  and  its  dis- 
tribution in  this  minute 
shell,  which  has  been  made  out  by  careful  microscopic  study,  affords 
a  valuable  clue  to  its  meaning  in  the  gigantic  fossil  organism 
Eozoon  canadense.  The  resemblance  which  Calcarina  bears  to  the 
radiate  forms  of  Tinoporus  (fig.  623),  which  are  often  found  with 
them  in  the  same  dr edgings,  is  frequently  extremely  striking  ;  and 
in  their  early  growth  the  two  can  scarcely  be  distinguished,  since 
both  commence  in  a  '  Rotaline '  spire  with  radiating  appendages  ; 
but  whilst  the  successive  chambers  of  Calcarina  continue  to  be 
added  on  the  same  plane,  those  of  Tinoporus  are  heaped  up  in  less 
regular  piles. 

Certain  beds  of  Carboniferous  limestone  in  Russia  are  entirely 
made  up,  like  the  more  modern  Nummulitic  limestone,  of  an  aggre- 
gation of  the  remains  of  a  peculiar  type  of  Foraminifera,  to  which 
the  name  Fusulina  (indicative  of  its  fusiform  or  spindle-like  shape) 
has  been  given  (fig.  625).  In  general  aspect  and  plan  of  growth  it 
so  much  resembles  Alveolina  that  its  relationship  to  that  type  would 
scarcely  be  questioned  by  the  superficial  observer.  But  when  its 
mouth  is  examined  it  is  found  to  consist  of  a  single  slit  in  the 
middle  of  the  lip ;  and  the  interior,  instead  of  being  minutely 
divided  into  chamberlets,  is  found  to  consist  of  a  regular  series  of 
simple  chambers ;  while  from  each  of  these  proceeds  a  pair  of 


FIG.  624. — Section  of  Eotalia  Schroeteriana  near 
its  base  and  parallel  to  it,  showing,  «,  a,  the 
radiating  interseptal  canals ;  6,  their  internal 
bifurcations ;  c,  a  transverse  branch ;  d,  tubulated 
wall  of  the  chambers. 


826  .MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

elongated  extensions,  which  correspond  to  the  '  alar  prolongations ' 
of  other  spirally  growing  Foraminifera,  but  which,  instead  of  wrapping 
round  the  preceding  whorls,  are  prolonged  in  the  direction  of  the 
axis  of  the  spire,  those  of  each  whorl  projecting  beyond  those  of  the 
preceding,  so  that  the  shell  is  elongated  with  every  increase  in  its 
diameter.  Thus  it  appears  that  in  its  general  plan  of  growth 
Fusulina  bears  much  the  same  relation  to  a  symmetrical  Rotaline  or 
Nummuline  shell  that  Alveolina  bears  to  Orbiculina  ;  and  this  view 
of  its  affinities  is  fully  confirmed  by  the  Author's  microscopic  exami- 
nation of  the  structure  of  its  shell.  For  although  the  Fusulina 
limestone  of  Russia  has  undergone  a  degree  of  metamorphism, 
which  so  far  obscures  the  tubulation  of  its  component  shells  as  to 
prevent  him  from  confidently  affirming  it,  yet  the  appearances  he 
could  distinguish  were  decidedly  in  its  favour.  And  having  since 
received  from  Dr.  C.  A.  White  specimens  from  the  Upper  Coal 
Measures  of  Iowa,  U.S.A..  which  are  in  a  much  more  perfect  state  of 


FIG.  625. — Section  of  Fusulina  limestone. 

preservation,  he  is  able  to  state  with  certainty,  not  only  that  Fusulina 
is  tubular,  but  that  its  tubulation  is  of  the  large  coarse  nature  that 
marks  its  affinity  rather  to  the  Rotaline  than  to  the  Nummuline 
series.  This  type  is  of  peculiar  interest  as  having  long  been  regarded 
as  the  oldest  form  of  Foraminifera  which  was  known  to  have  occurred 
in  sufficient  abundance  to  form  rocks  by  the  aggregation  of  its  in- 
dividuals. It  will  be  presently  shown,  however,  that  in  point  both 
of  antiquity  and  of  importance  it  is  far  surpassed  by  another. 

Nummulmidae. — All  the  most  elaborately  constructed,  and  the 
greater  part  of  the  largest,  of  the  '  vitreous  '  Foraminifera  belong  to 
the  group  of  which  the  well-known  Nummulite  may  be  taken  as  the 
representative.  Various  plans  of  growth  prevail  in  the  family  ; 
but  its  distinguishing  characters  consist  in  the  completeness  of  the 
wall  that  surrounds  each  segment  of  the  body  (the  septa  being 
generally  double  instead  of  single),  the  density  and  fine  porosity  of 
the  shell-substance,  and  the  presence  of  an  '  intermediate  skeleton/ 


POLYttTOMELLA  827 

with  a  *  canal  system  '  for  its  nutrition.  It  is  true  that  these  cha- 
racters are  also  exhibited  in  the  highest  of  the  Rotaline  series,  whilst 
they  are  deficient  in  the  genus  Amphistegina,  which  connects  the 
Nummuliiie  series  with  the  Rotaline  ;  but  the  occurrence  of  such 
modifications  in  their  border  forms  is  common  to  other  truly  natural 
groups.  With  the  exception  of  Amphistegina,  all  the  genera  of  this 
family  are  symmetrical  in  form,  the  spire  being  nautiloid  in  such 
as  follow  that  plan  of  growth,  whilst  in  those  which  follow  the 
cyclical  plan  there  is  a  constant  equality  on  the  two  sides  of  the 
median  plane  ;  but  in  AmphiAtgina  there  is  a  reversion  to  the 
Rotalian  type  in  the  turbinoid  form  of  its  spire,  as  in  the  characters 
already  specified,  although  its  general  conformity  to  the  Nummuline 
type  is  such  as  to  leave  no  reasonable  doubt  as  to  its  title  to  be 
placed  in  this  family.  Notwithstanding  the  want  of  symmetry  of 
its  spire,  it  accords  with  Operculina  and  Nummulites  in  having  its 
chambers  extended  by  '  alar  prolongations '  over  each  surface  of 
the  previous  whorl ;  but  on  the  under  side  these  prolongations  are 
almost  entirely  cut  off  from  the  principal  chambers,  and  are  so  dis- 
placed as  apparently  to  alternate  with  them  in  position,  so  that  M. 
d'Orbignv,  supposing  them  to  constitute  a  distinct  series  of  chambers, 
described  its  plan  of  growth  as  a  biserial  spire,  and  made  this  the 
character  of  a  separate  order.1 

The  existing  Nummulinidce  are  almost  entirely  restricted  to 
tropical  climates  ;  but  a  beautiful  little  form,  Polystomella  crispa, 
the  representative  of  a  genus  that  presents  the  most  regular  and 
complete  development  of  the  '  canal  system '  anywhere  to  be 
met  with,  is  common  on  our  own  coasts.  The  peculiar  surface- 
marking  shown  in  the  figure  consists  in  a  strongly  marked 
ridge-aiid-furrow  plication  of  the  shelly  wall  of  each  segment  along 
its  posterior  margin,  the  furrows  being  sometimes  so  deep  as  to 
resemble  fissures  opening  into  the  cavity  of  the  chamber  beneath. 
No  such  openings',  however,  exist,  the  only  communication  which 
the  sarcode-body  of  any  segment  has  with  the  exterior  being 
either  through  the  fine  tubuli  of  its  shelly  walls  or  through  the 
row  of  pores  that  are  seen  in  front  view  along  the  inner  margin 
of  the  septal  plane,  collectively  representing  a  fissured  aperture 
divided  by  minute  bridges  of  shell.  The  meaning  of  the  plication  of 
the  shelly  wall  comes  to  be  understood  when  we  examine  the  con- 
formation of  the  segments  of  the  sarcode-body,  which  may  be  seen 
in  the  common  Polystomella  crispa  by  dissolving  away  the  shell  of 
fresh  specimens  by  the  action  of  dilute  acid,  but  which  may  be  better 
studied  in  such  internal  casts  (fig.  620)  of  the  sarcode-body  and 
canal  system  of  the  large  P.  craticulata  of  the  Australian  coast  as 
may  sometimes  be  obtained  by  the  same  means  from  dead  shells 
which  have  undergone  infiltration  with  ferruginous  silicates.2  Here 

1  For  an  account  of  this  curious  modification  of  the  Nummuline  plan  of  growth, 
the  real  nature  of  which  was  first  elucidated  by  Messrs.  Parker  and  Rupert  Jones, 
see  the  Author's  Introduction  to  the  Study  of  the  Foraminifera  (published  by  the 
Ray  Society). 

2  It  was  by  Professor  Ehrenberg  that  the  existence  of  such  '  casts  '  in  the  Green- 
sands  of  various  geological  periods  (from  the  Silurian  to  the  Tertiary)  was  first 
pointed  out,   in   his    memoir    '  Ueber   den   Griinsand    und   seine  Erlauterung   des 


828  MICKOSCOPIC   FORMS   OF  ANIMAL  LIFE 

we  see  that  the  segments  of  the  sarcode-body  are  smooth  along  their 
anterior  edge  b,  61,  but  that  along  their  posterior  edge,  a,  they  are 
prolonged  backwards  into  a  set  of  '  retral  processes  ; '  and  these  pro- 
cesses lie  under  the  ridges  of  the  shell,  whilst  the  shelly  wall  dips 
down  in  to.  the  spaces  between  them,  so  as  to  form  the  furrows  seen 
on  the  surface.  The  connections  of  the  segments  by  stolons,  c,  c1, 
passing  through  the  pores  at  the  inner  margin  of  each  septum,  are 
also  admirably  displayed  in  such  *  casts.'  But  what  they  serve  most 
beautifully  to  demonstrate  is  the  canal  system,  of  which  the  distri- 
bution is  here  most  remarkably  complete  and  symmetrical.  At  d, 
dl,  d2  are  seen  three  turns  of  a  spiral  canal  which  passes  along  one 
end  of  all  the  segments  of  the  like  number  of  convolutions,  whilst  a 
corresponding  canal  is  found  on  the  side  which  in  the  figure  is  under- 
most ;  these  two  spires  are  connected  by  a  set  of  meridional  canals, 
e,  el,  e2,  which  pass  down  between  the  two  layers  of  the  septa  that 


FIG.  626.— Internal  cast  of  Polystomella  craticulata :  a,  retral  processes 
proceeding  from  the  posterior  margin  of  one  of  the  segments  ;  b,  bl,  smooth 
anterior  margin  of  the  same  segment ;  c,  c1,  stolons  connecting  successive 
segments,  and  uniting  themselves  with  the  diverging  branches  of  the  meri- 
dional canals;  d,  dlt  d2,  three  turns  of  one  of  the  spiral  canals;  e,  el,  e2, 
three  of  the  meridional  canals;  /,/1,/2,  their  diverging  branches. 

divide  the  segments ;  whilst  from  each  of  these  there  passes  off 
towards  the  surface  a  set  of  pairs  of  diverging  branches,/,/1,/2,  which 
open  upon  the  surface  along  the  two  sides  of  each  septal  band,  the 
external  openings  of  those  on  its  anterior  margin  being  in  the  fur- 
rows between  the  retral  processes  of  the  next  segment.  These  canals 
appear  to  be  occupied  in  the  living  state  by  prolongations  of  the 
sarcode-body  ;  and  the  diverging  branches  of  those  of  each  convolu- 
tion unite  themselves,  when  this  is  inclosed  by  another  convolution, 

organischen  Lebens,'  in  Abhandhmgen  der  konic/l.  Akad.  der  Wissenschaften, 
Berlin,  1855.  It  was  soon  afterwards  shown  by  the  late  Professor  Bailey  (Quart.  Journ. 
Microsc.  Sci.  vol.  v.  1857,  p.  83)  that  the  like  infiltration  occasionally  takes  place  in 
recent  Foraminifera,  enabling  similar  '  casts  '  to  be  obtained  from  them  by  the  solu- 
tion of  their  shells  in  dilute  acid  ;  the  Author,  as  well  as  Messrs.  Parker  and  Rupert 
Jones,  soon  afterwards  obtained  most  beautiful  and  complete  internal  casts  from 
recent  Foraminifera  brought  from  various  localities.  A  number  of  Greensands  yield- 
ing similar  casts  were  collected  on  the  '  Challenger '  Expedition,  the  most  notable  from 
the  coast  of  Australia. 


POLYSTOMELLA 


829 


with  the  stolon  processes  connecting  the  successive  segments  of  the 
latter,  as  seen  at  c[.  There  can  be  little  doubt  that  this  remarkable 
development  of  the  canal  system  has  reference  to  the  unusual  amount 
of  shell -substance  which  is  deposited  as  an  '  intermediate  skeleton ' 
upon  the  layer  that  forms  the  proper  walls  of  the  chambers,  and 


FIG.  627. — C y clod ypeus— external  surface  and  vertical  and  horizontal  sections. 

which  fills  up  with  a  solid  '  boss '  what  would  otherwise  be  the  de- 
pression at  the  umbilicus  of  the  spire.  The  substance  of  this  '  boss ' 
is  traversed  by  a  set  of  straight  canals,  which  pass  directly  from  the 
spiral  canal  beneath,  towards  the  external  surface,  where  they  open 
in  little  pits,  as  is  shown  in  Plate  XIX,  23,  the  umbilical  boss 
in  P.  crispa,  ho\vever,  being  much  smaller  in  proportion  than  it 


FIG.  628. — Operculina  laid  open  to  show  its  internal  structure :  a,  marginal 
cord  seen  in  cross-section  at  a';  6,  b,  external  walls  of  the  chambers; 
c,  c,  cavities  of  the  chambers ;  c',  c',  their  alar  prolongations ;  d,  <7,  septa 
divided  at  d'  d'  and  at  d"  so  as  to  lay  open  the  interseptal  canals,  the 
general  distribution  of  which  is  seen  in  the  septa  e,  e ;  the  lines  radiating 
from  e,  e  point  to  the  secondary  pores  ;  g,  g,  non-tubular  columns. 

is  in  P.  craticulata.  There  is  a  group  of  Foraminifera  to  which  the 
term  Nonionina  is  properly  applicable,  that  is  probably  to  be  con- 
sidered as  a  sub-genus  of  Polystomella,  agreeing  with  it  in  its  general 
conformation,  and  especially  in  the  distribution  of  its  canal  system, 
but  differing  in  its  aperture,  which  is  here  a  single  fissure  at  the 
inner  edge  of  the  septal  plane,  and  in  the  absence  of  the  '  retral 


830  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

processes '  of  the  segments  of  the  sarcode-body,  the  external  walls  of 
the  chambers  being  smooth.  This  form  constitutes  a  transition  to 
the  ordinary  Nummuline  type,  of  which  Polysiomella  is  a  more  aber- 
rant modification. 

The  Nummuline  type  is  most  characteristically  represented  at  the 
present  time  by  the  genus  Operculina,  which  is  so  intimately  united 
to  the  true  Nummulite  by  intermediate  forms  that  it  is  not  easy  to 
separate  the  two,  notwithstanding  that  their  typical  examples  are 
widely  dissimilar.  The  former  genus  (fig.  628)  is  represented  on  our 
own  coast  and  in  northern  seas  by  very  small  and  feeble  forms,  but 
it  attains  a  much  higher  development  in  the  tropics,  where  its 
diameter  sometimes  reaches  one-fourth  of  an  inch.  The  shell  is  a 
flattened  nautiloid  spire,  the  breadth  of  whose  earlier  convolutions 
increases  in  a  regular  progression,  but  of  which  the  last  convolution 
(in  full-grown  specimens)  usually  flattens  itself  out  like  that  of 
Peneroplis,  so  as  to  be  very  much  broader  than  the  preceding.  The 
external  walls  of  the  chambers,  arching  over  the  .spaces  between  the 
septa,  are  seen  at  6,  b ;  and  these  are  bounded  at  the  outer  edge  of 


FIG.  629. — Calcarina  laid  open  to  show  its  internal  structure  :  a,  chambered 
portion ;  6,  intermediate  skeleton ;  c,  one  of  the  radiating  prolongations 
proceeding  from  it,  with  extensions  of  the  canal  system. 

each  convolution  by  a  peculiar  band,  «,  termed  the  '  marginal  cord.' 
This  cord,  instead  of  being  perforated  by  minute  tubuli  like  those 
which  pass  from  the  inner  to  the  outer  surface  of  the  chamber- walls 
without  division  or  inosculation  (fig.  632),  is  traversed  by  a  system 
of  comparatively  large  inosculating  passages  seen  in  cross-section  at 
a',  and  these  form  part  of  the  canal  system  to  be  presently  de- 
scribed. The  principal  cavities  of  the  chambers  are  seen  at  c,  c ; 
while  the  '  alar  prolongations '  of  those  cavities  over  the  surface  of 
the  preceding  whorl  are  shown  at  c',  c' '.  The  chambers  are  separated 
by  the  septa  d,  d,  d,  formed  of  two  laminae  of  shell,  one  belonging 
to  each  chamber,  and  having  spaces  between  them  in  which  lie  the 
'  interseptal  canals,'  whose  general  distribution  is  seen  in  the  septa 
marked  e,  e,  and  whose  smaller  branches  are  seen  irregularly  divided 
in  the  septa  d',  d',  whilst  in  the  septum  d"  one  of  the  principal 
trunks  is  laid  open  through  its  whole  length.  At  the  approach  Of 
each  septum  to  the  marginal  cord  of  the  preceding  is  seen  the 
narrow  fissure  which  constitutes  the  principal  aperture  of  commimi- 


NUMMULITES  831 

cation  between  the  chambers  ;  in  most  of  the  septa,  however,  there 
are  also  some  isolated  pores  (to  which  the  lines  point  that  radiate 
from  e,  e)  varying  both  in  number  and  position.  The  inter septal 
canals  of  each  septum  take  their  departure  at  its  inner  extremity 
from  a  pair  of  spiral  canals,  of  which  one  passes  along  each  side  of 
the  marginal  cord  ;  and  they  communicate  at  their  outer  extremity 
with  the  canal  system  of  the  '  marginal  cord,'  as  shown  in  fig.  634. 
The  external  walls  of  the  chambers  are  composed  of  the  same  finely 
tubular  shell-substance  that  forms  them  in  the  Nummulite  ;  but,  as 
in  that  genus,  not  only  are  the  Septa  themselves  composed  of  vitreous 
non-tubular  substance,  but  that  wrhich  lies  over  them,  continuing 
them  to  the  surface  of  the  shell,  has  the  same  character,  showing 
itself  externally  in  the  form  sometimes  of  continuous  ridges,  some- 
times of  rows  of  tubercles,  which  mark  the  position  of  the  septa 
beneath.  These  non-tubular  plates  or  columns  are  often  traversed 
by  branches  of  the  canal  system,  as  seen  at  </,  g.  Similar  columns 
of  non-tubular  substance,  of  which  the  ."ummits  show  themselves  as 
tubercles  on  the  surface,  are  not  unfrequently  seen  between  the 
septal  bands,  giving  a  variation  to  the  surface-marking  which,  taken 
in  conjunction  with  variations  in  general  conformation,  might  be 
fairly  held  sufficient  to  characterise  distinct  species,  were  it  not  that 
on  a  comparison  of  a  great  number  of  specimens  these  variations 
are  found  to  be  so  gradational  that  no  distinct  line  of  demarcation 
can  be  drawn  between  the  individuals  which  present  them. 

The  genus  Nummulites,  though  represented  at  the  present  time 
by  small  and  comparatively  infrequent  examples,  was  formerly  de- 
veloped to  a  vast  extent,  the  Nummulitic  limestone,  chiefly  made  up 
by  the  aggregation  of  its  remains  (the  material  of  which  the  Pyramids 
are  built),  forming  a  band,  often  1,800  miles  in  breadth  and  frequently 
of  enormous  thickness,  that  may  be  traced  from  the  Atlantic  shores 
of  Europe  and  Africa,  through  Western  Asia  to  Northern  India  and 
China,  and  likewise  over  vast  areas  of  North  America  (fig.  630). 
The  diameter  of  a  large  proportion  of  fossil  Nummulites  ranges 
between  half  an  inch  and  an  inch ;  but  there  are  some  whose 
diameter  does  not  exceed  T^th  of  an  inch,  whilst  others  attain  the 
gigantic  diameter  of  4^  inches.  Their  typical  form  is  that  of  a 
double-convex  lens  ;  but  sometimes  it  much  more  nearly  approaches 
the  globular  shape,  whilst  in  other  cases  it  is  very  much  flattened  ; 
and  great  differences  exist  in  this  respect  among  individuals  of  what 
must  be  accounted  one  and  the  same  species.  Although  there  are 
some  Nummulites  which  closely  approximate  Opereulince  in  their 
mode  of  growth,  yet  the  typical  forms  of  this  genus  present  certain 
well-marked  distinctive  peculiarities.  Each  convolution  is  so  com- 
pletely invested  by  that  which  succeeds  it.  and  the  external  wall  or 
spiral  lamina  of  the  new  convolution  is  so  completely  separated  from 
that  of  the  convolution  it  incloses  by  the  '  alar  prolongations '  of  its 
own  chambers  (the  peculiar  arrangement  of  which  will  be  presently 
described),  that  the  spire  is  scarcely  if  at  all  visible  on  the  external 
surface.  It  is  brought  into  view,  however,  by  splitting  the  Num- 
mulite  through  the  median  plane,  which  may  often  be  accom- 
plished simply  by  striking  it  on  one  edge  with  a  hammer,  the  opposite 


MICROSCOPIC  FORMS   OF  ANIMAL  LIFE 


edge  being  placed  on  a  firm  support  ;  or,  if  this  method  should  not 
succeed,  by  heating  it  in  the  flame  of  a  spirit-lamp,  and  then  throw- 
ing it  into  cold  water  or  striking  it  edgeways.  Nummulites  usually 
show  many  more  turns,  and  a  more  gradual  rate  of  increase  in  the 
breadth  of  the  spire,  than  Foraminifera  generally  :  this  will  be  appa- 
rent from  an  examination  of  the  vertical  section  shown  in  fig.  631, 
which  is  taken  from  one  of  the  commonest  and  most  characteristic 


FIG.  630. — A,  piece  of  Nummulitic  limestone  from  Pyrenees, 
showing  Nummulites  laid  open  by  fracture  through  median 
plane ;  B,  Orbitoides. 

fossil  examples  of  the  genus,  and  which  shows  no  fewer  than  ten  convo- 
lutions in  a  fragment  that  does  not  nearly  extend  to  the  centre  of  the 
spire.  This  section  also  shows  the  complete  inclosure  of  the  older 
convolutions  by  the  newer,  and  the  interposition  of  the  alar  prolonga- 
tions of  the  chambers  between  the  successive  layers  of  the  spiral 
lamina.  These  prolongations  are  variously  arranged  in  different 


FIG.  631. — Vertical  section  of  portion  of  Nummulites  Icemgata :  a,  margin 
of  external  whorl ;  b,  one  of  the  outer  row  of  chambers  ;  c,  c,  whorl  invested 
by  a  ;  d,  one  of  the  chambers  of  the  fourth  whorl  from  the  margin ;  e,  e', 
marginal  portions  of  the  inclosed  whorls ;  /,  investing  portions  of  outer 
whorl ;  g,  g,  spaces  left  between  the  investing  portion  of  successive  whorls  ; 
h,  h,  sections  of  the  partitions  dividing  these. 

examples  of  the  genus  ;  thus  in  some,  as  N.  distans,  they  keep  their 
own  separate  course,  all  tending  radially  towards  the  centre  ;  in 
others,  as  JV.  Icevigata,  their  partitions  inosculate  with  each  other,  so 
as  to  divide  the  space  intervening  between  each  layer  and  the  next 
into  an  irregular  network,  presenting  in  vertical  section  the  appear- 
ance shown  in  fig.  631  ;  whilst  in  N.  garansensis  they  are  broken 


NUMMCLITES 


833 


up  into  a  number  of  chamberlets  having  little  or  no  direct  communi- 
cation with  each  other. 

Notwithstanding  that   the  inner  chambers  are  thus  so  deeply 
buried  in  the  mass  of  investing  whorls,  yet  there  is  evidence  that 


FIG.  632. — Portion  of  a  thin,  section  of  Nummulites  Icevigata  taken  in  the 
direction  of  the  preceding,  highly  magnified  to  show  the  minute  structure 
of  the  shell :  a,  a,  portions  of  the  ordinary  shell-substance  traversed  by 
parallel  tubuli;  &,  b,  portions  forming  the  marginal  cord,  traversed  by 
diverging  and  larger  tubuli ;  c,  one  of  the  chambers  laid  open ;  d,  d,  d, 
pillars  of  solid  substance  not  perforated  by  tubuli. 

the  segments  of  sarcode  which  they  contained  were  not  cut  off  from 
communication  with  the  exterior,  but  that  they  may  have  retained 
their  vitality  to  the  last.  The  shell  itself  is  almost  every- 
where minutely  porous,  being  penetrated  by  parallel  tubuli,  which 
pass  directly  from  one  surface  to  the  other.  These  tubes  are  shown, 
as  divided  lengthwise  by  a  vertical  section,  in  fig.  632,  a,  a  ;  whilst 
the  appearance  they  present  when  cut  across  in  a  horizontal  section 
is  shown  in  fig.  633,  the 
transparent  shell -substance 
«,  a,  a  being  closely  dotted 
with  minute  punctatioiis 
which  mark  their  orifices. 
In  that  portion  of  the  shell, 
however,  which  forms  the 
margin  of  each  whorl  (fig. 
632,  6,  b),  the  tubes  are  larger, 
and  diverge  from  each  other 
at  greater  intervals ;  and  it 
is  shown  by  horizontal  sections  FIG.  633. — Portion  of  horizontal  section  of 

that  they  communicate  freely      Nttmmnlites (showing  the  structure  of  the 

J  J         walls   and  of  the   septa  of  the  chambers: 

«>  a,  a,  portion  of  the  wall  covering  three 
chambers,  the  punctations  of  which  are  the 
orifices  of  tubuli ;  b,  b  septa  between  these 
chambers  containing  canals  which  send  out 
lateral  branches,  c,  c,  entering  the  chambers 
by  larger  orifices,  one  of  which  is  seen  at  d. 


with  each  other  laterally,  so 
as  to  form  a  network  such  as 
is  seen  at  6,  b,  fig.  634.  At 
certain  other  points,  d,  d,  d, 
fig.  632,  the  shell -substance 


is  not  perforated  by  tubes,  but 

is  peculiarly  dense  in  its  texture,  forming  solid  pillars  which  seem 
to  strengthen  the  other  parts ;  and  in  Nummulites  whose  surfaces 
have  been  much  exposed  to  attrition,  it  commonly  happens  that  the 
pillars  of  the  superficial  layer,  being  harder  than  the  ordinary  shell- 
substance,  and  being  consequently  less  worn  down,  are  left  as 

3n 


834 


MICKOSCOPIC   FORMS   OF  ANIMAL   LIFE 


FiG."634,— Internal  cast  of  two  of  the  cham- 
bers of  Nummulites  striata,  with  the 
network  of  canals,  &,  in  the  marginal 
cord  communicating  with  canals  passing 
between  the  chambers. 


prominences,  the  presence  of  which  has  often  been  accounted  (but 
erroneously)  as  a  specific  character.  The  successive  chambers  of  the 
same  whorl  communicate  with  each  other  by  a  passage  left  between 

the  inner  edge  of  the  partition 
that  separates  them  and  the 
'  marginal  cord '  of  the  pre- 
ceding whorl ;  this  passage  is 
sometimes  a  single  large  broad 
aperture,  but  is  more  com- 
monly formed  by  the  more  or 
less  complete  coalescence  of 
several  separate  perforations, 
as  is  seen  in  fig.  631,  b.  There 
is  also,  as  in  Operculina,  a 
variable  number  of  isolated 
pores  in  most  of  the  septa, 
forming  a  secondary  means  of 
communication  between  the 
chambers.  The  canal  system 
of  Nummulites  seems  to  be  ar- 
ranged upon  essentially  the 
same  plan  as  that  of  Oper- 
culina ;  its  passages,  however, 

are  usually  more  or  less  obscured  by  fossilising  material.  A  careful 
examination  will  generally  disclose  traces  of  them  in  the  middle  of 
the  partitions  that  divide  the  chambers  (fig.  633.  b,  b),  while  from 
these  may  be  seen  to  proceed  the  lateral  branches  (c,  c),  which,  after 
burrowing  (so  to  speak)  in  the  walls  of  the  chambers,  enter  them 
by  large  orifices  (d).  These  '  interseptal '  canals,  and  their  communi- 
cation with  the  inosculating  system  of  passages  excavated  in  the 

marginal  cord,  are  extremely 
well  seen  in  the  *  internal  cast ' 
represented  in  fig.  634. 

A  very  interesting  modifi- 
cation of  the  Nummuline  type 
is  presented  in  the  genus 
Heterostegina  (fig.  635),  which 
bears  a  very  strong  resemblance 
to  Orbiculina  in  its  plan  of 
growth,  whilst  in  every  other 
respect  it  is  essentially  dif- 
ferent. If  the  principal  cham- 
bers of  an  Operculina  were 
divided  into  chamberlets  by 
secondary. partitions  in  a  direc- 
tion transverse  to  that  of  the 
principal  septa,  it  would  be 
converted  into  a  Heterostegina, 

just  as  a  Peneroplis  would  be  converted  by  the  like  subdivision  into 
an  Orbiculina.  Moreover,  we  see  in  Heterostegina,  as  in  Orbiculina, 
a  great  tendency  to  the  opening  out  of  the  spipe  with  the  advance  of 


NUMMULITES 


835 


age ;  so  that  the  apertural  margin  extends  round  a  large  part  of  the 
shell,  which  thus  tends  to  become  discoidal.  And  it  is  not  a  little 
curious  that  we  have  in  this  series  another  form,  Cycloclypeus,  which 
bears  exactly  the  same  relation  to  Heterostegina  that  Orbitolites  does 
to  Orbiculina,  in  being  constructed  upon  the  cyclical  plan  from  the 
commencement,  its  chamberlets  being  arranged  in  rings  around  a 
central  chamber.  This  remarkable  genus,  at  present  only  known  in 
the  recent  condition  by  specimens  dredged  at  considerable  depths 
from  the  coast  of  Borneo  and  at  bne  or  two  points  in  the  Western 
Pacific,  is  perhaps  the  largest  of  existing  Foraminifera,  some  speci- 
mens of  its  discs  in  the  British  Museum  having  a  diameter  of  two 
and  a  quarter  inches.  Notwithstanding  the  difference  of  its  plan 
of  growth,  it  so  precisely  accords  with 
the  Nummuline  type  in  every  cha- 
racter which  essentially  distinguishes 
the  genus  that  there  cannot  be  a 
doubt  of  the  intimacy  of  their  rela- 
tionship. It  will  be  seen  from  the 
examination  of  that  portion  of  the 
figure  which  shows  Cycloclypeus  in 
vertical  section  that  the  solid  layers 
of  shell  by  which  the  chambered  por- 
tion is  inclosed  are  so  much  thicker, 
and  consist  of  so  many  more  lamellae 
in  the  central  portion  of  the  disc 
than  they  do  nearer  its  edge,  that 
new  lamellae  must  be  progressively 
added  to  the  surfaces  of  the  disc 
concurrently  with  the  addition  of  new 
rings  of  chamberlets  to  its  margin. 
These  lamellae,  however,  are  closely 
applied  one  to  the  other  without  any 
intervening  spaces  ;  and  they  are  all  FIG.  636.— Section  of  Orlitoldcs 
traversed  by  columns  of  non-tubular  Fortisii,  parallel  to  ^  the  surface, 
substance,  which  spring  from  the 
septal  bands,  and  gradually  increase  layer.' 
in  diameter  with  their  approach  to 
the  surface,  from  which  they  project  in  the  central  portion  of  the 
disc  as  glistening  tubercles.1 

The  Nummulitic  limestone  of  certain  localities  (as  the  south-west 
of  France,  Southern  Germany,  North-Eastern  India,  &c.)  contains  a 
vast  abundance  of  discoidal  bodies  termed  Orbitoides  (fig.  630,  B), 
which  are  so  similar  to  Nummulites  as  to  have  been  taken  for  them, 
but  which  bear  a  mueh  closer  resemblance  to  Cycloclypeus.  These 
are  only  known  in  the  fossil  state  ;  and  their  structure  can  only  be 
ascertained  by  the  examination  of  sections  thin  enough  to  be  trans- 
lucent. When  one  of  these  discs  (which  vary  in  size,  in  different 
species,  from  that  of  a  fourpenny-piece  to  that  of  half  a  crown  or 
even  larger)  is  rubbed  down  so  as  to  display  its  internal  organisation. 

1  Dr.  L.  Rhumbler's  '  Entwurf  eines  natiirlichen  Systems  der  Thalamophoren ' 
(Nachr.  Ges.  Gottingen.  1891,  p.  51)  is  chiefly  based  on  palaeontological  considerations. 

3  H  2 


836 


MICROSCOPIC   FORMS   OF   ANIMAL  LIFE 


two  different  kinds  of  structure  are  usually  seen  in  it,  one  being 
composed  of  chamberlets  of  very  definite  form,  quadrangular  in  some 
species,  circular  in  others,  arranged  with  a  general  but  not  constant 
regularity  in  concentric  circles  (figs.  636,  637,  b,  b) ;  the  other,  less 


FIG.  637. — Portions  of  the  section  of  Orbitoides  Fortisii,  shown  in  fig.  636, 
more  highly  magnified  :  a,  superficial  layer ;  b,  median  layer. 

transparent,  being  formed  of  minuter  chamberlets  which  have  no 
such  constancy  of  form,  but  which  might  almost  be  taken  for  the 
pieces  of  a  dissected  map  (a,  a).  In  the  upper  and  lower  walls  of 
these  last,  minute  punctations  may  be  observed,  which  seem  to  be 


FIG.  638.— Vertical  section  of  Orbitoides  Fortisii,  showing  the  large 
central  chamber  at  a,  and  the  median  layer  surrounding  it, 
covered  above  and  below  by  the  superficial  layers. 

the  orifices  of  connecting  tubes  whereby  they  are  perforated.  The 
relations  of  these  two  kinds  of  structure  to  each  other  are  made 
evident  by  the  examination  of  a  vertical  section  (fig.  638),  which 
shows  that  the  portion  b,  figs.  636,  637,  forms  the  median  plane, 
its  concentric  circles  of  chamberlets  being  arranged  round  a  large 
central  chamber,  as  in  Cycloclypeus ;  whilst  the  chamberlets  of  the 

portion  a  are  irregularly  superposed  one 
upon  the  other,  so  as  to  form  several 
layers  which  are  most  numerous  towards 
the  centre  of  the  disc,  and  thin  away 
gradually  towards  its  margin.  The  dis- 
position and  connections  of  the  cham- 
berlets of  the  median  layer  in  Orbitoides 
seem  to  correspond  very  closely  with 
those  which  have  been  already  described 
as  prevailing  in  Cycloclypeus,  the  most 
satisfactory  indications  to  this  effect 
being  furnished  by  the  silicious  '  internal 
casts '  to  be  met  with  in  certain  Green- 
sands,  which  afford  a  model  of  the  sar- 
code-body  of  the  animal.  In  such  a 

fragment  (fig.  639)  we  recognise  the  chamberlets  of  three  successive 
zones,  a,  af,  a",  each  of  which  seems  normally  to  communicate  by 
one  or  two  passages  with  the  chamberlets  of  the  zone  internal  and 
external  to  its  own  ;  whilst  between  the  chamberlets  of  the  same 


FIG.  639. — Internal  cast  of  por- 
tion of  median  plane  of  Orbi- 
toides Fortisii,  showing,  at 
a  a,  a'  a',  a",  a",  six  chambers 
of  each  of  three  zones,  with 
their  mutual  communications ; 
and  at  b  b,  b'  b',  b"  b",  portions 
of  three  annular  canals. 


EOZOON  837 

zone  there  seems  to  be  no  direct  connection.  They  are  brought  into 
relation,  however,  by  means  of  annular  canals,  which  seem  to  repre- 
sent the  spiral  canals  of  the  Nummulite,  and  of  which  the  '  internal 
casts '  are  seen  at  b  b,  bf  bf,  bff  ~b" . 

A  most  remarkable  fossil,  referable  to  the  foraminiferal  type, 
was  discovered  in  strata  much  older  than  the  very  earliest 
that  were  previously  known  to  contain  organic  remains  ;  and  the 
determination  of  its  real  character  may  be  regarded  as  one  of 
the  most  interesting  results  of  microscopic  research.  This  fossil, 
which  has  received  the  name  Eozoon  canadense  (fig.  640),  is  found 
in  beds  of  Serpentine  limestone  that  occur  near  the  base  of  the 


FIG.  640. — Vertical  section  of  Eozoon   canadense,  showing  alternation  of 
calcareous  (light)  and  serpentinous  (dark)  lamellae. 

Laurentian  formation  of  Canada,  which  has  its  parallel  in  Europe  in 
the  '  fundamental  gneiss '  of  Bohemia  and  Bavaria,  and  in  the  very 
earliest  stratified  rocks  of  Scandinavia  and  Scotland.  These  beds 
are  found  in  many  parts  to  contain  masses  of  considerable  size,  but 
usually  of  indeterminate  form,  disposed  after  the  manner  of  an 
ancient  coral  reef,  and  consisting  of  alternating  layers — frequently 
numbering  from  50  to  100 — of  carbonate  of  lime  and  serpentine 
(silicate  of  magnesia).  The  regularity  of  this  alternation  and  the 
fact  that  it  presents  itself  also  between  other  calcareous  and  silicious 
minerals  having  led  to  a  suspicion  that  it  had  its  origin  in  organic 
structure,  thin  sections  of  well-preserved  specimens  were  submitted 
to  microscopic  examination  by  the  late  Sir  W.  Dawson,  of  Montreal, 


838  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

who  at  once  recognised  its  foraminiferal  nature,1  the  calcareous 
layers  presenting  the  characteristic  appearances  of  true  shell,  so  dis- 
posed as  to  form  an  irregularly  chambered  structure,  and  frequently 
traversed  by  systems  of  ramifying  canals  corresponding  to  those  of 
Calcarina  ;  whilst  the  serpentinous  or  other  silicious  layers  were 
regarded  by  him  as  having  been  formed  by  the  infiltration  of  sili- 
cates in  solution  into  the  cavities  originally  occupied  by  the  sarcode- 
body  of  the  animal — a  process  of  whose  occurrence  at  various  geo- 
logical periods,  and  also  at  the  present  time,  abundant  evidence  has 
already  been  adduced.  Having  himself  taken  up  the  investigation 
(at  the  instance  of  Sir  William  Logan),  the  Author  was  not  only  able 
to  confirm  Dr.  Dawson's  conclusions,  but  to  adduce  new  and  im- 
portant evidence  in  support  of  them.2  Although  this  determination 
has  been  called  in  question,  on  the  ground  that  some  resemblance  to 
the  supposed  organic  structure  of  Eozoon  is  presented  by  bodies  of 
purely  mineral  origin,3  yet,  as  it  lias  been  accepted  not  only  by  most 
of  those  whose  knowledge  of  foraminiferal  structure  gives  weight  to 
their  judgment  (among  whom  the  late  Professor  Max  Schultze  may 
be  specially  named),  but  also  by  geologists  who  have  specially 
studied  the  micro-mineralogical  structure  of  the  older  Metamorphic 
rocks,4  the  Author  feels  justified  in  here  describing  Eozoon  as 
he  believes  it  to  have  existed  when  it  originally  extended  itself  as 
an  animal  growth  over  vast  areas  of  the  sea-bottom  in  the  Laurent  inn 
epoch. 

Whilst  essentially  belonging  to  the  Nummuline  group,  in  virtue 
of  the  fine  tubulation  of  the  shelly  layers  forming  the  '  proper  wall ' 
of  its  chambers,  Eozoon  is  related  to  various  types  of  recent  Forti- 
minifera  in  its  other  characters.  For  in  its  indeterminate  zoophytic 
mode  of  growth  it  agrees  with  Polytrema  in  the  incomplete  separa- 
tion of  its  chambers  ;  it  has  its  parallel  in  Carpentaria  ;  whilst  in  the 
high  development  of  its  '  intermediate  skeleton  '  and  of  the  '  canal 
system  '  by  which  this  is  formed  and  nourished,  it  finds  its  nearest 
representative  in  Calcarina.  Its  calcareous  layers  were  so  super- 
posed one  upon  another  as  to  include  between  them  a  succession 

1  This  recognition  was  due,  as  Dr.  Dawson  has  explicitly  stated  in  his  original 
memoir  (Quart.  Journ.  of  Geol.  Soc.  vol.  xxi.  p»54),  to  his  acquaintance,  not  merely 
with  the  Author's  previous  researches  on  the  minute  structure  of  the  Foramiiiifera, 
but  with  the  special  characters  presented  by  thin  sections  of  Calcarina  which  had 
been  transmitted  to  him  by  the  Author.     Dr.  Dawson  has  given  an  account  of  the 
geological  and  mineral ogical  relations  of  Eozoon,  as  well  as  of  its  organic  structure,  in 
a  small  book  entitled  The  Dawn  of  Life. 

2  For  a  fuller  account  of  the  results  of  the  Author's  own  study  of  Eozoon,  and  of  the 
basis  on  which  the  above  reconstruction  is  founded,  see  his  papers  in  Quart.  Journ. 
of  Geol.  Soc.  vol.  xxi.  p.  59,  and  vol.  xxii.  p.  219,  and  in  the  Intellectual  Observer, 
vol.  vii.  1865,  p.  278 ;  and  his  '  Further  Researches '  in  Ann.  of  Nat.  Hist.  June  1874. 

5  See  the  memoirs  of  Professors  King  and  Rowney  in  Quart.  Journ.  of  Geol.  Soc. 
vol.  xxii.  p.  185,  &nd.Ann.  of  Nat.  Hist.  May  1874. 

4  Among  these  the  Author  is  permitted  to  mention  Professor  Geikie,  of  Edinburgh, 
who  has  thus  studied  the  older  rocks  of  Scotland,  and  Professor  Bonney  of  London,  who 
has  made  a  like  study  of  the  Cornish  and  other  Serpentines.  By  both  these  eminent 
authorities  he  is  assured  that  they  have  met  with  no  purely  mineral  structure  in  the 
least  resembling  Eozoon,  either  in  its  regular  alternation  of  calcareous  and  serpen- 
tinous lamellae,  or  in  the  dendritic  extensions  of  the  latter  into  the  former  ;  and  while 
they  accept  as  entirely  satisfactory  the  doctrine  of  its  organic  origin  maintained  by 
the  Author,  they  find  themselves  unable  to  conceive  of  any  inorganic  agency  by  which 
such  a  structure  could  have  been  produced. 


EOZOON 


839 


of  'storeys'  of  chambers  (fig.  641,  A1.  A1,  A2,  A2),  the  chambers 
of  each  '  storey '  usually  opening  one  into  another,  as  at  a,  a,  like 
apartments  en  suite,  but  being  occasionally  divided  by  complete  septa, 
as  at  b,  b.  These  septa  are  traversed  by  passages  of  communication 
between  the  chambers  which  they  separate,  resembling  those  which, 
in  existing  types,  are  occupied  by  stolons  connecting  together  the 
segments  of  the  sarcode-body.  Each  layer  of  shell  consists  of  two 
finely  tubulated  or  '  Nummuline  '  lamellae,  B,  B,  which  form  the 
boundaries  of  the  chambers  beneath  and  above,  serving  (so  to  speak) 
as  the  ceiling  of  the  former,  and  as  the  floor  of  the  latter ;  and  of 
an  intervening  deposit  of  homogeneous  shell-substance  C,  0,  which 
constitutes  the  'inter- 
mediate skeleton.'  The 
tubuli  of  this  'Num- 
muline '  layer  (fig.  643) 
are  usually  filled  up  (as 
in  the  Nummulites 
of  the  '  Nummulitic 
limestone  ')  by  mineral 
infiltration,  so  as  in 
transparent  sections  to 
present  a  fibrous  ap- 
pearance ;  but  it  for- 
tunately happens  that 
through  their  having 
in  some  cases  escaped 
infiltration  the  tubu- 

IflHnn      i«     ««      rh'efinM-         FlG-  641.— Portion  of  the  calcareous  shell  of  Eozoon 

canadense   as  it  would  appear  if   the  serpentine 

as  it  IS   even   in   recent  that  fills  its  chambers  were  dissolved  away:  A1,  A1, 

Nummuline  shells   (fio\  chambers  of  lower  storey  opening  into  each  other 

643),  bearing  a  singu- 
lar resemblance  in  its 
occasional  waviness  to 
that  of  the  crab's  claw. 
The  thickness  of  this 
interposed  layer  varies 
considerably  in  differ- 
ent parts  of  the  same  mass,  being  in  general  greatest  near  its 
base  and  progressively  diminishing  towards  its  upper  surface. 
The  '  intermediate  skeleton '  is  occasionally  traversed  by  large 
passages  (D),  which  seem  to  establish  a  connection  between  the 
successive  layers  of  chambers  ;  and  it  is  penetrated  by  arborescent 
systems  of  canals  (E,  E),  which  are  often  distributed  both  so 
extensively  and  so  minutely  through  its  substance  as  to  leave 
very  little  of  it  without  a  branch.  These  canals  take  their  origin, 
not  directly  from  the  chambers,  but  from  irregular  lacunas  or 
interspaces  between  the  outside  of  the  proper  chamber-walls  and 
the  'intermediate  skeleton,'  exactly  as  in  Calcarina,  the  exten- 
sions of  the  sarcode-body  which  occupied  them  having  apparently 
been  formed  by  the  coalescence  of  the  pseudopodial  filaments  that 
passed  through  the  tubulated  lamellae. 


at  a,  <z,  but  occasionally  separated  by  a  septum, 
6,  6 ;  A2,  A2,  chambers  of  upper  storey ;  B,  B, 
proper  walls  of  the  chambers,  formed  of  a  finely 
tubular  or  Nummuline  substance;  C,  C,  inter 
mediate  skeleton,  occasionally  traversed  by  large 
stolon-passages,  D,  connecting  the  chambers  of 
different  storeys,  and  penetrated  by  the  arbores- 
cent systems  of  canals,  E,  E,  E. 


840  MICKOSCOPIC   FORMS   OF   ANIMAL  LIFE 

In  the  fossilised  condition  in  which  Eozoon  is  most  commonly 
found,  not  only  the  cavities  of  the  chambers,  but  the  canal  systems 
to  their  smallest  ramifications  are  filled  up  by  the  silicious  infiltra- 
tion which  has  taken  the  place  of  the  original  sarcode-body,  as  in  the 
cases  already  cited,  and  thus  when  a  piece  of  this  fossil  is  subjected 
to  the  action  of  dilute  acid,  by  which  its  calcareous  portion  is  dis- 
solved away,  we  obtain  an  internal  cast  of  its  chambers  and  canal 
system  (fig.  642),  which,  though  altogether  dissimilar  in  arrangement, 
is  essentially  analogous  in  character  to  the  '  internal  casts '  repre- 
sented in  figs.  622,  626.  This  cast  presents  us,  therefore,  with  a 
model  in  hard  serpentine  of  the  soft  sarcode-body  which  originally 
occupied  the  chambers,  and  extended  itself  into  the  ramifying  canals, 
of  the  calcareous  shell ;  and,  like  that  of  Polystomella,  it  affords  an 
even  more  satisfactory  elucidation  of  the  relations  of  these  parts 
than  we  could  have  gained  from  the  study  of  the  living  organism. 


FIG.  642. — Decalcified  portion  of  Eozoon  canadense  shell,  showing  the  ser- 
pentinous  internal  cast  of  the  chambers,  canals,  and  tubuli  of  the  original, 
presenting  an  exact  model  of  the  animal  substance  which  originally  filled 
them. 

We  see  that  each  of  the  layers  of  serpentine,  forming  the  lower  part 
of  such  a  specimen,  is  made  up  of  a  number  of  coherent  segments, 
which  have  only  undergone  a  partial  separation ;  these  appear  to 
have  extended  themselves  horizontally  without  any  definite  limit, 
but  have  here  and  there  developed  new  segments  in  a  vertical  direc- 
tion, so  as  to  give  origin  to  new  layers.  In  the  spaces  between  these 
successive  layers,  which  were  originally  occupied  by  the  calcareous 
shell,  we  see  the  *  internal  casts '  of  the  branching  canal  system, 
which  give  us  the  exact  models  of  the  extensions  of  the  sarcode-body 
that  originally  passed  into  them.  But  this  is  not  all.  In  specimens 
in  which  the  Nummuline  layer  constituting  the  *  proper  wall '  of  the 
chambers  was  originally  well  preserved,  and  in  which  the  decalcifying 
process  has  been  carefully  managed  (so  as  not,  by  too  rapid  an  evolu- 
tion of  carbonic  acid  gas,  to  disturb  the  arrangement  of  the  serpen 
tinous  residuum),  that  layer  is  represented  by  a  thin  white  film 
covering  the  exposed  surfaces  of  the  segments  ;  the  superficial  aspect 


EOZOOX 


841 


of  which,  as  well  as  its  sectional  view,  is  shown  in  fig.  642.  And 
when  this  layer  is  examined  with  a  sufficient  magnifying  power  it  is 
found  to  consist  of  extremely  minute  needle-like  fibres  of  serpentine, 
which  sometimes  stand  upright,  parallel,  and  almost  in  contact  with 
each  other,  like  the  fibres  of  asbestos  (so  that  the  film  which  they 
form  has  been  termed  the  '  asbestiform  layer '),  but  which  are  fre- 
quently grouped  in  converging  brush-like  bundles,  so  as  to  be  very 
close  to  each  other  in  certain  spots  at  the  surface  of  the  film,  whilst 
widely  separated  in  others.  Now  these  fibres,  which  are  less  than 
IQQO  Oth  of  an  inch  in  diameter,  are  the  '  internal  casts  '  of  the  tubuli 
of  the  Nummuline  layer  (a  precise  parallel  to  them  being  presented  in 
the  '  internal  cast '  of  a  recent  Amphistegina  which  was  in  the  Author's 

Cession)  ;  and  their  arrangement  presents  all  the  varieties  which 
j  been  mentioned  as  existing  in  the  shells  of  Operculina.     Thus 


FIG.  643. — Vertical  section  of  a  portion  of  one  of  the  calcareous  lamellae  of 
Eozoon  canadense  :  a  a,  Nummuline  layer  perforated  by  parallel  tubuli, 
which  show  a  flexure  along  the  line  a'  a' ;  beneath  this  is  seen  the  inter- 
mediate skeleton,  c,  c,  traversed  by  the  large  canals,  b,  b,  and  by  oblique 
cleavage  planes,  which  extend  also  into  the  Nummuline  layer. 

these  delicate  and  beautiful  silicious  fibres  represent  those  pseudo- 
podial  threads  of  sarcode  which  originally  traversed  the  minutely 
tubular  walls  of  the  chambers  ;  and  a  precise  model  of  the  most 
ancient  animal  of  which  we  have  any  knowledge,  notwithstanding 
the  extreme  softness  and  tenuity  of  its  substance,  is  thus  presented 
to  us  with  a  completeness  that  is  scarcely  even  approached  in  any 
later  fossil. 

In  the  upper  part  of  the  '  decalcified '  specimen  shown  in  fig.  642 
it  is  to  be  observed  that  the  segments  are  confusedly  heaped  together 
instead  of  being  regularly  arranged  in  layers,  the  lamellated  mode 
of  growth  having  given  place  to  the  acervuline.  This  change  is  by 
no  means  uncommon  among  Foraminifera,  an  irregular  piling 
together  of  the  chambers  being  frequently  met  with  in  the  later 
growth  of  types  whose  earlier  increase  takes  place  upon  some  much 


842        MICKOSCOPIC  FORMS  OF  ANIMAL  LIFE 

more  definite  plan.  After  what  fashion  the  earliest  development  of 
Eozoon  took  place,  we  have  at  present  no  knowledge  whatever ;  but 
in  a  young  specimen  which  has  been  recently  discovered  it  is  obvious 
that  each  successive  *  storey  '  of  chambers  was  limited  by  the  closing 
in  of  the  shelly  layer  at  its  edges,  so  as  to  give  to  the  entire  fabric  a 
definite  form  closely  resembling  that  of  a  straightened  Peneroplis. 
Thus  it  is  obvious  that  the  chief  peculiarity  of  Eozoon  lay  in  its 
capacity  for  indefinite  extension,  so  that  the  product  of  a  single  germ 
might  attain  a  size  comparable  to  that  of  a  massive  coral.  Now  this, 
it  will  be  observed,  is  simply  due  to  the  fact  that  its  increase  by 
gemmation  takes  place  continuously,  the  new  segments  successively 
budded  off  remaining  in  connection  with  the  original  stock,  instead 
of  detaching  themselves  from  it  as  in  Foraminifera  generally.  Thus 
the  little  Globigerina  forms  a  shell  of  which  the  number  of  chambers 
does  not  usually  seem  to  increase  beyond  sixteen,  any  additional 
segments  detaching  themselves  so  as  to  form  separate  shells  ;  but  by 
the  repetition  of  this  multiplication  the  sea-bottom  of  large  areas  of 
the  Atlantic  Ocean  at  the  present  time  has  come  to  be  covered  with 
accumulations  of  Globigerince,  which,  if  fossilised,  would  form  beds  of 
limestone  not  less  massive  than  those  which  have  had  their  origin  in 
the  growth  of  Eozoon.  The  difference  between  the  two  modes  of 
increase  may  be  compared  to  the  difference  between  a  herb  and  a 
tree.  For  in  the  herb  the  individual  organism  never  attains  any 
considerable  size,  its  extension  by  gemmation  being  limited  ;  though 
the  aggregation  of  individuals  produced  by  the  detachment  of  its  buds 
(as  in  a  potato-field)  may  give  rise  to  a  mass  of  vegetation  as  great 
as  that  formed  in  the  largest  tree  by  the  continuous  putting  forth  of 
new  buds. 

It  has  been  hitherto  only  in  the  Laurentian  serpentine  lime- 
stone of  Canada  that  Eozoon  has  presented  itself  in  such  a  state  of 
preservation  as  fully  to  justify  the  assumption  of  its  organic  nature. 
But  from  the  greater  or  less  resemblance  which  is  presented  to  this 
by  serpentine-limestones  occurring  in  various  localities  among  strata 
that  seem  the  geological  equivalents  of  the  Canadian  Laurentians,  it 
seems  a  justifiable  conclusion  that  this  type  was  very  generally  dif- 
fused in  the  earlier  ages  of  the  earth's  history,  and  that  it  had  a 
large  (and  probably  the  chief)  share  in  the  production  of  the  most 
ancient  calcareous  strata,  separating  carbonate  of  lime  from  its  solu- 
tion in  ocean  water,  in  the  same  manner  as  do  the  polypes  by  whose 
growth  coral  reefs  and  islands  are  being  upraised  at  the  present  time. 

An  elaborate  work,  '  Der  Bau  des  Eozoon  Canadense'  (1878), 
has  been  recently  published  by  Professor  Mb'bius,  of  Kiel,  in  which 
the  structure  of  Eozoon  is  compared  with  that  of  various  types  of 
Foraminifera,  and,  as  it  differs  from  that  of  every  one  of  them,  is 
affirmed  not  to  be  organic  at  all,  but  purely  mineral.  Upon  this  the 
Author  would  remark,  that  if  the  validity  of  this  mode  of  reasoning 
be  admitted,  any  fossil  whose  structure  does  not  correspond  with  that 
of  some  existing  type  is  to  be  similarly  rejected.  Thus  the  Stroma- 
topora  of  Silurian  and  Devonian  rocks,  which  some  palaeontologists 
regard  as  a  coral,  others  as  polyzoary,  others  as  a  calcareous  sponge, 
and  others  as  a  foraminifer,  would  not  be  a  fossil  at  all,  because  it 


EOZOON  843 

differs  from  every  known  living  form.  Yet  the  suggestion  that  it  is 
of  mineral  origin  would  be  scouted  as  absurd  by  every  palaeontologist. 
Again  it  is  urged  by  Professor  Mobius  that  as  the  supposed  canal 
system  of  Eozoon  has  not  the  constancy  and  regularity  of  distribu- 
tion which  it  presents  in  existing  Foraminifera,  it  must  be  accounted 
a  mineral  infiltration.  To  this  the  Author  would  reply — (1)  that 
a  prolonged  and  careful  study  of  this  'canal  system,'  in  a  great 
variety  of  modes,  with  an  amount  of  material  at  his  disposal  many 
times  greater  than  Professor  Mobius  could  command,  has  satisfied 
him  that  in  well-preserved  specimens  the  canal  system,  so  far  from 
being  vague  and  indefinite,  has  a  very  regular  plan  of  distribution  ; 
(2)  that  this  plan  does  not  differ  more  from  the  arrangements 
characteristic  of  the  several  types  of  existing  Foraminifera  than 
these  differ  from  each  other,  its  general  conformity  to  them  being 
such  as  to  satisfy  Professor  Max  Schultze  (one  of  the  ablest  students 
of  the  group)  of  its  foraminiferal  character  ;  and  (3)  that  not  only 
does  the  distribution  of  the  canal  system  of  Eozoon  differ  in  certain 
essential  features  from  every  form  of  mineral  infiltration  hitherto 
brought  to  light,  but  that  canal  systems  in  no  respect  differing  from 
each  other  in  distribution  are  occupied  by  different  minerals]  a  fact 
which  seems  conclusively  to  point  to  their  pre-existence  in  the  cal- 
careous layers,  and  the  subsequent  penetration  of  these  minerals  into 
the  passages  previously  occupied  by  sarcode — precisely  as  has 
happened  in  those  '  internal  casts '  of  existing  Foraminifera  which 
Professor  Mobius  altogether  ignores. 

The  argument  for  the  foraminiferal  nature  of  Eozoon  is  essentially 
a  cumulative  one,  resting  on  a  number  of  independent  probabilities^ 
no  one  of  which,  taken  separately,  has  the  cogency  of  a  proof;  yet 
the  accordance  of  them  all  with  that  hypothesis  has  an  almost 
demonstrative  value,  no  other  hypothesis  accounting  at  once  for  the 
whole  assemblage  of  facts.1 

Collection  and  Selection  of  Foraminifera. — Many  of  the  Fora- 
minifera attach  themselves  in  the  living  state  to  seaweeds,  zoophytes, 
itc.  ;  and  they  should  therefore  be  carefully  looked  for  on  such 
bodies,  especially  when  it  is  desired  to  observe  their  internal  organi- 
sation and  their  habits  of  life.  They  are  often  to  be  collected  in 

1  The  above  account  of  Eozoon  is  allowed  to  stand  as  Dr.  Carpenter's  name  has 
become  so  intimately  connected  with  the  view,  now  not  commonly  held,  that  the  body 
has  an  animal  origin.  It  may  be  noted  that  Prof.  J.  W.  Gregory,  wlio  has  had  an 
opportunity  of  examining  the  so-called  Tudor  specimen  of  Eozoon,  communicated 
to  the  Geological  Society,  on  March  11,  1891,  a  paper,  of  which  the  following  is  an 
abstract : — 

After  careful  examination  of  all  the  slides  and  figures,  and  after  consideration  of 
Sir  W.  Dawaon's  interpretation,  the  author  is  absolutely  unable  to  recognise  in  the 
specimen  any  trace  of  the  '  proper  wall,'  '  canals,'  or  '  stolon  passages,'  which  are 
claimed  to  occur  in  Eozoon,  or  any  reasons  for  regarding  the  calcite  bands  as  the 
'  intermediate  skeleton '  of  a  foraminifer.  There  are  points  in  Sir  W.  Dawson's 
figure  which  might  pass  as  '  stolon  passages,'  but  they  appear  very  different  in  a 
photograph,  and  the  specimen  agrees  with  the  latter.  The  author,  however,  gives 
reasons  for  concluding  that  the  case  against  the  organic  origin  of  the  Tudor  specimen 
does  not  rest  on  negative  evidence  alone ;  for,  though  the  rock  is  much  contorted,  the 
twin  lamellae  and  cleavage-planes  of  the  calcite  are  not  bent ;  and  the  fact  that  the 
crystalline  bands  cut  across  the  bedding-planes  further  shows  their  secondary  origin. 
The  rock  in  which  the  specimen  was  found  is  not  '  Lower  Laurentian,'  and  is  included 
by  Messrs.  Selwyn  and  Vennor  in  the  Huronian. 


844  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

much  larger  numbers,  however,  from  the  sand  or  mud  dredged  up 
from  the  sea-bottom,  or  even  from  that  taken  from  between  the  tide- 
marks.  In  a  paper  containing  some  valuable  hints  on  this  subject  } 
Mr.  Legg  mentions  that,  in  walking  over  the  Small-mouth  Sand, 
which  is  situated  on  the  north  side  of  Portland  Bay,  he  observed 
the  sand  to  be  distinctly  marked  with  white  ridges,  many  yards 
in  length,  running  parallel  with  the  edge  of  the  water ;  and  upon 
examining  portions  of  these,  he  found  Forarninifera  in  considerable 
abundance.  One  of  the  most  fertile  sources  of  supply  that  our  own 
coasts  afford  is  the  ooze  of  the  oyster-beds,  in  which  large  numbers 
of  living  specimens  will  be  found  ;  the  variety  of  specific  forms,  how- 
ever, is  usually  not  very  great.  In  separating  these  bodies  from  the 
particles  of  sand,  mud,  &c.,  with  which  they  are  mixed,  various 
methods  may  be  adopted  in  order  to  shorten  the  tedious  labour  of 
picking  them  out  one  by  one  under  the  simple  microscope  ;  and  the 
choice  to  be  made  among  these  will  mainly  depend  upon  the  condi- 
tion of  the  Foraminifera,  the^  importance  (or  otherwise)  of  obtaining 
them  alive,  and  the  nature  of  the  substances  with  which  they  are 
mingled.  Thus,  if  it  be  desired  to  obtain  living  specimens  from  the 
oyster-ooze  for  the  examination  of  their  soft  parts,  or  for  preservation 
in  an  aquarium,  much  time  will  be  saved  by  stirring  the  mud  (which 
should  be  taken  from  the  surface  only  of  the  deposit)  in  a  jar  with 
water,  and  then  allowing  it  to  stand  for  a  few  moments ;  for  the 
finer  particles  will  remain  diffused  through  the  liquid,  while  the 
coarser  will  subside ;  and,  as  the  Foraminifera  (in  the  present  case) 
will  be  among  the  heavier,  they  will  be  found  at  the  bottom  of  the 
vessel  with  comparatively  little  extraneous  matter,  after  this  opera- 
tion has  been  repeated  two  or  three  times.  It  would  always  be  well 
to  examine  the  first  deposit  let  fall  by  the  water  that  has  been 
poured  away,  as  this  may  contain  the  smaller  and  lighter  forms  of 
Foraminifera.  But  supposing  that  it  be  only  desired  to  obtain  the 
dead  shells  from  a  mass  of  sand  brought  up  by  the  dredge,  a  very 
different  method  should  be  adopted.  The  whole  mass  should  be 
exposed  for  some  hours  to  the  heat  of  an  oven,  and  be  turned  over 
several  times,  until  it  is  found  to  have  been  thoroughly  dried 
throughout ;  and  then,  after  being  allowed  to  cool,  it  should  be 
stirred  in  a  large  vessel  of  water.  The  chambers  of  their  shells 
being  now  occupied  by  air  alone  (for  the  bodies  of  such  as  were 
alive  will  have  shrunk  up  almost  to  nothing),  the  Foraminifera  will 
be  the  lightest  portion  of  the  mass ;  and  they  will  be  found  floating 
on  the  water,  while  the  particles  of  sand  &c.  subside.  Another 
method,  devised  by  Mr.  Legg,  consists  in  taking  advantage  of  the 
relative  sizes  of  different  kinds  of  Foraminifera  and  of  the  substances 
that  accompany  them.  This,  which  is  especially  applicable  to  the  sand 
and  rubbish  obtainable  from  sponges  (which  may  be  got  in  large  quan- 
tity from  the  sponge -merchants),  consists  in  sifting  the  whole  aggre- 
gate through  successive  sieves  of  wire-gauze,  commencing  with  one 
of  ten  wires  to  the  inch,  which  will  separate  large  extraneous  particles, 
and  proceeding  to  those  of  twenty,  forty,  seventy,  and  a  hundred 
wires  to  the  inch,  each  (especially  that  of  seventy)  retaining  a  much 
1  Trans,  of  Microsc.  Soc.  ser.  ii.  vol.  ii.  1854,  p.  19. 


COLLECTING  FOEAMINIFERA  845 

larger  proportion  of  foraminiferal  shells  than  of  the  accompanying 
particles  ;  so  that,  a  large  portion  of  the  extraneous  matter  being  thus 
got  rid  of,  the  final  selection  becomes  comparatively  easy.  Certain 
forms  of  Foraminifera  are  found  attached  to  shells,  especially  bivalves 
(such  as  the  Chamidw)  with  foliated  surfaces  ;  and  a  careful  exami- 
nation of  those  of  tropical  seas,  when  brought  home  '  in  the  rough/ 
is  almost  sure  to  yield  most  valuable  results.  The  final  selection  of 
specimens  for  mounting  should  always  be  made  under  some  appropriate 
form  of  single  microscope,  a  fitie  camel-hair  pencil,  with  the  point 
wetted  between  the  lips,  being  the  instrument  which  may  be  most  con- 
veniently and  safely  employed,  even  for  the  most  delicate  specimens. 
In  mounting  Foraminifera  as  microscopic  objects  the  method  to  be 
adopted  must  entirely  depend  upon  whether  they  are  to  be  viewed 
by  transmitted  or  by  reflected  light.  In  the  former  case  they  should 
be  mounted  in  Canada  balsam,  the  various  precautions  to  prevent 
the  retention  of  air-bubbles,  which  have  been  already  described,  being 
carefully  observed.  In  the  latter  no  plan  is  so  simple,  easy,  and 
effectual  as  attaching  them  with  a  little  gum  to  wooden  slides. 
They  should  be  fixed  in  various  positions,  so  as  to  present  all 
the  different  aspects  of  the  shell,  particular  care  being  taken 
that  its  mouth  is  clearly  displayed ;  and  this  may  often  be  most 
readily  managed  by  attaching  the  specimen  sideways  to  the  wall  of 
the  circular  depression  of  the  slide.  Or  the  specimens  may  be 
attached  to  discs  fitted  for  being  held  in  a  disc-holder ;  whilst  for 
the  examination  of  specimens  in  every  variety  of  position  Mr.  R. 
Beck's  disc-holder  will  be  found  extremely  convenient.  Where,  as 
will  often  happen,  the  several  individuals  differ  considerably  from 
one  another,  special  care  should  be  taken  to  arrange  them  in  series 
illustrative  of  their  range  of  variation  and  of  the  mutual  connections 
of  even  the  most  diverse  forms.  For  the  display  of  the  internal 
structure  of  Foraminifera  it  will  often  be  necessary  to  make  extremely 
thin  sections,  in  the  manner  already  described  ;  and  much  time  will 
be  saved  by  attaching  a  number  of  specimens  to  the  glass  slide  at 
once  and  by  grinding  them  down  together.  For  the  preparation  of 
sections,  however,  of  the  extreme  thinness  that  is  often  required, 
those  which  have  been  thus  reduced  should  be  transferred  to 
separate  slides  and  finished  off  each  one  by  itself. 

For  the  collection  and  examination  of  fossil  Foraminifera,  which 
are  of  great  interest  and  importance,  the  following  suggestions  will 
be  of  use  ;  they  are  the  result  of  the  ripe  experience  of  Mr.  F. 
Chapman : 

Perhaps  the  foraminiferous  clays  are  the  most  satisfactory  for 
those  who  desire  to  collect  foraminifera.  Ordinary  clays  require  to 
be  slowly  and  thoroughly  dried,  broken  into  small  pieces  of  about  a 
cubic  inch  or  so,  and  placed  in  a  vessel  of  water  with  steep  sides. 
After  some  little  time  the  material  will  be  found  to  have  become 
disintegrated.  The  vessel  should  then  be  shaken  round,  and  after 
the  coarser  particles  have  subsided  the  fine  muddy  portion  may  be 
poured  off.  The  materials  should  again  be  shaken  with  very  little 
water,  and  more  water  should  then  be  added  so  as  to  cleanse  the 
mud,  and  the  decanting  process  afterwards  repeated.  If  this  be  done 


846  MICROSCOPIC   FORMS   OF   ANIMAL  LIFE 

several  times  a  fine  sand  with  foraminiferal  and  other  shells  will  be 
obtained.  This  can  be  then  dried  and  sifted  in  the  manner  already 
described  for  the  sands  from  modern  deposits.  To  insure  obtaining 
the  minutest  shells,  the  water  which  is  poured  off  should  be  passed 
through  a  fine  cambric  or  silken  sieve. 

The  following  are  some  of  the  more  productive  of  the  fossiliferous 
deposits  : 

Weathered  surfaces  of  carboniferous  limestone  and  seams  of  clay 
in  the  joints  of  it. 

Clay  from  the  lias  formation. 

Gault  clay  especially  from  the  upper  zones. 

The  softer  beds  of  the  upper  chalk  and  especially  the  phosphatic 
chalk  of  Taplow.  which  washes  down  easily. 

Foraminifera  may  be  fixed  by  gum  arabic  with  three  drops  of 
glycerine  added  to  the  ounce,  or  with  gum  tragacanth,  which  has  the 
advantage  of  drying  with  a  dead  surface. 

SECTION  II. — RADIOLARIA. 

It  has  been  shown  that  one  series  of  forms  belonging  to  the 
rhizopod  type  is  characterised  by  the  radiating  arrangement  of  their 
rod-like  pseudopodia,  suggesting  the  designation  Heliozoa  or  *  sun- 
animalcules  ; '  and  that  even  among  those  fresh-water  forms  that  do 
not  depart  widely  from  the  common  Actinophrys  there  are  some 
whose  bodies  are  inclosed  in  a  complete  silicious  skeleton.  Now 
just  as  the  reticularian  type  of  rhizopod  life  culminates  in  the  marine 
calcareous-shelled  Foraminifera,  so  does  the  heliozoic  type  seem  to 
culminate  in  the  marine  Radiolaria;  which,  living  for  the  most 
part  near  the  surface  of  the  ocean,  form  silicious  skeletons  (often  of 
marvellous  symmetry  and  beauty)  that  fall  to  the  bottom  on  the 
death  of  the  animals  that  produced  them,  and  may  remain  unchanged, 
like  those  of  the  diatoms,  through  unlimited  periods  of  time.  Some 
of  these  skeletons,  mingled  with  those  of  diatoms,  had  been  detected 
by  Professor  Ehrenberg  in  the  midst  of  various  deposits  of  foramini- 
feral origin,  such  as  the  calcareous  Tertiaries  of  Sicily  and  Greece, 
and  of  Oran  in  Africa;  and  he  established  for  them  the  group  of 
Polycystina,  to  which  he  was  able  also  to  refer  a  beautiful  series  of 
forms  making  up  nearly  the  whole  of  a  silicious  sandstone  prevail- 
ing through  an  extensive  district  in  the  island  of  Barbadoes  (fig.  644). 
Nothing,  however,  was  known  of  the  nature  of  the  animals  that 
formed  them  until  they  were  discovered  and  studied  in  the  living  state 
by  Professor  J.  Miiller,1  who  established  the  group  of  Radiolaria, 
including  therein,  with  the  Polycystina  of  Ehrenberg,  the  Acantho- 
metrina  first  recognised  by  himself,  and  the  Thalassicolla  which  had 
been  dis3overed  by  Professor  Huxley.  Not  long  afterwards  appeared 
the  magnificent  and  '  epoch-making '  work  of  Professor  Haeckel ; 2 

1  'Ueber  die  Thalassicollen,   Polycystinen,    und    Acantliometren  des   Mittel- 
meeres,'  in  Abhandlungen  der  Jconigl.  Akad.  der  Wissensch.  zu  Berlin,  1858,  and 


separately  published ;  also  '  Ueber  die  im  Hafen  von  Messina  beobachteten  Poly- 
cystinen,'  in  the  Monatsberichte  of  the  Berlin  Academy  for  1855,  pp.  671-676. 

2  Die  Badiolarien  (Rhizopoda  Kadiaria),   Berlin,    1862.     This   great   work   has 
lately  been  followed  by  a  gigantic  monograph  published  in  the  c  Challenger  'Reports, 


EADIOLARIA  847 

and  since  that  time  much  has  been  added  by  various  observers  to 
our  knowledge  of  this  group,  which  still  remains,  however,  very 
imperfect. 

Each  individual  radiolarian  consists  of  two  portions  of  coloured 
or  colourless  sarcode — one  portion  nucleated  and  central,  the  other 
portion  peripheral,  and  almost  always  containing  certain  yellow 
corpuscles.  These  two  portions  are  separated  by  a  membrane  called 
the  capsule  ;  but  this  is  so  porous  as  to  allow  of  their  free  communi- 
cation with  each  other.  The  infrier  central  capsule  is  also  the  special 


\ 


FIG.  644.  —  Fossil  Eadiolaria  from  Barbadoes  :  a,  Podocyrtis  mitra  ;  b, 
Rhabdolithus  sceptrum\  c,  Lychnocaniu  m  falciferum  ;  d,  Eucyrtidium 
tiibulus  ;  e,  Flustrella  concentrica  ;  /,  Lychnocanium  lucerna  ;  g,  Eucyr- 
tidium  elegans  ;  h,  Dictyospyris  clatlirus  ;  i,  Eucyrtidium  Mongolfieri  ; 
&,  Stephanolithis  spinescens  ;  I,  S.  nodosa  ;  ?n,  Lithocyclia  ocellus  ;  n, 
Cephalolithis  sylvina  ;  o,  Podocyrtis  cothurnata]  p, 


organ  of  reproduction,  for  it  is  the  intracapsular  protoplasm,  with 
the  nuclei  imbedded  in  it,  which  serves  for  the  formation  of  flagellate 
spores;  the  outer  capsule  has  the  special  office  of  protecting  and 
providing  nourishment  for  the  cell.1  The  pseudopoclia  radiate  in  all 
directions  (fig.  645)  from  the  deeper  portion  of  the  extracapsular 
sarcode  ;  they  have  generally  much  persistency  of  direction  and  very 

which  extends  over  1,800  pages,  and  is  illustrated  by  140  plates.  In  it  are  described 
4,318  species,  of  which  3,508  are  new  to  science. 

1  The  structure  of  the  central  capsule  of  Aulacantha  has  been  carefully  worked 
out  by  W.  Karawaiew,  in  Zool.  Anzeig.  xviii.  (1895),  p.  286  and  p.  293. 


848 


MICROSCOPIC    FORMS   OF   ANIMAL  LIFE 


little  flexibility  ;  in  some  species  (but  not  ordinarily)  they  branch 
and  anastomose,  while  in  others  they  are  inclosed  in  hollow 
rods  that  form  part  of  the  silicious  skeleton,  and  issue  forth  from 
the  extremities  of  these.  A  flow  of  granules  takes  place  among 
them  ;  and  the  mode  in  which  they  obtain  food-particles  (consisting 
of  diatoms  and  other  minute  algae,  marine  infusoria,  &c.),  and  draw 
them  into  the  sarcode-bodies  of  the  radiola-rians,  appears  to  corre- 
spond entirely  with  their  action  in  Actinophrys  and  other  Heliozoa. 
The  yellow  cells,  or  Zooxanthellce,  as  K.  Brandt  has  proposed  to 
call  them,  so  often  seen  in  these  cells,  are  not  confined  to  Radiolavia, 


FIG.  645. — Polycystina:  A,  Haliomma  hystrix  ;  B,  Pterocanium,  with  animal. 

for  they  are  found  also  in  Actiniae  and  various  other  invertebrates ; 
nor  are  they  always  present  in  examples  studied  ;  they  are  now  com- 
pletely recognised l  as  algae  which  form  a  '  symbiotic  '  relation  with 
their  host,  the  animal  profiting  by  the  removal  of  its  waste  products 
by  its  messmate,  by  the  oxygen  which  its  guest  evolves  in  sunlight, 
and  by  the  food-material  it  provides  after  death,  while  the  plant 
feeds  on  the  waste  of  the  animal. 

In  most  Radiolaria  skeletal  structures  are  developed  in  the 
sarcode-body,  either  inside  or  outside  the  capsule,  or  in  both  positions ; 
sometimes  in  the  form  of  investing  networks  having  more  or  less  of 
a  spheroidal  form  (fig.  647,  l,  2),  or  of  radiating  spines,  3,  or  of 
combinations  of  these,  4,  5.  But  in  many  cases  the  skeleton  consists 
only  of  a  few  scattered  spicules  ;  and  this  is  especially  the  case  in 
certain  large  composite  forms  or  '  colonies '  (fig.  652),  which  may 


1   See  especially  K.  Brandt,  Verhandl.  Physiol.  Gesellsch.  Berlin,  1881-82,  p.  22 ; 
Mitth.  Zool.  Stat.  Neapel,  iv.  p.  191 ;  P.  Geddes,  Nature,  xxv.  p.  303. 


POLYCYSTINA 


849 


consist  of  as  many  ;is  a  thousand  zijoids  aggregated  together  in  various 

forms,  discoidal,  cylindrical,  spheroidal,  chain-like,  or  even  necklace- 
like.  The  'colonies '  seem  to  be  produced,  like  the  multiple  segments 
of  the  bodies  of  Forammifera,  by  the  non-sexual  multiplication  of  a 
primordial  zooid  ;  but  whether  this  multiplication  takes  place  by 
fission,  or  by  the  budding  oft'  of  portions  of  the  sarcode-body,  has 
not  yet  been  clearly  made  out.  The  emission  of  flagellated  zoospores. 
provided  with  a  very  large  nucleus,  and  in  some  cases  with  a  rod- 
like  crystal,  has  been  observed  in-hiaiiy  radiolarians  ;  but  of  the  mode 
in  which  they  are  produced,  and  of  their  subsequent  history,  very 
little  is  at  present  known.  Until  the  structure  and  life  history  of 
the  animals  of  this  very  interesting  type  shall  have  been  more  fully 
elucidated,  no  satisfactory  classification  of  them  can  be  framed  ;  and 
nothing  more  will  be  here  attempted  than  to  indicate  some  of  the 
principal  forms  under  which  the  radiolarian  type  presents  itself.1 

Discida. — Among  the 
beautiful  silk-ions  struc- 
tures which  are  met  with 
in  the  radiolarian  sand- 
stone of  Barbadoes  (fig. 
644)  there  is  none  more 
interesting  than  the  ske- 
leton of  Astromina  (fig. 
648),  in  which  we  have  a 
remarkable  example  of 
the  range  of  variation  that 
is  compatible  with  con- 
formity to  a  general  plan 
of  structure.  As  in  other 
forms  of  Haeckel's  group 
of  Discida,  there  is  in 
this  skeleton  a  combina- 
tion of  radial  and  of  cir- 
cumferential parts,  the 
former  consisting  of  solid 
spoke-like  rods,  whilst  the 
latter  is  composed  of  a  silicious  network  more  or  less  completely 
filling  up  the  spaces  between  the  rays.  The  radial  part  of  the  skele- 
ton predominates  in  the  beautiful  four-rayed  example  represented  at 
D,  having  the  form  of  a  cross  with  equal  arms  ;  whilst  in  F  and  G  it 
still  shows  itself  very  conspicuously,  though  the  spaces  between  the 
rays  are  in  great  part  filled  up  by  the  circumferential  network.  In  the 
five-rayed  specimens  A  and  B,  on  the  other  hand,  the  radial  portion 
is  much  less  developed,  while  the  circumferential  becomes  more  dis- 
coidal.  And  in  C  and  E,  while  the  circumferential  network  forms  a 
pentagonal  disc,  the  radial  portion  is  represented  only  by  solid  projec- 
tions at  its  angles.  The  transition  between  the  extreme  forms  is 
found  to  be  so  gradual  when  a  number  of  specimens  are  compared 
that  110  lines  of  specific  distinction  can  be  drawn  between  them  ;  and 

1  Considerable  attention  has  been  given  to  the  question  of  the  classification  of 
the  Radiolari*  by  Haeckel  and  by  R.  Hertwig,  Jcnaisclie  DenkscJn-.  ii.  1879,  p.  129. 

3  i 


FIG.  646.— Polycystina :  A,  Podocyrtis  Schom- 
B,  Rhop<iloc«niiini  ornatiun. 


850 


MICROSCOPIC    FORMS   OF   ANIMAL   LIFE 


the  difference  in  the  number  of  rays  is  probably  of  no  more  account  in 
these  low  forms  of  animal  life  than  it  is  in  the  discoidal  diatoms. 
Other  discoidal  forms,  showing  a  like  combination  of  radial  and 
circumferential  parts,  are  represented  in  figs.  649  and  650,  and  also 
in  fig.  644,  e,  mi. 


FIG.  647. — Various  forms  of  Eadiolaria  (after  Haeckel) :  1,  Etlunosphtera 
siphonoj)Jiora  ;  2,  Actinomma  inerme;  3,  Acanthometra  xipliicaittlia  ; 
4,  Atucrmosphcera  oligacantlia  ;  5,  Cladococcus  viniinnlix. 

Entosphasrida. — In  this  group  the  silicious  shell  is  spheroidal, 
and  is  formed  within  the  capsule ;  and  it  is  not  traversed  by  radii, 
although  prolongations  of  the  shell  often  extend  themselves  radially 


POLYCY8TIXA 


8Sl 


outwards,  as  in  Oladococcas  (fig.  647,  5).  Sometimes  the  central 
sphere  is  inclosed  in  two,  three,  or  even  more  concentric  spheres 
connected  by  radii,  as  in  the  beautiful  Actinomma  (fig.  647,  2),  re- 
minding us  of  the  wonderful  concentric  spheres  carved  in  ivory  by 


FIG.  648. — Varietal  modifications  of  Astro) 


the  Chinese.     One  of  the  most  common  examples  of  this  group  is 
the  Haliomma  Htimboldtii  (fig.  651),  in  which  the  shell  is  double. 

Polycystina. — This  name,  which  originally  included  the  preceding 
group,  is  now  restricted  to  those  which  have  the  shell  formed  outside 


FIG.  GW.—Perichlaniyflium  prcctcxtn m,  FIG.  Q50.—Stylodyctya  gracilis. 

the  capsule.  This  shell  may,  as  in  the  preceding,  be  a  simple  sphere 
composed  of  an  open  silicious  network,  as  in  Ethmosphcera  (fig.  647,  l) ; 
or  it  may  consist  of  two  or  three  concentric  spheres  connected  by 
radii  ;  or,  again,  it  may  put  forth  radial  outgrowths,  which  sometimes 

3  i2 


852  MICROSCOPIC   FOBMS   OF  ANIMAL  LIFE 

extend  themselves  to  several  times  the  diameter  of  the  shell,  and 
ramify  more  or  less  minutely,  as  in  Arachnosphcera  (fig.  647,  4).  But 
more  frequently  the  shell  opens  out  at  one  pole  into  a  form  more  or 
less  bell-like,  as  in  Podocyrtis  (fig.  646,  A,  and  fig.  644,  a,  o),  Eliopalo- 
canium  (fig.  646,  B),  and  Pterocanium  (fig  645,  B) ;  or  it  may 
be  elongated  into  a  somewrhat  cylindrical  form,  one  pole  remaining 
closed,  while  the  other  is  more  or  less  contracted,  as  in  Eucyrtidium 
(fig.  644,  d,  (/,  /).  The  transition  between  these1  forms,  again,  proves 
to  be  as  gradational,  when  many  specimens  are  compared,1  as  it  is 
among  Foraminifera. 

Acanthometrina. — In  this  group  the  animal  is  not  inclosed  within 
a  shell,  but  is  furnished  with  a  very  regular  skeleton,  composed 
of  elongated  spines,  which  radiate  in  all  directions  from  a  common 
centre  (fig.  645,  A).  The  soft  sarcode-body  is  spherical  in  form,  and 
occupies  the  spaces  left  between  the  bases  of  these  spines,  which 
are  sometimes  partly  inclosed  (as  in  the  species  represented)  by 
transverse  projections.  The  '  capsule '  is  pierced  by  the  pseudo- 
podia,  whose  convergence  may  be  traced  from  without  inwards, 

afterwards  passing  through  it  ;  and  it 
is  itself  enveloped  in  a  layer  of  less 
tenacious  protoplasm,  resembling  that 
of  which  the  pseudopodia  are  composed. 
One  species,  the  Acanthometra  echin- 
oides,  which  presents  itself  to  the  naked 
eye  as  a  crimson-red  point,  the  dia- 
meter of  the  central  part  of  its  body 
being  about  T^^ths  of  an  inch,  is  very 
common  on  some  parts  of  the  coast  of 
Norway,  especially  during  the  preva- 
lence of  westerly  winds;  and  the 
FIG.  651.—Haliomma  Humboldtii.  Author  has  himself  met  with  it  abun- 
dantly near  Shetland,  in  the  floating 

brown  masses  termed  madre  by  the  fishermen  (who  believe  them 
to  furnish  food  to  the  herring),  which  consists  mainly  of  this 
Acanthometra  mingled  with  Entomostraca. 

Phaeodaria. — Among  the  most  important  of  the  Radiolaria 
collected  by  the  '  Challenger '  are  the  comparatively  large  (as  much  as 
1  mm.  in  diameter)  single-celled  forms  which  are  remarkable  for  the 
constant  presence  of  large  dark  brown  granules,  which  are  scattered 
irregularly  round  the  central  capsule  and  cover  the  greater  part  of 
its  outer  surface.  The  nucleus  is  large,  the  capsular  membrane  is 
always  double,  and  is  pierced  by  one  or  more  large  openings  ;  the 
whole  cell  is  inclosed  in  a  thick  gelatinous  covering,  and  there  is 
nearly  always  a  well -developed  extracapsular  silicious  skeleton, 
according  to  the  structure  of  which  the  group  is  subdivided.2 

Collozoa. — To  this  group  belong  those  remarkable  composite 
forms  which,  exhibiting  the  characteristic  rarliolarian  type  in  their 

1  The  general  plan  of  structure  of  the  Polycystina,  and  the  signification  of  their 
immense  variety  of  forms,  were  ably  discussed  by  Dr.  Wallich  in  the  Trans,  of  the 
Microsc.  Soc.  n.s.  vol.  xiii.  1865,  p.  75. 

2  On  reproduction  in  this  group,  cf.  A.  Borgert,  Zool.  Anzeig.  xix.  (189(5),  p.  307. 


RADIOLAKIA 


853 


individual  zooids,  are  aggregated  into  masses  in  which  the  skeleton 
is  represented  only  by  scattered  spicules,  as  in  Spkcerozown  (fig.  652) 
and  Thalassicolla.  These  'sea-jellies,'  which  so  abound  in  the  seas  of 
warm  latitudes  as  to  be  among  the  commonest  objects  collected  by 
the  tow-net,  are  small  gelatinous  rounded  bodies,  of  very  variable 
size  and  shape,  but  usually  either  globular  or  discoidal.  Externally 
thev  are  invested  by  a  layer  of  condensed  sarcode,  which  sends  forth 
pseudopodial  extensions  that  commonly  stand  out  like  rays,  but 
sometimes  inosculate  with  each  other  so  as  to  form  a  network.  To- 
wards the  inner  surface  of  this  coat  are  scattered  a  great  number  of 
oval  bodies  resembling  cells  having  a  tolerably  distinct  membraniform 
wall  and  a  conspicuous  round  central  nucleus.  Each  of  these  bodies 
appears  to  be  without  any  direct  connection  with  the  rest,  but  it 
serves  as  a  centre  round  which  a  number  of  minute  yellowish-green 
vesicles  are  disposed. 
Each  of  these  groups  is 
protected  by  a  silicious 
skeleton,  which  some- 
times consists  of  separate 
spicules  (as  in  fig.  652), 
but  which  may  be  a  thin 
perforated  sphere,  like 
that  of  certain  Poly- 
cystina,  sometimes  ex- 
tending itself  into  radial 
prolongations.  The  in- 
ternal portion  of  each 
mass  is  composed  of  an 
aggregation  of  large 
vesicle-like  bodies  im- 
bedded in  a  softer  sar- 
codic  substance.1 

From  the  researches 
made  during  the  '  Chal- 
lenger '  Expedition,  it 
appears  that  the  Radiolaria  are  very  widely  diffused  through  the 
waters  of  the  ocean,  some  forms  being  more  abundant  in  tropical 
and  others  in  temperate  seas  ;  and  that  they  live  not  only  at  or  near 
the  surface,  but  also  at  considerable  depths.  Their  silicious  skeletons 
accumulate  in  some  localities  (in  which  the  calcareous  remains  of 
Foraminifera  are  wanting)  to  such  an  extent  as  to  form  a  '  radio- 
larian  ooze  ; '  and  it  is  obvious  that  the  elevation  of  such  a  deposit 
into  dry  land  would  form  a  bed  of  silicious  sandstone  resembling  the 
well-known  Barbadoes  rock,  which  is  said  to  attain  a  thickness  of 
1,100  feet,  or  a  similar  rock  of  yet  greater  thickness  in  the  Nicobar 

1  See  Professor  Huxley  (to  whom  we  owe  our  first  knowledge  of  these  forms)  in  Ann. 
Nat.  Hist.  ser.  ii.  vol.  viii.  1851,  p.  433 ;  also  Professor  Miiller,  of  Berlin,  in  Quart.  Journ. 
Microsc.  Sci.  vol.  iv.  1856,  p.  72,  and  in  his  treatise  Ueber  die  TJialassicoUen,  Poly- 
cystinen,  und  Acanthometren  des  Mittelmeeres,  the  magnificent  work  of  Professor 
Ha,eckel,  Die  Radiolarieii,  and  the  monograph  by  K.  Brandt,  published  in  the  Fauna 
und  Flora  des  Golfes  von  Neapel,  1885,  'Die  kolonrebildenden  Kadiolarien 
(Spheerozoeen)  des  Golfes  von  Neapel.' 


FIG.  652. — Sphcerozoum  ovodimare. 


854  MICROSCOPIC   FORMS   OF  ANIMAL  LIFE 

Islands.  Few  .microscopic  objects  are  more  beautiful  than  an 
assemblage  of  the  most  remarkable  forms  of  the  Barbadian  Poly- 
cystina  (fig.  644),  especially  when  seen  brightly  illuminated  upon 
a  black  ground ;  since  (for  the  reason  formerly  explained)  their 
solid  forms  then  become  much  more  apparent  than  they  are  when 
these  objects  are  examined  by  light  transmitted  through  them.  And 
when  they  are  mounted  in  Canada  balsam  the  black-ground  illu- 
mination is  much  to  be  preferred  for  the  purpose  of  display, 
although  minute  details  of  structure  can  be  better  made  out  when 
they  are  viewed  as  transparent  objects  with  higher  powers.  Many 
of  the  more  solid  forms  when  exposed  to  a  high  temperature  on  a 
slip  of  platinum  foil  undergo  a  change  in  aspect  which  renders  them 
peculiarly  beautiful  as  opaque  objects,  their  glassy  transparence 
giving  place  to  an  enamel-like  opacity.  They  may  then  be  mounted 
on  a  black  ground  and  illuminated  either  with  a  side  condenser  or 
with  the  parabolic  speculum.  No  class  of  object  is  more  suitable 
than  these  to  the  binocular  microscope,  its  stereoscopic  projection 
causing  them  to  be  presented  to  the  mind's  eye  in  complete  relief, 
so  as  to  bring  out  with  the  most  marvellous  and  beautiful  effect  all 
their  delicate  sculpture.1 

1  For  a  fuller  description  of  the  fossil  forms  of  this  group  see  Professor  Ehrenberg's 
memoirs  in  the  Monatsberichte  of  the  Berlin  Academy  for  1846, 1847,  and  1850  ;  also 
his  Microgeologie,  1854 ;  and  Ann.  of  Nat.  Hist.  vol.  xx.  1847.  The  best  method  of 
separating  the  Polycystina  from  the  Barbadoes  sandstone  it?  described  by  Mr.  Fur- 
long in  the  Quart.  Journ.  of  Microsc.  Sci.  n.s.  vol.  i.  1861,  p.  64. 


855 


CHAPTER  XY 

SPONGES  AND  ZOOPHYTES 

I.  SPOXGES 

WE  now  leave  the  PROTOZOA  and  commence  the  study  of  the  METAZOA, 
or  those  forms  in  which  the  egg-cell  undergoes  subdivision,  the  result- 
ing elements  of  which  do  not  separate  or  lead  an  independent 
existence,  but  combine  to  form  an  organic  whole,  various  parts 
undertaking  various  functions.  Of  these  Metazoa  the  simplest  ex- 
amples are  to  be  found  among  SPONGES.  The  determination  of  the 
real  character  of  the  animals  of  this  class  has  been  entirely  effected  by 
the  microscopic  examination  of  their  minute  structure  ;  for  until  this 
came  to  be  properly  understood,  not  only  was  the  general  nature  of 
these  organisms  entirely  misapprehended,  but  they  were  regarded 
by  many  naturalists  as  having  no  certain  claim  to  a  place  in  the 
animal  kingdom.  What  that  place  is,  is,  to  some  extent,  still  an 
open  question,1  but  it  may  now  be  unhesitatingly  affirmed  that  a 
sponge  is  an  aggregate  of  protozoic  units  only  in  the  sense  in  which 
,ill  -Met; i /on  are  composed  of  cells  ;  some  of  these  cells  have  a  striking 
resemblance  to  the  collared  Flayellata  (fig.  585),  whilst  others  re- 
semble Amcebce  (fig.  577).  These  units  are  held  together  by  a  con- 
tinuous connective-tissue-like  substance  which  clothes  the  skeletal 
framework  that  represents  our  usual  idea  of  a  sponge,  and  is  itself 
made  up  of  distinct  cellular  elements.  In  the  simpler  forms  of 
sponges,  however,  this  frame\vork  is  altogether  absent  ;  in  others  it 
is  represented  only  by  calcareous  or  silicious  '  spicules,'  which  are 
dispersed  through  the  sarcodic  substance  (fig.  654,  B) ;  in  others, 
again,  the  skeleton  is  a  keratose  (horny)  network,  which  may  be 
entirely  destitute  (as  in  our  ordinary  sponge)  of  any  mineral  support, 
but  which  is  often  strengthened  by  calcareous  or  silicious  spicules 
(fig.  654)  ;  whilst  in  what  may  be  regarded  as  the  highest  types  of 
the  group,  the  silicious  component  of  the  skeleton  increases,  and  the 
keratose  diminishes  until  the  skeleton  consists  of  a  beautiful  silicious 
network  resembling  spun  glass.  But  whatever  may  be  the  condi- 
tion of  the  skeleton,  that  of  the  body  that  clothes  it  remains 

1  For  an  instructive  discussion  on  this  point,  consult  Prof.  E.  A.  Minchin's  essay 
on  '  The  Position  of  Sponges  in  the  Animal  Kingdom'  in  Science  Papers,  i.  (n.s.) 
(1897),  to  which  is  appended  a  useful  list  of  works  on  the  subject.  Some  axithors 
demur  to  the  association  of  sponges  with  other  Metazoa,  and  Professor  Sollas  has  sug- 
gested the  use  of  the  group-name  Parazoa.  See  also  Treatise  on  Zoology,  vol.  ii. 
London,  1900. 


856 


SPONGES  AND   ZOOPHYTES 


essentially  the  same  ;  and  the  peculiarity  that  chiefly  distinguishes 
the  sponge-colony  from  the  plant-like  colonies  of  the  flagellate 
Infusoria  is  that  whilst  the  latter  extend  themselves  outwards  by 
repeated  ramification,  sending  their  zooid-bearing  branches  to 
meet  the  water  they  inhabit,  the  surface  of  the  former  extends 
itself  inwards,  forming  a  system  of  passages  and  cavities  lined  by 
these  and  the  amoeboid  cells,  through  which  a  current  of  water  is 
drawn 'in  to  meet  them  by  the  action  of  the  flagella.  The  minute 
pores  (fig.  653,  b,  b)  with  which  the  surface  a,  a  of  the  living  sponge 
is  beset  lead  to  incurrent  passages  that  open  into  chambers  lying 
beneath  it  (c.  c),  and  open  into  the  '  ampullaceous  sacs,'  or,  as  they 
are  now  called,  *  flagellate  chambers,'  from  the  presence  round 
their  walls  of  the  flagellate  or  collared  cells.  The  water  drawn 
in  by  their  agency  is  driven  outwards  through  a  system  of 
excurrent  canals,  which,  uniting  into  larger  trunks,  proceed  to  the 

oscula  or  projecting 
vents,  d,  from  each  of 
which,  during  the 
active  life  of  the 
sponge,  a  stream  of 
water,  carrying  out  ex- 
crementitious  matter, 
is  continually  issuing. 
The  in-current  brings 
into  the  chambers 
both  food-material  and 
FIG.  653. -Diagrammatic  section  of  a  sponge:  «,  a,  OXygen  ;  and  from  the 
superficial  layer  ;  b,  inhalant  apertures  or  pores  ;  c,  <-,  J  °  ,  .  , 

flagellated  chambers;  rl,  exhalant  oscule  ;  e,  deeper    manner        ill        winch 
substance  of  the  sponge.  coloured    particles  ex- 

perimentally    diffused 

through  the  water  wherein  a  sponge  is  living  are  received  into  its 
protoplasmic  substance,  it  seems  clear  that  the  nutrition  of  the  entire 
fabric  is  the  resultant  of  the  feeding  action  of  the  flagellate  units, 
each  of  which  takes  in,  after  its  kind,  the  food -particles  brought  by 
the  current  of  water,  and  imparts  the  product  of  its  digestion  of  them 
to  the  general  sarcodic  mass. 

The  continuous  substance  that  clothes  the  skeleton  of  the 
sponge  and  constitutes  the  chief  part  of  its  living  body  includes 
great  numbers  of  stellate  granular  cells.  Their  long  slender  pseudo- 
podia,  radiating  towards  those  of  their  neighbours,  often  unite 
together  to  form  a  complex  network  ;  on  the  chief  parts  of  the 
course  of  the  water-way  they  become  fusiform  in  shape  and  con- 
tractile in  function,  and  it  is  by  their  agency  that  the  continual 
contractions  and  expansions  of  the  oscula  are  produced,  which  are 
very  characteristic  of  the  living  sponge.  As  was  first  shown  by 
Professor  C.  Stewart,  sensory  organs,  formed  of  groups  of  cells 
with  long  projecting  filaments,  are  to  be  seen  on  the  surface  of 
many  sponges.  Any  one  of  these  amceboids,  again,  detached  from 
the  mass,  may  lay  the  foundation  of  a  new  '  colony.'  In  the 
aggregate  mass  produced  by  its  continuous  segmentation  certain 
globular  clusters  are  distinguishable,  each  having  a  cavity  in 


SPONGES  857 

it's  interior  ;  and  the  amceboicls  that  form  the  wall  of  this  cavity 
become  metamorphosed  into  collared  flagellate  cells  whose  flagella 
project  into  it.  Thus  is  formed  one  of  the  characteristic  *  ampul - 
laceous  sacs,'  which,  at  first  closed,  afterwards  communicates  with 
the  exterior,  on  the  one  hand  by  an  mcurrent  passage,  and  on  the 
other  with  the  excurrent  canal-system  leading  to  the  oscula.  Be- 
sides this  reproduction  by  '  microspores,'  there  is  another  form 
of  non-sexual  reproduction  by  macrospores,  which  are  clusters  of 
amceboids  encysted  in  firm  capsules,  frequently  strengthened  on 
their  exterior  by  a  layer  of  spicules  of  very  peculiar  form.  These 
'  seed-like  bodies,'  which  answer  to  the  encysted  states  of  many 
protophytes,  are  met  with  in  the  substance  of  the  sponge,  chiefly  in 
winter  ;  and  after  being  set  free  through  the  oscula  they  give  exit 
to  their  contained  .amceboids.  eacn  of  which  may  found  a  newT  colony. 
A  true  process  of  sexual  generation,  moreover,  is  known  to  take  place 
in  sponges,  certain  of  the  amceboids,  like  certain  cells  of  Volrosc, 
becoming  '  sperm-cells,'  and  developing  spermatozoa  by  the  meta- 
morphosis of  their  nuclei  ;  while  others  become  '  germ-cells,' 
developing  themselves  by  segmentation  (wrhen  fertilised)  into  the 
bodies  known  as  '  ciliated  gemmules,'  which  are  set  free  from  the 
walls  of  the  canals,  swim  forth  from  the  vents,  and  for  a  time  move 
actively  through  the  water.  In  a  word,  there  is  true  sexual  repro- 
duction by  ova  and  spermatozoa,  as  in  all  animals  that  are  not 
Protozoa.1 

The  arrangement  of  the  keratose  reticulation  in  the  sponges  with 
which  we  are  most  familiar  may  be  best  made  out  by  cutting  thin 
slices  of  a  piece  of  sponge  submitted  to  firm  compression,  and  view- 
ing these  slices,  mounted  upon  a  dark  ground,  with  a  low  magnifying 
power  under  incident  light.  Such  sections,  thus  illuminated,  are 
not  merely  striking  objects,  but  serve  to  show  very  characteristically 
the  general  disposition  of  the  larger  canals  and  of  the  smaller  pores 
with  which  they  communicate.  In  the  ordinary  sponge  the  fibrous 
skeleton  is  almost  entirely  destitute  of  spicules,  the  absence  of 
which,  in  fact,  is  one  important  condition  of  that  flexibility  and 
compressibility  on  which  its  uses  depend.  When  spicules  exist  in 
connection  with  such  a  skeleton,  they  are  usually  either  altogether 
imbedded  in  the  fibres,  or  are  implanted  into  them  at  their  bases  ; 
but  smaller  and  simpler  sponges,  such  as  Grantia,  have  no  horny 
skeleton,  and  their  calcareous  spicules  are  imbedded  in  the  general 
substance  of  the  body.  Sponge-spicules  are  much  more  fre- 
quently silicious  than  calcareous  ;  and  the  variety  of  forms  pre- 
sented by  the  silicious  spicules  is  much  greater  than  that  which 
we  find  in  the  comparatively  small  division  in  which  they  are 
composed  of  carbonate  of  lime.  The  long  needle-like  spicules,  which 
are  extremely  abundant  in  several  sponges,  lying  close  together 
in  bundles,  are  sometimes  straight,  sometimes  slightly  curved  ; 
they  are  sometimes  pointed  at  both  ends,  sometimes  at  one  only ; 
one  or  both  ends  may  be  furnished  with  a  head  like  that  of  a 

1  See  Chapter  V.  of  Mr.  Saville  Kent's  Manual  of  the  Infusoria,  and  Chapter  V.  of 
vol.  i.  of  Mr.  Balfour's  Comparative  Embryology,  as  well  as  Professor  Haeckel's  im- 
portant work  on  the  Calcareous  Sponges. 


858 


SPONGES  AND  ZOOPHYTES 


pin  or  may  cany  three  or  more  diverging  points  which  sometimes 
curve  back  so  as  to  form  hooks.  When  the  spicules  project  from  the 
horny  framework  they  are  usually  somewhat  conical  in  form,  and 
their  surface  is  often  beset  with  little  spines  arranged  at  regular  inter - 


M 

-nic. 


FIG.  654. — A,  section  through  PhaJcellia  ventilabrutii,  var.  connexida-,  taken  at  right 
angles  to  the  surface,  to  show  the  arrangement  of  the  parts  of  a  sponge :  p,  pores 
on  the  surface  leading  to  z'e,  the  inhalant  canals,  then  to  the  flagellated  chambers, 
fc,  and  thence  to  the  exhalant  canals,  ec,  to  o,  the  oscula  in  the  dermal  membrane. 
dm.  B,  more  highly  magnified  view  of  the  internal  portion  (choanosome)  of 
Axinella  paradoxa  x  290  :  me,  so-called  mesodermal  cells.  Other  letters  as  in  A. 
(After  Kidley  and  Dendy.) 


SPONGE-SPICULES 


859 


vals,  giving  them  a  jointed  appearance.1  The  more  recent  authorities 
on  Sponges,  such  as  Professor  Sollas  arid  Messrs.  Ridley  and  Dendy, 
have  recognised  that  in  the  present  state  of  our  knowledge  the  spicules 
which  are  ordinarily  found  in  silicious  Sponges  belong  to  one  of  two 
groups,  which,  as  they  differ  considerably  in  size,  may  be  called 
megascleres  (or,  more  correctly,  megaloscleres)  and  microscleres.  It  is 
to  the  definite  arrangement  of  the  former  that,  with  or  without  the 
addition  of  spongin,  the  sponge  owes  its  definite  skeleton  ;  the  micro- 
scleres give  consistency  to  the 
tissue  of  the  sponge,  and  are  ir- 
regularly scattered  throughout 
its  substance.  If  we  desire  to 
give  them  physiological  names* 
we  may  call  the  megaloscleres 
skeletal  spicules,  and  the  micro- 
scleres flesh-spicules.  If  we 
bear  in  mind  that  in  the 
opinion  of  the  most  competent 
spongiologists  the  polyaxial 
spicules  are  the  most  primi- 
tive, there  is  no  practical 
objection  to  our  noticing  them 
in  the  reverse  order,  a  method 
which  will  be  found  to  conduce 
to  simplicity  of  description. 
In  the  examination  of  spicules, 
it  is  necessary,  first  of  all,  to 
distinguish  between  axes  and 
rays ;  thus  in  the  Monaxonida 
the  megaloscleres  have  but  a 
single  axis,  but  the  growth 
from  the  point  of  origin  may 
be  on  either  side,  when  we 
have  two-rayed  or  diactinal 
megaloscleres,  or  it  may  extend 
in  one  direction  only,  when  the 
scleres  are  said  to  be  monactinal.  In  the  Calcispongiae  there  are 
three  axes  and  three  rays ;  but  in  some  sponges,  such  as  Venus's 
flower-basket,  the  growth  is  along  both  directions  of  the  axes,  so 
that  while  there  are  three  axes  there  are  six  rays,  or  the  spicules 
are  hexactinellid.  In  others,  such  as  Geodia  and  the  Lithistid 
Sponges,  there  are  four  axes,  whence  such  forms  are  called  tetraxonid. 

1  A  minute  account  of  the  various  forms  of  spicules  contained  in  Sponges  is  given 
by  Mr.  Bowerbank  in  his  first  memoir  'On  the  Anatomy  and  Physiology  of  the 
Spongiadae'  in  Phil.  Trans.  1858,  pp.  279-332;  and  in  his  Monograph  of  the 
British  Spongiadce,  published  by  the  Kay  Society.  The  Calcareous  Sponges  have 
been  made  by  Professor  Haeckel  the  subject  of  an  elaborate  monograph,  Die  Kalk- 
schwdmme,  Berlin,  1872.  For  enumerations  and  classifications  of  the  various  kinds 
of  spicules,  see  Professor  Sollas,  art.  '  Sponges,'  in  the  9th  edition  of  the  Encycl. 
Britannica,  and  Messrs.  Kidley  and  Dendy,  Beport  on  the  '  Challenger'  Monnx- 
<ni  id  a,  pp.  xv-xxi. 


-at 


L..I 
2 

FIG.  655. — Structure  of  the  chelae  of  Mo- 
naxonid  Sponges:  1,  tridentate  anisochela 
from  in  front ;  la,  from  the  side  ;  2,  2a, 
front  and  side  views  of  a  palmate  isochela ; 
t,  t',  tubercle ;  at,  at',  anterior  tooth  or 
palm  ;  It,  It',  lateral  tooth  or  palm  ;  s,  shaft ; 
/',  fimbria.  (After  Eidley  and  Dendy.) 


860  SPONGES   AND   ZOOPHYTES 

Lastly  there  are  the  least  regular  megaloscleres,  which  may  be  multi- 
radiate  or  spherical. 

As  is  well  known,  the  flesh-spicules  are  of  the  most  varied  forms, 
and  it  is  a  matter  of  some  difficulty  so  to  group  them  as  to  render 
them  more  easy  of  comprehension  by  the  student.  Messrs.  Ridley  and 
Dendy  suggest  that,  provisionally  at  any  rate,  we  should  regard  them 
as  (1)  simple  linear,  (2)  hooked,  (3)  stellate.  The  first  may  be  pointed 
at  either  end,  and  these  are  often  spinous,  or  they  may  be  long  and 
hair-like,  and  be  or  not  be  arranged  in  bundles  (dragmata)  ;  a  common 
form  is  that  of  a  bow,  and  these,  again,  are  sometimes  arranged  in 
bundles,  which  have  been  formed  within  one  and  the  same  cell. 
The  hooked  forms  may  be  simple  sigmiform  microscleres,  or  the 
shape  may  be  complicated  by  the  inner  margin  of  the  shaft  or  hook 
thinning  out  to  a  fine  knife-edge.  The  most  complex  forms  of  this 
group  are  the  microscleres  which  the  just-quoted  authors  denominate 
chelce.  They  describe  these  scleres  (fig.  655)  as  having  a  more  or 
less  curved  shaft  (s),  which  bears  at  each  end  a  variable  number  of 

sharply  recurved  processes  (at,  at',  It,  It'), 
which  they  call  the  '  teeth,'  or  if  broad 
and  expanded  the  '  palms  ; '  these  are  con- 
nected with  the  shaft  by  a  buttress-like 
xsoo  f/l/^b  n  projection,  which  is  generally  so  trans- 
parent as  to  be  with  difficulty  made  out. 
The  shaft  itself  is  frequently  drawn  out 
at  the  side  into  wing-like  processes  or 
fimbriae  (/).  If  the  two  ends  of  the 

FIG.  656. — Amsochelse  of  Clado-  -,  -.  •       z.  i         •* 

rhiea  inverse,,  showing  n,  the  spicule  are  equal,  we  have  isochelw  ;  if  un- 
nucleus  of  the  mother-cell  of  equal,  anisochelcc .  The  stellate  micro- 
the  spicule,  from  in  front,  a,  scieres  may  be  spiral,  have  a  shaft  with 
and  from  the  side,  o.  x  300.  .  -,*••  i*  i  •  i  i  A  -^i 

(After  Ridley  and  Dendy.)         spmose  whorls,  or  a  cylindrical  shaft  with 
a    toothed    whorl    at   either   end.     The 

spicules  of  sponges  cannot  be  considered,  like  the  raphides  of  plants, 
as  mere  deposits  of  mineral  matter  in  a  crystalline  state  ;  for,  like 
all  other  parts  of  the  organism,  they  are  of  cellular  origin  (fig.  656), 
and  the  special  cells  which  produce  them  are  distinguished  as  silico- 
blasts  ;  in  this  there  is  first  developed  a  central  organic  thread 
around  which  concentric  layers  of  silica  or  chalk  are  laid  down.1 

There  is  an  extremely  interesting  group  of  Sponges  in  which  the 
horny  skeleton  is  entirely  replaced  by  a  silicious  framework  of  great 
firmness  and  of  singular  beauty  of  construction.  This  framework 
may  be  regarded  as  fundamentally  consisting  of  an  arrangement  of 
six-rayed  spicules,  the  extensions  of  which  come  to  be,  as  it  were, 
soldered  to  one  another ;  and  hence  the  group  is  distinguished  as 
sexradiate.  Of  this  type  the  beautiful  EuplecteUa  of  the  Manila 
seas — which  was  for  a  long  time  one  of  the  greatest  of  zoological 
rarities,  but  which  now,  under  the  name  of  '  Venus's  flower-basket,' 
is  a  common  ornament  of  our  drawing-rooms — is  one  of  the  most 
characteristic  examples.2  Another  example  is  presented  by  the 

1  For  a  compendious  statement  of  the  characters  of  sponge-spicules  see  pp.  82-4 
of  Mr.  A.  Sedgvvick's  Student's  Text-booh  of  Zoology,  London,  1898. 

2  The  structure  and  arrangement  of  the  soft  parts  of  ILnplectella  aspergillum  have 


SPONGES  86 I 

Holtenia  Carpenter  i,  of  which  four  .specimens,  dredged  up  from  a 
depth  of  530  fathoms  between  the  Faroe  Islands  and  the  north  of 
Scotland,  w^ere  among  the  most  valuable  of  the  '  treasures  of  the 
deep '  obtained  during  the  first  deep-sea  exploration  (1868)  carried 
out  by  Sir  Wyville  Thomson  and  the  Author.  This  is  a  tin  nip-shaped 
body,  with  a  cavity  in  its  interior,  the  circular  mouth  of  which  is 
surrounded  with  a  fringe  of  elongated  silicious  spicules  ;  whilst  from 
its  base  there  hangs  a  sort  of  beard  of  silicious  threads  that  extend 
themselves,  sometimes  to  a  length  of  several  feet,  into  the  Atlantic 
mud  on  which  these  bodies  are  found.  The  framework  is  much 
more  massive  than  that  of  Euplectella,  but  it  is  not  so  exclusively 
mineral ;  for  if  it  be  boiled  in  nitric  acid  it  is  resolved  into  separate 
spicules,  these  being  not  soldered  together  by  silicious  continuity, 
but  held  together  by  animal  matter.  Besides  the  regular  sex- 
radiate  spicules,  there  is  a  remarkable  variety  of  other  forms,  which 
have  been  fully  described  and  figured  by  Sir  Wyville  Thomson.1 
One  of  the  greatest  features  of  interest  in  this  Holtenia  is  its 
singular  resemblance  to  the  Ventriodites  of  the  Cretaceous  formation. 
Subsequent  investigations  have  shown  that  it  is  very  widely  diffused, 
and  that  it  is  only  one  of  several  deep-sea  forms,  including  some 
of  singularly  beautiful  structure,  which  are  the  existing  repre- 
sentatives of  the  old  ventriculite  type.  One  of  these  was  previously 
known  from  being  occasionally  cast  up  on  the  shore  of  Barbadoes 
after  a  storm.  This  Dictyocalyx  puniiceus  has  the  shape  of  a  mush- 
room, the  diameter  of  its  disc  sometimes  ranging  to  a  foot.  A  small 
portion  of  its  reticulated  skeleton  is  a  singularly  beautiful  object 
when  viewed  with  incident  light  under  a  low  magnifying  power. 

With  the  exception  of  the  genus  ftpongilla  and  its  allies,  all 
known  sponges  are  marine,  but  they  differ  very  much  in  habit  of 
growth.  For  whilst  some  can  only  be  obtained  by  dredging  at  con- 
siderable depths,  others  live  near  the  surface,  whilst  others  attach 
themselves  to  the  surfaces  of  rocks,  shells,  Arc.  between  the  tide- 
marks.  The  various  species  of  Grantia  in  which,  of  all  the  marine 
sponges,  the  flagellate  cells  can  most  readily  be  observed,  belong  to 
this  last  category.  They  have  a  peculiarly  simple  structure,  each 
being  a  sort  of  bag  whose  wall  is  so  thin  that  no  system  of  canals  is 
required,  the  water  absorbed  by  the  outer  surface  passing  directly 
towards  the  inner,  and  being  expelled  by  the  mouth  of  the  bag.  The 
flagella  may  be  plainly  distinguished  with  a  ^-inch  objective  on  some 
of  the  cells  of  the  gelatinous  substance  scraped  from  the  interior  of 
the  bag ;  or  they  may  be  seen  in  situ  by  making  very  thin  trans- 
verse sections  of  the  substance  of  the  sponge.  It  is  by  such  sections 
alone  that  the  internal  structure  of  sponges,  and  the  relation  of 
their  spicular  and  horny  skeletons  to  their  fleshy  substance,  can  be 
demonstrated.  They  are  best  made  by  the  imbedding  process.  In 
order  to  obtain  the  spicules  in  an  isolated  condition,  the  animal 
matter  must  be  got  rid  of  either  by  incineration  or  by  chemical 

been  investigated  by  Prof.  F.  E.  Schulze,  Trans.  Royal  Soc.  of  Edinburgh,  xxix. 
p.  661. 

-  See  his  elaborate  memoir  in  Phil.  Trans.  1870,  and  his  Depths  of  the  Sea 
1872  p.  71. 


862 


SPONGES   AND   ZOOPHYTES 


reagents.  The  latter  method  is  preferable,  as  it  is  difficult  to  free 
the  mineral  residue  from  carbonaceous  particles  by  heat  alone.  If 
(as  is  commonly  the  case)  the  spicules  are  silicioas,  the  sponge  may 
be  treated  with  strong  nitric  or  nitre-muriatic  acid,  until  its  animal 
substance  is  dissolved  away ;  if,  on  the  other  hand,  they  are  cal- 
careous, a  strong  solution  of  potass  may  be  employed  instead  of  the 
acid.  The  operation  is  more  rapidly  accomplished  by  the  aid  of 
heat  ;  but  if  the  saving  of  time  be  not  of  importance,  it  is  preferable 
on  several  accounts  to  dispense  with  it.  The  spicules,  when  obtained 
in  a  separate  state,  should  be  mounted  in  Canada  balsam.  Sponge 
tissue  may  often  be  distinctly  recognised  in  sections  of  agate, 
chalcedony,  and  other  silicious  concretions,  as  will  be  more  fully 
stated  hereafter.1 

II.  ZOOPHYTES  (CCELENTERA) 

Under  the   general  designation  Zoophytes  it  will  be  still  con- 
venient to  group  those  animals  which  form  composite  skeletons  or 

*  polyparies '  of  a  more  or  less 
plant-like  character,  associating 
with  them  the  Acalephs,  which 
are  now  known  to  be  the  '  sexual 
zooids'  of  polypes,  but  excluding 
the  Polyzoa  on  account  of  their 
very  different  structure,  not- 
withstanding their  zoophytic 
forms  and  habits  of  life.  The 
animals  belonging  to  this  group 
may  be  considered  as  formed 
upon  the  primitive  yastrula 
type,  their  gastric  cavity 
(though  sometimes  extending 
itself  almost  indefinitely)  being 
lined  by  the  original  endoderm, 
and  their  surface  being  covered 
by  the  original  ectoderm,  and 
these  two  lamella?  not  being 
separated  by  the  interposition 
of  any  >ody-cavity  or  ccelom. 
It  is  a  fact  of  great  interest 
FIG.  657.— Longitudinal  section  of  the  body  that  although  the  product  of 
of  a  hydra  killed  in  full  digestion:  ec,  the  development  of  a  morula  is 
ectoderm;  en,  eiidoderm ',  mp  muscular  here  a  distinctly  individualised 
processes ;  a,  a  diatom ;  j ,  rood.  (Alter  ,  .  r  •  i  i 

T.  J.  Parker.)  PorvPe>  m  which  several  mutu- 

ally dependent  parts  make    up 

a  single  organic  whole,  yet  these  parts  still  retain  much  of  their 
independent  protozoic  life  ;  which  is  manifested  in  two  very  re- 
markable modes.  In  the  first  place,  the  digestive  sac  is  observed 
to  be  lined  by  a  layer  of  amoeboid  cells,  which  send  out  pseudopodial 

1  A  complete  and  valuable  handbook  to  the  Sponges  has  been  published  by 
Dr.  G.  C.  Vosmaer  as  vol.  ii.  of  Bromi's  Klassen  und  Ordnungen  des  Thierreichs, 
Leipzig,  1887.  Compare  also  the  article  by  Professor  Sollas  in  the  ninth  edition  of  the 


CCELENTERA  863 

prolongations  into  its  cavity  (fig.  657)  by  whose  agency  (it  may  be 
pretty  certainly  affirmed)  the  nutrient  material  is  first  introduced 
into  the  body-substance.  This  process  of  ;  intracellular  digestion  ' 
was  first  noticed  by  Professor  Allman  in  the  beautiful  hydroid  polype 
Mt/riothela ; l  the  like  has  been  since  shown  by  Mr.  Jeffery  Parker 
to  be  true  of  the  ordinary  Hydra  ;  -  and  Professor  E.  Ray  Lankester 
has  made  the  same  observation  upon  the  curious  little  Medusa  (Lint/to- 
codium),  which  lives  in  fresh- water  tanks  in  this  country,  whither 
it  has  undoubtedly  been  introduced  ;  while  the  observations  of 
Tvrukeiiberg  have  shown  that  a  similar  process  obtains  among  the  sea- 
anemones.3  (It  may  be  mentioned  in  this  connection,  that  Metschni- 
koff  has  seen  the  cells  which  line  the  alimentary  canal  of  the  lower 
planarian  worms  gorging  themselves  with  coloured  food-particles, 
exactly  in  the  manner  of  Amcefow  and  the  liver-fluke,  and  that  a 
number  of  larvre  are  known  to  obtain  their  nourishment  in  the  same 
way.4)  The  second  '  survival '  of  protozoic  independence  is  shown 
in  the  extraordinary  power  possessed  by  Hydra,  Actinia,  etc.  of 
reproducing  the  entire  organism  from  a  mere  fragment.  This  great 
division  includes  the  two  principal  groups  the  HYDROZOA  and  the 
ACTIXOZOA,  the  former  comprehending  the  Polypes,  and  the  latter 
the  Anemones.  In  the  Hydrozoa  the  mouth  is  placed  on  a  projecting 
oral  cone,  while  in  the  Aiithozoa  it  is  sunk  below  the  level  of  the  oral 
circlet  of  tentacles,  and  the  cavity  developed  from  and  connected 
with  the  digestive  cavity  separates  its  wrall  from  the  body-wall  arid 
is  traversed  by  a  series  of  vertical  partitions  or  septa.  As  most  of 
the  hydroid  polypes  are  essentially  microscopic  animals,  they  need 
to  be  described  with  some  minuteness  ;  whilst  in  regard  to  the 
Actinozoa  those  points  only  will  be  dwelt  on  which  are  of  special 
interest  to  the  microscopist. 

Hydrozoa. — The  type  of  this  group  is  the  Hydra,  or  fresh-water 
polype,  a  very  common,  inhabitant  of  pools  and  ditches,  where  it  is 
most  commonly  to  be  found  attached  to  the  leaves  or  stems  of  aquatic 
plants,  floating  pieces  of  stick,  Arc.  Two  species  are  common  in  this 
country,  the  //.  virldls  or  green  polype,  and  the  H.  vulyaris,  which 
is  usually  orange-brown,  but  sometimes  yellowish  or  red  (its  colour 
being  liable  to  some  variation  according  to  the  nature  of  the  food 
on  which  it  has  been  subsisting)  ;  a  third  less  common  species,  the 
H.fusca,  is  distinguished  from  both  the  preceding  by  the  length  of 
its  tentacles,  which  in  the  former  are  scarcely  as  long  as  the  body, 
whilst  in  the  latter  they  are.  when  fully  extended,  many  times  longer 

Encyclopedia  lirit<innir«  :  the  '  CliuUenger'  licjiorts  by  Professor  Schulze,  Messrs. 
Ridley  and  Dendy,  Polejaeff,  and  Sollas ;  and  the  numerous  memoirs  of  Professors 
O.  Schmidt  and  Schulze.  More  recently  important  additions  to  our  knowledge  of 
Sponges  have  been  made  by  Prof.  Yves  Delage  and  Monsieur  E.  Topsent  in  the 
Arch.  Zool.  Exptr.  ct  (rt'-n.  189-2-5,  and  by  Dr.  O.  Maas  in  the  Mitth.  Zool.  Stat. 
Neapel,  x.  and  elsewhere. 

1  Phil.  Trans.  1875,  p.  552.  It  should  be  noted  that  the  late  Professor  Claus 
called  attention  to  the  ingestion  of  foreign  bodies  by  amoeboid  cells  of  Monophyes 
in  1874.  See  his  Schriften  Zool.  Inltalts  I  Wien,  1874!,  p.  30. 

-  Proc.  of  Hoy.  Soc.  vol.  xxx.  1880,  p.  (>1. 

"   Quart.  Jo//r)i.  Mir  rase.  Sci.  n.s.  vol.  xx.  1880,  p.  871. 

4  Consult  an  interesting  article  on  'Intercellular  Digestion,'  by  Metschnikoff,  in 
Revue  Sclent  ifiqiic,  ser.  iii.  vol.  xi.  p.  683. 


864 


SPONGES   AND   ZOOPHYTES 


(fig.  658). 1  The  body  of  the  Hydra  consists  of  a  simple  bag  or  sac, 
which  may  be  regarded  as  a  stomach,  and  is  capable  of  varying  its 
shape  and  dimensions  in  a  very  remarkable  degree,  sometimes  ex- 
tending itself  in  a  straight  line  so  as  to  form  a  long  narrow  cylinder, 
at  other  times  being  seen  (when  empty)  as  a  minute  contracted 
globe,  whilst,  if  distended  with  food,  it  may  present  the  form  of  an 
inverted  flask  or  bottle,  or  even  of  a  button.  At  the  upper  end  of 
this  sac  is  a  central  opening,  the  mouth  ;  and  this  is  surrounded  by 

a  circle  of  tentacles  or  'arms,' 
usually  from  six  to  teninnumber, 
wrhich  are  arranged  with  great 
regularity  around  the  orifice. 
The  body  is  prolonged  at  its  lower 
end  into  a  narrow  base,  which 
is  furnished  with  a  suctorial  disc, 
and  the  Hydra  usually  attaches 
itself  by  this,  while  it  allowrs  its 
tendril -like  tentacles  to  float 
freely  in  the  water.  The  wall 
of  the  body  is  composed  of  two 
layers  of  cells ;  and  between 
these,  which  are  the  ectoderm 
and  endoderm,  there  is  a  deli- 
cate intermediate  layer,  which 
forms  the  supporting  lamella.'2 
The  arms  are  made  up  of  the 
same  materials  as  the  body  ; 
but  their  surface  is  beset  with 
little  wart-like  prominences, 
which,  when  carefully  examined, 
are  found  to  be  composed  of 
clusters  of  '  thread-cells,'  having 
a  single  large  cell  with  a  long 
spiculum  in  the  centre  of  each. 
The  structure  of  these  thread - 
cells  or  '  urticating  organs'  will 
be  described  hereafter ;  at  pre- 
FIG.  658.— Hydra  fusca,  with  a young  bud  sent  it  will  be  enough  to  point 
at  b,  and  a  more  advanced  bud  at  c.  out  that  fafe  apparatus,  repeated 

many  times  on  each  tentacle,  is 

doubtless  intended  to  give  to  the  organ  a,  great  prehensile  power, 
the  minute  filaments  forming  a,  rough  surface  adapted  to  prevent 
the  object  from  readily  slipping  out  of  the  grasp  of  the  arm,  whilst 
the  central  spicule  or  '  dart '  is  projected  into  its  substance,  probably 
conveying  into  it  a  poisonous  fluid  secreted  by  a  vesicle  at  its  base. 

1  On  the  specific  characters  of  Hydra  consult  Haacke,  Jenaisclie  Zeitsclir.  xiv. 
p.  133;  and  Jickeli,  Zool.  Anzeig.  v.  p.  491. 

*  To  this  intermediate  layer,  Mr.  G.  C.  Bourne  applies  the  term  mesoglcea.  For  an 
account  of  its  variations  and  structure  among  the  Coelentera,  and  a  discussion  of  its 
homology  with  the  mesoderm  of  higher  Metazoa,  see  his  essay  on  Fangia  in  vol. 
xxvii.  of  the  Quart.  Journ.  Microsc.  8ci.  n.s. 


HYDKOZOA 


865 


The  latter  inference  is  founded  upon  the  oft-repeated  observation 
that  if  the  living  prey  seized  by  the  tentacles  have  a  body  destitute 
of  hard  integument,  as  is  the  case  with  the  minute  aquatic  worms 
which  constitute  a  large  part  of  its  aliment,  this  speedily  dies, 
even  though,  instead  of  being  swallowed,  it  escapes  from  their  grasp  ; 
whilst,  on  the  other  hand,  minute  Entomostraca,  insects,  and  other 
animals  or  ova,  with  hard  envelopes,  may  escape  without  injury,  even 
after  having  been  detained  for  some  time  in  the  polype's  embrace. 
The  contractility  of  the 
tentacles  (the  interior 
of  which  is  traversed  by 
a  canal  that  communi- 
cates with  the  cavity 
of  the  stomach)  is  very 
remarkable,  especially 
in  the  Hydra  fusca, 
whose  arms,  when  ex- 
tended in  search  of 
prey,  are  not  less  than 
seven  or  eight  inches  in 
length  ;  whilst  they  are 
sometimes  so  contract- 
ed, when  the  stomach  xx:< 

fe 


is  filled  with  food,  as 
to  appear  only  like  little 
tubercles  around  its  en- 
trance. By  means  of 
these  instruments  the 
Hydra  is  enabled  to 
draw  its  support  from 
animals  whose  activity, 
as  compared  with  its 
own  slight  powers  of 
locomotion,  might  have 
been  supposed  to  re- 
move  them  altogether 
from  its  reach  ;  for 
when,  in  its  movements 
through  the  water,  a 
minute  worm  or  a  water- 
flea  happens  to  touch 
one  of  the  tentacles  of 

the  polype,  spread  out  as  these  are  in  readiness  for  prey,  it  is 
immediately  seized  by  this ;  other  arms  are  soon  coiled  around  it, 
and  the  unfortunate  victim  is  speedily  conveyed  to  the  stomachr 
within  which  it  may  frequently  be  seen  to  continue  moving  for 
some  little  time.  Soon,  however,  its  struggles  cease,  and  its  outline 
is  obscured  by  a  turbid  film,  which  gradually  thickens,  so  that  at 
last  its  form  is  wholly  lost.  The  soft  parts  are  soon  completely  dis- 
solved, and  the  harder  indigestible  portions  are  rejected  through  the 
mouth.  A  second  orifice  has  been  observed  at  the  lower  extremity 

3  K 


FIG.  659. — Campanularia  gelatinosa. 


866 


SPONGES   AND   ZOOPHYTES 


of  the  stomach ;  but  this  would  not  seem  to  be  properly  regarded  as 
anal,  since  it  is  not  used  for  ihe  discharge  of  such  exuvi?e ;  it  is 
probably  rather  to  be  considered  as  representing,  in  the  Hydra,  the 
entrance  to  that  ramifying  cavity  which,  in  the  compound  Hydrozoa, 
brings  into  mutual  connection  the  lower  extremities  of  the  stomachs 
of  all  the  individual  polypes. 

The  ordinary  mode  of  reproduction  in  this  animal  is  by  a  '  gemma- 
tion '  resembling  that  of  plants.  Little  bud-like  processes  (fig.  658, 
6,  c)  developed  from  its  external  surface  gradually  come  to  resemble 
the  parent  in  character,  and  to  possess  a  digestive  sac,  mouth,  and 
tentacles  ;  for  a  long  time,  however,  their  cavity  is  connected  with 
that  of  the  parent,  but  at  last  the  communication  is  cut  off  by  the 

closure  of  the  canal  of  the  foot- 
stalk, and  the  young  polype  quits 
its  attachment  and  goes  in  quest 
of  its  own  maintenance.  A 
second  generation  of  buds  is 
sometimes  observed  on  the  young 
polype  before  quitting  its  parent ; 
and  as  many  as  nineteen  young 
Hydrce  in  different  stages  of 
development  have  been  seen  thus 
connected  with  a  single  original 
stock  (fig.  660).  This  process 
takes  place  most  rapidly  under 
the  influence  of  warmth  and 
abundant  food  ;  it  is  usually  sus- 
pended in  winter,  but  may  be 
made  to  continue  by  keeping  the 
polypes  in  a  warm  situation  and 
well  supplied  with  food.  Another 
very  curious  endowment  seems  to 
depend  on  the  same  condition — 
the  extraordinary  power  which 
one  portion  possesses  of  repro- 
ducing the  rest.  Into  whatever 
number  of  parts  a  Hydra  may 
divided,  each  may  retain  its 
vitality,  and  give  origin  to  a 

new  and  entire  fabric  ;  so  that  thirty  or  forty  individuals  may 
be  formed  by  the  section  of  one.  The  Hydra  also  propagates  itself, 
however,  by  a  truly  sexual  process,  the  fecundating  apparatus,  or 
vesicle  producing  '  sperm-cells,'  and  the  ovum  (containing  the  '  germ- 
cell,'  imbedded  in  a  store  of  nutriment  adapted  for  its  early  develop- 
ment), being  both  evolved  in  the  substance  of  the  walls  of  the 
stomach — the  male  apparatus  forming  a  conical  projection  just 
beneath  the  arms,  while  the  female  ovary,  or  portion  of  the  body- 
substance  in  which  the  ovum  is  generated,  has  the  form  of  a 
knob  protruding  from  the  middle  of  its  length.  It  would  appear 
that  sometimes  one  individual  Hydra  develops  only  the  male  cysts 
or  sperm-cells,  while  another  develops  only  the  female  cysts  or  ovi- 


FIG.  660.— Hydra  fusca  in  gemmation  ; 
mouth :  6,  base ;  c.  origin  of  one  of  the  n 
buds.  be 


HYDROZOA  867 

sacs  ;  but  the  general  rule  seems  to  be  that  the  same  individual 
forms  both  organs.  The  fertilisation  of  the  ova,  however,  cannot 
take  place  until  after  the  rupture  of  the  spermatic  cyst  and  of  the 
ovisac,  by  which  the  contents  of  both  are  set  free.  The  autumn  is 
the  chief  time  for  the  development  of  the  sexual  organs,  but  they 
also  present  themselves  in  the  earlier  part  of  the  year,  chiefly  be- 
tween April  and  July.  According  to  Eeker,  the  eggs  of  H.  viridis 
produced  early  in  the  season  run  their  course  in  the  summer  of 
the  same  year  ;  while  those  produced  in  the  autumn  pass  the  winter 
without  change.  When  the  ovum  is  nearly  ripe  for  fecundation 
the  ovary  bursts  its  ectodermal  covering,  and  remains  attached  by 
a  kind  of  pedicle.  It  seems  t^  be  at  this  stage  that  the  act  of 
fecundation  occurs ;  a  very  strong  elastic  shell  or  capsule  then 
forms  round  the  ovum,  the  surface  of  which  is  in  some  cases  studded 
with  spine-like  points,  in  others  tuberculated,  the  divisions  between 
the  tubercles  being  polygonal.  The  ovum  finally  drops  from  its 
pedicle,  and  attaches  itself  by  means  of  a  mucous  secretion,  till  the 
hatching  of  the  young  Hydra,  which  comes  forth  provided  with  four 
rudimentary  tentacles  like  buds.  The  Hydra  possesses  the  power  of 
free  locomotion,  being  able  to  remove  from  the  spot  to  which  it  has 
attached  itself  to  any  other  that  may  be  more  suitable  to  its  wants  ; 
its  changes  of  place,  however,  seem  rather  to  be  performed  under  the 
influence  of  light,  towards  which  the  Hydra  seeks  to  move  itself,  than 
with  reference  to  the  search  after  food.1 

The  compound  Hydroids  may  be  likened  to  a  Hydra  whose 
gemmae,  instead  of  becoming  detached,  remain  permanently  connected 
with  the  parent  ;  and  as  these  in  their  turn  may  develop  gemma? 
from  their  own  bodies,  a  structure  of  more  or  less  arborescent 
character,  termed  a  poly  par y,  may  be  produced.  The  form  which 
this  will  present,  and  the  relation  of  the  component  polypes  to  each 
other,  will  depend  upon  the  mode  in  which  the  gemmation  takes 
place  ;  in  all  instances,  however,  the  entire  cluster  is  produced  by 
continuous  growth  from  a  single  individual ;  and  the  stomachs  of  the 
several  polypes  are  united  by  tubes,  which  proceed  from  the  base  of 
each,  along  the  stalk  and  branches,  to  communicate  with  the  cavity 
of  the  central  stem.  Whatever  may  be  the  form  taken  by  the  stem 
and  branches  constituting  the  polypary  of  a  hydroid  colony,  they  will 
be  found  to  be,  or  to  contain,  fleshy  tubes  having  two  distinct  layers, 
the  inner  (endoderm)  having  nutritive  functions  ;  the  outer  (ecto- 
derm) usually  secreting  a  hard  cortical  layer,  and  thus  giving  rise 
to  fabrics  of  various  forms.  Between  these  a  muscular  coat  is  some- 
times noticed.  The  fleshy  tube,  whether  single  or  compound,  is  called 
a  ceenosarc,  and  through  it  the  nutrient  matter  circulates.  The 
*  zboids,'  or  individual  members  of  the  colony,  are  of  two  kinds  :  one 
the  polypite,  or  alimentary  zooid,  resembling  the  Hydra  in  essential 

1  A  very  full  account  of  the  structure  and  development  of  Hydra  has  been 
published  by  Kleinenberg,  of  whose  admirable  monograph  a  summary  is  given  by 
Professor  Allman,  with  valuable  remarks  of  his  own,  in  Qua rt.  Journ.  Microsc.  Sci.  n.s. 
vol.  xiv.  1874,  p.  1.  See  also  the  important  paper  by  the  late  Mr.  Jeffery  Parker 
already  cited.  On  the  chlorophyll  corpuscles  of  H.  viridis  consult  Brandt,  Mittli. 
Zool.  Stot.  Neapel,  iv.  p.  191 ;  Hamann,  Zool.  Anzeig.  vi.  p.  367;  and  Lankester 
Quart.  Journ.  Microsc.  Sci.  n.s.  xxii.  p.  229. 

3K2 


868  SPONGES   AND   ZOOPHYTES 

structure,  and  more  or  less  in  aspect ;  the  other,  the  gonozooid,  or 
sexual  zooid,  developed  at  certain  seasons  only,  in  buds  of  particular 
shape.1 

The  simp  lest  division  of  the  Hydroida  is  that  adopted  by  Mr. 
Hincks,2  who  groups  them  under  the  sub-order  Athecata  and  Thecata, 
the  latter  being  again  divided  into  the  Tktcapkora  and  the  Gymno- 
chroa.  In  the  first,  neither  the  'polypites'  nor  the  sexual  zooids 
bear  true  protective  cases ;  in  the  second  the  polypites  are  lodged  in 
cells,  or,  as  Mr.  Hincks  prefers  to  call  them,  calycles,  many  of  which 
resemble  exquisitely  formed  crystal  cups,  variously  ornamented,  and 
sometimes  furnished  with  lids  or  opercula;  in  the  third,  which  con- 
tains the  Hydras,  there  is  no  polypary,  and  the  reproductive  zooids 
(gonozooids)  are  always  fixed  and  developed  in  the  body-walls.  Ac- 
cording to  Mr.  Hincks,  the  two  sexes  are  sometimes  borne  on  the 
same  colony,  but  more  commonly  the  zoophyte  is  direcious.  The 
cases,  however,  are  much  less  rare  than  has  been  supposed  in  which 
both  male  and  female  are  mingled  on  the  same  shoots.  The  sexual 
zooids  either  remain  attached,  and  discharge  their  contents  at 
maturity,  or  become  free  and  enter  upon  an  independent  existence. 
The  free  forms  nearly  always  take  the  shape  of  Medusce  (jelly-fish), 
swimming  by  rhythmical  contractions  of  their  bell  or  umbrella.  The 
digestive  cavity  is  in  the  handle  (manubrium)  of  the  bell ;  and  the 
generative  elements  (sperm-cells  or  ova)  are  developed  either  between 
the  membranes  of  the  manubrium  or  in  special  sacs  in  the  canals 
radiating  from  it.  The  ova,  when  fertilised  by  the  spermatozoa, 
undergo  '  segmentation '  according  to  the  ordinary  type,  the  whole 
yolk-mass  subdividing  successively  into  two,  four,  eight,  sixteen, 
thirty-two  or  more  parts,  until  a  '  mulberry  mass '  is  formed  ;  this 
then  begins  to  elongate  itself,  its  surface  being  at  first  smooth  and 
showing, a  transparent  margin,  but  afterwards  becoming  clothed  with 
cilia,  by  whose  agency  these  little  planulce,  closely  resembling  ciliated 
Infusoria,  first  move  about  within  the  capsule,  and  then  swim  forth 
freely  when  liberated  by  the  opening  of  its  mouth.  At  this  period 
the  embryo  can  be  made  out  to  consist  of  an  outer  and  an  inner 
layer  of  cells,  with  a  hollow  interior  ;  after  some  little  time  the  cilia 
disappear,  and  one  extremity  becomes  expanded  into  a  kind  of  disc 
by  which  it  attaches  itself  to  some  fixed  object ;  a  mouth  is  formed, 
and  tentacles  sprout  forth  around  it ;  and  the  body  increases  in  length 
and  thickness,  so  as  gradually  to  acquire  the  likeness  of  one  of  the 
parent  polypes,  after  which  the  '  polypary '  characteristic  of  the  genus 
is  gradually  evolved  by  the  successive  development  of  polype-buds 
from  the  first-formed  polype  and  its  subsequent  offsets.  The  Medusae 
of  these  polypes  (fig.  663)  belong  to  the  division  called  '  naked-eyed,' 
on  account  of  the  eye-spots  usually  seen  surrounding  the  margin  of 
the  bell  at  the  base  of  the  tentacles. 

A  characteristic  example  of  this  production  of  medusa-like 
'  gonozooids  '  is  presented  by  the  form  termed  Syrtcoryne  Sarsii  (fig. 

1  A  useful  list  of  the  principal  terms  used  in  describing  hydroids,  with  definitions, 
will  be  found  oil  pp.  16  and  17  of  Professor  Allman's  Rc2)ort  on  the  Hydroida  (Plu- 
•intil(iriidce)  of  the  Challenger. 

-  History  of  British  Hydroid  Zoophytes,  18(58. 


DEVELOPMENT  OF  HYDROZOA 


869 


661)  belonging  to  the  sub-order  Athecata.  At  A  is  shown  the  ali- 
mentary zooid,  or  polypite,  with  its  tentacles,  and  at  B  the  succes- 
sive stages  a,  5,  c,  of  the  sexual  zooids,  or  medusa-buds.  When 
sufficiently  developed  the  Medusa  swims  away,  and  as  it  grows  to 
maturity  enlarges  its  manubrium,  so  that  it  hangs  below  the  bell. 
The  Medusae  of  the  genus  Syncoryne  (as  now  restricted)  have  the 
form  named  Sarsia  in  honour  of  the  Swedish  naturalist  Sars.  Theii 
normal  character  is  that  of  free  swimmers  ;  but  Agassiz,  ascertained 
that  in  some  cases  towards  the 
end  of  the  breeding  season  the 
sexual  zooids  remain  fixed,  and 
mature  their  products  while  at- 
tached to  the  zoophyte.1  This 
latter  condition  of  the  sexual 
zooids  is  very  common  amongst 
the  Hydroirla  ;  and  various  inter- 
mediate stages  may  be  traced  in 
different  genera  between  the 
mode  in  which  the  gonozooids 
are  produced  in  the  common 
Hydra,  as  already  described,  and 
that  of  Syncoryne.  In  Tubu- 
lar ia  the  gonozooids,  though 
permanently  attached,  are  fur- 
nished with  swimming  bells, 
1  mving  four  tubercles  repre- 
senting marginal  tentacles.  A 
common  and  interesting  species, 
Tutnda/ria  indivisa,  receives  its 
specific  name  from  the  infre- 
quency  with  which  branches  are 
given  off  from  the  stems,  these  for 
the  most  part  standing  erect  and 
parallel,  like  the  stalks  of  corn, 
upon  the  base  to  which  they  are  F  66l. -Development  of  Medusa-bud 

,,J_J ,1 1  TVU'  T ..j.ZC-.'l         _••_  A; 


in  Syncoryne  Sarsii :  A,  an  ordinary 
polype,  with  its  club-shaped  body  covered 
with  tentacles  ;  B,  a  polype  putting  forth 
inedusoid  gemmae  ;  a,  a  very  young  bud  ; 
b,  a  bud  more  advanced,  the  quadran- 
gular form  of  which,  with  the  four 
nuclei  whence  the  cirrhi  afterwards 
spring,  is  shown  at  d;  c,  a  bud  still 
more  advanced. 


attached.  This  beautiful  zoo- 
phyte, which  sometimes  grows 
between  the  tide-marks,  but  is 
more  abundantly  obtained  by 
dredging  in  deep  water,  often 
attains  a  size  which  renders  it 
scarcely  a  microscopic  object,  its 
stems  being  sometimes  no  less 

than  a  foot  in  height  and  a  line  in  diameter.  Several  curious 
phenomena,  however,  are  brought  into  view  by  microscopic  examina- 
tion. The  polype-stomach  is  connected  with  the  cavity  of  the 
stem  by  a  circular  opening,  which  is  surrounded  by  a  sphincter  ; 
and  an  alternate  movement  of  dilatation  and  contraction  takes 
place  in  it,  fluid  being  apparently  forced  up  from  below,  and  then 
expelled  again,  after  which  the  sphincter  closes  in  preparation  for 

1  Hincks,  op.  cit.  p.  49. 


8/0  SPONGES  AND   ZOOPHYTES 

a  recurrence  of  the  operation,  this,  as  observed  by  Mr.  Lister, 
being  repeated  at  intervals  of  eighty  seconds.  Besides  the  foregoing 
movement,  a  regular  flow  of  fluid,  carrying  with  it  solid  particles  of 
various  sizes,  may  be  observed  along  the  whole  length  of  the  stem, 
passing  in  a  somewhat  spiral  direction.  It  is  worthy  of  mention 
here  that  when  a  Tubularia  is  kept  in  confinement  the  polype-heads 
almost  always  drop  off  after  a  few  days,  but  are  soon  renewed 
by  a  new  growth  from  the  stem  beneath ;  and  this  exuviation  and 
regeneration  may  take  place  many  times  in  the  same  individual.1 

It  is  in  the  families  Gampanulariida  and  Sertulariida  (whose 
polyparies  are  commonly  known  as  'corallines')  that  the  horny 
branching  fabric  attains  its  completest  development,  not  only  afford- 
ing an  investment  to  the  stem,  but  forming  cups  or  cells  for  the 
protection  of  the  polypites,  as  well  as  capsules  for  the  reproductive 
gonozb'oids.  Both  these  families  thus  belong  to  the  sub-order  Thecata. 
In  the  Gampanulariida  the  polype-cells  are  campanulate  or  bell- 
shaped,  and  are  borne  at  the  extremities  of  ringed  stalks  (fig.  659,  c) ; 
in  the  Sertulariida,  on  the  other  hand,  the  polype-cells  lie  along  the 
stem  and  branches,  attached  either, to  one  side  only,  or  to  both  sides 
(fig.  662).  In  both  the  general  structure  of  the  individual  polypes 
(fig.  659,  B,  d)  closely  corresponds  with  that  of  the  Hydra  ;  and  the 
mode  in  which  they  obtain  their  food  is  essentially  the  same.  Of 
the  products  of  digestion,  however,  a  portion  finds  its  way  down  into 
the  tubular  stem,  for  the  nourishment  of  the  general  fabric  ;  and 
very  much  the  same  kind  of  circulatory  movement  can  be  seen  in 
Campamdaria  as  in  Tubularia,  the  circulation  being  most  vigorous 
in  the  neighbourhood  of  growing  parts.  It  is  from  the  '  ccenosarc ' 
(fig.  659,  f)  contained  in  the  stem  and  branches  that  new  polype- 
buds  (b)  are  evolved  ;  these  carry  before  them  (so  to  speak)  a  portion 
of  the  horny  integument,  which  at  first  completely  invests  the  bud ; 
but  as  the  latter  acquires  the  organisation  of  a  polype,  the  case 
thins  away  at  its  most  prominent  part,  and  an  opening  is  formed 
through  which  the  young  polype  protrudes  itself. 

The  origin  of  the  reproductive  capsules  or  '  gonothecre '  (e)  is 
exactly  similar,  but  their  destination  is  very  different.  Within 
them  are  evolved,  by  a  budding  process,  the  generative  organs  of 
the  zoophyte  ;  and  these  in  the  Campanulariida  may  either  develop 
themselves  into  the  form  of  independent  .medusoids,  which  com- 
pletely detach  themselves  from  the  stock  that  bore  them,  make  their 
way  out  of  the  capsule,  and  swim  forth  freely,  to  mature  their 
sexual  products  (some  developing  sperm-cells,  and  others  ova),  and 
give  origin  to  a  new  generation  of  polypes ;  or,  in  cases  in  which 
the  medusoid  structure  is  less  distinctly  pronounced,  may  not  com- 
pletely detach  themselves,  but  (like  the  flower-buds  of  a  plant)  expand 
one  after  another  at  the  mouth  of  the  capsule,  withering  and  drop- 
ping off  after  they  have  matured  their  generative  products.  In  the 
fiertulariida,  on  the  other  hand,  the  medusan  conformation  is  wanting, 
as  the  gonozooids  are  always  fixed  ;  the  reproductive  cells  (fig.  662, #), 
which  were  shown  by  Professor  Edward  Forbes  to  be  reallv  meta- 

1  The  British  Tulnlariida  form  the  subject  of  a  most  complete  and  beautiful 
monograph  by  the  late  Professor  Allman,  published  by  the  Kay  Society. 


COLLECTING  ZOOPHYTES 


87I 


morphosed  branches,  developing  in  their  interior  certain  bodies  which 
were  formerly  supposed  to  be  ova,  but  which  are  now  known  to  be 
'  medusoids  '  reduced  to  their  most  rudimentary  condition.  Within 
these  are  developed — in  separate  gonothecse,  sometimes  perhaps  on 
distinct  polyparies — spermatozoa  and  ova  ;  and  the  latter  are  ferti- 
lised by  the  entrance  of  the  former  whilst  still  contained  within 
their  capsules.  The  fertilised  ova,  whether  produced  in  free  or  in 
attachecl  medusoids,  develop  themselves  in  the  first  instance  into 
ciliated  '  gemmules,'  or  planulse,  which  soon  evolve  themselves  into 
true  polypes,  from  every  one  of  which  a  new  composite  polypary 
may  spring. 

There  are  few  parts  of  our  coast  which  will  not  supply  some  or 
other  of  the  beautiful  and 
interesting  forms  of  zoo- 
phytic  life  which  have  been 
thus  briefly  noticed,  with- 
out any  more  trouble  in 
searching  for  them  than 
that  of  examining  the  sur- 
faces of  rocks,  stones,  sea- 
weeds, and  dead  shells 
between  the  tide-marks. 
Many  of  them  habitually 
live  in  that  situation ;  and 
others  are  frequently  cast 
up  by  the  waves  from  the 
deeper  waters,  especially 
after  a  storm .  Many  kinds, 
however,  can  only  be  ob- 
tained by  means  of  the 
dredge.  Of  the  remarkable 
forms  dredged  by  the  'Chal- 
lenger' mention  can  only 
be  made  here  of  the  gigantic 
Tubularian — Monocaulus — 
the  stem  of  which  measured 
seven  feet  four  inches, 
while  there  was  a  spread 
of  nine  inches  from  tip  to  tip  of  the  extended  tentacles,  and  of  the 
elegant  Streptocmdus  pulchewimus,  in  which  by  the  twisting  of 
the  stem  the  ultimate  ramules  are  thrown  into  *  a  graceful  and 
beautiful  spiral.'  For  observing  them  during  their  living  state,  no 
means  is  so  convenient  as  the  zoophyte -trough.  In  mounting  com- 
pound Hydrozoa,  as  well  as  Polyzoa,  it  will  be  found  of  great 
advantage  to  place  the  specimens  alive  in  the  cells  they  are  per- 
manently to  occupy,  and  to  then  add  osmic  acid  drop  by  drop  to 
the  sea-water  ;  this  has  the  effect  of  causing  the  protrusion  of  the 
animals,  and  of  rendering  their  tentacles  rigid.  The  liquid  may  be 
withdrawn,  and  replaced  by  Goadby's  solution,  Deane's  gelatine, 
glycerin  jelly,  weak  spirit,  diluted  glycerin,  a  mixture  of  spirit  and 
glycerin  with  sea- water,  or  any  other  menstruum,  by  means  of 


FIG.   662. — Sertularia   cupressina :  A,   natural 
size  ;  B,  portion  magnified. 


872  SPONGES  AND  ZOOPHYTES 

the  syringe  ;  and  it  is  well  to  mount  specimens  in  several  dif- 
ferent menstrua,  marking  the  nature  and  strength  of  each,  as 
some  forms  are  better  preserved  by  one  and  some  by  another.1 
An  excellent  method  of  preservation  has  been  discovered  by 
M.  Foettinger 2  in  the  use  of  chloral  hydrate :  when  all  the 
polypes  in  a  vessel  containing  100  c.c.  of  water  are  fully  expanded 
some  crystals  of  chloral  hydrate  are  to  be  dropped  into  the 
vessel ;  these  dissolve  rapidly  and  gradually  diffuse  through  the 
water.  About  ten  minutes  later  a  little  more  chloral  should  be 
added,  and  in  three-quarters  of  an  hour  the  whole  colony  will  be 
found  to  have  become  insensible  ;  the  advantage  of  this  method 
lies  in  the  fact  that  the  action  is  merely  narcotic,  and  .that  the  tissues 
are  not  affected.  When  the  influence  is  so  complete  that  irritation 
fails  to  produce  retraction  of  the  polypes  the  colony  may  be  put  into 
alcohol.  The  size  of  the  cell  must  of  course  be  proportioned  to  that 
of  the  object ;  and  if  it  be  desired  to  mount  such  a  specimen  as  may 
serve  for  a  characteristic  illustration  of  the  mode  of  growth  of  the 
species  it  represents,  the  large  shallow  cells,  whose  walls  are  made 
by  cementing  four  strips  of  glass  to  the  plate  that  forms  the  bottom, 
will  generally  be  found  preferable.  The  horny  polyparies  of  the 
Sertulariida,  when  mounted  in  Canada  balsam,  are  beautiful  objects 
for  the  polariscope  ;  but  in  order  to  prepare  them  successfully  some 
nicety  of  management  is  required.  The  following  are  the  outlines 
of  the  method  recommended  by  Dr.  Golding  Bird,  who  very  success- 
fully practised  it.  The  specimens  selected,  which  should  not  exceed 
two  inches  in  length,  are  first  to  be  submitted,  while  immersed  in 
water  of  120°,  to  the  vacuum  of  an  air-pump.  The  ebullition 
which  will  take  place  within  the  cavities  will  have  the  effect  of  free- 
ing the  polyparies  from  dead  polypes  and  other  animal  matter ;  and 
this  cleansing  process  should  be  repeated  several  times.  The 
specimens  are  then  to  be  dried,  by  first  draining  them  for  a  few 
seconds  on  bibulous  paper,  and  then  by  submitting  them  to  the 
vacuum  of  an  air-pump,  within  a  thick  earthenware  ointment-pot 
fitted  with  a  cover,  which  has  been  previously  heated  to  about  200°  ; 
by  this  means  the  specimens  are  very  quickly  and  completely  dried, 
the  water  being  evaporated  so  quickly  that  the  cells  and  tubes 
hardly  collapse  or  wrinkle.  The  specimens  are  then  placed  in 
camphine,  and  again  subjected  to  the  exhausting  process  for  the 
displacement  of  the  air  by  that  liquid ;  and  when  they  have  been 
thoroughly  saturated,  they  should  be  mounted  in  Canada  balsam  in 
the  usual  mode.  When  thus  prepared  they  become  very  beautiful 
transparent  objects  for  low  magnifying  powers ;  and  they  present  a 
gorgeous  display  of  colours  when  examined  by  polarised  light,  with 
the  interposition  of  a  plate  of  selenite,  the  effect  being  much  en 
hanced  by  the  use  of  black-ground  illumination. 

No  result  of  microscopic  research  was  more  unexpected  than 
the  discovery  of  the  close  relationship  subsisting  between  the 
hydroid  Zoophytes  and  the  medusoid  Acalephce  (or  'jelly-fish  ').  We 
now  know  that  the  small  free-swimming  medusoids  belonging  to 

1  See  Mr.  J.  W.  Morris  in  Quart.  Journ.  of  Microsc.  Sci.  n.s.  vol.  ii.  1862,  p.  116. 

2  Archives  de  Biologie,  vi.  p.  115. 


JELLY-FISHES 


873 


the  '  naked-eye  '  group,  of  which  Thaumantias  (fig.  663)  may  be 
taken  as  a  representative,  are  really  to  be  considered  as  the  detached 
sexual  apparatus  of  the  zoophytes  from  which  they  have  been 


FIG.  663. — A,  Thaumantias  pilosella,  one  of  the '  naked-eye  '  Medusae  :  a,  a, 
oral  tentacles ;  6,  stomach  ;  c1,  gastro-vascular  canals,  having  the  ovaries, 
d  d,  on  either  side,  and  terminating  in  the  marginal  canal,  e  e.  B,  Thau- 
mantias Eschscholtzii,  Haeckel. 

budded  off,  endowed  with  independent  organs  of  nutrition  and 
locomotion,  whereby  they  become  capable  of  maintaining  their  own 
existence  and  of  developing  their  sexual  products.  The  general  con- 
formation of  these  organs  will  be  understood  from  the  accompany- 
ing figure.  Many  of  this  group  are  very  beautiful  objects  for 


8/4  SPONGES  AND    ZOOPHYTES 

microscopic  examination,  being  small  enough  to  be  viewed  entire  in 
the  zoophyte-trough.     There  are  few  parts  of  the  coast  on  which  they 
may  not  be  found,  especially  on  a  calm  warm  day,  by  skimming  the 
surface  of  the  sea  with  the  tow-net  ;  and  they  are  capable  of  being 
stained  and  preserved  in  cells  after  being  hardened  by  osmic  acid. 
The  history  of  the  large  and  highly  developed  Medusce1  or  ACA- 
LEPH/E  which  are  commonly  known  as  'jelly-fish  '  is  essentially  simi- 
lar ;  for  their  progeny  have  been  ascertained  to  develop  themselves 
in  the  first  instance  under  the  polype  form,  and  to  lead  a  life  which 
in  all  essential  respects  is  zoophytic  ;  their  development  into  Medusae 
taking  place  only  in  the  closing  phase  of  their  existence,  and  then 
rather  by  gemmation  from  the  original  polype  than  by  a  metamor- 
phosis of  its  own  fabric.     The  huge  Rhizostoma  found  commonly 
swimming  round  our  coasts,  and  the  beautiful  Chrysaora  remarkable 
for  its  long  ;  furbelows '  which  act  as  organs  of  prehension,  are  oceanic 
acalephs  developed  from  very  small  polypites,  which  fix  themselves 
by  a  basal  cup  or  disc.     The  embryo  emerges  from  the  cavity  of  its 
parent,  within  which  the  first  stages  of  its  development  have  taken 
place,  in  the  condition  of  a  ciliated  '  planula,'  of  rather  oblong  form, 
very  closely  resembling  an  infusory  animalcule,  but  destitute  of  a 
mouth.     One  end  soon  contracts  and  attaches  itself,  however,  so  as 
to  form  a    foot ;  the  other  enlarges   and  opens  to  form  a  mouth, 
four  tubercles  sprouting  around  it  which  grow  into  tentacles  ;  whilst 
a  slit  in  the  midst  of  the  central  cells  gives  rise  to  the  cavity  of  the 
stomach.     Thus  a  hydra-like  polype  is  formed,  which  soon  acquires 
many  additional  tentacles  ;  and  this,  according  to  the  observations 
of  Sir  J.  G.  Dalyell  on  the  Hydra-tuba,  which  is  the  polype  stage  of 
the  Chrysaora  and  other  jelly-fish,  leads  in  every  important  particular 
the  life  of  a  Hydra  ;  propagates  like  it  by  repeated  gemmation,  so 
that  whole  colonies  are  formed  as  offsets  from  a  single  stock  ;  and 
can  be  multiplied  like  it  by  artificial  division,  each  segment  develop- 
ing itself  into  a  perfect  Hydra.     There  seems  to  be  no  definite  limit 
to  its  continuance  in  this  state,  or  to  its  power  of  giving  origin  to 
new  polype-buds  ;  but  when  the  time  comes  for  the  development  of 
its   sexual  gonozooids,  the  polype  quits  its  original  condition  of  a 
minute  bell   with   slender   tentacles   (fig.    664),   assumes   a   cylin- 
drical   form,    and    elongates  itself  considerably  ;  a   constriction  or 
indentation  is  then  seen  around  it,  just  below  the  ring  which  encircles 
the  mouth  and  gives  origin  to  the  tentacles  ;  and  similar  constrictions 
are  soon  repeated  round  the  lower  parts  of  the  cylinder,  so  as  to  give 
to  the  whole  body  somewhat  the  appearance  of  a  rouleau  of  coins  ; 
a  sort  of  fieshy  bulb,  a  (fig.  664,  II),  somewhat  of  the  form  of  the 
original  polype,  being  still  left  at  the   attached    extremity.      The 
number  of  circles  is  indefinite,  and  all  are  not  formed  at  once,  new 
constrictions  appearing  below,  after  the  upper  portions  have  been  de- 
tached ;  as  many  as  thirty  or  even  forty  have  thus  been  produced  in 
one  specimen.    The  constrictions  then  gradually  deepen,  so  as  to  divide 
the  cylinder  into  a  pile  of  saucer-like  bodies,  the   division   being 

1  See  Professor  Glaus,  Untersuchungen  iiber  die  Organisation  undEntwickelung 
der  Medusen,  Prague  and  Leipzig,  1883,  and  Miss  Ida  H.  Hyde,  '  Eiitwickelungs- 
geschichte  einiger  Scyphomedusen,'  in  Zeitsclir.  f.  wiss.  Zool.  Iviii.  p.  531. 


REPRODUCTION  OF  ACALEPHS  875 

most  complete  above,  and  the  upper  discs  usually  presenting  some 
increase  in  diameter  ;  and  whilst  this  is  taking  place  the  edges  of 
the  discs  become  divided  into  lobes,  each  lobe  soon  presenting  the 
cleft  with  the  supposed  rudimentary  eye  at  the  bottom  of  it,  which 
is  to  be  plainly  seen  in  the  detached  Medusae  (fig.  665,  C).  Up  to 
this  period,  the  tentacles  of  the  original  polype  surmount  the  highest 
of  the  discs ;  but  before  the  detachment  of  the  topmost  disc,  this 
circle  disappears,  and  a  new  one  is  developed  at  the  summit  of  the 
bulb  which  remains  at  the  base  of  the  pile.  At  last  the  topmost 
and  largest  disc  begins  to  exhibit  a  sort  of  convulsive  struggle ;  it 


FIG.  664. — I,  two  Hijdrce  tnbce  (Scypliistoma-si&ge)  of  Cyanea 
capillata,  with  two  (a,  b)  undergoing  fission  (Strobila-st&ge). 
II,  a  and  b  of  fig.  I  three  days  later.  In  a  the  tentacles  are 
developed  beneath  the  lowest  of  the  Epliyrce,  from  the  stalk 
of  the  Strobila,  which  will  persist  as  a  Hydra  tube.  (After 
Van  Beneden.) 

becomes  detached,  and  swims  freely  away ;  and  the  same  series  of 
changes  takes  place  from  above  downwards,  until  the  whole  pile  of 
discs  is  detached  and  converted  into  free-swimming  Medusae.  But 
the  original  polypoid  body  still  remains,  and  may  return  to  its 
original  polype-like  mode  of  gemmation,  becoming  the  progenitor  of 
a  new  colony,  every  member  of  which  may  in  its  turn  bud  off  a  pile 
of  Medusa  discs. 

The  bodies  thus  detached  have  all  the  essential  characters  of  the 
adult  Medusae.  Each  consists  of  an  umbrella-like  disc  divided  at 
its  edge  into  a  variable  number  of  lobes,  usually  eight ;  and  of  a 


8;6 


SPONGES  AND   ZOOPHYTES 


stomach,  which  occupies  M  considerable  proportion  of  the  disc,  and 
projects  downwards  in  the  form  of  a  proboscis,  in  the  centre  of  which 
is  the  quadrangular  mouth  (fig.  665,  A,  B).  As  the  animal  advances 
towards  maturity  the  intervals  between  the  segments  of  the  border 
of  the  disc  gradually  fill  up,  so  that  the  divisions  are  obliterated ; 
tubular  prolongations  of  the  stomach  extend  themselves  over  the 
disc ;  and  from  its  borders  there  sprout  forth  tendril-like  filaments 
which  hang  down  like  a  fringe  around  its  margin.  From  the  four 
angles  of  the  mouth,  which,  even  in  the  youngest  detached  animal, 
admits  of  being  greatly  extended  and  protruded,  prolongations  are 
put  forth,  which  form  the  four  large  tentacles  of  the  adult.  The 
young  Medusas  are  very  voracious,  and  grow  rapidly,  so  as  to  attain 


FIG.  665. — Development  of  Chrysaora  from  Hydra  tuba  :  A, 
detached  individual  viewed  sideways,  and  enlarged,  showing 
the  proboscis  a,  and  b  the  bifid  lobes ;  B,  individual  seen  from 
above,  showing  the  bifid  lobes  of  the  margin,  and  the  quadri- 
lateral mouth  ;  C,  one  of  the  bifid  lobes  still  more  enlarged, 
showing  the  rudimentary  eye  (?)  at  the  bottom  of  the  cleft ; 
D,  group  of  young  Medusae,  as  seen  swimming  in  the  water, 
of  the  natural  size. 

a  very  large  size.  The  Cyanece  and  Chrysaorce,  which  are  common  all 
round  our  coasts,  often  have  a  diameter  of  from  six  to  fifteen  inches  ; 
while  Rhizostoma  sometimes  reaches  a  diameter  of  from  two  to  three 
feet.  The  quantity  of  solid  matter,  however,  which  their  fabrics  con- 
tain is  extremely  small.  It  is  not  until  adult  age  has  been  attained 
that  the  generative  organs  make  their  appearance,  in  four  chambers 
disposed  around  the  stomach,  which  are  occupied  by  plaited  mem- 
branous ribbons  containing  sperm-cells  in  the  male  and  ova  in  the 
female  j  and  the  embryos  evolved  from  the  latter,  when  they  have 
been  fertilised  by  the  agency  of  the  former,  repeat  the  extraordinary 
cycle  of  phenomena  which  has  been  now  described,  developing  them- 
selves in  the  first  instance  into  hydroid  polypes,  from  which  medusoids 
are  subsequently  budded  off. 


ACTINOZOA  877 

This  cycle  of  phenomena  is  one  of  those  to  which  the  term  '  alter- 
nation of  generations'  was  applied  by  Steenstrup,1  who  brought 
together  under  this  designation  a  number  of  cases  in  which  genera- 
tion A  does  not  produce  a  form  resembling  itself,  but  a  different  form, 
B ;  whilst  generation  B  gives  origin  to  a  form  which  does  not  re- 
semble itself,  but  returns  to  the  form  A,  from  which  B  itself  sprang. 
It  was  early  pointed  out,  however,  by  the  Author  2  that  the  term 
'  alternation  of  generations '  does  not  appropriately  represent  the 
facts  either  of  this  case  or  of  any  of  the  other  cases  grouped  under 
the  same  category,  the  real  fact  being  that  the  two  organisms,  A 
and  B,  constitute  two  stages  in  the  life-history  of  one  generation, 
and  the  production  of  one  form,  from  the  other  being  in  only  one 
instance  by  a  truly  generative  or  sexual  act,  whilst  in  the  other  it  is 
by  a  process  of  gemmation  or  budding.  Thus  the  Medusa3  of  both 
orders  (the  '  naked-eyed  '  and  the  '  covered-eyed '  of  Forbes)  are  de- 
tached flower-buds,  so  to  speak,  of  the  hydroid  zoophytes  which  bud 
them  off,  the  zoophytic  phase  of  life  being  the  most  conspicuous  in 
such  Thecata  as  C'ampanulariida  and  /Sertulariida,  whose  Medusa- 
buds  are  of  small  size  and  simple  conformation,  and  not  unfrequently 
do  not  detach  themselves  as  independent  organisms  ;  whilst  the 
Medusan  phase  of  life  is  the  most  conspicuous  in  the  ordinary  Acalephs, 
their  zoophytic  stage  being  passed  in  such  obscurity  as  only  to  be 
detected  by  careful  research.  The  Author's  views  on  this  subject, 
which  were  at  first  strongly  contested  by  Professor  E.  Forbes  and 
other  eminent  zoologists,  have  now  come  to  be  generally  adopted.3 

Actinozoa. — Of  this  group  the  common  sea-anemones  may  be 
taken  as  types,  constituting,  with  their  allies,  the  order  Zoantharia, 
or  helianthoid  polypes,  which  have  numerous  tentacles  disposed  in 
several  rows.  Next  to  them  come  the  Alcyonaria,  consisting  of 
those  whose  polypes,  having  always  eight  broad  short  tentacles, 
present  a  star-like  aspect  when  expanded  ;  as  is  the  case  with  various 
composite  sponge-like  bodies,  unpossessed  of  any  hard  skeleton,  which 
inhabit  our  own  shores,  and  also  with  the  red  coral  and  the  Tubipora 
of  warmer  seas,  which  have  a  stony  skeleton  that  is  internal  in  the 
first  case  and  external  in  the  second,  as  also  with  the  sea  pens  and 
the  Gorgonice  or  sea-fans.  A  third  order,  Rugosa,  consists  of  fossil 
corals,  whose  stony  polyparies  are  intermediate  in  character  between 
those  of  the  two  preceding.  And  lastly,  the  Ctenophora,  free-swim- 
ming gelatinous  animals,  many  of  which  are  beautiful  objects  for 
the  microscope,  are  by  some  zoologists  ranked  with  the  Actinozoa.4 

Of  the  Zoantharia  the  common  Actinia  or  '  sea-anemone '  mav 
be  taken  as  the  type,  the  individual  polypites  of  all  the  composite 
fabrics  included  in  the  group  being  constructed  upon  the  same  model.5 
In  by  far  the  larger  proportion  of  these  zoophytes,  the  bases  of  the 

1  See  his  treatise  on  The  Alternation  of  Generations,  a  translation  of  which  has 
been  published  by  the  Ray  Society. 

2  Brit,  and  For.  Med.  Chir.  Review,  vol.  i.  1848,  p.  192  et  seq. 

5  Compare  Huxley,  Anatomy  of  Invert ebrated  Animals,  p.   133;  and  Balfour, 
Comparative  Embryology,  i.  p.  isi. 

4  Professor  Haeckel,  led  by  the  study  of  Ctenaria  ctenophora,  associates  the 
Ctenophora  with  the  Hydrozoa  (Sitzungtber.  Jenaische  GesellscJtaft,  May  16,  1879). 

5  On  the  anatomy  of  Actinia  and  its  allies,  see  O.  and  E.  Hertwig's  monograph 
in  vols.  xiii.  and  xiv.  of  the  Jenaische  Zeitschrift. 


878  SPONGES  AJND   ZOOPHYTES 

poly  pit  es,  as  well  as  the  soft  flesh  that  connects  together  the  members 
of  aggregate  masses,  are  consolidated  by  calcareous  deposit  into  stony 
corals ;  and  the  surfaces  of  these  are  beset  with  '  cells,'  usually  of  a 
nearly  circular  form,  each  having  numerous  vertical  plates  or  lamella 
radiating  from  its  centre  towards  its  circumference,  which  are 
formed  by  the  consolidation  of  the  lower  portions  of  the  radiating 
partitions  that  divide  the  space  intervening  between  the  stomach  and 
the  general  integument  of  the  animal  into  separate  chambers.  This 
arrangement  is  seen  on  a  large  scale  in  the  Fungia  or  *  mushroom- 
coral  '  of  tropical  seas,  which  is  the  stony  base  of  a  solitary  anemone- 
like  animal ;  on  a  far  smaller  scale,  it  is  seen  in  the  little  Caryo- 
phyllia,  a  like  solitary  anemone  of  our  own  coasts,  which  is  scarcely 
distinguishable  from  an  Actinia  by  any  other  character  than  the 
presence  of  this  disc,  and  also  on  the  surface  of  many  of  those  stony 
corals  known  as  '  madrepores  ; '  whilst  in  some  of  these  the  indivi- 
dual polype-cells  are  so  small  that  the  lamellated  arrangement  can 
only  be  made  out  when  they  are  considerably  magnified.  Portions 
of  the  surface  of  such  corals,  or  sections  taken  at  a  small  depth,  are 
very  beautiful  objects  for  low  powers,  the  former  being  viewed  by 
reflected  and  the  latter  by  transmitted  light.  And  thin  sections  of 
various  fossil  corals  of  this  group  are  very  striking  objects  for  the 
lower  powers  of  the  oxy-hydrogen  microscope.  An  exceedingly  use- 
ful method  of  preparing  sections  of  corals  has  been  devised  by  Dr.  G. 
von  Koch  ;  the  corals  with  all  their  soft  parts  in  plate  are  hardened 
in  absolute  alcohol,  and  then  placed  in  a  solution  of  copal  in  chloro- 
form. After  thorough  permeation  they  are  taken  out  and  dried 
slowly  until  the  masses  become  quite  hard.  These  masses  may  now 
be  cut  into  sections  with  a  fine  saw  and  rubbed  down  on  a  whetstone 
in  the  ordinary  manner  ;  after  staining,  the  sections  may  be  mounted 
in  Canada  balsam.  The  great  value  of  this  method  lies  in  the  fact 
that  by  it  the  soft  and  hard  parts  are  retained  in  their  proper  rela- 
tions with  each  other.1 

The  chief  point  of  interest  to  the  microscopist,  however,  in  the 
structure  of  these  animals  lies  in  the  extraordinary  abundance  and 
high  development  of  those  '  filiferous  capsules,'  or  '  thread-cells,'  the 
presence  of  which  on  the  tentacles  of  the  hydroid  polypes  has  been 
already  noticed,  and  which  are  also  to  be  found,  sometimes  sparingly 
sometimes  very  abundantly,  in  the  tentacles  surrounding  the  mouth 
of  the  Medusae,  as  well  as  on  other  parts  of  their  bodies.  If  a 
tentacle  of  any  of  the  sea-anemones  so  abundant  on  our  coasts  (the 
smaller  and  more  transparent  kinds  being  selected  in  preference)  be 
cut  off,  and  be  subjected  to  gentle  pressure  between  the  two  glasses 
of  the  aquatic  box  or  the  compressoriuin,  multitudes  of  little  dart- 
like  organs  will  be  seen  to  project  themselves  from  its  surface  near 
its  tip  ;  and  if  the  pressure  be  gradually  augmented,  many  additional 
darts  will  every  moment  come  into  view.  Not  only  do  these  organs 
present  different  forms  in  different  species,  but  even  in  one  and  the 
same  individual  very  strongly  marked  diversities  are  shown,  of 
which  a  few  examples  are  given  in  fig.  666.  At  A,  B,  C,  D  is 
shown  the  appearance  of  the  '  filiferous  capsules,'  whilst  as  yet  the 
1  See  Zoologischer  Anzeiger,  i.  p.  36 ;  and  Proc.  Zijol.  Soc.  London,  1880,  p.  24. 


ALCYONAKIA 


879 


\J 


thread  lies  coiled  up  in  their  interior ;  and  at  E,  F,  G,  H  are  seen 
a  few  of  the  most  striking  forms  which  they  exhibit  when  the  thread 
or  dart  has  started  forth.  These  thread-cells  are  found  not  merely  in 
the  tentacles  and  other  parts  of 
the  external  integument  of  Ac- 
tinozoa,  but  also  in  the  long  fila- 
ments which  lie  in  coils  within 
the  chambers  that  surround  the 
stomach,  in  contact  with  the 
sexual  organs  which  are  attached 
to  the  lamellae  dividing  the  cham- 
bers. The  latter  sometimes  con- 
tain '  sperm-cells  '  and  sometimes 
ova,  the  two  sexes  being  here 
divided,  not  united  in  the  same 
individual.  What  can  be  the 
office  of  the  filiferous  filaments 
thus  contained  in  the  interior  of 
the  body  it  is  difficult  to  guess 
at.  They  are  often  found  to  pro- 
trude from  rents  in  the  external 
tegument,  when  any  violence  has 
been  used  in  detaching  the  animal 
from  its  base  ;  and  when  there  is 
no  external  rupture  they  are  often 
forced  through  the  wall  of  the 
stomach  into  its  cavity,  and  may 
be  seen  hanging  out  of  the  mouth. 
The  largest  of  these  capsules,  in 
their  unprotected  state,  are  about 
^^th  of  an  inch  in  length  ;  while 
the  thread  or  dart,  in  Corynactis 
Allmanni,  when  fully  extended 
is  not  less  than  Jth  of  an  inch, 
or  thirty-seven  times  the  length 
of  its  capsule.1 

Of  the  Alcyonaria  a  character- 
istic example  is  found  in  the  Alcy- 
onium  digitatum  of  our  coasts  ;  FIG.  666.— Filiferous  capsules  of  Acti- 
a  lobed  sponge-like  mass,  covered  nozoa:  A,  B,  Cori/nactis  Allmanni; 
with  a  tough  skin,  which  is  com-  ct>  ,E>  ?>  OaryophyUiaamithiii  D,  G, 

T     -,  ,         . ,  „      Actinia  crassicornis ;  H,  Actinia  can- 

monly  known  under  the  name  of     dida. 

1  dead-man's     toes,'     or    by    the 

more   elegant   name  of  '  mermaid's  fingers.'     When  a  specimen  of 

this  is  first  torn  from  the  rock  to  which  it  has  attached  itself,  it 

contracts  into  an  unshapely  mass,  whose  surface  presents  nothing 

1  See  Mr.  Gosse's  Naturalises  Rambles  on  the  Devonshire  Coast,  and  Professor 
Mb'bius,  '  Ueber  den  Bau  u.s.w.  der  Nesselkapseln  einiger  Polypen  mid  Quallen,'  in 
AbhandL  Naturw.  Vereins  zu  Hamburg,  Band  v.  1866.  On  the  relations  of  stinging 
cells  to  the  nervous  system,  see  Dr.  v.  Lendenfeld,  Quart.  Journ.  of  Microsc.  Sci.  n.s. 
xxvii.  p.  393.  On  the  stinging  cells  of  Coelentera  generally,  see  N.  Iwanzoff  in  Bull. 
Soc.  Moscow,  1896,  pp.  95  aiido'23. 


88o 


SPONGES   AND   ZOOPHYTES 


but  a  series  of  slight  depressions  arranged  with  a  certain  regularity. 
But  after  being  immersed  for  a  little  time  in  a  jar  of  sea- water  the 
mass  swells  out  again,  and  from  every  one  of  these  depressions  an 
eight-armed  polype  is  protruded,  '  which  resembles  a  flower  of  ex- 
quisite beauty  and  perfect  symmetry.  In  specimens  recently  taken, 
each  of  the  petal-like  tentacula  is  seen  with  a  hand-glass  to  be  fur- 
nished with  a  row  of  delicately  slender  pinnce  or  filaments,  fringing 
each  margin,  and  arching  onwards  ;  and  with  a  higher  power  these 
pinnae  are  seen  to  be  roughened  throughout  their  whole  length  with 
numerous  prickly  rings.  After  a  day's  captivity,  however,  the  petals 
shrink  up  into  short,  thick,  unshapely  masses,  rudely  notched  at  their 
edges.'  (Gosse.)  When  a  mass  of  this  sort  is  cut  into  it  is  found 
to  be  channelled  out  somewhat  like  a  sponge  by  ramifying  canals ;  the 
vents  of  which  open  into  the  stomachal  cavities  of  the  polypes,  which 
are  thus  brought  into  free  communication  with  each  other,  a  cha- 
racter that  especially  distinguishes  this  order.  A  movement  of  fluid 
is  kept  up  within  these  canals  (as  may  be  distinctly  seen  through 


FIG.  667.— Spicules  of  Alcyoniun 
and  Gorgonia. 


FIG.  668. — A,  spicules  of  Gorgonia  guttata 
B,  spicules  of  Muricea  elongate/. 


their  transparent  bodies)  by  means  of  cilia  lining  the  internal  surfaces 
of  the  polypes  ;  but  no  cilia  can  be  discerned  on  their  external  sur- 
faces. The  tissue  of  this  spongy  polypidom  is  strengthened  through- 
out, like  that  of  sponges,  with  mineral  spicules  (always,  however,  cal- 
careous), which  are  remarkable  for  the  elegance  of  their  forms  ;  these 
are  disposed  with  great  regularity  around  the  bases  of  the  polypes, 
and  even  extend  part  of  their  length  upwards  on  their  bodies.  In 
the  Gorgonia  or  sea-fan,  whilst  the  central  part  of  the  polypidom  is 
consolidated  into  a  horny  axis,  the  soft  flesh  which  clothes  this  axis 
is  so  full  of  tuberculated  spicules,  especially  in  its  outer  layer,  that, 
when  this  dries  up,  they  form  a  thick  yellowish  or  reddish  incrusta- 
tion upon  the  horny  stem.  This  crust  is,  however,  so  friable  that  it 
may  be  easily  rubbed  down  between  the  fingers,  and  when  examined 
with  the  microscope  it  is  found  to  consist  of  spicules  of  different 
shapes  and  sizes,  more  or  less  resembling  those  shown  in  figs.  667,  668, 
sometimes  colourless,  but  sometimes  of  a  beautiful  crimson,  yellow, 


CTENOPHOEA 


88l 


or  purple.     These  spieules  are  best  seen  by  black-ground  illumination, 
especially  when  viewed  by  the  binocular  microscope.     They  are,  of 

course,  to  be  sepa- 
rated from  the 
animal  substance  in 
the  same  manner  as 
the  calcareous  spi- 
eules of  sponges ; 
and  they  should  be 
mounted,  like  them, 

//S^5»W   c  in  Canada  balsam. 

The  spieules  always 
possess  an  organic- 
basis,  as  is  proved 
by  the  fact  that 
when  their  lime  is 
dissolved  by  dilute 
acid  a  gelatinous- 
looking  residuum  is 
left  which  preserves 
the  form  of  the 
spicule. 

The  Ctenophora, 
or  '  comb-bearers.' 
are  so  named  from 
the  comb- like  ar- 
rangement of  the 
rows  .  of  tiny 


FIG.  669. — 1.  Eu-plokamis  station  is,  with  its  tentacles 
extended,  about  twice  the  natural  size:  m,  mouth;  c, 
ctenophoral  plate  ;  t,  tentacular  apparatus.  (After  Chun.) 
2.  Diagrammatic  view  of  Hormijjhora  plumosa,  seen 
from  the  aboral  pole :  c,  as  before ;  tv,  tentacular  vessel ; 
PPt  polar  plates.  (After  Chun.) 


FIG.  670.  —  Beroi-' 
Forskalii,  show- 
ing the  ^-tubular 
prolongations  of 
the  stomach. 


'  paddles '  by  the  movement  of  which  the  bodies  of  these  animals  are 
propelled.     A  very  beautiful  and  not  uncommon  representative  of 

SL 


882  SPONGES   AND   ZOOPHYTES 

this  order  is  furnished  by  the  Cydippe1  p  ileus  (compare  fig.  669),  very 
commonly  known  as  the  Beroe,  which  designation,  however,  properly 
appertains  to  another  animal  (fig.  670)  of  the  same  grade  of  organisa- 
tion. The  body  of  Cydippe  is  a  nearly  globular  mass  of  soft  jelly, 
usually  about  f  ths  of  an  inch  in  diameter,  and  it  may  be  observed, 
even  with  the  naked  eye,  to  be  marked  by  eight  bright  bands, 
which  proceed  from  pole  to  pole  like  meridian  lines.  These  bands 
are  seen  with  the  microscope  to  be  formed  of  rows  of  flattened 
filaments,  far  larger  than  ordinary  cilia,  but  lashing  the  water  in 
the  same  manner  ;  they  sometimes  act  quite  independently  of  one 
another,  so  as  to  give  to  the  body  every  variety  of  motion,  but 
sometimes  work  all  together.  If  the  sunlight  should  fall  upon  them 
when  they  are  in  activity,  they  display  very  beautiful  iridescent 
colours.  In  addition  to  these  ;  paddles '  the  Cydippe  is  furnished 
with  a  pair  of  long  tendril-like  filaments,  rising  from  the  bottom  of 
a  pair  of  cavities  in  the  posterior  part  of  the  body,  and  furnished 
with  lateral  branches  ;  within  these  cavities  they  may  lie  doubled  up, 
so  as  not  to  be  visible  externally ;  and  when  they  are  ejected,  which 
often  happens  quite  suddenly,  the  main  filaments  first  come  forth,  and 
the  lateral  tendrils  subsequently  uncoil  themselves,  to  be  drawn  in 
again  and  packed  up  within  the  cavities  with  almost  equal  sudden- 
ness. The  mouth  of  the  animal  situated  at  one  of  the  poles  leads' 
first  to  a  quadrifid  cavity  bounded  by  four  folds  which  seem  to  repre- 
sent the  oral  proboscis  of  the  ordinary  Medusa?  (fig.  664)  ;  and  this 
leads  to  the  true  stomach,  which  passes  towards  the  opposite  pole, 
near  to  which  it  bifurcates,  its  branches  passing  towards  the  polar 
surface  on  either  side  of  a  little  body  which  has  every  appearance  of 
being  a  nervous  ganglion,  and  which  is  surmounted  externally  by  a 
fringe-like  apparatus  that  seems  essentially  to  consist  of  sensory 
tentacles.2  From  the  cavity  of  the  stomach  tubular  prolongations 
pass  off  beneath  the  ciliated  bands,  very  much  as  in  the  true  Beroe. 
These  may  easily  be  injected  with  coloured  liquids  by  the  intro- 
duction of  the  extremity  of  a  fine-pointed  glass  syringe  into  the 
mouth.  The  liveliness  of  this  little  creature,  which  may  sometimes 
be  collected  in  large  quantities  at  once  by  the  stick-net,  renders  it  a 
most  beautiful  subject  for  observation  when  due  scope  is  given  to  its 
movements ;  but  for  the  sake  of  microscopic  examination,  it  is  of 
course  necessary  to  confine  these.  Various'  species  of  true  Beroe,3 
some  of  them  even  attaining  the  size  of  a  small  lemon,  are  occasionally 
to  be  met  with  on  our  coasts,  in  all  of  which  the  movements  of  the 

1  More  correctly  Hormiphora. 

-  It  is  commonly  stated  that  the  two  branches  of  the  alimentary  canal  open  on 
the  surface  by  two  pores  situated  in  the  hollow  of  the  fringe,  one  on  either  side  of  the 
nervous  ganglion.  The  Author,  however,  has  not  been  able  to  satisfy  himself  of  the 
existence  of  such  excretory  pores  in  the  ordinary  Cijdippe  or  Beroe,  although  he  has 
repeatedly  injected  their  whole  alimentary  canal  and  its  extensions,  and  has  atten- 
tively watched  the  currents  produced  by  ciliary  action  in  the  interior  of  the  bifurcat- 
ing prolongations,  which  currents  always  appear  to  him  to  return  as  from  caeeal 
extremities.  He  is  himself  inclined  to  believe  that  this  arrangement  has  reference 
solely  to  the  nutrition  of  the  nervous  ganglion  and  tentacular  apparatus,  which  lies 
imbedded  (so  to  speak)  in  the  bifurcation  of  the  alimentary  canal,  so  as  to  be  able 
to  draw  its  supply  of  nutriment  direct  from  that  cavity. 

5  On  the  anatomy  of  Beroe,  see  Eimer,  Zo'ologische  Sttuli.cn  atif  Capri.  I.  Ueber 
Beroe  ovatus,  Leipzig,  187:5. 


C(ELENTEKA  883 

body  are  effected  by  the  like  agency  of  paddles  arranged  in  meridional 
bands.  These  are  splendidly  luminous  in  the  dark,  and  the  lumi- 
nosity is  retained  even  by  fragments  of  their  bodies,  being  augmented 
by  agitation  of  the  water  containing  them.  All  the  Ctenophora  are 
reproduced  from  eggs,  and  are  already  quite  advanced  in  their  deve- 
lopment by  the  time  they  are  hatched.  Long  before  they  escape, 
indeed,  they  swim  about  with  great  activity  within  the  walls  of  their 
diminutive  prison,  their  rows  of  locomotive  paddles  early  attaining 
a  large  size,  although  the  long  flexile  tentacles  of  Cydippe  are  then 
only  short  stumpy  protuberances.  By  Cceloplana  and  Cteiwplana 
the  Ctenophora  appear  to  be  allied  to  the  Planarian  Worms.1 

Those  who  may  desire  to  acquire  a  "more  systematic  and  detailed  acquaintance 
with  the  zoophyte  group  may  be  especially  referred  to  the  following  treatises  and 
memoirs,  in  addition  to  those  already  cited,  and  to  the  various  recent  systematic 
treatises  on  zoology :— Dr.  Johnston's  History  of  British  Zoophytes ;  Professor  Milne- 
Edwards's  '  Recherches  sur  les  Polypes,'  and  his  '  Histoire  des  Corallaires '  (in  the 
Suites  a  Buff  on),  Paris,  1857 ;  Professor  Van  Beneden, '  Sur  les  Tubulaires '  and '  Sur  les 
Campanulaires,'  in  Mem.  de  I'Acad.  Hoy.  de  Bruxelles,  torn,  xvii.,  and  his '  Recherches 
sur  1'Hist.  Nat.  des  Polypes  qui  frequenteiit  les  Cotes  de  Belgique,'  op.  cit.  torn. 
xxxvi. ;  Sir  J.  G.  Dalyell's  Hare  and  Bemarkable  Animals  of  Scotland,  vol.  i. ; 
Trembley's  Mem.  pour  serv/r  <'i  fjiinfoin-  d'un  genre  de  Polype  d'eau  douce]  M. 
Hollard's  'Monographic  du  Genre  Actinia1  in  Ann.  des  Sci.  Nat.  ser.  iii.  torn.  xv. ; 
Professor  Max  Schultze,  '  On  the  Male  Reproductive  Organs  of  Campanularia  genicu- 
lata '  in  Quart.  Jon  rn.  Mirr.  Sci.  vol.  iii.  1855,  p.  59;  Professor  P.  E.  Schulze's  memoirs 
on  Cordijlopliora  lacustris,  Leipzig,  1871,  and  on  Syncoryne,  1873 :  Professor  Agassiz's 
beautiful  monograph  on  American  Medusae,  forming  the  third  volume  of  his  Contri- 
butions to  the  Natural  History  of  the  United  States  of  America ;  Mr.  Hincks's 
British  Hydroid-  Zoophytes ;  Professor  Allman's  admirable  memoirs  on  Cordylophora, 
iind  Myriothela  in  the  Phil.  Trans,  for  1853  and  1875 ;  Professor  Lacaze-Duthiers's 
Hist.  Nat.  du  Corail,  Paris,  1864,  and  his  essays  on  the  Development  of  Corals,  in 
vols.  i.  and  ii.  of  the  Archives  de  Zool.  experimental ;  Professor  J.  R.  Greene's 
Manual  of  the  Sub-kingdom  Coelenterata,  which  contains  a  bibliography  very  com- 
plete to  the  date  of  its  publication,  and  the  articles  '  Actinozoa,'  '  Ctenophora,'  and 
'  Hydrozoa  '  in  the  supplement  to  the  Natural  History  Division  of  the  English  Cyclo- 
pcedia.  The  Ctenophora  are  specially  treated  of  in  vol.  iii.  of  Professor  Agassiz's 
Contributions  to  the  Natural  History  of  the  United  States.  See  also  Professor 
Alex.  Agassiz's  Seaside  Studies  in  Natural  History  and  his  Illustrated  Catalogue 
of  the  Museum  of  Comparative  Zoology  at  Harvard  College',  Professor  James 
Clark  in  American  Journal  of  Science,  ser.  ii.  vol.  xxxv.  p.  348;  Dr.  D.  Macdonald  in 
Trans.  EOIJ.  Soc  Edinb.  vol.  xxiii.  p.  515  ;  Mr.  H.  N.  Moseley,  '  On  the  Structure  of 
a  Species  of  Millepora,'  in  Phil.  Trans.  1877,  p.  117,  and  '  On  the  Structure  of  the 
Stylasterida,'  ibid.  1878,  p.  425  ;  and  011  the  Acaleplice,  Professor  Haeckel's  Beitrdge 
zur  Naturgeschichte  der  H/jdroinedusen  ;  the  masterly  work  of  the  brothers  Hertwig, 
Das  Nervensystem  und  die  Sinnesorgane  der  Medusen,  1878 ;  and  the  memoir  of 
Professor  Schafer,  '  On  the  Nervous  System  of  Aurelia  aunt  a,'  in  Phil.  Trans.  1878, 
p.  563.  Of  later  treatises  Professor  Ray  Lankester's  article  011  Hydrozoa,  in  the  9th 
edition  of  the  Encyclopedia  Britannica;  the  'Challenger*  Reports  of  Professor 
Altaian  011  the  Hydroida  (Pluniulariidse  only),  Professor  Haeckel  on  the  Medusae, 
Professor  Moseley  011  Deep-sea  Corals,  Dr.  R.  Hertwig  on  the  Actiniaria,  Professors 
E.  P.  Wright  and  Studer  011  the  Alcyoiiaria,  and  Mr.  George  Brook  on  the  Antipatharia ; 
the  monographs  by  Dr.  A.  Andres  on  Actiniae  and  by  Dr.  C.  Chun  on  Ctenophora, 
published  in  the  Fauna  und  Flora  des  Golfes  von  Neapel,  should  be  consulted. 
Dr.  Chun  has  made  some  progress  with  a  general  account  of  the  Ccelentera  in  Bronn's 
'  Thierreich,'  Bd.  ii.  Abth.  2.  On  fresh-water  Medusa?,  see  Mr.  R.  T.  Giinther 
iu  Quart.  Jount.  M/t-r.  Sri.  xxxvi.  p.  284. 


1  See  Korotneff,  ZritxcJn:  f.  wise.  Z.1,,1.  xliii.  p.  -242,  and  Dr.  A.  Willey   Quart. 
Journ.  Micr.  Sci.  xxxix.  p.  S23. 


3  L  2 


884 


CHAPTER   XYI 

ECHINODEBMA 

As  we  ascend  the  scale  of  animal  life,  we  meet  with  such  a  rapid 
advance  in  complexity  of  structure  that  it  is  no  longer  possible  to 
acquaint  oneself  with  any  organism  by  microscopic  examination 
of  it  as  a  whole  ;  and  the  dissection  or  analysis  which  becomes 
necessary,  in  order  that  each  separate  part  may  be  studied  in  detail, 
belongs  rather  to  the  comparative  anatomist  than  to  the  ordinary 
microscopist.  This  is  especially  the  case  with  the  Echinus  (sea- 
urchin),  Asterias  (star-fish),  and  other  members  of  the  class  Echino- 
derma,  of  whose  complex  organisation  even  a  general  account 
would  be  quite  foreign  to  the  purpose  of  this  work.  Yet  there  are 
certain  parts  of  their  structure  which  furnish  microscopic  objects  of 
such  beauty  and  interest  that  they  cannot  by  any  means  be  passed 
by  ;  while  the  study  of  their  embryonic  forms,  which  can  be  pro- 
secuted by  any  seaside  observer,  brings  into  view  an  order  of  facts 
of  the  highest  scientific  interest. 

It  is  in  the  structure  of  that  calcareous  skeleton  which  exists 
under  some  form  in  nearly  every  member  of  this  class  that  the  ordi- 
nary microscopist  finds  most  to  interest  him.  This  attains  its  highest 
development  in  the  Echinoidea,  in  which  it  forms  a  box-like  shell  or 
'  test,'  composed  of  numerous  polygonal  plates  jointed  to  each  other 
with  great  exactness,  and  beset  on  its  external  surface  with  l  spines,' 
which  may  have  the  form  of  prickles  of  no  great  length,  or  may  be 
stout  club-shaped  bodies,  or,  again,  may  be  very  long  and  slender 
rods.  The  intimate  structure  of  the  shell  is  everywhere  the  same  ; 
for  it  is  composed  of  a  network,  which  consists  of  carbonate  of  lime 
with  a  very  small  quantity  of  animal  matter  as  a  basis,  and  which 
extends  in  every  direction  (i.e.  in  thickness  as  well  as  in  length  and 
breadth),  its  areolce  or  interspaces  freely  communicating  with  each 
other  (figs.  671,  672).  These  '  areolae,'  and  the  solid  structure  which 
surrounds  them,  may  bear  an  extremely  variable  proportion  one  to 
the  other  ;  so  that  in  two  masses  of  equal  size  the  one  or  .the  other 
may  greatly  predominate ;  and.  the  texture  may  have  either  a  re- 
markable lightness  and  porosity,  if  the  network  be  a  very  open  one, 
like  that  of  fig.  671,  or  may  possess  a  considerable  degree  of  com- 
pactness, if  the  solid  portion  be  strengthened.  Generally  speaking, 
the  different  layers  of  this  network,  which  are  connected  together 
by  pillars  that  pass  from  one  to  the  other  in  a  direction  perpendicu- 


STKUCTURE   OF  ECHINOLDS  885 

lar  to  their  plane,  are  so  arranged  that  the  perforations  in  one  shall 
correspond  to  the  intermediate  solid  structure  in  the  next ;  and  their 
transparence  is  such  that  when  we  are  examining  a  section  thin 
enough  to  contain  only  two  or  three  such  layers,  it  is  easy,  by 
properly  focussing  the  microscope,  to  bring  either  one  of  them  into 
distinct  view.  From  this  very  simple  but  very  beautiful  arrange- 
ment, it  comes  to  pass  that  the  plates  of  which  the  entire  *  test '  is 
made  up  possess  a  very  considerable  degree  of  strength,  notwith- 
standing that  their  porousness  is  such  that  if  a  portion  of  a  fractured 
edge,  or  any  other  part  from  which  the  investing  membrane  has 
been  removed,  be  laid  upon  fluid  of  almost  any  description,  this  will 
l>e  rapidly  sucked  up  into  its  substance.  A  very  beautiful  example 
of  the  same  kind  of  calcareous  skeleton,  having  a  more  regular  con- 
formation, is  furnished  by  the  disc  or  *  rosette  '  which  is  contained 
in.  the  tip  of  every  one  of  the  tubular  suckers  put  forth  by  the  living 
Echinus  from  the  '  ambulacral  pores '  that  are  seen  in  the  rows  of 


FIG.  671. — Section  of  shell  of  Echinus  FIG.  672. — Transverse  section  of  cen- 

showing  the   calcareous  network  of  tral  portion  of  spine  of  Heterocen- 

which  it  is  composed  :  a  a,  portions  trotus,  showing  its  more  open  net- 

of  a  deeper  layer.  work. 

smaller  plates  interposed  between  the  larger  spine-bearing  plates  of 
its  box-like  shell.  If  the  entire  disc  be  cut  off,  and  be  mounted 
when  dry  in  Canada  balsam,  the  calcareous  rosette  may  be  seen 
sufficiently  wrell ;  but  its  beautiful  structure  is  better  made  out  when 
the  animal  membrane  that  incloses  it  has  been  got  rid  of  by  boiling 
in  a  solution  of  caustic  potass ;  and  the  appearance  of  one  of  the 
five  segments  of  which  it  is  composed,  when  thus  prepared,  is  shown 
in  fig.  674. 

The  most  beautiful  display  of  this  reticulated  struct ure,  however, 
is  shown  in  the  conformation  of  the  l  spines '  of  Echinus,  Cidaris,  &c., 
in  which  it  is  combined  with  solid  ribs  or  pillars,  disposed  in  such  a 
manner  as  to  increase  the  strength  of  these  organs,  a  regular  and 
elaborate  pattern  being  formed  by  their  intermixture,  which  shows 
considerable  variety  in  different  species.  When  we  make  a  thin 
transverse  section  of  almost  any  spine  belonging  to  the  genus 
Echinus  (the  small  spines  of  our  British  species,  however,  being 
exceptional  in  this  respect)  or  its  immediate  allies,  we  see  it  to  be 


886 


ECHINODEKMA 


made  up  of  a  number  of  concentric  layers,  arranged  in  a  manner  that 
strongly  reminds  us  of  the  concentric  rings  of  an  exogenous  tree 


FIG.  678.— Transverse  section  of  spine  of  Eclitnoinctra. 

(fig.  673).     The  number  of  these  layers  is  extremely  variable,  de- 
pending not  merely  upon  the  age  of  the  spine,  but  (as  will  presently 

appear)  upon  the  part  of 
its  length  from  which  the 
section  happens  to  be 
taken.  The  centre  is 
usually  occupied  by  a 
very  open  network  (fig. 
672) ;  and  this  is  bounded 
by  a  row  of  transparent 
spaces  (like  those  at  a  a! , 
b  bf,  c  c',  &c.,  fig.  675), 
which  on  a  cursory  in- 
spection might  be  sup- 
posed to  be  void,  but  are 
found  on  closer  examina- 
tion to  be  the  sections  of 
solid  ribs  or  pillars,  which 

run  in  the  direction  of  the  length  of  the  spine,  and  form  the  exterior 
of  every   layer.     Their   solidity   becomes   very   obvious   when   we 


FIG.  674.— One  of  the  segments  of  the  calcareous 
skeleton  of  an  ambulacral  disc  of  Echinus. 


SPINES   OF  ECHlNOIDS  887 

either  examine  a  section  of  a  spine  whose  substance  is  pervaded  (MS 
often  happens)  with  a  colouring  matter  of  some  depth,  or  when  we 
look  at  a  very  thin  section  by  black-ground  illumination.  Around 
the  innermost  circle  of  these  solid  pillars  there  is  another  layer  of 
the  calcareous  network,  which  again  is  surrounded  by  another  circle 
of  solid  pillars  ;  and  this  arrangement  may  be  repeated  many  times, 
as  shown  in  fig.  675,  the  outermost  row  of  pillars  forming  the 
projecting  ribs  that  are  commonly  to  be  distinguished  on  the  surface 
of  the  spine.  Around  the  cup-shaped  base  of  the  spine  is  a  membrane 
which  is  continuous  with  that  covering  the  surface  of  the  shell,  and 
serves  not  merely  to  hold  down  the  cup  upon  the  tubercle  over  which 
it  works,  but  also  by  its  contractility  to  move  the  spine  in  any  required 
direction.  The  increase  in  size  of  the  spine  appears  to  be  due  to  the 
protoplasmic  substance  which  fills  up  the  spaces  in  the  open  network 
of  the  spine  and  other  skeletal  structures.  Each  new  formation 
completely  ensheathes  the  old,  not  merely  surrounding  the  part  pre- 
viously formed,  but  also  projecting  considerably  beyond  it ;  and  thus 
it  happens  that  the  number  of  layers  shown  in  a  transverse  section 


FIG.  675. — Portion   of   transverse  section  of   spine  of  Heterocentrotua 
mammillatus. 

will  depend  in  part  upon  the  place  of  that  section.  For  if  it  cross 
near  the  base,  it  will  traverse  every  one  of  the  successive  layers  from 
the  very  commencement ;  whilst  if  it  cross  near  the  apex,  it  will 
traverse  only  the  single  layer  of  the  last  growth,  notwithstanding 
that,  in  the  club-shaped  spines,  this  terminal  portion  may  be  of  con- 
siderably larger  diameter  than  the  basal ;  and  in  any  intermediate 
part  of  the  spine,  so  many  layers  will  be  traversed  as  have  been 
formed  since  the  spine  first  attained  that  length.  The  basal  portion 
of  the  spine  is  enveloped  in  a  reticulation  of  a  very  close  texture 
without  concentric  layers,  forming  the  cup  or  socket  which  works 
over  the  tubercle  of  the  shell. 

Their  combination  of  elegance  of  pattern  with  richness  of  colour- 
ing renders  well-prepared  specimens  of  these  spines  among  the  most 
beautiful  objects  that  the  microscopist  can  anywhere  meet  with. 
The  large  spines  of  the  various  species  of  the  genus  Heterocentrotus 
furnish  sections  most  remarkable  for  size  and  elaborateness,  as  well 
as  for  depth  of  colour  (in  which  last  point,  however,  the  deep  purple 
spines  of  Echinus  lividus  are  pre-eminent) ;  but  for  exquisite 


888  ECH1NODEEMA 

neatness  of  pattern  there  are  no  spines  that  can  approach  those 
of  Echinometra  (fig.  673).  The  spines  of  Stomopneustes  variolcuris 
are  also  remarkable  for  their  beauty.  No  succession  of  concentric 
layers  is  seen  in  the  spines  of  the  British  Echini,  probably  be- 
cause (according  to  the  opinion  of  the  late  Sir  J.  G.  Daly  ell)  these 
spines  are  cast  off  and  renewed  every  year,  each  new  formation 
thus  going  to  make  an  entire  spine,  instead  of  making  an  addition 
to  that  previously  existing.  Most  curious  indications  are  some- 
times afforded  by  sections  of  Echinus-spines  of  an  extraordinary 
power  of  reparation  inherent  in  these  bodies.  For  irregularities 
are  often  seen  in  the  transverse  sections  which  can  be  accounted 
for  in  no  other  way  than  by  supposing  the  spines  to  have  received 
an  injury  when  the  irregular  part  was  at  the  exterior,  and  to 
have  had  its  loss  of  substance  supplied  by  the  growth  of  new 


FIG.  676. — Transverse  section  of  a  spine  of  Goniocidaris  florigera, 
which  shows  that  the  prickles  on  the  spine  are  formed,  not  by  the 
crust  only,  but  also  by  the  inner  reticular  tissue.  (From  Bell.) 

tissue,  over  which  the  subsequent  layers  have  been  formed  as  usual. 
And  sometimes  a  peculiar  ring  may  be  seen  upon  the  surface  of  a 
spine,  which  indicates  the  place  of  a  complete  fracture,  all  beyond 
it  being  a  new  growth,  whose  unconformableness  to  the  older  or 
basal  portion  is  clearly  shown  by  a  longitudinal  section.1  The  spines 
of  Cidaris  present  a  marked  departure  from  the  plan  of  structure 
exhibited  in  Echinus  ;  for  not  only  are  they  destitute  of  concentric 
layers,  but  the  calcareous  network  which  forms  their  principal 
substance  is  incased  in  a  solid  calcareous  sheath  perforated  with 
tubules,  which  seems  to  take  the  place  of  the  separate  pillars  of  the 
Echini.  This  is  usually  found  to  close  in  the  spine  at  its  tip  also ; 

1  See  the  Author's  description  of  such  reparations  in  the  Monthly  Microscopical 
Journal,  vol.  iii.  1870,  p.  225. 


SPINES  ;  PEDICELLAKI^:  889 

and  thus  it  would  appear  that  the  entire  spine  must  be  formed  at 
once,  since  no  addition  could  be  made  either  to  its  length  or  to  its 
diameter,  save  on  the  outside  of  the  sheath,  where  it  is  never  to  be 
found.  The  sheath  itself  often  rises  up  in  prominent  points  or 
ridges  on  the  surface  of  these  spines  ;  but,  as  is  shown  in  fig.  676, 
the  reticular  portion  may  have  a  share  in  the  formation  of  the  rings. 
This  view  of  the  mode  of  formation  of  the  Cidarid  spine  is  con- 
tested by  Professor  Jeffrey  Bell,  who  has  brought  forward l  evidence 
to  show  that  if  two  spines  of  different  sizes  be  taken  from  two 
examples  of  Cidaris  metularia,  also  differing  in  size,  the  quantity  of 
solid  calcareous  sheath  seen  in  transverse  section  is  proportionately 
less  in  the  larger  than  in  the  smaller  spine  ;  from  this  he  concludes 
that  the  growth  is  due  to  the  internal  reticulated  portion  rather 
than  to  the  outer  crust.  The  slender,  almost  filamentary  spines 


FIG.  677. — Spine  of  Sputangus. 

of  Spatangus  (fig.  677)  and  the  innumerable  minute  hair-like  pro- 
cesses attached  to  the  shell  of  Clypeaster  are  composed  of  the  like 
regularly  reticulated  substance  ;  2  and  these  are  very  beautiful  objects 
for  the  lower  powers  of  the  microscope,  when  laid  upon  a  black 
ground  and  examined  by  reflected  light  without  any  further  prepara- 
tion. It  is  interesting  also  to  find  that  the  same  structure  presents 
itself  in  the  curious  Pedicellarice  (forceps-like  bodies  often  mounted  on 
long  stalks),  which  are  found  on  the  surface  of  many  Echinida  and 
Asterida,  and  the  nature  of  which  was  formerly  a  source  of  much 
perplexity  to  naturalists,  some  having  maintained  that  they  were 
parasites,  whilst  others  considered  them  as  proper  appendages  of  the 
Echinus  itself.  The  complete  conformity  which  exists  between  the 
structure  of  their  skeleton  and  that  of  the  animal  to  which  they  are 
attached  removes  all  doubt  of  their  being  truly  appendages  to  it,  as 
observation  of  their  actions  in  the  living  state  would  indicate.3 

1  Journ.  Roy.  Microsc.  Soc.  1884,  p.  845. 

2  A  number  of  rare  spines  are  described  and  figured  by  Prof.  H.  W.  Mackintosh 
in  vols.  xxvi.  (p.  475)  and  xxviii.  (pp.  241  and  259)  of  the  Trans.  Boy.  Irish  Academy. 

5  Prof.  Alex.  Agassiz  has  shown  the  relations  of  the  Pedicellariae  to  the  spines. 
Much  information  regarding  the  various  forms  of  these  curious  bodies  will  be  found 
in  Professor  Perrier's  memoir  in  the  Ann.  Sc.  Nat,  (5),  vols.  xii.  and  xiii. ;  Mr  Sladen's 


890 


ECHINODERMA 


Another  example  of  the  same  structure  is  found  in  the  peculiar 
framework  of  plates  which  surrounds  the  interior  of  the  oral  orifice 
of  the  shell,  and  which  includes  the  five  teeth  that  may  often  be  seen 
projecting  externally  through  that  orifice,  the  whole  forming  what 
is  known  as  the  '  lantern  of  Aristotle.'  The  texture  of  the  plates 
or  jaws  resembles  that  of  the  shell  in  every  respect,  save  that  the 
network  is  more  open ;  but  that  of  the  teeth  differs  from  it  so  widely 
as  to  have  been  likened  to  that  of  the  bone  and  dentine  of  vertebrate 
animals.  The  careful  investigations  of  Mr.  James  Salter,1  however, 
have  fully  demonstrated  that  the  appearances  which  have  suggested 
this  comparison  are  to  be  otherwise  explained,  the  plan  of  structure 
of  the  tooth  being  essentially  the  same  as  that  of  the  shell,  although 
greatly  modified  in  its  working  out.  The  complete  tooth  has  some- 


FIG.  678.— Structure  of  the  tooth  of  Echinus  :  A,  vertical  section,  showing 
the  form  of  the  apex  of  the  tooth  as  produced  by  wear,  and  retained  by 
the  relative  hardness  of  its  elementary  parts ;  a,  the  clear  condensed  axis ; 
b,  the  body  formed  of  plates ;  c,  the  so-called  enamel ;  d,  the  keel.  B, 
commencing  growth  of  the  tooth,  as  seen  at  its  base,  showing  its  two  sys- 
tems of  plates ;  the  dark  appearance  in  the  central  portion  of  the  upper 
part  is  produced  by  the  incipient  reticulations  of  the  flabelliform  processes. 
C,  transverse  section  of  the  tooth,  showing  at  a  the  ridge  of  the  keel ;  at  b 
its  lateral  portion,  resembling  the  shell  in  texture ;  at  c  c,  the  enamel. 

what  the  form  of  that  of  the  front  tooth  of  a  rodent,  save  that  its 
concave  side  is  strengthened  by  a  projecting  '  keel,'  so  that  a  trans- 
verse section  of  the  tooth  presents  the  form  of  a  _|_.  This  keel  is 
composed  of  cylindrical  rods  of  carbonate  of  lime,  having  club-shaped 
extremities  lying  obliquely  to  the  axis  of  the  tooth  (fig.  678,  A,  d)  ; 
these  rods  do  not  adhere  very  firmly  together,  so  that  it  is  difficult 
to  keep  them  in  their  places  in  making  sections  of  the  part.  The 

essay  in  Ann.  and  Mag.  Nat.  Hist.  (5),vi.  p.  101 ;  andM.  Foettinger's  paper  in  vol.  ii. 
p.  455  of  the  Archives  de  Biologic. 

1  See  his  memoir,  '  On  the  Structure  and  Growth  of  the  Tooth  of  Echinus,'  in 
Phil.  Trans,  for  1861,  p.  387.  See  also  Ofiesbrecht,  '  Der  feiiiere  Bau  der  Seeigel- 
zk'hne,'  Morph.  Jahrbuch,  vi.  p.  79. 


CALCAKEOUS  TISSUE  891 

convex  surface  of  the  tooth  (c,  c,  c)  is  covered  with  a  firmer  layer, 
which  has  received  the  name  of  '  enamel.'  This  is  composed  of 
shorter  rods,  also  obliquely  arranged,  but  having  a  much  more 
intimate  mutual  adhesion  than  we  find  among  the  rods  of  the  keel. 
The  principal  part  of  the  substance  of  the  tooth  (A,  b)  is  made  up  of 
what  may  be  called  the  *  primary  plates.'  These  are  triangular  plates 
of  calcareous  shell-substance,  arranged  in  two  series  (as  shown  at 
B),  and  constituting  a  sort  of  framework  with  which  the  other  parts 
to  be  presently  described  become  connected.  These  plates  may  be 
seen  by  examining  the  growing  base  of  an  adult  tooth  that  has 
been  preserved  with  its  attached  soft  parts  in  alcohol,  or  (which  is 
preferable)  by  examining  the  base  of  the  tooth  of  a  fresh  specimen, 
the  minuter  the  better.  The  lengthening  of  a  tooth  below,  as  it 
is  worn  away  above,  is  mainly  effected  by  the  successive  addition  of 
new  *  primary  plates.'  To  the  outer  edge  of  the  primary  plates  at 
some  little  distance  from  the  base  we  find  attached  a  set  of  lappet - 
like  appendages,  which  are  formed  of  similar  plates  of  calcareous 
shell-substance,  and  are  denominated  by  Mr.  Salter  *  secondary 
plates.'  Another  set  of  appendages  termed  '  flabelliform  processes  ' 
is  added  at  some  little  distance  from  the  growing  base  ;  these  consist 
of  elaborate  reticulations  of  calcareous  fibres,  ending  in  fan-shaped 
extremities.  And  at  a  point  still  further  from  the  base  we  find  the 
different  components  of  the  tooth  connected  together  by  *  soldering 
particles,'  which  are  minute  calcareous  discs  interposed  between  the 
previously  formed  structures ;  and  it  is  by  the  increased  develop- 
ment of  this  connective  substance  that  the  intervening  spaces  are 
narrowed  into  the  semblance  of  tubuli  like  those  of  bone  or  dentine. 
Thus  a  vertical  section  of  the  tooth  comes  to  present  an  appearance 
very  like  that  of  the  bone  of  a  vertebrate  animal,  with  its  lacunae, 
canaliculi.  and  lamella  ;  but  in  a  transverse  section  the  body  of  the 
tooth  bears  a  stronger  resemblance  to  dentine  ;  whilst  the  keel  and 
enamel  layer  more  resemble  an  oblique  section  of  Pinna  than  any 
other  form  of  shell-structure. 

The  calcareous  plates  which  form  the  less  compact  skeletons  of 
the  Asteroidea  ('  star-fish  '  and  their  allies)  and  of  the  Ophiuroidea 
('sand-stars'  and  'brittle  stars')  have  the  same  texture  as  those  of 
the  shell  of  Echinus.  And  this 
presents  itself,  too,  in  the  spines  or 
prickles  of  their  surface  when 
these  (as  in  the  great  Goniaster 
equestris  or  'knotty  cushion-star') 
are  large  enough  to  be  furnished 
with  a  calcareous  framework.  An 
example  of  this  kind,  furnished  by 
the  Astrophyton,  is  represented  in 
fig.  679.  The  spines  with  which  FlG  679.  -Calcareous  plate  and  claw 
the  arms  of  the  species  of  Ophiothrix  of  Astrovlujton. 

('  brittle  star  ')  are  beset  are  often 

remarkable  for  their  beauty  of  conformation  ;  those  of  0.  penta- 
phyllum,  one  of  the  most  common  kinds,  might  serve  (as  Professor 
E.  Forbes  justly  remarked),  in  point  of  lightness  and  beauty,  as 


892  ECHINODERMA 

models  for  the  spire  of  a  cathedral.  These  are  seen  to  the  greatest 
advantage  when  mounted  in  Canada  balsam,  and  viewed  by  the 
binocular  microscope  with  black-ground  illumination.  It  is  inter- 
esting to  remark  that  the  minute  tooth  of  Ophiothrix  clearly  exhibits, 
with  scarcely  any  preparation,  that  gradational  transition  between 
the  ordinary  reticular  structure  of  the  shell  and  the  peculiar  sub- 
stance of  the  tooth  which  in  the  adult  tooth  of  the  Echinus  can 
only  be  traced  by  making  sections  of  it  near  its  base.  The  tooth  of 
Ophiothrix  may  be  mounted  in  balsam  as  a  transparent  object  with 
scarcely  any  grinding  down  ;  and  it  is  then  seen  that  the  basal  por- 
tion of  the  tooth  is  formed  upon  the  open  reticular  plan  characteristic 
of  the  '  shell,'  whilst  this  is  so  modified  in  the  older  portion  by  sub- 
sequent addition  that  the  upper  part  of  the  tooth  has  a  bone-like 
character. 

The  calcareous  skeleton  is  very  highly  developed  in  the  Crinoidea, 
their  stems  and  branches  being  made  up  of  a  calcareous  network 
closely  resembling  that  of  the  shell  of  the  Echinus.  This  is  extremely 
well  seen,  not  only  in  the  recent  Pentacrinus  asterius,  a  somewhat  rare 
animal  of  the  West  Indian  seas,  but  also  in  a  large  proportion  of 
the  fossil  crinoids,  whose  remains  are  so  abundant  in  many  of  the 
older  geological  formations ;  for,  notwithstanding  that  these  bodies 
have  been  penetrated  in  the  act  of  fossilisation  by  a  mineral  infiltra- 
tion, which  seems  to  have  substituted  itself  for  the  original  fabric 
(a  regularly  crystalline  cleavage  being  commonly  found  to  exist  in 
the  fossil  stems  of  Encrinites,  &c.,  as  in  the  fossil  spines  of  Echinida), 
yet  their  organic  structure  is  often  most  perfectly  preserved.1  In 
the  circular  stems  of  Encrinites  the  texture  of  the  calcareous  net- 
work is  uniform,  or  nearly  so,  throughout ;  but  in  the  pentangular 
Pentacrini  a  certain  figure  or  pattern  is  formed  by  variations  of 
texture  in  different  parts  of  the  transverse  section.2 

The  minute  structure  of  the  shells,  spines,  and  other  solid  parts 
of  the  skeleton  of  Echinoderma  can  only  be  displayed  by  thin 
sections  made  upon  the  general  plan  already  described  in  Chapter  VII. 
But  their  peculiar  texture  requires  that  certain  precautions  should 
be  taken :  in  the  first  place,  in  order  to  prevent  the  section  from 
breaking  whilst  being  reduced  to  the  desirable  thickness ;  and  in 
the  second,  to  prevent  the  interspaces  of  the  network  from  being 
clogged  by  the  particles  abraded  in  the  reducing  process.  An  illus- 
tration of  a  section  cut  from  a  spine  of  Echhwnietra  is  given  in 
fig.  673.  A  section  of  the  shell,  spine,  or  other  portion  of  the 
skeleton  should  first  be  cut  with  a  fine  saw,  and  be  rubbed  on  a  flat 
file  until  it  is  about  as  thin  as  ordinary  card,  after  which  it  should 
be  smoothed  on  one  side  by  friction  with  water  on  a  Water-of-Ayr 

1  The  calcareous  skeleton  even  of  living  Echiiioderms  has  a  crystalline  aggregation, 
as  is  very  obvious  in  the  more  solid  spines  of  Echinoinetr(s,  &c. ;  for  it  is  difficult,  in 
sawing  these  across,  to  avoid  their  tendency  to  cleavage  in  the  oblique  plane  of 
calcite.     And  the  Author  is  informed  by  Mr.  Sorby  that  the  calcareous  deposit  which 
fills  up  the  areolae  of  the  fossilised  skeleton  has  always  the  same  crystalline  system 
with  the  skeleton  itself,  as  is  shown  not  merely  by  the  uniformity  of  their  cleavage, 
but  by  their  similar  action  on  polarised  light. 

2  See  figs.  74-76  of  the  Author's  memoir  on   '  Shell  Structure  '  in  the  Report  of 
the  British  Association,  1847. 


PREPARING    SPINES  893 

stone.  It  should  then,  after  careful  washing,  be  dried,  first  on  white 
blotting-paper,  afterwards  by  exposure  for  some  time  to  a  gentle 
heat,  so  that  no  water  may  be  retained  in  the  interstices  of  the  net- 
work which  would  oppose  the  complete  penetration  of  the  Canada 
balsam.  Next,  it  is  to  be  attached  to  a  glass  slip  by  balsam  hardened 
in  the  usual  manner ;  but  particular  care  should  be  taken,  first,  that 
the  balsam  be  brought  to  exactly  the  right  degree  of  hardness,  and 
second,  that  there  be  enough  not  merely  to  attach  the  specimen  to 
the  glass,  but  also  to  saturate  its  substance  throughout.  The  right 
degree  of  hardness  is  that  at  which  the  balsam  can  be  with  difficulty 
indented  by  the  thumb-nail  ;  if  it  be  made  harder  than  this,  it  is 
apt  to  chip  off  the  glass  in  grinding,  so  that  the  specimen  also  breaks 
away ;  and  if  it  be  softer,  it  holds  the  abraded  particles,  so  that 
the  openings  of  the  network  become  clogged  with  them.  If,  when 
rubbed  down  nearly  to  the  required  thinness,  the  section  appears  to 
be  uniform  and  satisfactory  throughout,  the  reduction  may  be  com- 
pleted without  displacing  it ;  but  if  (as  often  happens)  some  inequality 
in  thickness  should  be  observable,  or  some  minute  air-bubbles  should 
show  themselves  between  the  glass  and  the  under  surface,  it  is  de- 
sirable to  loosen  the  specimen  by  the  application  of  just  enough  heat 
to  melt  the  balsam  (special  care  being  taken  to  avoid  the  production 
of  fresh  air-bubbles)  and  to  turn  it  over  so  as  to  attach  the  side 
last  polished  to  the  glass,  taking  care  to  remove  or  to  break  with 
the  needle  point  any  air- bubbles  that  there  may  be  in  the  balsam 
covering  the  part  of  the  glass  on  which  it  is  laid.  The  surface  now 
brought  uppermost  is  then  to  be  very  carefully  ground  down, 
special  care  being  taken  to  keep  its  thickness  uniform  through  every 
part  (which  may  be  even  better  judged  of  by  the  touch  than  by  the 
eye),  and  to  carry  the  reducing  process  far  enough,  without  carrying 
it  too  far.  Until  practice  shall  have  enabled  the  operator  to  judge 
of  this  by  passing  his  finger  over  the  specimen,  he  must  have  con- 
tinual recourse  to  the  microscope  during  the  latter  stages  of  his 
work  ;  and  he  should  bear  constantly  in  mind  that,  as  the  specimen 
will  become  much  more  translucent  when  mounted  in  balsam  and 
covered  with  glass  "than  it  is  when  the  ground  surface  is  exposed,  he 
need  not  carry  his  reducing  process  so  far  as  to  produce  at  once  the 
entire  translucence  he  aims  at,  the  attempt  to  accomplish  which 
would  involve  the  risk  of  the  destruction  of  the  specimen.  In 
'  mounting '  the  specimen  liquid  balsam  should  be  employed,  and 
only  a  very  gentle  heat  (not  sufficient  to  produce  air-bubbles  or  to 
loosen  the  specimen  from  the  glass)  should  be  applied  ;  and  if,  after 
it  has  been  mounted,  the  section  should  be  found  too  thick,  it  will 
be  easy  to  remove  the  glass  cover  and  to  reduce  it  further,  care  being 
taken  to  harden  to  the  proper  degree  the  balsam  which  has  been 
newly  laid  on. 

If  a  number  of  sections  are  to  be  prepared  at  once  (which  it  is 
often  useful  to  do  for  the  sake  of  economy  of  time,  or  in  order  to 
compare  sections  taken  from  different  parts  of  the  same  spine),  this 
may  be  most  readily  accomplished  by  laying  them  down,  when  cut 
off  by  the  saw,  without  any  preliminary  preparation  save  the  blow- 
ing of  the  calcareous  dust  from  their  surfaces,  upon  a  thick  slip  of 


894  ECHINODERMA 

glass  well  covered  with  hardened  balsam  ;  a  large  proportion  of  its 
surface  may  thus  be  occupied  by  the  sections  attached  to  it,  the 
chief  precaution  required  being  that  all  the  sections  come  into 
equally  close  contact  with  it.  Their  surfaces  may  then  be  brought 
to  an  exact  level  by  rubbing  them  down,  first  upon  a  flat  piece  of 
grit  (which  is  very  suitable  for  the  rough  grinding  of  such  sections) 
and  then  upon  a  large  Water-of-Ayr  stone  whose  surface  is  ;  true.' 
When  this  level  has  been  attained  the  ground  surface  is  to  be  well 
washed  and  dried,  and  some  balsam  previously  hardened  is  to  be 
spread  over  it,  so  as  to  be  sucked  in  by  the  sections,  a  moderate  heat 
being  at  the  same  time  applied  to  the  glass  slide  ;  and  when  this 
has  been  increased  sufficiently  to  loosen  the  sections  without  over- 
heating the  balsam,  the  sections  are  to  be  turned  over,  one  by  one, 
so  that  the  ground  surfaces  are  now  to  be  attached  to  the  glass  slip, 
special  care  being  taken  to  press  them  all  into  close  contact  with  it. 
They  are  then  to  be  very  carefully  rubbed  down,  until  they  are 
nearly  reduced  to  the  required  thinness ;  and  if,  on  examining  them 
from  time  to  time,  their  thinness  should  be  found  to  be  uniform 
throughout,  the  reduction  of  the  entire  set  may  be  completed  at  once  ; 
and  when  it  has  been  carried  sufficiently  far,  the  sections,  loosened  by 
warmth,  are  to  be  taken  up  on  a  camel-hair  brush  dipped  in  turpen- 
tine and  transferred  to  separate  slips  of  glass  whereon  some  liquid 
balsam  has  been  previously  laid,  in  which  they  are  to  be  mounted  in 
the  usual  manner.  It  more  frequently  happens,  however,  that,  not- 
withstanding every  care,  the  sections,  when  ground  in  a  number 
together,  are  not  of  uniform  thickness,  owing  to  some  of  them  being 
underlain  by  a  thicker  stratum  of  balsam  than  others ;  and  it  is 
then  necessary  to  transfer  them  to  separate  slips  before  the  reducing 
process  is  completed,  attaching  them  with  hardened  balsam,  and 
finishing  each  section  separately. 

A  very  curious  internal  skeleton,  formed  of  detached  plates  or 
spicules,  is  found  in  many  members  of  this  class,  often  forming  an 
investment  like  a  coat  of  mail  to  some  of  the  viscera,  especially  the 
ovaries.  The  forms  of  these  plates  and  spicules  are  generally  so 
diverse,  even  in  closely  allied  species,  as  to  afford  very  good  differ- 
ential characters.  This  subject  is  one  that  has  been  as  yet  but  very 
little  studied,  Mr.  Stewart  being  the  only  microscopist  who  has  given 
much  attention  to  it,1  but  it  is  well  worthy  of  much  more  extended 
research. 

It  now  remains  for  us  to  notice  the  curious  and  often  very  beau- 
tiful structures  which  represent,  in  the  class  ffolothurioidea,  the  solid 
calcareous  skeleton  of  the  classes  already  noticed.  The  greater 
number  of  the  animals  belonging  to  this  order  are  distinguished  by 
the  flexibility  and  absence  of  firmness  of  their  envelopes ;  and  ex- 
cepting in  the  case  of  the  various  species  which  have  a  set  of  cal- 
careous plates,  disposed  around  the  wall  of  the  pharynx,  we  do  not  find 
among  them  any  representation,  that  is  apparent  to  the  unassisted 
eye,  of  that  skeleton  which  constitutes  so  distinctive  a  feature  of  the 

1  See  his  memoir   in  the   Liinifan    Transactions,   xxv.  p.    365;  see   also   Bell, 
Journ.  Hoij.  Microbe.  Soc.  1882,  p.  '227. 


HOLOTHURIAN   SPICULES 


895 


class  generally.1  But  a  microscopic  examination  of  their  integument 
at  once  brings  to  view  the  existence  of  great  numbers  of  minute 
isolated  plates,  every  one  of  them  presenting  the  characteristic  re- 
ticulated structure,"  which  are  set  with  greater  or  less  closeness  in 


FIG.  680. — Holothurioidea :  I,Stichopus  Kef  erst  ei  nil ;  «,  calcareous 
plate  of  same;  6,  c,  calcareous  plates  of  Holothuria  vaffabunda  ; 
(1,  the  same  of  H.  inh<il>His\  e,  the  same  of  H.  botellus;  f,  of  H. 
pardalis :  g,  of  H.  edit  fin. 

the  substance  of  the  skin.  Various  forms  of  the  plates  which  thus 
present  themselves  in  Holothuria  are  shown  in  fig.  680. 2  In  the 
Synapta,  one  of  the  long-bodied  forms  of  this  order,  which  abounds 
in  the  Mediterranean  Sea,  and  of  which  two  species  (the  8.  digitata 


FIG.  681. — Calcareous  skeleton  of  S//>Hij>t«  :  A,  plate  imbedded  in 
skin ;  B,  the  same,  with  its  anchor-like  spine  attached  ;  C,  anchor- 
like  spine  separated. 

and  >V.  inhcerens)  occasionally  occur  upon  our  own  coasts,3  the  cal- 
careous plates  of  the  integument  have  the  regular  form  shown  at  A, 
fig.  681  ;  and  each  of  these  carries  the  curious  anchor-like  appendage, 
C,  which  is  articulated  to  it  by  the  notched  piece  at  the  foot,  in  the 

1  For  an  account  of  a  very  remarkable  form   see  Moseley  '  On  the  Pharynx  of  an 
unknown  Holothurian,  of  the  family  Dendrochirotae,  in  which  the  calcareous  skeleton 
is  remarkably  developed,'  Quart.  Jourti.  Microsc.  Sci.  n.s.  xxiv.  p.  255. 

2  For  figures  of  the  spicules  of  British  Holothurians,  see  Bell,  Catalogue  of  the 
British  Echinod&rms,  London,  1892,  pis.  i.-vi. 

3  '  On  the  spicules  of  Syiiapta,  together  with  some  general  remarks  on  the  archi- 
tecture of  Echmoderm  spicules,'  consult  R.  Semon,  Mitth.  Zool.  Stat.  Ne^el,  vii. 
p.  272.     An  excellent  summary  of  our  knowledge  of  the  spicules  of  Holothurians  is 
given  by  Prof.  Ludwig  in  his  volume  in  Bronn's  TJiierreich,  pp.  35-61. 


896  ECHINODEKMA 

manner  shown  (in  side  view)  at  B.  The  anchor-like  appendages 
project  from  the  surface  of  the  skin,  and  may  be  considered  as  re- 
presenting the  spines  of  Echinida.  Nearly  allied  to  the  Synapta  is 
the  Chiridota,  the  integument  of  which  is  entirely  destitute  of 
'  anchors,'  but  is  furnished  with  very  remarkable  wheel-like  plates  ; 
those  represented  in  fig.  682  are  found  in  the  skin  of  Chiridota 
violacea,  a  species  inhabiting  the  western  parts  of  the  Indian  Ocean. 
These  '  wheels  '  are  objects  of  singular  beauty  and  delicacy,  being 
especially  remarkable  for  the  very  minute  notching  (scarcely  to  be 
discerned  in  the  figure  without  the  aid  of  a  magnifying  glass)  which 
is  traceable  round  the  inner  margin  of  their  '  tires.'  There  can  be 
scarcely  any  reasonable  doubt  that  almost  every  member  of  this  class 
has  some  kind  of  calcareous  skeleton  disposed  in  a  manner  conform- 
able to  the  examples  now  cited  ;  and  it  is  now  generally  acknow- 
ledged that  the  marked  peculiarities  by  which  they  are  respectively 
distinguished  are  most  useful  in  the  determination  of  genera  and 
species.1  The  plates  may  be  obtained  separately  by  the  usual 

method  of  treating  the  skin 
with  a  solution  of  potass,  and 
they  should  be  mounted  in 
Canada  balsam.  But  their  posi- 
tion in  the  skin  can  only  be 
ascertained  by  making  sections 
of  the  integument  both  vertical 
and  parallel  to  its  surface  ;  and 

, .      ,     these    sections,    when    dry,    are 
FIG.  682. — Wheel-like  plates  from  skin  of  J  ' 

Chiridota  violacea,  most    advantageously    mounted 

in  the  same  medium,  by  which 

their  transparence  is  greatly  increased.  All  the  objects  of  this  class 
are  most  beautifully  displayed  by  the  black-ground  illumination,  and 
their  solid  forms  are  seen  with  increased  effect  under  the  binocular. 
The  black-ground  illumination  applied  to  very  thin  sections  of  Echinus 
spines  brings  out  some  effects  of  marvellous  beauty  ;  and  even  in  these 
the  solid  form  of  the  network  connecting  the  pillars  is  better  seen 
with  the  binocular  than  it  can  be  with  the  ordinary  microscope.2 

Echinoderm  Larvae. — We  have  now  to  notice  that  most  remark- 
able set  of  objects  furnished  to  the  microscopic  inquirer  by  the  larval 
states  of  this  class  ;  for  our  first  knowledge  of  which  we  were  in- 
debted to  the  painstaking  and  widely  extended  investigations  of 
Professor  J.  Miiller.3  All  that  our  limits  permit  is  a  notice  of  two  of 
the  most  curious  forms  of  these  larvae  by  way  of  sample  of  the  won- 

1  No  systematic  account  of  a  species  of  Holothurian  can  be  regarded  as  complete 
which  does  not  contain  an  account  of  the  form  of  its  spicules,  when  these  are  present. 
Figures  of  various  forms  will  be  found  in  Professor  Semper's  Beisen  im  Archipel  tier 
Philippinen  :  Holothurien,  Dr.  Theel's  '  Challenger  '  Reports,  and  the  memoirs  of 
Professors  Bell,  Ludwig,  and  Selenka. 

2  It  may  be  here  pointed  out  that  the  reticulated  appearance  is  sometimes  de- 
ceptive, what  seems  to  be  solid  network  being  in  many  instances  a  hollow  network 
of  passages  channelled  out  in  a  solid  calcareous  substance.     Between  these  two  con- 
ditions, in  which  the  relation  between  the  solid  framework  and  the  intervening  space 
is  completely  reversed,  there  is  every  intermediate  gradation. 

5  Of  later  works  consult  especially  the  '  Selections  from  Embryological  Mono- 
graphs, ii.  Echiuodermata,' edited  by  Mr.  A.  Agassiz,  in  vol.  ix.  of  the  M<-nu>i>-*  of  tlic 
Museum  of  Comparative 


LARVAL   ECHINODEKMS 


897 


derful  phenomena  which  his  researches  brought  to  light,  and  to  which 
the  attention  of  microscopists  who  have  the  opportunity  of  studying 
them  should  be  the  more  assiduously  directed,  as  even  the  most  deli- 
cate of  these  organisms  have  been  found  capable  of  such  perfect 
preservation  as  to  admit  of  being  studied,  when  mounted  as  pre- 
parations, even  better  than  when  alive.  The  larval  zooids  have,  by 
secondary  adaptations  to  their  mode  of  life,  acquired  a  type  quite 
different  from  that  which  characterises  the  adults ;  for  instead  of  a 
radial  symmetry  they  exhibit  a  bilateral,  the  two  sides  being  pre- 
cisely alike,  and  each  having  a  ciliated  fringe  along  the  greater  part 
or  the  whole  of  its  length.  The 
two  fringes  are  united  by  a 
superior  and  an  inferior  trans- 
verse ciliated  band,  and  be- 
tween these  two  the  mouth  of 
the  zb'oid  is  always  situated. 
The  external  forms  of  these 
larvae,  however,  vary  in  a  most 
remarkable  degree,  owing  to  the 
unequal  evolution  of  their  dif- 
ferent parts ;  and  there  is  also 
a  considerable  diversity  in  the 
several  orders  as  to  the  propor- 
tion of  the  fabric  of  the  larva 
which  enters  into  the  compo- 
sition of  the  adult  form.  When 
the  young  begins  to  acquire  the 
characters  of  the  fully  developed 
star-fish  and  sea-urchin,  the 
parts  which  are  not  retained 
shrivel  up,  and  their  substance 
goes  to  feed  the  young  form. 

One  of  the  most  remarkable 
forms  of  Echinoderm  larvae  is 
that  which  has  received  the 
name  of  Bipinnaria  (fig.  683), 
from  the  symmetrical  arrange- 
ment of  its  natatory  organs.  The  mouth  (a),  which  opens  in  the 
middle  of  a  transverse  furrow,  leads  through  an  oesophagus,  a',  to  a 
large  stomach,  around  which  the  body  of  a  star-fish  is  developing 
itself;  and  on  one  side  of  this  mouth  are  observed  the  intestinal 
tube  and  anus  (b).  On  either  side  of  the  anterior  portion  of  the 
body  are  six  or  more  narrow  fin-like  appendages,  which  are  fringed 
with  cilia  ;  and  the  posterior  part  of  the  body  is  prolonged  into 
a  sort  of  pedicle,  bilobed  towards  its  extremity,  which  also  is 
covered  with  cilia.  The  organisation  of  this  larva  seems  completed, 
and  its  movements  through  the  water  become  very  active,  before 
the  mass  at  its  anterior  extremity  presents  anything  of  the  aspect  of 
the  star-fish,  in  this  respect  corresponding  with  the  movements  of 
the  Pluteus  of  the  Echinoidea.  The  temporary  mouth  of  the  larva 
does  not  remain  as  the  permanent  mouth  of  the  star-fish ;  for  the 

3  M 


FIG.  683. — Bipinnaria  asterigera,  or  larva 
of  star-fish  :  a,  mouth  ;  a',  oesophagus ;  6, 
intestinal  tube  and  anal  orifice ;  c,  furrow 
in  which  the  mouth  is  situated  ;  d  d',  bi- 
lobed peduncle  ;  1,  2,  3,  4,  5, 6,  7,  ciliated 
arms. 


898  ECHINODERMA 

oesophagus  of  the  latter  enters  on  what  is  to  become  the  dorsal  side  of 
its  body,  and  the  true  mouth  is  subsequently  formed  by  the  thinning 
away  of  the  integument  on  its  ventral  surface.  The  young  star-fish 
is  separated  from  the  Bipinnarian  larva  by  the  forcible  contractions 
of  the  connecting  stalk,  as  soon  as  the  calcareous  consolidation  of  its 
integument  has  taken  place  and  its  true  mouth  has  been  formed,  but 
long  before  it  has  attained  the  adult  condition ;  and  as  its  ulterior 
development  has  not  hitherto  been  observed  in  any  instance,  it  is  not 
yet  known  what  are  the  species  in  which  this  mode  of  evolution 
prevails.  The  larval  zooid  continues  active  for  several  days  after  its 
detachment ;  and  it  is  possible,  though  perhaps  scarcely  probable, 
that  it  may  develop  another  asteroid  by  a  repetition  of  this  process 
of  gemmation. 

In  the  Bipinnaria,  as  in  other  larval  zooids  of  the  Aster oidea, 
there  is  no  internal  calcareous  framework  ;  such  a  framework,  how- 
ever, is  found  in  the  larvae  of  the  Echinoidea  and  Ophiuroidea,  of 
which  the  form  delineated  in  fig.  684  is  an  example.  The  embryo 
issues  from  the  ovum  as  soon  as  it  has  attained,  by  repeated  '  seg- 
mentation '  of  the  yolk,  the  condition  of  the  '  mulberry-mass,'  and 
the  superficial  cells  of  this  are  covered  with  cilia  by  whose  agency 
it  swims  freely  through  the  water.  So  rapid  are  the  early  processes 
of  development  that  no  more  than  from  twelve  to  twenty-four 
hours  intervene  between  fecundation  and  the  emersion  of  the  embryo, 
the  division  into  two,  four,  or  even  eight  segments  taking  place 
within  three  hours  after  impregnation.  Within  a  few  hours  after 
its  emersion  the  embryo  changes  from  the  spherical  into  a  sub- 
pyramidal  form  with  a  flattened  base ;  and  in  the  centre  of  this 
base  is  a  depression,  which  gradually  deepens,  so  as  to  form  a  mouth 
that  communicates  with  a  cavity  in  the  interior  of  the  body  which 
is  surrounded  by  a  portion  of  the  yolk-mass  that  has  returned  to  the 
liquid  granular  state.  Subsequently  a  short  intestinal  tube  is  found, 
with  an  anal  orifice  opening  on  one  side  of  the  body.  The  pyramid 
is  at  first  triangular,  but  it  afterwards  becomes  quadrangular  ;  and 
the  angles  are  greatly  prolonged  round  the  mouth  (or  base),  whilst 
the  apex  of  the  pyramid  is  sometimes  much  extended  in  the  opposite 
direction,  but  is  sometimes  rounded  off  into 'a  kind  of  dome  (fig. 
684,  A).  All  parts  of  this  curious  body,  and  especially  its  most 
projecting  portions,  are  strengthened  by  a  framework  of  thread-like 
calcareous  rods  (e).  In  this  condition  the  embryo  swims  freely 
through  the  water,  being  propelled  by  the  action  of  the  cilia,  which 
clothe  the  four  angles  of  the  pyramid  and  its  projecting  arms,  and 
which  are  sometimes  thickly  set  upon  two  or  four  projecting  lobes 
(/) ;  and  it  has  received  the  designation  of  Pluteus.  The  mouth  is 
usually  surrounded  by  a  sort  of  proboscis,  the  angles  of  which  are 
prolonged  into  four  slender  processes  (g,  g,  g,  </),  shorter  than  the  four 
outer  legs,  but  furnished  with  a  similar  calcareous  framework. 

The  first  indication  of  the  production  of  the  young  Echinus  from 
its  '  pluteus '  is  given  by  the  formation  of  a  circular  disc  (fig.  684, 
A,  c)  on  one  side  of  the  central  stomach  (b)  ;  and  this  disc  soon 
presents  five  prominent  tubercles  (B),  which  subsequently  become 
elongated  into  tubular  processes,  which  will  form  the  'sucking- 


LAKVAL  ECHINI 


899 


feet '  of  the  adult.  The  disc  gradually  extends  itself  over  the  stomach, 
and  between  its  tubules  the  rudiments  of  spines  are  seen  to  protrude 
(D) ;  these,  with  the  tubules,  increase  in  length,  so  as  to  project  against 
the  envelope  of  the  pluteus,  and  to  push  themselves  through  it ;  whilst, 
at  the  same  time,  the  original  angular  appendages  of  the  pluteus 
diminish  in  size,  the 
ciliary  movement  be- 
comes  less  active,  being 
superseded  by  the  action 
of  the  suckers  and  spines, 
arid  the  mouth  of  the 
pluteus  closes  up.  By 
the  time  that  the  disc 
has  grown  over  half  of 
the  gastric  sphere,  very 
little  of  the  pluteus  re- 
mains, except  some  of 
the  slender  calcareous 
rods,  and  the  number 
of  suckers  and  spines 
rapidly  increases.  The 
calcareous  framework  of 
the  shell  at  first  consists, 
like  that  of  the  star- 
fishes, of  a  series  of 
isolated  networks  de- 
veloped between  the 
cirrhi,  and  upon  these 
rest  the  first  formed 
spines  (D).  But  they 
gradually  become  more 
consolidated,  and  extend 
themselves  over  the 
granular  mass,  so  as  to 
form  the  series  of  plates 
constituting  the  shell. 
The  mouth  of  the  Echi- 
nus (which  is  altogether 
distinct  from  that  of  the 
pluteus)  is  formed  at 
that  side  of  the  granular 


FIG.  684.— Embryonic  development  of  Echinus :  A 
Pluteus  larva  at  the  time  of  the  first  appearance 
of  the  disc ;  «,  mouth,  in  the  midst  of  the  four- 
pronged  proboscis  ;  6,  stomach ;  c,  Echinoid  disc  ; 
d,  d,  d,  d,  four  arms  of  the  pluteus-body ;  e,  cal- 
careous framework ;  /,  ciliated  lobes ;  g\  g,  g,  g, 
ciliated  processes  of  the  proboscis ;  B,  disc  with 
the  first  indication  of  the  sucking- f eet ;  C,  disc, 


with  the  origin  of  the  spines  between  the  tubular 
sucking-feet ;  D,  more  advanced  disc,  with  the  feet, 
<7,  and  spines,  #,  projecting  considerably  from  the 
surface.  (N.B.— In  B,  C,  and  D,  the  Pluteus  is  not 
represented,  its  parts  having  undergone  no  change, 
save  in  becoming  relatively  smaller.) 


mass  over  which  the 
shell  is  last  extended; 
and  the  first  indication 
of  it  consists  in  the  ap- 
pearance of  the  five  cal- 
careous concretions,  which  are  the  summits  of  the  five  portions  of 
the  framework  of  jaws  and  teeth  that  surround  it.  All  traces  of 
the  original  pluteus  are  now  lost;  and  the  larva,  which  now 
presents  the  general  aspect  of  an  Echinoid  animal,  gradually 
augments  in  size,  multiplies  the  number  of  its  plates,  cirrhi,  and 

3  M  2 


900  ECHINODERMA 

spines,  evolves  itself  into  its  particular  generic  and  specific  type, 
and  undergoes  various  changes  of  internal  structure  tending  to 
the  development  of  the  complete  organism.1 

An  excellent  summary  of  the  developmental  history  of  the 
several  Echinoderm  types,  with  references  to  the  principal  memoirs 
which  treat  of  it,  will  be  found  in  Chapter  XX.  of  Mr.  Balfour's 
'  Comparative  Embryology,'  and  in  Professor  A.  Lang's  '  Jahrbuch 
der  vergleichenden  Anatomic,' which  has  been  translated  into  English.2 
In  collecting  the  free-swimming  larvae  of  Echinoderma  the  stick- 
net  should  be  carefully  employed  in  the  manner  already  described, 
and  the  search  for  them  is  of  course  most  likely  to  be  successful  in 
those  localities  in  which  the  adult  forms  of  the  respective  species 
abound,  and  on  warm  calm  days,  in  which  they  seem  to  come  to  the 
surface  in  the  greatest  numbers.  The  following  mode  of  preparing 
and  mounting  them  has  been  kindly  communicated  to  the  Author 
by  Mr.  Percy  Sladen  : — '  For  killing  and  preserving  echinoderm  zooids, 
I  have  come  to  prefer  either  osmic  acid  or  the  picro-sulphuric  mix- 
ture of  Kleinenberg  of  one-third  strength.  The  latter,  of  course, 
destroys  all  calcareous  structures  ;  but  the  soft  parts  are  preserved 
in  a  wonderful  manner.  If  the  diluted  Kleinenberg's  mixture  is 
used,  let  the  zooids  remain  in  it  for  one  or  two  hours;  then  wash 
them  thoroughly  in  70  per  cent,  spirit,  until  all  trace  of  acid  is  re- 
moved ;  then  stain  ;  then  again  wash  in  70  per  cent,  spirit,  transfer 
them  to  90  per  cent,  spirit  for  some  hours,  and  lastly  to  absolute 
alcohol.  Transfer  them  from  this  to  oil  of  cloves  ;  and  finally  mount 
in  Canada  balsam  in  the  usual  manner.  If  osmic  acid  be  used,  place 
three  or  four  of  the  living  zooids  in  a  watch-glass  of  sea-water,  and 
add  a  drop  of  the  1  per  cent,  solution.  They  should  not  remain  even 
in  this  weak  solution  for  more  than  a  minute,  and  should  then  be 
thoroughly  washed  in  a  superabundance  of  35  per  cent,  spirit,  to  pre- 
vent the  deposit  of  crystals  of  salt  consequent  on  the  action  of  the 
osmic  acid.  Then  transfer  the  specimens  to  70  per  cent,  spirit,  and 
proceed  as  in  the  other  case.' 

One  of  the  most  interesting  to  the  microscopist  of  all  Echino- 
derma is  the  Antedon*  (more  generally  known  as  Comatula),  or 
;  feather-star'  (fig.  685),  which  is  the  commonest  existing  representa- 
tive of  the  great  fossil  series  of  Crinoidea,  or  '  lily-stars,'  that  were 
among  the  most  abundant  types  of  this  class  in  the  earlier  epochs  of 
the  world's  history.  Like  these,  the  young  of  Antedon  is  attached 
by  a  stalk  to  a  fixed  base,  part  of  which  is  shown  in  fig.  686  ;  but 
when  it  has  arrived  at  a  certain  stage  of  development  it  drops  off  from 
this  like  a  fruit  from  its  stalk,  and  the  animal  is  thenceforth  free  to 
move  through  the  ocean  water  it  inhabits.  It  can  swim  with  con- 

1  Abbreviated  development,  in  which  there  is  no  free-swimming  larva,  is  now 
known  to  be  more  common  than  was  once  supposed  :  among  Holothurians  Cncu 
maria    crocea,   among   Ophiuroids  Ophiacantha  vivipara,  and  among  Echinoids 
Hemiaster  cavernosus  may  be  cited  as  examples. 

2  Those  who  wish  to  carry  their  study  further  must  consult  the  recent  memoirs 
of  Mr.  Bury,  Prof.  MacBride,  and  Dr.  Willey,  and  that  of  Dr.  T.  Mortensen,  Die 
Echinodermenlarven  der  Plankton  Expedition  (Kiel  and  Leipzig,  1898),  in  which 
there  is  a  systematic  revision  of  the  Echinoderm  larvae  already  known. 

5  See  the  Author  s  '  Researches  on  the  Structure,  Physiology,  and  Development 
of  Antedon  rosaceus,'  Part  L,  in  Phil.  Trans.  1866,  p.  671. 


ANTEDON 


901 


siderable  activity,  but  it  exerts  this  power  chiefly  to  gain  a  suitable 
place  for  attaching  itself  by  means  of  the  jointed  prehensile  cirrhi 
put  forth  from  the  aboral  (under)  side  of  the  central  disc  (fig.  685)  ; 
so  that,  notwithstanding  its  locomotive  power,  it  is  nearly  as  station- 
ary in  its  free  adult  condition  as  it  is  in  its  earlier  jpentacrinoid 
stage.  The  pentacrinoid  larva  l — first  discovered  by  Mr.  J.  V. 
Thompson,  of  Cork,  in  1823,  but  originally  supposed  by  him  to  be  a 
permanently  attached  Crinoid — forms  a  most  beautiful  object  for  the 
lower  powers  of  the  microscope,  when  well  preserved  in  fluid,  and 
viewed  by  a  strong  incident  light  (fig.  686,3) ;  and  a  series  of  specimens 
in  different  stages  of  development  shows  most  curious  modifications 
in  the  form  and  arrangement  of  the  various  component  pieces 
of  its  calcareous  skeleton.  Jn  its  earlier  stage  (fig.  686,  l)  the 
body  is  inclosed  in  a 
calyx  composed  of 
two  circles  of  plates, 
namely,  five  basals, 
forming  a  sort  of 
pyramid  whose  apex 
points  downwards,  and 
is  attached  to  the 
highest  joint  of  the 
stem ;  and  five  orals 
superposed  on  these, 
forming  when  closed 
a  like  pyramid  whose 
apex  points  upwards, 
but  usually  separating 
to  give  passage  to  the 
tentacles,  of  which  a 
circlet  surrounds  the 
mouth.  In  this  con- 
dition there  is  no 
rudiment  of  arms.  In 
the  more  advanced 
stage  shown  at  2, 

the  arms  have  begun  to  make  their  appearance,  and  the  skeleton 
when  carefully  examined  is  found  to  consist  of  the  following  pieces, 
as  shown  in  fig.  686,  l,  b,  b,  the  circlet  of  basals  supported  on  the 
top  of  the  stem  ;  r1,  the  circle  of  first  radials,  now  interposed  between 
the  basals  and  the  orals,  and  alternating  with  both  ;  between  two 
of  these  is  interposed  the  single  anal  plate  a  ;  whilst  they  support 
the  second  and  the  third  radials  (r2,  r5),  from  the  latter  of  which 
the  bifurcating  arms  spring ;  finally,  between  the  second  radials  we 
see  the  five  orals  lifted  from  the  basals  on  which  they  originally 
rested  by  the  interposition  of  the  first  radials.  In  the  more  advanced 
stage  shown  in  fig.  686,  3,  we  find  the  highest  joint  of  the  stem 

1  The  pentacrinoid  larvae  of  Antedon  have  been  found  abundantly  (attached  to 
seaweeds  and  zoophytes)  at  Millport,  on  the  Clyde,  and  in  Lamlash  Bay,  Arran;  in 
Kirkwall  Bay,  Orkney ;  in  Lough  Strangford,  near  Belfast,  and  in  the  Bay  of  Cork  ; 
and  at  Ilfracombe  and  in  Salccmbe  Bay,  Devon. 


FIG.   685. — Antedon    (Comatula),    or    feather-star, 
seen  from  its  aboral  side. 


902 


ECHINODERMA 


beginning  to  enlarge,  to  form  the  centro-dorsal  plate  (2,  c  d),  from 
which  are  beginning  to  spring  the  dorsal  cirrhi  (cir)  that  serve  to 


FIG.  686.— Pentacrinoid  larva  of  Antedon.  1.  Skeleton  of  early  pentacrinoid, 
under  black-ground  illumination,  showing  its  component  plates :  b,  b, 
basals,  articulated  below  to  the  highest  point  of  the  stem;  rl,  r1,  first 
radials,  between  two  of  which  is  seen  the  single  anal  plate,  a  ;  r!,  second 
radials  ;  r5,  third  radials,  giving  off  the  bifurcating  arms  at  their  summit ; 
o,  o,  orals.  2,  3.  Back  and  front  views  of  a  more  advanced  pentacrinoid, 
as  seen  by  incident  light,  one  of  the  pair  of  arms  being  cut  away  in  fig.  8 
in  order  to  bring  the  mouth  and  its  surrounding  parts  into  view  :  b,  b, 
basals ;  r1,  r2,  r5,  first,  second,  and  third  radials ;  a,  anal,  now  carried 
upwards  by  the  projection  of  the  vent,  v  ;  o,  o,  orals;  cir,  dorsal  cirrhi, 
developed  from  the  highest  joint  of  the  stem. 


ANTEDON  903 

anchor  the  animal  when  it  drops  from  the  stem  ;  this  supports  the 
basals,  on  which  rest  the  first  radials  (rl)  ;  whilst  the  anal  plate  is 
now  lifted  nearly  to  the  level  of  the  second  radials  (r2)  by  the 
development  of  the  anal  funnel  or  vent  to  which  it  is  attached.  The 
oral  plates  are  not  at  first  apparent,  as  they  no  longer  occupy  their 
first  position  ;  but  on  being  carefully  looked  for  they  are  found  still 
to  form  a  circlet  around  the  mouth  (3,  o,  o),  not  having  undergone 
any  increase  in  size,  whilst  the  visceral  disc  and  the  calyx  in  which 
it  is  lodged  have  greatly  extended.  These  oral  plates  finally  dis- 
appear by  absorption  ;  while  the  basals  are  at  first  concealed  by  the 
great  enlargement  of  the  centro-dorsal  (which  finally  extends  so  far 
as  to  conceal  the  first  radials  also)  ;  and  at  last  undergo  metamor- 
phosis into  a  beautiful  '  rosette,  >which  lies  between  the  cavity  of  the 
centro-dorsal  and  that  of  the  calyx.  In  common  with  other  members 
of  its  class,  the  Antedon  is  represented  in  its  earliest  phase  of  develop- 
ment by  a  free-swimming  '  larval  zooid '  or  pseudembryo^  which  was 
first  observed  by  Busch,  and  has  been  since  carefully  studied  by 
Professors  Wyville  Thomson ]  and  Goette.2  This  zooid  has  an 
elongated  egg-like  form,  and  is  furnished  with  transverse  bands  of 
cilia  and  with  a  mouth  and  anus  of  its  own.  After  a  time,  how- 
ever, rudiments  of  the  calcareous  plates  forming  the  stem  and  calyx 
begin  to  show  themselves  in  its  interior ;  a  disc  is  then  formed  at  the 
posterior  extremity  by  which  it  attaches  itself  to  a  seaweed  (very 
commonly  Laminaria),  zoophyte,  or  polyzoary  ;  the  calyx  containing 
the  true  stomach,  with  its  central  mouth  surrounded  by  tentacles,  is 
gradually  evolved ;  and  the  sarcodic  substance  of  the  pseudembryo, 
by  which  this  calyx  and  the  rudimentary  stem  were  originally  in- 
vested, gradually  shrinks,  until  the  young  pentacrinoid  presents 
itself  in  its  characteristic  form  and  proportions.3 

1  '  On  the  Development  of  Antedon  rosaceus'  in  Phil.  Trans,  for  1865,  p.  513. 

2  Archiv  f.  mikrosk.  Anat.  Bd.  xii.  p.  583. 

5  The  general  results  of  the  Author's  own  later  studies  of  this  most  interesting 
type  (the  key  to  the  life-history  of  the  entire  geological  succession  of  Crinoidea)  are 
embodied  in  a  notice  communicated  to  the  Proceedings  of  the  Royal  Society  for 
1876,  p.  211,  and  in  a  subsequent  note,  p.  451.  Of  the  further  contributions  recently 
made  to  our  knowledge  of  it  the  memoir  of  Dr.  H.  Ludwig  '  Zur  Anatomic  der 
Crinoideen  '  (Leipzig,  1877),  forming  part  of  his  Morphologische  Studien  an  Echino- 
dermen,  is  the  most  important.  Those  who  wish  to  carry  further  their  study  of  the 
Crinoidea  should  consult  the  two  monographs  by  Dr.  P.  Herbert  Carpenter  in  the 
'  Challenger '  Reports. 


COLLEGE    OF    DENTISTRY 
UNIVERSITY  OF  CALIFORNIA 


COLLEGE    OF"   DENTISTRY 
UNIVERSITY  OF  CALIFORNIA 


ANSj 
CHAPTER  XVII 

POLYZOA  AND   TUNIC  AT  A 

As  in  previous  editions  of  this  work  the  Author  followed  the  once 
prevalent  habit  of  regarding  the  Polyzoa  and  Tunicata  as  structurally 
allied,  and  as  it  would  be  necessary  to  entirely  recast  the  work  were 
the  two  groups  to  be  now  otherwise  dealt  with,  and  as,  finally,  there 
is  no  real  inconvenience  or  impropriety  in  discussing  them  in  one 
chapter,  it  is  proposed  to  continue,  with  this  word  of  warning,  the 
original  arrangement  of  the  Author.  Some  members  of  both  these 
groups  are  found  on  almost  every  coast,  and  are  most  interesting 
objects  for  anatomical  examination,  as  well  as  for  observation  in  tin- 
living  state.1 

Polyzoa. — The  group  which  is  known  under  this  name  to  many 
British  naturalists  (corresponding  with  that  which  by  Continental 
zoologists  is  designated  Sryozoa)  was  formerly  ranked  as  an  order  of 
zoophytes,  and  it  has  been  entirely  by  microscopic  study  that  its  com- 
paratively high  organisation  has  been  ascertained.  The  animals  of 
the  Polyzoa,  in  consequence  of  their  universal  tendency  to  multipli- 
cation by  gemmation,  are  seldom  or  never  found  solitary,  but  form 
clusters  or  colonies  of  various  kinds ;  and  as  each  is  inclosed  in  either 
a  horny  or  a  calcareous  sheath  or  'cell,'  a  composite  structure  is 
formed,  closely  corresponding  with  the  '  polypidom '  of  a  zoophyte, 
which  has  been  appropriately  designated  the  polyzoary.  The  indi- 
vidual cells  of  the  polyzoary  are  sometimes  only  connected  with  each 
other  by  their  common  relation  to  a  creeping  stem  or  stolon,  as  in 
Laguncula  (fig.  687);  but  more  frequently  they  bud  forth  directly. 
one  from  another,  and  extend  themselves  in  different  directions  over 
plane  surfaces,  as  is  the  case  with  Flustrce,  Lepralia?,,  &c.  (fig.  688) ; 
whilst  not  unfrequeiitly  the  polyzoary  develops  itself  into  an  arbores- 
cent structure  (fig.  689),  which  may  even  present  somewhat  of  the 
density  and  massiveness  of  the  stony  corals.  Each  individual  is  com- 
posed externally  of  a  sort  of  sac,  of  which  the  outer  or  tegument  a  ry 
layer  is  either  simply  membranous,  or  is  horny,  or  in  some  instances 
calcified,  so  as  to  form  the  cell ;  this  investing  sac  is  lined  by  a  more 
delicate  membrane,  which  closes  its  orifice,  and  which  then  becomes 
continuous  with  the  wall  of  the  alimentary  canal ;  this  lies  freely  in 
the  visceral  sac,  floating  (as  it  were)  in  the  liquid  which  it  contains. 
The  principal  features  in  the  structure  of  this  group  will  be  best 
understood  from  the  examination  of  a  characteristic  example,  such  as 
the  Laguncula  repens,  which  is  shown  in  the  state  of  expansion  at  A, 
fig.  687,  and  in  the  state  of  contraction  at  B  and  C.  The  mouth  is 

1  For  a  good  general  account  see  Dr.  Harmer  in  vol.  ii.  of  the  Cambridge  Natural 
History,  1896, 


POLYZOA 


905 


surrounded  by  a  circle  of  tubular  tentacles,  which  are  clothed  by 
vibratile  cilia;  these  tentacles,  in  the  species  we  are  considering, 
\;ir\  from  ten  to  twelve  in  number,  but  in  some  other  instances  they 
are  more  numerous.  l'>\ 
the  ciliary  investment 
of  the  tentacles  the 
Polyzoa  are  at  once  dis- 
tinguishable from  those 
hydroid  polypes  to 
which  they  bear  a 
superficial  resemblance  •. 
and  with  which  they 
were  at  one  time  con- 
founded ;  and  accord- 
ingly, while  still  ranked 
among  zoophytes,  they 
w c- 1 -e  character] se<  1  as 
r'lliitbraehiate.  The  ten- 
tacula, are  seated  upon 
an  annular  disc,  which 
i>  termed  the  1<>/>I><> 
/ilnn-e,  and  which  forms 
the  roof  of  the  visceral 
or  perigastric  cavity  ; 
and  this  cavity  extends 
itself  into  the  interior  of 
the  tentacula,1  through 
perforations  in  the  lo- 
phophore, as  is  shown  at 
I),  fig.  687, representing 
a  portion  of  the  ten- 
tacular circle  on  a 
larger  scale,  a,  a  being 
the  tentacula,  b  b  their 
internal  canals,  c  the 
muscles  of  the  tentacula, 
d  the  lophophore,  and  e 
its  retractile  muscles. 
The  mouth  situated  in 
the  centre  of  the  lopho- 
phore, as  shown  at  A, 
leads  to  a  funnel-shaped 
cavity  or  pharynx,  6, 
which  is  separated  from 
the  oesophagus,  d,  by  a 
valve  at  c  ;  and  this  oeso- 
phagus opens  into  the 


•\flftt 


FIG.  687. — Structure  of  Laguncitla  repens  (Van  Bene- 
den).  A,  polypide  expanded  ;  B,  polypide  retracted; 
C,  another  view  of  the  same,  with  the  visceral 
apparatus  in  outline,  that  the  manner  in  which  it 
is  doubled  on  itself,  with  the  tentacular  crown  and 
muscular  system,  may  be  more  distinctly  seen: 
a  a,  tentacula ;  6,  pharynx ;  r,  pharyngeal  valve ; 
d,  oesophagus ;  e,  stomach ;  /",  its  pyloric  orifice ; 
g,  cilia  on  its  inner  surface  ;  h,  biliary  follicles  lodged 
in  its  wall ;  i ,  intestine  ;  k,  particles  of  excremen- 
titious  matter  ;  I,  anal  orifice  ;  m,  testis  ;  n,  ovary  ; 
o,  ova  lying  loose  in  the  perivisceral  cavity ;  p,  out- 
let for  their  discharge  ;  q,  spermatozoa  in  the  peri- 
visceral  cavity ;  r,  s,  t,  u,  v,  w,  x,  muscles.  D,  por- 
tion of  the  lophophore  more  enlarged :  a  a,  tenta- 
cula ;  b  6,  their  internal  canals ;  c,  their  muscles ; 
d,  lophophore ;  e,  its  retractor  muscles. 


stomach,  e,  which  occu- 
pies a  considerable  part  of  the  visceral  cavity.     (In  the  Bowerbankia 

1  This  communication  between  the  tentacular  and  visceral  cavities  is  denied  by 
Dr.  Vigelius,  who  has  recently  made  a  careful  search  for  it. 


906 


POLYZOA   AND   TUN1CATA 


and  some  other  Polyzoa  a  muscular  stomach  or  gizzard  for  the  tri- 
turation  of  the  food  intervenes  between  the  oesophagus  and  the 
true  digestive  stomach.)  The  walls  of  the  stomach,  A,  have  consider- 
able thickness,  and  the  epithelial  cells  which  line  them  seem  to  have 
the  character  of  a  rudimentary  digestive  gland.  This,  however,  is 
more  obvious  in  some  other  members  of  the  group.  The  stomach  is 
lined,  especially  at  its  upper  part,  with  vibratile  cilia,  as  seen  at  c,  g  ; 
and  by  the  action  of  these  the  food  is  kept  in  a  state  of  constant 
agitation  during  the  digestive  process.  From  the  upper  part  of  the 
stomach,  which  is  (as  it  were)  doubled  upon  itself,  the  intestine  (i) 
opens,  by  a  pyloric  orifice,  /,  which  is  furnished  with  a  regular  valve  ; 
within  the  intestine  are  seen  at  k  particles  of  excrementitious  matter 
which  are  discharged  by  the  anal  orifice  at  I.  No  special  circulating 
apparatus  here  exists  ;  but  the  liquid  which  fills  the  cavity  that  sur- 
rounds the  viscera  con- 
tains the  nutritive 
matter  which  has  been 
prepared  by  the  diges- 
tive operation.  and 
which  has  transuded 
through  the  walls  of 
the  alimentary  canal  ; 
a  few  corpuscles  of  ir- 
regular size  are  seen  to 
float  in  it.  No  other 
respiratory  organs  exist 
than  the  tentacula,  into 
whose  cavity  the  nutri- 
tive fluid  is  probably 
sent  from  the  peri- 
visceral  cavity  for  aera- 
tion by  the  current  of 
water  that  is  continu- 
FIG.  688.— Cells  of  Polyzoa:  A,  Mastigophora  Hynd-  a]iv  flowi^or  nvpr  thpm 
manniiB,  Cribrilina  figularis;  cf  Umbonula  ^ *owmg  over  m< 
verrucosa.  The  production  of 

gemmce    or    buds    may 

take  place  either  from  the  bodies  of  the  polypides  themselves,  which 
is  what  always  happens  when  the  cells  are  in  mutual  apposition, 
or  from  the  connecting  stem  or  '  stolon,'  where  the  cells  are  distinct 
one  from  the  other,  as  in  Laguncula.  In  the  latter  case  there  is 
first  seen  a  bud-like  protuberance  of  the  horny  external  integu- 
ment, into  which  the  soft  membranous  lining  prolongs  itself  ;  the 
cavity  thus  formed,  however,  is  not  to  become  (as  in  Hydra  and  its 
allies)  the  stomach  of  the  new  zooid,  but  it  constitutes  the  chamber 
surrounding  the  digestive  viscera,  which  organs  have  their  origin 
in  a  thickening  of  the  lining  membrane  that  projects  from  one  side 
of  the  cavity  into  its  interior,  and  gradually  shapes  itself  into  the 
alimentary  canal  with  its  tentacular  appendages.  Of  the  produc- 
tion of  gemmae  from  the  polypides  themselves  the  best  examples  are 
furnished  by  the  Flustrce  and  their  allies.  From  a  single  cell  of  the 
Flustrse  five  such  buds  may  be  sent  off,  which  develop  themselves 


POLYZOA  907 

into  new  polypides  around  it ;  and  these  in  their  turn  produce  buds 
from  their  unattached  margins,  so  as  rapidly  to  augment  the  number 
of  cells.  To  this  extension  there  seems  no  definite  limit,  and  it  often 
happens  that  the  cells  in  the  central  portion  of  the  leaf-like  expan- 
sion of  a  Flustra  are  devoid  of  contents  and  have  lost  their  vitality, 
whilst  the  edges  are  in  a  state  of  active  growth.1  Independently  of 
their  propagation  by  gemmation,  the  Polyzoa  have  a  true  sexual 
generation,  the  sexes,  however,  being  usually,  if  not  invariably,  united 
in  the  same  polypides.  The  sperm-cells  are  developed  in  a  glandular 
body,  the  testis,  m,  which  lies  beneath  the  base  of  the  stomach,  or 
they  are  developed  from  large  portions  of  the  inner  surface  of  the 
body- wall ;  when  mature  they  runture,  and  set  free  the  spermatozoa, 
q  q,  which  swim  freely  in  the  liquid  of  the  visceral  cavity.  The  ova, 
on  the  other  hand,  are  formed  in  an  ovarium,  n,  which  is  lodged  in 
the  membrane  lining  the  tegumentary  sheath  near  its  outlet  or  is 
placed  near  the  end  of  the  csecal  process  of  the  stomach  ;  the  ova, 
having  escaped  from  this  into  the  visceral  cavity,  as  at  0,  are  fer- 
tilised by  the  spermatozoa  which  they  there  meet  with,  and  are 
finally  discharged  by  an  outlet  atjt?,  beneath  the  tentacular  circle. 

These  creatures  possess  a  considerable  number  of  muscles,  by 
which  their  bodies  may  be  projected  from  their  sheaths,  or  drawn 
within  them ;  of  these  muscles,  r,  s,  t,  u,  v,  w,  x,  the  direction  and 
points  of  attachment  sufficiently  indicate  the  uses  ;  they  are  for  the 
most  part  retractors,  serving  to  draw  in  and  double  up  the  body,  to 
fold  together  the  circle  of  tentacula,  and  to  close  the  aperture  of  the 
sheath,  when  the  animal  has  been  completely  withdrawn  into  its 
interior.  The  projection  and  expansion  of  the  animal,  on  the  con- 
trary, appear  to  be  chiefly  accomplished  by  a  general  pressure  upon 
the  sheath,  which  will  tend  to  force  out  all  that  can  be  expelled  from 
it.  The  tentacles  themselves  are  furnished  with  distinct  muscular 
fibres,  by  which  their  separate  movements  seem  to  be  produced.  At 
the  base  of  the  tentacular  circle,  just  above  the  anal  orifice,  is  a  small 
body  (seen  at  A,  a),  which  is  a  nervous  ganglion  ;  as  yet  no  branches 
have  been  distinctly  seen  to  be  connected  with  it  in  this  species  ;  but 
its  character  is  less  doubtful  in  some  other  Polyzoa.  Besides  the 
independent  movements  of  the  individual  polypides,  other  movements 
may  be  observed,  which  are  performed  by  so  many  of  them  simulta- 
neously as  to  indicate  the  existence  of  some  connecting  agency  ;  and 
such  connecting  agency,  it  is  affirmed  by  Dr.  Fritz  Miiller,2  is  fur- 
nished by  what  he  terms  a  '  colonial  nervous  system.'  In  a  Seria- 
laria  having  a  branching  polyzoary  that  spreads  itself  on  seaweeds 
over  a  space  of  three  or  four  inches,  he  states  that  a  nervous 
ganglion  may  be  distinguished  at  the  origin  of  each  branch,  and 
another  ganglion  at  the  origin  of  each  polypide-bud,  all  these 
ganglia  being  connected  together,  not  merely  by  principal  trunks, 

1  For  further  details  consult  Haddon  'On  Budding  in  Polyzoa,'  Quart.  Jovrn. 
Microsc.  Sci.  xxiii.  p.  516.  Embryonic  fission  has  been  observed  by  Harmer  in  Crisia 
and  Lichenopora. 

-  See  his  memoir  in  Wiegmann's  Archiv,  1860,  p.  311,  translated  in  Quart.  Joiirn. 
of  Microsc.  Sci.  n.s.  vol.  i.  1861,  p.  300  ;  Rev.  T.  Hincks's  'Note  on  the  Movements  of 
the  Vibracula  in  Caberea  boryi,  and  on  the  supposed  common  Nervous  System  in 
the  Polyzoa,'  Quart.  Journ.  Microsc.  Sci.  xviii.  p.  1. 


9O8  POLYZOA   AND   TUNICATA 

but  also  by  plexuses  of  nerve-fibres,  which  may  be  distinctly  made 
out  with  the  aid  of  chromic  acid  in  the  cylindrical  joints  of  the  poly- 
zoary.  His  views,  however,  are  not  now  accepted,  observers  of 
great  histological  experience  maintaining  that  what  he  regards  as 
nerve-fibres  are  only  connective  tissue. 

Of  all  the  Polyzoa  of  our  own  coasts  the  Membraniporidce,  or 
'  sea-mats '  (Flustra,  Membranipora),  are  the  most  common  ;  these 
present  flat  expanded  surfaces  resembling  in  form  those  of  many  sea- 
weeds (for  which  they  are  often  mistaken),  but  exhibiting,  when 
viewed  with  even  a  low  magnifying  power,  a  most  beautiful  network, 
which  at  once  indicates  their  real  character.  The  cells  are  generally 
arranged  on  both  sides,  and  it  was  calculated  by  Dr.  Grant  that  as 
a  single  square  inch  of  an  ordinary  Flustra  contains  1,800  such  cells, 
and  as  an  average  specimen  presents  about  ten  square  inches  of 
surface,  it  will  consist  of  no  fewer  than  18,000  polypides.  The  want 
of  transparence  in  the  cell-wall,  however,  and  the  infrequency  with 
which  the  animal  projects  its  body  far  beyond  the  mouth  of  the  cell, 
render  the  species  of  this  genus  less  favourable  subjects  for  micro- 
scopic examination  than  are  those  of  the  Bowerbankia,  a  polyzoon 
with  a  trailing  stem  and  separated  cells  like  those  of  Laguncula,  which 
is  very  commonly  found  clustering  around  the  base  of  masses  of 
Mustrse.  It  was  in  this  that  many  of  the  details  of  the  organisation 
of  the  interesting  group  we  are  considering  were  first  studied  by  Dr. 
A.  Farre,  who  discovered  it  in  1837,  and  subjected  it  to  a  far  more 
minute  examination  than  any  polyzoon  had  previously  received  ;  l 
and  it  is  one  of  the  best  adapted  of  all  the  marine  forms  yet  known 
for  the  display  of  the  beauties  and  wonders  of  this  type  of  organisa- 
tion. The  Alcyonidium,  however,  is  one  of  the  most  remarkable  of 
all  the  marine  forms  for  the  comparatively  large  size  of  the  tentacular 
crowns,  these,  when  expanded,  being  very  distinctly  visible  to  the 
naked  eye,  and  presenting  a  spectacle  of  the  greatest  beauty  when 
viewed  under  a  sufficient  magnifying  power.  The  polyzoary  of  this 
genus  has  a  spongy  aspect  and  texture,  very  much  resembling  that  of 
certain  Alcyonian  zoophytes,  for  which  it  might  readily  be  mistaken 
when  its  contained  animals  are  all  withdrawn  into  their  cells  ;  when 
these  are  expanded,  however,  the  aspect  of  the  two  is  altogether 
different,  as  the  minute  plumose  tufts  which  then  issue  from  the 
surface  of  the  Alcyonidium,  making  it  look  as  if  it  were  covered  with 
the  most  delicate  downy  film,  are  in  striking  contrast  with  the  larger 
solid-looking  polypes  of  the  Alcyonium.  The  opacity  of  the  polyzoary 
of  the  Alcyonidium  renders  it  quite  unsuitable  for  the  examination  of 
anything  more  than  the  tentacular  crown  and  the  oesophagus  which 
it  surmounts,  the  stomach  and  the  remainder  of  the  visceral  appa- 
ratus being  always  retained  within  the  cell.  It  furnishes,  however, 
a  most  beautiful  object  for  the  binocular  microscope,  when  mounted 
with  all  its  polypides  expanded.2  Several  of  the  fresh- water  Polyzoa 
are  peculiarly  interesting  subjects  for  microscopic  examination,  alike 

1  See  his  memoir  '  On  the  Minute  Structure  of  some  of  the  Higher  Forms  of 
Polypi,'  in  the  Phil.  Trans,  for  1837,  p.  387. 

2  Mr.  J.  Lomas  has  detected  calcareous  spicules  in  Alcyonidium  gelatinosum, 
and  finds  that  they  are  more  abundant  in  older  than  in  younger  colonies.    See  Proceed- 
ings of  the  Liverpool  Geological  Society,  v.  p.  241. 


GKOUPS   OF  POLYZOA  909 

on  account  of  the  remarkable  distinctness  with  which  the  various 
parts  of  their  organisation  may  be  seen  and  the  very  beautiful  man- 
ner in  which  their  ciliated  tentacula  are  arranged  upon  a  deeply 
crescentic  or  horseshoe-shaped  lophophore.  By  this  peculiarity  the 
fresh-water  Polyzoa  are  distinguished  from  the  marine  ;  and  they, 
with  the  marine  Rhabdopleura,  may  be  further  distinguished  by  the 
possession  of  an  epistome,  or  moveable  process  above  the  mouth, 
whence  Professor  Allman  calls  them  the  Pkylactofamata,  as  com- 
pared with  the  others,  which  are  Gymnolcemata,  or  have  no  epistome. 
The  cells  of  the  Phylactolcemata  are  for  the  most  part  lodged  in  a 
sort  of  gelatinous  substratum  which  spreads  over  the  leaves  of 
aquatic  plants,  sometimes  forming  masses  of  considerable  size  ;  but 
in  the  very  curious  and  beautiful  Cristatella  the  polyzoarv  is  un- 
attached, so  as  to  be  capable  of  moving  freely  through  the  water.1 

In  the  marine  Polyzoa,  constituting  by  far  the  most  numerous 
division  of  the  class,  the  anus  opens  either  outside  (Ectoprocta)  or 
within  (Entoprocta)  the  circlet  of  tentacles  ;  the  former  comprise 
three  groups  : — I.  Cheilostomata,  in  which  the  mouth  of  the  cell  is 
sub-terminal,  or  not  quite  at  its  extremity  (fig.  688).  is  somewhat 
crescentic  in  form,  and  is  furnished  with  a  movable  (generally  mem- 
branous) lip.  which  closes  it  when  the  animal  retreats.  This  includes 
a  large  part  of  the  species  that  most  abound  on  our  own  coast,  not- 
withstanding their  wide  differences  in  form  and  habit.  Thus  the 
polyzoaries  of  some  (as  Flustra)  are  horny  and  flexible,  whilst  those 
of  others  (as  Eschara  and  Retepora)  are  so  penetrated  with  calcareous 
matter  as  to  be  quite  rigid ;  some  grow  as  independent  plant -like 
structures  (as  Bugula  and  Gemellaria),  whilst  others,  having  a  like 
arborescent  form,  creep  over  the  surfaces  of  rocks  or  stones  (as 
Hippothoa)  ;  and  others,  again,  have  their  cells  in  close  apposition, 
and  form  crusts  which  possess  no  definite  figure  (as  is  the  case  with 
Lepralia  and  Membranipora).  II.  The  second  order,  Cydostomata, 
consists  of  those  Polyzoa  which  have  the  mouth  at  the  termination  of 
tubular  calcareous  cells,  without  any  movable  appendage  or  lip  (fig. 
689).  This  includes  a  comparatively  small  number  of  genera,  of  which 
Crisia  and  Tubulipora  contain  the  largest  proportion  of  the  species 
that  occur  on  our  own  coasts.  III.  The  distinguishing  character  of 
the  third  order,  Ctenostomata,  is  derived  from  the  presence  of  a  comb- 
like  circular  fringe  of  bristles,  connected  by  a  delicate  membrane, 
around  the  mouth  of  the  cell,  when  the  animal  is  projected  from  it, 
this  fringe  being  drawn  in  when  the  animal  is  retracted.  The  poly- 
zoaries of  this  group  are  very  various  in  character,  the  cells  being 
sometimes  horny  and  separate  (as  in  Farrella  and  Boiverbankia), 
sometimes  fleshy  and  coalescent  (as  in  Alcyonidium).  IV.  In  the 
Entoprocta,  which  are  represented  by  Loxosoma  and  Pedicellina, 
and  are  doubtless  the  most  archaic  of  the  true  Polyzoa,  the  lopho- 
phore is  produced  upwards  on  the  back  of  the  tentacles,  uniting 
them  at  their  base  in  a  sort  of  muscular  calyx,  and  giving  to  the 
animal  when  expanded  somewhat  the  form  of  an  inverted  bell,  like 


9io 


POLYZOA  AND   TUNICATA 


that  of  Vorticella  (fig.  593).  As  the  Polyzoa  altogether  resemble 
hydroid  zoophytes  in  their  habits,  and  are  found  in  the  same  localities, 
it  is  not  requisite  to  add  anything  to  what  has  already  been  said 
respecting  the  collection,  examination,  and  mounting  of  this  very 
interesting  class  of  objects.1 

A  large  proportion  of  the  Cheilostomata  are  furnished  with  very 
peculiar  motile  appendages,  which  are  of  two  kinds,  avicularia  and 
vibracula.  The  avicularia  or  '  bird's  head  processes,'  so  named  from 
the  striking  resemblance  they  present  to  the  head  and  jaws  of  a  bird 

(fig.  689,  B),  are  generally, 
when  highly  differentiated, 
'  sessile '  upon  the  angles 
or  margins  of  the  cells, 
that  is,  are  attached  at 
once  to  them  without  the 
intervention  of  a  stalk,  as 
at  A,  being  either  *  pro- 
jecting' or '  immersed  ; '  but 
in  the  genera  Bugula  and 
Bicellaria,  where  they  are 
present  at  all,  they  are 
'  pedunculate,'  or  mounted 
on  foot-stalks  (B).  Under 
one  form  or  the  other,  they 
are  wanting  in  but  few  of 
the  genera  belonging  to 
this  order  ;  and  their  pre- 
sence or  absence  furnishes 
valuable  characters  for  the 
discrimination  of  species. 
Each  avicularium  has  two 
'  mandibles,'  of  which  one 

is  fixed,  like  the  upper  iaw 

FIG.  689.— A,  portion  of  Bicellana  cihata,  en-      f        i  •     i     ^i          ,-i^F      J 
larged  ;  B,  one  of  the  '  bird's  head  '  processes  of   O±   a    bird,   the   otner   mov- 
Bugula  avicularia,  more  highly  magnified,  and   able,  like  its  lower  jaw  ;  the 
seen  in  the  act  of  grasping  another.  latter  ig  Opened  and  closed 

by    two    sets    of    muscles 

which  are  seen  in  the  interior  of  the  '  head,'  and  between  them  is  a 
peculiar  body,  furnished  with  a  pencil  of  bristles,  which  is  probably  n 

i  For  a  more  detailed  account  of  the  structure  and  classification  of  the  marine 
Polyzoa  see  Professor  Van  Beneden's  '  Recherches  sur  les  Bryozoaires  de  la  cf>te 
d'Ostende'  in  Mem.  de  I'Acad.  Boy.  de  Bruxelles,  torn.  xvii. ;  Mr.  G.  Busk's 
Catalogue  of  the  Marine  Polyzoa  in  the  Collection  of  the  British  Musemn;  Mr. 
Hmcks's  British  Marine  Polyzoa,  1880;  and  Nitsche,' '  Beitrage  zur  Kenntiiiss  der 
Bryozoen,  m  Zeitschnftf.  wiss.  Zool.  Bde.  xx.  xxi.  xxiv.  Of  the  more  important 
recent  publications  we  may  note  Mr.  Busk's  Reports  on  the  Polyzoa  of  the  Chall  en  <,<•>• 
voyage ;  Mr.  Harmer,  On  the  Structure  and  Development  of  Loxosoma  '  and  'On 
the  Life-history  of  Pechcelhna,'  in  vols.  xxv.  and  xxvi.  of  the  Quart.  Journ.  of 
Microsc.  Sci. ;  J.  Barrois,  Recherches  sur  TEmbryologie  des  Bryozoaires,'  Lille  1877 
and  other  memoirs;  W.  J.  Vigelms,  '  Morphologische  Untersuchungeii  iiber  Flustra 
Membranaceo-truncata,'  Biolog.  Centralblatt,  iii.  p.  705  and  Bijdranen  tot  <!<• 
DierJcunde,  xi.  For  a  general  account  see  Professor  Ray  Lankester's  article '  Polyzoa  ' 
in  the  9th  edition  of  the  Encyclopedia  Britannica,  and  Dr.  Harmer's  work  alreadv 
referred  to. 


AVICULARIA  AND   VIBRACULA  91  I 

tactile  organ,  being  brought  forwards  when  the  mouth  is  open,  so 
that  the  bristles  project  beyond  it,  and  being  drawn  back  when  the 
mandible  closes.  The  aviciilaria  keep  up  a  continual  snapping  action 
during  the  life  of  the  polyzoary ;  and  they  may  often  be  observed  to 
lay  hold  of  minute  worms  or  other  bodies,  sometimes  even  closing 
upon  the  beaks  of  adjacent  organs  of  the  same  kind,  as  shown  at  B. 
In  the  pedunculate  forms,  besides  the  snapping  action,  there  is  a 
continual  rhythmical  nodding  of  the  head  upon  the  stalk  ;  and  few 
spectacles  are  more  curious  than  a  portion  of  the  polyzoary  of 
Bugula  avicularia  (a  very  common  British  species)  in  a  state  of 
active  vitality,  when  viewed  under  a  power  sufficiently  low  to  allow 
a  number  of  these  bodies  to  be  in  sight  at  once.  It  is  still  very 
doubtful  what  is  their  precise  function  in  the  economy  of  the  animal — 
whether  it  is  to  retain  within  the  reach  of  the  ciliary  current  bodies 
that  may  serve  as  food,  or  whether  it  is,  like  the  Pedicellarise  of 
Echini,  to  remove  extraneous  particles  that  may  be  in  contact  with 
the  surface  of  the  polyzoary.  The  latter  would  seem  to  be  the  func- 
tion of  the  vibracula,  which  are  long  bristle-shaped  organs  (fig.  688, 
A),  each  one  springing  at  its  base  out  of  a  sort  of  cup  that  contains 
muscles  by  which  it  is  kept  in  almost  constant  motion,  sweeping 
slowly  and  carefully  over  the  surface  of  the  polyzoary,  and  removing 
what  might  be  injurious  to  the  delicate  inhabitants  of  the  cells  when 
their  tentacles  are  protruded.1 

Tunicata. — The  zoological  position  of  the  Timieata,  which  has 
long  been  a  subject  of  great  discussion,  appears  to  be  now  approxi- 
mately settled ;  the  study  of  their  development  has  shown  that 
they  are  provided  with  a  notochord,  and  that  their  nervous  system 
follows  the  course  which  is  characteristic  of  what  are  often  called 
Vertebrata,  but  should  better  be  called  Chordata.  As  the  noto- 
chord is  always  restricted  to  the  hinder  part  of  the  body,  the 
Tunicata  may  be  called  Urochordata.  In  all  (except,  perhaps, 
Appendicularici)  there  are  distinct  signs  of  degeneration.  They  have 
been  named  Tunicata  from  the  inclosure  of  their  bodies  in  a  '  tunic,' 
which  is  sometimes  leathery  or  even  cartilaginous  in  its  texture,  and 
which  sometimes  includes  calcareous  spicules,  whose  forms  are  often 
very  beautiful.  They  are  often  found  to  resemble  the  Polyzoa  in 
their  tendency  to  produce  composite  structures  by  gemmation  ;  but  in 
their  habits  they  are  for  the  most  part  very  inactive,  exhibiting 
scarcely  anything  comparable  to  those  rapid  movements  of  expansion 
and  retraction  which  it  is  so  interesting  to  watch  among  the  Polyzoa  ; 
whilst,  with  the  exception  of  the  Salpidcv  and  other  floating  species 
which  are  chiefly  found  in  seas  warmer  than  those  that  surround  our 
coast,  and  the  curious  Appendicularla  to  be  presently  noticed,  they 
are  rooted  to  one  spot  during  all  but  the  earliest  period  of  their  lives. 
The  larger  forms  of  the  Ascidian  group,  which  constitutes  the  bulk 
of  the  class,  are  always  solitary ;  not  propagating  by  gemmation, 
except  in  the  case  of  the  Clavelinidae.  Although  of  special  importance 

1  See  Mr.  G.  Busk's  '  Remarks  on  the  Structure  and  Function  of  the  Avicularian 
and  Vibracular  Organs  of  Polyzoa '  in  Trans.  Microsc.  Soc.  ser.  ii.  vol.  ii.  1854, 
p.  26  ;  and  Mr.  A.  W.  Waters,  '  On  the  use  of  the  Avicularian  Mandible  in  the  Deter- 
mination of  the  Cheilostomatous  Bryozoa,'  Journ.  EOIJ.  Microsc.  Soc.  (2),  v.  p.  774. 


912  POLYZOA   AND   TUNICATA 

to  the  comparative  anatomist  and  the  zoologist,  this  group  does  not 
afford  much  to  interest  the  ordinary  microscopist,  except  in  the  pecu- 
liar actions  of  its  respiratory  and  circulatory  apparatus.  In  common 
with  the  composite  forms  of  the  group,  the  solitary  Ascidians  have 
a  large  branchial  sac,  with  fissured  walls,  resembling  that  shown  in 
figs.  690,  B,  and  692  ;  into  this  sac  water  is  admitted  by  the  oral 
orifice,  and  a  large  proportion  of  it  is  caused  to  pass  through  the 
fissures,  by  the  agency  of  the  cilia  with  which  they  are  fringed,  into 
a  surrounding  chamber,  whence  it  is  expelled  through  the  atriopore, 
or  opening  of  the  mantle.  This  action  may  be  distinctly  watched 
through  the  external  walls  in  the  smaller  and  more  transparent 
species ;  and  not  even  the  ciliary  action  of  the  tentacles  of  the  Polyzoa 
affords  a  more  beautiful  spectacle.  It  is  peculiarly  remarkable  in  one 
species  that  occurs  on  our  own  coasts,  the  Corella  parallelogramma,1 
in  which  the  wall  of  the  branchial  sac  is  divided  into  a  number  of 
areolse,  each  of  them  shaped  into  a  shallow  funnel ;  and  round  one 
of  these  funnels  each  branchial  fissure  makes  two  or  three  turns  of  a 
spiral.  When  the  cilia  of  all  these  spiral  fissures  are  in  active  move- 
ment at  once,  the  effect  is  most  singular.  Another  most  remarkable 
phenomenon  presented  throughout  the  group,  and  well  seen  in  the 
solitary  Ascidian  just  referred  to,  is  the  alternation  in  the  direction 
of  the  circulation.  The  heart,  which  lies  at  the  bottom  of  the 
branchial  sac,  has  its  one  end  connected  with  the  principal  trunk 
leading  to  the  body,  and  the  other  with  that  leading  to  the  branchial 
sac.  At  one  time  it  will  be  seen  that  the  blood  flows  from  the 
respiratory  apparatus  to  the  end  of  the  heart  in  which  its  trunk 
terminates,  which  then  contracts  so  as  to  drive  it  through  the  sys- 
temic trunk  to  the  body  at  large  ;  but  after  this  course  has  been  main- 
tained for  a  time  the  heart  ceases  to  pulsate  for  a  moment  or  two, 
and  the  course  is  reversed,  the  blood  flowing  into  the  heart  from 
the  body  generally,  and  being  propelled  to  the  branchial  sac.  After 
this  reversed  course  has  continued  for  some  time  another  pause 
occurs,  and  the  first  course  is  resumed.  The  length  of  time  inter- 
vening between  the  changes  does  not  seem  by  any  means  constant. 
It  is  usually  stated  at  from  half  a  minute  to  two  minutes  in  the  com- 
posite forms  ;  but  in  the  solitary  Corella  parallelogramma  (a  species 
very  common  in  Lamlash  Bay,  Arran),  the  Author  has  repeatedly 
observed  an  interval  of  from  five  to  fifteen  minutes,  and  in  some 
instances  he  has  seen  the  circulation  go  on  for  half  an  hour,  or  even 
longer,  without  change — always,  however,  reversing  at  last. 

The  compound  Ascidians  are  very  commonly  found  adherent  to 
seaweeds,  zoophytes,  and  stones  between  the  tide-marks ;  and  they 
present  objects  of  great  interest  to  the  microscopist,  since  the  small 
size  and  transparence  of  their  bodies  when  they  are  detached  from 
the  mass  in  which  they  are  imbedded  not  only  enable  their  structure 
to  be  clearly  discerned  without  dissection,  but  allow  many  of  their 
living  actions  to  be  watched.  Of  these  we  have  a  characteristic 
example  in  Amaroucium  proliferum,  of  which  the  form  of  the  com- 

1  See  Alder  in  Ann.  of  Nat.  Hist.  ser.  iii.  vol.  xi.  1863,  p.  157;  and  Hancock  in 
Journ.  Linn.  Soc.  ix.  p.  333. 


TUNICATA 


913 


posite  mass  and  the  anatomy  of  a  single  individual  are  displayed  in 
fig.  690.  Its  clusters  appear  almost  completely  inanimate,  exhibiting 
no  very  obvious  movements  when 
irritated;  but  if  they  be  placed 
when  fresh  in  sea-Water  a  slight 
pouting  of  the  orifices  will  soon  be 
perceptible,  and  a  constant  and 
energetic  series  of  currents  will  be 
found  to  enter  by  one  set  and  to  be 
ejected  by  the  other,  indicating  that 
all  the  machinery  of  active  life  is 
going  on  within  these  apathetic 
bodies.  In  the  family  Polydmidce 
to  which  this  genus  belongs  the 
body  is  elongated,  and  may  be 
divided  into  three  regions :  the  thorax 
(A),  which  is  chiefly  occupied  by  the 
respiratory  sac  ;  the  abdomen  (B), 
which  contains  the  digestive  appa- 
ratus ;  and  the  post-abdomen  (C),  in 
which  the  heart  and  generative 
organs  are  lodged.  At  the  summit 
of  the  thorax  is  seen  the  oral  orifice, 
c,  which  leads  to  the  branchial  sac  e ; 
this  is  perforated  by  an  immense 
number  of  slits,  which  allow  part  of 
the  water  to  pass  into  the  space 
between  the  branchial  sac  and  the 
muscular  mantle.  At  k  is  seen  the 


FIG.  690. — Compound  mass  of  Amaroiiciumproliferum  with  the  anatomy  of  a 
single  zooid  :  A,  thorax ;  B,  abdomen ;  C,  post-abdomen ;  c,  oral  orifice ; 
e,  branchial  sac  ;  /,  thoracic  blood-vessel ;  i,  atriopore ;  i',  projection  over- 
hanging it ;  j,  nervous  ganglion;  k,  oesophagus;  I,  stomach  surrounded  by 
digestive  tubuli ;  m,  intestine  ;  n,  anus  opening  into  the  cloaca  formed  by 
the  mantle  ;  o,  heart ;  o',  pericardium ;  p,  ovarium  ;  p',  egg  ready  to  escape ; 
q,  testis ;  r,  spermatic  canal ;  r',  termination  of  this  canal  in  the  cloaca. 

cesophagus,  which  is  continuous  with  the  lower  part  of  the  pha- 
ryngeal  cavity  ;  this  leads  to  the  stomach,  I,  which  is  surrounded 
by  glandular  follicles  ;  and  from  this  passes  off  the  intestine,  m,  which 
terminates  at  n  in  the  vent.  A  current  of  water  is  continually 

3  N 


914  POLYZOA  AND  TUNIC  ATA 

drawn  in  through  the  mouth  by  the  action  of  the  cilia  of  the  bran- 
chial sac  and  of  the  alimentary  canal ;  a  part  of  this  current  passes 
through  the  fissures  of  the  branchial  sac  into  the  peribranchial 
cavity,  and  thence  into  the  cloaca  ;  whilst  another  portion,  entering 
the  stomach  by  an  aperture  at  the  bottom  of  the  pharyiigeal  sac, 
passes  through  the  alimentary  canal,  giving  up  any  nutritive 
materials  it  may  contain,  and  carrying  away  with  it  any  excre- 
mentitious  matter  to  be  discharged ;  and  this  having  met  the 
respiratory  current  in  the  cloaca,  the  two  mingled  currents  pass  forth 
together  by  the  atrial  orifice,  i.  The  long  post-abdomen  is  principally 
occupied  by  the  large  ovarium,  p,  which  contains  ova  in  various  stages 
of  development.  These,  when  matured  and  set  free,  find  their  way 
into  the  cloaca,  where  two  large  ova  are  seen  (one  marked  p  and 
the  other  immediately  below  it)  waiting  for  expulsion.  In  this  posi- 
tion they  receive  the  fertilising  material  from  the  testis,  q.  which 
discharges  its  products  by  the  long  spermatic  canal,  r,  that  opens  into 
the  cloaca,  r  .  At  the  very  bottom  of  the  post -abdomen  we  find  the 
heart,  o,  inclosed  in  its  pericardium,  o' '.  In  the  group  we  are  now 
considering  a  number  of  such  animals  are  imbedded  together  in  a 
sort  of  gelatinous  mass,  and  covered  with  an  integument  common  to 
them  all ;  the  composition  of  this  gelatinous  substance  is  remarkable 
as  including  cellulose,  which  generally  ranks  as  a  vegetable  product. 
The  mode  in  which  new  individuals  are  developed  in  this  mass  is  by 
the  extension  of  stolons  or  creeping  stems  from  the  bases  of  those 
previously  existing  ;  and  from  each  of  these  stolons  several  buds  may 
be  put  forth,  every  one  of  which  may  evolve  itself  into  the  likeness 
of  the  stock  from  which  it  proceeded,  and  may .  in  its  turn  increase 
and  multiply  after  the  same  fashion. 

In  the  family  of  Didemnians  the  post-abdomen  is  absent,  the  heart 
and  generative  apparatus  being  placed  by  the  side  of  the  intestine  in  the 
abdominal  portion  of  the  body.  The  zooids  are  frequently  arranged 
in  star-shaped  clusters,  their  anal  orifices  being  all  directed  towards 
a  common  vent  which  occupies  the  centre.  This  shortening  is  still 
more  remarkable,  however,  in  the  family  of  Botryllians,  whose 
beautiful  stellate  gelatinous  incrustations  are  extremely  common  upon 
seaweeds  and  submerged  rocks  (fig.  691).  The  anatomy  of  these 
animals  is  very  similar  to  that  of  the  Amaroucium  already  described  ; 
with  this  exception,  that  the  body  exhibits  no  distinction  of  cavities, 
all  the  organs  being  brought  together  in  one,  which  must  be  con- 
sidered as  thoracic.  In  this  respect  there  is  an  evident  approximation 
towards  the  solitary  species.1 

This  approximation  is  still  closer,  however,  in  the  '  social '  Asci- 
dians,  or  Clavellinidce,  in  which  the  general  plan  of  structure  is 
nearly  the  same,  but  the  zooids  are  simply  connected  by  their  stolons 
instead  of  being  included  in  a  common  investment ;  so  that  their 
relation  to  each  other  is  very  nearly  the  same  as  that  of  the  poly- 

1  For  more  special  information  respecting  the  compound  Ascidians  see  espe- 
cially the  admirable  monograph  of  Professor  Milne-Edwards  on  that  group  ;  Mr.  Lister's 
memoir, '  On  the  Structure  and  Functions  of  Tubular  and  Cellular  Polypi,  and  of 
Ascidise,'  in  the  Phil.  Trans.  1834 ;  and  the  article  '  Tunicata,'  by  Professor  T.  Rupert 
Jones,  in  the  Cyclopcedia  of  Anatomy  and  Physiology.  More  recent  authorities 
are  cited  on  p.  918. 


TUNICATA 


915 


pides  of  Laguncula,  the  chief  difference  being  that  a  regular  cir- 
culation takes  place  through  the  stolon  in  the  one  case,  such  as  has 
no  existence  in  the  other.  A  better  opportunity  of  studying  the 
living  actions  of  the  Ascidians  can  scarcely  be  found  than  that  which 
is  afforded  by  the  genus  Perophora,  first  discovered  by  Mr.  Lister, 
which  occurs  not  unfrequently  on  the  south  coast  of  England  and  in 
the  Irish  Sea,  living  attached  to  seaweeds,  and  looking  like  an  assem- 
blage of  minute  globules  of  jelly,  dotted  with  orange  and  brown,  and 
linked  by  a  silvery  winding  thread.  The  isolation  of  the  body  of 
each  zooid  from  that  of  its  fellows,  and  the  extreme  transparence  of 
its  tunics,  not  only  enable  the  movements  of  fluid  within  the  body  to 
be  distinctly  discerned,  but  also  allow  the  action  of  the  cilia  that 
border  the  slits  of  the  respiratory  sac  to  be  clearly  made  out.  This 
sac  is  perforated  with  four  rows  of  narrow  oval  openings,  through 
which  a  portion  of  the  water  tha,t  enters  its  oral  orifice  escapes 


FIG.  (J91.—Botryllus  violaceus  \  A,  cluster  on  the  surface  of  a  Fucus  ; 
B,  portion  of  the  same  enlarged. 

into  the  space  between  the  sac  and  the  mantle,  and  is  thus  dis- 
charged immediately  by  the  atrial  funnel.  Whatever  little  particles, 
animate  or  inanimate,  the  current  of  water  brings  flow  into  the 
sac  unless  stopped  at  its  entrance  by  the  tentacles,  which  do  not 
appear  fastidious.  The  particles  which  are  admitted  usually  lodge 
somewhere  on  the  sides  of  the  sac,  and  then  travel  horizontally  until 
they  arrive  at  that  part  of  it  down  which  the  current  proceeds  to  the 
entrance  of  the  stomach,  which  is  situated  at  the  bottom  of  the 
sac.  Minute  animals  are  often  swallowed  alive,  and  have  been 
observed  darting  about  in  the  cavity  for  some  days,  without  any  ap- 
parent injury  either  to  themselves  or  to  the  creature  which  incloses 
them.  In  general,  however,  particles  which  are  unsuited  for  reception 
into  the  stomach  are  rejected  by  the  sudden  contraction  of  the  mantle 
(or  muscular  tunic),  the  atriopore  being  at  the  same  time  closed,  so 
that  they  ^  are  forced  out  by  a  powerful  current  through  the  oral 
orifice.  The  curious  alternation  of  the  circulation  that  is  character- 
istic of  the  class  generally  may  be  particularly  well  studied  in 
Perophora.  The  creeping  stalk  that  connects  the  individuals  of 

3N  2 


916 


POLYZOA   AND    TUNICATA 


any  group  contains  two  distinct  canals,  which  send  off  branches 
into  each  peduncle.  One  of  these  branches  terminates  in  the  heart, 
which  is  nothing  more  than  a  contractile  dilatation  of  the  principal 
trunk  ;  this  trunk  subdivides  into  vessels  (or  rather  sinuses,  which  are 
mere  channels  not  having  proper  walls  of  their  own),  of  which  some 
ramify  over  the  respiratory  sac,  branching  off  at  each  of  the  passages 
between  the  oval  slits,  whilst  others  are  first  distributed  to  the 
stomach  and  intestine,  and  to  the  soft  surface  of  the  mantle.  All 

these  reunite  so  as  to  form 
a  trunk,  which  passes  to  the 
peduncle  and  constitutes  the 
returning  branch.  Although 
the  circulation  in  the  dif- 
ferent bodies  is  brought 
into  connection  by  the  com- 
mon stem,  yet  that  of  each 
is  independent  of  the  rest, 
continuing  when  the  current 
through  its  own  foot -stalk  is 
interrupted  by  a  ligature  ; 
and  the  stream  which  re- 
turns from  the  branchial 
sac  and  the  viscera  is  then 
poured  into  the  posterior 
part  of  the  heart  instead 
of  entering  the  peduncle. 

The  development  of  the 
Ascidians,  the  early  stages 
of  which  are  observable 
whilst  the  ova  are  still 
within  the  cloaca  of  the 
parent,  presents  some  phe- 
nomena of  much  interest 


*•,  p.bi: 


to  the  microscopist  which 
alone  can  be  noticed  here. 
After  the  ordinary  repeated 
segmentation  of  the  yolk, 
whereby  a  '  mulberry  mass  ' 
is  produced,  a  sort  of  ring 


FIG.  692.— Diagrammatic  longitudinal  section  of 
Ascidia  showing  the  heart,  the  blood-vessels, 
the  branchial  sac,  the  alimentary  canal,  &c. 
from  the  left  side :  br.si.,  branchial  siphon : 
at.si.,  atrial  siphon ;  t.,  test ;  m.,  mantle ; 
br.s.,  branchial  sac;  p.br.,  peribranchial 
cavity;  cl.,  cloaca;  n.g ,  nerve  ganglion; 
tn.,  tentacle;  gl.,  neural  gland;  ce.a.,  ceso- 
phageal  aperture ;  st.,  stomach  ;  i.,  intestine ; 
r.,  rectum;  a.,  anus;  o.w.,  genital  organs; 

g.d.,   genital   ducts;  h.,  heart;  c.sp.,   cardio-      •  pnoirplino- its  rpntrnl 

splanchnic  vessel;  v.t.,  vessel  to  the  test; 
t.k.,  terminal  knob  on  vessel  in  test;  v.t'., 
vessel  from  the  test;  v.st.,  vessel  to  the 
stomach  &c. ;  v.m.,  vessel  to  the  mantle; 
v.m!.,  vessel  from  the  mantle ;  d.v.,  dorsal 
vessel ;  tr.,  transverse  vessel  of  branchial 
sac  ;  l.v .,  fine  longitudinal  vessel  of  branchial 
sac  ;  sg.,  stigmata  of  branchial  sac ;  v.v., 
ventral  vessel ;  br.c.,  branchio-cardiac 
vessel;  sp.br.,  splanchno-branchial  vessel. 
(After  Prof.  Herdman.) 


portion  ;  but  this  soon 
shows  itself  as  a  tapering 
tail-like  prolongation  from 
one  side  of  the  yolk,  which 
gradually  becomes  more 
and  more  detached  from 
it,  save  at  the  part  from 
which  it  springs.  Either 


whilst  the  egg  is  still  within 
the  cloaca,  or  soon  after  it  has  escaped  from  the  vent,  its  envelope 
bursts,  and  the  larva  escapes,  and  in  this  condition  it  presents  very 


TUNIC  AT  A  9 1  7 

much  the  appearance  of  a  tadpole,  the  tail  being  straightened 
out,  and  propelling  the  body  freely  through  the  water  by  its  lateral 
strokes.  The  centre  of  the  body  is  occupied  by  a  mass  of  liquid  yolk, 
and  this  is  continued  into  the  interior  of  three  prolongations  which 
extend  themselves  from  the  opposite  extremity,  each  terminating  in  a 
sort  of  sucker.  After  swimming  about  for  some  hours  with  an  active 
wriggling  movement,  the  larva  attaches  itself  to  some  solid  body  by 
means  of  one  of  these  suckers ;  if  disturbed  from  its  position,  it  at 
first  swims  about  as  before  ;  but  it  soon  completely  loses  its  activity, 
and  becomes  permanently  attached  ;  and  important  changes  manifest 
themselves  in  its  interior.  The  organs  and  tissues  which  constitute 
the  chief  part  of  the  future  animal  are  gradually  drawn  back,  so  that 
the  whole  of  it  is  concentrated  ftito  one  mass  ;  and  the  tail,  now  con- 
sisting only  of  the  gelatinous  envelope,  is  either  detached  entire  from 
the  body  by  the  contraction  of  the  connecting  portion,  or  withers, 
ami  is  tin-own  off  gradually  in  shreds.  The  shaping  of  the  internal 
organs  out  of  the  yolk  mass  takes  place  very  rapidly,  so  that  by  the 
end  of  the  second  day  of  the  sedentary  state  the  outlines  of  the 
branchial  sac  and  of  the  stomach  and  intestine  may  be  traced,  no 
external  orifices,  however,  being  as  yet  visible.  The  pulsation  of  the 
heart  is  first  seen  on  the  third  day,  and  the  formation  of  the  branchial 
and  anal  orifices  takes  place  on  the  fourth,  after  which  the  ciliary 
ciu-rents  are  immediately  established  through  the  branchial  sac  and 
alimentary  canal.  The  embryonic  development  of  other  Ascidians, 
solitary  as  well  as  composite,  takes  place  on  a  plan  essentially  the 
same  as  the  foregoing,  a  free  tadpole-like  larva  being  always  produced 
in  the  first  instance  with  the  curious  exception  of  some  species  of 
Molgula.1 

This  larval  condition  is  represented  in  a  very  curious  adult  free- 
swimniiiig  form,  termed  Appeiidicularia,  which  is  frequently  to  be 
taken  with  the  tow-net  on  our  own  coasts.  This  animal  has  an  oval 
or  flask-like  body,  which  in  large  specimens  attains  the  length  of 
one-fifth  of  an  inch,  but  which  is  often  not  more  than  one-fourth  or 
one-fifth  of  that  size.  It  is  furnished  with  a  tail-like  appendage 
three  or  four  times  its  own  length,  broad,  flattened,  and  rounded  at 
its  extremity  ;  and  by  the  powerful  vibrations  of  this  appendage  it  is 
propelled  rapidly  through  the  water.  The  structure  of  the  body  dif- 
fers greatly  from  that  of  the  Ascidians,  its  plan  being  much  simpler ; 
in  particular,  the  pharyngeal  sac  is  entirely  destitute  of  ciliated 
branchial  fissures  opening  into  a  surrounding  cavity  ;  but  two  canals, 

1  The  study  of  the  development  of  Ascidians  derived  a  new  interest  and  im- 
portance from  the  discovery,  made  by  Kowalevsky  in  1867,  that  their  free-swimming 
larvae  present  a  most  striking  parallelism  to  vertebrate  embryos,  in  exhibiting  the 
beginnings  of  a  spinal  marrow  and  a  notochord  ;  thus  bridging  over  the  gulf  that  was 
supposed  to  separate  them  from  Invertebrata,  and  (when  taken  in  connection  with 
the  curious  Ascidian  affinities  of  Amphioxus,  the  lowest  vertebrate  at  present  known) 
affording  strong  reason  for  belief  in  the  derivation  of  the  vertebrate  and  tunicate 
types  from  a  common  original.  See  his  memoir  '  Entwickelungsgeschichte  der 
einfachen  Ascidien '  in  Mem.  St.  Petersb.  Acad.  Sci.  torn.  x.  1867,  and  the  abstract 
of  it  in  Quart.  Journ.  Microsc.  Set.  x.  n.s.  1870,  p.  59  ;  also  Professor  Haeckel's  History 
of  Creation,  ii.  pp.  152,  200.  Further  information  will  be  found  in  chap.  ii.  of  vol. 
ii.  of  the  late  Professor  Balfour's  Comparative  Embryology,  and  an  application  of  the 
facts  of  development  to  the  philosophy  of  the  subject  in  Professor  Ray  Lankester's 
Degeneration  (London,  1880). 


91 8  POLYZOA   AND    TUNICATA 

one  on  either  side  of  the  entrance  to  the  stomach,  are  prolonged  from 
it  to  the  external  surface  ;  and  by  the  action  of  the  long  cilia  with 
which  these  are  furnished,  in  conjunction  with  the  cilia  of  the 
branchial  sac,  a  current  of  water  is  maintained  through  its  cavity. 
From  the  observations  of  Huxley,  however,  it  appears  that  the 
direction  of  this  current  is  by  no  means  constant ;  since,  although  it 
usually  enters  by  the  mouth  and  passes  out  by  the  ciliated  canals, 
it  sometimes  enters  by  the  latter  and  passes  out  by  the  former.  The 
caudal  appendage  has  a  central  axis  (notochord),  above  and  below 
which  is  a  ribbon-like  layer  of  muscular  fibres  ;  a  nervous  cord, 
sfcudded  at  intervals  with  minute  ganglia,  may  be  traced  along  its 
whole  length.  By  Mertens,  one  of  the  early  observers  of  this  animal, 
it  was  said  to  be  furnished  with  a  peculiar  gelatinous  envelope  or 
Ifaus  (house),  very  easily  detached  from  the  body,  and  capable  of 
being  re-formed  after  having  been  lost.  Notwithstanding  the  great 
numbers  of  specimens  which  have  been  studied  by  Muller,  Huxley, 
Leuckart,  and  Gegenbaur,  none  of  these  excellent  observers  has 
met  with  this  appendage;  but  it  Jias  been  since  seen  by  Allman, 
who  describes  it  as  an  egg-shaped  gelatinous  mass,  in  which 
the  body  is  imbedded,  the  tail  alone  being  free ;  whilst  from  either 
side  of  the  central  plane  there  radiates  a  kind  of  double  fan,  which 
seems  to  be  formed  by  a  semicircular  membranous  lamina  folded 
upon  itself.  It  was  surmised  by  Allman,  with  much  probability,  that 
this  curious  appendage  is  '  nidamental,'  having  reference  to  the 
development  and  protection  of  the  young ;  but  on  this  point  further 
observations  are  much  needed  ;  and  any  microscopist  who  may  meet 
with  Appendicularia  furnished  with  its  *  house  '  should  do  all  he  can 
to  determine  its  structure  and  its  relations  to  the  body  of  the 
animal.1 

1  For  details  in  respect  to  the  structure  of  Appendicularia,  see  Huxley  in  Phil. 
Trans,  for  1851,  and  in  Quart.  Journ.  of  Microsc.  Sci.  vol.  iv.  1856,  p.  181 ;  also 
Allman  in  the  same  journal,  vol.  vii.  1859,  p.  86 ;  Gegenbaur  in  Sieboldund  Kolliker's 
Zeitschrift,  Bd.  vi.  1855,  p.  406 ;  Leuckart's  Zoologische  Untersuchungen,  Heft  ii. 
1854 ;  Fol's  '  Etudes  sur  les  Appendiculaires  '  in  Archiv.  Zool.  exper.  torn.  i.  1872, 
p.  57';  the  three  memoirs  by  H.  Lohmann  published  in  1896.  For  the  Tunicata 
generally,  see  Professor  T.  Rupert  Jones  in  vol.  iv.  of  the  Cyclop,  of  Anatomy 
and  Physiology;  Professor  Herdman's  article,  'Tunicata,'  in  the  9th  edition 
of  the  Encyclopedia  Britannica ;  Mr.  Alder's  '  Observations  on  the  British 
Tunicata '  in  Ann.  of  Nat.  Hist.  ser.  iv.  vol.  xi.  1863,  p.  153 ;  and  Mr.  Hancock's 
memoir  '  On  the  Anatomy  and  Physiology  of  the  Tunicata  '  in  the  Journal  of  the 
Linnean  Society,  vol.  ix.  p.  309.  Great  additions  to  our  knowledge  have  been 
made  by  Professor  Herdman,  whose  reports  on  the  forms  collected  by  H.M.S. 
Challenger  should  be  consulted,  and  by  Professors  Van  Beneden  and  Julin  (see  espe- 
cially their  memoirs  in  the  Archives  de  Biologie) .  See  also  Eoule, '  Recherches  sur  les 
Ascidies  simples  des  cotes  de  Provence,'  Ann.  Museum  Marseilles,  ii. ;  Seeliger, 
'  Die  Entwickelungsgeschichte  der  Socialen  Ascidien,'  Jenaische  Zeitschr.  xviii. 
p.  528 ;  Salensky,  '  Neue  Untersuchungen  iiber  die  embryonale  Entwickelung  der 
Salpen,'  Mitth.  Zool.  Stat.  Neapel,  iv.  pp.  90,  327 ;  and  Ulianin,  '  Die  Arten  des 
Gattun'g  Doliolum  im  Golfe  von  Neapel,'  in  the  Fauna  und  Flora  des  Golfes  von 
Neapel,  x.  The  above  titles  by  no  means  exhaust  the  list  of  recent  important  memoirs 
on  Tunicata,  but  the  researches  of  Caullery,  Metcalf,  Pizon,  and  Seeliger  are  beyond 
the  scope  of  this  work.  The  last-named  has  commenced  a  systematic  account  of  the 
group  in  Bronn's  ThierreicJi. 


919 


CHAPTER   XVIII 

MOLLUSC  A  AND  BRACHTOPODA 
I 

THE  various  forms  of  *  shell-fish,'  with  their  *  naked '  or  shell-less 
allies,  furnish  a  great  abundance  of  objects  of  interest  to  the  micro- 
scopist,  of  which,  however,  the  greater  part  may  be  grouped  under 
three  heads — namely  (1)  the  structure  of  the  shell,  which  is  most 
interesting  in  the  COXCHIFERA  (or  LAMELLIBRANCHIATA)  and  BRACHIO- 
PODA,  in  both  of  which  classes  the  shells  are  '  bivalve,'  while  the  animals 
differ  from  each  other  essentially  in  general  plan  of  structure  ;  (2) 
the  structure  of  the  tongue  or  palate  of  the  GASTROPODA,  most  of  which 
have  '  univalve '  shells,  others,  however,  being  '  naked  ; '  (3)  the 
developmental  history  of  the  embryo,  for  the  study  of  which  certain 
of  the  Gastropods  present  the  greatest  facilities.  These  three  subjects, 
therefore,  will  be  first  treated  of  systematically,  and  a  few  miscella- 
neous facts  of  interest  will  be  subjoined. 

Shells  of  Mollusca, — These  investments  were  formerly  regarded 
as  mere  inorganic  exudations,  composed  of  calcareous  particles, 
cemented  together  by  animal  glue  ;  microscopic  examination,  how- 
ever, has  shown  that  they  possess  a  definite  structure,  and  that  this 
structure  presents  certain  very  remarkable  variations  in  some  of  the 
groups  of  which  the  molluscous  series  is  composed.  We  shall  first 
describe  that  which  may  be  regarded  as  the  characteristic  structure 
of  the  ordinary  bivalves,  taking  as  a  type  the  group  of  Margaritacece, 
which  includes  the  Meleagrina  or  *  pearl  oyster '  and  its  allies,  the 
common  Pinna  ranking  amongst  the  latter.  In  all  these  shells  we 
readily  distinguish  the  existence  of  two  distinct  layers  :  an  external, 
of  a  brownish- yellow  colour;  and  an  internal,  which  has  a  pearly 
or  '  nacreous '  aspect,  and  is  commonly  of  a  lighter  hue. 

The  structure  of  the  outer  layer  may  be  conveniently  studied  in 
the  shell  of  Pinna,  in  which  it  commonly  projects  beyond  the  inner, 
and  there  often  forms  lamirise  sufficiently  thin  and  transparent  to 
exhibit  its  general  characters  without  any  artificial  reduction.  If  a 
small  portion  of  such  a  lamina  be  examined  with  a  low  magnifying 
power  by  transmitted  light,  each  of  its  surfaces  will  present  very 
much  the  appearance  of  a  honeycomb  ;  whilst  its  broken  edge  exhibits 
an  aspect  which  is  evidently  fibrous  to  the  eye,  but  which,  when 
examined  under  the  microscope  with  reflected  light,  resembles  that 
of  an  assemblage  of  segments  of  basaltic  columns  (fig.  696).  This 
outer  layer  is  thus  seen  to  be  composed  of  a  vast  number  of  prisms, 
having  a  tolerably  uniform  size,  and  usually  presenting  an  approach 


920 


MOLLUSCA  AND  BRACHIOPODA 


FIG.  693. — Section  of  shell  of  Pinna,  taken 
transversely  to  the  direction  of  its  prism. 


to  the  hexagonal  shape.  These  are  arranged  perpendicularly  (or 
nearly  so)  to  the  surface  of  the  lamina  of  the  shell ;  so  that  its  thick- 
ness is  formed  by  their  length,  and  its  two  surfaces  by  their  extremi- 
ties. A  more  satisfactory  view  of  these  prisms  is  obtained  by  grinding 
down  a  lamina  until  it  possesses  a  high  degree  of  transparence,  the 

prisms  being  then  seen  (fig. 
693)  to  be  themselves  com- 
posed of  a  very  homogeneous 
substance,  but  to  be  sepa- 
rated by  definite  and 
strongly  marked  lines  of 
division.  When  such  a 
lamina  is  submitted  to  the 
action  of  dilute  acid,  so  as 
to  dissolve  away  the  car- 
bonate of  lime,  a  tolerably 
firm  and  consistent  mem- 
brane is  left,  which  exhibits 
the  prismatic  structure  just 
as  perfectly  as  did  the 
original  shell  (fig.  694),  its 

hexagonal  divisions  bearing  a  strong  resemblance  to  the  walls  of 
the  cells  of  the  pith  or  bark  of  a  plant.  By  making  a  section  of  the 
shell  perpendicularly  to  its  surface,  we  obtain  a  view  of  the  prisms 
cut  in  the  direction  of  their  length  (fig.  695) ;  these  are  frequently 
seen  to  be  marked  by  delicate  transverse  stria*  (fig.  696)  closely  re- 
sembling those  observable  011  the  prisms  of  the  enamel  of  teeth,  to 
which  this  kind  of  shell-structure  may  be  considered  as  bearing  a 
very  close  resemblance,  except  as  regards  the  mineralising  ingredient, 

If  a  similar  section  be  de- 
calcified by  dilute  acid,  the 
membranous  residuum  will 
exhibit  the  same  resem- 
blance to  the  walls  of  pris- 
matic cells  viewed  longitu- 
dinally, and  will  be  seen  to 
be  more  or  less  regularly 
marked  by  the  transverse 
striae  just  alluded  to.  It 
sometimes  happens  in  re- 
cent but  still  more  com- 
monly in  fossil  shells,  that 
the  decay  of  the  animal 
membrane  leaves  the  con- 
tained prisms  without  any  connecting  medium ;  as  they  are  then 
quite  isolated,  they  can  be  readily  detached  one  from  another ;  and 
each  one  may  be  observed  to  be  marked  by  the  like  striations, 
which,  when  a  sufficiently  high  magnifying  power  is  used,  are  seen 
to  be  minute  grooves,  apparently  resulting  from  a  thickening  of  the 
intermediate  wall  in  those  situations.  These  appearances  seem  best 
accounted  for  by  supposing  that  each  is  lengthened  by  successive 


FIG.  694. — Membranous  basis  of  the  same. 


STRUCT UKE   OF   SHELLS 


92I 


additions  at  its  base,  the  lines  of  junction  of  which  correspond  with 

the  transverse  striation  ;  and  this  view  corresponds  well  with  the 

fact  that  the  shell  -membrane  not  unfrequently  shows  a  tendency  to 

split  into  thin  laminae  along  the  lines  of  striation,  whilst  we  occa- 

sionally meet  with  an  excessively  thin  natural  lamina  lying  between 

the  thicker  prismatic  layers,  with 

one    of   which    it    would     have 

probably  coalesced  but  for  some 

accidental  cause  which  preserved 

its  distinctness.    That  the  prisms 

are  not  formed  in  their  entire 

length  at  once,  but  that  they  are 

progressively     lengthened     and 

consolidated  at  their  lower   ex- 

tremities,   would     appear     also 

from   the    fact   that   where  the 


FIG.  695. — Section  of  the  shell  of  Pinna 
in  the  direction  of  its  prisms. 


shell  presents  a  deep  colour  (as 

in   Pinna   nigrina)    this  colour 

is    usually    disposed  in    distinct 

strata,  the  outer  portion  of  each  layer  being  the  part  most  deeply 

tinged,  whilst  the  inner  extremities  of  the  prisms  are  almost  colour- 

less. 

This  *  prismatic  '  arrangement"  of  the  carbonate  of  lime  in  the 
shells  of  Pinna  and  its  allies  has  "been  long  familiar  to  concholo- 
gists,  and  regarded  by  them  as  the  result  of  crystallisation.  When 


FIG.  696. — Oblique  section  of  prismatic  shell-substance. 

it  was  first  more  minutely  investigated  by  Mr.  Bowerbaiik1  and  the 
Author,2  and  was  shown  to  be  connected  with  a  similar  arrangement 
in  the  membranous  residuum  left  after  the  decalcification  of  the  shell - 
substance  by  acid,  microscopists  generally  3  agreed  to  regard  it  as  a 
'  calcified  epidermis,'  the  long  prismatic  cells  being  supposed  to  be 
formed  by  the  coalescence  of  the  epidermic  cells  in  piles,  and  giving 

1  '  On  the  Structure  of  the  Shells  of  Molluscous  and  Conchiferous  Animals,'  in 
Trans.  Microsc.  Soc.  ser.  i.  vol.  i.  1844,  p.  123. 

2  '  On  the  Microscopic  Structure  of  Shells  '  in  Reports  of  British  Association  for 
1844  and  1847. 

••  See  Mr.  Quekett's  Histological  Catalogue  of  the  College  of  Surgeons'  Museum 
and  his  Lectures  on  Histology,  vol.  ii. 


922  MOLLUSC  A  AND  BRACHIOPODA 

their  shape  to  the  deposit  of  carbonate  of  lime  formed  within  them. 
The  progress  of  inquiry,  however,  has  led  to  an  important  modifica- 
tion of  this  interpretation,  the  Author  being  now  disposed  to  agree 
with  Huxley  l  in  the  belief  that  the  entire  thickness  of  the  shell 
is  formed  as  an  excretion  from  the  surface  of  the  epidermis,  and 
that  the  horny  layer  which  in  ordinary  shells  forms  their  external 
envelope  or  '  periostracum,'  2  being  here  thrown  out  at  the  same  time 
with  the  calcifying  material,  is  converted  into  the  likeness  of  a 
cellular  membrane  by  the  pressure  of  the  prisms  that  are  formed  by 
crystallisation  at  regular  distances  in  the  midst  of  it.  The  pecu- 
liar conditions  under  which  calcareous  concretions  form  themselves 
in  an  organic  matrix  have  been  carefully  studied  by  Mr.  Rainey 
and  Dr.  W.  M.  Ord,  of  whose  researches  some  account  will  be  given 
hereafter. 

The  internal  layer  of  the  shells  of  the  Margaritacece  and  some 
other  families  has  a  '  nacreous '  or  iridescent  lustre,  which  depends 
(as  Sir  D.  Brewster  has  shown 3)  upon  the  striation  of  its  surface 
with  a  series  of  grooved  lines,  which  usually  run  nearly  parallel  to 
each  other  (fig.  697).  As  these  lines  are  not  obliterated  by  any 
amount  of  polishing,  it  is  obvious  that  their  presence  depends  upon 
something  peculiar  in  the  texture  of  this  substance,  and  not  upon 
any  mere  superficial  arrangement.  When  a  piece  of  the  nacre  (com- 
monly known  as  '  mother  of- pearl ')  of  the  Meleagrina  or  '  pearl-oyster ' 
is  carefully  examined,  it  becomes  evident  that  the  lines  are  produced 
by  the  cropping  out  of  laminae  of  shell  situated  more  or  less  obliquely 
to  the  plane  of  the  surface.  The  greater  the  dip  of  these  laminae,  the 
closer  will  their  edges  be  ;  whilst  the  less  the  angle  which  they  make 
with  the  surface,  the  wider  will  be  the  interval  between  the  lines. 
When  the  section  passes  for  any  distance  in  the  plane  of  a  lamina,  no 
lines  will  present  themselves  on  that  space.  And  thus  the  appearance 
of  a  section  of  nacre  is  such  as  to  have  been  aptly  compared  by  Sir  J. 
Herschel  to  the  surface  of  a  smoothed  deal  board,  in  which  the  woody 
layers  are  cut  perpendicularly  to  their  surface  in  one  part,  and  nearly 
in  their  plane  in  another.  Sir  D.  Brewster  (loc.  cit.)  appears  to  have 
supposed  that  nacre  consists  of  a  multitude  of  layers  of  carbonate  of 
lime  alternating  with  animal  membrane,  and  that  the  presence  of 
the  grooved  lines  on  the  most  highly  polished  surface  is  due  to  the 
wearing  away  of  the  edges  of  the  animal  laminae,  whilst  those  of  the 
hard  calcareous  laminae  stand  out.  If  each  line  upon  the  nacreous 
surface,  however,  indicates  a  distinct  layer  of  shell-substance,  a  very 
thin  section  of  '  mother-of-pearl '  ought  to  contain  many  hundred 
laminae,  in  accordance  with  the  number  of  lines  upon  its  surface, 
these  being  frequently  no  more  than  ^^th  of  an  inch  apart.  But 
when  the  nacre  is  treated  with  dilute  acid,  so  as  to  dissolve  its  cal- 

1  See   his   article,    '  Tegumentary    Organs,'   in    Cyclopedia   of  Anatomy   and 
,  supplementary  volume,  pp.  489-492. 


The  periostracum  is  the  yellowish-brown  membrane  covering  the  surface  of 
many  shells,  which  is  often  (but  erroneously)  termed  their  epidermis. 

5  Phil.  Trans.  1814,  p.  397.— The  late  Mr.  Barton  (of  the  Mint)  succeeded  in 
producing  an  artificial  iridescence  on  metallic  buttons  by  drawing  closely  approxi- 
mating lines  with  a  diamond  point  upon  the  surface  of  the  steel  die  by  which  they 
were  struck. 


STRUCTUEE   OF   SHELLS  923 

careous  portion,  110  such  repetition  of  membranous  layers  is  to  be 
found  ;  on  the  contrary,  if  the  piece  of  nacre  be  the  product  of  one 
act  of  shell  formation,  there  is  but  a  single  layer  of  membrane.  This 
layer,  however,  is  found  to  present  a  more  or  less  folded  or  plaited 
arrangement,  and  the  lineation  of  the  nacreous  surface  may  perhaps 
be  thus  accounted  for.  A  similar  arrangement  is  found  in  pearls, 
which  are  rounded  concretions  projecting  from  the  inner  surface  of 
the  shell  of  Meleayrina,  and  possessing  a  nacreous  structure  corre- 
sponding to  that  of  ;  mother-of-pearl.'  Such  concretions  are  found  in 
many  other  shells,  especially  the  fresh-water  mussels,  Unio  and  Ano- 
don ;  but  these  are  usually  less  remarkable  for  their  pearly  lustre  ; 
and,  when  formed  at  the  edge  of  the  valves,  they  may  be  partly  or 
even  entirely  made  up  of  the  prismatic  substance  of  the  external 
layer,  and  may  be  consequently  altogether  destitute  of  the  pearly 
character. 

In  all  the  genera  of  the  Mwgaritacecv  we  find  the  external  layer 


FIG.  697. — Section  of  nacreous  lining  of  shell  of  Meleagrlna 
margaritifera  (pearl-oyster). 

of  the  shell  prismatic,  and  of  considerable  thickness,  the  internal 
layer  being  nacreous.  But  it  is  only  in  the  shells  of  a  few  families 
of  bivalves  that  the  combination  of  organic  with  mineral  components 
is  seen  in  the  same  distinct  form  ;  and  these  families  are  for  the  most 
part  nearly  allied  to  Pinna.  In  the  Unionidce  (or  'fresh-water 
mussels ')  nearly  the  whole  thickness  of  the  shell  is  made  up  of  the 
internal  or  '  nacreous '  layer  ;  but  a  uniform  stratum  of  prismatic 
substance  is  always  found  between  the  nacre  and  the  periostracum, 
really  constituting  the  inner  layer  of  the  latter,  the  outer  being 
simply  horny.  In  the  Ostreacece  (or  oyster  tribe),  also,  the  greater 
part  of  the  thickness  of  the  shell  is  composed  of  a  '  sub-nacreous  ' 
substance,  representing  the  inner  layer  of  the  shells  of  Margaritaceae, 
its  successively  formed  laminae,  however,  having  very  little  adhesion 
to  each  other ;  and  every  one  of  these  laminae  is  bordered  at  its  free 
edge  by  a  layer  of  the  prismatic  substance  distinguished  by  its 


924 


MOLLUSCA    AND   BBACHIOPODA 


brownish-yellow  colour.  In  these  and  some  other  cast's  a  distinct 
membranous  residuum  is  left  after  the  decalcificatioii  of  the  prismatic 
layer  by  dilute  acid  ;  and  this  is  most  tenacious  and  substantial 
where  (as  in  the  Margaritacece)  there  is  110  proper  periostracum. 
Generally  speaking,  a  thin  prismatic  layer  may  be  detected  upon  the 
external  surface  of  bivalve  shells,  where  this  has  been  protected 
by  a  periostracum,  or  has  been  prevented  in  any  other  mariner 
from  undergoing  abrasion  ;  thus  it  is  found  pretty  generally  in 
Chama,  Trigonia,  and  Solen,  and  occasionally  in  Anomia  and  Pecten. 
In  many  other  instances,  however,  nothing  like  a  cellular  struc- 
ture can  be  distinctly  seen  in  the  delicate  membrane  left  after  decal- 
cification  ;  and  in  such  cases  the  animal  basis  bears  but  a  very  small 
proportion  to  the  calcareous  substance,  and  the  shell  is  usually  ex- 
tremely hard.  This  hardness  ap- 
pears to  depend  upon  the  mineral 
arrangement  of  the  carbonate  of 
lime  ;  for  whilst  in  the  prismatic 
and  ordinary  nacreous  layer  this 
has  the  crystalline  condition  of 
calcife,  it  can  be  shown  in  the  hard 
shell  of  Pholas  to  have  the  arrange- 
ment of  arragonite,  the  difference 
between  the  two  being  made  evi- 
dent by  polarised  light.  A  very 
curious  appearance  is  presented  by 
a  section  of  the  large  hinge-tooth 
of  Mi/a  arenaria  (fig.  698),  in 
which  the  carbonate  of  lime  seems 
to  be  deposited  in  nodules  that 
possess  a  crystalline  structure  re- 
sembling that  of  the  mineral 
termed  wavellite.  Approaches  to 
this  curious  arrangement  are  seen  in  many  other  shells. 

There  are  several  bivalve  shells  which  almost  entirely  consist  of 
what  may  be  termed  a  sub-nacreous  substance,  their  polished 
surfaces  being  marked  by  lines,  but  these  lines  being  destitute  of 
that  regularity  of  arrangement  which  is  necessary  to  produce  the 
iridescent  lustre.  This  is  the  case,  for  example,  with  most  of  the 
Pectinidce  (or  scallop  tribe),  also  with  some  of  the  Mytiluwii'  (or 
mussel  tribe),  and  with  the  common  Oyster.  In  the  internal  layer 
of  by  far  the  greater  number  of  bivalve  shells,  however,  there  is  not 
the  least  approach  to  the  nacreous  aspect ;  nor  is  there  anything 
that  can  be  described  as  definite  structure  ;  and  the  residuum 
left  after  its  decalcificatiori  is  usually  a  structureless  '  basement 
membrane.' 

The  ordinary  account  of  the  mode  of  growth  of  the  shells  of 
bivalve  Mollusca — that  they  are  progressively  enlarged  by  the  depo- 
sition of  new  laminae,  each  of  which  is  in  contact  with  the  internal 
surface  of  the  preceding,  and  extends  beyond  it — does  not  express 
the  whole  truth  ;  for  it  takes  no  account  of  the  fact  that  most  shells 
are  composed  of  two  layers  of  very  different  texture,  and  does  not 


PIG.  698.— Section  of  hinge-tooth  of 
Mi/a  arenaria. 


SHELLS   OF   LAMELLIBRANCHS  925 

specify  whether  both  these  layers  are  thus  formed  by  the  entire 
surface  of  the  ;  mantle '  whenever  the  shell  has  to  be  extended,  or 
whether  only  one  is  produced.  An  examination  of  tig.  699  will 
clearly  show  the  mode  in  which  the  operation  is  effected.  This  figure 
represents  a  section  of  one  of  the  valves  of  Unio  occidens,  taken  per- 
pendicularly to  its  surface,  and  passing  from  the  margin  or  lip  (at 
the  left  hand  of  the  figure)  towards  the  hinge  (which  would  be  at 
some  distance  beyond  the  right).  This  section  brings  into  view  the 
two  substances  of  which  the  shell  is  composed,  traversing  the  outer 
or  prismatic  layer  in  the  direction  of  the  length  of  its  prisms,  and 
passing  through  the  nacreous  lining  in  such  a  manner  as  to  bring 
into  view  its  numerous  laminae,  separated  by  the  lines  a  a',  b  b',  c  c', 
Art-.  These  lines  evidently  indicate  the  successive  formations  of  this 
layer,  and  it  may  be  easily  shown  by  tracing  them  towards  the 
hinge  on  the  one  side  and  towards  the  margin  on  the  other,  that  at 
every  enlargement  of  the  shell  its  whole  interior  is  lined  by  a  new 
nacreous  lamina  in  immediate  contact  with  that  which  preceded  it. 


FIG.  699. — Vertical  section  of  the  lip  of  one  of  the  valves  of  the 
shell  of  Unio  :  a,  b,  c,  successive  formations  of  the  outer 
prismatic  layer  ;  a',  b',  c',  the  same  of  the  inner  nacreous  layer. 

The  number  of  such  laminae,  therefore,  in  the  oldest  part  of  the  shell 
indicates  the  number  of  enlargements  which  it  has  undergone.  The 
outer  or  prismatic  layer  of  the  growing  shell,  on  the  other  hand,  is 
only  formed  where  the  new  structure  projects  beyond  the  margin  of 
the  old  ;  and  thus  we  do  not  find  one  layer  of  it  overlapping  another 
except  at  the  lines  of  junction  of  two  distinct  formations.  When  the 
shell  has  attained  its  full  dimensions,  however,  new  laminae  of  both 
layers  still  continue  to  be  added,  and  thus  the  lip  becomes  thickened 
by  successive  formations  of  prismatic  structure,  each  being  applied 
to  the  inner  surface  of  the  preceding,  instead  of  to  its  free  margin. 
A  like  arrangement  may  be  well  seen  in  the  Oyster,  with  this  differ- 
ence, that  the  successive  layers  have  but  a  comparatively  slight 
adhesion  to  each  other.1 

The  shells  of  Terebratulw  and  of  most  other  Brachiopods  are 
distinguished  by  peculiarities  of  structure  which  differentiate  them 
from  those  of  the  Mollusca.  When  thin  sections  of  them  are 
microscopically  examined,  they  exhibit  the  appearance  of  long  flat- 
tened prisms  (fig.  700,  A,  6),  which  are  arranged  with  such  obliquity 

1   The  most  important  recent  work  on  the  shells  of  Lamellibranchs  is  that  of 
the  lately  deceased  F.  Bernard  ;  see  Bull.  Soc.  Geol.  France,  vols.  xxiii.  and  xxiv. 


926 


MOLLUSCA   AND   BKACHIOPODA 


that  their  rounded  extremities  crop  out  upon  the  inner  surface  of  the 
shell  in  an  imbricated  (tile-like)  manner  (a).  All  true  Terebratulidce, 
both  recent  and  fossil,  exhibit  another  very  remarkable  peculiarity ; 
namely,  the  perforation  of  the  shell  by  a  large  number  of  canals, 


FIG.  700.—  A,  internal  surface,  a,  and  oblique  section,  fc,  of  shell  of  Waldheimia 
australis  ;  B,  external  surface  of  the  same. 

which  generally  pass  nearly  perpendicularly  from  one  surface  to  the 
other  (as  is  shown  in  vertical  sections,  fig.  701),  and  terminate  inter- 
nally by  open  orifices  (fig.  700,  A),  whilst  externally  they  are  covered 

by  the  periost  raciun  (B)  .  Their 
diameter  is  greatest  towards 
the  external  surface,  where 
they  sometimes  expand  sud- 
denly, so  as  to  become  trum- 
pet-shaped ;  and  it  is  usually 
narrowed  rather  suddenly 
when,  as  sometimes  happens, 
a  new  internal  layer  is  formed 
as  a  lining  to  the  preceding 
(fig.  701,  A,  d  d).  Hence  the 
diameter  of  these  canals,  as 
shown  in  different  transverse 
sections  of  one  and  the  same 
shell,  will  vary  according  to 
^e  part  of  its  thickness  wlich 

the  section  happens  to  tra- 
verse.  The  shells  of  different 

<V«**  *  perforated  Sracto- 
pods,  however,  present  very 
striking  diversities  in  the  size  and  closeness  of  their  canals,  as  shown 
by  sections  taken  in  corresponding  parts  ;  three  examples  of  this 
kind  are  given  for  the  sake  of  comparison  in  figs.  702-704.  These 
canals  are  occupied  in  the  living  state  by  tubular  prolongations  of 
the  mantle,  whose  interior  is  filled  with  a  fluid  containing  minute 
cells  and  granules,  which,  from  its  corresponding  in  appearance  with 
the  fluid  contained  in  the  great  sinuses  of  the  mantle,  may  perhaps 


opening   by  large   trumpet-  shaped   orifices 
on  the  outer   surface,   and   contracting   at 


SHELLS  OF  ERACHTOPODA 


927 


be  considered  to  be  the  animal's  blood.  Of  their  special  function  in 
the  economy  of  the  animal  it  is  difficult  to  form  any  probable  idea  ; 
but  it  is  interesting  to  remark  (in  connection  with  the  hypothesis  of 
a  relationship  between  Brachiopods  and  Polyzoa)  that  they  seem  to 
have  their  parallel  in  extensions  of  the  perivisceral  cavity  of  many 
species  of  Flustra,  Eschara,  Lepralia,  &c.,  into  passages  excavated  in 
the  walls  of  the  cells  of  the  polyzoary.  Professor  Sollas  l  finds  in 
the  centre  of  these  prolongations  an  axial  fibre  which  can  be  traced 
backwards  to  the  nerve-cells  of  the  mantle  ;  at  the  distal  end  is  a 
terminal  cell  which  is  connected  by  a  fibril  with  the  axial  fibre,  and 
is  covered  externally  by  a  transparent  chitinous  layer  ;  save  for  the 
absence  (or  the  unproved  presence)  of  pigment  cells  we  should  be 
justified  in  regarding  the  processes  as  organs  which  are  sensitive  to 
luminous  impressions. 

In  the  family  Rhynchonellidce,  which   is    represented    by  only 
six  recent  species,  but  which  contains  a  very  large  proportion  of 


FIG.  702. 


FIG.  703. 


FIG.  704. 


FIG.  702.— Horizontal  section  of  shell  of  Terebratula  bullata  (fossil,  Oolite). 
FIG.  703.  „  „  Megerlia  lima  (fossil,  Chalk). 

FIG.  704.  „  „  Spiriferina  roStrata  (Triassic). 

• 

fossil  Brachiopods,  these  canals  are  almost  entirely  absent  ;  so 
that  the  uniformity  of  their  presence  in  the  Teretoatulidce,  and  their 
general  absence  in  the  Rhynchonellidce,  supply  a  character  of 
great  value  in  the  discrimination  of  the  fossil  shells  belonging 
to  these  two  groups  respectively.  Great  caution  is  necessary, 
however,  in  applying  this  test ;  mere  surface  markings  cannot  be 
relied  on ;  and  no  statement  on  this  point  is  worthy  of  reliance 
which  is  not  based  on  a  microscopic  examination  of  thin  sections  of 
the  shell.  In  the  families  Spiriferidce  and  Strophomenidce,  on  the 
other  hand,  some  species  possess  the  perforations,  whilst  others  are 
destitute  of  them  ;  so  that  their  presence  or  absence  there  serves  only 
to  mark  out  subordinate  groups.  This,  however,  is  what  holds  good 
in  regard  to  characters  of  almost  every  description  in  other  depart- 
ments of  natural  history;  a  character  which  is  of  fundamental 
importance  from  its  close  relation  to  the  general  plan  of  organisation 
in  one  group  being,  from  its  want  of  constancy,  of  far  less  account 
in  another.2 

1  Proc.  Boy.  Dublin  Soc.  v.  318. 

2  For  a  particular  account  of  the  Author's  researches  on  this  group  see  his  memoir 
on  the  subject,  forming  part  of  the  introduction  of  Mr.  Davidson's  Monograph  of  the 


928  MOLLUSCA  AND  BKACHIOPODA 

There  is  not  by  any  means  the  same  amount  of  diversity  in  the 
structure  of  the  shell  in  the  class  of  Gastropods,  a  certain  typical 
plan  of  construction  being  common  to  by  far  the  greater  number  of 
them.  The  small  proportion  of  animal  matter  contained  in  most  of 
these  shells  is  a  very  marked  feature  in  their  character,  and  it 
serves  to  render  other  features  indistinct,  since  the  residuum  left 
after  the  removal  of  the  calcareous  matter  is  usually  so  imperfect  as 
to  give  no  clue  whatever  to  the  explanation  of  the  appearances  shown 
by  sections.  Nevertheless,  the  structure  of  these  shells  is  by  no 
means  homogeneous,  but  always  exhibits  indications,  more  or  less 
clear,  of  a  definite  arrangement.  The  *  porcellanous '  shells  are  com- 
posed of  three  layers,  all  presenting  the  same  kind  of  structure,  but 
each  differing  from  the  others  in  the  mode  in  which  this  is  disposed. 
For  each  layer  is  made  up  of  an  assemblage  of  thin  laminae  placed 
side  by  side,  which  separate  one  from  another,  apparently  in  the 
planes  of  rhomboidal  cleavage,  when  the  shell  is  fractured  ;  and,  as 
was  first  pointed  out  by  Mr.  Bowerbank,  each  of  these  laminae  con- 
sists of  a  series  of  elongated  spicules  (considered  by  him  as  prismatic 
cells  filled  with  carbonate  of  lime)  lying  side  by  side  in  close  apposi- 
tion ;  and  these  series  are  disposed  alternately  in  contrary  directions, 
so  as  to  intersect  each  other  nearly  at  right  angles,  though  still 
lying  in  parallel  planes.  The  direction  of  the  planes  is  different, 
however,  in  the  three  layers  of  the  shell,  bearing  the  same  relation 
to  each  other  as  have  those  three  sides  of  a  cube  which  meet  each 
other  at  the  same  angle  ;  and  by  this  arrangement,  which  is  better 
seen  in  the  fractured  edge  of  the  Cyprcea  or  any  similar  shell  than 
in  thin  sections,  the  strength  of  the  shell  is  greatly  augmented.  A 
similar  arrangement,  obviously  answering  the  same  purpose,  has 
been  shown  by  the  late  Sir  John  Tomes  to  exist  in  the  enamel 
of  the  teeth  of  Rodentia,  and  by  Professor  Rolleston  in  that  of  the 
elephant. 

The  principal  departures  from  this  plan  of  structure  are  seen  in 
Patella,  Chiton,  Haliotis,  Turbo  and  its  allies,  and  in  the  '  naked ' 
Gastropods,  many  of  which  last,  both  terrestrial  and  marine,  have 
some  rudiment  of  a  shell.  Thus  in  the  common  slug,  Limax  rufus, 
a  thin  oval  plate  of  calcareous  texture  is  found  imbedded  in  the 
shield-like  fold  of  the  mantle  covering  the  fore  part  of  its  back  ;  and 
if  this  be  examined  in  an  early  stage  of  its  growth  it  is  found  to 
consist  of  an  aggregation  of  minute  calcareous  nodules,  generally 
somewhat  hexagonal  in  form,  and  sometimes  quite  transparent, 
whilst  in  other  instances  it  presents  an  appearance  closely  resembling 
that  delineated  in  fig.  698.  In  the  epidermis  of  the  mantle  of  some 
species  of  Doris,  on  the  other  hand,  we  find  long  calcareous  spicules, 
generally  lying  in  parallel  directions,  but  not  in  contact  with  each 
other,  giving  firmness  to  the  whole  of  its  dorsal  portion  ;  and  these 
are  sometimes  covered  with  small  tubercles,  like  the  spicules  of 

British  Fossil  Brachiopoda,  published  by  the  Palseontographical  Society.  A  very 
remarkable  example  of  the  importance  of  the  presence  or  absence  of  the  perforations 
in  distinguishing  shells  whose  internal  structure  shows  them  to  be  generically  dif- 
ferent, whilst  from  their  external  conformation  they  would  be  supposed  to  be  not 
only  generically  but  specifically  identical,  will  be  found  in  the  Ann.  Nat.  Hist. 
ser.  iii.  vol.  xx.  1867,  p.  68. 


SHELLS   OF  MOLLUSCA  929 

Gorgonia.  They  may  be  separated  from  the  soft  tissue  in  which 
they  are  imbedded  by  means  of  caustic  potash ;  and  when  treated 
with  dilute  acid,  whereby  the  calcareous  matter  is  dissolved  away, 
an  organic  basis  is  left,  retaining  in  some  degree  the  form  of  the 
original  spicule.  This  basis  seems  to  be  a  cell  in  the  earliest  stage  of 
its  formation,  being  an  isolated  particle  of  protoplasm  without  wall 
or  cavity,  and  the  close  correspondence  between  the  appearance  pre- 
sented bv  thin  sections  of  various  univalve  shells,  and  the  forms  of 
the  spicules  of  Doris,  seems  to  justify  the  conclusion  that  even  the 
most  compact  shells  of  this  group  are  constructed  out  of  the  like 
elements,  in  a  state  of  closer  aggregation  and  more  definite  arrange- 
ment, with  the  occasional  occurrence  of  a  layer  of  more  spheroidal 
bodies  of  the  same  kind,  like  those  forming  the  vestigial  shell  of 
Limax. 

The  structure  of  shells  generally  is  best  examined  by  making 
sections  in  different  planes  as  nearly  parallel  as  may  be  possible  to 
the  surfaces  of  the  shell,  and  other  sections  at  right  angles  to  these  ; 
the  former  may  be  designated  as  horizontal,  the  latter  as  vertical. 
Nothing  need  here  be  added  to  the  full  directions  for  making  such 
sections  which  have  already  been  given.  Many  of  them  are  beautiful 
and  interesting  objects  for  the  polariscope.  Much  valuable  informa- 
tion may  also  be  derived  from  the  examination  of  the  surfaces  pre- 
sented by  fracture.  The  membranous  residua  left  after  the  decalci- 
fication  of  the  shell  by  dilute  acid  may  be  mounted  in  weak  spirit  or 
in  Goadby's  solution. 

The  animals  composing  the  class  of  Cephalopoda  (cuttle-fish  and 
nautilus  tribe)  are  for  the  most  part  without  shells ;  and  the 
structure  of  the  few  that  we  meet  with  in  the  genera  Nautilus,  Argo- 
nauta  ('  paper  nautilus '),  and  Spirula  does  not  present  any  peculi- 
arities that  need  here  detain  us.  The  rudimentary  shell  or  sepiostaire 
of  the  common  cuttle-fish,  however,  which  is  frequently  spoken  of 
as  the  '  cuttle-fish  bone,'  exhibits  a  very  beautiful  and  remarkable 
structure,  such  as  causes  sections  of  it  to  be  very  interesting  micro- 
scopic objects.  The  outer  shelly  portion  of  this  body  consists  of 
horny  layers,  alternating  with  calcified  layers,  in  which  last  may  be 
seen  an  hexagonal  arrangement  somewThat  corresponding  with  that 
shown  in  fig.  698.  The  soft  friable  substance  that  occupies  the  hollow 
of  this  boat-shaped  shell  is  formed  of  a  number  of  delicate  calcareous 
plates  running  across  it  from  one  side  to  the  other  in  parallel 
directions,  but  separated  by  intervals  several  times  wider  than  the 
thickness  of  the  plates ;  and  these  intervals  are  in  great  part  filled 
up  by  what  appear  to  be  fibres  or  slender  pillars  passing  from  one 
plate  or  floor  to  another.  A  more  careful  examination  shows, 
however,  that,  instead  of  a  large  number  of  detached  pillars,  there 
exists  a  comparatively  small  number  of  very  thin  sinuous  laminae, 
which  pass  from  one  surface  to  the  other,  winding  and  doubling  upon 
themselves,  so  that  each  lamina  occupies  a  considerable  space.  Their 
precise  arrangement  is  best  seen  by  examining  the  parallel  plates, 
after  the  sinuous  laminae  have  been  detached  from  them,  the  lines 
of  junction  being  distinctly  indicated  upon  these.  By  this  arrange- 
ment each  layer  is  most  effectually  supported  by  those  with  which 

3o 


930  MOLLUSCA  AND   BKACHIOPODA 

it  is  connected  above  and  below,  and  the  sinuosity  of  the  thin 
intervening  laminse,  answering  exactly  the  same  purpose  as  the 
4  corrugation '  given  to  iron  plates  for  the  sake  of  diminishing  their 
flexibility,  adds  greatly  to  the  strength  of  this  curious  texture, 
which  is  at  the  same  time  lightened  by  the  large  amount  of  open 
space  between  the  parallel  plates  that  intervenes  among  the  sinu- 
osities of  the  laminse.  The  best  method  of  examining  this  structure 
is  to  make  sections  of  it  with  a  sharp  knife  in  various  directions, 
taking  care  that  the  sections  are  no  thicker  than  is  requisite  for 
holding  together  ;  these  may  be  mounted  on  a  black  ground  as 
opaque  objects,  or  in  Canada  balsam  as  transparent  objects,  under 
which  last  aspect  they  furnish  very  beautiful  objects  for  the  polari- 
scope. 

Palate    of   Cephalophorous    Molluscs. — The    organ    which    is 
sometimes    referred    to    under     this    designation,    and   sometimes 

as  the  'tongue,'  is  one  of  a 
very  singular  nature,  and 
cannot  be  likened  to  either 
the  tongue  or  the  palate  of 
higher  animals ;  it  is  best  to 
call  it  by  its  distinctive  name 
'  odontophore.'  For  it  is  a 
tube  that  passes  backwards 

and  downwards  beneath  the 
mouthj  closed  at  its  hinder 

end,  whilst  in  front  it  opens 
obliquely  upon  the  floor  of 

the  mouth,  being  (as  it  were) 
FIG.  705.— Portion  of  the  left  half  of  the  palate  „!.•+  j  eT1T,pflr|  mif,  o0  qo 

of  Helix  hortensis,  the  rows  of  teeth  near  sl">   UP   ana   sPr< 
the  edge  separated  from  each  other  to  show  to  form  a  nearly  flat  surface, 
their  form.  On  the  interior  of  the  tube, 

as  well  as  on  the  flat  expan- 
sion of  it,  we  find  numerous  transverse  rows  of  minute  teeth,  which 
are  set  upon  flattened  plates,  each  principal  tooth  sometimes 
having  a  basal  plate  of  its  own,  whilst  in  other  instances  one  plate 
carries  several  teeth.  Of  the  former  arrangement  we  have  an 
example  in  the  palate  of  many  terrestrial  Gastropods,  such  as  the 
snail  (Helix)  and  slug  (Limax),  in  which  the  number  of  plates  in 
each  row  is  very  considerable  (figs.  705,  706),  amounting  to  180 
in  the  large  garden  slug  (Limax  maximus) ;  whilst  the  latter  prevails 
in  many  marine  Gastropods,  such  as  the  common  whelk  (Buccinutn 
undatum),  the  palate  of  which  has  only  three  plates  in  each  row,  one 
bearing  the  small  central  teeth,  and  the  two  others  the  large  lateral 
teeth  (fig.  709).  The  length  of  the  palatal  tube  and  the  number  of 
rows  of  teeth  vary  greatly  in  different  species.  Generally  speaking, 
the  tube  of  the  terrestrial  Gastropods  is  short,  and  is  contained 
entirely  within  the  nearly  globular  head ;  but  the  rows  of  teeth 
being  closely  set  together  are  usually  very  numerous,  there  being 
frequently  more  than  100,  and  in  some  species  as  many  as  160  or 
170;  so  that  the  total  number  of  teeth  may  mount  up,  as  in  Helix 
pomatia,  to  21,000,  and  in  Limax  maximus  to  26,800.  The  trans- 


PALATES   OF  GASTKOPODA 


931 


FIG.  706. — Palate  of  Hyalinia  cellaria. 


verse  rows  are  usually  more  or  less  curved,  as  shown  in  fig.  706, 
whilst  the  longitudinal  rows  are  quite  straight,  and  the  curvature 
takes  its  departure  on  each  side  from  a  central  longitudinal  row,  the 
teeth  of  which  are  symmetrical,  whilst  those  of  the  lateral  portions 
of  each  transverse  row  present 
a  modification  of  that  symmetry, 
the  prominences  on  the  inner 
side  of  each  tooth  being  sup- 
pressed, whilst  those  on  the  outer 
side  are  increased  ;  this  modifica- 
tion may  be  observed  to  augment 
in  degree  as  we  pass  from  the* 
central  line  towards  the  edges. 

The  palatal  tube  of  the 
marine  Gastropods  is  generally 
longer,  and  its  teeth  larger, 
and  in  many  instances  it  extends 

far  beyond  the  head,  which  may,  indeed,  contain  but  a  small 
part  of  it.  Thus  in  a  common  limpet  (Patella)  we  find  the  principal 
part  of  the  tube  to  lie  folded  up,  but  perfectly  free,  in  the  abdominal 
cavity,  between  the  greatly  elongated  intestine  and  the  muscular 
foot ;  and  in  some  species  its  length  is  twice  or  even  three  times  as 
great  as  that  of  the  entire  animal.  In  a  large  proportion  of  cases 
these  palates  exhibit  a  very  marked  separation  between  the  central 
and  the  lateral  portions  (figs. 
707,  708),  the  teeth  of  the  cen- 
tral band  being  frequently  small 
and  smooth  at  their  edges, 
whilst  those  of  the  lateral  are 
large  and  serrated.  The  palate 
of  Trochus  zizyphimts,  repre- 
sented in  fig.  707,  is  one  of  the 
most  beautiful  examples  of  this 
form,  not  only  the  large  teeth 
of  the  lateral  bands,  but  the 
delicate  leaf-like  teeth  of  the 
central  portion  having  their 
edges  minutely  serrated.  A  yet 
more  complex  type,  however,  is 
found  in  the  palate  of  Haliotis, 
in  which  there  is  a  central  band 

of  teeth  having  nearly  straight    FIG.  707. — Palate  of  Trochus  zizypliinus. 
edges  instead  of  points  ;  then,  on 

each  side,  a  lateral  band  consisting  of  large  teeth  shaped  like  those 
of  the  shark  ;  and  beyond  this,  again,  another  lateral  band  on  either 
side,  composed  of  several  rows  of  smaller  teeth.  Very  curious 
differences  also  present  themselves  among  the  different  species  of 
the  same  genus.  Thus  in  Doris  pilosa  the  central  band  is  almost 
entirely  wanting,  and  each  lateral  band  is  formed  of  a  single  row 
of  very  large  hooked  teeth,  set  obliquely  like  those  of  the  lateral 
band  in  fig.  707  ;  whilst  in  Doris  tuberculata  the  central  band  is  the 

3  o  2 


932 


MOLLUSCA  AND  BBACHIOPODA 


part  most  developed,  and  contains  a  number  of  rows  of  conical  teeth, 
standing  almost  perpendicularly,  like  those  of  a  harrow  (fig.  708). 

Many  other  varieties  might  be  described  did  space  permit ;  but 
we  must  be  content  with  adding  that  the  form  and  arrangement  of 
the  teeth  of  these  '  palates '  afford  characters  of  great  value  in  classi- 
fication, as  was  first  pointed  out  by  Professor  Loven  (of  Stockholm) 
in  1847,  and  has  been  since  very  strongly  urged  by  Dr.  J.  E.  Gray, 
who  considers  that  the  structure  of  these  organs  is  one  of  the  best 
guides  to  the  natural  affinities  of  the  species,  genera,  and  families  of 
this  group,  since  any  important  alteration  in  the  form  or  position  of 
the  teeth  must  be  accompanied  by  some  corresponding  peculiarity  in 
the  habits  and  food  of  the  animal.1  Hence  a  systematic  examination 
and  delineation  of  the  structure  and  arrangement  of  these  organs,  by 
the  aid  of  the  microscope  and  camera  lucida,  would  be  of  the  greatest 
service  to  this  department  of  natural  history.  The  short  thick  tube 

of  Limax  and  other  terrestrial 
Gastropods  appears  adapted  for 
the  trituration  of  the  food  pre- 
viously to  its  passing  into  the 
oesophagus  ;  for  in  these  animals 
we  find  the  roof  of  the  mouth 
furnished  with  a  large  strong 
horny  plate,  against  which  the 
flat  end  of  the  tongue  can  work. 
On  the  other  hand,  the  flattened 
portion  of  the  palate  of  Bucci- 
num  (whelk)  and  its  allies  is 
used  by  these  animals  as  a  file, 
with  which  they  bore  holes 
through  the  shells  of  the  molluscs 
that  serve  as  their  prey ;  this 

they  are  enabled  to  effect  by  everting  that  part  of  the  proboscis- 
shaped  mouth  whose  floor  is  formed  by  the  flattened  part  of  the 
tube,  which  is  thus  brought  to  the  exterior,  and  by  giving  a  kind  of 
sawing  motion  to  the  organ  by  means  of  the  alternate  action  of 
two  pairs  of  muscles — a  protractor  and  a  retractor — which  put 
forth  and  draw  back  a  pair  of  cartilages  whereon  the  tongue  is 
supported,  and  also  elevate  and  depress  its  teeth.  The  use  of  the 
long  blind  tubular  part  of  the  palate  in  these  Gastropods  is  that 
of  a  '  cavity  of  reserve,'  from  which  a  new  toothed  surface  may  be 
continually  supplied  as  the  old  one  is  worn  away — somewhat  as  the 
front  teeth  of  the  rodents  are  constantly  being  regenerated  from  the 
surface  of  the  pulps  which  occupy  their  hollow  conical  bases — as  fast 
as  they  are  rubbed  down  at  their  edges,  or  as  a  nail  is  constantly 
being  worn  away  at  its  free  end,  and  fashioned  anew  in  its 
1  bed.' 

The  preparation  of  these  palates  for  the  microscope  can,  of  course, 
be  only  accomplished  by  carefully  dissecting  them  from  their  attach- 
ments within  the  head  ;  and  it  will  be  also  necessary  to  remove  the 
membrane   that  forms  the  sheath  of  the  tube,  when  this  is  thick 
1  Ann.  Nat,  Hist.  ser.  ii.  vol.  x.  1852,  p.  413. 


FIG.  708. — Palate  of  Doris  tuberculata. 


DEVELOPMENT    OF   MOLLUSC  A  933 

enough  to  interfere  with  its  transparence.  The  tube  itself  should  be 
slit  up  with  a  pair  of  fine  scissors  through  its  entire  length,  and 
should  be  so  opened  out  that  its  expanded 
surface  may  be  a  continuation  of  that 
which  forms  the  floor  of  the  mouth.  The 
mode  of  mounting  it  will  depend  upon  the 
manner  in  which  it  is  to  be  viewed.  For 
the  ordinary  purposes  of  microscopic  ex- 
amination no  method  is  so  good  as  mount- 
ing in  fluid,  either  weak  spirit  or  Goadby's 
solution  answering  very  well.  But  many 
of  these  palates,  especially  those  of  the 
marine  Gastropods,  become  most  beautiful 
objects  for  the  polariscope  when  they  are 
mounted  in  Canada  balsam,  the  form 
and  arrangement  of  the  teeth  being  very 
strongly  brought  out  by  it  (fig.  709),  and 

a  gorgeous  play  of  colours  being  exhibited  _^ 
when  a  selenite  plate  is  placed  behind  the  FIG    709._Palate  of   Buc< 
object,  and  the  analysing  prism  is  made  to       numundatum  as  seen  under 
rotate.1  polarised  light. 

Development  of  Molluscs. — Leaving  to 

the  scientific  embryologist  the  large  field  of  study  that  lies  open  to 
him  in  this  direction,2  the  ordinary  microscopist  will  find  much  to 
interest  him  in  the  observation  of  certain  special  phenomena  of 
which  a  general  account  will  be  here  given.  Attached  to  the  gills  of 
fresh-water  mussels  (Unio  and  Anodon)  there  are  often  found  in  the 
spring  or  early  summer  minute  bodies  which,  when  first  observed, 
were  described  as  parasites,  under  the  name  of  Glochidia,  but  are 
now  known  to  be  their  own  progeny  in  an  early  phase  of  develop- 
ment. When  they  are  expelled  from  between  the  valves  of  their 
parent,  they  attach  themselves  in  a  peculiar  manner  to  the  fins  and 
gills  of  fresh- water  fish.  In  this  stage  of  the  existence  of  the  young 
Anodon,  its  valves  are  provided  with  curious  barbed  or  serrated 
hooks  (fig.  710,  A),  and  are  continually  snapping  together,  until 
they  have  inserted  their  hooks  into  the  skin  of  the  fish,  which  seems 
so  to  retain  the  barbs  as  to  prevent  the  reopening  of  the  valves.  In 
this  stage  of  its  existence  no  internal  organ  is  definitely  formed, 
except  the  strong  '  adductor '  muscle  (aad)  which  draws  the  valves 
together,  and  the  long,  slender  byssus-filament  (&?/)  which  makes 
its  appearance  while  the  embryo  is  still  within  the  egg-mem- 
brane, lying  coiled  up  between  the  lateral  lobes.  The  hollow  of 
each  valve  is  filled  with  a  soft  granular-looking  mass,  in  which 
are  to  be  distinguished  what  are  perhaps  the  rudiments  of  the 

1  For   additional   details   on  the   organisation   of   the  palate    and   teeth   of  the 
Gastropod  molluscs,  see  Mr.   W.  Thomson  in  Cyclop.  Anat.  and  Physiol.  vol.  iv. 
pp.  1142,  1143,  and  in  Ann.  Nat.  Hist.  ser.  ii.  vol.  vii.  p.  86 ;  Professor  Troschel,  Das 
Gebiss  der  Schneclten,  Berlin,  1856-79  ;  A.  Riicker,  '  Ueber  die  Bildung  der  Radula 
bei  Helix  pomatia,'  Bericht  oberhess.  Gesellsch.  Giessen,  xxii.  p.  209  ;  P.  Geddes, '  On 
the  Mechanism  of  the  Odontophore  in  certain  Molluscs,'  Trans.  Zb'ol.  Soc.  x.  p.  485. 

2  See  Balfour's  Comparative  Embryology,  vol.  i.  chap.  ix.     More  recent  text- 
books of  embryology,  such  as  that  of  Professor  Korschelt  and  Heider,  need  not  here 
be  specifically  cited. 


934  MOLLUSCA  AND  BKACHIOPODA 

branchi.e  and  of  oral  tentacles ;  but  their  nature  can  only  be  cer- 
tainly determined  by  further  observation,  which  is  rendered  difficult 
by  the  opacity  of  the  valves.  By  keeping  a  supply  of  fish,  however, 
with  these  embryos  attached,  the  entire  history  of  the  development 
of  the  fresh- water  mussel  may  be  worked  out.1 

In  certain  members  of  the  class  Gastropoda  the  history  of  em- 
bryonic development  presents  numerous  phenomena  of  great  interest. 
The  eggs  (save  among  the  terrestrial  species)  are  usually  deposited  in 
aggregate  masses,  each  inclosed  in  a  common  protective  envelope  or 
nidamentum.  The  nature  of  this  envelope,  however,  varies  greatly  ; 
thus,  in  the  common  Limnceus  stagnalis,  or  '  water-snail,'  of  our  ponds 
and  ditches  it  is  nothing  else  than  a  mass  of  soft  jelly,  about  the  size 
of  a  sixpence,  in  which  from  fifty  to  sixty  eggs  are  imbedded,  and 
which  is  attached  to  the  leaves  or  stems  of  aquatic  plants ;  in  the 
Buccinum  undatum,  or  common  whelk,  it  is  a  membranous  case, 


•p.  ad 


FIG.  710. — A,  Glochidium  immediately  after  it  is  hatched  :  ad,  ad- 
ductor ;  sh,  shell ;  by,  byssus-cord  ;  s,  sense-organs.  B,  the  same 
after  it  has  been  on  the  fish  for  some  weeks  :  br,  branchiae  ;  auv, 
auditory  sac;  /,  food;  a.ad  and  p. ad,  anterior  and  posterior 
adductors ;  al,  mesenteron  ;  mt,  mantle. 

connected  with  a  considerable  number  of  similar  cases  by  short  stalks, 
so  as  to  form  large  globular  masses  which  may  often  be  picked  up  on 
our  shores,  especially  between  April  and  June ;  in  the  Pwrpura 
lapillus,  or  '  rock-whelk,'  it  is  a  little  flask-shaped  capsule,  having 
a  firm  horny  wall,  which  is  attached  by  a  short  stem  to  the  surface 
of  rocks  between  tide  marks,  great  numbers  being  often  found 
standing  erect  side  by  side ;  whilst  in  the  JSTudibranchiate  order 
generally  (consisting  of  the  Doris,  JSolis,  and  other  '  sea-slugs ')  it 
forms  a  long  tube  with  a  membranous  wall,  in  which  immense 
numbers  of  eggs  (even  half  a  million  or  more)  are  packed  closely 
together  in  the  midst  of  a  jelly-like  substance,  this  tube  being  disposed 
in  coils  of  various  forms,  which  are  usually  attached  to  seaweeds  or 
zoophytes.  The  course  of  development,  in  the  first  and  last  of  these 
instances,  may  be  readily  observed  from  the  very  earliest  period  down 

1  See  the  Kev.  W.  Houghton,  '  On  the  Parasitic  Nature  of  the  Fry  of  the  Ano- 
donta  cygnea,'  in  Quart.  Journ.  Microsc.  Sci.  n.s.  vol.  ii.  1861,  p.  162,  and  especially 
Balfour,  op.  cit.  pp.  220-223.  On  the  embryonal  byssus-gland  of  Anodonta,  see 
J.  Carriere,  Zoolog.  Anzeig.  vii.  p.  41. 


DEVELOPMENT   OF   DORIS 


935 


to  that  of  the  emersion  of  the  embryo,  owing  to  the  extreme  trans- 
parence of  the  nidamentum  and  of  the  egg-membranes  themselves. 
The  first  change  which  will  be  noticed  by  the  ordinary  observer  is 
the  '  segmentation '  of  the  yolk-mass,  which  divides  itself  (after  the 
manner  of  a  cell  undergoing  binary  subdivision)  into  two  parts,  each 
of  these  two  into  two  others,  and  so  on  until  a  m&rula,  or  mulberry- 
like  mass  of  minute  yolk-segments,  is  produced  (fig.  711,  A-F), 
which  is  converted  by  '  invagination '  into  a  '  gastrula,'  whose  form 


FIG.  711. — Embryonic  development  of  Doris  bilamellata  :  A,  ovum,  consist- 
ing of  enveloping  membrane,  a,  and  yolk,  b  ;  B,  C,  D,  E,  F,  successive 
stages  of  segmentation  of  yolk ;  G,  first  marking  out  of  the  shape  of  the 
embryo ;  H,  embryo  on  the  eighth  day  ;  I,  the  same  on  the  ninth  day ;  K,  the 
same  on  the  twelfth  day,  seen  on  the  left  side  at  L  ;  M,  still  more  advanced 
embryo,  seen  at  N  as  retracted  within  its  shell ;  a,  position  of  shell-gland ; 
c,  c,  ciliated  lobes ;  d,  foot ;  g,  hard  plate  or  operculum  attached  to  it ; 
h,  stomach ;  i,  intestine ;  m,  n,  masses  (glandular  ?)  at  the  sides  of  the 
oesophagus;  o,  heart  (?) ;  s,  retractor  muscle  (?) ;  t,  situation  of  funnel; 
v,  membrane  enveloping  the  body  ;  x,  auditory  vesicles ;  y,  mouth. 


936  MOLLUSCA  AND  BEACHIOPODA 

is  shown  at  G.  This  'gastrula'  soon  begins  to  exhibit  a  very  curious 
alternating  rotation  within  the  egg,  two  or  three  turns  being  made 
in  one  direction,  and  the  same  number  in  a  reverse  direction  :  this 
movement  is  due  to  the  cilia  fringing  a  sort  of  fold  of  the  ecto- 
derm termed  the  velum,  which  afterwards  usually  gives  origin  to  a 
pair  of  large  ciliated  lobes  (H-L,  c)  resembling  those  of  Rotifers. 
The  velum  is  so  little  developed  in  Limnceus,  however,  that  its 
existence  was  commonly  overlooked  until  recognised  by  Professor 
Ray  Lankester,1  who  also  has  been  able  to  distinguish  its  fringe  of 
minute  cilia.  This,  however,  has  only  a  transitory  existence ;  and 
the  later  rotation  of  the  embryo,  which  presents  a  very  curious 
spectacle  when  a  number  of  ova  are  viewed  at  once  under  a  low 
magnifying  power,  is  due  to  the  action  of  the  cilia  fringing  the  head 
and  foot. 

A  separation  is  usually  seen  at  an  early  period  between  the 
anterior  or  'cephalic'  portion,  and  the  posterior  or  'visceral'  portion, 
of  the  embryonic  mass,  and  the  development  of  the  former  advances 
with  the  greater  activity.  One  of  the  first  changes  which  are  seen  in 
it  consists  in  its  extension  into  a  sort  of  fin-like  membrane  on  either 
side,  the  edges  of  which  are  fringed  with  long  cilia  (fig.  711,  H-L,  c), 
whose  movements  may  be  clearly  distinguished  whilst  the  embryo  is 
still  shut  up  within  the  egg  ;  at  a  very  early  period  may  also  be  dis- 
cerned the '  auditory  vesicles'  (K,  x)  or  rudimentary  organs  of  hearing, 
which  scarcely  attain  any  higher  development  in  these  creatures 
during  the  whole  of  life  ;  and  from  the  immediate  neighbourhood  of 
these  is  put  forth  a  projection,  which  is  afterwards  to  be  evolved  into 
the  '  foot '  or  muscular  disc  of  the  animal.  While  these  organs  are 
making  their  appearance,  the  shell  is  being  formed  on  the  surface  of 
the  posterior  portion,  appearing  first  as  a  thin  covering  over  its  hinder 
part  and  gradually  extending  itself  until  it  becomes  large  enough  to 
inclose  the  embryo  completely,  when  this  contracts  itself.  The 
ciliated  lobes  are  best  seen  in  the  embryos  of  Nudibranchs ;  and  the 
fact  of  the  universal  presence  of  a  shell  in  the  embryos  of  that  group 
is  of  peculiar  interest,  as  it  is  destined  to  be  cast  off  very  soon  after 
they  enter  upon  active  life.  These  embryos  may  be  seen  to  move 
about,  as  freely  as  the  narrowness  of  their  prison  permits,  for  some 
time  previous  to  their  emersion ;  and  when  set  free  by  the  rupture 
of  the  egg-cases  they  swim  forth  with  great  activity  by  the  action 
of  their  ciliated  lobes — these,  like  the  'wheels'  of  Rotif era,  serving  also 
to  bring  food  to  the  mouth,  which  is  at  that  time  unprovided  with 
the  reducing  apparatus  subsequently  found  in  it.  The  same  is  true 
of  the  embryo  of  Lymnceus,  save  that  its  swimming  movements  are 
less  active,  in  consequence  of  the  non-development  of  the  ciliated 
lobes  ;  and  the  currents  produced  by  the  cilia  that  fringe  the  head 
and  the  orifice  of  the  respiratory  sac  seem  to  have  reference  chiefly 
to  the  provision  of  supplies  of  food  and  of  aerated  water  for  respira- 

1  See  his  valuable  '  Observations  on  the  Development  of  Limnceits  stagnalis  and 
on  the  early  stages  of  other  Mollusca '  in  Quart.  Journ.  -Microsc.  Sci.  October  1874 ; 
and  '  On  the  Developmental  History  of  the  Mollusca,'  Phil.  Trans.  1875.  See  also 
Lereboullet,  '  Recherches  sur  le  Developpement  du  Limne'e,'  in  Ann.  des  Sci.  Nat. 
Zool.  4e  se'rie,  torn,  xviii.  p.  47. 


DEVELOPMENT   OF  PUEPUEA 


937 


tion.  The  disappearance  of  the  cilia  has  been  observed  by  Mr.  Hogg 
to  be  coincident  with  the  development  of  the  teeth  to  a  degree  suf- 
ficient to  enable  the  young  water-snail  to  crop  its  vegetable  food ; 
and  he  has  further  ascertained  that  if  the  growing  animal  be  kept  in 
fresh  water  alone  for  some  time,  without  vegetable  matter  of  any 
kind,  the  gastric  teeth  are  very  imperfectly  developed,  and  the  cilia 
are  still  retained.1 

A  very  curious  modification  of  the  ordinary  plan  of  development 
is  presented  in  Purpura  lapillus,  and  it  is  probable  that  something 
of  the  same  kind  exists  also  in  Buccinum,  as  well  as  in  other  Gas- 
tropods of  the  same  extensive  order1  (Pectinibratwhiata).  Each  of 
the  capsules  already  described  contains  from  500  to  600  egg-like 
bodies  (fig.  712,  A)  imbedded  in>a  viscid  gelatinous  substance  ;  but 
only  from  twelve  to  thirty  embryos  usually  attain  complete  develop- 
ment, and  it  is  obvious,  from  the  large  comparative  size  which  these 
attain  (fig.  713,  B),  that  each  of 
them  must  include  an  amount  of 
substance  equal  to  that  of  a  great 
number  of  the  bodies  originally 
found  within  the  capsule.  The 
explanation  of  this  fact  (long 
since  noticed  by  Dr.  J.  E.  Gray 
in  regard  to  Buccinum)  seems  to 
be  as  follows.  Of  those  500  or 
600  egg-like  bodies,  only  a  small 
part  are  fertile  ova,  the  remainder 
being  unfertilised  eggs,  the  yolk 
material  of  which  serves  for  the 
nutrition  of  the  embryos  in  the  FIG. 
later  stages  of  their  intracapsular 
life.  The  distinction  between 
them  manifests  itself  at  a  very 
early  period,  even  in  the  first 
segmentation ;  for,  while  the  latter 

divide  into  two  equal  hemispheres  (fig.  712,  B),  the  fertilised  ova 
divide  into  a  larger  and  a  smaller  segment  (D) ;  in  the  cleft  between 
these  are  seen  the  minute  *  directive  vesicles,'  which  appear  to  be 
always  double,  although  from  being  seen  '  end  on,'  only  one  may 
be  visible  ;  and  near  these  is  generally  to  be  seen  a  clear  space 
in  each  segment.  The  difference  is  still  more  strongly  marked  in 
the  subsequent  divisions  ;  for  whilst  the  cleavage  of  the  infertile 
eggs  goes  on  irregularly,  so  as  to  divide  each  into  from  fourteen  to 
twenty  segments,  having  no  definiteness  of  arrangement  (0,  E,  F,  G), 
that  of  the  fertile  ova  takes  place  in  such  a  manner  as  to  mark  out 
the  distinction  already  alluded  to  between  the  '  cephalic '  and  the 
'visceral'  portions  of  the  mass  (H),  and  the  evolution  of  the 
former  into  distinct  organs  very  speedily  commences.  In  the  first 
instance  a  narrow  transparent  border  is  seen  around  the  whole 
embryonic  mass,  which  is  broader  at  the  cephalic  portion  (I) ;  next, 


712. — Early  stages  of  embryonic 
development  of  Purpura  lapillus:  A, 
egg-like  spherule ;  B,  C,  E,  F,  G,  suc- 
cessive stages  of  segmentation  of  yolk- 
spherules  ;  D,  H,  I,  J,  K,  successive 
stages  of  development  of  early  embryos. 


See  Trans.  Microsc.  Soc.  ser.  ii.  vol.  ii.  1854,  p.  93. 


938 


MOLLUSCA  AND   BKACHIOPODA 


this  border  is  fringed  with  short  cilia,  and  the  cephalic  extension 
into  two  lobes  begins  to  show  itself;  and  then  between  the  lobes  a 
large  mouth  is  formed,  opening  through  a  short  wide  oesophagus, 
the  interior  of  which  is  ciliated,  into  the  visceral  cavity,  occu- 
pied as  yet  only  by  the  yolk-particles  originally  belonging  to  the 
ovum  (K). 

Whilst  these  developmental  changes  are  taking  place  in  the  embryo, 
the  whole  aggregate  of  segments  formed  by  the  yolk-cleavage  of  the 
infertile  eggs  coalesces  into  one  mass,  as  shown  at  A,  fig.  713  ;  and 
the  embryos  are  often,  in  the  first  instance,  so  completely  buried 
within  this  as  only  to  be  discoverable  by  tearing  its  portions  asunder  ; 
but  some  of  them  may  commonly  be  found  upon  its  exterior,  and 
those  contained  in  one  capsule  very  commonly  exhibit  the  different 


FIG.  713. — Later  stages  of  embryonic  development  of  Purpura  lapillus. 
A,  conglomerate  mass  of  vitelline  segments,  to  which  were  attached  the 
embryos  a,  6,  c,  d,  e.  B,  full-sized  embryo  in  more  advanced  stage  of 
development. 


stages  of  development  represented  in  fig.  712,  H-K.  After  a  short 
time,  however,  it  becomes  apparent  that  the  most  advanced  embryos 
are  beginning  to  swallow  the  yolk  segments  of  the  conglomerate  mass, 
and  capsules  will  not  unfrequently  be  met  with  in  which  embryos 
of  various  sizes,  as  a,  b,  c,  d,  e  (fig.  713,  A),  are  projecting  from  its 
surface,  their  difference  of  size  not  being  accompanied  by  advance  in 
development,  but  merely  depending  upon  the  amount  of  this  *  supple- 
mental' yolk  which  the  embryos  have  respectively  gulped  down. 
For  during  the  time  in  which  they  are  engaged  in  appropriating  this 
additional  supply  of  nutriment,  although  they  increase  in  size,  yet 
they  scarcely  exhibit  any  other  change  ;  so  that  the  large  embryo, 
fig.  713,  e,  is  not  apparently  more  advanced,  as  regards  the  formation 
of  its  organs,  than  the  small  embryo,  fig.  712,  K.  So  soon  as  this 
operation  has  been  completed,  however,  and  the  embryo  has  attained 
its  full  bulk,  the  evolution  of  .its  organs  takes  place  very  rapidly  ;  the 


DEVELOPMENT   OF   PURPUKA  939 

ciliated  lobes  are  much  more  highly  developed,  being  extended  in  a 
long  sinuous  margin,  so  as  almost  to  remind  the  observer  of  the 
'  wheels '  of  Rotifera,  and  being  furnished  with  very  long  cilia  (fig. 
713,  B) ;  the  auditory  vesicles,  the  tentacula,  the  eyes,  and  the  foot 
successively  make  their  appearance  ;  a  curious  rhythmically  contractile 
vesicle  is  seen,  just  beneath  the  edge  of  the  shell  in  the  region  of  the 
neck,  which  may,  perhaps,  serve  as  a  temporary  heart ;  a  little  later 
the  real  heart  may  be  seen  pulsating  beneath  the  dorsal  part  of  the 
shell ;  and  the  mass  of  yolk -segments  of  which  the  body  is  made  up 
gradually  shapes  itself  into  the  various  organs  of  digestion,  respira- 
tion, &c.,  during  the  evolution  of  which  (and  while  they  are  as  yet  far 
from  complete)  the  capsule  thins  away  at  its  summit  and  the  embryos 
make  their  escape  from  it. l  > 

It  happens  not  unfrequently  that  one  of  the  embryos  which  a 
capsule  contains  does  not  acquire  its  '  supplemental '  yolk  in  the 
manner  now  described,  and  can  only  proceed  in  its  development  as  far 
as  its  original  yolk  will  afford  it  material ;  and  thus,  at  the  time  when 
the  other  embryos  have  attained  their  full  size  and  maturity,  a  strange- 
looking  creature,  consisting  of  two  large  ciliated  lobes  with  scarcely 
the  rudiment  of  a  body,  may  be  seen  in  active  motion  among  them. 
This  may  happen,  indeed,  not  only  to  one,  but  to  several  embryos 
within  the  same  capsule,  especially  if  their  number  should  be  con- 
siderable ;  for  it  sometimes  appears  as  if  there  were  not  food  enough 
for  all,  so  that,  whilst  some  attain  their  full  dimensions  and  complete 
development,  others  remain  of  unusually  small  size,  without  being 
deficient  in  any  of  their  organs ;  and  others,  again,  are  more  or  less 
completely  abortive — the  supply  of  supplemental  yolk  which  they 
have  obtained  having  been  too  small  for  the  development  of  their 
viscera,  although  it  may  have  afforded  what  was  needed  for  that  of 
the  ciliated  lobes,  eyes,  tentacles,  auditory  vesicles,  and  even  the 
foot — or,  on  the  other  hand,  no  additional  supply  whatever  having 
been  acquired  by  them,  so  that  their  development  has  been  arrested 
at  a  still  earlier  stage.  These  phenomena  are  of  so  remarkable  a 
character  that  they  furnish  an  abundant  source  of  interest  to  any 
niicroscopist  who  may  happen  to  be  spending  the  months  of  August 
and  September  in  a  locality  in  which  the  Purpura  abounds ;  since, 
by  opening  a  sufficient  number  of  capsules,  no  difficulty  need  be 
experienced  in  arriving  at  all  the  facts  which  have  been  noticed  in 
this  brief  summary.2  It  is  much  to  be  desired  that  such  microscopists 

1  The  Author  thinks  it  worth  while  to  mention  the  method  which  he  has  found 
most  convenient  for  examining  the  contents  of  the  egg-capsules  of  Purpura,  as  he 
believes  that  it  may  be  advantageously  adopted  in  many  other  cases.  This  consists 
in  cutting  off  the  two  ends  of  the  capsule  (taking  care  not  to  cut  far  into  its  cavity), 
and  in  then  forcing  a  jet  of  water  through  it  by  inserting  the  end  of  a  fine-pointed 
syringe  into  one  of  the  orifices  thus  made,  so  as  to  drive  the  contents  of  the  capsule 
before  it  through  the  other.  These  should  be  received  into  a  shallow  cell  and  first 
examined  under  the  simple  microscope.  For  some  further  observations  on  the  de- 
velopment of  Purpura,  see  Professor  Haddon,  '  Notes  on  the  Development  of  the 
Mollusca,'  Quart.  Journ.  Microsc.  Sci.  xxii.  p.  367. 

-  Fuller  details  on  this  subject  will  be  found  in  the  Author's  account  of  his  re- 
searches in  Trans.  Microsc.  Soc.  ser.  ii.  vol.  iii.  1855,  p.  17.  His  account  of  the 
process  was  called  in  question  by  MM.  Koren  and  Danielssen,  who  had  previously 
given  an  entirely  different  version  of  it,  but  was  fully  confirmed  by  the  observations 
of  Dr.  Dyster.  See  Ann.  Nat.  Hist.  ser.  ii.  vol.  xx.  1857,  p.  16.  The  independent 


940  MOLLUSCA  AND   BRACHIOPODA 

as  possess  the  requisite  opportunity  would  apply  themselves  to  the 
study  of  the  corresponding  history  in  other  Pectiiiibranchiate  Gastro- 
pods, with  a  view  of  determining  how  far  the  plan  now  described 
prevails  through  the  order.  And  now  that  these  molluscs  have  been 
brought  not  only  to  live,  but  to  breed,  in  artificial  aquaria,  it  may  be 
anticipated  that  a  great  addition  to  our  knowledge  of  this  part  of 
their  life-history  will  ere  long  be  made. 

Ciliary  Motion  on  Gills. — There  is  no  object  that  is  better 
suited  to  exhibit  the  general  phenomena  of  ciliary  motion  than  a 
portion  of  the  gill  of  some  bivalve  mollusc.  The  Oyster  will  answer 
the  purpose  sufficiently  well ;  but  the  cilia  are  much  larger  on  the 
gills  of  the  Mussel  (Mytilus),1  as  they  are  also  on  those  of  the  Anodon 
or  common  '  fresh-water  mussel '  of  our  ponds  and  streams.  Nothing 
more  is  necessary  than  to  detach  a  small  portion  of  one  of  the  ribbon- 
like  bands  which  will  be  seen  running  parallel  with  the  edge  of  each 
of  the  valves  when  the  shell  is  opened,  and  to  place  this,  with  a 
little  of  the  liquor  contained  within  the  shell,  upon  a  slip  of  glass — 
taking  care  to  spread  it  out  sufficiently  with  needles  to  separate  the 
bars  of  which  it  is  composed,  since  it  is  on  the  edges  of  these,  and 
round  their  knobbed  extremities,  that  the  ciliary  movement  presents 
itself — and  then  covering  it  with  a  thin  glass  disc.  Or  it  will  be 
convenient  to  place  the  object  in  the  aquatic  box,  which  will  enable 
the  observer  to  subject  it  to  any  degree  of  pressure  that  he  may  find 
convenient.  A  magnifying  power  of  about  120  diameters  is  amply 
sufficient  to  afford  a  general  view  of  this  spectacle  ;  but  a  much 
greater  amplification  is  needed  to  bring  into  view  the  peculiar  mode  in 
which  the  stroke  of  each  cilium  is  made.  Few  spectacles  are  more 
striking  to  the  unprepared  mind  than  the  exhibition  of  such  won- 
derful activity  as  will  then  become  apparent  in  a  body  which  to  all 
ordinary  observation  is  so  inert.  This  activity  serves  a  double  pur- 
pose ;  for  it  not  only  drives  a  continual  current  of  water  over  the 
surface  of  the  gills  themselves,  so  as  to  effect  the  aeration  of  the 
blood,  but  also  directs  a  portion  of  this  current  to  the  mouth,  so 
as  to  supply  the  digestive  apparatus  with  the  aliment  afforded  by 
the  Diatomacece,  Infusoria,  &c.  which  it  carries  in  with  it. 

Organs  of  Sense  of  Molluscs. — Some  of  the  minuter  and  more 
rudimentary  forms  of  the  special  organs  of  sight,  hearing,  and  touch 
which  the  molluscous  series  presents  are  very  interesting  objects  of 
microscopic  examination.  Thus,  just  within  the  margin  of  each  valve 
of  Pecten,  we  see  (when  we  observe  the  animal  in  its  living  state 
under  water)  a  row  of  minute  circular  points  of  great  brilliancy,  each 
surrounded  by  a  dark  ring ;  these  are  the  eyes  with  which  this 
creature  is  provided,  and  by  which  its  peculiarly  active  movements 
are  directed.  Each  of  them,  when  their  structure  is  carefully  exa- 
mined, is  found  to  be  protected  by  a  sclerotic  coat  with  a  transparent 

observations  of  M.  Claparede  on  the  development  of  Neritina  fluviatilis  (Midler's 
Archiv,  1857,  p.  109,  and  abstract  in  Ami.  of  Nat.  Hist.  ser.  ii.  vol.  xx.  1857,  p.  196) 
showed  the  mode  of  development  in  that  species  to  be  the  same  in  all  essential  par- 
ticulars as  that  of  Purpura.  The  subject  has  again  been  recently  studied  with  great 
minuteness  by  Selenka,  Niederlandisches  Archiv  fur  Zoologie,  Bd.  i.  July  1862. 

1  This  shellfish  may  be  obtained,  not  merely  at  the  seaside,  but  likewise  at  the 
shops  of  the  fishmongers  who  supply  the  humbler  classes,  even  in  Midland  towns. 


SENSE-ORGANS   OF  MOLLUSCA  941 

cornea  in  front,  and  to  possess  a  coloured  iris  (having  a  pupil)  that 
is  continuous  with  a  layer  of  pigment  lining  the  sclerotic,  a  crystalline 
lens  and  vitreous  body,  and  a  retinal  expansion  proceeding  from  an 
optic  nerve  which  passes  to  each  eye  from  the  trunk  that  runs  along 
the  margin  of  the  mantle.1  Professor  H.  N.  Moseley  made  the 
interesting  discovery  that  many  of  the  Chitonidce  are  provided  with 
a  large  number  of  minute  eyes  on  the  exposed  areas  of  the  outer 
surfaces  of  their  shells ;  as  the  fibres  of  the  optic  nerve  are  directed 
to  the  rods  from  behind  these  eyes  are  of  the  ordinary  invertebrate 
type,  and  differ  therein  from  the  just  mentioned  eyes  of  Pecten,  or 
those  which  are  found  on  the  back  of  Onchidium,  which  resemble 
the  vertebrate  retina  in  having  the  optic  fibres  inserted  into  the  front 
aspect  of  the  layer  of  rods.2  Eyes*  of  still  higher  organisation  are 
borne  upon  the  head  of  most  Gastropod  molluscs,  generally  at  the 
base  of  one  of  the  pairs  of  tentacles,  but  sometimes,  as  in  the  Snail 
and  Slug,  at  the  points  of  these  organs.  In  the  latter  case  the  ten- 
tacles are  furnished  with  a  very  peculiar  provision  for  the  protection 
of  the  eyes  ;  for  when  the  extremity  of  either  of  them  is  touched  it 
is  drawn  back  into  the  basal  part  of  the  organ,  much  as  the  finger  of 
a  glove  may  be  pushed  back  into  the  palm.  The  retraction  of  the 
tentacle  is  accomplished  by  a  strong  muscular  band,  which  arises 
within  the  head  and  proceeds  to  the  extremity  of  the  tentacles ; 
whilst  its  protrusion  is  effected  by  the  agency  of  the  circular  bands 
with  which  the  tubular  wall  of  the  tentacle  is  itself  furnished,  the 
inverted  portion  being  (as  it  were)  squeezed  out  by  the  contraction 
of  the  lower  part  into  which  it  has  been  drawn  back.  The  structure 
of  the  eyes  and  the  curious  provision  just  described  may  easily  be 
examined  by  snipping  off  one  of  the  eye-bearing  tentacles  with  a  pair 
of  scissors.  None  but  the  Cephalopod  molluscs  have  distinct  organs 
of  hearing ;  but  rudiments  of  such  organs  may  be  found  in  most 
Gastropods  (fig.  711,  K,  x),  attached  to  some  part  of  the  nervous 
collar  that  surrounds  the  oesophagus,  and  even  in  many  bivalves,  in 
connection  with  the  nervous  ganglion  imbedded  in  the  base  of  the 
foot.  These  '  auditory  vesicles,'  as  they  are  termed,  are  minute  sac- 
culi,  each  of  which  contains  a  fluid,  wherein  are  suspended  a  number 
of  minute  calcareous  particles  (named  otoliths,  or  ear- stones),  which 
are  kept  in  a  state  of  continual  movement  by  the  action  of  cilia 
lining  the  vesicles.  This  'wonderful  spectacle,'  as  it  was  truly 
designated  by  its  discoverer  Siebold,  may  be  brought  into  view 
without  any  dissection  by  submitting  the  head  of  any  small  and  not 
very  thick-skinned  Gastropod,  or  the  young  of  the  larger  forms,  to 
gentle  compression  under  the  microscope  and  transmitting  a  strong 
light  through  it.  The  very  early  appearance  of  the  auditory  vesicles 
in  the  embryo  Gastropod  has  been  already  alluded  to.  Those  who 
have  the  opportunity  of  examining  young  specimens  of  the  common 
Pecten  will  find  it  extremely  interesting  to  watch  the  action  of  the 

1  See  Mr.  S.  J.  Hickson  on  '  The  Eye  of  Pecten  '  in  Quart.  Journ.  Microsc.  Sci. 
vol.  xx.  n.s.  1880,  p.  448,  and  K.  E.  Schreiner,  '  Die  Augen  bei  Pecten  und  Lima,' 
Bergens  Mus.  Aarbog,  1896,  no.  1. 

2  See  Professor  Moseley '  On  the  Presence  of  Eyes  in  the  Shells  of  certain  Chitonidse 
and  on  the  Structure  of  these  Organs,'  in  Quart.  Journ.  Microsc.  Sci.  xxv.  p.  37. 


942  MOLLUSCA  AND  BRACHIOPODA 

very  delicate  tentacles  which  they  have  the  power  of  putting  forth 
from  the  margin  of  their  mantle,  the  animal  being  confined  in  a 
shallow  cell,  or  in  the  zoophyte  trough  ;  and  if  the  observer  should 
be  fortunate  enough  to  obtain  a  specimen  so  young  that  the  valves 
are  quite  transparent,  he  will  find  the  spectacle  presented  by  the 
ciliary  movement  of  the  gills,  as  well  as  the  active  play  of  the  foot 
(of  which  the  adult  can  make  no  such  use),  to  be  worthy  of  more 
than  a  cursory  glance.1 

Chromatophores  of  Cephalopods. — Almost  any  species  of  cuttle- 
fish (8epia)  or  squid  (Loligo)  will  afford  the  opportunity  of  examining 
the  very  curious  provision  which  their  skin  contains  for  changing  its 
hue.  This  consists  in  the  presence  of  numerous  large '  pigment-cells,' 
containing  colouring  matter  of  various  tints,  the  prevailing  colour, 
however,  being  that  of  the  fluid  of  the  ink-bag.  These  pigment-cells 
may  present  very  different  forms,  being  sometimes  nearly  globular, 
whilst  at  other  times  they  are  flattened  and  extended  into  radiating 
prolongations  ;  and,  by  the  peculiar  contractility  with  which  they  are 
endowed,  they  can  pass  from  one  to  the  other  of  these  conditions,  so 
as  to  spread  their  coloured  contents  over  a  comparatively  large 
surface,  or  to  limit  them  within  a  comparatively  small  area.  Very 
commonly  there  are  different  layers  of  these  pigment-cells,  their  con- 
tents having  different  hues  in  each  layer ;  and  thus  a  great  variety  of 
coloration  may  be  given  by  the  alteration  in  the  form  of  the  cells  of 
which  one  or  another  layer  is  made  up.  It  is  curious  that  the 
changes  in  the  hue  of  the  skin  appear  to  be  influenced,  as  in  the  case 
of  the  chameleon,  by  the  colour  of  the  surface  with  which  it  may  be 
in  proximity.  The  alternate  contractions  and  extensions  of  these 
pigment-cells,  or  chromatophores^  may  be  easily  observed  in  a  piece  of 
skin  detached  from  the  living  animal  and  viewed  as  a  transparent 
object,  since  they  will  continue  for  some  time  if  the  skin  be  placed 
in  sea- water.  And  they  may  also  be  well  seen  in  the  embryo  cuttle- 
fish, which  will  sometimes  be  found  in  a  state  of  sufficient  advance- 
ment in  the  grape-like  eggs  of  these  animals  attached  to  sea-\veeds, 
zoophytes,  &c.  The  eggs  of  the  small  cuttle-fish  termed  the  Sepiola, 
which  is  very  common  on  our  southern  coasts,  are  imbedded,  like  those 
of  the  Doris,  in  gelatinous  masses  which  are  attached  to  seaweeds, 
zoophytes,  &c.  ;  and  their  embryos,  when  near  maturity,  are  ex- 
tremely beautiful  and  interesting  objects,  being  sufficiently  trans- 
parent to  allow  the  action  of  the  heart  to  be  distinguished,  as  well  as 
to  show  most  advantageously  the  changes  incessantly  occurring  in 
the  form  and  hue  of  the  '  chromatophores.'  2 

1  Much  valuable  information  concerning  the  sensory  organs  of  molluscs  will  be 
found  in  Dr.  H.  Simroth's  memoir,  '  Ueber  die  Sinneswerkzeuge  unserer  einheimi- 
schen  Weichthiere,'  Zeitschr.  fur  iviss.  Zb'ol.  xxvi.  p.  227. 

-  For  further  information  regarding  the  chromatophores  see  an  essay  by  Dr. 
Klemensiewicz  in  the  Sitzungsberichte  of  the  Vienna  Academy,  vol.  Ixxviii.  p.  7, 
and  Krukenberg,  Vergl.  physiol.  Studien,  1880. 

The  following  works  and  memoirs  on  the  Mollusca  generally  may  be  consulted  by 
the  student :  S.  P.  Woodward,  A  Manual  of  the  Mollusca,  3rd  ed.  London,  1875 ; 
Keferstein,  in  Bronn's  Klassen  und  Ordnungen  des  Thierreichs ',  the  article  '  Mollusca,' 
by  Professor  Bay  Lankester,  in  the  9th  edition  of  the  Encyclopedia  Britannica ; 
M.  P.  •  Fischer's  Manuel  de  Conchyliologie,  Paris,  1881-87 ;  and  the  Rev.  A.  H. 
Cooke's  volume  in  the  Cambridge  Natural  History  ;  as  well  as  the  numerous  reports 
on  the  Mollusca  collected  by  H.M.S.  Challenger. 


943 


CHAPTER   XIX 

WORMS 

I 

UNDER  the  general  designation  of  Worms  many  naturalists  still 
group  a  number  of  Metazoa,  which  differ  considerably  among  them- 
selves, and  exhibit  on  the  one  hand  very  simple,  and  on  the  other 
somewhat  complex  plans  of  organisation ;  the  assemblage  is,  indeed, 
hardly  anything  else  than  a  zoological  lumber-room,  from  which, 
with  the  progress  of  research,  group  after  group  may  be  expected  to 
be  removed.  Among  others  there  are  included  in  it  the  Entozoa  or 
intestinal  worms,  the  Rotifera  or  wheel-animalcules,  Turbellaria,  and 
Annulata,  each  of  which  furnishes  many  objects  for  microscopic 
examination  that  are  of  the  highest  scientific  interest.  As  our 
business,  however,  is  less  with  the  professed  morphologist  than  with 
the  general  inquirer  into  the  minute  wonders  and  beauties  of  Nature, 
we  shall  pass  over  these  classes  (the  Rotifera  having  been  already 
treated  of  in  detail,  Chapter  XIII)  with  only  a  notice  of  such  points  as 
are  likely  to  be  specially  deserving  the  attention  of  observers  of  the 
latter  order. 

Entozoa. — This  term  is  one  which  has  been  applied  to  such  worms 
as  are  parasitic  within  the  bodies  of  other  animals,  and  which  obtain 
their  nutriment  by  the  absorption  of  the  juices  of  these,  thus 
bearing  a  striking  analogy  to  the  parasitic  Fungi.1  The  most  re- 
markable feature  in  their  structure  consists  in  the  entire  absence  or 
the  extremely  low  development  of  their  nutritive  system,  and  the 
extraordinary  development  of  their  reproductive  apparatus.  Thus 
in  the  common  Tcenia  ('  tape-worm '),  which  may  be  taken  as  the  type 
of  the  Cestoid  group,  there  is  neither  mouth  nor  stomach,  the  so-called 
'  head '  being  merely  an  organ  for  attachment,  whilst  the  segments  of 
the  *  body '  contain  repetitions  of  a  complex  generative  apparatus, 
the  male  and  female  sexual  organs  being  so  united  in  each  as  to 
enable  it  to  fertilise  and  bring  to  maturity  its  own  very  numerous 
eggs  ;  and  the  chief  connection  between  these  segments  is  established 
by  two  pairs  of  longitudinal  canals,  which  appear  to  represent  the 
'  water-vascular  system,'  whose  simplest  condition  has  been  noticed 
in  the  wheel-animalcule.  Few  among  the  striking  results  of  micro- 
scopic inquiry  have  been  more  curious  than  the  elucidation  of  the 
real  nature  of  the  bodies  formerly  denominated  cystic  Entozoa,  which 

1  The  most  important  work  on  human  entozoic  parasites  is  that  by  Professor 
Leuckart,  Die  menscJiliclien  Parasiten,  of  which  a  second  edition  is  now  in  course 
of  publication ;  of  this  the  first  portion  has  been  translated  into  English  by 
Mr.  W.  E.  Hoyle. 


944  WORMS 

had  been  previously  ranked  as  a  distinct  group.  These  are  not 
found,  like  the  preceding,  in  the  cavity  of  the  alimentary  canal  of 
the  animals  they  infest,  but  always  occur  in  the  substance  of  solid 
organs,  such  as  the  glands,  muscles,  &c.  They  present  themselves  to 
the  eye  as  bags  or  vesicles  of  various  sizes,  sometimes  occurring 
singly,  sometimes  in  groups ;  but  upon  careful  examination  each 
vesicle  is  found  to  bear  upon  some  part  a  '  head '  furnished  with 
booklets  and  suckers  ;  and  this  may  be  either  single,  as  in  Cysticercus 
(the  entozoon  whose  presence  gives  to  pork  what  is  known  as  the 
1  measly'  disorder),  or  multiple,  as  in  Ccenurus,  which  is  developed  in 
the  brain,  chiefly  of  sheep,  where  it  gives  rise  to  the  disorder  known 
as  *  the  staggers.'  Xow,  in  none  of  these  cystic  forms  has  any 
generative  apparatus  ever  been  discovered,  and  hence  they  are  ob- 
viously to  be  considered  as  imperfect  animals.  The  close  resemblance 
between  the  '  heads '  of  certain  Cysticerci  and  that  of  certain  Tcenice 
first  suggested  that  the  two  might  be  different  states  of  the  same 
animal ;  and  experiments  made  by  those  who  have  devoted  them- 
selves to  the  working  out  of  this  curious  subject  have  led  to  the 
assured  conclusion  that  the  cystic  Entozoa  are  nothing  else  than 
cestoid  worms,  whose  development  has  been  modified  by  the 
peculiarity  of  their  position,  the  large  bag  being  formed  by  a  sort 
of  dropsical  accumulation  of  fluid  when  the  young  are  evolved  in  the 
midst  of  solid  tissues ;  whilst  the  very  same  bodies,  conveyed  into  the 
alimentary  canal  of  some  carnivorous  animal  which  has  fed  upon  the 
flesh  infested  with  them,  begin  to  bud  forth  the  generative  segments, 
the  long  succession  of  which,  united  end  to  end,  gives  to  the  entire 
series  a  band-like  aspect. 

Other  forms  of  Entozoa  belong  to  the  Nematoid  or  thread-like 
order — of  which  the  common  Ascaris  may  be  taken  as  a  type  ;  one 
species  of  this  (the  A .  lumbricoides  or  '  round  worm ')  is  a  common 
parasite  in  the  small  intestine  of  man,  while  another  (the  Oxyuris 
vermicularis  or  *  thread-worm ')  is  found  rather  in  the  lower  bowel — 
and  they  are  much  less  profoundly  degraded  in  their  organisation ; 
they  have  a  distinct  alimentary  canal,  which  commences  with  a  mouth 
at  the  anterior  extremity  of  the  body,  and  which  terminates  by  an  anal 
orifice  near  the  other  extremity ;  and  they  also  possess  a  regular 
arrangement  of  circular  and  longitudinal  muscular  fibres  by  which 
the  body  can  be  shortened,  elongated,  or  bent  in  any  direction.  The 
smaller  Nematode  worms,  by  some  or  other  of  which  almost  every 
vertebrated  animal  is  infested,  are  so  transparent  that  every  part  of 
their  internal  organisation  may  be  made  out,  especially  with  the 
assistance  of  the  compressor,  without  any  dissection ;  and  the  study 
of  the  structure  and  actions  of  their  generative  apparatus  has  yielded 
many  very  interesting  results,  especially  in  regard  to  the  first  forma- 
tion of  the  ova,  the  mode  of  their  fertilisation,  and  the  history  of 
their  subsequent  development.1  Some  of  the  worms  belonging  to 
this  group  are  not  parasitic  in  the  bodies  of  other  animals,  but  live 
in  the  midst  of  dead  or  decomposing  vegetable  matter.  Others,  such 
as  Gordius  or  the  *  hair-worm,'  are  parasitic  for  the  greater  part  of 

1  See  particularly  the  various  recent  memoirs  of  Van  Beneden  and  of  Boveri,  based 
on  a  study  of  Ascaris  megalocephala. 


XEMATODES  AND  TKEMATODES  945 

their  existence,  but  leave  their  host  for  the  purpose  of  maturing 
their  generative  products ;  in  these  later  stages  the  Gordius  is  fre- 
quently found  in  large  knot -like  masses  (whence  its  name)  in  the 
water  or  mud  of  the  pools  inhabited  by  the  insects  in  which  the 
earlier  stages  were  passed.  The  Anguillulce  are  little  eel-like  worms, 
of  which  one  species,  A.  fluviatilis,  is  very  often  found  in  fresh  water 
amongst  Desmidice,  Confervce,  &c.,  also  in  wet  moss  and  moist  earth, 
and  sometimes  also  in  the  alimentary  canals  of  snails,  frogs,  fishes, 
insects,  and  larger  worms ;  whilst  an  allied  species,  Tylenchus  tritici, 
is  met  with  in  the  ears  of  wheat  affected  with  the  blight  termed  the 
'cockle;'  another,  the  A.  glutinis  (A.  aceti),  is  found  in  sour  paste, 
and  was  often  found  in  stale  vinegar,  until  the  more  complete 
removal  of  mucilage  and  the  acfdition  of  sulphuric  acid,  in  the 
course  of  the  manufacture,  rendered  this  liquid  a  less  favourable 
*  habitat '  for  these  little  creatures.  A  writhing  mass  of  any  of  these 
species  of  '  eels '  is  one  of  the  most  curious  spectacles  which  the 
microscopist  can  exhibit  to  the  unscientific  observer ;  and  the 
capability  which  they  all  possess  (in  common  with  Rotifers  and 
Tardigrades)  of  revival  after  desiccation,  at  a  very  remote  interval, 
enables  him  to  command  the  spectacle  at  any  time.  A  grain  of 
wheat  within  which  these  worms  (often  erroneously  called  Vibriones) 
are  being  developed  gradually  assumes  the  appearance  of  a  black 
peppercorn ;  and  if  it  be  divided  the  interior  will  be  found  almost 
completely  filled  with  a  dense  white  cottony  mass,  occupying  the 
place  of  the  flour,  and  leaving  merely  a  small  place  for  a  little 
glutinous  matter.  The  cottony  substance  seems  to  the  eye  to  consist 
of  bundles  of  fine  fibres  closely  packed  together  ;  but  on  taking  out 
a  small  portion,  and  putting  it  under  the  microscope  with  a  little 
water  under  a  thin  glass  cover,  it  will  be  found  after  a  short  time  (if 
not  immediately)  to  be  a  wriggling  mass  of  life,  the  apparent  fibres 
being  really  Anguillulce  or  '  eels '  of  the  microscopist.  If  the  seeds 
be  soaked  in  water  for  a  couple  of  hours  before  they  are  laid  open, 
the  eels  will  be  found  in  a  state  of  activity  from  the  first ;  their 
movements,  however,  are  by  no  means  so  energetic  as  those  of  the 
A.  glutinis,  or  '  paste  eel.'  This  last  frequently  makes  its  appearance 
spontaneously  in  the  midst  of  paste  that  is  turning  sour ;  but  the 
best  means  of  securing  a  supply  for  any  occasion  consists  in  allowing 
a  portion  of  any  mass  of  paste  in  which  they  may  present  themselves 
to  dry  up,  and  then,  laying  this  by  so  long  as  it  may  not  be  wanted, 
to  introduce  it  into  a  mass  of  fresh  paste,  which  if  it  be  kept  warm 
and  moist  will  be  found  after  a  few  days  to  swarm  with  these  curious 
little  creatures. 

Besides  the  foregoing  orders  of  Entozoa,  the  Trematode  group, 
which  is  more  closely  allied  to  the  Cestoda  than  to  the  Nematodes, 
must  be  named  ;  of  this  the  Distoma  hepaticum,  or  '  fluke,'  found 
in  the  livers  of  sheep  affected  with  the  *  rot,'  is  a  typical  example. 
Into  the  details  of  the  structure  of  this  animal,  which  has  the 
general  form  of  a  sole,  there  is  no  occasion  for  us  here  to  enter ; 
it  is  remarkable,  however,  for  the  branching  form  of  its  diges- 
tive cavity,  which  extends  throughout  almost  the  entire  body,  very 
much  as  in  the  allied  Planarice  (fig.  714) ;  and  also  for  the  curious 

3p 


946  WORMS 

phenomena  of  its  development,  several  distinct  forms  being  passed 
through  between  one  sexual  generation  and  another.  These  have 
been  especially  studied  in  the  Distoma,  which  infests  Paludina, 
the  ova  of  which  are  not  developed  into  the  likeness  of  their 
parents,  but  into  minute  worm-like  bodies,  which  seem  to  be  little 
else  than  masses  of  cells  inclosed  in  a  contractile  integument,  no 
formed  organs  being  found  in  them ;  these  cells,  in  their  turn,  are 
developed  into  independent  larvae,  which  escape  from  their  contain- 
ing cyst  in  the  condition  of  free  ciliated  animalcules ;  in  this  con- 
dition they  remain  for  some  time,  and  then  imbed  themselves  in 
the  mucus  that  covers  the  tail  of  the  mollusc,  in  which  they  undergo 
a  gradual  development  into  true  Distomata ;  and  having  thus  ac- 
quired their  perfect  form,  they  penetrate  the  soft  integument,  and 
take  up  their  habitation  in  the  interior  of  the  body.  Thus  a  con- 
siderable number  of  Distomata  may  be  produced  from  a  single  ovum 
by  a  process  of  cell-multiplication  in  an  early  stage  of  its  develop- 
ment. In  some  instances  the  free  ciliated  larvae  are  provided  with 
pigment-spots  or  rudimentary  optic  organs,  although  these  organs  are 
wanting  in  the  fully  developed  Distoma,  the  peculiar  '  habitat '  of 
which  would  render  them  useless.1 

Turbellaria. — This  group  of  animals,  which  is  distinguished  by 
the  presence  of  cilia  over  the  entire  surface  of  the  body,  contains 
forms  which  are  among  the  simplest  of  those  in  which  the  Metazoic 
organisation  obtains.  It  deserves  special  notice  here  chiefly  on  ac- 
count of  the  frequency  with  which  the  worms  of  the  Planarian 
tribe  present  themselves  among  collections  both  of  marine  and  of 
fresh-water  animals  (particular  species  inhabiting  either  locality) 
and  on  account  of  the  curious  organisation  which  many  of  these 
possess.  Most  of  the  members  of  this  tribe  have  elongated,  flattened 
bodies,  and  move  by  a  sort  of  gliding  or  crawling  action  over  the 
surfaces  of  aquatic  plants  and  animals.  Some  of  the  smaller  kinds 
are  sufficiently  transparent  to  allow  of  their  internal  structure  being 
seen  by  transmitted  light,  especially  when  they  are  slightly  com- 
pressed ;  and  the  opposite  figure  (fig.  714)  displays  the  general 
conformation  of  their  principal  organs  as  thus  shown.  The  body 
has  the  flattened  sole-like  shape  of  the  Trematode  Entozoa  ;  its 
mouth,  which  is  situated  at  a  considerable  distance  from  the  anterior 
extremity  of  the  body,  is  surrounded  by.  a  circular  sucker  that  is 
applied  to  the  living  surface  from  which  the  animal  draws  its  nutri- 
ment ;  and  the  buccal  cavity  (b)  opens  into  a  short  oesophagus  (c) 
which  leads  at  once  to  the  cavity  of  the  stomach.  This  cavity  does 
not  give  origin  to  any  intestinal  tube,  nor  is  it  provided  with  any 
second  orifice  ;  but  a  large  number  of  ramifying  canals  are  prolonged 
from  it,  which  carry  its  contents  into  every  part  of  the  body.  This 
seems  to  render  unnecessary  any  system  of  vessels  for  the  circulation 
of  nutritive  fluid  ;  and  the  two  principal  trunks,  with  connecting 
and  ramifying  branches,  which  may  be  observed  in  them  may  be 

1  On  the  development  and  life-history  of  the  '  Liver-fluke  '  see  Professor  A.  P. 
Thomas,  Quart.  Journ.  Microsc.  Sci.  xxiii.  p.  1 ;  and  K.  Leuckart,  Arcliiv  fur  Natur- 
gesch.  xlviii.  p.  80.  On  its  anatomy,  see  Dr.  P.  Sommer,  Zeitschr.  fur  wiss.  Zool. 
xxxiv. 


PLANAEIA 


947 


regarded  in  the  light  of  a  gastro-vascular  system,  the  function  of 
which  is  not  only  digestive,  but  also  circulatory.  Both  sets  of  sexual 
organs  are  combined  in  the  same  individuals,  though  the  congress 
of  two,  each  impregnating  the  ova  of  the  other,  seems  to  be  gene- 
rally necessary.  The  ovaria,  as 
in  the  Entozoa,  extend  through 
a  large  part  of  the  body,  their 
ramifications  proceeding  from 
the  two  oviducts  (&,  &),  wThich 
have  a  dilatation  (I)  at  their 
point  of  junction.  The  Pla- 
narice  T  do  not  multiply  by  eggs 
alone  ;  for  they  occasionally  un- 
dergo spontaneous  fission  in  a 
transverse  direction,  each  seg- 
ment becoming  a  perfect  animal ; 
and  an  artificial  division  into 
two  or  even  more  parts  may  be 
practised  with  a  like  result.  In 
fact,  the  power  of  the  Planariw 
to  reproduce  portions  which 
have  been  removed  seems  but 
little  inferior  to  that  of  the 
Hydra ;  a  circumstance  which 
is  peculiarly  remarkable  when 
the  much  higher  character  of 
their  organisation  is  borne  in 
rnind.  They  possess  a  distinct 
pair  of  nervous  ganglia  (/,/), 
from  which  branches  proceed  to 
various  parts  of  the  body ;  and 
in  the  neighbourhood  of  these 
are  usually  to  be  observed  a 
number  (varying  from  two  to 
forty)  of  ocelli  or  rudimentary 
eyes,  each  having  its  refracting 
body  or  crystalline  lens,  its  pig- 


FIG.  714.— Structure  of  Polycelis  levi- 
gatus  (a  Planarian  worm) :  a,  mouth, 
surrounded  by  its  circular  sucker;  b, 
buccal  cavity ;  c,  oesophageal  orifice  ; 
d,  stomach ;  e,  ramifications  of  gastric 
canals ;  /,  cephalic  ganglia  and  their 
nervous  filaments ;  g,  g,  testes ;  h, 
vesicula  seminalis ;  i,  male  genital 
canal;  k,  k,  oviducts;  I,  dilatation  at 
their  point  of  junction ;  m,  female 
genital  orifice. 


ment-layer,  its  nerve-bulb,  and 

its   cornea-like   bulging   of  the 

skin.     The  integument  of  many 

of    these   animals    is    furnished 

with   cells   containing   rods   or   spindles   which   are   very   possibly 

comparable  to  the  '  thread-cells '  of  zoophytes.2 

Annulata,— This  class  includes  all  the  higher  kinds  of  worm -like 
animals,  the  greater  part  of  which  are  marine,  though  there  is  one 
well-marked  group  the  members  of  which  inhabit  fresh  water  or  live 

See  Balfour's  Comparative  Embryology,  vol.  i.  pp.  159-162. 
-  For  further  information  regarding  the  Turbellaria  consult  Dr.  L.  Graff's  article 


naturelle  des  Turbellaries, 

Lille,  1879.     On  transverse  fission,  see  Bell,  Journ.  Boy.  Microsc.  Soc.  (2)  vi.  p  1107 

8p2 


'.MY/. 

lie, 


948 


WORMS 


on  land.  The  body  in  this  class  is  usually  elongated  and  nearly 
always  presents  a  well-marked  segmental  division,  the  segments 
being  for  the  most  part  similar  and  equal  to  each  other,  except  at 
the  two  extremities;  though  in  some,  as  the  leech  and  its  allies, 

the  segmental  division  is  very  in- 
distinctly seen,  on  account  of  the 
general  softness  of  the  integument. 
A  large  portion  of  the  marine  An- 
nelids have  special  respiratory  ap- 
pendages, into  which  the  fluids  of 
the  body  are  sent  for  aeration,  and 
these  are  situated  upon  the  head 
(fig.  715)  in  those  species  which 
(like  the  Serpula,  Terebella,  Sabel- 
laria,  <fcc.)  have  their  bodies  inclosed 
by  tubes,  either  formed  of  a  shelly 
substance  produced  from  their  own 
surface,  or  built  up  by  the  agglutina- 
tion of  grains  of  sand,  fragments  of 
shell,  &c. ; 1  whilst  they  are  distri- 
buted along  the  two  sides  of  the  body 
in  such  as  swim  freely  through  the 
water,  or  crawl  over  the  surfaces  of 
rocks,  as  is  the  case  with  the  Nereidce, 
or  simply  bury  themselves  in  the 
sand,  as  the  Arenicola  or  '  lob-worm.' 
In  these  respiratory  appendages  the 
circulation  of  the  fluids  may  be  dis- 
tinctly seen  by  microscopic  exami- 
nation ;  and  these  fluids  are  of  two 
kinds  :  first,  a  colourless  fluid,  con- 
taining numerous  cell-like  cor- 
puscles, which  can  be  seen  in  the 
smaller  and  more  transparent 

that 


FIG.    715. — Circulating     apparatus    of 


"Terebellaconchilega:  a,  labial  ring;  species    to    occupy    the    space 

b,  b,  tentacles;  c,  first  segment  of  intervenes  between  the  outer  sur- 

the  trunk ;  d,  skin  of  the  back ;  e  face    of  the   alimentary  canal    and 

pharynx:  f,  intestine ;  q, longitudinal  ,-,  ,,      ,,    .,       i r     ,  ,    . 

musclesof  the  inferior  surface  of  the  the  inner  wall  of  the  body,  and  to 

body;  h,  glandular  organ;  i,  organs  pass  from  this  into  Canals  which 
of  generation;./,  feet;  k,  k,  branchise  ;  often  ramify  extensively  in  the 
L  dorsal  vessel  acting  as  a  respiratory  .  *  J 

heart;  m,  dorso-intestinal  vessel;  respiratory  organs,  but  are  never 
n,  venous  sinus  surrounding  oesopha-  furnished  with  a  returning  series 

of  passages ;  and  second,  a  fluid 
which  is  usually  red,  contains  few 
floating  particles,  and  is  inclosed  in 
a  system  of  proper  vessels  that  communicates  with  a  central  pro- 
pelling organ,  and  not  only  carries  the  fluid  away  from  this,  but  also 
brings  it  back  again.  In  Terebella  we  find  a  distinct  provision  for  the 


gus ;  n',  inferior  intestinal  vessel; 
o,  o,  ventral  trunk ;  p,  lateral  vascular 
branches. 


1  For  an  interesting  account  of  the  formation  of  these  tubes  see  Mr.  A.  T.  Watson's 
paper  in  Journ.  Boy.  Micr.  Soc.  1890,  p.  685. 


DEVELOPMENT  OF    WORMS  949 

aeration  of  both  fluids  ;  for  the  first  is  transmitted  to  the  teiidril- 
like  tentacles  which  surround  the  mouth  (fig.  715,  £>,  6),  whilst  the 
second  circulates  through  the  beautiful  arborescent  gill-tufts  (k,  k) 
situated  just  behind  the  head.  The  former  are  covered  with  cilia,  the 
action  of  \vhich  continually  renews  the  stratum  of  water  in  contact 
with  them,  wrhilst  the  latter  are  destitute  of  these  organs ;  and  this 
seems  to  be  the  general  fact  as  to  the  several  appendages  to  which 
these  two  fluids  are  respectively  sent  for  aeration,  the  nature  of  their 
distribution  varying  greatly  in  the  different  members  of  the  class.  In 
the  observation  of  the  beautiful  spectacle  presented  by  the  respiratory 
circulation  of  the  various  kinds  of  Annulates  which  swarm  on  most 
of  our  shores,  and  in  the  examination  of  what  is  going  on  in  the 
interior  of  their  bodies  (where  this  is  rendered  possible  by  their 
transparence),  the  microscopist  will  find  a  most  fertile  source  of 
interesting  occupation ;  and  he  may  easily,  with  care  and  patience, 
make  many  valuable  additions  to  our  present  stock  of  knowledge  on 
these  points.  There  are  many  of  these  marine  worms  in  which 
the  appendages  of  various  kinds  put  forth  from  the  sides  of  their 
bodies  furnish  very  beautiful  microscopic  objects ;  as  do  also  the 
different  forms  of  teeth,  jaws,  &c.  with  which  the  mouth  is  com- 
monly armed  in  the  free  or  non-tubicolar  species,  which  are 
eminently  carnivorous. 

The  early  history  of  their  development  is  extremely  curious ; 
for  many  come  forth  from  the  egg  in  a  condition  very  little 
more  advanced  than  the  ciliated  gemmules  of  polypes,  consist- 
ing of  a  globular  mass  of  untransformed  cells,  certain  parts  of 
whose  surface  are  covered  with  cilia,  which  ordinarily  become 
arranged  in  one  or  more  definite  rings ;  in  a  few  hours,  however, 
this  embryonic  mass  elongates,  and  the  indications  of  a  segmental 
division  become  apparent,  the  head  being  (as  it  were)  marked  off 
in  front,  whilst  behind  this  is  a  large  segment  thickly  covered  with 
cilia,  then  a  narrower  and  non-ciliated  segment,  and  lastly  the 
caudal  or  tail  segment,  which  is  furnished  with  cilia.  A  little 
later  a  new  segment  is  seen  to  be  interposed  in  front  of  the 
caudal,  and  the  dark  internal  granular  mass  shapes  itself  into  the 
outline  of  an  alimentary  canal.1  The  number  of  segments  pro 
gressively  increases  by  the  interposition  of  new  ones  between  the 
caudal  and  its  preceding  segments;  the  various  internal  organs 
become  more  and  more  distinct,  eye-spots  make  their  appearance, 
little  bristly  appendages  are  put  forth  from  the  segments,  and 
the  animal  gradually  assumes  the  likeness  of  its  parent ;  a  few 
days  being  passed  by  the  tubicolar  kinds,  however,  in  the  actively 

1  A  most  curious  transformation  once  occurred  within  the  Author's  experience 
in  the  larva  of  an  Annelid,  which  was  furnished  with  a  broad  collar'  or  disc  fringed 
with  very  long  cilia,  and  showed  merely  an  appearance  of  segmentation  in  its  hinder 
part ;  for  in  the  course  of  a  few  minutes,  during  which  it  was  not  under  observation, 
this  larva  assumed  the  ordinary  form  of  a  marine  worm  three  or  four  times  its  pre- 
vious length,  and  the  ciliated  disc  entirely  disappeared.  An  accident  unfortunately 
prevented  the  more  minute  examination  of  this  worm,  which  the  Author  would  have 
otherwise  made  ;  but  he  may  state  that  he  is  certain  that  there  was  no  fallacy  as  to 
the  fact  above  stated,  this  larva  having  been  placed  by  itself  in  a  cell,  on  purpose 
that  it  might  be  carefully  studied,  and  having  been  only  laid  aside  for  a  short  time 
whilst  other  selections  were  being  made  from  the  same  gathering  of  the  tow-net. 


950 


WOKMS 


moving  condition,  before  they  settle  down  to  the  formation  of  a 
tube.1 

To  carry  out  any  systematic  observations  on  the  embryonic 
development  of  Annulata  the  eggs  should  be  searched  for  in  the 
situations  which  these  animals  haunt ;  but  in  places  where  Amiu- 
lata  abound  free-swimming  larvae  are  often  to  be  obtained  at  the 
same  time  and  in  the  same  manner  as  small  Medusae  ;  and  there  is 
probably  no  part  of  our  coasts  oft'  which  some  very  curious  forms 
may  not  be  met  with.  The  following  may  be  specially  mentioned 
as  departing  widely  from  the  ordinary  type,  and  as  in  themselves 
extremely  beautiful  objects  :  The  Actinotrocha,  which  is  now  known 

to  be  the  young  stage  of  the  Gephyrean 
worm  Phoronis  (fig.  716),  bears  a 
strong  resemblance  in  many  particulars 
to  the  '  bipinnarian '  larva  of  a  star- 
fish, having  an  elongated  body,  with 
a  series  of  ciliated  tentacles  (d)  sym- 
metrically arranged  ;  these  tentacles, 
however,  proceed  from  a  sort  of  disc 
which  somewhat  resembles  the  '  lopho- 
phore '  of  certain  Polyzoa.  The  mouth 
(e)  is  concealed  by  a  broad  but  pointed 
hood  or  '  epistome '  (a),  which  some- 
times closes  down  upon  the  tentacular 
disc,  but  is  sometimes  raised  and  ex- 
tended forwards.  The  nearly  cylin- 
drical body  terminates  abruptly  at  the 
other  extremity,  where  the  anal  orifice 
of  the  intestine  (b)  is  surrounded  by  a 
circlet  of  very  large  cilia.  This  animal 
swims  with  great  activity,  sometimes 
by  the  tentacular  cilia,  sometimes  by 
the  anal  circlet,  sometimes  by  both 
combined  ;  and  besides  its  movement 
of  progression  it  frequently  doubles 
FIG.  HQ.-Actinotrocha  branchi-  itself  together,  so  as  to  bring  the  anal 
ata :  a.  epistome  or  hood;  o,  .*?  ,  , ,  .  ,  ,  .  , 

anus ;  c,   stomach ;  d    ciliated    extremity  and  the  epistome  almost  into 
tentacles ;  e,  mouth.  contact.     It  js  so  transparent  that  the 

whole  of  its  alimentary  canal  may  be 

as  distinctly  seen  as  that  of  Laguncula ;  and,  as  in  that  polyzoon, 
the  alimentary  masses  often  to  be  seen  within  the  stomach  (c)  are 
kept  in  a  continual  whirling  movement  by  the  agency  of  cilia,  with 
which  its  walls  are  clothed.2  An  even  more  extraordinary  departure 
from  the  ordinary  type  is  presented  by  the  larva  which  has  received 
the  name  Pilidium  (fig.  717),  its  shape  being  that  of  a  helmet,  the 

1  For  further  information  on  this  subject  see  Balfour's  Comparative  Embryology, 
vol.  i.  chap.  xii.  and  the  memoirs  there  cited. 

2  '  Ueber  Pilidium  und  Actinotrocha '  in  Midler's  Archiv,  1858,  p.  293.     For 
more   recent   observations   upon   the   latter   creature,   see    Balfour's    Comparative 
Embnjology,  vol.  i.  pp.  299-302 ;  and  a  paper  011  '  The  Origin  and  Significance  of  the 
Metamorphosis  of  Actinotrocha,1  by  Mr.  E.  B.  Wilson   (of  Baltimore),  in   Quart. 
Journ.  Microsc.  Sci.  April  1881. 


OF   WORMS 


951 


plume  of  which  is  replaced  by  a  single  long  bristle-like  appendage 
that  is  in  continual  motion,  its  point  moving  round  and  round  in  a 
circle.  This  curious  organism,  first  noticed  by  Johannes  Miiller,  has 
been  since  ascertained  to  be  the  larva  of  some  species  of  the  Nemer- 
tine  worms,  which  belong  to  the  division  Anopla,  a  group  in  which 
there  are  no  stylets  to  the  proboscis.1 

Among    the   animals    captured    by   the    tow-net    the    marine 
zoologist    will    not    be    unlikely    to    meet    with    a    worm    which, 


FIG.  IVl.—Pilidium  gyrans .  A,  young,  showing  at  a  the  alimentary 
canal,  and  at  b  the  rudiment  of  the  Nemertid ;  B,  more  advanced 
stage  of  the  same ;  C,  newly  freed  Nemertid. 

although  by  no  means  microscopic  in  its  dimensions,  is  an  admirable 
subject  for  microscopic  observation,  owing  to  the  extreme  trans- 
parence of  its  entire  body,  which  is  such  as  to  render  it  difficult  to 
be  distinguished  when  swimming  in  a  glass  jar  except  by  a  very 
favourable  light.  This  is  the  Tomopteris,  so  named  from  the 
division  of  the  lateral  portions  of  its  body  into  a  succession  of  wing- 
like  segments  (fig.  718,  B),  each  of  them  carrying  at  its  extremity  a 
pair  of  pinnules,  by  the  movements  of  which  it  is  rapidly  propelled 
through  the  water.  The  full-grown  animal,  which  measures  nearly 

1  See  especially  Leuckart  and  Pagenstecher's  '  Untersuchungen  tiber  niedere 
Seethiere  '  in  Miiller's  Archiv,  1853,  p.  569  ;  and  Balfour,  op.  cit.  p.  165.  The  Author 
has  frequently  met  with  Pilidium  in  Lamlash  Bay. 


952 


WOEMS 


an  inch  in  length,  has  first  a  curious  pair  of  '  frontal  horns '  pro- 
jecting laterally  from  the  head,  so  as  to  give  the  animal  the  appear- 


FIG.  718.— Structure  and  development  of  Tomopteris  onisciformis  :  A,  portion 
of  caudal  prolongations,  containing  the  spermatic  sacs,  a  a ;  B,  adult  male 
specimen ;  C,  hinder  part  of  adult  female  specimen,  more  enlarged,  showing 
ova,  lying  freely  in  the  perivisceral  cavity  and  its  caudal  prolongation ;  D, 
ciliated  canal,  commencing  externally  in  the  larger  and  smaller  rosette-like 
discs,  a,  & ;  E,  one  of  the  pinnulated  segments,  showing  the  position  of  the 
ciliated  canal,  c,  and  its  rosette-like  discs,  a,  b ;  showing  also  the  incipient 
development  of  the  ova,  d,  at  the  extremity  of  the  segment ;  F,  cephalic  gan- 
glion, with  its  pair  of  auditory  (?)  vesicles,  a  a,  and  its  two  ocelli,  b  b  ;  G-,  very 
young  Tomopteris,  showing  at  a  a  the  larval  antennae ;  b  b,  the  incipient 
long  antennas  of  the  adult ;  c,  d,  e,  /,  four  pairs  of  succeeding  pinnulated 
segments,  followed  by  bifid  tail. 


TOMOPTEEIS  953 

ance  of  a  '  hammer-headed '  shark ;  behind  these  there  is  a  pair  of 
very  long  antennae,  in  each  of  which  we  distinguish  a  rigid  bristle- 
like  stem  or  seta,  inclosed  in  a  soft  sheath,  and  moved  at  its  base 
by  a  set  of  muscles  contained  within  the  lateral  protuberances  at 
the  head.  Behind  these  are  about  sixteen  pairs  of  the  ordinary 
pinnulated  segments,  of  which  the  hinder  ones  are  much  smaller 
than  those  in  front,  gradually  lessening  in  size  until  they  become 
almost  rudimentary ;  and  where  these  cease  the  body  is  continued 
onwards  into  a  tail-like  prolongation,  the  length  of  which  varies 
greatly  according  as  it  is  contracted  or  extended.  This  prolongation, 
however,  bears  four  or  five  pairs  of  very  minute  appendages,  and 
the  intestine  is  continued  to  its  very  extremity,  so  that  it  is  really 
to  be  regarded  as  a  continuation  of  the  body.  In  the  head  we  find, 
between  the  origins  of  the  antennae,  a  ganglionic  mass,  the  component 
cells  of  which  may  be  clearly  distinguished  under  a  suificient  mag- 
nifying power,  as  shown  at  F  ;  seated  upon  this  are  two  pigment- 
spots  (b,  6),  each  bearing  a  double  pellucid  lens-like  body,  wrhich  are 
obviously  rudimentary  eyes ;  wThilst  imbedded  in  its  anterior  por- 
tion are  two  peculiar  nucleated  vesicles,  a,  a,  which  are  probably 
the  rudiments  of  some  other  sensory  organs.  On  the  under  side  of 
the  head  is  situated  the  mouth,  which,  like  that  of  many  other 
Annelids,  is  furnished  with  a  sort  of  proboscis  that  can  be  either 
projected  or  drawn  in ;  a  short  oesophagus  leads  to  an  elongated 
stomach,  which,  when  distended  with  fluid,  occupies  the  whole 
cavity  of  the  central  portion  of  the  body,  as  shown  in  fig.  B,  but 
which  is  sometimes  so  empty  and  contracted  as  to  be  like  a  mere 
cord,  as  shown  in  fig.  C.  In  the  caudal  appendage,  however,  it  is 
always  narrowed  into  an  intestinal  canal ;  this,  when  the  appendage 
is  in  an  extended  state,  as  at  C,  is  nearly  straight ;  but  when  the 
appendage  is  contracted,  as  seen  at  B,  it  is  thrown  into  convolutions. 
The  perivisceral  cavity  is  occupied  by  fluid,  in  which  some  minute 
corpuscles  may  be  distinguished ;  and  these  are  kept  in  motion  by 
cilia  wrhich  clothe  some  parts  of  the  outer  surface  of  the  alimentary 
canal  and  line  some  part  of  the  wall  of  the  body.  No  other  more 
special  apparatus,  either  for  the  circulation  or  for  the  aeration  of 
the  nutrient  fluid,  exists  in  this  curious  worm,  unless  we  are  to 
regard  as  subservient  to  the  respiratory  function  the  ciliated  canal 
which  may  be  observed  in  each  of  the  lateral  appendages  except 
the  five  anterior  pairs.  This  canal  commences  by  two  orifices  at 
the  base  of  the  segment,  as  shown  at  fig.  E,  b,  and  on  a  larger  scale 
at  fig.  D  ;  each  of  these  orifices  (D,  a,  b)  is  surrounded  by  a  sort  of 
rosette,  and  the  rosette  of  the  larger  one  (a)  is  furnished  with 
radiating  ciliated  ridges.  The  two  branches  incline  towards  each 
other,  and  unite  into  a  single  canal  that  runs  along  for  some  dis- 
tance in  the  wall  of  the  body,  and  then  terminates  in  the  perivisceral 
cavity,  and  the  direction  of  the  motion  of  the  cilia  which  line  it  is 
from  without  inwards. 

The  reproduction  and  developmental  history  of  this  Annelid 
present  many  points  of  great  interest.  The  sexes  appear  to  be 
distinct,  ova  being  found  in  some  individuals  and  spermatozoa  in 
others.  The  development  of  the  ova  commences  in  certain  *  germ- 


954  WORMS 

cells '  situated  within  the  extremities  of  the  pinnulated  segments, 
where  they  project  inwards  from  the  wall  of  the  body  ;  these,  when 
set  free,  float  in  the  fluid  of  the  perivisceral  cavity  and  multiply 
themselves  by  self-division  ;  and  it  is  only  after  their  number  has 
thus  been  considerably  augmented  that  they  begin  to  increase  in 
size  and  to  assume  the  characteristic  appearance  of  ova.  In  this 
stage  they  usually  fill  the  perivisceral  cavity,  not  only  of  the  body, 
but  of  its  caudal  extension,  as  shown  at  C  ;  and  they  escape  from 
it  through  transverse  fissures  which  form  in  the  outer  wall  of  the 
body  at  the  third  and  fourth  segments.  The  male  reproductive 
organs,  on  the  other  hand,  are  limited  to  the  caudal  prolongation, 
where  the  sperm-cells  are  developed  within  the  pinnulated  append- 
ages, as  the  germ-cells  of  the  female  are  within  the  appendages  of 
the  body.  Instead  of  being  set  free,  however,  into  the  perivisceral 
cavity,  they  are  retained  within  a  saccular  envelope  forming  a  testis 
(A,  a,  a)  which  fills  up  the  whole  cavity  of  each  appendage ;  and 
within  this  the  spermatozoa  may  be  observed,  when  mature,  in 
active  movement.  They  make  their  escape  externally  by  a  passage 
that  seems  to  communicate  with  the  smaller  of  the  two  just  men- 
tioned rosettes  ;  but  they  also  appear  to  escape  into  the  perivisceral 
cavity  by  an  aperture  that  forms  itself  when  the  spermatozoa  are 
mature.  Whether  the  ova  are  fertilised  while  yet  within  the  body 
of  the  female  by  the  entrance  of  spermatozoa  through  the  ciliated 
canals,  or  after  they  have  made  their  escape  from  it,  has  not  yet 
been  ascertained.  Of  the  earliest  stages  of  embryonic  development 
nothing  whatever  is  yet  known  ;  but  it  has  been  ascertained  that 
the  animal  passes  through  a  larval  form,  which  differs  from  the 
adult  not  merely  in  the  number  of  the  segments  of  the  body  (which 
successively  augment  by  additions  at  the  posterior  extremity),  but 
also  in  that  of  the  antennae.  At  G  is  represented  the  earliest  larva 
hitherto  met  with,  enlarged  as  much  as  ten  times  in  proportion  to 
the  adult  at  B  ;  and  here  we  see  that  the  head  is  destitute  of  the 
frontal  horns,  but  carries  a  pair  of  setigerous  antennae,  a,  a,  behind 
which  there  are  five  pairs  of  bifid  appendages,  6,  c,  d,  e,f,  in  the 
first  of  which,  5,  one  of  the  pinnules  is  furnished  with  a  seta.  In 
more  advanced  larvae  having  eight  or  ten  segments  this  is  developed 
into  a  second  pair  of  antennae  resembling  the  first ;  and  the  animal 
in  this  stage  has  been  described  as  a  distinct  species,  T.  quadricornis. 
At  a  more  advanced  age,  however,  the  second  pair  attains  the 
enormous  development  shown  at  B,  and  the  first  or  larval  antennae 
disappear,  the  setigerous  portions  separating  at  a  sort  of  joint  (Gr,  a, 
«),  whilst  the  basal  projections  are  absorbed  into  the  general  wall 
of  the  body.  This  beautiful  creature  has  been  met  with  on  so  many 
parts  of  our  coast  that  it  cannot  be  considered  at  all  uncommon, 
and  the  microscopist  can  scarcely  have  a  more  pleasing  object  for 
study.1  Its  elegant  form,  its  crystal  clearness,  and  its  sprightly, 
graceful  movements  render  it  attractive  even  to  the  unscientific 

1  See  the  memoirs  of  the  Author  and  M.  Claparede  in  vol.  xxii.  of  the  Linnean 
Transactions  and  the  authorities  there  referred  to ;  also  a  memoir  by  Dr.  F. 
Vdjdovsky  in  Zeitschrift  f.  Wiss.  Zool.  Bd.  xxxi.  1878. 


NAIS 


955 


observer  ;  whilst  it  is  of  special  interest  to  the  morphologist  as  one 
of  the  simplest  examples  yet  known  of  the  Annelid  type. 

To  one  phenomenon  of  the  greatest  interest  presented  by  various 
small  marine  Annelids  the  attention  of  the  microscopist  should  be 
specially  directed ;  this  is  their  luminosity,  which  is  not  a  steady 
glow  like  that  of  the  glow-worm  or  fire-fly,  but  a  series  of  vivid 
scintillations  (strongly  resembling  those  produced  by  an  electric 
discharge  through  a  tube  spotted  with  tinfoil),  that  pass  along  a 
considerable  number  of  segments,  lasting  for  an  instant  only,  but 
capable  of  being  repeatedly  excited  by  any  irritation  applied  to  the 
body  of  the  animal.  These  scintillations  may  be  discerned  under 
the  microscope,  even  in  separate  segments,  when  they  are  subjected 
to  the  irritation  of  a  needle-point  or* a  gentle  pressure  ;  and  it  has  been 
ascertained  by  the  careful  observations  of  M.  de  Quatrefages  that 
they  are  given  out  by  the  muscular  fibres  in  the  act  of  contraction.1 

Among  the  fresh- water  Annelids  those  most  interesting  to  the 
microscopist  are  the  worms  of  the  Nais  tribe,  which  are  common  in 
our  rivers  and  ponds,  living  chiefly  amidst  the  mud  at  the  bottom, 
and  especially  among  the  roots  of  aquatic  plants.  Being  blood-red 
'in  colour,  they  give  to  the  surface  of  the  mud,  when  they  protrude 
themselves  from  it  in  large  numbers  and  keep  the  protruded  portion 
of  their  bodies  in  constant  undulation,  a  very  peculiar  appearance  ; 
but  if  disturbed  they  withdraw  themselves  suddenly  and  completely. 
These  worms,  from  the  extreme  transparency  of  their  bodies,  present 
peculiar  facilities  for  microscopic  examination,  and  especially  for  the 
study  of  the  internal  circulation  of  the  red  liquid  commonly  con- 
sidered as  blood.  There  are  here  no  external  respiratory  organs,  and 
the  thinness  of  the  general  integument  appears  to  supply  all  needful 
facility  for  the  aeration  of  the  fluids.  One  large  vascular  trunk  (dorsal ) 
may  be  seen  lying  above  the  intestinal  canal,  and  another  (ventral)  be- 
neath it,  and  each  of  these  enters  a  contractile  dilatation,  or  heart- 
like  organ,  situated  just  behind  the  head.  The  fluid  moves  forwards 
in  the  dorsal  trunk  as  far  as  the  heart,  which  it  enters  and  dilates  ; 
and  wThen  this  contracts  it  propels  the  fluid  partly  to  the  head  and 
partly  to  the  ventral  heart,  which  is  distended  by  it.  The  ventral 
heart,  contracting  in  its  turn,  sends  the  blood  backwards  along  the 
ventral  trunk  to  the  tail,  whence  it  passes  towards  the  head  as 
before.  In  this  circulation  the  stream  branches  oft'  from  each  of 
the  principal  trunks  into  numerous  vessels  proceeding  to  different 
parts  of  the  body,  which  then  return  into  the  other  trunk ;  and 
there  is  a  peculiar  set  of  vascular  coils,  hanging  down  in  the  peri- 
visceral  cavity  that  contains  the  corpusculated  liquid  representing 
the  true  blood,  which  seem  specially  destined  to  convey  to  it  the 
aerating  influence  received  by  the  red  fluid  in  its  circuit,  thus 
acting  (so  to  speak)  like  internal  gills.  The  Naiad  worms  have 
been  observed  to  undergo  spontaneous  division  during  the  summer 
months,  a  new  head  and  its  organs  being  formed  for  the  posterior- 
segment  behind  the  line  of  constriction  before  its  separation  from 

1  See  his  memoirs  on  the  Annelida  of  La  Manche  in  Ann.  des  tici.  Nat.  ser.  ii. 
Zool.  torn.  xix.  and  ser.  iii.  Zool.  torn.  xiv. ;  and  Professor  Mclntosh  in  Nature, 
xxxii.  p.  478. 


956  WOKMS 

the  anterior.1  In  the  Leech  tribe  the  dental  apparatus  with  which 
the  mouth  is  furnished  is  one  of  the  most  curious  among  their 
points  of  minute  structure,  and  the  common  '  medicinal '  leech 
affords  one  of  the  most  interesting  examples  of  it.  What  is 
commonly  termed  the  '  bite '  of  the  leech  is  really  a  saw-cut,  or 
rather  a  combination  of  three  saw-cuts,  radiating  from  a  common 
centre.  If  the  mouth  of  the  leech  be  examined  with  a  hand- 
magnifier,  or  even  with  the  naked  eye,  it  will  be  seen  to  be  a 
triangular  aperture  in  the  midst  of  a  sucking  disc,  and  on  turning 
back  the  lips  of  that  aperture  three  little  white  ridges  are  brought 
into  view.  Each  of  these  is  the  convex  edge  of  a  horny  semicircle, 
strengthened  by  a  deposit  of  carbonate  of  lime  which  is  bordered  by 
a  row  of  eighty  or  ninety  minute  hard  and  sharp  teeth ;  whilst 
the  straight  border  of  the  semicircle  is  imbedded  in  the  muscular 
substance  of  the  disc,  by  the  action  of  which  it  is  made  to  move 
backwards  and  forwards  in  a  saw-like  manner,  so  that  the  teeth  are 
enabled  to  cut  into  the  skin  to  which  the  suctorial  disc  has  affixed 
itself.2 

1  See  Professor  A.  G.  Bourne,  'On  Budding  in  the  Oligocheeta,'  Report  Brit. 
Assoc.  1885,  p.  1096. 

2  Among  the  various  sources  of  information  as  to  the  anatomy  and  physiology  of 
the  Annelids  the  following  may  be  specially  mentioned  :  the  '  Histoire  Naturelle  des 
Anneles  Marins  et  d'Eau  douce '  of  M.  de  Quatrefages,  forming  part  of  the  Suites  a 
Buffon ;  the  successive  admirable  monographs  of  the  late  Professor  Ed.  Claparede, 
Recherches  Anatotniqu.es  sur  les  Annelides,  Turbellaries,  Opalines  et  Gregarines, 
observes  dans  les  Hebrides,  Geneva,  1861 ;  Recherches  Anatomiques  sur  les  Oligo- 
chetes,  Geneva,  1862  ;  Beobachtungen  iiber  Anatomie  und  Entwickelungsgeschichte 
wirbelloser  Thiere  an  der  Kilste  von  Normandie,  Leipzig,  1863 ;  and  Les  Annelides 
Chetopodes  du  Golfe  de  Naples,  Geneva,  1868-70  ;  the  monograph  of  Dr.Ehlers,  Die 
Borsienwiirmer  (Annelida  Chcetopoda),^  1864-68.    With  the  exception  of  Professor 
Mclntosh's  article  in  the  Encyclopedia  Britannica,   and  the  various  articles  on 
'  Worms '  in  the  Cambridge  Natural  History,  which  can  be  warmly  commended  to 
the  student,  most  of  the  recent  papers  on  Annelids  have  dealt  with  small  groups  only, 
but  of  these  a  very  large  number  has  appeared.     For  the  descriptions  of  new  forms 
the  memoirs  of  Grube,  Mclntosh,  and  St.  Joseph  are  especially  to  be  consulted; 
Hatschek,  Kleinenberg,  and  Salensky  have  written  the  most  important  contributions 
to  our  knowledge  of  development ;  Benham,  Bergh,  Bourne,  Eisig,  Meyer,  Perrier, 
and  Whitman  have,  among  others,  added  to  our  knowledge  of  their  anatomy  and 
morphology. 


957 


CHAPTER  XX 

CRUSTACEA 
I 

PASSING  to  the  division  of  Arthropods,  in  which  the  body  is 
furnished  with  distinctly  articulated  or  jointed  limbs,  some  of  which 
are  always  modified  to  serve  as  mouth-organs,  we  come  first  to  the 
class  of  Crustacea,  which  ordinarily  includes  (when  used  in  its 
most  comprehensive  sense)  all  those  animals  belonging  to  this  group 
which  are  fitted  for  aquatic  respiration,  though  the  king-crab 
(Limulus)  seems  to  have  closer  relations  to  the  scorpions,  and  the 
Pycnogonids  to  the  spiders.  It  thus  comprehends  a  very  extensive 
range  of  forms  ;  for  although  we  are  accustomed  to  think  of  the  crab, 
lobster,  cray-fish,  and  other  well-known  species  of  the  order  Decapoda 
(ten-footed)  as  its  typical  examples,  yet  all  these  belong  to  the  highest 
of  its  many  orders  ;  and  among  the  lower  are  many  of  a  far  simpler 
structure,  not  a  few  which  would  not  be  recognised  as  belonging  to 
the  class  at  all  were  it  not  for  the  information  given  by  the 
study  of  their  development  as  to  their  real  nature,  which  is  far  more 
apparent  in  their  early  than  it  is  in  their  adult  condition.  Many 
of  the  inferior  kinds  of  Crustacea  are  so  minute  and  transparent 
that  their  whole  structure  may  be  made  out  by  the  aid  of  the 
microscope  without  any  preparation ;  this  is  the  case,  indeed,  with 
nearly  the  whole  group  of  Entomostraca,  and  with  the  larval  forms 
even  of  the  cra£>,  and  its  allies ;  and  we  shall  give  our  first  atten- 
tion to  these,  afterwards  noticing  such  points  in  the  structure  of  the 
larger  kinds  as  are  likely  to  be  of  general  interest. 

A  curious  example  of  the  reduction  of  an  elevated  type  to  a 
very  simple  form  is  presented  by  the  group  of  Pycnogonida,  or  no- 
body crabs,  some  of  the  members  of  which  may  be  found  by  atten- 
tive search  in  almost  every  locality  where  seaweeds  abound,  it 
being  their  habit  to  crawl  (or  rather  to  sprawl)  over  the  surfaces  of 
these,  and  probably  to  imbibe  as  food  the  gelatinous  substance  with 
which  they  are  invested.1  The  general  form  of  their  bodies  (fig. 
719)  usually  reminds  us  of  that  of  some  of  the  long-legged  crabs, 
the  abdomen  being  almost  or  altogether  deficient,  whilst  the  head  is 
very  small,  and  fused  (as  it  were)  into  the  thorax ;  so  that  the  last- 
named  region,  with  the  members  attached  to  it,  constitutes  nearly 
the  whole  bulk  of  the  animal.  The  head  is  extended  in  front  into 

r  T  It  is  remarkable  that  very  large  forms,  of  this  group,  sometimes  extending  to 
more  than  twelve  inches  across,  have  been  brought  up  from  great  depths  of  the  sea. 


958 


CRUSTACEA 


a  proboscis-like  projection,  at  the  extremity  of  which  is  the  narrow 
orifice  of  the  mouth,  which  draws  in  the  semi-fluid  aliment.  Instead 
of  being  furnished  (as  in  the  higher  crustaceans)  with  two  pairs  of 
antennae  and  numerous  pairs  of  *  foot-jaws,'  it  has  but  a  single  pair 
of  either ;  it  also  bears  four  minute  ocelli,  or  rudimentary  eyes,  set 
at  a  little  distance  from  each  other  on  a  sort  of  tubercle.  From 
the  thorax  proceed  four  pairs  of  legs,  each  composed  of  several  joints, 
and  terminated  by  a  hooked  claw ;  and  by  these  members  the 
animal  drags  itself  slowly  along,  instead  of  walking  actively  upon 
them  like  a  crab.  The  mouth  leads  to  a  very  narrow  oesophagus 
(a),  which  passes  back  to  the  central  stomach  (b)  situated  in  the 


FIG.  719. — Ammothea  pycnogonoides  :  a,  narrow  oesophagus  ; 
&,  stomach;  c,  intestine  ;  d,  digestive  caeca  of  the  foot-jaws  ; 
e,  e,  digestive  caeca  of  the  legs. 

midst  of  the  thorax,  from  the  hinder  end  of  which  a  narrow  intes- 
tine (c)  passes  off.  to  terminate  at  the  posterior  extremity  of  the 
body.  From  the  central  stomach  five  pairs  of  crecal  prolongations 
radiate,  one  pair  (d)  entering  the  foot-jaws,  the  other  four  (e,  e) 
penetrating  the  legs,  and  passing  along  them  as  far  as  the  last  joint 
but  one  ;  and  those  extensions  are  covered  with  a  layer  of  brownish - 
yellow  granules,  which  are  probably  to  be  regarded  as  a  digestive 
gland.  The  stomach  and  its  caecal  prolongations  are  continually 
executing  peristaltic  movements  of  a  very  curious  kind  ;  for  they 
contract  and  dilate  with  an  irregular  alternation,  so  that  a  flux  and 
reflux  of  their  contents  is  constantly  taking  place  between  the 
central  portion  and  its  radiating  extensions.  The  peri  visceral  space 
between  the  widely  extended  stomach  and  the  walls  of  the  body  and 


PYCNOGONIDA;  ENTOMOSTRACA  959 

limbs  is  occupied  by  a  transparent  liquid,  in  which  are  seen  floating 
a  number  of  minute  transparent  corpuscles  of  irregular  size  ;  and 
this  fluid,  which  represents  the  blood,  is  kept  in  continual  motion, 
not  only  by  the  general  movements  of  the  animal,  but  also  by  the 
actions  of  the  digestive  apparatus  ;  since,  whenever  the  caecum  of 
any  one  of  the  legs  undergoes  dilatation,  a  part  of  the  circum- 
ambient liquid  will  be  pressed  out  from  the  cavity  of  that  limb, 
either  into  the  thorax  or  into  some  other  limb  whose  stomach  is 
contracting.  The  fluid  must  obtain  its  aeration  through  the  general 
surface  of  the  body,  as  there  are  no  special  organs  of  respiration. 
The  nervous  system  consists  of  a  single  ganglion  in  the  head  (formed 
by  the  coalescence  of  a  pair),  and  of  another  in  the  thorax  (formed 
by  the  coalescence  of  four  pairs),*  with  which  the  cephalic  ganglion 
is  connected  in  the  usual  mode,  namely,  by  two  nervous  cords  which 
diverge  from  each  other  to  embrace  the  oesophagus.  In  the  study 
of  the  very  curious  phenomena  exhibited  by  the  digestive  apparatus, 
as  well  as  of  the  various  points  of  internal  conformation  which  have 
been  described,  the  achromatic  condenser  will  be  found  useful,  even 
with  the  1-inch,  f-inch,  or  ^-inch  objectives  ;  for  the  imperfect 
transparence  of  the  bodies  of  these  animals  renders  it  of  importance 
to  drive  a  large  quantity  of  light  through  them,  and  to  give  to  this 
light  such  a  quantity  as  shall  sharply  define  the  internal  organs.1 

Entomostraca. — This  group  of  crustaceans,  many  of  the  existing 
members  of  which  are  of  such  minute  size  as  to  be  only  just  visible  to 
the  naked  eye,  is  distinguished  by  the  fact  that  they  never  have  more 
than  three  pairs  of  their  appendages  converted  into  mouth-organs, 
nor  possess  any  appendage  on  such  segments  as  may  lie  behind  the 
generative  orifices.  The  segments  into  which  the  body  is  divided 
are  frequently  very  numerous,  and  are  for  the  most  part  similar  to 
each  other ;  but  there  is  a  marked  difference  in  regard  to  the 
appendages  which  they  bear,  and  to  the  mode  in  which  these 
minister  to  the  locomotion  of  the  animals.  For  in  what  have  been 
called  the  Lophyropoda,  or  'bristly-footed'  tribe,  a  small  number  of 
legs  not  exceeding  five  pairs  have  their  function  limited  to  locomotion, 
the  respiratory  organs  being  attached  to  the  parts  in  the  neighbour- 
hood of  the  mouth  ;  whilst  in  the  Branchiopoda,  or  'gill- footed  '  tribe, 
the  members  (known  as  '  fin-feet ')  serve  both  for  locomotion  and  for 
respiration,  and  the  number  of  these  is  commonly  large,  being  in  Apus 
as  many  as  sixty  pairs.  The  character  of  their  movements  differs 
accordingly  ;  for  whilst  all  the  members  of  the  first-named  tribe  dart 
through  the  water  in  a  succession  of  jerks,  so  as  to  have  acquired  the  com- 
mon name  of  'water-fleas,'  those  among  the  latter  which  possess  a  great 

1  Certain  points  of  resemblance  borne  by  Pycnogonida  to  spiders  make  the 
careful  study  of  their  development  a  matter  of  special  interest  and  importance,  as 
there  is  some  reason  to  regard  them  rather  as  Arachnida  adapted  to  a  marine 
habitat  than  as  Crustacea.  See  Balfour's  Comparative  Embryologtj,  pp.  448,  449, 
and  the  authorities  there  referred  to.  The  most  recent  additions  to  the  literature 
of  the  Pycnogonids  are  Dr.  A.  Dohrn's  Die  Pantopoden  des  Golfes  von  Neapel 
&c.,  Leipzig,  1881 ;  Dr.  P.  P.  C.  Hoek's  '  Report  on  the  Pycnogonida  of  the  Challenger,' 
1881,  and  his  '  Nouvelle  Etude  sur  les  Pycnogonides,'  in  Archives  de  Zool.  Exper.  ix. 
p.  445  ;  and  Professor  G.  O.  Sars's  report  in  the  Zoology  of  the  Norwegian  North  Sea 
Expedition. 


960  CRUSTACEA 

number  of  '  fin-feet '  swim  with  an  easy  gliding  movement,  sometimes 
on  their  back  alone  (as  is  the  case  with  Branchipus)  and  sometimes 
with  equal  facility  on  the  back,  belly,  or  sides  (as  is  done  by  Artemia 
salina,  the  '  brine-shrimp ').  Some  of  the  most  common  forms  of 
both  tribes  will  now  be  briefly  noticed. 

The  first  group  contains  two  orders,  of  which  the  first,  Ostracoda, 
is  distinguished  by  the  complete  inclosure  of  the  body  in  a  bivalve 
shell,  by  the  small  number  of  legs,  and  by  the  absence  of  an  external 
egg-sac.  One  of  the  best  known  examples  is  the  little  Cypria,  which 
is  a  common  inhabitant  of  pools  and  streams ;  this  may  be  recognised 
by  its  possession  of  two  pairs  of  antennae,  the  first  having  numerous 
joints  with  a  pencil-like  tuft  of  filaments,  and  projecting  forwards 
from  the  front  of  the  head,  whilst  the  second  has  more  the  shape  of 
legs,  and  is  directed  downwards,  and  by  the  limitation  of  its  legs  to 
two  pairs,  of  which  the  posterior  does  not  make  its  appearance  outside 
the  shell,  being  bent  upwards  to  give  support  to  the  ovaries.  The 
valves  are  generally  opened  widely  enough  to  allow  the  greater  part 
of  both  pairs  of  antennae  and  of  the  front  pair  of  legs  to  pass  out 
between  them ;  but  when  the  animals  are  alarmed,  they  draw  these 
members  within  the  shell,  and  close  the  valves  firmly.  They  are 
very  lively  creatures,  being  almost  constantly  seen  in  motion,  either 
swimming  by  the  united  action  of  their  foot-like  antennae  and  legs, 
or  walking  upon  plants  and  other  solid  bodies  floating  in  the  water. 
Nearly  allied  to  the  preceding  is  Cy there,  whose  body  is  furnished 
with  three  pairs  of  legs,  all  projecting  out  of  the  shell,  and  whose 
superior  antennae  are  destitute  of  the  filamentous  brush  ;  this  genus 
is  almost  entirely  marine,  and  some  species  of  it  may  almost  in- 
variably be  met  with  in  little  pools  among  the  rocks  between  the 
tide-marks,  creeping  about  (but  not  swimming)  amongst  Conferva' 
and  Corallines.  There  is  abundant  evidence  of  the  former  existence 
of  Crustacea  of  larger  size  than  any  now  existing,  for  in  certain 
fresh-water  strata,  both  of  the  Secondary  and  Tertiary  series,  we  find 
layers,  sometimes  of  great  extent  and  thickness,  which  are  almost 
entirely  composed  of  the  fossilised  shells  of  Cyprides ;  whilst  in 
certain  parts  of  the  chalk,  which  was  a  marine  deposit,  the  remains 
of  bivalve  shells  resembling  those  of  Cythere  present  themselves 
in  such  abundance  as  to  form  a  considerable  part  of  its  substance.1 

In  the  order  Copepoda  there  is  a  jointed  shell  forming  a  kind 
of  buckler  or  carapace  that  almost  entirely  incloses  the  head  and 
thorax,  an  opening  being  left  beneath,  through  which  the  appendages 
project ;  and  there  are  five  pairs  of  legs,  mostly  adapted  for  swim- 
ming, the  fifth  pair,  however,  being  rudimentary  in  the  genus  Cyclops, 
the  commonest  example  of  the  group.  This  genus  receives  its  name 
from  possessing  only  a  single  eye,  or  rather  a  single  cluster  of  ocelli ; 
which  character,  however,  it  has  in  common  with  the  two  genera 
already  named,  as  well  as  with  Daphnia,  and  with  many  other 
Entomostraca.  It  contains  numerous  species,  some  of  which  belong 

1  On  the  recent  British  Ostracoda  see  the  monograph  by  G.  S.  Brady  in  vol.  xxvi. 
of  the  Transactions  of  the  Linnean  Society  of  London ;  compare  also  Zenker, 
'  Monographic  der  Ostracoden,'  Arcliiv  fur  Naturg.  xx.  1854.  Glaus  has  an  essay  on 
the  development  of  Cypris,  Marburg,  1868  ;  see  also  Dr.  Brady's '  Challenger  Report.' 


ENTOMOSTRACA 


96l 


to  the  fresh  water,  whilst  others  are  marine.  The  fresh-water 
species  often  abound  in  the  muddiest  and  most  stagnant  pools, 
as  well  as  in  the  clearest  springs.  Of  the  marine  species  some 
are  to  be  found  in  the  localities  in  which  the  Cythere  is  most 
abundant,  whilst  others  inhabit  the  open  ocean,  and  must  be  col- 
lected by  the  tow- net.  The  body  of  the  Cyclops  is  soft  and  gela- 
tinous, and  it  is  composed  of  two  distinct  parts,  a  thorax  (fig.  720,  a) 
and  an  abdomen  (b),  of  which  the  latter,  being  comparatively  slender, 
is  commonly  considered  as  a  tail,  though  tra versed  by  the  intestine, 
which  terminates  near  its 
extremity.  The  head,  which 
coalesces  with  the  thorax, 
bears  one  very  large  pair 
of  antennae  (c),  possessing 
numerous  articulations  and 
furnished  with  bristly  ap- 
pendages, and  another  small 
pair  (cl) ;  it  is  also  furnished 
with  a  pair  of  mandibles  or 
true  jaws  and  with  two 
pairs  of  '  maxillae,'  of  which 
the  hinder  pair  is  the  longer 
and  more  abundantly  sup- 
plied with  bristles.  The 
legs  (e)  are  all  beset  with 
plumose  tufts,  as  is  also  the 
tail  (/,  /)  which  is  borne  at 
the  extremity  of  the  ab- 
domen. On  either  side  of 
the  abdomen  of  the  female, 
there  is  often  to  be  seen  an 
egg  -  capsule  (B) ;  within 
which  the  ova,  after  be- 
ing fertilised,  undergo  the 
earlier  stages  of  their  de- 
velopment. The  Cyclops  is 
a  very  active  creature,  and 
strikes  the  water  in  swimming,  not  merely  with  its  legs  and  tail 
but  also  with  its  antennae.  The  rapidly  repeated  movements  of  its 
feet-jaws  serve  to  create  a  whirlpool  in  the  surrounding  water,  by 
which  minute  animals  of  various  kinds,  and  even  its  own  young,  are 
brought  to  its  mouth  to  be  devoured.1 

The  tribe  of  Branchiopoda  is  divided  also  into  two  groups,  of 
which  the  Cladocera  present  the  nearest  approach  to  the  preceding, 
having  a  bivalve  carapace,  no  more  than  from  four  to  six  pairs  of 
legs,  two  pairs  of  antennae,  of  which  one  is  large  and  branched  and 
adapted  for  swimming,  and  a  single  eye.  The  commonest  form  of 

1  See  for  British  forms  Professor  G-.  S.  Brady's  Monograph  of  the  free  and 
semi-parasitic  Copepoda  of  the  British  Islands,  published  by  the  Kay  Society, 
1878-80,  and  Mr.  I.  C.  Thompson's  accounts  of  those  collected  near  the  Isle  of  Man, 
published  by  the  Liverpool  Biological  Society. 

3Q 


FIG.  720. — A,  female  of  Cyclops  quadricornis : 
a,  body  ;  b,  tail ;  c,  antenna ;  d,  antennule ;  e, 
feet ;  /,  plumose  setae  of  tail.  B,  tail,  with 
external  egg-sacs.  C,  D,  E,  F,  G,  successive 
stages  of  development  of  young. 


962  CRUSTACEA 

this  is  the  Daphnia,  pulex,  which  is  sometimes  called  the  ;  arborescent 
water-flea/  from  the  branching  form  of  its  antennae.  It  is  very 
abundant  in  many  ponds  and  ditches,  coming  to  the  surface  in  the 
mornings  and  evenings  and  in  cloudy  weather,  but  seeking  the 
depths  of  the  water  during  the  heat  of  the  day.  It  swims  by 
taking  short  springs  ;  and  feeds  on  minute  particles  of  vegetable  sub- 
stances, but  does  not,  however,  reject  animal  matter  when  offered. 
Some  of  the  peculiar  phenomena  of  its  reproduction  will  be  presently 
described. 

The  other  group,  Phyllopoda,  includes  those  Branchiopoda  whose 
body  is  divided  into  a  great  number  of  segments,  nearly  all  of  which 
are  furnished  with  leaflike  appendages,  or  *  fin-feet.'  The  two 
families  which  this  group  includes,  however,  differ  considerably  in 
their  conformation  ;  for  in  that  of  which  the  genera  Apus  and  Nebalia  l 
are  representatives,  the  body  is  inclosed  in  a  shell,  either  shield- like 
or  bivalve,  and  the  feet  are  generally  very  numerous ;  whilst  in  that 
which  contains  Branchipus  and  Artemia,  the  body  is  entirely  unpro- 
tected, and  the  number  of  pairs  of  feet  does  not  exceed  eleven.  The 
Apus  caiwriformis.;  which  is  an  animal  of  comparatively  large  size,  its 
entire  length  being  about  2^  inches,  is  an  inhabitant  of  stagnant 
waters ;  but  although  occasionally  very  abundant  in  particular  pools, 
or  ditches,  it  is  not  to  be  met  with  nearly  so  commonly  as  the  Ento- 
mostraca  already  noticed ;  in  this  country,  indeed,  it  is  exceedingly 
rare.  It  is  recognised  by  its  large  oval  carapace,  which  covers  the 
head  and  body  like  a  shield  ;  by  the  nearly  cylindrical  form  of  its 
body,  which  is  composed  of  thirty  articulations,  and  by  the  large 
number  of  its  appendages,  which  amount  to  about  sixty  pairs.  The 
number  of  joints  in  these  is  so  great  that  in  a  single  individual  they 
may  be  safely  estimated  at  not  less  than  two  millions.  These  organs, 
however,  are  for  the  most  part  small ;  and  the  instruments  chiefly 
used  by  the  animal  for  locomotion  are  the  first  pair  of  feet,  which  are 
very  much  elongated  (bearing  such  a  resemblance  to  the  principal 
antennae  of  other  Entomostraca  as  to  be  commonly  ranked  in  the 
same  light),  and  are  distinguished  as  rami  or  oars.  With  these  they 
can  swim  freely  in  any  position ;  but  when  the  rami  are  at  rest,  and 
the  animal  floats  idly  on  the  water,  its  fin-feet  may  be  seen  in  in- 
cessant motion,  causing  a  sort  of  whirlpool  in  the  water,  and  bringing 
to  the  mouth  the  minute  animals  (chiefly  the  smaller  Entomostraca 
inhabiting  the  same  localities)  that  serve  for  its  food.  The  Branchipus 
stagnalis  has  a  slender,  cylindriform,  and  very  transparent  body,  of 
nearly  an  inch  in  length,  furnished  with  eleven  pairs  of  fin-feet,  but 
is  destitute  of  any  protecting  envelope  ;  its  head  is  furnished  with  a 
pair  of  very  curious  prehensile  organs,  which  are  really  modified 
antennae,  whence  it  has  received  the  name  of  Cheirocephalus ;  but 

1  Professor  Glaus  has  pointed  out  the  relations  of  Nebalia  to  the  Malacostraca,  or 
higher  division  of  the  Crustacea,  and  has  suggested  for  the  group  which  they  re- 
present the  name  of  Leptostraca.  See  the  Zeitscfir.  fur  wiss.  Zool.  1872,  p.  323  ; 
Claus,  Untersuchungen  zur  Erforschung  der  genealogischen  Grundlage  des 
Crustaceen- Systems,  Wien,  1876,  as  well  as  '  Ueber  den  Organismus  der  Nebaliiden 
und  die  systematische  Stellung  der  Leptostraken,'  in  Arl.  Zool.  Inst.  Wien.  viii. 
(1889),  pp.  1-148,  15  pis. ;  but  a  different  view  is  taken  by  Professor  G.  O.  Sars  in  his 
Keport  on  the  Challenger  Phyllocarida. 


ENTOMOSTRACA  963 

these  are  not  used  by  it  for  the  seizure  of  prey,  as  the  food  of  this 
animal  is  vegetable,  but  to  clasp  the  female  in  the  act  of  copulation. 
The  Sranchipus  or  Cheirocephalus  is  certainly  the  most  beautiful  and 
elegant  of  all  the  Entomostraca,  being  rendered  extremely  attractive 
to  the  view  by  '  the  uninterrupted  undulatory  wavy  motion  of  its 
graceful  branchial  feet,  slightly  tinged  as  they  are  with  a  light  red- 
dish hue,  the  brilliant  mixture  of  transparent  bluish-green  and  bright 
red  of  its  prehensile  antennae,  and  its  bright  red  tail  with  the  beauti- 
ful plumose  setre  springing  from  it.'  Unfortunately,  however,  it  is 
a  very  rare  animal  in  this  country.  The  Artemia  salina,  or  '  brine- 
shrimp,'  is  an  animal  of  very  similar  organisation,  and  almost 
equally  beautiful  in  its  appearance  and  movements,  but  of  smaller 
size,  its  body  being  about  half  an  inch  in  length.  Its  '  habitat '  is 
very  peculiar,  for  it  is  only  found  in  the  salt-pans  or  brine-pits  in 
which  sea- water  is  undergoing  concentration  (as  at  Lymington) ;  and 
in  these  situations  it  is  sometimes  so  abundant  as  to  communicate  a 
red  tinge  to  the  liquid. 

Some  of  the  most  interesting  points  in  the  history  of  the  Ento- 
mostraca lie  in  the  peculiar  mode  in  which  their  generative  function 
is  performed,  and  in  their  tenacity  of  life  when  desiccated,  in  which 
last  respect  they  correspond  with  many  Rotifers.  By  this  pro- 
vision they  escape  being  completely  exterminated,  as  they  might 
otherwise  soon  be,  by  the  drying  up  of  the  pools,  ditches,  and  other 
small  collections  of  water  which  constitute  their  usual  habitats. 
We  do  not,  of  course,  imply  that  the  adult  animals  can  bear  a  com- 
plete desiccation,  although  they  will  preserve  their  vitality  in  mud 
that  holds  the  smallest  quantity  of  moisture ;  but  their  eggs  are 
more  tenacious  of  life,  and  there  is  ample  evidence  that  these  will 
become  fertile  on  being  moistened,  after  having  remained  for  a  long 
time  in  the  condition  of  fine  dust.  Most  Entomostraca,  too,  are 
killed  by  severe  cold,  and  thus  the  whole  race  of  adults  perishes 
every  winter  ;  but  their  eggs  seem  unaffected  by  the  lowest  tempera- 
ture, and  thus  continue  the  species,  which  would  be  otherwise  ex- 
terminated. Again,  we  frequently  meet  in  this  group  with  that 
agamic  reproduction  which  we  have  seen  to  prevail  so  extensively 
among  the  lower  forms.  In  many  species  there  is  a  double 
mode  of  multiplication,  the  sexual  and  the  non-sexual.  The 
former  takes  place  at  certain  seasons  only,  the  males  (which  are 
often  so  different  in  conformation  from  the  females  that  they  would 
not  be  supposed  to  belong  to  the  same  species  if  they  were  not  seen 
in  actual  congress)  disappearing  entirely  at  other  times.  The  latter, 
on  the  other  hand,  continues  at  all  periods  of  the  year,  so  long  as 
warmth  and  food  are  supplied,  and  is  repeated  many  times  so  as  to 
give  origin  to  as  many  successive  '  broods.'  Further,  a  single  act  of 
impregnation  may  serve  to  fertilise,  not  merely  the  ova  wrhich  are 
then  mature  or  nearly  so,  but  all  those  subsequently  produced  by 
the  same  female,  wrhich  are  deposited  at  considerable  intervals.  In 
these  two  modes  the  multiplication  of  these  little  creatures  is  carried 
on  with  great  rapidity,  the  young  animal  speedily  coming  to  maturity 
and  beginning  to  propagate,  so  that,  according  to  the  computation 
of  Jurine,  founded  upon  data  ascertained  by  actual  observation,  a 

3Q  2 


964  CKUSTACEA 

single  fertilised  female  of  the  common  Cyclops  quadricornis  may  be 
the  progenitor  in  one  year  of  4,442,189,120  young.1 

The  eggs  of  some  Entomostraca  are  deposited  freely  in  the  water, 
or  are  carefully  attached  in  clusters  to  aquatic  plants ;  but  they  are 
more  frequently  carried  for  some  time  by  the  parent  in  special 
receptacles  developed  from  the  posterior  part  of  the  body  ;  and  in 
many  cases  they  are  retained  there  until  the  young  are  ready  to 
come  forth,  so  that  these  animals  may  be  said  to  be  ovo- viviparous. 
In  Daphnia  the  eggs  are  received  into  a  large  cavity  between  the 
back  of  the  animal  and  its  shell,  and  there  the  young  undergo  almost 
their  whole  development,  so  as  to  come  forth  in  a  form  nearly 
resembling  that  of  their  parent.  Soon  after  their  birth  a  moult  or 
exuviation  of  the  shell  takes  place,  and  the  egg-coverings  are  cast 
off  with  it.  In  a  very  short  time  afterwards  another  brood  of  eggs 
is  seen  in  the  cavity  and  the  same  process  is  repeated,  the  shell 
being  again  exuviated  after  the  young  have  been  brought  to  maturity. 
At  certain  times,  however,  the  Daphnia  may  be  seen  with  a  dark 
opaque  substance  within  the  back  of  the  shell,  wrhich  has  been  called 
the  ephippium,  from  its  resemblance  to  a  saddle.  This,  when  care- 
fully examined,  is  found  to  be  of  dense  texture,  and  to  be  composed 
of  a  mass  of  hexagonal  cells ;  and  it  contains  two  oval  bodies,  each 
consisting  of  an  ovum  covered  with  a  horny  casing,  enveloped  in  a 
capsule  which  opens  like  a  bivalve  shell.  From  the  observations  of 
Sir  J.  Lubbock,2  it  appears  that  the  ephippium  is  really  only  an 
altered  portion  of  the  carapace,  its  outer  valve  being  a  part  of  the 
outer  layer  of  the  epidermis,  and  its  inner  valve  the  corresponding 
part  of  the  inner  layer.  The  development  of  the  ephippial  eggs  takes 
place  at  the  posterior  part  of  the  ovaries,  and  is  accompanied  by  the 
formation  of  a  greenish-brown  mass  of  granules ;  and  from  this 
situation  the  eggs  pass  into  the  receptacle  formed  by  the  new  cara- 
pace, where  they  become  included  between  the  two  layers  of  the 
ephippium.  This  is  cast  off,  in  process  of  time,  with  the  rest  of  the 
skin,  from  which,  however,  it  soon  becomes  detached ;  and  it  con- 
tinues to  envelope  the  eggs,  generally  floating  on  the  surface  of 
the  water  until  they  are  hatched  with  the  returning  warmth  of 
spring.  This  curious  provision  obviously  affords  protection  to 
the  eggs  which  are  to  endure  the  severity  of  winter  cold ;  and  an 
approach  to  it  may  be  seen  in  the  remarkable  firmness  of  the 
envelopes  of  the  *  winter  eggs '  of  some  Rotifera.  There  seems  a 
strong  probability,  from  the  observations  of  Sir  J.  Lubbock  (now 
Lord  Avebury),  that  the  '  ephippial '  eggs  are  true  sexual  products, 
since  males  are  to  be  found  at  the  time  when  the  ephippia  are  de- 
veloped ;  whilst  it  is  certain  that  the  ordinary  eggs  can  be  produced 
non-sexually,  and  that  the  young  which  spring  from  them  can  multi- 
ply the  race  in  like  manner.  The  young  which  are  produced  from 
the  ephippial  eggs  seem  to  have  the  same  power  of  continuing  the 

1  For  an  interesting  account  of  the  parthenogenetic  development  of  Apus  and  its 
allies  see  the  sixth  of  Von  Siebold's  Beitrdge  zur  Parthenogenesis  der  Arthropoden 
(Leipzig,  1871). 

2  '  An  account  of  the  two  Methods   of   Reproduction   in   Daplmia,   and  of  the 
Structure  of  the  Ephippium,'  in  Phil.   Trans.  1857,  p.  79.     On  the  '  summer-egg  '  of 
Daphnia  see  Lebedinsky,  Zool.  Anzeig.  xiv.  p.  149. 


ENTOMOSTKACA  965 

race   by  non-sexual   reproduction    as   the  young   developed  under 
ordinary  circumstances. 

In  most  Entomostraca  the  young  at  the  time  of  their  emersion 
from  the  egg  differ  considerably  from  the  parent,  especially  in  having 
only  the  thoracic  portion  of  the  body  as  yet  developed,  and  in  pos- 
sessing but  a  small  number  of  locomotor  appendages  (see  fig.  720, 
C-G)  ;  the  visual  organs,  too,  are  frequently  wanting  at  first.  The 
process  of  development,  however,  takes  place  with  great  rapidity, 
the  animal  at  each  successive  moult  (which  process  is  very  commonly 
repeated  at  intervals  of  a  day  or  two)  presenting  some  new  parts, 
and  becoming  more  and  more  like  its  parent,  which  it  very  early 
resembles  in  its  power  of  multiplication,  the  female  laying  eggs 
before  she  has  attained  her  own  full  size.  Even  when  the  Entomo- 
straca have  attained  their  full  growth,  they  continue  to  exuviate 
their  shell  at  short  intervals  during  the  whole  of  life ;  and  this 
repeated  moulting  seems  to  prevent  the  animal  from  being  injured, 
or  its  movements  obstructed,  by  the  overgrowth  of  parasitic  animal- 
cules and  conferva,  weak  and  sickly  individuals  being  frequently 
seen  to  be  so  covered  with  such  parasites  that  their  motion  and  life 
are  soon  arrested,  apparently  because  they  have  not  strength  to  cast 
off  and  renew  their  envelopes.  The  process  of  development  appears 
to  depend  in  some  degree  upon  the  influence  of  light,  being  retarded 
when  the  animals  are  secluded  from  it ;  but  its  rate  is  still  more 
influenced  by  heat ;  and  this  appears  also  to  be  the  chief  agent  that 
regulates  the  time  which  elapses  between  the  moultings  of  the  adult, 
these,  in  Daphnia,  taking  place  at  intervals  of  two  days  in  warm 
summer  weather,  whilst  several  days  intervene  between  them  when 
the  weather  is  colder.  The  cast  shell  carries  with  it  the  sheaths  not 
only  of  the  limbs  and  plumes,  but  of  the  most  delicate  hairs  and 
setae  which  are  attached  to  them.  If  the  animal  have  previously 
sustained  the  loss  of  a  limb,  it  is  generally  renewed  at  the  next  naoult, 
as  in  higher  Crustacea.1 

Forming  part  of  the  entomostracous  group  is  the  tribe  of 
suctorial  Crustacea,2  which  for  the  most  part  live  as  parasites  upon 
the  exterior  of  other  animals  (especially  fish),  whose  juices  they 
imbibe  by  means  of  the  peculiar  proboscis-like  organ  which  takes 
in  them  the  place  of  the  jaws  of  other  crustaceans;  whilst  other 
appendages,  representing  the  foot-jaws,  are  furnished  with  hooks, 
by  which  these  parasites  attach  themselves  to  the  animals  from 
whose  juices  they  derive  their  nutriment.  Many  of  the  suctorial 
Crustacea  bear  a  strong  resemblance,  even  in  their  adult  condition, 
to  other  Entomostraca;  but  more  commonly  it  is  between  the 
earlier  forms  of  the  two  that  the  resemblance  is  the  closest, 
most  of  the  Suctoria  undergoing  such  extraordinary  changes  in  their 

1  For   a  systematic   and  detailed  account   of   this   group   Dr.   Baird's   Natural 
History  of  the  British  Entomostraca,  published  by  the  Bay  Society  in  1849,  must 
still  be  recommended.     The  numerous   essays  by   Professor  Glaus  should  also  be 
consulted. 

2  It  is  now  generally  recognised  that  these  should  be  placed  with  the  Copepoda, 
which  may  be  divided  into  the  Eucopepoda  and  the  Branchiura ;  the  former  are 
divisible  into  the  Gnathostomata,  most  of  which  are  non-parasitic,  and  have  been 
already  described  under  Copepoda,  and  the  Siphonostomata,  of  which  Lemcsa  is  an 
example. 


966  CRUSTACEA 

progress  towards  the  adult  condition  that,  if  their  complete  forms 
were  alone  attended  to,  they  might  be  excluded  from  the  class 
altogether,  as  was  (in  fact)  done  by  many  earlier  zoologists.  Of  the 
suctorial  Crustacea  which  form  the  group  Branchiura  may  be 
specially  mentioned  the  Argulus  foliaceus,  which  attaches  itself  to 
the  surface  of  the  bodies  of  fresh-water  fish,  such  as  the  stickleback, 
and  is  commonly  known  under  the  name  of  the  '  fish-louse.'  This 
animal  has  its  body  covered  with  a  large  firm  oval  shield,  which 
does  not  extend,  however,  over  the  posterior  part  of  the  abdomen. 
The  mouth  is  armed  with  a  pair  of  styliform  mandibles;  and  on 
each  side  of  the  proboscis  there  is  a  large,  short,  cylindrical  ap- 
pendage, terminated  by  a  curious  sort  of  sucking-disc,  with  another 
pair  of  longer  jointed  members,  terminated  by  prehensile  hooks. 
These  two  pairs  of  appendages,  which  are  probably  to  be  considered 
as  representing  the  foot-jaws,  are  followed  by  four  pairs  of  legs, 
which,  like  those  of  the  branchiopods,  are  chiefly  adapted  for 
swimming ;  and  the  tail,  also,  is  a  kind  of  swimmeret.  This  little 
animal  can  leave  the  fish  upon  which  it  feeds,  and  then  swims  freely  in 
the  water,  usually  in  a  straight  line,  but  frequently  and  suddenly 
changing  its  direction,  and  sometimes  turning  over  and  over  several 
times  in  succession.  The  stomach  is  remarkable  for  the  large  csecal 
prolongations  which.it  sends  out  on  either  side,  immediately  beneath 
the  shell ;  for  these  subdivide  and  ramify  in  such  a  manner  that  they 
are  distributed  almost  as  minutely  as  the  caecal  prolongations  of  the 
stomach  of  the  Planar  ia  (fig.  714).  The  proper  alimentary  canal,  how- 
ever, is  continued  backwards  from  the  central  cavity  of  the  stomach,  as 
an  intestinal  tube,  which  terminates  in  an  anal  orifice  at  the  extremity 
of  the  abdomen.  A  far  more  remarkable  departure  from  the  typical 
form  of  the  class  is  shown  in  the  Lerncea,  which  is  found  attached 
to  the  gills  of  fishes.  This  creature  has  a  long  suctorial  proboscis  ; 
a  short  thorax,  to  which  is  attached  a  single  pair  of  legs,  which  meet 
at  their  extremities,  where  they  bear  a  sucker  which  helps  to  give 
attachment  to  the  parasite  ;  a  large  abdomen  ;  and  a  pair  of  pendent 
egg-sacs.  In  its  adult  condition  it  buries  its  anterior  portion  in  the 
soft  tissue  of  the  animal  it  infests,  and  appears  to  have  little  or  no 
power  of  changing  its  place.  But  the  young,  when  they  come  forth 
from  the  egg,  are  as  active  as  the  young  of  Cyclops  (fig.  720,  C.  D), 
which  they  much  resemble ;  and  only  attain  the  adult  form  after  a 
series  of  metamorphoses,  in  which  they  cast  off  their  locomotive 
members  and  eyes.  It  is  curious  that  the  original  form  is  retained 
with  comparatively  slight  change  by  the  males,  which  increase  but 
little  in  size,  and  are  so  unlike  the  females  that  no  one  would  suppose 
the  two  to  belong  to  the  same  family,  much  less  to  the  same  species, 
but  for  the  study  of  their  development.1 

From  the  parasitic   suctorial  Crustacea   the   transition   is  not 

1  As  the  group  of  suctorial  Crustacea  is  interesting  rather  to  the  professed 
naturalist  than  to  the  amateur  microscopist,  even  an  outline  view  of  it  would  be  un- 
suitable to  the  present  work ;  and  the  Author  would  -refer  such  of  his  readers  as  may 
desire  to  study  it  to  the  excellent  treatise  by  Dr.  Baird  already  referred  to.  Of  the 
numerous  recent  essays  and  memoirs  those  of  Professor  Glaus  should  by  all  means 
be  consulted.  Mr.  P.  W.  Bassett-Smith,  Staff-surgeon  E.N.,  has  in  the  last  few  years 
published  several  interesting  papers. 


CIKEIPEDIA 


967 


really  so  abrupt  as  it  might  at  first  sight  appear  to  the  group  of 
Cirripedia,  consisting  of  the  barnacles  and  their  allies  ;  for  these, 
like  many  of  the  Suctoria,  are  fixed  to  one  spot  during  the  adult 
portion  of  their  lives,  but  come  into  the  world  in  a  condition  that 
bears  a  strong  resemblance  to  the  early  state  of  many  other 
Crustacea.  The  departure  from  the  ordinary  crustacean  type  in 
the  adults  is,  in  fact,  so  great  that  it  is  not  surprising  that  zoolo- 
gists in  general  should  have  ranked  them  in  a  distinct  class,  their 
superficial  resemblance  to  the  Mollusca,  indeed,  having  caused  most 
systematists  to  place  them  in  that  series,  until  due  weight  was 
given  to  those  structural  features  which  mark  their  *  articulated ' 
character.  We  must  limit  ourselves,  in  our  notice  of  this  group, 
to  that  very  remarkable  part  of  their  history,  the  microscopic 
study  of  which  has  contributed  most  essentially  to  the  elucidation 


FIG.  721. — Development  of  Balanus  balanoides :  A,  earliest 
form ;  B,  larva  after  second  moult ;  C,  side  view  of  the  same ; 
D,  stage  immediately  preceding  the  loss  of  activity;  a, 
stomach  (?) ;  b,  nucleus  of  future  attachment  (?). 

of  their  real  nature.  The  observations  of  Mr.  J.  Y.  Thompson,1  with 
the  extensions  and  rectifications  which  they  have  subsequently 
received  from  others  (especially  Mr.  Spence  Bate2  and  Mr.  Dar- 
win 3),  show  that  there  is  no  essential  difference  between  the  early 
forms  of  the  sessile  Cirripeds  (Balanidce  or  'acorn-shells')  and  of  the 
pedunculated  (Lepadidce  or  '  barnacles ')  ;  for  both  are  active  little 
animals  (fig.  721,  A),  possessing  three  pairs  of  legs  and  a  pair  of 
compound  eyes,  and  having  the  body  covered  with  an  expanded 
carapace,  like  that  of  many  entomostracous  crustaceans,  so  as  in  no 

1  Zoological  Besearches,  No.  IV.  1830,  and  Phil.  Trans.  1885,  p.  355. 

2  'On  the  Development   of  the  Cirripedia'  in  Ann.  Nat.  Hist.  ser.  ii.  vol.  viii. 
1851,  p.  324. 

5  Monograph  of  the  Sub-Class  Cirripedia,  published  by  the  Ray  Society. 


968  CKUSTACEA 

essential  particular  to  differ  from  the  larva  of  Cyclops  (fig.  720,  C). 
After  going  through  a  series  of  metamorphoses,  one  stage  of  which 
is  represented  in  fig.  721,  B,  C,  these  larvae  come  to  present  a  form, 
D,  which  reminds  us  strongly  of  that  of  Daphnia,  the  body  being 
inclosed  in  a  shell  composed  of  two  valves,  which  are  united  along 
the  back,  whilst  they  are  free  along  their  lower  margin,  where  they 
separate  for  the  protrusion  of  a  large  and  strong  anterior  pair  of 
prehensile  limbs,  provided  with  an  adhesive  sucker  and  hooks,  and 
of  six  pairs  of  posterior  legs  adapted  for  swimming.  This  bivalve 
shell,  with  the  members  of  both  kinds,  is  subsequently  thrown  off; 
the  animal  then  attaches  itself  by  its  head,  a  portion  of  which,  in 
the  barnacle,  becomes  excessively  elongated  into  the  '  peduncle '  of 
attachment,  whilst  in  Balanus  it  expands  into  a  broad  disc  of 
adhesion  ;  the  first  thoracic  segment  sends  backwards  a  prolongation 
which  arches  over  the  rest  of  the  body,  so  as  completely  to  inclose 
it,  and  of  which  the  exterior  layer  is  consolidated  into  the  '  multi- 
valve  '  shell  ;  whilst  from  the  other  thoracic  segments  are  evolved 
the  six  pairs  of  cirri,  from  whose  peculiar  character  the  name  of 
the  group  is  derived.  These  are  long,  slender,  many-jointed,  tendril- 
like  appendages,  fringed  with  delicate  filaments  covered  with 
cilia,  whose  action  serves  both  to  bring  food  to  the  mouth  and  to 
maintain  aerating  currents  in  the  water.  The  balani  are  peculiarly 
interesting  objects  in  the  aquarium  on  account  of  the  pumping 
action  of  their  beautiful  feathery  appendages,  which  may  be  watched 
through  a  tank  microscope ;  and  their  cast  skins,  often  collected  by 
the  tow-net,  are  well  worth  mounting.1 

Malacostraca. — The  chief  points  of  interest  to  the  microscopist 
in  the  more  highly  organised  forms  of  Crustacea  are  furnished  by 
the  structure  of  the  exoskeleton,  and  by  the  phenomena  of  meta- 
morphosis, both  which  may  be  best  studied  in  the  commonest  kinds. 
The  exoskeleton  of  the  Decapods  in  its  most  complete  form  consists 
of  three  strata,  viz.  1,  a  horny  structureless  layer  covering  the 
exterior  ;  2,  an  areolated  stratum  ;  and  3,  a  laminated  tubular  sub- 
stance. The  innermost  and  even  the  middle  layers,  however,  may  be 
altogether  wanting ;  thus,  in  the  larval  forms  known  as  Phyllosomata 
or  'glass-crabs,'  the  envelope  is  formed  by  the  transparent  horny 
layer  alone  ;  and  in  many  of  the  small  crabs  belonging  to  the  genus  Por- 
tunus  the  whole  substance  of  the  carapace  beneath  the  horny  invest- 
ment presents  the  areolated  structure.  It  is  in  the  large  thick-shelled 
crabs  that  we  find  the  three  layers  most  differentiated.  Thus  in 
the  common  Cancer  pagurus  we  may  easily  separate  the  structure- 
less horny  covering  after  a  short  maceration  in  dilute  acid ;  the 
areolated  layer,  in  which  the  pigmentary  matter  of  the  coloured 
parts  of  the  shell  is  chiefly  contained,  may  be  easily  brought  into 
view  by  grinding  away  from  the  intwr  side  as  flat  a  piece  as  can  be 
selected,  having  first  cemented  the  outer  surface  to  the  glass  slide, 
and  by  examining  this  with  a  magnifying  power  of  250  diameters, 
driving  a  strong  light  through  it  with  the  achromatic  condenser  ; 

1  Valuable  details  as  to  the  structure  of  this  group  will  be  found  in  Dr.  P.  P.  C. 
Hoek's  report  on  the  Cirripeds  collected  by  H.M.S.  Challenger.  Compare,  also, 
M.  Nussbaum,  Anatomische  Studien,  Bonn,  1890. 


MALACOSTRACA  969 

whilst  the  tubular  structure  of  the  thick  inner  layer  may  be  readily 
demonstrated  by  means  of  sections  parallel  and  perpendicular  to  its 
surface.  This  structure,  which  resembles  that  of  dentine,  save  that 
the  tubuli  do  not  branch,  but  remain  of  the  same  size  through  their 
whole  course,  may  be  particularly  well  seen  in  the  black  extremity 
of  the  claw,  which  (apparently  from  some  peculiarity  in  the  mole- 
cular arrangement  of  its  mineral  particles)  is  much  denser  than  the 
rest  of  the  shell,  the  former  having  almost  the  semi-transparence 
of  ivory,  whilst  the  latter  has  a  chalky  opacity.  In  a  transverse 
section  of  the  claw  the  tubuli  may  be  seen  to  radiate  from  the  central 
cavity  towards  the  surface,  so  as  very  strongly  to  resemble  their 
arrangement  in  a  tooth  ;  and  the  resemblance  is  still  further  increased 
by  the  presence,  at  tolerably  regular  intervals,  of  minute  sinuosities 
corresponding  with  the  laminations  of  the  shell,  which  seem,  like 
the  '  secondary  curvatures '  of  the  dentinal  tubuli,  to  indicate  suc- 
cessive stages  in  the  calcification  of  the  animal  basis.  In  thin 
sections  of  the  areolated  layer  it  may  be  seen  that  the  apparent 
walls  of  the  areolse  are  merely  translucent  spaces  from  which  the 
tubuli  are  absent,  their  orifices  being  abundant  in  the  intervening 
spaces.1  The  tubular  layer  rises  up  through  the  pigmentary  layer 
of  the  crab's  shell  in  little  papillary  elevations,  which  seem  to  be 
concretionary  nodules;  and  it  is  from  the  deficiency  of  the  pig- 
mentary layer  at  these  parts  that  the  coloured  portion  of  the  shell 
derives  its  minutely  speckled  appearance.  Many  departures  from 
this  type  are  presented  by  the  different  species  of  decapods  ;  thus 
in  the  prawns  there  are  large  stellate  pigment-spots  resembling 
those  of  frogs,  the  colours  of  which  are  often  in  remarkable  con- 
formity with  those  of  the  bottom  of  the  rock-pools  frequented  by 
these  creatures ;  whilst  in  the  shrimps  there  is  seldom  any  distinct 
trace  of  the  areolated  layer,  and  the  calcareous  portion  of  the  skele- 
ton is  disposed  in  the  form  of  concentric  rings,  which  seem  to  be  the 
result  of  the  concretionary  aggregation  of  the  calcifying  deposit.2 

It  is  a  very  curious  circumstance  that  a  strongly  marked  dif- 
ference exists  between  crustaceans  that  are  otherwise  very  closely 
allied  in  regard  to  the  degree  of  change  to  which  their  young  are 
subject  in  their  progress  towards  the  adult  condition.  For,  whilst 
the  common  crab,  lobster,  spiny  lobster,  prawn,  and  shrimp 
undergo  a  regular  metamorphosis,  the  young  of  the  crayfish  and 
some  land-crabs  come  forth  from  the  egg  in  a  form  which  corre- 
sponds in  all  essential  particulars  with  that  of  their  parents. 
Generally  speaking,  a  strong  resemblance  exists  among  the  young 
of  all  the  species  of  decapods  which  undergo  a  metamorphosis,  whether 
they  are  afterwards  to  belong  to  the  macrurous  (long-tailed)  or  to 
the  brachyurous  (short-tailed)  division  of  the  group ;  and  the  forms 

1  The  Author  is  now  quite  satisfied  of  the  correctness  of  the  interpretation  put  by 
Professor  Huxley  (see  his  article,  '  Tegumentary  Organs,'  in  the  Cyclop.  Anat.  and 
Phys.  vol.  v.  p.  487),  and  by  Professor  W.  C.  Williamson  ( '  On  some  Histological 
Features  in  the  Shells  of  Crustacea'  in   Quart.  Journ.  Microsc.  Sci.  vol.  viii.  1860, 
p.  38)  upon  the  appearances  which  he  formerly  described  (Report  of  British  Asso- 
ciation for  1847,  p.  128)  as  indicating  a  cellular  structure  in  this  layer. 

2  Consult   Braun,   '  Ueber  die   histologischen   Vorgaiige  bei   der   Hautung   von 
Astacus  fluviatilis,'  Arbeit.  Zool.  Inst.  Wurzburg,  ii.  p.  121. 


970  CRUSTACEA 

of  these  larvae  are  so  peculiar,  and  so  entirely  different  from  any  of 
those  into  which  they  are  ultimately  to  be  developed,  that  they  were 
considered  as  belonging  to  a  distinct  genus,  Zoea,  until  their  real 
nature  was  first  ascertained  by  Mr.  J.  Y.  Thompson.  Thus,  in  the 
earliest  state  of  Carcinus  mcenas  (small  edible  crab),  we  see  the  head 
and  thorax,  which  form  the  principal  bulk  of  the  body,  included 
within  a  large  carapace  or  shield  (fig.  722,  A)  furnished  with  a  long 
projecting  spine,  beneath  which  the  fin-feet  are  put  forth  ;  whilst 
the  abdominal  segments,  narrowed  and  prolonged,  carry  at  the  end 
a  flattened  tail-fin,  by  the  strokes  of  which  upon  the  water  the  pro- 
pulsion of  the  animal  is  chiefly  effected.  Its  condition  is  hence 
comparable,  in  almost  all  essential  particulars,  to  that  of  Cyclops. 
In  the  case  of  the  lobster,  prawn,  and  other  '  macrurous '  species, 
the  metamorphosis  chiefly  consists  in  the  separation  of  the  loco- 
motor  and  respiratory  organs,  true  legs  being  developed  from  the 
thoracic  segments  for  the  former,  and  true  gills  (concealed  within  a 
special  chamber  formed  by  an  extension  of  the  carapace  beneath  the 


FIG.  722.— Metamorphosis  of  Carcinus  mcenas:  A,  first  or  Zoea 
stage ;  B,  second  or  Megalopa  stage ;  C,  third  stage,  in  which 
it  begins  to  assume  the  adult  form  ;  D,  perfect  form. 

body)  for  the  latter  ;  while  the  abdominal  segments  increase  in  size 
and  become  furnished  with  appendages  (false  feet)  of  their  own.  In 
the  crabs,  or  '  brachyurous '  species,  on  the  other  hand,  the  altera- 
tion is  much  greater  ;  for,  besides  the  change  first  noticed  in  the 
thoracic  members  and  respiratory  organs,  the  thoracic  region  becomes 
much  more  developed  at  the  expense  of  the  abdominal,  as  seen  at 
B,  in  which  stage  the  larva  is  remarkable  for  the  large  size  of  its 
eyes,  and  hence  received  the  name  of  Megalopa,  when  it  was  sup- 
posed to  be  a  distinct  type.  In  the  next  stage,  C,  we  find  the 
abdominal  portion  reduced  to  an  almost  rudimentary  condition,  and 
bent  under  the  body  ;  the  thoracic  limbs  are  more  completely  adapted 
for  walking,  save  the  first  pair,  which  are  developed  into  chelce  or 
pincers  ;  and  the  little  creature  entirely  loses  the  active  swimming 
habits  which  it  originally  possessed,  and  takes  on  the  mode  of  life 
peculiar  to  the  adult.1 

In  collecting  minute  Crustacea  the  ring-net  should  be  used  for 

1  On  the  metamorphoses  of  Crustacea  and  Cirripedia,  see  especially  the  TTnter- 
suchungen  uber   Crustaceen   of    Professor    Glaus,   Vienna,    1876.     A   number    of 


COLLECTING  CRUSTACEA  971 

the  fresh-water  species,  and  the  tow-net  for  the  marine.  In  localities 
favourable  for  the  latter  the  same  ;  gathering '  will  often  contain 
multitudes  of  various  species  of  Entomostraca,  accompanied  perhaps 
by  the  larva?  of  higher  Crustacea,  echinoderm  larvae,  annelid  larvae, 
and  the  smaller  Medusca.  The  water  containing  these  should  be  put 
into  a  large  glass  jar,  freely  exposed  to  the  light ;  and,  after  a  little 
practice,  the  eye  will  become  so  far  habituated  to  the  general  appear- 
ance and  modes  of  movement  of  these  different  forms  of  animal  life 
as  to  be  able  to  distinguish  them,  one  from  the  other.  In  selecting 
any  specimen  for  microscopic  examination  the  dipping-tube  will  be 
found  invaluable.  The  collector  will  frequently  find  Megalopa  larvae, 
recognisable  by  the  brightness  of  their  two  black  eye-spots,  on  the  sur- 
face of  floating  leaves  of  Zoster  a,  *  The  study  of  the  metamorphosis 
will  be  best  prosecuted,  however,  by  obtaining  the  fertilised  eggs, 
which  are  carried  about  by  the  females,  and  watching  the  history  of 
their  products.  For  preserving  specimens,  whether  of  Entomostraca 
or  of  larvae  of  the  higher  Crustacea,  the  Author  would  recommend 
sterilised  glycerin -jelly  as  the  best  medium. 

interesting  facts  and  speculations  on  the  Crustacea  will  be  found  in  F.  Mailer's  Facts 
and  Argument?  for  Darwin  (London,  1869).  The  work  of  Keichenbach  on  the 
Development  of  the  Crayfish  is  contained  in  vol.  xxix.  of  the  Zeitschr.f.  wiss.  Zool. 
p.  123,  1877,  and  vol.  xiv.  of  the  Abhandl.  Senckenberg.  Naturf.  Gesells.  1886.  See 
also  the  essay,  by  "W.  K.  Brooks,  On  the  Development  of  Lucifer,  in  Phil.  Trans. 
1882,  p.  57.  Mr.  F.  H.  Herrick's  memoir  on  the  American  Lobster  (Bull.  U.S.  Fish. 
Comm.  xv.  [1895]  )  contains  matter  of  much  interest.  Professor  Sars's  fully  illustrated 
monograph  of  the  Crustacea  of  Norway  is  being  steadily  and  rapidly  published. 


9/2 


CHAPTER  XXI 

INSECTS  AND  ARACHNID  A 

THERE  is  no  class  in  the  whole  animal  kingdom  which  affords  to  the 
microscopist  such  a  wonderful  variety  of  interesting  objects,  and 
such  facilities  for  obtaining  an  almost  endless  succession  of  novelties, 
as  that  of  insects.  For  in  the  first  place,  the  number  of  different 
kinds  that  may  be  brought  together  (at  the  proper  time)  with  ex- 
tremely little  trouble  far  surpasses  that  which  any  other  group  of 
animals  can  supply  to  the  most  painstaking  collector ;  then,  again, 
each  specimen  will  afford  to  him  who  knows  how  to  employ  his 
materials  a  considerable  number  of  microscopic  objects  of  very 
different  kinds  ;  and  thirdly,  although  some  of  these  objects  require 
much  care  and  dexterity  in  their  preparation,  a  large  proportion 
may  be  got  out,  examined,  and  mounted  with  very  little  skill  or 
trouble.  Take,  for  example,  the  common  house-fly ;  its  eyes  may 
be  easily  mounted,  one  as  a  transparent,  the  other  as  an  opaque 
object ;  its  antennae,  although  not  such  beautiful  objects  as  those  of 
many  other  Diptera,  are  still  well  worth  examination  ;  its  tongue  or 
'proboscis'  is  a  peculiarly  interesting  object,  though  requiring  some 
care  in  its  preparation ;  its  spiracles,  which  may  be  easily  cut  out 
from  the  sides  of  its  body,  have  a  very  curious  structure;  its 
alimentary  canal  affords  a  very  good  example  of  the  minute  distri- 
bution of  the  trachea! ;  its  wing,  examined  in  a  living  specimen 
newly  come  forth  from  the  pupa  state,  exhibits  the  circulation  of 
the  blood  in  the  '  nervures,'  and  when  dead  shows  a  most  beautiful 
play  of  iridescent  colours,  and  a  remarkable  ar eolation  of  surface, 
when  examined  by  light  reflected  from  its  surface  at  a  particular 
angle ;  its  foot  has  a  very  peculiar  conformation,  which  is  doubtless 
connected  with  its  singular  power  of  walking  over  smooth  surfaces 
in  direct  opposition  to  the  force  of  gravity,  while  the  structure 
and  physiology  of  its  sexual  apparatus,  with  the  history  of  its  develop- 
ment and  metamorphoses,  would  of  itself  suffice  to  occupy  the  whole 
time  of  an  observer  who  should  desire  thoroughly  to  work  it  out,  not 
only  for  months,  but  for  years.1  Hence,  in  treating  of  this  department 
in  such  a  work  as  the  present,  the  Author  labours  under  the  embarras 
des  richesses ;  for,  to  enter  into  such  a  description  of  the  parts  of  the 
structure  of  insects  most  interesting  to  the  microscopist  as  should 

1  See  Mr.  Lowne's  valuable  treatise  on  The  Anatomy  and  Physiology  of  the 
Blow-fly,  1870  ;  second  edition  1891-4. 


MOUNTING-  INSECTS  973 

be  at  all  comparable  in  fulness  with  the  accounts  which  it  has  been 
thought  desirable  to  give  of  other  classes  would  swell  out  the 
volume  to  an  inconvenient  bulk  ;  and  no  course  seems  open  but  to 
limit  the  treatment  of  the  subject  to  a  notice  of  the  kinds  of 
objects  which  are  likely  to  prove  most  generally  interesting,  with  a 
few  illustrations  that  may  serve  to  make  the  descriptions  more  clear, 
and  with  an  enumeration  of  some  of  the  sources  whence  a  variety 
of  specimens  of  each  class  may  be  most  readily  obtained.  And  this 
limitation  is  the  less  to  be  regretted,  since  there  already  exist  in 
our  language  numerous  elementary  treatises  on  entomology,  wherein 
the  general  structure  of  insects  is  fully  explained,  and  the  conforma- 
tion of  their  minute  parts  as  seen  with  the  microscope  is  adequately 
illustrated.1 

A  considerable  number  of  the  smaller  insects— especially  those 
belonging  to  the  orders  Coleoptera  (beetles),  ffieuroptera  (dragon-fly, 
May-fly,  &c.),  Hymenoptera  (bee,  wrasp,  &c.),  and  Diptera  (two-winged 
flies) — may  be  mounted  entire  as  opaque  objects  for  low  magnifying 
powers,  care  being  taken  to  spread  out  their  legs,  wings,  &c.,  so  as 
adequately  to  display  them,  which  may  be  accomplished,  even  after 
they  have  dried  in  other  positions,  by  softening  them  by  steeping 
them  in  hot  water,  or,  where  this  is  objectionable,  by  exposing 
them  to  steam.  Directions  on  this  point,  applicable  to  small  and 
large  insects  alike,  may  be  found  in  various  text -books  of  ento- 
mology. There  are  some,  however,  whose  translucence  allows  them 
to  be  viewed  as  transparent  objects,  and  these  are  either  to 
be  mounted  in  Canada  balsam  or  in  Dean's  medium,  glycerin 
jelly,  or  Farrant's  gum,  according  to  the  degree  in  which  the  horny 
opacity  of  their  integument  requires  the  assistance  of  the  balsam  to 
facilitate  the  transmission  of  light  through  it,  or  the  softness  and 
delicacy  of  their  textures  render  an  aqueous  medium  more  desirable. 
Thus  an  ordinary  flea  or  bug  will  best  be  mounted  in  balsam ;  but 
the  various  parasites  of  the  louse  kind,  with  some  or  other  of  which 
almost  every  kind  of  animal  is  affected,  should  be  set  up  in  some  of 
the  'media.'  Some  of  the  aquatic  Iarva3  of  the  Diptera  and  Neuro- 
ptera,  which  are  so  transparent  that  their  whole  internal  organisa- 
tion can  be  made  out  without  dissection,  are  very  beautiful  and 
interesting  objects  when  examined  in  the  living  state,  especially 
because  they  allow  the  circulation  of  the  blood  and  the  action  of  the 
dorsal  vessel  to  be  discerned.  Among  these  there  is  none  prefer- 
able to  the  larva  of  the  Ephemera  marginata  (day-fly),  which  is  dis- 
tinguished by  the  possession  of  a  number  of  beautiful  appendages 
on  its  body  and  tail,  and  is,  moreover,  an  extremely  common 
inhabitant  of  our  ponds  and  streams.  This  insect  passes  two  or 
even  three  years  in  its  larval  state,  and  during  this  time  it 
repeatedly  throws  off  its  skin ;  the  cast  skin,  when  perfect,  is  an 
object  of  extreme  beauty,  since,  as  it  formed  a  complete  sheath  to 
the  various  appendages  of  the  body  and  tail,  it  continues  to  exhibit 
their  outlines  with  the  utmost  delicacy ;  and  by  keeping  these  larvae 

1  An  excellent  introduction  to  the  study  of  insects  will  be  found  in  The  Structure 
and  Life-history  of  the  Cockroach,  by  L.  C.  Miall  and  A.  Denny  (London,  1886). 
See  also  Dr.  D.  Sharp  in  the  Cambridge  Natural  History. 


974  INSECTS   AND   AKACHNIDA 

in  an  aquarium,  and  by  mounting  the  entire  series  of  their  cast 
skins,  a  record  is  preserved  of  the  successive  changes  they  undergo. 
Much  care  is  necessary,  however,  to  extend  them  upon  slides  in  con- 
sequence of  their  extreme  fragility ;  and  the  best  plan  is  to  place 
the  slip  of  glass  under  the  skin  whilst  it  is  floating  on  water,  and  to 
lift  the  object  out  upon  the  slide.  Thin  sections  of  insects,  cater- 
pillars, £c.,  which  bring  the  internal  parts  into  view  in  their  normal 
relations,  may  be  cut  with  the  microtome  by  first  soaking  the  body 
(as  suggested  by  Dr.  Halifax)  in  thick  gum-mucilage,  which  passes 
into  its  substance,  and  gives  support  to  its  tissues,  and  then  inclos- 
ing it  in  a  casing  of  melted  paraffin  made  to  fit  the  cavity  of  the 
section-instrument. 

Structure  of  the  Integument, — In  treating  of  these  separate  parts 
of  the  organisation  of  insects  which  furnish  the  most  interesting 
objects  of  microscopic  study  we  may  most  appropriately  commence 
with  their  integument  and  its  appendages  (scales,  hairs,  &c.).  The 
body  and  members  are  closely  invested  by  a  hardened  skin,  which 
acts  as  their  skeleton,  and  affords  points  of  attachment  to  the  muscles 
by  which  their  several  parts  are  moved,  being  soft  and  flexible,  how- 
ever, at  the  joints.  This  skin  is  usually  more  or  less  horny  in  its 
texture,  and  is  consolidated  by  the  animal  substance  termed  chitine, 
as  well  as  in  some  cases  by  a  small  quantity  of  mineral  matter.  It 
is  in  the  Coleoptera  that  it  attains  its  greatest  development,  the 
*  dermo-skeleton '  of  many  beetles  being  so  firm  as  not  only  to  confer 
upon  them  an  extraordinary  power  of  passive  resistance,  but  also  to 
enable  them  to  put  forth  enormous  force  by  the  action  of  the  power- 
ful muscles  which  are  attached  to  it.  The  outer  layer  of  this  dermo- 
skeleton  is  continuous,  the  cells  which  secrete  it  lying  beneath  the 
parallel  lamina?  of  which  it  is  made  up ;  on  the  surface  the  chitinous 
cuticle  may  be  seen  to  be  marked  out  into  a  number  of  polygonal 
(usually  hexagonal)  areas  which  correspond  to  the  subjacent  secret- 
ing cells.  Of  this  we  have  a  very  good  example  in  the  superficial 
layers  (fig.  737,  B)  of  the  thin  horny  lamella?  or  blades  which 
constitute  the  terminal  portion  of  the  antenna  of  the  cockchafer, 
this  layer  being  easily  distinguished  from  the  intermediate  portion 
(A)  of  the  lamina  by  careful  focussing.  In  many  beetles  the  hexa- 
gonal areolation  of  the  surface  is  distinguishable  when  the  light  is 
reflected  from  it  at  a  particular  angle,  even  when  not  discernible  in 
transparent  sections.  The  integument  of  the  common  red  ant 
exhibits  the  hexagonal  cellular  arrangement  very  distinctly  through- 
out ;  and  the  broad  flat  expansion  of  the  leg  of  the  Crabro  ('  sand- 
wasp')  affords  another  beautiful  example  of  a  distinctly  cellular 
arrangement  of  the  outer  layer  of  the  integument.  The  inner  layer, 
however,  which  constitutes  the  principal  part  of  the  thickness  of  the 
horny  casing  of  the  beetle  tribe,  seldom  exhibits  any  distinct  organi- 
sation, though  it  may  be  usually  separated  into  several  lamella?, 
which  are  sometimes  traversed  by  tubes  that  pass  into  them  from 
the  inner  surface,  and  extend  towards  the  outer  without  reach- 
ing it. 

Tegumentary  Appendages. — The  surface  of  the  insects  is  often 
beset,  and  is  sometimes  completely  covered,  with  appendages  having 


INTEGUMENT  975 

either  the  form  of  broad  flat  scales  or  that  of  hairs  more  or  less 
approaching  the  cylindrical  shape,  or  some  form  intermediate  be- 
tween the  two.  The  scaly  investment  is  most  complete  among  the 
Lepidoptera  (butterfly  and  moth  tribe),  the  distinguishing  character 
of  the  insects  of  this  order  being  derived  from  the  presence  of  a 
regular  layer  of  scales  upon  each  side  of  their  large  membranous 
wings.  It  is  to  the  peculiar  coloration  of  the  scales  that  the  various 
hues  and  figures  are  due,  by  which  these  wings  are  so  commonly 
distinguished,  all  the  scales  on  one  patch  (for  example)  being  green, 
those  of  another  red,  and  so  on;  for  the  subjacent  membrane 
remains  perfectly  transparent  and  colourless  when  the  scales  have 
been  brushed  off  from  its  surface.  Each  scale  seems  to  be  composed 
of  two  or  more  membranous  lantallse,  often  with  an  intervening 
deposit  of  pigment,  on  which,  especially  in  Lepidoptera,  their  colour 
depends.  Certain  scales,  however,  especially  in  the  beetle  tribe, 
have  a  metallic  lustre,  and  exhibit  brilliant  colours  that  vary  with 
the  mode  in  which  the  light  glances  from  them ;  and  this  '  irides- 
cence,' which  is  specially  noteworthy  in  the  scales  of  the  Curculio 
imperialis  ('  diamond  beetle '),  seems  to  be  a  purely  optical  effect, 
depending  either  (like  the  prismatic  hues  of  a  soap-bubble)  on  the 
extreme  thinness  of  the  membranous  lamellae,  or  (like  those  of 
'  mother-of-pearl ')  on  a  lineation  of  surface  produced  by  their  corru- 
gation. Each  scale  is  furnished  at  one  end  with  a  sort  of  handle  or 

*  pedicle '  (figs.  723,  724),  by  which  it  is  fitted  into  a  minute  socket 
attached  to  the  surface  of  the  insect ;  and  on  the  wings  of  Lepido- 
ptera these  sockets  are  so  arranged  that  the  scales  lie  in  very  regular 
rows,  each  row  overlapping  a  portion  of  the  next,  so  as  to  give  to 
their  surface,  when  sufficiently  magnified,  very  much  the  appearance 
of  being  tiled  like  the  roof  of  a  house.     Such  an  arrangement  is  said 
to  be  '  imbricated.'     The  forms  of  these  scales  are  often  very  curious, 
and  frequently  differ  a  good  deal  on  the  several  parts  of  the  wings 
and  of  the  body  of  the  same  individual,  being  usually  more  expanded 
on  the  former  and  narrower  and  more  hairlike  on  the  latter.     A 
peculiar  type  of  scale,  which  has  been  distinguished  by  the  designa- 
tion plumule,  is  met  with  among  the  Pier  idee,  one  of  the  principal 
families  of  the  diurnal  Lepidoptera.     The  '  plumules  '  are  not  flat, 
but  cylindrical  or  bellows- shaped,  and  are  hollow  ;  they  are  attached 
to  the  wing  by  a  bulb  at  the  end  of  a  thin  elastic  peduncle  that 
differs  in  length  in  different  species,  and  proceeds  from  the  broader, 
not  from  the  narrower  end  of  the  scale ;  whilst  the  free  extremity 
usually  tapers  off  and  ends  in  a  kind  of  brush,  though  sometimes  it 
is  broad  and  has  its  edge  fringed  with   minute   filaments.     These 

*  plumules ,'  which  are  peculiar  to  the  males,  are  found  on  the  upper 
surface  of  the  wings,  partly  between  and  partly  under  the  ordinary 
scales.     They  seem  to  be  represented  among  the  Lyccenidce  by  the 
'  battledore'  scales  to  be  presently  described.1 

The  peculiar  markings  exhibited  by  many  of  the  scales  very  early 
attracted  the  attention  of  opticians  engaged  in  the  application   of 

1  See  Mr.  Watson's  memoirs  '  On  the  Scales  of  Battledore  Butterflies,'  in  Monthly 
Microscopical  Journal,  ii.  pp.  73,  314. 


976  INSECTS  AND   ARACHNIDA 

achromatism  to  the  microscope ;  for,  as  the  clearness  and  strength 
with  which  they  could  be  shown  were  found  to  depend  on  the 
decree  to  which  the  angular  aperture  of  an  objective  could  be  opened 
without  sacrifice  of  perfect  correction  for  spherical  and  chromatic 
aberration,  such  scales  proved  very  serviceable  as  'tests.'  The 
Author  can  well  remember  the  time  when  those  of  the  Morpho  Mem- 
laus  (fig.  723),  the  ordinary  and  '  battledore '  scales  of  the  Polyom- 
matus  Argus  (figs.  724,  725),  and  the  scales  of  the  Lepisma  saccharina 
(fig.  726),  which  are  now  only  used  for  testing  objects  of  loiv  or 
medium  power,  were  the  recognised  tests  for  objects  of  high  power ; 
while  the  exhibition  of  alternating  light  and  dark  bands  on  a 
Podura  scale  was  regarded  as  a  first -rate  performance.  It  is  easy 
for  anyone  possessed  of  a  good  apochromatic  objective  of  6  mm. 
(i-  inch)  to  obtain  all  the  characteristic  features  of  the  scale  ;  but 

the  determination  of  the  method  of 
construction  of  the  scale  and  the  proper 
interpretation  of  the  '  markings '  is  a 
matter  that  the  wise  microscopist  will 
prefer  to  relegate  to  the  days  when  the 
apertures  of  our  best  present  lenses  will 
be  looked  upon  comparatively  as  we  now 
look  upon  the  earliest  achromatic  ob- 
jectives. No  one  can  give  a  fairly 
comprehensive  and  satisfactory  sugges- 
tion of  the  true  nature  of  the  Podura 
scale,  and  yet  on  no  one  object  has 
microscopy  lavished  so  much  labour  for 
so  many  years. 

The  easier  test  scales  are  furnished 
by  the  Lepidoptera  (butterflies  and 
moths),  and  among  the  most  beautiful 
of  these,  both  for  colour  and  for  regu- 
larity of  marking,  are  those  of  the 
Morpho  Menelaus  (fig.  723).  These  are 
of  a  rich  blue  tint,  and  exhibit  strong 
longitudinal  striae,  which  seem  due  to 
ribbed  elevations  of  one  of  the  superficial  layers.  There  is  also  an 
appearance  of  transverse  striation,  which  cannot  be  seen  at  all  with 
an  inferior  objective,  but  becomes  very  decided  with  a  good  objective 
of  medium  focus ;  and  this  is  found,  when  submitted  to  the  test  of  a 
high  power  and  good  illumination,  to  depend  upon  the  presence  of 
transverse  thickenings  or  corrugations  (fig.  723),  probably  on  the  in- 
ternal surface  of  one  of  the  membranes.  The  large  scales  of  the  Poly- 
ommatus  Argus  ('azure  blue '  butterfly)  resemble  those  of  the  Menelaus 
in  form  and  structure,  but  are  more  delicately  marked  (fig.  724). 
Their  ribs  are  more  nearly  parallel  than  those  of  the  Menelaus  scale, 
and  do  not  show  the  same  transverse  striation.  When  one  of  these 
scales  lies  partly  over  another,  the  effect  of  the  optical  intersection 
of  the  two  sets  of  ribs  at  an  oblique  angle  is  to  produce  a  set  of 
interrupted  striations  (6),  very  much  resembling  those  of  the  Podura 
scale.  The  same  butterfly  furnishes  smaller  scales,  which  are  com- 


FIG.  723.— Scale  of  Morpho 
Menelaus. 


SCALES 


977 


in  only  termed  the  '  battledore '  scales,  from  their  resemblance  in 
form  to  that  object  (fig.  724,  a).  These  scales,  which  occur  in  the 
males  of  several  genera  of  the  family  Lycasnidas,  and  present  a 
considerable  variety  of  shape, l  are  marked  by  narrow  longitudinal 
ribbings,  which  at  intervals  seem  to  expand  into  rounded  or  oval 
elevations  that  give  to  the  scales  a  dotted  appearance  (fig.  725) ;  at 
the  lower  part  of  the  scale,  however,  these  dots  are  wanting. 
Dr.  Anthony  describes  and  figures  them  as  elevated  bodies,  some- 
what resembling  dumb-bells  or  shirt-studs,  ranged  along  the  ribs, 
and  standing  out  from  the  general  surface.2  Other  good  observers, 
however,  whilst  recognising  the  stud-like  bodies  described  by  Dr. 
Anthony,  regard  them  as  not  projecting  from  the  external  surface 
of  the  scale,  but  as  interposed  between  its  two  lamellae ; 3  and  this 
view  seems  to  the  Author  to  be  more  conformable  than  Dr.  Anthony's 
to  general  probability. 

The  more  difficult  '  test  scales '  are  furnished  by  little  wingless 
insects  ranked  together  by  Latreille  in  the  order  Thysanura,  but 


FIG.  724. — Scales  of  Pol yominat us  Argus 
(azure  blue)  :  a,  battledore  scale ;  b, 
interference  striae. 


FIG.  725.— Battledore  scale  of 
Polyommatus  Argus  (azure 
blue). 


now  separated  by  Sir  John  Lubbock,4  on  account  of  important 
differences  in  internal  structure,  into  the  two  groups  Collembola  and 
true  Thysanura.  Of  the  former  of  these  the  Lepismidce  constitute 
the  typical  family;  and  the  scale  of  the  common  Lepisma  saccha- 
rina,  or  'sugar-louse,'5  very  early  attracted  the  attention  of 

1  See  Watson,  foe.  cit. 

-  '  The  Markings  on  the  Battledore  Scales  of  some  of  the  Lepidoptera  '  in  Monthly 
Microscopical  Journal,  vol.  vii.  1872,  pp.  1,  250. 

5  See  '  Proceedings  of  the  Microscopical  Society,'  op.  cit.  p.  278. 

4  See  his  Monograph  of  the  Collembola  and  Thysanura,  published  by  the  Ray 
Society,  1872. 

•"'  This  insect  may  be  found  in  most  old  houses,  frequenting  damp  warm  cupboards, 
and  especially  such  as  contain  sweets ;  it  may  be  readily  caught  in  a  small  pill-box, 
which  should  have  a  few  pinholes  in  the  lid;  and  if  a  drop  of  chloroform  be  put 
over  the  holes  the  inmate  will  soon  become  insensible,  and  may  be  then  turned  out 
upon  a  piece  of  clean  paper,  and  some  of  its  scales  transferred  to  a  slip  of  glass  by 
simply  pressing  this  gently  on  its  body. 

3  R 


978 


INSECTS   AND   AKACHNIDA 


microscopists  on  account  of  its  beautiful  shell-like  sculpture.  When 
viewed  under  a  low  magnifying  power  it  presents  a  beautiful 
'  watered-silk '  appearance,  which,  with  higher  amplification,  is  found 
to  depend  (as  Mr.  R.  Beck  first  pointed  out) 1  upon  the  intersection 
of  two  sets  of  strife,  representing  the  different  structural  arrange- 
ments of  its  two  superficial  membranes.  One  of  its  surfaces  (since 
ascertained  by  Mr.  Joseph  Beck 2  to  be  the  under  or  attached 
surface  of  the  scale)  is  raised,  either  by  corrugation  or  thickening, 
into  a  series  of  strongly  marked  longitudinal  ribs,  which  run  nearly 
parallel  from  one  end  of  the  scale  to  the  other,  and  are  particularly 
distinct  at  its  margins  arid  at  its  free  extremity ;  whilst  the  other 

surface  (the  free  or  outer,  according 
to  Mr.  J.  Beck)  presents  a  set  of  less 
definite  corrugations,  radiating  from 
the  pedicle,  where  they  are  strongest, 
towards  the  sides  and  free  extremity 
of  the  scale,  and  therefore  crossing 
the  parallel  ribs  at  angles  more  or 
less  acute  (fig.  726).  It  was  further 
pointed  out  by  Mr.  R.  Beck  that  the 
intersection  of  these  twro  sets  of  cor- 
rugations at  different  angles  produces 
most  curious  effects  upon  the  appear- 
ances which  optically  represent  them. 
For  where  the  diverging  ribs  cross 
the  longitudinal  ribs  very  obliquely, 
as  they  do  near  the  free  extremity  of 
the  scale,  the  longitudinal  ribs  seem 
broken  up  into  a  series  of  '  excla- 
mation markings,'  like  those  of  the 
Podura ;  but  where  the  crossing  is 
transverse  or  nearly  so,  as  at  the 
sides  of  the  scale,  an  appearance  is 
presented  as  of  successions  of  large 
bright  beads.  The  conclusion  drawn 
by  the  Messrs.  Beck,  that  these  in- 
terrupted appearances  are  '  produced 

by  two  sets  of  uninterrupted  lines  on  different  surfaces,'  has  been 
confirmed  by  the  careful  investigations  of  Mr.  Moorhouse.3  The 
minute  beaded  structure  observed  by  Dr.  Royston-Pigott 4  alike  in 
the  ribs  and  in  the  intervening  spaces  may  now  be  certainly  re- 
garded as  an  optical  effect  of  diffraction.  In  the  scale  of  a  type 
nearly  allied  to  Lepisma,  the  Machilis  polypoda,  the  very  distinct 
ribbing  (fig.  727)  is  produced  by  the  corrugation  of  the  under  mem- 
branous lamina  alone,  the  upper  or  exposed  lamina  being  smooth, 
with  the  exception  of  slight  undulations  near  the  pedicle,  and  the 
cross-markings  being  due  to  structure  between  the  superposed 


FIG.  726.— Scale  of  Lepisma 
saccharina. 


1  The  Achromatic  Microscope,  p.  50. 

2  See  his  appendix  to  Sir  John  Lubbock's  Monograph. 


Monthly  Microscopical  Journal,  vol.  xi.  1874,  p.  13,  and  vol.  xviii.  1877,  p.  31. 
4  Ibid.  vol.  ix.  1873,  p.  63. 


SCALES 


979 


I   1 


membranes,  probably  a  deposit  on  the  interior  surface  of  one  or  both 
of  them.1 

Although  the  Podivridce  and  Ltpismidce  now  rank  as  distinct 
families,  yet  they  approximate  sufficiently  in  general  organisation, 
MS  well  as  in  habits,  to  justify  the  expectation  that  their  scales 
would  be  framed  upon  the  same  plan.  The  Poduridce  are  found 
amidst  the  sawdust  of  wine-cellars,  in  garden  tool-houses,  or  near 
decaying  wood,  and  derive  their  popular  name  of  '  spring-tails ' 
from  the  possession  by  many  of  them  of  a  curious  caudal  appen- 
dage by  which  they  can  leap  like  fleas.  This  is  particularly 
well  developed  in  the  species  now  designated  Lejndoci/rtus  curvi- 
collis,  which  furnishes  what  are  ordinarily  known  as  '  Podura  '  scales. 
*  When  full  grown  and  unrubbed,*  says  Sir 
John  Lubbock,  '  this  species  is  very  beauti- 
ful, and  reflects  the  most  gorgeous  metallic 
tints.'  Its  scales  are  of  different  sizes  and 
of  different  degrees  of  strength  of  marking 
(fig.  728,  A,  B),  and  are  therefore  by  no 
means  of  uniform  value  as  tests.  The 
general  appearance  of  their  surface,  under 
a  power  not  sufficient  to  resolve  their  mark- 
ings, is  that  of  watered  silk,  light  and  dark 
bands  passing  across  it  with  wavy  irregu- 
larity ;  but  a  well-corrected  objective  of 
very  moderate  aperture  now  suffices  to  re- 
solve every  dark  band  into  a  row  of  dis- 
tinct '  exclamation  marks.'  A  certain 
longitudinal  continuity  may  be  traced  be- 
tween the  '  exclamation  marks '  in  the 
ordinary  test  scale  ;  but  this  is  much  more 
apparent  in  other  scales  from  the  same 
species  (fig.  729),  as  well  as  in  the 
scales  of  various  allied  types,  which  were 
carefully  studied  by  the  late  Mr.  R.  Beck.2 
In  certain  other  types,  indeed,  the  scales 
have  very  distinct  longitudinal  parallel 
ribs,  sometimes  with  regularly  disposed 
cross-bars ;  these  ribs,  being  confined  to  one 
surface  only  (that  which  is  in  contact  with 
the  body),  are  not  subject  to  any  such  interference  with  their  optical 
continuity  as  has  been  shown  to  occur  in  Lepisma ;  but  more  or  less 
distinct  indications  of  radiating  corrugations  often  present  them- 
selves. The  appearance  of  the  interrupted  '  exclamation  marks ' 
Mr.  J.  Beck  considers  to  be  due  '  to  irregular  corrugations  of  the 
outer  surface  of  the  under  membrane,  to  slight  undulations  on  the 
outer  surface  of  the  upper  membrane,  and  to  structure  between  the 
superposed  membranes.'  It  has,  indeed,  been  stated  by  Mr.  Joseph 

1  See  Mr.  Joseph  Beck  in  Sir  J.  Lubbock's  Monograph,  p.  255. 

2  Trans.  Microsc.  Soc.  n.s.  vol.  x.  1862,  p.  83.     See  also  Mr.  Joseph  Beck,  in  the 
appendix  to  Sir  John  Lubbock's  Monograph,  and  in  Monthly  Microscopical  Journal, 
iv.  p.  253. 

3  K  2 


FIG.  727. — Scale  of  Machilis 
polypoda. 


980 


INSECTS   AND   ARACHNIDA 


Beck  that  the  scales  of  a  lepidopterous  insect  belonging  to  the  genus 
Mormo,  which  under  a  low  power  present  the  watered-silk  appear- 
ance seen  in  the  Podura  scale,  under  a  -1  in.  obj.  show  the  'exclama- 
tion markings/  whilst  under  a  y1,,  in.  obj.  they  exhibit  distinct  ribs 
from  pedicle  to  apex,  thus  showing  in  one  scale  how  the  appearances 
run  from  one  into  the  other.1 

The  'hairs  of  many  insects,  and  still  more  of  their  larvae,  are 
very  interesting  objects  for  the  microscope  on  account  of  their 
branched  or  tufted  conformation,  this  being  particularly  remarkable 
in  those  with  which  the  common  hairy  caterpillars  are  so  abundantly 
beset.  Some  of  these  afford  very  good  tests  for  the  perfect  correction 
of  objectives.  Thus  the  hair  of  the  bee  is  pretty  sure  to  exhibit 


FIG.  728. — Test  scales  of  Lepidocyrtiis  curvi- 
collis  :  A,  large  strongly  marked  scale  ;  B, 
small  scale,  more  faintly  marked. 


FIG.  729.— Ordinary  scale 
of  Lepidocyrtns  curvi- 
collis. 


strong  prismatic  colours  if  the  chromatic  aberration  should  not  have 
been  exactly  neutralised ;  and  that  of  the  larva  of  a  Dermestes 
(commonly,  but  erroneously,  termed  the  '  bacon-beetle ')  was  once 
thought  a  very  good  test  of  denning  power,  and  is  still  useful  for 
this  purpose.  It  has  a  cylindrical  shaft  (fig.  730,  B)  with  closely  set 
whorls  of  spiny  protuberances,  four  or  five  on  each  whorl ;  the 
highest  of  these  whorls  is  composed  of  mere  knobby  spines  ;  and  the 
hair  is  surmounted  by  a  curious  circle  of  six  or  seven  large  filaments, 
attached  by  their  pointed  ends  to  its  shaft,  whilst  at  their 'free  ex- 
tremities they  dilate  into  knobs.  An  approach  to  this  structure  is 
seen  in  the  hairs  of  certain  Myriopods  (centipedes,  galley-worms,  etc.), 
of  which  an  example  is  shown  in  fig.  730,  A ;  but  a  beautiful  photo- 

1  Journ.  Boy.  Microsc.  Soc.  vol.  ii.  1879,  p.  810.  On  the  subject  generally 
Dr.  A.  Spuler's  'Beitrag  zur  Kemitniss  des  feineren  Baues  .  .  .  der  Fliigelbedeckung 
der  Schmetterlinge,'  in  ZooL  JaJtrb.  Anat.  viii.  should  be  consulted. 


HAIRS 


981 


micrograph  of  the  hair  of  Poly.t:emi8  lagurus,  of  the  family  Polij- 
desmidcK  (order  ChUognatha),  is  given  in  fig.  6  of  the  frontispiece. 
This  is  one  of  the  finest  test  objects  for  medium  powers.  Other 
minute  forms  of  this  class  are  most  beautiful  objects  under  the 
binocular  microscope  on  account  of  the  remarkable  structure  and 
regular  arrangement  of  their  hairs. 

In  examining  the  integument  of  insects  and  its  appendages 
parts  of  the  surface  may  be  viewed  either  by  reflected  or  transmitted 
light,  according  to  their  degree  of  transparence  and  the  nature  of 
their  covering.  The  beetle  and  butterfly  tribes  furnish  the  greater 
number  of  the  specimens  suitable  to  be  viewed  as  opaque  objects  ; 
and  nothing  is  easier  than  to  mqunt  portions  of  the  elytra  of  the 
former  (usually  the  most  showy  parts  of  their 
bodies),  or  of  the  wings  of  the  latter,  in  the  ^  * 

manner  described  in  Chapter  VII.  The  tribe 
of  Curculionidce,  in  which  the  surface  is  beset 
with  scales  having  the  most  varied  and  lustrous 
hues,  is  distinguished  among  Coleoptera  for  the 
brilliancy  of  the  objects  it  affords,  the  most 
remarkable  in  this  respect  being  the  well-known 
Curculio  imperialis^  or '  diamond  beetle '  of  South 
America,  parts  of  whose  elytra,  when  properly 
illuminated  and  looked  at  with  a  low  power, 
show  like  clusters  of  jewels  flashing  against  a 
dark  velvet  ground.  In  many  of  the  British 
Curculionidce,  which  are  smaller  and  far 
less  brilliant,  the  scales  lie  at  the  bottom  of 
little  depressions  of  the  surface  ;  and  if  the 
elytra  of  the  diamond  beetle  be  carefully 
examined,  it  will  be  found  that  each  of  the 
clusters  of  scales  which  are  arranged  upon  it 
in  rows  seems  to  rise  out  of  a  deep  pit  which 
sinks  in  by  its  side.  The  transition  from  scales 
to  hairs  is  extremely  wrell  seen  by  comparing 
the  different  parts  of  the  surface  of  the  diamond 
beetle  with  each  other.  The  beauty  and  bril- 
liancy of  many  objects  of  this  kind  are  increased 
by  mounting  them  in  cells  in  Canada  balsam, 
even  though  they  are  to  be  viewed  with  reflected 

light  ;  other  objects,  however,  are  rendered  less  attractive  by  this 
treatment ;  and  in  order  to  ascertain  whether  it  is  likely  to  improve 
or  to  deteriorate  the  specimen,  it  is  a  good  plan  first  to  test  some 
other  portion  of  the  body  having  scales  of  the  same  kind  by  touching 
it  with  turpentine,  and  then  to  mount  the  part  selected  as  an  object, 
either  in  balsam  or  dry,  according  as  the  turpentine  increases  or 
diminishes  the  brilliancy  of  the  scales  on  the  spot  to  which  it  was 
applied.  Portions  of  the  wings  of  Lepidoptera  are  best  mounted  as 
opaque  objects  without  any  other  preparation  than  gumming  them 
flat  down  to  the  disc  of  the  wooden  slide,  care  being  taken  to  avoid 
disturbing  the  arrangement  of  the  scales  and  to  keep  the  objects, 
when  mounted,  as  secluded  as  possible  from  dust.  In  selecting  such 


FIG.  730.— A,  hair  of 
Myriojjod]  B,  hair  of 
Dermestes. 


982 


INSECTS   AND   ARACHNID  A 


portions  it  is  well  to  choose  those  which  have  the  brightest  and 
the  most  contrasted  colours,  exotic  butterflies  being  in  this  respect 
usually  preferable  to  British  ;  and  before  attaching  them  to  their 
slides  care  should  be  taken  to  ascertain  in  what  position,  with  the 
arrangement  of  light  ordinarily  used,  they  are  seen  to  the  best  ad- 
vantage, and  to  fix  them  there  accordingly.  Whenever  portions  of  the 
integument  of  insects  are  to  be  viewed  as  transparent  objects,  for  the 
display  of  their  intimate  structure,  they  should  be  mounted  in 
Canada  balsam,  after  soaking  for  some  time  in  turpentine,  since  this 
substance  has  a  peculiar  effect  in  increasing  their  translucence.  Not 
only  the  horny  cases  of  perfect  insects  of  various  orders,  but  also  of 
those  of  their  pupae,  are  worthy  of  this  kind  of  study  ;  and  objects 
of  great  beauty  (such  as  the  chrysalis  case  of  the  emperor  moth),  as 
well  as  of  scientific  interest,  are  sure  to  reward  such  as  may  prose- 
cute it  with  any  assiduity.  Further  information  may  often  be  gained 
by  softening  such  parts  in  potash  and  viewing  them  in  fluid.  The 

scales  of  the  wings  of  Lepido- 
ptera  &c.  are  best  transferred 
to  the  slide  by  simply  pressing  a 
portion  of  the  wing  either  upon 
the  slip  of  glass  or  upon  the 
cover ;    if  none  should  adhere 
the  glass  may  first  be  gently 
breathed    on.     Some  of  them 
are  best  seen  when  examined 
'  dry,'  whilst  others  are  more 
clear  when  mounted  in  fluid  ; 
and   for  the  determination  of 
their  exact  structure  it  is  well 
to  have  recourse  to  both  these 
FIG.  731.— Head  and  compound  eyes  of  the  methods.      Hairs,  on  the  other 
bee,  showing  the  ocellites  in   situ   on  one  i*anj      QT,a     v.ocf     ™™1T,forl     ;,, 
side,   A,   and  displaced    on   the   other,   B  ;  n' 
a,  a,  a,  stemmata;  &,  6,  antennae.  balsam. 

Parts  of  the  Head.— The 

eyes  of  insects,  situated  upon  the  upper  and  outer  part  of  the  head, 
are  usually  very  conspicuous  organs,  and  are  frequently  so  large  as 
to  touch  each  other  in  front  (fig.  731).  We  find  in  their  structure 
a  remarkable  example  of  that  multiplication  of  similar  parts  which 
seems  to  be  the  predominating  '  idea '  in  the  conformation  of  arti- 
culated animals  ;  for  each  of  the  large  protuberant  bodies  which  we 
designate  as  an  eye  is  really  a  *  compound '  eye,  made  up  of  many 
hundred  or  even  many  thousand  minute  conical  ocelli  (B).  Ap- 
proaches to  this  structure  are  seen  in  Entomostraca ;  but  the 
number  of  '  ocellites '  thus  grouped  together  is  usually  small. 
In  the  higher  Crustacea,  however,  the  '  ocelli '  are  very  numerous ; 
and  their  compound  eyes  are  constructed  upon  the  same  general 
plan  as  those  of  insects,  though  their  shape  and  position  are  often 
very  peculiar.  The  individual  ocelli  are  at  once  recognised  when 
the  'compound  eyes'  are  examined  under  even  a  low  magnifying 
power  by  the  'faceted'  appearance  of  the  surface  (fig.  731,  A), 
which  is  marked  out  by  very  regular  divisions  either  into  hexagons 


EYES 


983 


cornea;  6,  transparent  pyramids 
surrounded  with  pigment ;  c,  fibres 
of  the  optic  nerve  ;  d,  trunk  of  the 
optic  nerve. 


or  squares;  each  facet  is  the  ;  corneule '  of  a  separate  ocellite,  and 
has  a  convexity  of  its  own;  hence,  by  counting  the  facets,  we 
can  ascertain  the  number  of  ocelli 
in  each  ;  compound  eye.'  In  the  two 
eyes  of  the  common  fly  there  are 
as  many  as  4,000 ;  in  those  of  the 
cabbage-butterfly  there  are  about 
17,000;  in  the  dragon-fly  24,000; 
and  in  the  Mordella  beetle  25,000. 
The  structure  of  the  arthropod  eye 
is  best  explained  by  a  comparative 
account  of  the  various  stages  'of 
complication  which  it  presents. 
In  various  larvse  the  cuticular 
layer  is  modified  to  form  a  single 
lens,  behind  which  are  simple,  sepa-  FlG.  732.-Diagram  of  a  section  of  the 
rate,  elongated  hypodermic  cells,  composite  eye  of  Melolontha  vul- 
some  of  which  are  continuous  with  9aris  (cockchafer) :  a,  facets  of  the 
fine  branches  of  the  optic  nerve ; 
these  may  be  called  retinal  'cells. 
The  next  stage  in  complication  is 
seen  when  these  last  combine  to  form 
groups,  *  retinulse  ; '  the  sensitive 
cells  may  become  divided  into  twro 
regions,  an  outer  one,  which  is 
'  vitreous '  and  refractive  in  function, 
while  the  inner  part  remains  sensi- 
tive ;  the  cornea!  surface  may  be- 
come broken  up  into  a  number  of 
facets,  each  of  wilich  corresponds  to 
one  of  the  '  pyramids '  so  formed, 
and  within  the  retinula  there  may 
be  differentiated  a  rhabdom  (see  fig. 
733)  formed  by  the  nerve-rod. 

After  traversing  the  pyramids 
the  rays  reach  the  extremities  of 
the  fibres  of  the  optic  nerve,  which 
are  surrounded,  like  the  pyramid, 
by  pigmentary  substance.  Thus  the 
rays  which  have  passed  through  the 
several  '  corneules '  are  prevented 
from  mixing  with  each  other;  and 
110  rays,  save  those  which  pass  ill  FlG-  733.— Part  of  the  compound  eye 
, ,  J  '  f  ,  i  .  ,  of  Phniqanea  ;  the  retinal  cells  are 

the  axes  of  the  pyramids,  can  reach      seen  to  be  united  into  a  retinula  (r) 

the  fibres  of  the  optic  nerve.    Hence, 

it  is  evident  that,  as  no  two  ocelli 

011  the    same   side    (fig.    731)   have 

exactly  the  same  axis,  no  two    can 

receive  their    rays  from   the   same    point  of  an  object ;    and    thus, 

as    each    compound '  eye   is   immovably    fixed    upon    the  head,    the 

combined  action  of  the  entire  aggregate  will  probably  afford  but 


which  is  differentiated  into  a  rhab- 
dom (m)  posteriorly  ;  cc,  crystalline 
cone ;  f,  facet  of  compound  eye  ; 
pg,  pigment.  (After  Grenacher.) 


984 


INSECTS  AND   AKACHNIDA 


a  single  image,  resembling  that  which  we  obtain  by  means  of  our 
single  eyes.  This  judgment  has  received  a  confirmation  as  unex- 
pected as  it  is  complete  and  beautiful.  The  subject  of  the  real 
nature  of  compound  vision  can  be  considered  no  longer  a  matter  of 
doubt.  We  have  as  complete  evidence  of  its  character  as  \ve  have 
of  that  of  vision  by  vertebrate  eyes.  It  is  to  Professor  S.  Exner, 
of  Vienna,  that  we  are  indebted  for  the  striking  though  simple 
results.  He  has  been  engaged  for  years  on  cognate  researches, 
and  has  at  length  succeeded  in  taking  a  photo-micrograph  of  the 
image  presented  at  the  back  of  a  compound  insect  eye  in  precisely 
the  same  manner  as  a  similar  photograph  might  be  taken  with 
the  retina  removed  at  the  back  of  the  eye  of  one  of  the  higher- 
vertebrates. 

The  demonstration  was  satisfactorily  made,  and  the  present  Editor 
is  indebted  for  a  knowledge  of  the  following  details  to  the  courtesy 
of  a  private  communication  from  Professor  Exner. 

The  general  result  of  the  researches  on  the  subject  is  presented 
in  fig.  734,  which  is  the  image  at  the  back  of  the  compound  eye 

of  Lampyris  splendidula 
(fire-fly),  in  the  position 
in  which  it  would  be  por- 
trayed upon  the  retina,  but 
magnified  120  diameters. 
On  to  the  window  pane  a 
letter  R  cut  out  in  black 
was  fixed ;  the  distance  of 
the  window  from  the  eye 
was  225  cm.,  while  the  dis- 
tance of  the  church  from  the 
window  through  which  it  is 
seen  in  the  magnified  image 
was  135  paces. 

The  result  is  unmistak- 
able ;  there  may  appear  to  be 
some  matters  of  interest  still 
needing  interpretation,  but 
these  are  explained  in  the 
monograph  by  Exner,  giving 
the  complete  details  of  the 
method  he  adopted  and  the 
mathematical  explanation  of 
the  results  he  obtained.  The 
rectitude  of  the  image  and 
the  reversion  of  the  R  are 
certainly  noteworthy ;  and 
as  a  contribution  to  our 
knowledge  of  the  physiology 

of  sight  in  insects  and  other  animals  with  compound  eyes,  the  im- 
portance of  the  result  obtained  by  the  ingenuity  and  skill  of  Professor 
Exner  is  great,  giving  us  a  new  start  on  solid  ground.  The  mathe- 
matics of  the  question  are  fully  discussed  by  Exner  in  a  memoir,  to 


FIG.  784. — Image  of  a  window  with  the 
letter  R  on  one  of  its  panes,  and  a  church 
beyond,  taken  through  the  compound  eye 
of  Lampyris  splendidula,  and  magnified 
120  diams. 


EYES 


985 


which  the  student  must  be  referred  for  complete  information.1  The 
kind  of  image  formed  by  the  compound  eye  lias  long  been  a  matter 
of  discussion  amongst  physiologists.2 

The  process  of  taking  the  photo-micrograph  copied  in  fig.  734 
was  this  :  The  eye  of  the  Lampyris  was  carefully  dissected  out  from 
the  head,  the  retina  and  pigment  removed  with  a  fine  camel-hair 
pencil,  and  the  back  of  the  eye  immersed  in  a  mixture  of  glycerin 
and  water,  possessing  a  refractive  index  of  1  "346 ;  this  was  already 
known  to  be  the  refractive  index  of  the  blood  of  the  Lampyris.  The 
whole  was  placed  upon  an  ordinary  cover-glass,  this  being  fixed  by 
its  edges  to  a  slide  or  object-carrier  with  a  circular  aperture  cut  in 
it,  as  in  fig.  735,  C  ;  a  is  the  slide  with  an  aperture  less  in  diameter 


I) 


C 


FIG.  735. — Diagrammatic  illustration  of  the  method  by  which 
the  image  in  fig.  734  was  photo-micrographed. 

than  the  cover-glass  b  cut  through  it;  c  is  the  fluid-medium  of 
^=1-346  in  which  the  back  parts  of  the  eye  are  immersed,  thus 
fulfilling  the  conditions  of  living  sight,  while  the  cornea  with  its 
lenses  is  shown  at  d,  being,  as  in  the  normal  state,  in  air.  If  the  eye 

1  Sitzungsber.  ATtad.  Wissensch.  Wien,  Bd.  xcviii.  (1889),  pp.  13,  143  ;  also  Die 
Pliysiologie  der  facettirten  Augen  von  Krebsen  und  Insecten  (Leipzig  und  Wien, 
1891). 

2  A   critical   history  of  the  discussion  will  be  found  in  Chapter  VII.  of  Sir  J. 
Lubbock's  Senses  of  Animals  (London,  1888),  and  in  Dr.  D.  Sharp's  Annual  Address 
to  the  Entomological  Society  of  London,  1888  (1889).    See  also  Mr.  A.  Mallock  in  Proc. 
Boy.  Soc.  Lond.  vol.  Iv.  p.  85.     The  question  of  the  physiology  of  the  compound  eye 
of  Arthropods   has   given   rise  to  much  discussion.     For  further  details  as  to  its 
structure  consult  Grenadier's  great  work,  Untersuchungen  uber  das  Sehorgan  der 
Artliropoden  &c.  (Gb'ttingen,  1879) ;  Carriere,  Die  Sehorgane  der  Thiere  &c.  (Munich 
and  Leipzig,  1885) ;  Hickson,  '  The  Eye  and  Optic  Tract  of  Insects,'  Quart.  Journ. 
Microsc.    Sci.  xxv.  p.  215;  Lankester  and  Bourne,  'The  Minute  Structure  of  the 
Lateral  and  Central  Eyes  of  Scorpio  and   Limulus,'   Quart.  Journ.  Microsc.  Sci. 
xxiii.  p.  177  ;  Lowne,  '  On  the  Compound  Vision  and  the  Morphology  of  the  Eye  in 
Insects,'  Trans.  Linn.  Soc.  (2),  ii.  p.  389  ;  Patten, '  Eyes  of  Molluscs  and  Arthropods,' 
Mitth.  Zool.  Stat.  Neapel,  vi. 


986  INSECTS   AND   ARACHNIDA 

be  now  examined  with  a  microscope  (the  C  of  Zeiss  was  employed), 
the  'lenses'  will  be  distinctly  seen,  but  if  the  focus  be  readjusted 
to  the  focal  plane  of  the  image  in  the  eye  this  image  will  be  seen 
and  magnified.  This  will  be  understood  from  I)  (fig.  735),  where 
e,  f  represent  the  image,  h  the  cornea  with  its  *  lenses '  g,  e'-f  being 
the  image  of  the  object  thrown  upon  the  position  from  which  the 
retina  has  been  removed,  and  which  is  now  made  the  focal  plane  of 
the  objective  employed. 

It  was  this  image  (e'-f)  which  was  photographed  in  the  ordinary 
manner  with  a  Zeiss  photo-micrographic  apparatus  and  the  C  object- 
glass.  The  manner  in  which  this  was  done  is  seen  diagrammatically 
at  E  (fig.  735),  where  i  indicates  the  cornea  of  the  eye  exposed 
to  air,  k  the  image  thrown  though  the  '  lenses '  as  a  unified 
picture  at  the  focal  point  of  the  microscope,  and  I  is  the  sensitised 
plate  on  which  the  image  was  photographed.  This  piece  of  admi- 
rable research  and  its  clear  results  have  a  value  not  only  physio- 
logical but  philosophical. 

Although  the  structure  already  described  may  be  considered  as 
typical  of  the  eyes  of  insects,  yet  there  are  various  departures  from 
it  (most  of  them  slight)  in  the  different  members  of  the  class. 
Thus  in  some  cases  the  posterior  surface  of  each  '  corneule '  is 
concave ;  and  a  space  is  left  between  it  and  the  iris-like  dia- 
phragm, which  seems  to  be  occupied  by  a  watery  fluid  or  '  aqueous 
humour.'  In  other  instances,  again,  this  space  is  occupied  by  a 
double-convex  body,  which  seems  to  represent  the  'crystalline 
lens,'  and  this  body  is  sometimes  found  behind  the  iris,  the  num- 
ber of  ocelli  being  reduced,  and  each  one  being  larger,  so  that  the 
cluster  presents  more  resemblance  to  that  of  spiders,  &c.  Besides 
their  '  compound '  eyes,  insects  usually  possess  a  small  number  of 
*  simple '  eyes  (termed  ocelli  or  stemmata)  seated  upon  the  top  of  the 
head  (fig.  731,  a,  a,  a).  Each  of  these  consists  of  a  single  very  con- 
vex corneule,  to  the  back  of  which  proceeds  a  bundle  of  rods  that 
are  in  connection  with  fibrils  of  the  optic  nerve.  Such  ocelli  are 
the  only  visual  organs  of  the  larvse  of  insects  that  undergo  complete 
metamorphosis,  the  '  compound '  eyes  being  only  developed  towards 
the  end  of  the  pupa  stage. 

Various  modes  of  preparing  and  mounting  the  eyes  of  insects 
may  be  adopted,  according  to  the  manner  wherein  they  are  to  be 
viewed.  For  the  observation  of  their  external  faceted  surface  by 
reflected  light  it  is  better  to  lay  down  the  entire  head,  so  as  to 
present  a  front  face  or  a  side  face,  according  to  the  position  of  the 
eyes,  the  former  giving  a  view  of  both  eyes  when  they  approach 
each  other  so  as  nearly  or  quite  to  meet  (as  in  fig.  731),  whilst  the 
latter  will  best  display  one  when  the  eyes  are  situated  more  at  the 
sides  of  the  head.  For  the  minuter  examination  of  the  '  corneules,' 
however,  these  must  be  separated  from  the  hemispheroidal  mass 
whose  exterior  they  form  by  prolonged  maceration,  and  the  pig- 
ment must  be  carefully  washed  away  by  means  of  a  fine  camel-hair 
brush  from  the  inner  or  posterior  surface.  In  flattening  them  out 
upon  the  glass  slide  one  of  two  things  must  necessarily  happen  : 
either  the  margin  must  tear  when  the  central  portion  is  pressed 


ANTENNA 


987 


down  to  a  level,  or,  the  margin  remaining  entire,  the  central  por- 
tion must  be  thrown  into  plaits,  so  that  its  corneules  overlap  one 
another.  As  the  latter  condition  interferes  with  the  examination 
of  the  structure  much  more  than  the  former  does,  it  should  be 
avoided  by  making  a  number  of  slits  in  the  margin  of  the  convex 
membrane  before  it  is  flattened  out.  Vertical  sections,  adapted  to 
demonstrate  the  structure  of  the  ocelli  and  their  relations  to  the 
optic  nerve,  can  be  only  made  when  the  insect  is  fresh  or  has  been 
preserved  in  strong  spirit.  Mr.  Lowne  recommends  that  the  head 
should  be  hardened  in  a  2  per  cent,  solution  of  chromic  acid,  and 
then  imbedded  in  cacao  butter ;  the  sections  must  be  cut  very  thin, 
and  should  be  mounted  in  Canadh,  balsam.  The  following  are  some 
of  the  insects  whose  eyes  are  best  adapted  for  microscopic  pre- 
parations ;  Coleoptera,  Cicindela,  Dytiscus,  Melolontha  (cockchafer), 
Lucanus  (stag-beetle) ;  Orthoptera,  Acheta  (house  arid  field  crickets), 
Locusta ;  Hemiptera,  Notonecta  (boat-fly) ;  JVe-uroptera,  Libellula 
(dragon-fly),  Agrion  ;  Hymenoptera,  Vespida?  (wasps)  and  Apidse 
(bees)  of  all  kinds  ;  Lepidoptera,  Vanessa  (various  species  of),  Sphinx 
ligustri  (priA'et  hawk-moth),  Bombyx  (silkworm  moth  and  its  allies)  ; 
Diptera,  Tabanus  (gad-fly),  Asilus,  Eristalis  (drone-fly),  Tipula  (crane- 
fly),  Musca  (house-fly),  and 
many  others. 

The  antenncv,  which  are 
the  two  jointed  appendages 
arising  from  the  upper  part 
of  the  head  of  insects  (fig. 
731,  b  6),  present  a  most 
wonderful  variety  of  confor- 
mation in  the  several  tribes 
of  insects,  often  differing 
considerably  in  the  several 
species  of  one  genus,  and 
even  in  the  two  sexes  of  the 
same  species.  Hence  the 
characters  which  they  afford 
are  extremely  useful  in  classi- 
fication, especially  since  their 
structure  must  almost  neces- 
sarily be  in  some  way  related 
to  the  habits  and  general 
economy  of  the  creatures  to 
which  they  belong,  although 
our  imperfect  acquaintance 
with  their  function  may  pre- 
vent us  from  clearly  discerning  this  relation.  Thus  among  the 
Coleoptera  we  find  one  large  family,  including  the  glow-worm,  fire- 
fly, skip-jack,  <fcc.,  distinguished  by  the  toothed  or  seriated  form  of 
the  antenna?,  and  hence  called  Serricornia  ;  in  another,  of  which  the 
burying-beetle  is  the  type,  the  antenna?  are  terminated  by  a  club- 
shaped  enlargement,  so  that  these  beetles  are  termed  Clavicornia  ; 
in  another,  again,  of  which  the  Hydrophilus,  or  large  water-beetle, 


FIG.  736. — Antenna  of  Melolontha 
(cockchafer). 


988 


INSECTS   AND   AKACHN1DA 


is  an  example,  the  antennae  are  never  longer,  and  are  commonly 
shorter,  than  one  of  the  pairs  of  palpi,  whence  the  name  of  Palpi - 
cornia  is  given  to  this  group  ;  in  the  very  large  family  that  includes 
the  Lucanlj  or  stag-beetles,  with  the  Scarabcei,  of  which  the  cockchafer 
is  the  commonest  example,  the  antenna?  terminate  in  a  set  of  leanike 
appendages,  which  are  sometimes  arranged  like  a  fan  or  the  leaves 
of  an  open  book  (fig.  736),  are  sometimes  parallel  to  each  other  like 
the  teeth  of  a  comb,  arid  sometimes  fold  one  over  the  other,  thence 
giving  the  name  Lamelllcornia ;  whilst  another  large  family  is 
distinguished  by  the  appellation  Longicornia,  from  the  great  length 
of  the  antenna?,  which  are  at  least  as  long  as  the  body,  and  often 
longer.  Among  the  Lepidoptera,  again,  the  conformation  of  the 
antennae  frequently  enables  us  at  once  to  distinguish  the  group  to 
which  any  specimen  belongs.  As  every  treatise  on  entomology  con- 
tains figures  and  descriptions  of  the  principal  types  of  conformation 
of  these  organs,  there  is  no  occasion  here  to  dwell  upon  them  longer 
than  to  specify  such  as  are  most  interesting  to  the  microscopist : 
Coleoptera,  Brachinus,  Calathus,  Harpalus,  Dytiscus,  Staphylimis, 
Philonthus,  Elater,  Lampyris,  Silpha,  Hydrophilus,  Aphodius, 
Melolontha,  Cetonia,  Curculio,  Necrophorus  ;  Orthoptera,  Forficula 
(earwig),  Blatta  (cockroach) ;  Lepidoptera,  Sphingidae  (hawk-moths), 
and  Noctuina  (moths)  of  various  kinds,  the  large  '  plumed  '  antennae 
of  the  latter  being  peculiarly  beautiful  objects  under  a  low  magni- 
fying power ;  Diptera,  Culicidae  (gnats  of  various  kinds),  Tipulidaa 
(crane-flies  and  midges),  Tabanus,  Eristalis,  and  Muscidae  (flies  of 

various    kinds).      All    the 

A  T>  larger   antennae,  when  not 

mounted  *  dry  '  as  opaque 
objects,  should  be  put  up  in 
balsam,  after  being  soaked 
for  some  time  in  tur- 
pentine ;  but  the  small 
feathery  antenna?  of  gnats 
and  midges  are  so  liable 
to  distortion  when  thus 
mounted  that  it  is  better 
to  set  them  up  in  fluid,  the 
head  with  its  pair  of  an- 
tennae being  thus  preserved 
together  when  not  too 
large.  A  curious  set  of  organs  is  to  be  discovered  in  the 
antennae  of  many  insects,  which  have  been  supposed  to  constitute 
collectively  an  apparatus  for  hearing.  Each  consists  of  a  cavity 
hollowed  out  in  the  horny  integument,  sometimes  nearly  spherical, 
sometimes  flask-shaped,  and  sometimes  prolonged  into  numerous 
extensions  formed  by  the  folding  of  its  lining  membrane ;  the  mouth 
of  the  cavity  seems  to  be  normally  closed  in  by  a  continuation  of  this 
membrane,  though  its  presence  cannot  always  be  satisfactorily  deter- 
mined ;  whilst  to  its  deepest  part  a  nerve-fibre  may  be  traced.  The 
expanded  lamellae  of  the  antennae  of  Melolontha  present  a  great  dis- 
play of  these  cavities,  which  are  indicated  in  fig.  737,  A,  by  the 


FIG.  787. — Minute  structure  of  leaflike  expan- 
sions of  antenna  of  MelolontJia :  A,  their  in- 
ternal layer ;  B,  their  superficial  layer. 


THE   MOUTH   OF  INSECTS  989 

small  circles  that  beset  almost  their  entire  area ;  their  form,  which 
is  very  peculiar,  can  here  be  only  made  out  by  vertical  sections  ;  but 
in  many  of  the  smaller  antenna?,  such  as  those  of  the  bee,  the 
cavities  can  be  seen  sidewise  without  any  other  trouble  than  that 
of  bleaching  the  specimen  to  render  it  more  transparent.1 

The  next  point  in  the  organisation  of  insects  to  which  the  atten- 
tion of  the  microscopist  may  be  directed  is  the  structure  of  the 
mouth.  Here,  again,  we  find  almost  infinite  varieties  in  the  details 
of  conformation ;  but  these  may  be  for  the  most  part  reduced  to  a 
small  number  of  types  or  plans,  which  are  characteristic  of  the  dif- 
ferent orders  of  insects.  It  is  among  the  Coleoptera,  or  beetles,  that 
we  find  the  several  parts  of  which  the  mouth  is  composed  in  their 
most  distinct  form  ;  for,  although  some  of  these  parts  are  much  more 
highly  developed  in  other  insects,  other  parts  may  be  so  much  altered 
or  so  little  developed  as  to  be  scarcely  recognisable.  The  Coleoptera 
present  the  typical  conformation  of  the  mandibulate  mouth,  which  is 
adapted  for  the  prehension  and  division  of  solid  substances  ;  and  this 
consists  of  the  following  parts  :  1,  a  pair  of  jaws,  termed  mandibles, 
frequently  furnished  with  powerful  teeth,  opening  laterally  on  either 
side  of  the  mouth,  and  serving  as  the  chief  instruments  of  manduca- 
tion ;  2,  a  second  pair  of  jaws,  termed  maxillce,  smaller  and  weaker 
than  the  preceding,  beneath  which  they  are  placed,  and  serving  to 
hold  the  food,  and  to  convey  it  to  the  back  of  the  mouth  ;  3,  an 
upper  lip,  or  labrum  ;  4,  a  lower  lip  or  labium  ;  5,  one  or  two  pairs 
of  small  jointed  appendages,  termed  palpi,  attached  to  the  maxilla?, 
and  hence  called  maxillary  palpi ;  6,  a  pair  of  labial  palpi.  The 
labium  2  is  often  composed  of  several  distinct  parts,  its  basal  portion 
being  distinguished  as  the  mentum  or  chin,  and  its  anterior  portion 
being  sometimes  considerably  prolonged  forwards,  so  as  to  form  an 
organ  which  is  properly  designated  the  ligula,  but  which  is  more 
commonly  known  as  the  '  tongue,'  though -not  really  entitled  to  that 
designation,  the  real  tongue  being  a  soft  and  projecting  organ  which 
forms  the  floor  of  the  mouth,  and  which  is  only  found  as  a  distinct 
part  in  a  comparatively  small  number  of  insects,  as  the  cricket.  This 
ligula  is  extremely  developed  in  the  fly  kind,  in  which  it  forms  the 
chief  part  of  what  is  commonly  called  the  '  proboscis '  (fig.  739)  ;  3 

1  See  the  memoir  of  Dr.  Hicks,  '  On  a  new  Structure  in  the  Antennae  of  Insects,' 
in  Trans.  Linn.  Soc.  xxii.  p.  147;  and  his  'Further  Remarks'  at  p.  383  of  the 
same  volume.  See  also  the  memoir  of  M.  Lespes, '  Sur  1'Appareil  auditif  des  Insectes,' 
ioi.Aim.de9  Sci.  Nat.  ser.  iv.  Zool.  torn.  ix.  p.  258  ;  and  that  of  M.  Claparede, '  Sur  le's 
pretendus  Organes  auditifs  des  Coleopteres  lamellicornes  et  autres  Insectes,'  in  Ann. 
des  Sci.  Nat.  se"r.  iv.  Zool.  torn.  x.  p.  236.  Dr.  Hicks  lays  great  stress  on  the  'bleach- 
ing process '  as  essential  to  success  in  this  investigation,  and  he  gives  the  following 
directions  for  performing  it :  Take  of  chlorate  of  potass  a  drachm,  and  of  water  a 
drachm  and  a  half ;  mix  these  in  a  small  wide  bottle  containing  about  an  ounce ;  wait 
five  minutes  and  then  add  about  a  drachm  and  a  half  of  strong  hydrochloric  acid. 
Chlorine  is  thus  slowly  developed,  and  the  mixture  will  retain  its  bleaching  power 
for  some  time.  For  an  account  of  Herr  F.  Ruland's  observations  see  Joum.  Hoy 
Micr  Soc.  1888,  p.  723. 

2  The  labium  and  the  labial  palps  are,  morphologically,  a  second  pair  of  maxillae 
which  have  undergone  more  or  less  fusion  of  the  basal  parts  along  the  median  line. 

•"'  The  representation  given  in  the  figure  is  taken  from  one  of  the  ordinary  pre- 
parations of  the  fly's  proboscis,  which  is  made  by  slitting  it  open,  flattening  it  out, 
and  mounting  it  in  balsam.  For  representations  of  the  true  relative  positions  of 
the  different  parts  of  this  wonderful  organ,  and  for  minute  descriptions  of  them,  the 


990 


INSECTS  AND   AEACHNIDA 


and  it  also  forms  the  'tongue'  of  the  bee  and  its  allies  (fig.  738). 
The  ligula  of  the  common  fly  presents  a  curious  modification  of  the 
ordinary  tracheal  structure,  the  purpose  of  which  is  not  apparent  ; 
for  instead  of  its  trachea?  being  kept  pervious,  after  the  usual 
fashion,  by  the  winding  of  a  continuous  spiral  fibre  through  their 
interior,  the  fibre  is  broken  into  rings,  and  these  rings  do  not  sur- 
round the  whole  tube,  but  are  terminated  by  a  set  of  arches  that  pass 
from  one  to  another  (fig.  739,  B).1  In  the  Diptera,  or  two-  winged 
flies  generally,  the  labrum,  maxilla?,  mandibles,  and  the  internal 
tongue  (where  it  exists)  are  •  converted  into  delicate  lancet-shaped 
organs  termed  setce,  which,  when  closed  together,  are  received  into 

a  hollow  on  the  upper  side  of  the 
labium  (fig.  739),  but  which  are 
capable  of  being  used  to  make 
punctures  in  the  skin  of  animals  or 
the  epidermis  of  plants,  whence 
the  juices  may  be  drawn  forth  by 
the  proboscis.  Frequently,  how- 
ever, two  or  more  of  these  organs 
may  be  wanting,  so  that  their 
number  is  reduced  from  six  to 
four,  three,  or  two.  In  the 
Hymenoptera  (bee  and  wasp  tribe) 
the  labrum  and  the  mandibles 
(fig.  738,  b)  much  resemble  those 
of  mandibulate  insects,  and  are 
used  for  corresponding  purposes  ; 
the  maxilla?  (c)  are  greatly  elon- 
gated, and  form,  when  closed,  a 
tubular  sheath  for  the  ligula  j  or 
'  tongue,'  through  which  the 


FIG.  738.—  Parts  of  the  mouth  of  Api 


honey  is  drawn  up  ;  the  labial 
palpi  (d)  also  are  greatly  de- 
\elopecl,  ^d  fold  together,  like 

ligula,  or  prolonged  labium,  com-   the   maxilla?,    so   as   to  form   an 
monly  termed  the  '  tongue.'  inner   sheath    for   the  '  tongue  ;  ' 

while  the  'ligula'  itself  (e)  is  a 

long  tapering  muscular  organ,  marked  by  an  immense  number  of 
short  annular  divisions,  and  densely  covered  over  its  own  length 
with  long  hairs.  It  is  not  tubular,  as  some  have  stated,  but  is 
solid  ;  when  actively  employed  in  taking  food  it  is  extended  to  a 

reader  is  referred  to  Mr.  Suffolk's  memoir,  '  On  the  Proboscis  of  the  Blow-fly,'  in 
Monthly  Microsc.  Journ.  i.  p.  331,  and  to  Mr.  Lowne's  treatise  on  The  Anatomy 
and  Physiology  of  the  Blow-fly. 

1  According  to  Dr.  Anthony  (Monthly  Microscopical  Journ.  vol.  xi.  p.  242),  these 
'  pseudo-tracheae  '  are  suctorial  organs,  which  can  take  in  liquid  alike  at  their  ex- 
tremities and  through  the  whole  length  of  the  fissure  caused  by  the  interruption  of 
the  rings,  the  edges  of  this  fissure  being  formed  by  the  alternating  series  of  '  ear-like 
appendages'  connected  with  the  terminal  'arches,'  the  closing  together  of  which 
converts  the  pseudo-tracheee  into  a  complete  tube.  Dr.  Anthony  considers  each  of 
these  ear-like  appendages  to  be  a  minute  sucker,  '  either  for  the  adhesion  of  the  fleshy 
tongue,  or  for  the  imbibition  of  fluids,  or  perhaps  for  both  purposes.'  The  point  is 
well  worthy  of  further  investigation. 


.MOUTH-PARTS   OF   INSECTS 


991 


FIG.  739. — A,  tongue  of  common  fly  :  a,  lobes  of  ligula  ;  b,  portion  inclosing 
the  lancets,  formed  by  the  metamorphosis  of  the  maxillae  ;  c,  maxillary 
palpi.  B,  a  portion  of  some  of  the  pseudo-tracheae  more  highly  magnified. 


992 


INSECTS   AND   ARACHNIDA 


great  distance  beyond  the  other  parts  of  the  mouth ;  but  when  at 
rest  it  is  closely  packed  up  and  concealed  between  the  maxilla?.  '  The 
manner/  says  Mr.  Newport,  '  in  which  the  honey  is  obtained  when 
the  organ  is  plunged  into  it  at  the  bottom  of  a  flower  is  by  "  lapping," 
or  a  constant  succession  of  short  and  quick  extensions  and  contrac- 
tions of  the  organ,  which  occasion  the  fluid  t©  accumulate  upon  it 
and  to  ascend  along  its  upper  surface,  until  it  reaches  the  orifice  of 
the  tube  formed  by  the  approximation  of  the  maxillae  above,  and  of 
the  labial  palpi  and  this  part  of  the  ligula  below.' 

By  the  plan  of  conformation  just  described  we  are  led  to  that 
which  prevails  among  the  Lepidoptera,  or  butterfly  tribe,  which, 
being  pre-eminently  adapted  for  suction,  is  termed  the  haustellate 
mouth.  In  these  insects  the  labium  and  mandibles  are  reduced  to 
three  minute  triangular  plates ;  whilst  the  maxillae  are  immensely 
elongated,  and  are  united  together  along  the  median  line  to  form 

the  haustellum,  or 
true  '  proboscis,' 
which  contains  a 
tube  formed  by  the 
junction  of  the  two 
grooves  that  are 
channelled  out 
along  their  mutu- 
ally applied  sur- 
faces, and  which 
serves  to  pump 
up  the  juices  of 
deep  cup-shaped 
flowers,  into  which 
the  size  of  their 
wings  prevents 
these  insects  from 
entering.  The 
length  of  this  haustellum  varies  greatly  :  thus  in  such  Lepidoptera  as 
take  no  food  in  their  perfect  state  it  is  a  very  insignificant  organ  ;  in 
some;  of  the  white  hawk-moths,  which  hover  over  blossoms  without 
alighting,  it  is  nearly  two  inches  in  length,  and  in  most  butterflies  and 
moths  it  is  about  as  long  as  the  body  itself;  in  Amphonyx,  one  of  the 
/Sphingidce,  it  is  more  than  nine  inches  long,  or  about  three  times  the 
length  of  the  body.  This  haustellum,  which,  when  not  in  use,  is 
coiled  up  in  a  spiral  beneath  the  mouth,  is  an  extremely  beautiful 
microscopic  object,  owing  to  the  peculiar  banded  arrangement  it  ex- 
hibits (fig.  740),  which  is  probably  due  to  the  disposition  of  its  muscles. 
In  many  instances  the  two  halves  may  be  seen  to  be  locked  together 
by  a  set  of  hooked  teeth,  which  are  inserted  into  little  depressions 
between  the  teeth  of  the  opposite  side.  Each  half,  moreover,  may 
be  ascertained  to  contain  a  trachea  or  air-tube,  and  it  is  probable, 
from  the  observations  of  Mr.  Newport,  that  the  sucking  up  of  the 
juices  of  a  flower  through  the  proboscis  (which  is  accomplished  with 
great  rapidity)  is  effected  by  the  agency  of  the  respiratory  apparatus. 
The  proboscis  of  many  butterflies  is  furnished,  for  some  distance  from 


FIG.  740. — Haustellum  (proboscis)  of  Vanessa. 


PAETS   OF   THE    BODY  993 

its  extremity,  with  a  double  row  of  small  projecting  barrel-shaped 
bodies  (shown  in  fig.  740),  which  are  surmised  by  Mr.  Newport 
(whose  opinion  is  confirmed  by  the  kindred  inquiries  of  Dr.  Hicks) 
to  be  organs  of  taste.  Numerous  other  modifications  of  the  structure 
of  the  mouth,  existing  in  the  different  tribes  of  insects,  are  wrell 
worthy  of  the  careful  study  of  the  microscopist ;  but  as  detailed 
descriptions  of  most  of  these  will  be  found  in  every  systematic  trea- 
tise on  entomology,  the  foregoing  general  account  of  the  principal 
types  must  suffice. 

Parts  of  the  Body. — The  conformation  of  the  several  divisions  of 
the  alimentary  canal  presents  such  a  multitude  of  diversities,  not 
only  in  different  tribes  of  insects,  but  in  different  states  of  the  same 
individual,  that  it  would  be  utterly  vain,  to  attempt  here  to  give 
even  a  general  idea  of  it,  more  especially  as  it  is  a  subject  of  far 
less  interest  to  the  ordinary  microscopist  than  to  the  professed 
anatomist.  Hence  we  shall  only  stop  to  mention  that  the  'muscular 
gizzard,'  in  which  the  oesophagus  very  commonly  terminates,  is  often 
lined  by  several  rows  of  strong  horny  teeth  for  the  reduction  of  the 
food,  which  furnish  very  beautiful  objects,  especially  for  the  bino- 
cular. These  are  particularly  developed  among  the  grasshoppers, 
crickets,  and  locusts,  the  nature  of  whose  food  causes  them  to  require 
powerful  instruments  for  its  reduction.1 

The  circulation  of  blood  may  be  distinctly  watched  in  many  of 
the  more  transparent  larvse,  and  may  sometimes  be  observed  in  the 
perfect  insect.  It  is  kept  up  by  a  '  dorsal  vessel '  (so  named  from 
the  position  it  always  occupies  along  the  middle  of  the  back  in  the 
thoracic  and  abdominal  regions),  which  really  consists  of  a  succession 
of  muscular  contractile  cavities,  one  for  each  segment,  opening  one 
into  another  from  behind  forwards,  so  as  to  form  a  continuous  trunk 
divided  by  valvular  partitions.  In  many  larvae,  however,  these 
partitions  are  very  indistinct ;  and  the  walls  of  the  '  dorsal  -vessel ' 
are  so  thin,  and  transparent  that  it  can  with  difficulty  be  made  out, 
a  limitation  of  the  light  by  the  diaphragm  being  often  necessary. 
The  blood  which  moves  through  this  trunk,  and  which  is  distributed 
by  it  to  the  body,  is  a  transparent  and  nearly  colourless  fluid,  carry- 
ing with  it  a  number  of  '  oat-shaped '  corpuscles,  by  the  motion  of 
which  its  flow  can  be  followed.2  The  current  enters  the  *  dorsal 
vessel '  at  its  posterior  extremity,  and  is  propelled  forwards  by  the 
contractions  of  the  successive  chambers,  being  prevented  from  moving 
in  the  opposite  direction  by  the  valves  between  the  chambers,  which 
only  open  forwards.  Arrived  at  the  anterior  extremity  of  the 
*  dorsal  vessel,'  the  blood  is  distributed  in  three  principal  channels  : 
a  central  one,  namely,  passing  to  the  head,  and  a  lateral  one  to  either 
side,  descending  so  as  to  approach  the  lower  surface  of  the  body. 
It  is  from  the  two  lateral  currents  that  the  secondary  streams 
diverge,  which  pass  into  the  legs  and  wings,  and  then  return  back 
to  the  main  stream  ;  and  it  is  from  these  also  that  in  the  larva 

1  The  student  who  desires  to  carry  further  the  stuffy  of  the  digestive  apparatus 
should  consult  Professor  Plateau's  memoir,  '  Recherches  sur  les   Phenomenes   de 
la  Digestion  chez  les  Insectes,'  Mem.  Acad .  Hoi/.  <lc  Jirhjiqne,  xli. 

2  On  the  blood-tissue  of   insects  consult  Mr.  AV.   ."\L  Wheeler  in  vol.  vi.  of  the 
American  journal  Psyche. 


994  INSECTS   AND   ARACHNIDA 

of  the  Ephemera  marginata  (day-fly),  the  extreme  transparence  of 
which  renders  it  one  of  the  best  of  all  subjects  for  the  observation 
of  insect  circulation,  the  smaller  currents  diverge  into  the  gill -like 
appendages  with  which  the  body  is  furnished.  The  blood-currents 
seem  rather  to  pass  through  channels  excavated  among  the  tissues 
than  through  vessels  with  distinct  walls.  In  many  aquatic  larvae, 
especially  those  of  the  Culicidce  (gnat  tribe),  the  body  is  almost 
entirely  occupied  by  the  visceral  cavity  ;  and  the  blood  may  be  seen 
to  move  backwards  in  the  space  that  surrounds  the  alimentary 
canal,  which  here  serves  the  purpose  of  the  channels  usually  exca- 
vated through  the  solid  tissues,  and  which  freely  communicates  at 
each  end  with  the  dorsal  vessel.  This  condition  strongly  resembles 
that  found  in  many  Annulata.1 

The  circulation  may  be  easily  seen  in  the  wings  of  many  insects 
in  their  pupa  state,  especially  in  those  of  the  Neuroptera  (such  as 
dragon-flies  and  day-flies),  which  pass  this  part  of  their  lives  under 
water  in  a  condition  of  activity,  the  pupa  of  Agrion  puella,  one  of 
the  smaller  dragon-flies,  being  a  particularly  favourable  subject  for 
such  observations.  Each  of  the  '  nervures '  of  the  wings  contains  a 
'  trachea '  or  air-tube,  which  branches  off  from  the  tracheal  system 
of  the  body  ;  and  it  is  in  a  space  around  the  trachea  that  the  blood 
maybe  seen  to  move  when  the  hard  framework  of  the  nervure  itself 
is  not  too  opaque.  The  same  may  be  seen,  however,  in  the  wings  of 
pupse  of  bees,  butterflies,  etc.,  which  remain  shut  up  motionless  in 
their  cases ;  for  this  condition  of  apparent  torpor  is  one  of  great 
activity  of  their  nutritive  system,  those  organs,  especially,  which 
are  peculiar  to  the  perfect  insect  being  then  in  a  state  of  rapid 
growth,  and  having  a  vigorous  circulation  of  blood  through  them. 
In  certain  insects  of  nearly  every  order  a  movement  of  fluid  may 
be  seen  in  the  wings  for  some  little  time  after  their  last  meta- 
morphosis ;  but  this  movement  soon  ceases  and  the  wings  dry  up. 
The  common  fly  is  as  good  a  subject  for  this  observation  as  can 
be  easily  found  ;  it  must  be  caught  within  a  few  hours  or  days  of  its 
first  appearance ;  and  the  circulation  may  be  most  conveniently 
brought  into  view  by  inclosing  it  (without  water)  in  the  aquatic- 
box,  and  pressing  down  the  cover  sufficiently  to  keep  the  body  at  rest 
without  doing  it  any  injury. 

The  respiratory  apparatus  of  insects  affords  a  very  interest- 
ing series  of  microscopic  objects ;  for,  with  great  uniformity  in  its 
general  plan,  there  is  almost  infinite  variety  in  its  details.  The 
aeration  of  the  blood  in  this  class  is  provided  for,  not  by  the  trans- 
mission of  the  fluid  to  any  special  organ  representing  the  lung  of  a 
vertebrated  animal  or  the  gill  of  a  mollusc,  but  by  the  introduction 
of  air  into  every  part  of  the  body,  through  a  system  of  minutely 
distributed  trachece,  or  air-tubes,  which  penetrate  even  the  smallest 
and  most  delicate  organs.  Thus,  as  we  have  seen,  they  pass  into 
the  haustellum,  or  '  proboscis.'  of  the  butterfly,  and  they  are  minutely 

1  See  the  memoirs  on  Corethra^lumicornis,  by  Professor  Rymer  Jones,  in  Trans. 
Microsc.  Soc.  n.s.  vol.  xv.  1867,  p.  99  ;  by  Professor  E.  Ray  Lankester,  in  the  Popular 
Science  Beview  for  October  1865  ;  and  by  Dr.  A.  Weismann,  in  Zeitsclir.  f.  wiss.  Zuol. 
Bd.  xvi.  p.  45.  On  the  circulatory  system  of  insects  consult  Graber,  '  Ueber  den  pro- 
pulsatorischen  Apparat  der  Insecten,'  Arcli.fur  mikr.  Anat.  ix.  p.  129. 


BESPIRATOKY   APPARATUS 


995 


distributed  in  the  elongated  labium  or  '  tongue '  of  the  %  (fig.  739) 
Their  general  distribution  is  shown  in  fig.  741,  where  we  see  two 
long  trunks  (/)  passing  from  one  end  of  the  body  to  the  other,  and 
connected  with  each  other  by  a  transverse  canal  in  every  segment ; 
these  trunks  communicate,  on  the  one  hand,  by  short  wide  passages 
with  the  'stigmata,'  'spiracles/  or  'breathing  pores'  (g),  through 
which  the  air  enters  and  is  discharged  ;  whilst  they  give  off  branches 
to  the  different  segments, 
which  divide  again  and 
again  into  ramifications  of 
extreme  minuteness.  They 
usually  communicate  also 
with  a  pair  of  air-sacs  (A) 
which  is  situated  in  the 
thorax ;  but  the  size  of 
these  (which  are  only  found 
in  the  perfect  insect,  no 
trace  of  them  existing  in 
the  larvae)  varies  greatly 
in  different  tribes,  being 
usually  greatest  in  those 
insects  which  (like  the  bee) 
can  sustain  the  longest  and 
most  powerful  flight,  and 
least  in  such  as  habitually 
live  upon  the  ground  or 
upon  the  surface  of  the 
water.  The  structure  of 
the  air-tubes  reminds  us 
of  that  of  the  'spiral 
vessels '  of  plants,  which 
seemed  destined  (in  part 
at  least)  to  perform  a 
similar  office :  for  within 
the  membrane  that  forms 
their  outer  wall  an  elastic 
fibre  winds  round  and 
round,  so  as  to  form  a 

spiral  closelv  resembling  Fl<*-  741.— Tracheal  system  of  Nepa  (water- 
in  its  nom'tinn  ami  fnno  scorpion) :  a,  head  ;  6,  first  pair  of  legs  ;  c,  first 

segment  of  thorax  ;  d,  second  pair  of  wings  ;  e, 

tions  the  spiral  wire  spring         second  pair  of  legs  ;  /,  tracheal  trunk ;  g,  one 
of  flexible  gas  pipes  :  with-         of  the  stigmata ;  h,  air-sac, 
in    this,    again,    however, 

there  is  another  membranous  wall  to  the  air-tubes,  so  that  the  spire 
winds  between  their  inner  and  outer  coats.  When  a  portion  of  one 
of  the  great  trunks  with  some  of  the  principal  branches  of  the 
tracheal  system  has  been  dissected  out,  and  so  pressed  in  mounting 
that  the  sides  of  the  tubes  are  flattened  against  each  other  (as  has 
happened  in  the  specimen  represented  in  fig.  742),  the  spire  forms 
two  layers  which  are  brought  into  close  apposition,  and  a  very 
beautiful  appearance,  resembling  that  of  watered  silk,  is  produced 

3s2 


996 


INSECTS   AND    ARACHNIDA 


by  the  crossing  of  the  two  sets  of  fibres,  of  which  one  overlies  the 
other.     That  this  appearance,  however,  is  altogether  an  optical  illu- 
sion may  be  easily  demonstrated  by  carefully  following  the  course 
of  any  one  of  the  fibres,  which  will  be  found  to  be  perfectly  regular. 
The  '  stigmata '  or  '  spiracles '  through  which  the  air  enters  the 
tracheal   system   are    generally    visible    on    the    exterior    of    tin- 
body  of  the  insect    (espe- 
cially    on     the     abdomi- 
nal segments)  as  a   series 
of      pores      along      each 
margin  of  the  under  sur- 
face.       In     most      larva-, 
nearly    every    segment    is 
provided  wTith  a  pair 
in      the      perfect 
several    of    them    remain 
closed,    especially    in    the 


but 

insect 


thoracic    region,    so    that 


FIG.  742.— Portion  of  a  large  trachea  of  Dytiscus, 
with  some  of  its  principal  branches. 


« 

^m      ^tflP^B  ^  their  number  is  often  con- 

£T     $Jf^  gprv.^          siderably     reduced.      The 

gM        \  structure  of  the   spiracle- 

nj     ;  H  varies  greatly  in  regard  t<  > 

complexity  in  different  in- 
sects ;  and  even  where  1  he 
general  plan  is  the  same 
the  details  of  conforma- 
tion are  peculiar,  so  thai 

perhaps  in  scarcely  any  two  species  are  they  alike.  Generally  speak- 
ing, they  are  furnished  with  some  kind  of  sieve  at  their  entrance  by 
which  particles  of  dust,  soot,  &c.,  which  would  otherwise  enter  the 
air-passages,  are  filtered  out;  and  this  sieve  may  be  formed  by 

the  interlacement  of  the 
branches  of  minute  arbo- 
rescent growths  from  the 
border  of  the  spiracle,  as 
in  the  common  fly  (fig. 
743),  or  in  the  J)i/tiscus ; 
or  it  may  be  a  membrane 
perforated  with  minute 
holes,  and  supported  upon 
a  framework  of  bars  that 
is  prolonged  in  like  manner 
from  the  thickened  margin 

FIG.  743. — Spiracle  of  common  fly.  of  the   aperture   (fig.  744), 

as    in    the    larva?    of    the 

Melolontha  (cockchafer).  Not  unfrequently  the  centre  of  the  aper- 
ture is  occupied  by  an  impervious  disc,  from  which  radii  proceed 
to  its  margin,  as  is  well  seen  in  the  spiracle  of  Tipula,  (crane- 
fly).1  In  those  aquatic  larvae  which  breathe  air  we  often  find  one 

1  Consult  Landois  and  Thiele,  '  Der  Tracheenverschluss  bei  den  Insecten,'  Zcit- 
schriftf.  wiss.  Zool.  xvii.  p.  187. 


RESPIRATORY  APPARATUS 


997 


of  the  spiracles  of  the  last  segment  of  the  abdomen  prolonged  into  a 
tube,  the  mouth  of  which  remains  at  the  surface  while  the  body  is 
immersed  ;  the  larvae  of  the  gnat  tribe  may  frequently  be  observed 
in  this  position. 

There  are  many  aquatic  larvae,  however,  which  have  an  entirely 
different  provision  for  respiration,  being  furnished  with  external  leaf- 
like  or  brush-like  appendages  into  which  the  tracheae  are  prolonged,  so 
that  by  absorbing  air  from  the  water  that  bathes  them  they  may  con- 
vey this  into  the  interior  of  the  body.  We  cannot  have  a  better  example 
of  this  than  is  afforded  by  the  larva  of  the  common  Ephemera  (day- 
fly),  the  body  of  which  is  furnished  with  a  set  of  branchial  appendages 
resembling  the  '  fin-feet '  of  branqfiiopods,  whilst  the  three-pronged 
tail  also  is  fringed  with  clusters  of  delicate  hairs  which  appear  to 
minister  to  the  same  function.  In  the  larva  of  the  Libellula 
(dragon-fly)  the  extension  of  the  surface  for  aquatic  respiration 
takes  place  within  the  termination  of  the  intestine,  the  lining 
membrane  of  which  is  folded  into  an  immense  number  of  plaits, 
each  containing  a  minutely  ramified  system  of  tracheae ;  the  water 
slowly  drawn  in  through  the  anus 
for  bathing  this  surface  is  ejected 
with  such  violence  that  the  body 
is  impelled  in  the  opposite  direc- 
tion ;  and  the  air  taken  up  by  its 
tracheae  is  carried  through  the 
system  of  air-tubes  of  which  they 
form  part  into  the  remotest  organs. 
This  apparatus  is  a  peculiarly  in- 
teresting object  for  the  microscope 
011  account  of  the  extraordinarily 
rich  distribution  of  the  tracheae  in 
the  intestinal  'folds.  FlG  744._Spiracle  of  larva  of 

The  main  trunks  of  the  tracheal  cockchafer, 

system,  with  their  principal  ramifi- 
cations, may  generally  be  got  out  with  little  difficulty  by  laying 
open  the  body  of  an  insect  or  larva  under  water  in  a  dissecting 
trough,  and  removing  the  whole  visceral  mass,  taking  care  to  leave 
as  many  as  possible  of  the  branches,  which  will  be  seen  pro- 
ceeding to  this  from  the  two  great  longitudinal  tracheae,  to  whose 
position  these  branches  will  serve  as  a  guide.  Mr.  Quekett  recom- 
mended the  following  as  the  most  simple  method  of  obtaining  a 
perfect  system  of  tracheal  tubes  from  a  larva.  A  small  opening 
having  been  made  in  its  body,  this  is  to  be  placed  in  strong  acetic 
acid,  which  will  soften  or  decompose  all  the  viscera  ;  and  the  tracheae 
may  then  be  well  washed  with  the  syringe,  and  removed  from  the 
body  with  the  greatest  facility,  by  cutting  away  the  connections  of 
the  main  tubes  with  the  spiracles  by  means  of  fine-pointed  scissors. 
In  order  to  mount  them  they  should  be  floated  upon  the  slide,  on 
which  they  should  then  be  laid  out  in  the  position  best  adapted  for 
displaying  them.  If  they  are  to  be  mounted  in  Canada  balsam  they 
should  be  allowed  to  dry  upon  the  slide,  and  should  then  be  treated 
in  the  usual  way  ;  but  their  natural  appearance  is  best  preserved 


998  INSECTS   AND   AKACHNIDA 

by  mounting  them  in  fluid  (weak  spirit  or  Goadby's  solution),  using 
a  shallow  cell  to  prevent  pressure.  The  finer  ramifications  of  the 
tracheal  system  may  generally  be  seen  particularly  well  in  the  mem- 
branous wall  of  the  stomach  or  intestine  ;  and  this,  having  been  laid  out 
and  dried  upon  the  glass,  may  be  mounted  in  balsam  so  as  to  keep  the 
tracheae  full  of  air  (whereby  they  are  much  better  displayed),  if  care 
be  taken  to  use  balsam  that  has  been  previously  thickened,  to  drop 
this  on  the  object  without  liquefying  it  more  than  is  absolutely 
necessary,  and  to  heat  the  slide  and  the  cover  (the  heat  may  be 
advantageously  applied  directly  to  the  cover  after  it  has  been  put 
on  by  turning  over  the  slide  so  that  its  upper  face  shall  look  down- 
ward) only  to  such  a  degree  as  to  allow  the  balsam  to  spread  and 
the  cover  to  be  pressed  down.  The  spiracles  are  easily  dissected  out 
by  means  of  a  pointed  knife  or  a  pair  of  fine  scissors  ;  they  should 
be  mounted  in  glycerin  jelly  when  their  texture  is  soft,  and  in 
balsam  when  the  integument  is  hard  and  horny. 

Wings, — These  organs  are  essentially  composed  of  an  extension 
of  the  external  membranous  layer  of  the  integument  over  a  frame- 
work formed  by  prolongations  of  the  inner  horny  layer,  within 
which  prolongations  tracheae  are  nearly  always  to  be  found,  whilst 
they  also  include  channels  through  which  blood  circulates  during 
the  growth  of  the  wing  and  for  a  short  time  after  its  completion. 
This  is  the  simple  structure  presented  to  us  in  the  wings  of  Neuro- 
ptera  (dragon-flies,  &c.),  Hymenoptera  (bees  and  wasps),  Diptera 
(two- winged  flies),  and  also  of  many  Homoptera  (Cicadce  and  Aphides) ; 
and  the  principal  interest  of  these  wings  as  microscopic  objects  lies 
in  the  distribution  of  their  '  veins  '  or  '  nervures '  (for  by  both  names 
are  the  ramifications  of  their  skeleton  known)  and  in  certain  points 
of  accessory  structure.  The  venation  of  the  wings  is  most  beautiful 
in  the  smaller  ISTeuroptera,  since  it  is  the  distinguishing  feature  of 
this  order  that  the  veins,  after  subdividing,  reunite  again,  so  as  to 
form  a  close  network ;  whilst  in  the  Hymenoptera  and  Diptera  such 
reunions  are  rare,  especially  towards  the  margins  of  the  wings,  and 
the  areolse  are  much  larger.  Although  the  membrane  of  which 
these  wings  are  composed  appears  perfectly  homogeneous  when 
viewed  by  transmitted  light,  even  with  a  high  magnifying  power, 
yet  when  viewed  by  light  reflected  obliquely  from  their  surfaces 
an  appearance  of  cellular  areolation  is  often  discernible  ;  this  is  well 
seen  in  the  common  fly,  in  which  each  of  these  areolae  has  a  hair  in 
its  centre.  In  order  to  make  this  observation,  as  well  as  to  bring 
out  the  very  beautiful  iridescent  hues  which  the  wings  of  many 
minute  insects  (as  the  Aphides')  exhibit  when  thus  viewred,  it  is  con- 
venient to  hold  the  wing  in  the  stage-forceps  for  the  sake  of  giving 
it  every  variety  of  inclination  ;  and  when  that  position  has  been 
found  which  best  displays  its  most  interesting  features,  it  should  be 
set  up  as  nearly  as  possible  in  the  same.  For  this  purpose  it  should 
be  mounted  on  an  opaque  slide,  but  instead  of  being  laid  down 
upon  its  surface  the  wing  should  be  raised  a  little  above  it,  its 
*  stalk  '  being  held  in  the  proper  position  by  a  little  cone  of  soft  wax, 
in  the  apex  of  which  it  may  be  imbedded.  The  wings  of  most 
Hymenoptera  are  remarkable  for  the  peculiar  apparatus  by  which 


WINGS;   SOUND-ORGANS  999 

those  of  the  same  side  are  connected  together,  so  as  to  constitute  in 
flight  but  one  large  wing  ;  this  consists  of  a  row  of  curved  booklets 
on  the  anterior  margin  of  the  posterior  wing,  which  lay  hold  of  the 
thickened  and  doubled  down  posterior  edge  of  the  anterior  wing. 
These  booklets  are  sufficiently  apparent  in  the  wings  of  the  common 
bee,  when  examined  with  even  a  low  magnifying  power ;  but  they 
are  seen  better  in  the  wasp,  and  better  still  in  the  hornet.  The 
peculiar  scaly  covering  of  the  wings  of  the  Lepidoptera  has  already 
been  noticed ;  but  it  may  here  be  added  that  the  entire  wings  of 
many  of  the  smaller  and  commoner  insects  of  this  order,  such  as  the 
Tineidce  or  '  clothes-moths,'  form  very  beautiful  opaque  objects  for 
low  powers,  the  most  beautiful  of  all  being  the  divided  wings  of 
the  Fissipennia  or  '  plumed  mot?hs,'  especially  those  of  the  genus 

us}- 

There  are  many  insects,  however,  in  which  the  wings  are  more  or 
less  consolidated  by  the  interposition  of  a  layer  of  horny  substance 
between  the  two  layers  of  membrane.  This  plan  of  structure  is 
most  fully  carried  out  in  the  Coleoptera  (beetles),  whose  anterior 
wings  are  metamorphosed  into  elytra  or  ;  wing-cases  ; '  and  it  is 
upon  these  that  the  brilliant  hues  by  which  the  integument  of  many 
of  these  insects  is  distinguished  are  most  strikingly  displayed.  In 
the  anterior  wings  of  the  Forfoulidce,  or  earwig  tribe,  the  cellular 
structure  may  often  be  readily  distinguished  when  they  are  viewed 
by  transmitted  light,  especially  after  having  been  mounted  in  Canada 
balsam.  The  anterior  wings  of  the  Orthoptera  (grasshoppers, 
crickets,  &c.),  although  not  by  any  means  so  solidified  as  those  of 
Coleoptera,  contain  a  good  deal  of  horny  matter ;  they  are  usually 
rendered  sufficiently  transparent,  however,  by  Canada  balsam  to  be 
viewed  with  transmitted  light ;  and  many  of  them  are  so  coloured 
as  to  be  very  showy  objects  (as  are  also  the  posterior  fan-like  wings) 
for  the  electric  or  gas  microscope,  although  their  large  size  and  the 
absence  of  any  minute  structure  prevent  them  from  affording  much 
interest  to  the  ordinary  microscopist.  We  must  not  omit  to  men- 
tion, however,  the  curious  sound-producing  apparatus  which  is 
possessed  by  most  insects  of  this  order,  and  especially  by  the  common 
house-cricket.  This  consists  of  the  '  tympanum,'  or  drum,  which  is 
a  space  on  each  of  the  upper  wings,  scarcely  crossed  by  veins,  but 
bounded  externally  by  a  large  dark  vein  provided  with  three  or  four 
longitudinal  ridges  ;  and  of  the  '  file '  or  '  bow,'  which  is  a  transverse 
horny  ridge  in  front  of  the  tympanum,  furnished  with  numerous 
teeth ;  and  it  is  believed  that  the  sound  is  produced  by  the  rubbing 
of  the  two  bows  across  each  other,  while  its  intensity  is  increased 
by  the  sound-board  action  of  the  tympanum.  The  wings  of  the 
Fidyoridce  (lantern- flies)  have  much  the  same  texture  as  those  of  the  • 
Orthoptera,  and  possess  about  the  same  value  as  microscopic  objects, 
differing  considerably  from  the  purely  membranous  wings  of  the 
Cicadce  and  Aphides,  which  are  associated  with  them  in  the  order 
Nomoptera.  In  the  order  Hemiptera,  to  which  belong  various  kinds 

1  Compare  the  recently  published  memoir  by  M.  Baer,  '  Ueber  Bau  und  Farben 
der  Fliigelschuppen  bei  Tagfaltern,'  in  Zeitschr.f.  wiss.  Ziiol.  Ixv.  (1898),  pp.  50-65, 
as  also  M.  von  Linden  on  the  development  of  the  markings,  pp.  1-50  of  the  same  volume. 


1000  INSECTS   AND   AKACHNIDA 

of  land  and  water  insects  that  have  a  suctorial  mouth  resembling 
that  of  the  common  bug,  the  wings  of  the  anterior  pair  are  usually 
of  parchmeiity  consistence,  though  membranous  near  their  tips,  and 
are  often  so  richly  coloured  as  to  become  very  beautiful  objects 
when  mounted  in  balsam  and  viewed  by  transmitted  light ;  this  is 
the  case  especially  with  the  terrestrial  vegetable-feeding  kinds,  such 
as  the  Pentatoma  and  its  allies,  some  of  the  tropical  forms  of  which 
rival  the  most  brilliant  of  the  beetles.  The  British  species  are  by 
no  means  so  interesting,  and  the  aquatic  kinds,  which,  next  to  the 
bed-bugs,  are  the  most  common,  always  have  a  dull  brown  or  almost 
black  hue  ;  even  among  these  last,  however,  of  which  the  Notonecta 
(water-boatman)  and  the  Nepa  (water-scorpion)  are  well -known 
examples,  the  wings  are  beautifully  variegated  by  differences  in  the 
depth  of  that  hue.  The  halteres  of  the  Diptera,  which  are  the  re- 
presentatives of  the  posterior  wings,  have  been  shown  by  Dr.  J.  B. 
Hicks  to  present  a  very  curious  structure,  which  is  found  also  in 
the  elytra  of  Coleoptera  and  in  many  other  situations,  consisting  in 
a  multitude  of  vesicular  projections  of  the  superficial  membrane,  to 
each  of  which  there  proceeds  a  nervous  filament,  that  conies  to  it 
through  an  aperture  in  the  tegumentary  wall  on  which  it  is  seated. 
Various  considerations  are  stated  by  Dr.  Hicks  which  lead  him  to 
the  belief  that  this  apparatus,  when  developed  in  the  neighbourhood 
of  the  spiracles  or  breathing  pores,  essentially  ministers  to  the  sense 
of  smell,  whilst,  when  developed  upon  the  palpi  and  other  organs  in 
the  neighbourhood  of  the  mouth,  it  ministers  to  the  sense  of  taste.1 

Feet. — Although  the  feet  of  insects  are  formed  pretty  much  on 
one  general  plan,  yet  that  plan  is  subject  to  considerable  modifica- 
tions in  accordance  with  the  habits  of  life  of  different  species.  The 
entire  limb  usually  consists  of  five  divisions,  namely,  the  coxa  or  hip, 
the  trochanter,  the  femur  or  thigh,  the  tibia  or  shank,  and  the  tarsus 
or  foot ;  and  this  last  part  is  made  up  of  several  successive  joints. 
The  typical  number  of  these  joints  seems  to  be  five,'*  but  that 
number  is  subject  to  reduction  ;  and  the  vast  order  Coleoptera  is 
subdivided  into  primary  groups,  according  as  the  tarsus  consists  of 
five,  four,  or  three  segments.  The  last  joint  of  the  tarsus  is  usually 
furnished  with  a  pair  of  strong  hooks  or  claws  (figs.  745,  746) ;  and 
these  are  often  serrated  (that  is,  furnished  with  saw-like  teeth), 
especially  near  the  base.  The  under  surface  of  the  other  joints  is 
frequently  beset  with  tufts  of  hairs,  which  are  arranged  in  various 
modes,  sometimes  forming  a  complete  *  sole ; '  this  is  especially  the 
case  in  the  family  Gurculionidce ;  a  pair  of  the  feet  of  the  '  diamond 
beetle '  mounted  so  that  one  shows  the  upper  surface  made  resplendent 
by  its  jewel-like  scales,  and  the  other  the  hairy  cushion  beneath,  is 
a  very  interesting  object.  In  many  insects,  especially  of  the  fly 
kind,  the  foot  is  furnished  with  a  pair  of  membranous  expansions 

1  See  his  memoir,  '  On  a  new  Organ  in  Insects,'  in  Journ.  Linn.  Soc.  vol.  i.  1856, 
p.  136;  his  'Further  Remarks  on  the  Organs  found  on  the  Bases  of  the  Halteres 
and  Wings   of   Insects,'  in   Trans.  Linn.  Soc.  xxii.  p.  141 ;  and  his  memoir,  '  On 
certain  Sensory  Organs  in   Insects    hitherto   undescribed,'   in    Trans.   Linn.   Soc. 
xxiii.  p.  189.  ,  Compare  also  the  interesting  memoir  of  Weinland,  in  Zeitschr.  f. 
wiss.  Zb'ol.  li.  (18SO>,  pp.  35-160,  5  pis. 

2  See,  however,  Professor  Huxley  (Anat.  of  Invertebrate  Animals,  p.  348),  who, 
regarding  the  'pulvillus'  of  the  cockroach  as  a  joint,  finds  the  number  to  be  six. 


FEET 


1001 


termed  pulvilli  (fig.  745)  ;  and  these  are  beset  with  numerous  hairs, 
each  of  which  has  a  minute  disc  at  its  extremity.  This  structure  is 
evidently  connected  with  the  power  which  these  insects  possess  of 
walking  over  smooth  surfaces  in  opposition  to  the  force  of  gravity ; 
yet  there  is  still  considerable  uncertainty  as  to  the  precise  mode  in 
which  it  ministers  to  this  faculty.  Some  believe  that  the  discs  act 
as  suckers,  the  insect  being  held  up  by  the  pressure  of  the  air  against 
their  upper  surface  when  a  vacuum  is  formed  beneath  ;  whilst  others 
maintain  that  the  adhesion  is  the  result  of  the  secretion  of  a  viscid 
liquid  from  the  under  side  of  the  foot.  The  careful  observations  of 
Mr.  Hepworth  have  led  him  to  a  conclusion  which  seems  in  harmony 
with  all  the  facts  of  the  case — namely,  that  each  hair-  is  a  tube  con- 
veying a  liquid  from  a  glandular  sacculus  situated  in  the  tarsus, 
and  that  when  the  disc-  is  applied  to  a  surface  the  pouring  forth  of 
this  liquid  serves  to  make  its  adhesion  perfect.  That  this  adhesion 
is  not  produced  by  atmospheric  pressure  alone  is  proved  by  the 
fact  that  the  feet  of  flies  continue  to  hold  on  to  the  interior  of  an 
exhausted  receiver ;  whilst. 
on  the  other  hand,  that  the 
feet  pour  forth  a  secreted 
fluid  is  evidenced  by  the 
marks  left  by  their  attach- 
ment on  a  clean  surface  of 
glass.  Although,  when  all 
the  hairs  have  the  strain 
put  upon  them  equally,  the 
adhesion  of  their  discs  suf- 
fices to  support  the  insect, 
yet  each  row  may  be  de- 
tached separately  by  the 
gradual  raising  of  the  tarsus 
arid  pulvilli,  as  when  we 
remove  a  piece  of  adhesive 
plaster  by  lifting  it  from 
the  edge  or  corner .  Flies  are 

often  found  adherent  to  window-panes  in  the  autumn,  their  reduced 
strength  not. being  sufficient  to  enable  them  to  detach  their  tarsi.1 
A  similar  apparatus  on  a  far  larger  scale  presents  itself  on  the  foot 
of  the  Dytiscus  (fig.  746,  A).  The  first  joints  of  the  tarsus  of  this 
insect  are  widely  expanded,  so  as  to  form  a  nearly  circular  plate, 
and  this  is  provided  with  a  very  remarkable  apparatus  of  suckers, 
of  which  one  disc  (a)  is  extremely  large,  and  is  furnished  with  strong 
radiating  fibres ;  a  second  (b)  is  a  smaller  one  formed  on  the  same 
plan  (a  third,  of  the  like  kind,  being  often  present) ;  whilst  the 
greater  number  are  comparatively  small  tubular  club-shaped  bodies, 
each  having  a  very  delicate  membranous  sucker  at  its  extremity,  as 
shown  on  a  larger  scale  at  B.  These  all  have  essentially  the  same 

1  See  Mr.  Hepworth's  communications  to  the  Quart.  Joiirn.  Microsc.  Sci.  vol.  ii. 
1854,  p.  158,  and  vol.  iii.  1855,  p.  :-512.  See  also  Mr.  Tuffen  West's  memoir  '  On  the 
Foot  of  the  Fly,'  in  Trans.  Linn.  Soc.  xxii.  p.  393;  Mr.  Lowne's  Anatomy  of 
the  Blow-fly ;  H.  Dewitz  in  Zoologischer  Anzeiger,  vi.  p.  273  ;  and  G.  Simmer- 
macher  in  Zeitschr.f.  iviss.  Zdol.  xl.  p.  481. 


FIG.  745.— Foot  of  fly. 


1002  INSECTS   AND   AKACHNIDA 

structure,  the  large  suckers  being  furnished,  like  the  hairs  of  the 
fly's  foot,  with  secreting  sacculi,  which  pour  forth  fluid  through  the 
tubular  footstalks  that  carry  the  discs,  whose  adhesion  is  thus 
secured ;  whilst  the  small  suckers  form  the  connecting  link  between 
the  larger  suckers  and  the  hairs  of  many  beetles,  especially  Curcu- 
lionidce.1  The  leg  and  foot  of  the  Dytiscus,  if  mounted  without 
compression,  furnish  a  peculiarly  beautiful  object  for  the  binocular 
microscope.  The  feet  of  caterpillars  differ  considerably  from  those 
of  perfect  insects.  Those  of  the  first  three  segments,  which  are 
afterwards  to  be  replaced  by  true  legs,  are  furnished  with  strong 
horny  claws ;  but  each  of  those  of  the  other  segments,  which  are 
termed  '  pro-legs,'  is  composed  of  a  circular  series  of  comparatively 
slender  curved  booklets,  by  which  the  caterpillar  is  enabled  to  cling 
to  the  minute  roughness  of  the  surface  of  the  leaves,  &c.,  on  which 
it  feeds.  This  structure  is  well  seen  in  the  pro-legs  of  the  common 
silkworm. 

Stings  and  Ovipositors. — The  insects  of  the  order  Hymenoptera 
are  all  distinguished  by  the  prolongation  of  the  antepenultimate  and 


FIG.  746. — A,  foot  of  Dytiscus,  showing  its  apparatus  of  suckers :  a,  b,  large 
suckers  ;  c,  ordinary  suckers.  B,  one  of  the  ordinary  suckers  more  highly 
magnified. 

penultimate  segments  of  the  abdomen  (the  eighth  and  ninth  ab- 
dominal segments  of  the  larva)  into  a  peculiar  organ,  which  in  one 
division  of  the  order  is  a  '  sting,'  and  in  the  other  is  an  *  ovipositor ' 
or  instrument  for  the  deposition  of  the  eggs,  which  is  usually  also 
provided  with  the  means  of  boring  a  hole  for  their  reception.  The 
former  group  consists  of  the  bees,  wasps,  ants,  &c. ;  the  latter  of  the 
saw-flies,  gall-flies,  ichneumon-flies,  &c.  These  two  sets  of  instru- 
ments are  not  so  unlike  in  structure  as  they  are  in  function.2  The 

1  See  Mr.  Lowne, '  On  the  so-called  Suckers  of  Dytiscus  and  the  Pulvilli  of  Insects,' 
in  Monthly  Microsc.  Journ.  v.  p.  267. 

2  See  Kraepelin,  '  Untersuchungen  iiber  den  Bau,  Mechanismus  und  Entwicke- 
lungsgeschichte  der  bienenartigen  Thiere,'  in  Zeitschr.  f.  Wiss.  Zool,  xxiii.  p.  289 ; 
Dewitz,  '  Ueber  Bau  und  Entwickelung  des  Stachels  und  der  Legescheide,'  op.  cit. 


STINGS   AND   OVIPOSITORS  1 003 

;  sting  '  is  usually  formed  of  a  pair  of  darts,  beset  with  barbed  teeth 
at  their  points,  and  furnished  at  their  roots  with  powerful  muscles, 
whereby  they  can  be  caused  to  project  from  their  sheath,  which  is  a 
horny  case  formed  by  the  prolongation  of  the  integument  of  the  last 
segment,  slit  into  two  halves,  which  separate  to  allow  the  protrusion 
of  the  sting  ;  whilst  the  peculiar  '  venom  '  of  the  sting  is  due  to  the 
ejection,  by  the  same  muscular  action,  of  a  poisonous  liquid,  from  a 
bag  situated  near  the  root  of  the  sting,  which  passes  down  a  canal 
excavated  between  the  darts,  so  as  to  be  inserted  into  the  puncture 
which  they  make.  The  stings  of  the  common  bee,  wasp,  and  hornet 
may  all  be  made  to  display  this  structure  without  much  difficulty  in 
the  dissection.  The  '  ovipositor  >  of  such  insects  as  deposit  their 
eggs  in  holes  ready-made,  or  in  soft  animal  or  vegetable  substances 
(as  is  the  case  with  the  Ichneumonidce),  is  simply  a  long  tube,  which 
is  inclosed,  like  the  sting,  in  a  cleft  sheath.  In  the  gall-flies 
(Cynipidce)  the  extremity  of  the  ovipositor  has  a  toothed  edge,  so 
as  to  act  as  a  kind  of  saw  whereby  harder  substances  may  be  pene- 
trated ;  and  thus  an  aperture  is  made  in  the  leaf,  stalk,  or  bud  of 
the  plant  or  tree  infested  by  the  particular  species,  in  which  the  egg 
is  deposited,  together  with  a  drop  of  fluid  that  has  a  peculiarly 
irritating  effect  upon  the  vegetable  tissues,  occasioning  the  production 
of  the '  galls,'  which  are  new  growths  that  serve  not  only  to  protect  the 
larvae,  but  also  to  afford  them  nutriment.  The  oak  is  infested  by 
several  species  of  these  insects,  which  deposit  their  eggs  in  different 
parts  of  its  fabric  ;  and  some  of  the  small  *  galls '  which  are  often 
found  upon  the  surface  of  oak-leaves  are  extremely  beautiful  objects 
for  the  lower  powers  of  the  microscope.  In  the  Tenthredinidce,  or 
'  saw-flies,'  and  in  their  allies,  the  Siricidce,  the  ovipositor  is  furnished 
with  a  still  more  powerful  apparatus  for  penetration,  by  means  of 
which  some  of  these  insects  can  bore  into  hard  timber.  This  consists 
of  a  pair  of  *  saws '  which  are  not  unlike  the  '  stings '  of  bees,  &c., 
but  are  broader  and  toothed  for  a  greater  length,  and  are  made  to 
slide  along  a  firm  piece  that  supports  each  blade,  like  the  '  back  '  of 
a  carpenter's  *  tenon-saw ; '  they  are  worked  alternately  (one  being 
protruded  while  the  other  is  drawn  back)  with  great  rapidity ;  but 
when  not  in  use  they  lie  in  a  fissure  beneath  a  sort  of  arch  formed 
by  the  terminal  segment  of  the  body.  When  a  slit  has  been  made 
by  the  working  of  the  saws  they  are  withdrawn  into  this  sheath ; 
the  ovipositor  is  then  protruded  from  the  end  of  the  abdomen  (the 
body  of  the  insect  being  curved  downwards),  and,  being  guided  into 
the  slit  by  a  pair  of  small  hairy  feelers,  there  deposits  an  egg.1 
Many  other  insects,  especially  of  the  order  Diptera,  have  very  pro- 
longed ovipositors,  by  means  of  which  they  can  insert  their  eggs 
into  the  integuments  of  animals  or  into  other  situations  in  which 
the  larvae  will  obtain  appropriate  nutriment.  A  remarkable  example 

xxv.  p.  174  ;  and  '  Ueber  Bau  und  Entwickelung  des  Stachels  der  Ameisen,'  op.  cit. 
xxviii.  p.  527. 

1  The  above  is  the  account  of  the  process  given  by  Mr.  J.  W.  Gooch,  who  has 
informed  the  Author  that  he  has  repeatedly  verified  the  statement  formerly  made  by 
him  (Science  Gossip,  Feb.  1,  1873),  that  the  eggs  are  deposited,  not,  as  originally 
stated  by  Keaumur,  by  means  of  a  tube  formed  by  the  coaptation  of  the  saws,  but 
through  a  separate  ovipositor,  protruded  when  the  saws  have  been  withdrawn. 


INSECTS   AND   ARACHNID  A 

of  this  is  furnished  by  the  gad-fly  (Tabanus),  whose  ovipositor  is 
composed  of  several  joints,  capable  of  being  drawn  together  or 
extended  like  those  of  a  telescope,  and  is  terminated  by  boring 
instruments  ;  and  the  egg  being  conveyed  by  its  means,  not  only 
into  but  through  the  integument  of  the  ox,  so  as  to  be  imbedded  in 
the  tissue  beneath,  a  peculiar  kind  of  inflammation  is  set  up  there, 
which  (as  in  the  analogous  case  of  the  gall-fly)  for.ms  a  nidus  appro- 
priate both  to  the  protection  and  to  the  nutrition  of  the  larva.  Other 
insects  which  deposit  their  eggs  in  the  ground,  such  as  the  locusts, 
have  their  ovipositors  so  shaped  as  to  answer  for  digging  holes  for 
their  reception.  The  preparations  which  serve  to  display  the  fore- 


FIG.  747. — Various  eggs,  chiefly  of  the  Mallopliaga  (Anoplura). 

going  parts  are  best  seen  when  mounted  in  balsam,  save  in  the 

of  the  muscles  and  poison-apparatus  of  the  sting,  which  are  better 

preserved  in  fluid  or  in  glycerin  jelly. 

The  sexual  organs  of  insects  furnish  numerous  objects  of  extreme 
interest  to  the  anatomist  and  physiologist ;  but  as  an  account  of 
them  would  be  unsuitable  to  the  present  work,  a  reference  to  n 
copious  source  of  information  respecting  one  of  their  most  curious 
features,  and  to  a  list  of  the  species  that  afford  good  illustrations, 
must  here  suffice.1  The  eggs  of  not  only  the  class  Insecta,  but  of 

1  See  the  memoirs  of  M.  Lacaze-Duthiers, '  Sur  1'Armure  Genitale  des  Insectes,'  in 
Ann.  des  Sci.  Nat.  ser.  iii.  Zool.  tomes  xii.  xiv.  xvii.  xviii.  xix. ;  and  M.  Ch.  Robin's 


EGGS  1005 

many  of  the  minuter  forms  of  the  class  AracJinida,  as  for  example 
the  Aca/rina,  or  mites  and  ticks,  present  to  those  who  are  in  search  of 
objects  of  beauty  a  wide  and  most  interesting  field.  In  fig.  747  we 
give  a  group  of  eggs,  all  but  the  central  form  being  eggs  or  organisms 
of  this  order.  It  is  thus  with  the  eggs  of  many  insects  ;  they  are 
objects  of  great  beauty,  on  account  of  the  regularity  of  their  form 
and  the  symmetry  of  the  markings  on  their  surface  (fig.  748).  The 
most  interesting  belong  for  the  most  part  to  the  order  Lepidoptera  ; 
and  there  are  few  among  these  that  are  not  worth  examination, 
some  of  the  commonest  (such  as  those  of  the  cabbage  butterfly, 


FIG.  748. — Eggs  of  butterflies  and  moths. 

which  are  found  covering  large  patches  of  the  leaves  of  that  plant) 
being  as  remarkable  as  any.  Those  of  the  puss-moth  (Cerura 
vinula),  the  privet  hawk-moth  (Sphinx  ligustri),  the  small  tortoise- 
shell  butterfly  (Vanessa  urticce),  the  meadow-brown  butterfly  (Hip- 
parchia  janira),  the  brimstone-moth  (Rumia  cratcegata),  and  the 
silkworm  (Bombyx  mori)  may  be  particularly  specified;  and,  from 
other  orders,  those  of  the  cockroach  (Blatta  orientalis),  field-cricket 
(Acheta  campestris),  water-scorpion  (Nepa  ranatra),  bug  (Cimsx 
lectularius),  cow-dung  fly  (Scatophaga  stercoraria),  and  blow-fly 

Memoire  stir  les  Objets  qui  peuvent  etre  conserves  en  Preparations  mirroscopiques 
(Paris,  185(5),  which  is  peculiarly  full  in  the  enumeration  of  the  objects  of  interest 
afforded  bv  the  class  of  Insects. 


1006  INSECTS   AND   AKACHNLDA 

(Mii£ca  vomitoria).1  In  order  to  preserve  these  eggs  they  should 
be  mounted  in  fluid  in  a  cell,  since  they  will  otherwise  dry  up,  and 
may  lose  their  shape.  They  are  very  good  objects  for  securing  some 
of  the  best  binocular  effects. 

The  remarkable  mode  of  reproduction  that  exists  among  the 
Aphides  must  not  pass  unnoticed  here,  from  its  curious  connection 
with  the  non-sexual  reproduction  of  Entomostraca  and  Roil f era,  as 
also  of  Hydra  and  Zoophytes  generally,  all  of  which  fall  specially, 
most  of  them  exclusively,  under  the  observation  of  the  microscopist. 
The  Aphides,  which  may  be  seen  in  the  spring  and  early  summer, 
and  which  are  commonly,  but  not  always,  wingless,  are  all  of  one 
sex,  and  give  birth  to  a  brood  of  similar  Aphides,  which  come  into 
the  world  alive,  and  before  long  go  through  a  like  process  of  multi- 
plication. As  many  as  from  seven  to  ten  successive  broods  may  thus 
be  produced  in  the  course  of  a  single  season ;  so  that  from  a  single 
Aphis  it  has  been  calculated  that  no  fewer  than  ten  thousand  million 
millions  may  be  evolved  within  that  period.  In  the  latter  part  of 
the  year,  however,  some  of  these  viviparous  Aphides  attain  their  full 
development  into  males  and  females ;  and  these  perform  the  true 
generative  process,  whose  products  are  eggs,  which,  when  hatched  in 
the  succeeding  spring,  give  origin  to  a  new  viviparous  brood  that 
repeat  the  curious  life-history  of  their  predecessors.  It  appears  from 
the  observations  of  Huxley2  that  the  broods  of  viviparous  Aphides 
originate  in  ova  which  are  not  to  be  distinguished  from  those 
deposited  by  the  perfect  winged  female.  Nevertheless,  this  non- 
sexual  or  agamic  reproduction  must  be  considered  analogous  rather 
to  the  '  gemmation '  of  other  animals  and  plants  than  to  their  sexual 
'  generation ; '  for  it  is  favoured,  like  the  gemmation  of  Hydra,  by 
warmth  and  copious  sustenance,  so  that  by  appropriate  treatment  the 
viviparous  reproduction  may  be  caused  to  continue  (as  it  would 
seem)  indefinitely,  without  any  recurrence  to  the  sexual  process. 
Further,  it  seems  now  certain  that  this  mode  of  reproduction  is  not 
at  all  peculiar  to  the  Aphides,  but  that  many  other  insects  ordinarily 
multiply  by  '  agamic '  propagation,  the  production  of  males  and  the 
performance  of  the  true  generative  act  being  only  an  occasional 
phenomenon  ;  and  the  researches  of  Professor  Siebold  have  led  him  to 
conclude  that  even  in  the  ordinary  economy  of  the  hive-bee  the  same 
double  mode  of  reproduction  occurs.  The  queen,  who  is  the  only 
perfect  female  in  the  hive,  after  impregnation  by  one  of  the  drones 
(or  males)  deposits  eggs  in  the  '  royal '  cells,  which  are  in  due  time 
developed  into  young  queens  ;  others  in  the  drone  cells,  which  become 
drones  ;  and  others  in  the  ordinary  cells,  which  become  workers  or 
neuters.  It  has  long  been  known  that  these  last  are  really  un- 
developed females,  which,  under  certain  conditions,  might  become 
queens ;  and  it  has  been  observed  by  bee-keepers  that  worker-bees, 
in  common  with  virgin  or  unimpregnated  queens,  occasionally  lay 

1  Compare  R.  Leuckart  in  Archiv  f.  Anat.  1853,  p.  90,  '  Ueber  die  Micropyle  und 
den  feinern  Bau  der  Schalenhaut  bei  den  Insecteneiern,'  and  A.  Brandt,   Ueber  das 
Ei  und  seine  Bildungstatte,  Leipzig,  1878. 

2  '  On  the  Agamic  Reproduction  and   Morphology   of  Aphis '  in  Trans.   Linn. 
Soc.  xxii.  p.  193.      For  observations  on  American  Aphides  see  various  papers  by 
Mr.  C.  M.  Weed  in  Pysclie  and  other  American  journals. 


DEVELOPMENT   OF  INSECTS  lOO? 

eggs  from  which  eggs  none  but  drones  are  ever  produced.  From  careful 
microscopic  examination  of  the  drone  eggs  laid  even  by  impregnated 
queens,  Siebold  drew  the  conclusion  that  they  have  not  received  the 
fertilising  influence  of  the  male  fluid,  which  is  communicated  to  the 
queen-eggs  and  worker-eggs  alone  ;  so  that  the  products  of  sexual 
generation  are  always  female,  the  males  being  developed  from  these 
by  a  process  which  is  essentially  one  of  gemmation.1 

The  embryonic  development  of  insects  is  a  study  of  peculiar 
interest  from  the  fact  that  it  may  be  considered  as  divided  (at 
least  in  such  as  undergo  a  '  complete  metamorphosis ')  into  two 
stages  that  are  separated  by  the  whole  active  life  of  the  larva — that, 
namely,  by  which  the  larva  is  produced  within  the  egg,  and  that  by 
which  the  imago  or  perfect  insect  is  produced  within  the  body  of 
the  pupa.  Various  circumstances  combine,  however,  to  render  the 
study  a  very  difficult  one  ;  so  that  it  is  not  one  to  be  taken  up  by 
the  inexperienced  microscopist.  The  following  summary  of  the 
history  of  the  process  in  the  common  blow- fly,  however,  will  pro- 
bably be  acceptable.  A  yaetmila  with  two  membranous  lamellae 
having  been  evolved  in  the  first  instance,  the  outer  lamella  very 
rapidly  shapes  itself  into  the  form  of  the  larva,  and  shows  a  well- 
marked  segmented  division.  The  alimentary  canal,  in  like  manner, 
shapes  itself  from  the  inner  lamella,  at  first  being  straight  and 
very  capacious,  including  the  whole  yolk,  but  gradually  becoming 
narrow  and  tortuous  as  additional  layers  of  cells  are  developed 
between  the  two  primitive  lamellae,  from  which  the  other  internal 
organs  are  evolved.  When  the  larva  comes  forth  from  the  egg  it 
still  contains  the  remains  of  the  yolk ;  it  soon  begins,  however,  to 
feed  voraciously ;  and  in  no  long  period  it  grows  to  many  thousand 
times  its  original  weight,  without  making  any  essential  progress  in 
development,  but  simply  accumulating  material  for  future  use.  An 
adequate  store  of  nutriment  (analogous  to  the  *  supplemental  yolk ' 
of  Purpura)  having  thus  been  laid  up  writhin  the  body  of  the 
larva,  it  resumes  (so  to  speak)  its  embryonic  development,  its  passage 
into  the  pupa  state,  from  which  the  imago  is  to  come  forth,  involving 
a  degeneration  of  all  the  larval  tissues ;  whilst  the  tissues  and 
organs  of  the  imago  'are  redeveloped  from  cells  which  originate 
from  the  disintegrated  parts  of  the  larva,  under  conditions  similar 
to  those  appertaining  to  the  formation  of  the  embryonic  tissues  from 
the  yolk.'  The  development  of  the  segments  of  the  head  and  body 
in  insects  generally  proceeds  from  the  corresponding  larval  segments ; 
but,  according  to  Dr.  "Weismann,  there  is  a  marked  exception  in  the 
case  of  the  Diptera  and  other  insects  whose  larvae  are  unfurnished 
with  legs,  their  head  and  thorax  being  newly  formed  from  '  imaginal 
discs,'  w^hich  adhere  to  the  nerves  and  tracheae  of  the  anterior 
extremity  of  the  larva  ;  2  and,  strange  as  this  assertion  may  seem, 

1  See  Professor  Siebold's  memoir,  On  True  Parthenogenesis  in  Moths  and  Bees, 
translated  by  W.  S.  Dallas  (London,  1857) ;  and  his  Beitrage  zur  Parthenogenesis 
der  Arthropoden  (Leipzig,  1871). 

-  See  his  '  Entwickelung  der  Dipteren  '  in  Zeitsclirift  f.  Wiss.  Zool.  xiii.  and  xiv. ; 
Mr.  Lowne's  Anatomy  of  the  Blow-fly  (1st  ed.),  pp.  6-9, 118-121  ;  and  A.  Kowalevsky, 
'  Beitrage  zur  Kenntnis  der  Nachembryonalen-Entwickelung  der  Musciden,'  Zeitschr. 
f.  Wiss.  Zool.  xliv.  p.  542. 


1008  INSECTS   AND   ARACHNID  A 

it   has    been    confirmed    by   the    subsequent    investigations  of  Mr. 
Lowne.1 

The  Arachnida,  or  scorpions  and  pseudo-scorpions,  and  the  Ara- 
neida  or  spiders,  present  much  that  is  of  interest  even  to  the  unscien- 
tific who  use  the  microscope  only  for  pleasure.  The  general  remarks 
which  have  been  made  in  regard  to  insects  are  equally  applicable 
to  these,  but  have  special  application  in  that  group  known  as  the 
Acarina,  consisting  of  the  mites  and  ticks.  Some  of  these  are 
parasitic,  and  are  popularly  associated  with  the  wingless  parasitic 
insects,  to  which  they  bear  a  strong  general  resemblance,  save  in 
having  eight  legs  instead  of  six.  The  Acarina  are  the  true  *  mites  ;  ' 
they  generally  have  the  legs  adapted  for  walking,  and  some  of  them 
are  of  active  habits.  The  common  cheese-mite,  as  seen  by  the  naked 
eye,  is  familiar  to  every  one  ;  yet  few  who  have  not  seen  it  under  a 
microscope  have  any  idea  of  its  real  conformation  and  movements  ; 
and  a  cluster  of  them,  cut  out  of  the  cheese  they  infest,  and  placed 
under  a  magnifying  power  sufficiently  low  to  enable  a  large  number 
to  be  seen  at  once,  is  one  of  the  most  amusing  objects  flint  can  be 
shown  to  the  young.  There  are  many  other  species,  which  closely  re- 
semble the  cheese-mite  in  structure  and  habits,  but  which  feed  upon 
different  substances  ;  and  some  of  these  are  extremely  destructive. 

The  Acarina  are  the  smallest  of  the  Arihropoda,  arid  are  specially 
well  fitted  for  microscopical  examination  ;  indeed,  with  the  exception 
of  the  Ixodidce  (including  the  Argosince),  which  attain  a  substantial 
size,  particularly  in  tropical  countries,  but  little  can  be  learnt 
respecting  them  without  such  aid ;  as  far  as  is  at  present  known, 
other  mites  are  not  larger  in  hot  countries  than  in  Europe.  Mam 
species  make  beautiful  objects  for  the  microscope,  and  may  be  well 
preserved,  the  hard-bodied  specimens  in  balsam  without  heat  or 
pressure,  the  soft-bodied  in  glycerin  or  glycerin  jelly ;  e.g.  the 
nymphs  of  Leiosoma  palmacinctum,  Tegeocranus  cepheiformis,  T. 
dentatus,  and  the  adults  of  Glyciphagus  pliimiger  and  G.  palmifer 
are  admirable.  They  are  all  British,  and  are  found  respectively  on 
lichen  at  the  Land's  End,  on  the  fallen  bark  and  needles  of  fir-trees, 
on  fallen  oak-wood,  in  the  fodder  in  stables,  and  on  cellar- walls. 
Many  of  the  Trombidiidw  and  Hydrachnidce  also  are  very  beautiful ; 
and  the  Dermaleichi,  especially  the  males,  and  such  creatures  as 
Myobia,  Listrophorus,  tfoc.,  are  extremely  curious.  With  the  excep- 
tion of  the  Pnytoptidce,  all  Acarina  in  the  adult  stage  have  eight 
legs  and  the  constriction  between  cephalo-thorax  and  abdomen  is 
far  less  marked  than  in  insects  and  spiders — in  many  genera  it  is 
wholly  lost.  The  sexes  are  distinct  and  often  very  different  from 
each  other ;  the  reproduction  is  oviparous  or  ovo-viviparous — pos- 
sibly in  rare  and  exceptional  instances  viviparous.  The  ova  are 
usually  elliptical  or  oval  ;  in  those  which  have  a  hard  shell  a 
curious  stage  known  as  the  '  deutovium '  exists ;  as  the  egg  increases 
in  size  the  shell  splits  into  two  symmetrical  halves,  which  remain 
attached  to  the  lining  membrane,  but  are  widely  separated,  the 

1  Keference  should  be  made  to  Professor  Btitschli's  observations  in  Morplwl . 
Jahrbttch,  xiv.  p.  170,  and  Dr.  Voeltzkow's  paper  in  Arbeit.  Zool.  Zool.  Inst.  Wiir;.- 
burg,  ix.  p.  1. 


PLATE  XX 


.Acapina. 


Newman  oliromo. 


MITES  1009 

membrane  becoming  the  external  covering  in  the  space  left.  The 
eggs  of  the  so-called  stone-mite  (Petrobia  lapidum)  are  discoidal  and 
sculptured ;  they  occasionally  appear  in  countless  numbers  over  a 
large  space  of  ground  in  a  single  night,  making  the  place  look 
whitewashed  ;  they  have  been  mistaken  for  fungi  and  called  Crate- 
rium  pyriforme ;  they  are  good  microscopical  objects.  The  larvae 
of  all  Acarina,  except  PJiytoptus  and  possibly  Dermanyssus,  are 
hexapod ;  the  fourth  pair  of  legs  is  absent.  The  nymphal  stage  is 
usually  the  principal  period  of  growth ;  occasionally,  however,  it  is 
wanting.  The  nymph  is  an  active  chrysalis,  as  in  the  Orthoptera ;  it 
usually  undergoes  several  ecdyses.  In  many  species  of  the  Oribatidce 
the  whole  skin  is  not  cast,  but  splits  round  the  edge  of  the  body, 
and  the  dorso-abdominal  portion  remains  attached  to  the  new  skin  ; 
often  it  has  a  row  of  elegant  spines  or  hairs  round  its  edge  ;  thus 
after  two  or  three  ecdyses  these  spines  form  concentric  rings  on  the 
notogaster  (Plate  XXI,  fig.  2).  In  the  Trombidiidce,  Tyroglyphi,  &c. 
the  nymphs  usually  greatly  resemble  the  adults ;  in  the  Oribatidce 
they  are  often  totally  different,  and  every  intermediate  stage  occurs. 
The  change  from  nymph  to  adult  is  usually  preceded  by  an  inert 
period. 

The  number  and  variety  of  the  families,  and  the  differences  in 
the  external  form  and  internal  anatomy,  are  so  great  and  so  endless 
that  it  is  impossible  here  to  do  more  than  indicate  a  few  leading 
features  and  refer  to  a  few  examples  of  interest.  The  caput  is,  of 
course,  fused  with  the  thorax,  but  sometimes  a  constriction  at  the 
base  of  the  rostrum  gives  a  false  appearance  of  there  being  a  distinct 
head.  The  trophi  are  extremely  different  in  the  respective  families, 
or  even  genera.  In  the  more  highly  organised  of  the  Gamasidce 
almost  all  the  parts  which  exist  in  the  most  elaborate  insect-mouths 
except  the  labial  palpi  may  be  found ;  they  are  well  described  by 
M.  Megnin.1  A  large  oral  tube  is  formed  by  the  ankylosed 
maxillae  and  probably  upper  lip  and  lingua.  Up  the  centre  of  this 
tube  the  mandibles  pass  freely ;  they  are  very  long  and  chelate ; 
the  first  joint  is  simply  cylindrical ;  the  second  similar,  but  having 
the  fixed  chela  at  its  distal  end ;  the  third  is  the  movable  chela. 
They  are  capable  of  being  projected  far  beyond  the  body,  or  of  being 
withdrawn  wholly  within  it,  the  muscles  which  withdraw  them 
often  arising  from  quite  the  posterior  end  of  the  body.  These  man- 
dibles are  different  in  the  two  sexes,  and  those  of  the  male  often 
have  most  remarkable  appendages.  One  of  the  best  examples  is 
that  of  Gamasus  terribilis,  a  species  found  in  moles'  nests  by  Mr. 
Michael.  Professor  Canestrini,  of  Padua,  also  has  figured  some  very 
singular  forms.  In  the  Oribatidce,  Tetranychus,  the  Sarcoptidce, 
&c.  the  mandibles  are  also  chelate,  but  of  two  joints  only,  shorter, 
more  powerful,  and  not  capable  of  such  great  protrusion.  In  the 
Hydrachnidce,  Trombidiince,  &c.  the  mandible  is  not  chelate,  but 
the  terminal  joint  shuts  back  like  a  clasp-knife,  as  in  the  poison- 
fangs  of  spiders.  Other  forms  of  mandible  are  found.  The  maxillae 
are  large  toothed  crushing  organs  in  the  Oribatidce ;  they  are  very 

1  Journ.  de  I'Anat.  et  de  la  PhysioL  Kobin,  May  1876. 

3  T 


10 10  INSECTS   AND   ARACHNID  A 

strongly  developed  in  Hoplophora,  which  is  a  wood-boring  creature. 
In  other  families  they  are  more  commonly  joined,  forming  a  maxil- 
lary lip  with  a  flexible  edge  for  sucking  purposes.     The  maxillary 
palpi  vary  greatly  ;  in  the  Sarcoptidce,  Myobia,  &c.  they  are  anky- 
losed  to  the  lip  ;  in  the  Phytopti  Nalepa  is  of  opinion  that  they  are 
needle-like  piercing  organs,  but  these  may  well  be  the  maxillae.     In 
some  predatory  forms,  as  Cheyletus,  Trombidium,  &c.,  they  assume 
great  importance,  being   the   raptorial   organs ;    in   the   first  they 
are  extremely  large  and  powerful  and  work  horizontally ;  they  are 
provided   with   a  number  of  long  chitinous  spines  and   comb-like 
appendages   of    a    very   singular   character.     In    Trombidium   the 
ultimate  joint  is  articulated  at  the  base  of,  or  part  of  the  way  down, 
the  penultimate,  forming  a  species  of  chela.     In  Bdella  the  palpi 
are  long  thin  organs,  carried  upward  and  backward,  and  have  the 
appearance  of  antennae.     The  joints   of  the  legs   are   from   three 
(Demodex)  to  seven  (some  Trombidiidce  and  Gamasidce)  ;  five  is  the 
most  usual  number.     They  are  terminated  by  a  sucker  as  in  the  Sar- 
coptidce,  where  it  is  often  very  large ;  or  by  a  claw  or  claws,  or  both 
together.     In  some  parasitic  species  the  claws  are  developed  in  a 
special  manner  for  holding  the  hairs  of  the  host ;  thus  Myobia  has 
the  claw  of  the  first  leg  flattened  out  so  as  to  form  a  broad  lamina, 
which  curls  round  the  hair  and  presses  it  against  a  chitinous  peg  on 
the  tarsus ;  Myocoptes  has  a  similar  arrangement  on  the  third  leg. 
Both  these  genera  contain  species  which  are  parasites  of  the  mouse, 
and  easily  obtained.     In  the  Oribatidce,   Tyroglyphi,  &c.    the   legs 
are  all  strictly  walking  organs  ;  but  in  Cheyletus,  most  Gamasidce, 
&c.  the  firsfc  pair  are  tactile,  and  not  used  in  locomotion.     The  legs 
generally  correspond  on  the  two  sides  of  the  body,  but  in  Freyana 
heteropus,  an  extraordinary  parasite  of  the  cormorant  discovered  by 
Mr.  Michael  (Plate  XXII,  fig.  3),  the  second  leg  of  the  male  is  developed 
to  a  much  greater  extent  on  one  side  than  on  the    other,  and   is 
supported  by  a  different  sternal  skeleton   on   the   two    sides ;    the 
strangest  fact  is  that  it  is  not  always  the  same  side  that  is  thus 
developed ;  it  is  usually  the  left,  but  occasionally  the  right.     The 
integument  of  the  Acarina  is  almost  always  soft  in  the  immature 
forms ;  in  the  adults  it  is  hard  and  chitinised  in  the  Oribatidce  and 
most  Gamasidce ;  partly  so  in  the  Ixodidce ;  and  usually  soft  in  most 
other  families,  and  often  minutely  striated.     The  hairs  and  other 
appendages  of  the  integument  of  a  similar  nature  are  often  very 
characteristic  and  extraordinary.     In  the  nymph  of  Leiosoma  palma- 
cinctum  they  are  large  scale-like  processes  of  a  Japanese-fan  shape, 
which  entirely  cover  up  and  conceal  the  body  of  the  creature ;  a 
leaf-like  form  is  also  common.     In  Glycijmagus  plumiger  they  are 
elegant  plumes ;  in  some  Sarcoptidce,  e.g.  Symbiotes  tripilis,  some  of 
the  simple  setiform  hairs  are  three  times  the  length  of  the  body ; 
in  the  Trombidiidce  the  body-hairs  are  often  extremely  fanciful.     The 
setiform  hairs  are  the  principal  organs  of  touch,  those  on  the  front 
legs  being  specially  important.     So  sensitive  are  they  that  Cheyletus 
and  some  Gamasids,  which  are  predatory  and  capture  such  active 
creatures  as  Thysanaridce,  are    entirely  eyeless,  and   trust    to   the 
tactile  sense  only.     Haller  was  of  opinion  that  certain  specialised 


Ac  arm  a. 


Wes  t,Ne  wrma/ 


MITES  10 I J 

hairs  had  an  auditory  function.  In  the  Ixodidce  a  singular  drum- 
like  structure  in  the  first  leg  has  been  considered  by  Haller  and 
others  to  be  the  hearing  organ ;  while  in  the  Oribatidce  that  organ 
appears  to  be  located  in  the  pseudo-stigmata,  two  paired  organs  at 
the  side  of  the  cephalo-thorax  which  were  long  taken  for  true  stig- 
mata. The  Qamasidce,  Oribatidce.  Tyroglyphidce,  Sarcoptidce,  &c. 
are  entirely  without  special  organs  of  vision.  The  Hyclrachnidce 
have  two  pairs  of  simple  eyes,  each  pair  being  so  close  together  as 
to  look  like  a  single  eye.  The  Trombidiidce  mostly  have  simple 
eyes,  the  number  and  position  of  which  vary  with  the  species. 
As  to  internal  anatomy  it  should*  be  noted  that  there  is  almost 
endless  variety.  The  alimentary  canal  most  commonly  consists  of  a 
long  thin  oesophagus,  provided  with  distensor  muscles  on  each  side, 
so  as  to  make  it  a  sucking  organ  ;  it  usually  passes  right  through  or 
close  under  the  great  ganglion  known  as  the  brain ;  in  some  species, 
as  Damceus  geniculatus,  the  oesophagus  is  followed  by  a  large  pro- 
ventriculus,  but  this  is  not  usual ;  it  more  commonly  leads  directly 
into  the  ventriculus,  which  generally  is  a  principal  viscus,  and  in 
most  families  furnished  with  more  or  less  glandular  crecal  ap- 
pendages, not  numerous,  but  often  very  large,  occasionally  larger 
than  the  organ  itself.  A  valve  in  many  cases  separates  the  ven- 
triculus from  the  hind-gut,  which  is  commonly  divided  into  what 
may  be  called  colon  and  rectum.  In  the  Gamasidce  a  single  very 
large  Malpighian  vessel  on  each  side  of  the  body  enters  between 
the  two  last-named  divisions  of  the  alimentary  canal.  These  vessels 
run  right  along  the  side  of  the  body,  and  strong  pulsation  may  be 
seen  in  them.  In  the  Oribatidce  they  are  absent,  their  function 
being  apparently  performed  by  supercoxal  glands.  The  Tyro- 
glyphidce,  /Sarcoplidce,  Pliytoptidce ,  &c.  are  without  special  respira- 
tory organs ;  the  Oribatidce  and  some  Uropoda  have  simple  un- 
branched  tracheae,  much  in  the  same  condition  as  those  of  Peripatus. 
The  other  Gamasidce,  the  Trombidiidce,  Cheyletidce,  Ixodidce,  &c. 
usually  have  branched  tracheae,  like  insects  ;  air-sacs  are  occasion- 
ally found,  but  not  anything  like  the  trachea!  lungs  or  gills  (so 
called)  of  spiders  and  scorpions.  The  principal  nerve-centre  is 
much  concentrated,  and  consists  usually  of  either  a  large  supra- 
cesophageal  and  smaller  suboesophageal  ganglion  joined  by  com- 
missures ;  or,  more  frequently,  the  whole  forms  one  mass  which  is 
pierced  by  the  oesophagus,  which  may  be  pulled  out,  leaving  a  neat 
round  hole  ;  the  nerves,  of  course,  radiate  from  this  mass,  but  there 
is  not  space  here  to  describe  their  course.  A  pulsating  organ  of  the 
nature  of  the  dorsal  vessel  of  insects,  but  much  shorter,  and  with 
only  one  or  two  pairs  of  ostia,  has  been  detected  in  some  Gamasidce, 
and  in  Ixodes,  first  by  Kramer  and  afterwards  by  Winkler  and 
Glaus ;  it  has  a  median  aorta  running  forward  ;  it  is  best  seen  in 
life  in  young  specimens  still  transparent ;  it  lies  at  the  rear  of  the 
ventriculus,  near  the  dorsal  surface.  Nothing  of  the  nature  of  a 
heart  has  yet  been  discovered  in  other  Acarina.  The  reproductive 
organs  are,  perhaps,  most  frequently  of  the  '  ring  '  type,  well  known 
in  the  Arachnida ;  thus  in  female  Oribatidce  they  consist  of  a  central 
ovary,  with  an  oviduct  springing  from  near  each  end,  in  which  the 

3x2 


10 1  2  INSECTS   AND   ARACHNIDA 

eggs  are  matured ;  the  oviducts  both  terminate  in  an  unpaired 
vagina,  whence  the  eggs  pass  into  a  long,  membranous,  extensible 
ovipositor,  often  wrinkled  or  striated  with  singular  fineness  and 
beauty.  The  external  aperture  is  closed  by  chitinous  folding  doors. 
A  more  or  less  similar  arrangement  may  be  found  in  most  Gamasidce, 
Hydrachnidce,  &c.,  but  without  the  ovipositor.  Spermathecse  are 
often  found  in  the  Gamasidce,  Tyroglyphidce,  &c.,  and  accessory 
glands  frequently  accompany  the  vagina  in  almost  all  families.  The 
male  system  varies  greatly,  but  is  frequently  constructed  on  similar 
lines,  preserving  somewhat  of  the  '  ring  '  form. 

The  principal  families  into  which  the  Acarina  are  divided  are  as 
follows  : — 

The  Gamasidce,  which  in  the  adult  stage  are  mostly  pro- 
vided with  a  hard  chitinous  cuticle  in  all  parts  of  the  body.  They 
are  mostly  predatory,  but  the  females  and  young  are  often  parasitic. 
Pteroptus  and  Dermanyssus,  however,  are  more  leathery  in  texture, 
and  are  parasitic  during  their  whole  lives,  the  former  on  bats,  the 
latter  on  birds.  This  family  have  the  true  stigmata,  one  on  each 
side  of  the  ventral  surface,  usually  between  the  second  and  third 
pairs  of  legs  ;  these  do  not  communicate  directly  with  the  external 
air,  but  have  a  long  tubular  peritreme  in  the  chitin  of  the  ventral 
surface,  often  very  elaborate  in  form,  and  emerging  to  the  air  usually 
between  the  first  and  second  legs.  This  is  highly  characteristic  of 
the  family. 

The  Ixodidce,  or  ticks,  most  of  which  are  probably  primarily 
vegetable  feeders,  but  will,  when  opportunity  offers,  attach  them- 
selves to  animals  by  sinking  their  long  serrated  rostral  projection 
into  the  skin,  have  a  single  ventral  stigma  on  each  side,  com- 
municating directly  with  the  air  by  a  large  cullender-plate,  which 
is  an  interesting  microscopical  object.  The  males  have  the  dorsal 
surface  of  the  abdomen  almost  entirely  covered  by  a  chitinous  plate, 
which  is  much  smaller  in  the  females;  but  the  leathery  portion  of 
the  abdomen  in  that  sex  is  capable  of  great  distension  for  the  pur- 
pose of  permitting  the  suction  of  animal  juices.  The  Argasidce 
must  be  included  in  this  group  ;  their  tenacity  of  life  and  power  of 
existing  without  food  are  marvellous  ;  their  bite  is  severe,  but  the 
terrible  stories  told  of  the  results  of  the  bite  of  the  Persian  Argas 
have  not  been  supported  on  investigation. 

The  Oribatidce  are  mostly  wholly  chitinised,  the  chitin  being 
very  hard  and  brittle.  The  stigmata  are  in  the  acetabula  of  the 
legs.  The  pseudo-stigmata  (hearing  organs)  of  this  family  have 
been  before  referred  to.  Oribatidce  are  vegetable  feeders,  living  in 
moss,  lichen,  fungus,  dead  wood,  under  bark  of  trees,  &c.,  and  some 
few  species  on  aquatic  plants.  They  are  widely  distributed  from 
the  arctic  regions  to  the  equatorial.  Hoplophora  has  the  power  of 
withdrawing  the  legs  wholly  within  the  carapace,  and  then  shutting 
down  the  cephalo-thorax  against  the  abdomen,  so  as  to  close  the 
opening,  when  it  appears  like  a  chitinous  ball ;  from  this  power  it 
has  been  called  the  '  box-mite.'  The  sexes  have  not  any  external 
difference. 

The  Trombidiidce  are  a  large  and  varied  group,  mostly  predatory 


MITES  1013 

and  with  soft,  often  velvety  skins,  frequently  of  scarlet  and  other 
brilliant  colours.  The  large  Trombidium  holosericum  is  a  well-known 
microscopical  object.  The  Tetranychi  are  usually  included  in  this 
family  ;  they  are,  however,  rather  doubtful  members  ;  they  are  the  'red- 
spiders  '  of  our  greenhouses,  much  dreaded  by  horticulturists.  Each 
foot  is  provided  with  about  four  singular  hairs  with  round  knobs  at 
the  end.  Bryobia  is  an  allied  genus  found  in  great  numbers  on  ivy 
&c.  in  gardens  and  is  a  beautiful  object.  The  hexapod  larvae  of  several 
species  of  Trombidium  often  attach  themselves  temporarily  to  the  skin 
of  animals,  including  man,  and  produce  intolerable  itching.  They  were 
supposed  by  the  earlier  Acarologists  to  be  all  one  species,  and  to 
be  adult,  and  to  form  a  distinct  "family ;  they  were  called  Leptus 
autumncdis,  and  are  known  in  England  as  the  '  harvest-bug/  and 
in  France  as  the  rouget.  The  Bdellidce  are  also  included  in  this 
family  ;  some  authors  also  include  the  Cheyleti,  which,  however,  seem 
to  need  a  separate  family,  having  many  curious  characters,  including 
the  dorsal  position  of  the  male  organs. 

The  Hydrachnidce,  or  water-mites,  as  well  as  the  Trombidiidce, 
have  the  two  stigmata  in  the  rostrum ;  the  legs  are  swimming 
organs,  the  sexes  often  very  different ;  they  live  in  fresh  water  and 
are  often  parasitic  in  their  immature,  but  not  in  the  adult  stages. 
They  are  mostly  soft-bodied  and  often  of  brilliant  colours. 

The  Limnocaridce  are  sometimes  treated  as  a  sub-family  of  the 
Hydrachnidce,  but  are  crawling,  not  swimming  creatures,  and  are 
found  in  fresh  water ;  but  the  ffalicaridce,  which  either  constitute 
a  sub-family  of,  or  are  closely  associated  with  them,  are  marine, 
and  are  much  found  among  Hydrozoa,  on  which  they  probably 
prey. 

The  parasitic  Myobiidce  are  by  some  included  in  the  Cheyletidce ; 
the  differences,  however,  are  very  considerable.  They  are  the  last 
tracheate  family. 

The  Tyroglyphidce  are  the  cheese-mite  family ;  they  are  far  the 
most  destructive  of  all  Acarina,  swarming  in  countless  numbers  and 
devouring  hay,  cheese,  drugs,  growing  plants  and  roots,  &c. ;  the 
genus  Glyciphagus  contains  many  singular  and  interesting  forms,  as 
G.  platygaster  and  G.  Krameri,  found  in  moles'  nests.  It  is  in  this 
family  that  the  curious  hypopial  stage  exists ;  some  of  the  indi- 
viduals of  some  species,  instead  of  following  the  ordinary  life-history, 
are  changed  at  one  ecdysis  into  a  totally  different-looking  creature, 
with  a  highly  chitinised  cuticle  and  rudimentary  mouth-organs, 
which  can  endure  draught  and  other  unfavourable  circumstances 
which  would  kill  the  ordinary  form.  They  attain  the  same  adult 
stage  as  other  individuals.  The  Hypopus  is  provided  with  adhesive 
suckers  whereby  it  attaches  itself  temporarily  to  other  creatures,  and 
this  serves  for  the  distribution  of  the  species. 

The  Tarsonemidce  are  minute  creatures,  some  leaf-miners,  some 
parasitic  on  bees  &c. 

The  Sarcoptidce  are  divided  into  two  great  sub-families,  the  Sar- 
coptince,  or  itch-mites,  of  which  the  well-known  Sarcoptes  scabiei  of  man 
(Plate  XXII,  fig.  4)  is  the  type,  and  the  Analgesince,  or  bird-parasite 
mites;  all  have  soft  bodies  with  finely  striated  cuticles.  Sarcoptes 


IOI4  INSECTS  AND   ARACHNID  A 


iei  is  a  minute  creature  of  almost  circular  form,  the  female  of 
which  burrows  under  the  epidermis,  causing  the  disease.  The  mite  is 
found  at  the  end  of  the  burrow,  not  in  the  pustule  at  its  commence- 
ment. The  first  two  pairs  of  legs  and  the  third  leg  of  the  male 
are  terminated  by  suckers,  the  other  legs  by  long  bristles.  The 
male  is  smaller  than  the  female.  The  Analgesince  (Dermaleichi)  are 
a  very  large  and  curious  group  ;  the  males  often  differ  greatly  from 
the  females,  and  the  skin  is  often  greatly  strengthened  by  chitinous 
plates  and  structures.  The  species  are  not  always  parasitic  on  one 
bird  only  ;  often  the  same  species  may  be  found  on  numerous  birds, 
while  several  species  frequently  live  on  the  same  bird  ;  they  are  not 
usually  supposed  to  be  injurious  to  the  birds ;  they  are  found  on  the 
feathers. 

The  Phytoptidce  are  extremely  minute  creatures  living  in  galls 
which  they  form  on  the  leaves  and  twigs  of  numerous  trees  and 
plants  ;  they  are  elongated  in  form  with  the  two  hind  pairs  of  legs 
abortive ;  there  is  but  little  variety  among  them.  Slightly  resem- 
bling them  in  general  form,  but  very  different  in  other  respects,  is 
Demodex  folliculorum,  which  is  found  in  the  sebaceous  follicles  of 
the  human  skin,  particularly  the  nose.  Those  follicles,  which  are 
enlarged  and  whitish  with  a  terminal  exterior  black  spot,  may  be 
forced  out  by  pressure,  and  the  Acarus  will  often  be  found  within. 
Similar  parasites  exist  on  the  dog  and  pig. 

There  are  numerous  other  curious  and  interesting  forms  which 
cannot  be  included  in  any  of  the  families  mentioned  above. 

The  number  of  objects  furnished  to  the  microscopist  by  the 
spider  tribe  is  very  large  from  a  biological  point  of  view,  although 
mere  objects  of  microscopical  interest  popularly  are  not  so  numerous 
as  in  insects.  Their  eyes  exhibit  a  condition  intermediate  between 
that  of  insects  and  crustaceans  and  that  of  vertebrata,  for  they  are 
simple  like  the  *  stemmata '  of  the  former,  usually  number  from  six 
to  eight,  are  sometimes  clustered  together  in  one  mass,  but  more 
frequently  disposed  separately ;  while  they  present  a  decided  ap- 
proach in  internal  structure  to  the  type  characteristic  of  the  visual 
organs  of  the  latter. 

The  structure  of  the  mouth  is  always  mandibulate,  and  is  less 
complicated  than  that  of  the  mandibulate  insects.  The  respiratory 
apparatus  is  not  tracheal,  as  in  insects  and  some  Acarina,  but  is 
constructed  upon  a  very  different  plan,  for  the  '  stigmata,'  which 
are  usually  four  in  number  on  each  side,  open  upon  a  like  number 
of  respiratory  sacculi,  each  of  which  contains  a  series  of  leaf-like 
folds  of  ^ its  lining  membrane  upon  which  the  blood  is  distributed  so 
as  to  afford  a  large  surface  to  the  air. 

In  the  structure  of  the  limbs,  the  principal  point  worthy  of 
notice  is  the  peculiar  appendage  with  which  they  usually  terminate, 
for  the  strong  claws,  with  a  pair  of  which  the  last  joint  of  the 
foot  is  furnished,  have  their  edges  cut  into  comb-like  teeth,  which 
appear  to  be  used  by  the  animal  as  cleansing  instruments,  and  in 
many  cases  for  the  manipulation  of  the  silk  of  their  snares.  But  a 
feature  deserving  study  by  the  microscopist  is  the  physical  cause  of 
the  exquisite  sensitiveness  of  these  <  feet.'  By  resting  these  upon  a 


PLATE  XXII. 


West,  Newir.aji  lilL 


AcarincL. 


SPIDERS  1015 

trap-line  of  silk  carried  to  her  den,  she  can,  by  a  veritable  telegraphy, 
discover  instantly,  not  only  the  fact  that  there  is  prey  upon  her 
snare,  but  the  exact  spot  in  the  web  of  the  snare  in  which  that 
prey  is  entangled.  In  the  same  way,  by  seizing  certain  tightly 
stretched  threads  communicating  with  the  main  lines  of  the  snare, 
she  can  discover  in  an  instant  the  presence  and  position  of  her  prey, 
though  far  beyond  the  reach  of  vision. 

The  most  characteristic  and  interesting  part  in  the  special 
organisation  of  the  spider  is  the  '  spinning  apparatus/  by  means  of 
which  its  often  elaborately  > 

constructed  webs  arc  pro- 
duced. These  consist  of 
'  spinnerets  '  on  the  ex- 
terior of  the  body  and 
glandular  organs  lying 
within  the  abdomen  ;  it  is 
by  them  that  the  silk  from 
which  all  the  elements  of 
the  snare  are  produced  is 
secreted. 

Of  these  glands   there 
are  two  pairs   which   are 
sac-like   in    form,    with   a        FlG-  749.— Foot,  with  comb-like  claws,  of  the 
coiled    tube     opening    'di-  common  spider  (^dra). 

rectly  on  the  spinnerets  : 

there  are  three  pairs,  of  a  convoluted  appearance,  opening  on  the 
hinder  spinnerets  ;  and  there  are  three  of  a  sinuous  tubular  form 
opening  on  the  hinder  and  middle  spinnerets.  Beyond  these  there 
are  respectively  200  and  400  smaller  glands,  which  open  on  the 
front,  middle,  and  hinder  spinnerets.  They  all  terminate  in  tubes 
of  great  delicacy,  through  which  the  silk  is  drawn  at  the  will  of  the 
spinster  ;  and,  while  the  scaffolding  or  framework  of  the  web  of 
Epeira  is  double  and  hardens  rapidly  in  air  (fig.  750,  A),  those  which 
lie  across  the  polygons  of 

the  scaffolding  are  stud-      A 

ded  at  regular  intervals 

with  viscid  globules,  as  ^ 

seen  in  fig.  750,  B  ;  and      B     •  ~ ' O 

it  is  to  these  viscid  glo- 
bules that  the  peculiarly  FlG-  750.— Ordinary  thread  (A)  and  viscid 
adhesive  character  of  the  thread  (B)  of  the  common  8plder" 
web  is  due. 

The  usual  number  of  the  spinnerets  is  six.  They  are  little  teat- 
like  processes  crowned  with  silk  tubes.  They  are  movable  at  the 
will  of  the  spider,  and  can  be  erected  or  depressed,  and  one,  many, 
or  all  of  the  tubes  crowning  a  spinneret  may  be  caused  to  exude 
and  have  drawn  from  it  or  them  the  silk  as  the  spider  determines. 
There  can  be  no  doubt  that  there  is  a  difference  in  the  silk  secreted 
by  different  glands,  and  its  appropriate  employment  is  a  part  of 
the  skill  of  the  spider. 

It  is  certain  that  the  silken  threads  of  a  snare  are  of  two  kinds ; 


I0l6  INSECTS  AND   ARACHNID  A 

(1)  that  which  rapidly  hardens  on  contact  with  the  air,  and  which 
is  employed  in  the  construction  of  the  framework  of  the  snare ;  and 

(2)  a  viscid  silk  with  which  the  entangling  meshes  by  which  prey  is 
caught  are  put  in.     The  latter  present  beautiful  objects  for  popular 
observation,  because  the  thread   has  strung   upon  it,    as  it   were, 
innumerable   pearl-like   globules   in  which    the  viscidity   remains. 
These  beads  are  produced   after   the   thread   is   drawn   out   by   a 
special  vibratory  action  set  up  in  the  thread  by  the  spider. 

The  eggs  of  spiders  are  not  objects  of  special  optical  interest, 
but  they  afford  opportunities  for  good  embryological  work,1  and  the 
habits  of  spiders  offer  a  good  scope  for  industrious  study  in  the  field.2 

1  See  the  work  of  Kishinouyi  in  Journ.  Coll.  Sci.  Imp.  Univ.  Japan,  vol.  iv. 

2  See  particularly  McCook's  American   Spiders   and    their   Spinning    Work, 
Philadelphia,  1889  and  1890,  and  the  various  papers  of  Mr.  and  Mrs.  Peckham  in  the 
American  journals. 


IO1/ 


CHAPTER   XXII 

VEBTEBBATED  ANIMALS 

WE  are  now  arrived  at  the  highest  division  of  the  animal  kingdom, 
in  which  the  bodily  fabric  attains  its  greatest  development,  not  only 
as  to  completeness,  but  also  as  to  size ;  and  it  is  in  most  striking 
contrast  with  the  class  we  have  been  last  considering.  Since  not 
only  the  entire  bodies  of  vertebrated  animals,  but,  generally  speak- 
ing, the  smallest  of  their  integral  parts,  are  far  too  large  to  be  viewed 
as  microscopic  objects,  we  can  study  their  structure  only  by  a 
separate  examination  of  their  component  elements ;  and  it  seems, 
therefore,  to  be  a  most  appropriate  course  to  give  under  this  head  a 
sketch  of  the  microscopic  characters  of  those  primary  tissues  of 
which  their  fabric  is  made  up,  and  which,  although  they  may  be 
traced  with  more  or  less  distinctness  in  the  lower  tribes  of  animals, 
attain  their  most  complete  development  in  this  group.1 

Although  there  would  at  first  sight  appear  but  little  in  common 
between  the  simple  bodies  of  those  humble  Protozoa  which  con- 
stitute the  lowest  types  of  the  animal  series,  and  the  complex 
fabric  of  man  or  other  vertebrates,  yet  it  seems  certain  that  in 
the  latter,  as  in  the  former,  the  process  of  *  formation  '  is  essentially 
carried  on  by  the  instrumentality  of  protoplasmic  substance,  univer- 
sally diffused  through  it  in  such  a  manner  as  to  bear  a  close  resem- 
blance to  the  pseudopodial  network  of  the  rhizopod  ;  whilst  the 
tissues  produced  by  its  agency  lie,  as  it  were,  on  the  outside  of 
this,  bearing  the  same  kind  of  relation  to  it  as  the  foraminiferal 
shell  does  to  the  sarcodic  substance  wrhich  fills  its  cavities  and 
extends  itself  over  its  surface.  For,  as  was  first  pointed  out  by 
Dr.  Beale,2  the  smallest  living  *  elementary  part '  of  every  organised 

1  This  sketch  is  intended,  not  for  the   professional   student,   but   only   for  the 
amateiir  microscopist  who  wishes  to  gain  some  general  idea  of  the  elementary  struc- 
ture of  his  own  body  and  of  that  of  vertebrate  animals  generally.     Those  who  wish 
to  go  more  deeply  into  the  inquiry  are  referred  to  the  following.     The  translation  of 
Strieker's  Manual  of  Histology,  published  by  the    New    Sydenham    Society ;    the 
translation  of  the  4th  edition  of  Professor  Frey's  Histology  and  Histo-Chemistry  of 
Man ;  the  '  General  Anatomy '  of  the  10th  edition  of   Quoin's  Anatomy,   1893,  by 
Professor  Schafer;  and  the  Atlas  of  Histology,  by  Dr.  Klein  and  Mr.  Noble  Smith. 

2  Professor  Beale's  views  are  most  systematically  expounded  in  his  lectures  On  the 
Structure  of  the  Simple  Tissues  of  the  Human  Body,  1861 ;  in  his  How  to  work 
with  the  Microscope,  5th  edition,  1880 ;  and  in  the  introductory  portion  of  his  new 


I0l8  VERTEBRATED   ANIMALS 

fabric  is  composed  of  organic  matter  in  two  states  :  the  protoplasmic 
(which  he  termed  germinal  matter),  possessing  the  power  of  selecting 
pabulum  from  the  blood,  and  of  transforming  this  either  into  the 
material  of  its  own  extension  or  into  some  product  which  it 
elaborates ;  whilst  the  other,  which  may  be  termed  formed  material, 
may  present  every  gradation  of  character  from  a  mere  inorganic 
deposit  to  a  highly  organised  structure,  but  is  in  every  case  altogether 
incapable  of  self-increase.  A  very  definite  line  of  demarcation  can 
be  generally  drawn  between  these  two  substances  by  the  careful  use 
of  the  staining  process  ;  but  there  are  many  instances  in  which  there 
is  the  same  gradation  between  the  one  and  the  other  as  we  have 
formerly  noticed  between  the  '  endosarc '  and  the  '  ectosarc '  of  the 
Amoeba.  Thus  it  is  on  the  protoplasmic  component  that  the  exist- 
ence of  every  form  of  animal  organisation  essentially  depends ; 
since  it  serves  as  the  instrument  by  which  the  nutrient  material 
furnished  by  the  blood  is  converted  into  the  several  forms  of  tissue. 
Like  the  sarcodic  substance  of  the  rhizopods,  it  seems  capable  of  in- 
definite extension ;  and  it  may  divide  and  subdivide  into  independ- 
ent portions,  each  of  which  may  act  as  the  instrument  of  formation 
of  an  '  elementary  part.'  Two  principal  forms  of  such  elementary 
parts  present  themselves  in  the  fabric  of  the  higher  animals, 
viz.  cells  and  fibres  (which  are  modified  cells)  ;  and  it  will  be 
desirable  to  give  a  brief  notice  of  these  before  proceeding  to  describe 
those  more  complex  tissues  which  are  the  products  of  a  higher- 
elaboration. 

The  cells  of  which  a  few  animal  tissues  are  essentially  composed 
consist,  in  some  cases,  of  the  same  parts  as  the  typical  cell  of  the 
plant,  viz.  a  definite  *  cell-wall,'  inclosing  '  cell-contents '  and  a 
'  nucleus,'  which  is  the  seat  of  its  formative  activity.  It  is  of  such 
cells,  retaining  more  or  less  of  their  characteristic  spheroidal  shape, 
that  every  mass  of  fat,  whether  large  or  small,  is  chiefly  made  up. 
In  a  large  number  of  cases  the  cell  shows  itself  in  a  somewhat 
different  form,  the  '  elementary  part '  being  a  corpuscle  of  proto- 
plasm of  which  the  exterior  has  undergone  a  slight  consolidation, 
like  that  which  constitutes  the  '  primordial  utricle '  of  the  vegetable 
cell  or  the  '  ectosarc '  of  the  Amoeba,  but  in  which  there  is  no  proper 
distinction  between  *  cell-wall '  and  'cell-contents.'  This  condition, 
which  is  characteristically  exhibited  by  the  nearly  globular  colourless 
corpuscles  of  the  blood,  appears  to  be  common  to  all  cells  in  the  in- 
cipient stage  of  their  formation,  and  the  progress  of  their  develop- 
ment consists  in  the  gradual  differentiation  of  their  parts,  the  '  cell- 
wall  '  becoming  distinctly  separated  from  the  '  cell-contents,'  and 
these  from  the  *  nucleus,'  and  the  original  protoplasm  being  very 


edition  of  Todd  and  Bowman's  Physiological  Anatomy,  1867.  The  principal  results 
of  the  inquiries  of  German  histologists  on  this  point  are  well  stated  in  a  paper  by 
Dr.  Duffin  on  '  Protoplasm,  and  the  Part  it  plays  in  the  Actions  of  Living  Beings '  in 
Quart.  Journ.  Microsc.  Sci.  n.s.  vol.  iii.  1863,  p.  251.  The  Author  feels  it  necessary, 
however,  to  express  his  dissent  from  Professor  Beale's  views  in  one  important  particular, 
viz.  his  denial  of  '  vital '  endowments  to  the  '  formed  material '  of  any  of  the  tissues  ; 
since  it  seems  to  him  illogical  to  designate  contractile  muscular  fibre  (for  example) 
as  '  dead,'  merely  because  it  has  not  the  power  of  self-reparation. 


CELLS  IOI9 

commonly  replaced  more  or  less  completely  by  some  special  product 
(such  as  fat  in  the  cells  of  adipose  tissue,  or  haemoglobin  in  the  red 
corpuscles  of  the  blood),  in  which  cases  the  nucleus  often  disappears 
altogether.  In  the  earlier  stages  of  cell-development  multiplication 
takes  place  with  great  activity  by  a  duplicative  subdivision  that 
corresponds  in  all  essential  particulars  with  that  of  the  plant-cell, 
as  is  well  seen  in  cartilage,  a  section  of  which  will  often  exhibit  in 
one  view  the  successive  stages  of  the  process.1  Whether  *  free  '  cell- 
multiplication  ever  takes  place  in  the  higher  animals  is  at  present 
uncertain. 

A  large  part  of  the  fabric  of  £he  higher  animals  is  made  up  of 
fibrous  tissues,  which  serve  to  bind  together  the  other  components, 
and  which,  when  consolidated  by  calcareous  deposit,  constitute  the 
substance  of  the  skeleton.  In  these  the  relation  of  the  '  germinal 
matter '  and  the  '  formed  material '  presents  itself  under  an  aspect 
which  seems  at  first  sight  very  different  from  that  just  described. 
A  careful  examination,  however,  of  those  'connective  tissue  cor- 
puscles '  that  have  long  been  distinguished  in  the  midst  of  the  fibres 
of  which  these  tissues  are  made  up,  shows  that  they  are  the  equi- 
valents of  the  corpuscles  of  '  germinal  matter,'  which  in  the  previous 
instance  came  to  constitute  cell-nuclei,  and  that  the  fibres  hold  the 
same  relation  to  them  that  the  '  walls '  and  '  contents '  of  cells  do  to 
their  germinal  corpuscles.  The  transition  from  the  one  type  to  the 
other  is  wrell  seen  in  fibro-cartilage,  in  which  the  so-called  '  inter- 
cellular substance'  is  often  as  fibrous  as  tendon.  The  difference 
between  the  two  types,  in  fact,  seems  essentially  to  consist  in  this, 
that,  whilst  the  segments  of  '  germinal  matter '  which  form  the  cell- 
nuclei  in  cartilage  and  in  other  cellular  tissues  are  completely 
isolated  from  each  other,  each  being  completely  surrounded  by  the 
product  of  its  own  elaborating  action,  those  which  form  the  i  con- 
nective-tissue corpuscles '  are  connected  together  by  radiating  pro- 
longations that  pass  between  the  fibres,  so  as  to  form  a  con- 
tinuous network  closely  resembling  that  formed  by  the  pseudo- 
podia  of  the  rhizopod.  Of  this  we  have  a  most  beautiful  example 
in  bone ;  for  whilst  its  solid  substance  may  be  considered  as 
connective  tissue  solidified  by  calcareous  deposit,  the  '  lacuna? '  and 
'  canal iculi '  which  are  excavated  in  it  (fig.  752)  give  lodgment  to  a 
set  of  radiating  corpuscles  closely  resembling  those  just  described ; 
and  these  are  centres  of  '  germinal  matter,'  which  appear  to  have  an 
active  share  in  the  formation  and  subsequent  nutrition  of  the  osseous 
texture.  In  dentine  (or  tooth-substance)  we  seem  to  have  another 

1  Great  attention  has  lately  been  given  by  many  able  observers  to  the  changes 
which  take  place  in  the  nucleus  before  and  during  its  cleavage.  A  full  account  of 
these  is  contained  in  Professor  Strassburger's  Zellbildung  und  ZelWteilung,  1880. 
See  also  Dr.  Klein's  '  Observations  on  the  Structure  of  Cells  and  Nuclei '  in  Quart. 
Journ.  Microsc.  Sci.  n.s.  vol.  xviii.  1878,  p.  315,  and  vol.  xix.  1879,  pp.  125,  404;  and 
chap.  xliv.  of  his  Atlas  of  Histology.  The  numerous  essays  of  Flemming,  in  the 
Archivf.  mikr.  Anat.  from  1875  to  1890  ;  Gruber,  on  the  Nucleus  of  Protozoa,  in  vol.  xl. 
of  the  Zeitschr.f.  Wiss.  Zool.;  .and  Carnoy,  in  La  Cellule,  may  be  studied  by  those 
who  desire  to  carry  further  the  history  of  the  cell.  A  remarkable  series  of  observa- 
tions have  followed  the  publication  of  Professor  E.  van  Bcneden's  work  on  the  egg 
of  Ascaris  megalocephala  in  Bull.  Acad.  Boij.  Sci.  Belg.  xiv.  pp.  215-95. 


1020  VERTEBRATED   ANIMALS 

form  of  the  same  thing,  the  walls  of  its  '  tubuli '  and  the  '  inter- 
tubular  substance '  being  the  '  formed  material '  that  is  produced 
from  thread-like  prolongations  of  'germinal  matter'  issuing  from 
its  pulp,  and  continuing  during  the  life  of  the  tooth  to  occupy  its 
tubes  ;  just  as  in  the  Foramina/era  we  have  seen  a  minutely  tubular 
structure  to  be  formed  around  the  individual  threads  of  sarcode 
which  proceeded  from  the  body  of  the  contained  animal.  It  may 
now  be  asserted,  indeed,  that  the  bodies  of  even  the  highest  animals 
are  everywhere  penetrated  by  that  protoplasmic  substance  of  which 
those  of  the  lowest  and  simplest  are  entirely  composed  ;  and  that 
this  substance,  which  forms  a  continuous  network  through  almost 
every  portion  of  the  fabric,  is  the  main  instrument  of  the  formation, 
nutrition,  and  reparation  of  the  more  specialised  or  differentiated 
tissues.  As  it  is  the  purpose  of  this  work,  not  to  instruct  the 
professional  student  in  histology  (or  the  science  of  the  tissues), 
but  to  supply  scientific  information  of  general  interest  to  the 
ordinary  microscopist,  no  attempt  will  here  be  made  to  do  more 
than  describe  the  most  important  of  those  distinctive  characters 
which  the  principal  tissues  present  when  subjected  to  microscopic 
examination ;  and  as  it  is  of  no  essential  consequence  what  order  is 
adopted,  we  may  conveniently  begin  with  the  structure  of  the 
skeleton,1  which  gives  support  and  protection  to  the  softer  parts  of 
the  fabric. 

Bone. — The  microscopic  characters  of  osseous  tissue  may  some- 
times be  seen  in  a  very  thin  natural  plate  of  bone,  such  as  in  that 
forming  the  scapula  (shoulder-blade)  of  a  mouse ;  but  they  are  dis- 
played more  perfectly  by  artificial  sections,  the  details  of  the  arrange- 
ment being  dependent  upon  the  nature  of  the  specimen  selected  and 
the  direction  in  which  the  section  is  made.  Thus  when  the  shaft  of 
a  '  long '  bone  of  a  bird  or  mammal  is  cut  across  in  the  middle  of  its 
length,  we  find  it  to  consist  of  a  hollow  cylinder  of  dense  bone, 
surrounding  a  cavity  which  is  occupied  by  an  oily  marrow ;  but  if 
the  section  be  made  nearer  its  extremity  we  find  the  outside  wall 
gradually  becoming  thinner,  whilst  the  interior,  instead  of  forming 
one  large  cavity,  is  divided  into  a  vast  number  of  small  chambers, 
partially  divided  by  a  sort  of  'lattice  work'  of  osseous  fibres,  but 
communicating  with  each  other  and  with  the  cavity  of  the  shaft, 
and  filled  like  it  with  marrow.  In  the  bones  of  reptiles  and  fishes, 
on  the  other  hand,  this  'cancellated'  structure  usually  extends 
throughout  the  shaft,  which  is  not  so  completely  differentiated  into 
solid  bone  and  medullary  cavity  as  it  is  in  the  higher  Vertebrata. 
In  the  most  developed  kinds  of  '  flat '  bones,  again,  such  as  those  of 
the  head,  we  find  the  two  surfaces  to  be  composed  of  dense  plates  of 
bone,  with  a  '  cancellated  '  structure  between  them  ;  whilst  in  the  less 
perfect  type  presented  to  us  in  the  lower  Yertebrata,  the  whole 
thickness  is  usually  more  or  less  '  cancellated,'  that  is,  divided  up 
into  minute  medullary  cavities.  When  we  examine,  under  a  low 
magnifying  power,  a  longitudinal  section  of  a  long  bone,  or  a  section 

1  This  term  is  used  in  its  most  general  sense,  as  including  not  only  the  proper 
internal  skeleton,  but  also  the  hard  parts  protecting  the  exterior  of  the  body,  which 
form  the  dermal  skeleton. 


STRUCTURE    OF   BONE 


IO2I 


of  a  flat  bone  parallel  to  its  surface,  we  find  it  traversed  by  numerous 
canals,  termed  Haversian  after  their  discoverer  Havers,  which  are  in 
connection  with  the  central  cavity,  and  are  filled  like  it  with  marrow. 
In  the  shafts  of  *  long  '  bones  these  canals  usually  run  in  the  direction 
of  their  length,  but  are  connected  here  and  there  by  cross-branches  ; 
whilst  in  the  flat  bones  they  form  an  irregular  network.  On  apply- 
ing a  higher  magnifying  power  to  a  thin  transverse  section  of  a  long 
bone  we  observe  that  each  of  the  canals  whose  orifices  present  them- 
selves in  the  field  of  view  (fig.  751)  is  the  centre  of  a  rod  of  bony 
tissue  (1 ),  usually  more  or  less  circular  in  its  form,  which  is  arranged 
around  it  in  concentric  rings,  resembling  those  of  an  exogenous 
stem.  These  rings  are  marked  out  and  divided  by  circles  of  little 
dark  spots,  which,  when  closely  examined  (2),  are  seen  to  be  minute 
flattened  cavities  excavated  in  the  solid  substance  of  the  bone,  from 
the  two  flat  sides  of  which 
pass  forth  a  number  of 
extremely  minute  tubules, 
one  set  extending  inwards, 
or  in  the  direction  of  the 
centre  of  the  system  of 
rings,  and  the  other  out- 
wards, or  in  the  direction 
of  its  circumference  ;  and 
by  the  inosculation  of  the 
tubules  (or  canaliculi)  of 
the  different  rings  with 
each  other  a  continuous 
communication  is  esta- 
blished between  the  cen- 
tral Haversian  canal  and 
the  outermost  part  of  the 
bony  rod  that  surrounds 
it,  which  doubtless  minis- 
ters to  the  nutrition  of 

the  texture.  Blood-vessels  are  traceable  into  the  Haversian  canals, 
but  the  '  canaliculi '  are  far  too  minute  to  carry  blood-corpuscles  ;  they 
are  occupied,  however,  in  the  living  bone  by  threads  of  protoplasmic 
substance,  which  bring  the  segments  of  '  germinal  matter  '  contained 
in  the  lacunae  into  communication  with  the  walls  of  the  blood- 
vessels. 

The  minute  cavities  or  lacunce  from  which  the  canaliculi  proceed 
(fig.  752)  are  highly  characteristic  of  true  osseous  tissue,  being  never 
deficient  in  the  minutest  parts  of  the  bones  of  the  higher  Yertebrata, 
although  those  of  fishes  are  occasionally  destitute  of  them.  The  dark 
appearance  which  they  present  in  sections  of  a  dried  bone  is  not  due 
to  opacity,  but  is  simply  an  optical  effect,  dependent  (like  the  black- 
ness of  air-bubbles  in  liquids)  upon  the  dispersion  of  the  rays  by  the 
highly  refracting  substance  that  surrounds  them.  The  size  and 
form  of  the  lacunae  differ  considerably  in  the  several  classes  of  Yer- 
tebrata, and  even  in  some  instances  in  the  orders,  so  that  it  is  often 
possible  to  determine  the  group  to  which  a  bone  belonged  by  the 


FIG.  751. — Minute  structure  of  bone  as  seen  in 
transverse  section :  1,  a  rod  surrounding  an 
Haversian  canal,  3.  showing  the  concentric 
arrangement  of  the  lamellae  ;  2,  the  same,  with 
the  lacunae  and  canaliculi ;  4,  portion  of  the 
lamellse  parallel  with  the  external  surface. 


IO22 


VERTEBRATED   ANIMALS 


microscopic  examination  of  even  a  minute  fragment  of  it.  The 
following  are  the  average  dimensions  of  the  lacunae,  in  characteristic 
examples  drawn  from  four  principal  divisions,  expressed  in  fractions 
of  an  inch  : — 


Man  . 
Ostrich 
Turtle  . 
Conger-eel 


Long  Diameter 
1-1440  to  1-2400 
1-1333  „  1-2250 
1-375     „  1-1150 
1-550     „,   1-1135 


Short  Diameter 
1-4000  to  1-8000 
1-5425  „  1-9850 
1-4500  „  1-5840 
1-4500  „  1-8000 


The  lacunae  of  birds  are  thus  distinguished  from  those  of  mam- 
mals by  their  somewhat  greater  length  and  smaller  breadth,   but 

they  differ  still  more  in  the 
remarkable  tortuosity  of  their 
canaliculi,  which  wind  back- 
wards and  forwards  in  a  very 
irregular  manner.  There  is  an 
extraordinary  increase  in  length 
in  the  lacunae  of  reptiles,  with- 
out a  corresponding  increase  in 

FIG.  752. -Lacunae  of  osseous  substance :       breadth  ;  and  this  is  also  seen 
a,  central  cavity ;  b,  its  ramifications.  in  some  fishes,    though    in    ge- 

neral the  lacunae  of  the  latter 

are  remarkable  for  their  angularity  of  form  and  the  fewness  of  their 
radiations,  as  shown  in  fig.  753,  which  represents  the  lacunae  and 
canaliculi  in  the  bony  scale  of  the  Lepidosteus  ('  bony  pike '  of  the 
North  American  lakes  and  rivers),  with  wrhich  the  bones  of  its  in- 
ternal skeleton  perfectly  agree  in  structure.  The  dimensions  of  the 
lacunse  in  any  bone  do  not  bear  any  relation  to  the  size  of  the  animal 


FIG.  753. — Section  of  the  bony  scale  of  Lepidosteus  :  a,  show- 
ing the  regular  distribution  of  the  lacunae  and  of  the  connecting 
canaliculi ;  b,  small  portion  more  highly  magnified. 


to  which  it  belonged  ;  thus  there  is  little  or  no  perceptible  difference 
between  their  size  in  the  enormous  extinct  Iguanodon  and  in  the 
smallest  lizard  now  inhabiting  the  earth.  But  they  bear  a  close  rela- 
tion to  the  size  of  the  blood-corpuscles  in  the  several  classes ;  and 
this  relation  is  particularly  obvious  in  the  *  perennibranchiate ' 
Batrachia,  the  extraordinarily  large  size  of  whose  blood-corpuscles 
will  be  presently  noticed. 


Proteus   . 

Siren 

Menopoma 

Lepidosiren 

Pterodactyle 


TEETH  1023 

Long  Diameter  Short  Diameter 

1-570  to  1-980  1-885    to  1-1200 

1-290  .,  1-480  1-540     „  1-975 

1-450  „  1-700  1-1300  „  1-2100 

1-375  „  1-494  1-980     „  1-2200 

1-445  „  1-1185  1-4000  „  1-5225 ' 


In  preparing  sections  of  bone  it  is  important  to  avoid  the  pene- 
tration of  the  Canada  balsam  into  the  interior  of  the  lacunae  and 
canaliculi,  since  when  these  are  filled  by  it  they  become  almost 
invisible.  Hence  it  is  preferable  mot  to  employ  this  cement  at  all, 
except  it  may  be  in  the  first  instance,  but  to  rub  down  the  section 
beneath  the  finger,  guarding  its  surface  with  a  slice  of  cork  or  a  slip 
of  gutta-percha,  and  to  give  it  such  a  polish  that  it  may  be  seen  to 
advantage  even  when  mounted  dry.  As  the  polishing,  however, 
occupies  much  time,  the  benefit  which  is  derived  from  covering  the 
surfaces  of  the  specimen  with  Canada  balsam  may  be  obtained 
without  the  injury  resulting  from  the  penetration  of  the  balsam  into 
its  interior,  by  adopting  the  following  method.  A  quantity  of 
balsam  proportioned  to  the  size  of  the  specimen  is  to  be  spread  upon 
a  glass  slip,  and  to  be  rendered  stifFer  by  boiling,  until  it  becomes 
nearly  solid  when  cold  ;  the  same  is  to  be  done  to  the  thin  glass 
cover ;  next,  the  specimen  being  placed  on  the  balsamed  surface  of 
the  slide,  and  being  overlain  by  the  balsamed  cover,  such  a  degree  of 
warmth  is  to  be  applied  as  will  suffice  to  liquefy  the  balsam  without 
causing  it  to  flow  freely,  and  the  glass  cover  is  then  to  be  quickly 
pressed  down,  and  the  slide  to  be  rapidly  cooled,  so  as  to  give  as 
little  time  as  possible  for  the  penetration  of  the  liquefied  balsam  into 
the  lacunar  system.  The  same  method  may  be  employed  in  making 
sections  of  teeth.2  The  study  of  the  ossein  or  organic  basis  of  bone 
should  be  pursued  by  macerating  a  fresh  bone  in  dilute  nitre-hydro- 
chloric acid,  then  steeping  it  for  some  time  in  pure  water,  and 
tearing  thin  shreds  from  the  residual  substance,  which  will  be 
found  to  consist  of  an  imperfectly  fibrillated  material,  allied  in  its 
essential  constitution  to  the  '  white  fibrous '  tissue. 

Teeth. — The  intimate  structure  of  the  teeth  in  the  several  classes 
and  orders  of  Yertebrata  presents  differences  which  are  no  less 
remarkable  than  those  of  their  external  form,  arrangement,  and  suc- 
cession. It  will  obviously  be  impossible  here  to  do  more  than  sketch 
some  of  the  most  important  of  these  varieties.  The  principal  part  of 
the  substance  of  all  teeth  is  made  up  of  a  solid  tissue  that  has  been 
appropriately  termed  dentine.  In  sharks  as  in  many  other  fishes 
the  general  structure  of  this  dentine  is  extremely  similar  to 
that  of  bone,  the  tooth  being  traversed  by  numerous  canals,  which 
are  continuous  with  the  Haversian  canals  of  the  subjacent  bone,  and 
receive  blood-vessels  from  them  (fig.  754),  while  each  of  these  canals 

1  See  Professor  J.  Quekett's  memoir  on  this  subject  in  the  Trans.  Microsc.  Soc. 
ser.  i.  vol.  ii. ;  and  his  more  ample  illustration  of  it, in  the  Illustrated  Catalogue  of 
the  Histological  Collection  in  the  Museum  of  the  Royal  College  of  Surgeons, 
vol.  ii. 

2  Some  useful  hints  on  the  mode  of  making  these  preparations  will  be  found  in 
the  Quart.  Journ   Microsc.  Sci.  vol.  vii.  1859,  p.  258. 


IO24 


VERTEBRATED   ANIMALS 


is  surrounded  by  a  system  of  tubuli  (fig.  755),  which  radiate  into 
the  surrounding  solid  substance.  These  tubuli,  however,  do  not  enter 
lacunse,  nor  is  there  any  concentric  annular  arrangement  around  the 
medullary  canals  ;  but  each  system  of  tubuli  is  continued  onwards, 
through  its  own  division  of  the  tooth,  the  individual  tubes  sometimes 
giving  off  lateral  branches,  whilst  in  other  instances  their  trunks 
bifurcate,  This  arrangement  is  peculiarly  well  displayed,  when 
sections  of  teeth  construct ed:  upon  this  type  are  viewed  as  opaque 
objects  (fig.  756).  In  the  teeth  of  the  higher  Yertebrata,  however, 
we  usually  find  the  centre  excavated  into  a  single  cavity  (fig.  757), 
and  the  remainder  destitute  of  vascular  canals  :  but  there  are  inter- 
mediate cases  (as  in  the  teeth  of  the  great  fossil  sloths)  in  which  the 
inner  portion  of  the  dentine  is  traversed  by  prolongations  of  this 
cavity,  conveying  blood-vessels,  which  do  not  pass  into  the  exterior 


FIG.  754. — Perpendicular  section  of 
tooth  of  Lemma,  moderately  en- 
larged, showing  network  of  me- 
dullary canals. 


FIG.  755.— Transverse  section  of  por- 
tion of  tooth  oiPristis,  more  highly 
magnified,  showing  orifices  of  me- 
dullary canals,  with  systems  of 
radiating  and  inosculating  tubuli. 


layers.  The  tubuli  of  the  *  non- vascular '  dentine,  which  exists  by 
itself  in  the  teeth  of  nearly  all  mammalia,  and  which  in  the  elephant 
is  known  as  '  ivory,'  all  radiate  from  the  central  cavity,  and  pass 
towards  the  surface  of  the  tooth  in  a  nearly  parallel  course.  Their 
diameter  at  their  largest  part  averages  j^^th  of  an  inch;  their 
smallest  branches  are  immeasurably  fine.  The  tubuli  in  their  course 
present  greater  and  lesser  undulations ;  the  former  are  few  in  number, 
but  the  latter  are  numerous ;  and  as  they  occur  at  the  same  part  of 
the  course  of  several  contiguous  tubes  they  give  rise  to  the  appearance 
of  lines  concentric  with  the  centre  of  radiation.  These  'secondary 
curvatures '  probably  indicate  in  dentine,  as  in  the  crab's  shell,  suc- 
cessive stages  of  calcification.  The  tubuli  are  occupied,  during  the 
life  of  the  tooth,  by  delicate  threads  of  protoplasmic  substance,  ex- 
tending into  them  from  the  central  pulp. 

Two  other  substances,  one  of  them  harder  and  the  other  softer 


TEETH 


1025 


opaque  object. 


ray), 


tooth    of 
viewed    as    an 


than  dentine,  are  frequently  found  associated  with  it  ;  the  former  is 

termed  enamel,  and  the  latter  cementum  or  crusta  petrosa.    The  enamel 

is  composed  of  long  prisms,  closely  resembling  those  of  the  *  prismatic  ' 

shell-substance  formerly  described,  but  on  a  far  more  minute  scale,  the 

diameter  of  the  prisms  not  being  more  in  man  than  -^Vyth  of  an 

inch.     The  length  of  the  prisms  corresponds  with  the  thickness  of 

the  layer  of  enamel  ;  and  the  ' 

two  surfaces  of  this  layer  pre- 

sent the  ends  of  the  prisms, 

the  form  of  which  usually  ap- 

proaches the  hexagonal.     The 

course  of  the  enamel  prisms  is 

more  or  less  wavy,  and  they 

are  marked  by  numerous  trans- 

verse striae,  resembling  those 

of    the    prismatic    shell-sub- 

stance, and  probably  origina- 

ting in  the  same  cause  —  the 

coalescence  of  a  series  of  shorter 

prisms  to  form  the  lengthened 

prism.       In  man  and  in  car-    pIG   756.—  Transverse  section    of 

nivorous  animals  the  enamel       Myliobates    (eagle 

covers  the  crown  of  the  tooth 

only,   with   a   simple   cap   or 

superficial  layer  of  tolerably 

uniform  thickness  (fig.  757,  «), 

which   follows  the  surface  of 

the  dentine  in  all  its  inequali- 

ties ;  and  its  component  prisms 

are  directed  at  right  angles  to 

that  surface,  their  inner  ex- 

tremities resting  in  slight  but 

regular  depressions  on  the  ex- 

terior of  the  dentine.     In  the 

teeth    of    many    herbivorous 

animals,  however,  the  enamel 

forms  (with  the  cementum)  a 

series  of  vertical  plates  which 

dip  down  into  the  substance 

of  the  dentine,  and  present  pio_  757>_  vertical  section  of  human  molar 
their  edges  alternately  with  it  tooth:  a,  enamel;  &,  cementum  or  crusta 
at  the  grinding  surface  of  the  petrosa;  c,  dentine  or  ivory;  d  osseous 
tooth;  and  there  is  in  such 
teeth  no  continuous  layer  of 
enamel  over  the  crown.  This 
arrangement  provides  by  the  unequal  ivear  of  these  three  sub- 
stances (of  which  the  enamel  is  the  hardest,  and  the  cementum  the 
softest)  for  the  constant  maintenance  of  a  rough  surface,  adapted  to 
triturate  the  tough  vegetable  substances  on  which  these  animals  feed. 
Though  the  enamel  is  not  always  present,  it  has  been  shown  by  Mr. 
Charles  Tomes  that  the  germ  from  which  it  is  formed  always  appears 

3  u 


at  outer  part  of  dentine. 


1026  VEKTEBRATED   ANIMALS 

in  the  embryonic  tooth  ;  and  he  has  further  shown  that  it  is  much 
more  frequently  present  than  used  to  be  supposed.  The  cementum, 
or  crusta  petrosa,  has  the  characters  of  true  bone,  possessing  its  dis- 
tinctive stellate  lacunse  and  radiating  canaliculi.  Where  it  exists 
in  small  amount  we  do  not  find  it  traversed  by  medullary  canals  ; 
but,  like  dentine,  it  is  occasionally  furnished  with  them,  and  thus 
resembles  bone  in  every  particular.  These  medullary  canals  enter 
its  substance  from  the  exterior  of  the  tooth,  and  consequently  pass 
towards  those  which  radiate  from  the  central  cavity  in  the  direction 
of  the  surface  of  the  dentine,  where  this  possesses  a  similar  vascu- 
larity,  as  was  remarkably  the  case  in  the  teeth  of  the  great  extinct 
Megatherium.  In  the  human  tooth,  however,  the  cementum  has  no 
such  vascular ity,  but  forms  a  thin  layer  (fig.  757,  b),  which  envelopes 
the  root  of  the  tooth  commencing  near  the  termination  of  the  cap 
of  enamel.  In  the  teeth  of  many  herbivorous  mammals  it  dips 
down  with  the  enamel  to  form  the  vertical  plates  of  the  interior  of 
the  tooth;  and  in  the  teeth  of  the  Edentata,  as  well  as  of  many 
reptiles  and  fishes,  it  forms  a  thick  continuous  envelope  over  the 
whole  surface,  until  worn  away  at  the  crown.1 

Dermal  Skeleton. — The  skin  of  fishes,  of  a  few  amphibians,  of 
most  reptiles,  and  of  few  mammals,  is  strengthened  by  plates  of  a 
horny,  cartilaginous,  bony,  or  even  enamel-like  texture,  which  are 
sometimes  fitted  together  at  their  edges,  so  as  to  form  a  continuous 
box-like  envelope ;  whilst  more  commonly  they  are  so  arranged  as 
partially  to  overlie  one  another,  like  the  tiles  on  a  roof;  and  it  is 
in  this  latter  case  that  they  are  usually  known  as  scales.  Although 
we  are  accustomed  to  associate  in  our  minds  the  *  scales '  of  fishes 
with  those  of  reptiles,  yet  essentially  different  structures  have  been 

included  under  this  name, 
those  of  the  former  and  of 
many  of  the  latter  being 
developed  in  the  substance 
of  the  true  skin  (with  a 
layer  of  which,  in  addition 
to  the  epidermis,  they  are 
always  covered),  and  bear- 
ing a  resemblance  to  car- 
tilage and  bone  in  their 
FIG.  758.— Portion  of  skin  of  sole,  viewed  as  an  texture  and  composition  ; 
opaque  object.  whilst  others,  such  as  the 

scales  of  snakes  or  the  tor- 
toise-shell, are  formed  upon  the  surface  of  the  true  skin,  and  are 
to  be  considered  as  analogous  to  nails,  hoofs,  &c.  and  other  'epi- 
dermic appendages.'  In  nearly  all  the  existing  fishes  the  scales  are 
flexible,  being  but  little  consolidated  by  calcareous  deposit ;  and  in 
some  species  they  are  so  thin  and  transparent  that,  as  they  do  not 
project  obliquely  from  the  surface  of  the  skin,  they  can  only  be 
detected  by  raising  the  superficial  layer  of  the  skin  and  searching 

1  The  student  is  recommended  to  consult  Mr.  C.  S.  Tomes's  Manual  of  Dental 
Anatomy,  Human  and  Comparative. 


SCALES   OF   FISHES 


1027 


beneath  it,  or  by  tearing  off  the  entire  thickness  of  the  skin  and 
looking  for  them  near  its  under  surface.  This  is  the  case,  for 
example,  with  the  common  eel,  and  with  the  viviparous  blenny ;  of 
either  of  which  fish  the  skin  is  a  very  interesting  object  when  dried 
and  mounted  in  Canada  balsam,  the  scales  being  seen  imbedded  in 
its  substance,  whilst  its  outer  surface  is  studded  with  pigment-cells. 
Generally  speaking,  however,  the  posterior  extremity  of  each  scale 
projects  obliquely  from  the  general  surface,  carrying  before  it  the 
thin  membrane  that  incloses  it,  which  is  studded  with  pigment- 
cells  ;  and  a  portion  of  the  skin  ofj, almost  any  fish,  but  especially  of 
such  as  have  scales  of  the  ctenoid  kind  (that  is,  furnished  at  their 
posterior  extremities  with  comb-like  teeth,  fig.  759),  when  dried 
with  its  scales  in  sit  if.,  is  a  very  beautiful  opaque  object  for  the  low 
powers  of  the  microscope  (fig.  758),  especially  with  the  binocular 
arrangement.  Care  must  be  taken,  however,  that  the  light  is  made 
to  glance  upon  it  in  the  most  advan- 
tageous manner,  since  the  brilliance  with 
which  it  is  reflected  from  the  comb-like 
projections  entirely  depends  upon  the 
angle  at  which  it  falls  upon  them.  The 
only  appearance  of  structure  exhibited  by 
the  thin  flat  scale  of  the  eel,  when  ex- 
amined microscopically,  is  the  presence  of 
a  layer  of  isolated  spheroidal  transparent 
bodies,  imbedded  in  a  plate  of  like  trans- 
parence ;  these,  from  the  researches  of 
Professor  W.  C.  Williamson  l  upon  other 
scales,  appear  not  to  be  cells  (as  they 
might  readily  be  supposed  to  be),  but  con- 
cretions of  carbonate  of  lime.  When  the 
scale  of  the  eel  is  examined  by  polarised 
light  its  surface  exhibits  a  beautiful  St. 

Andrew's  cross  ;  and  if  a  plate  of  selenite  „ 

T         i    i    i  •     i     -,  i  FIG.  759. — Scale  of  sole,  viewed 

is    placed    behind    it,    and  the  analysing        as  a  transparent  object, 
prism  be  made  to  revolve,  a  remarkable 
play  of  colours  is  presented, 

In  studying  the  structure  of  the  more  highly  developed  scales, 
we  may  take  as  an  illustration  that  of  the  carp,  in  which  two  very 
distinct  layers  can  be  made  out  by  a  vertical  section,  with  a  third 
but  incomplete  layer  interposed  between  them.  The  outer  layer  is 
composed  of  several  concentric  laminae  of  a  structureless  trans- 
parent substance  like  that  of  cartilage ;  the  outermost  of  these 
laminae  is  the  smallest,  and  the  size  of  the  plates  increases  pro- 
gressively from  without  inwards,  so  that  their  margins  appear  on  the 
surface  as  a  series  of  concentric  lines  ;  and  their  surfaces  are  thrown 
into  ridges  and  furrows,  which  commonly  have  a  radiating  direction. 
The  inner  layer  is  composed  of  numerous  laminae  of  a  fibrous 

1  See  his  elaborate  memoirs,  '  On  the  Microscopic  Structure  of  the  Scales  and 
Dermal  Teeth  of  some  Ganoid  and  Placoid  Fish,'  in  Phil.  Trans.  1849  ;  and  '  Investi- 
gations into  the  Structure  and  Development  of  the  Scales  and  Bones  of  Fishes,'  in 
Phil.  Trans.  1851. 

3  u  2 


1028  VERTEBKATED   ANIMALS 

structure,  the  fibres  of  each  lamina  being  inclined  at  various  angles 
to  those  of  the  lamina  above  and  below  it.  Between  these  two  layers 
is  interposed  a  stratum  of  calcareous  concretions,  resembling  those 
of  the  scale  of  the  eel ;  these  are  sometimes  globular  or  spheroidal, 
but  more  commonly  *  lenticular,'  that  is,  having  the  form  of  a  double 
convex  lens.  The  scales  which  resemble  those  of  the  carp  in  having 
a  form  more  or  less  circular,  and  in  being  destitute  of  comb-like 
prolongations,  are  called  cycloid ;  and  such  are  the  characters  of 
those  of  the  salmon,  herring,  roach,  &c.  The  structure  of  the  ctenoid 
scales  (fig.  759),  which  we  find  in  the  sole,  perch,  pike,  &c.,  does  not 
differ  essentially  from  that  of  the  cycloid,  save  as  to  the  projection 
of  the  comb- like  teeth  from  the  posterior  margin ;  and  it  does  not 
appear  that  the  strongly  marked  division  which  Professor  Agassiz 
has  attempted  to  establish  between  the  '  cycloid '  and  the  '  ctenoid ' 
orders  of  fishes,  on  the  basis  of  this  difference,  is  in  harmony 
with  their  general  organisation.  Scales  of  every  kind  may  become 
consolidated  to  a  considerable  extent  by  the  calcification  of  their 
soft  substance  ;  but  they  never  present  any  approach  to  the  true 
bony  structure,  such  as  is  shown  in  the  two  orders  to  be  next  ad- 
verted to. 

In  the  ganoid  scales,  on  the  other  hand,  the  whole  substance  of 
the  scale  is  composed  of  a  material  which  is  essentially  bony  in  its 
nature,  its  intimate  structure  being  always  comparable  to  that  of  one 
or  other  of  the  varieties  which  present  themselves  in  the  bones  of  the 
vertebrate  skeleton,  and  being  very  frequently  identical  with  that 
of  the  bones  of  the  same  fish,  as  is  the  case  with  the  Lepidosteus  (fig. 
753),  one  of  the  few  existing  representatives  of  this  order,  which,  in 
former  ages  of  the  earth's  history,  comprehended  a  large  number  of 
important  families.  Their  name  (from  yaj'oc,  splendour)  is  bestowed 
on  account  of  the  smoothness,  hardness,  and  high  polish  of  the  outer 
surface  of  the  scales,  which  are  due  to  the  presence  of  a  peculiar  layer 
that  has  been  likened  to  the  enamel  of  teeth.  The  scales  of  this 
order  are  for  the  most  part  angular  in  their  form,  and  are  arranged 
in  regular  rows,  the  posterior  edges  of  each  slightly  overlapping  the 
anterior  ones  of  the  next,  so  as  to  form  a  very  complete  defensive 
armour  to  the  body.  The  scales  of  the  placoid  type,  which  charac- 
terise the  existing  sharks  and  rays,  with  their  fossil  allies,  are 
irregular  in  their  shape,  and  very  commonly  do  not  come  into  mutual 
contact,  but  are  separately  imbedded  in  the  skin,  projecting  from  its 
surface  under  various  forms.  In  the  rays  each  scale  usually  consists 
of  a  flattened  plate  of  a  rounded  shape,  with  a  hard  spine  projecting 
from  its  centre ;  in  the  sharks  (to  which  tribe  belongs  the  '  dog-fish ' 
of  our  own  coast)  the  scales  have  more  of  the  shape  of  teeth.  This 
resemblance  is  not  confined  to  external  form  ;  for  their  intimate 
structure  strongly  resembles  that  of  dentine,  their  dense  substance 
being  traversed  by  tubuli,  which  extend  from  their  centre  to  their 
circumference  in  minute  ramifications,  without  any  trace  of  osseous 
lacunae.  These  tooth-like  scales  are  often  so  small  as  to  be  invisible 
to  the  naked  eye  ;  but  they  are  well  seen  by  drying  a  piece  of  the  skin 
to  which  they  are  attached,  and  mounting  it  in  Canada  balsam  ;  and 
they  are  most  brilliantly  shown  by  the  assistance  of  polarised  light. 


HAIR  IO29 

A  like  structure  is  found  to  exist  in  the  '  spiny  rays  '  of  the  dorsal  fin, 
which,  also,  are  parts  of  the  dermal  skeleton  ;  and  these  rays  usually 
have  a  central  cavity  filled  with  medulla,  from  which  the  tubuli 
radiate  towards  the  circumference.  This  structure  is  very  well  seen 
in  thin  sections  of  the  fossil  *  spiny, rays.'  which,  with  the  teeth  and 
scales,  are  often  the  sole  relics  of  the  vast  multitudes  of  sharks  that 
must  have  swarmed  in  the  ancient  seas,  their  cartilaginous  internal 
skeletons  having  entirely  decayed  aAvay.  In  making  sections  of  bony 
scales,  spiny  rays,  &c.  the  method  must  be  followed  which  has  been 
already  detailed  under  the  head  of  bone.1 

The  scales  of  reptiles,  the  feathers  of  birds,  and  the  hairs,  hoofs, 
jiails,  claws,  and  horns  (when  not  bony)  of  'mammals  are  all  epi- 
dermic appendages ;  that  is,  they  are  produced  upon  the  surface,  not 
within  the  substance  of  the  true  skin,  and  are  allied  in  structure  to 
the  epidermis,  being  essentially  composed  of  aggregations  of  cells 
filled  with  horny  matter  and  frequently  much  altered  in  form.  This 
structure  may  generally  be  made  out  in  horns,  nails,  &c.  with  little 
difficulty  by  treating  thin  sections  of  them  with  a  dilute  solution  of 
soda,  which  after  a  short  time  causes  the  cells  that  had  been 
flattened  into  scales  to  resume  their  globular  form.  The  most 
interesting  modifications  of  this  structure  are  presented  to  us  in 
hairs  and  in  feathers ;  which  forms  of  clothing  are  very  similar  to 
each  other  in  their  essential  nature,  and  are  developed  in  the  same 
manner — viz.  by  an  increased  production  of  epidermic  cells  at  the 
bottom  of  a  flask-shaped  follicle,  which  is  formed  in  the  substance 
of  the  true  skin,  and  which  is  supplied  with  abundance  of  blood 
by  a  special  distribution  of  vessels  to  its  walls.  "When  a  hair  is 
pulled  out  '  by  its  root,'  its  base  exhibits  a  bulbous  enlargement, 
of  which  the  exterior  is  tolerably  firm,  whilst  its  interior  is  occu- 
pied by  a  softer  substance,  which  is  known  as  the  i  pulp ; '  and  it 
is  to  the  continual  augmentation  of  this  pulp  in  the  deeper  part 
of  the  follicle,  and  to  its  conversion  into  the  peculiar  substance  of 
the  hair  when  it  has  been  pushed  upwards  to  its  narrow  neck,  that 
the  growth  of  the  hair  is  due.  The  same  is  true  of  feathers,  the  stems 
of  which  are  but  hairs  on  a  larger  scale  ;  for  the  l  quill '  is  the  part 
contained  within  the  follicle  answering  to  the  *  bulb '  of  the  hair ; 
and  whilst  the  outer  part  of  this  is  converted  into  the  peculiarly  solid 
horny  substance  forming  the  '  barrel '  of  the  quill,  its  interior  is 
occupied,  during  the  whole  period  of  the  growth  of  the  feather,  with 
the  soft  pulp,  only  the  shrivelled  remains  of  which,  however,  are 
found  within  it  after  the  quill  has  ceased  to  grow. 

Although  the  hairs  of  different  mammals  differ  greatly  in  the 
appearances  they  present,  we  may  generally  distinguish  in  them 
two  elementary  parts — viz.  a  cortical  or  investing  substance,  of  a 
dense  horny  texture,  and  a  medullary  or  pith -like  substance,  usually 
of  a  much  softer  texture,  occupying  the  interior.  The  former  can 

1  For  further  information  regarding  the  scales  of  fishes,  see  the  papers  by  O 
Hertwig  in  vol.  viii.  of  the  Jenaische  Zeitschrift,  and  vols.  ii.  and  v.  of  the 
Morpholog.  Jahrbuch.  A  condensed  summary  of  our  knowledge,  from  the  more 
recent  standpoint,  will  be  found  in  Dean's  Fishes,  Living  and  Fossil  (New  York, 
1895),  pp.  23-6. 


1030 


VERTEBRATED   ANIMALS 


sometimes  be  distinctly  made  out  to  consist  of  flattened  scales 
arranged  in  an  imbricated  manner,  as  in  some  of  the  hairs  of  the 
sable  (fig.  760) ;  whilst  in  the  same  hairs,  the  medullary  substance 
is  composed  of  large  spheroidal  cells.  In  the  musk  deer,  on  the 
other  hand,  the  cortical  substance  is  nearly  undistinguishable,  and 


FIG.  760. — Hair  of  sable,  showing  large 
rounded  cells  in  its  interior,  covered 
by  imbricated  scales  or  flattened  cells. 


FIG.  761. — Hair  of  musk-deer,  consist- 
ing almost  entirely  of  polygonal  cells. 


almost  the  entire  hair  seems  made  up  of  thin -walled  polygonal  cells 
(fig.  761).  The  hair  of  the  reindeer,  though  much  larger,  has  a  very 
similar  structure ;  and  its  cells,  except  near  the  root,  are  occupied 
with  hair  alone,  so  as  to  seem  black  by  transmitted  light,  except 
when  penetrated  by  the  fluid  in  which  they  are  mounted.  In  the 
hair  of  the  mouse,  squirrel,  and  other  small  rodents  (fig.  762,  A,  B), 

the  cortical  substance  forms  a 
tube,  which  we  see  crossed  at 
intervals  by  partitions  that 
are  sometimes  complete, 
sometimes  only  partial ; 
these  are  the  walls  of  the  single 
or  double  line  of  cells,  of  which 
the  medullary  substance  is  made 
up.  The  hairs  of  the  bat  tribe 
are  commonly  distinguished  by 
the  projections  on  their  surface, 
which  are  formed  by  extensions 
of  the  component  scales  of  the 
cortical  substance :  these  are 
particularly  well  seen  in  the 
hairs  of  one  of  the  Indian 
species,  which  has  a  set  of 
whorls  of  long  narrow  leaflets 
(so  to  speak)  arranged  at 
regular  intervals  on  its  stem  (C).  In  the  hair  of  the  peccary  (fig.  763) 
the  cortical  envelope  sends  inwards  a  set  of  radial  prolongations,  the 
interspaces  of  which  are  occupied  by  the  polygonal  cells  of  the  medul- 
lary substance  ;  and  this,  on  a  larger  scale,  is  the  structure  of  the 
'  quills  '  of  the  porcupine,  the  radiating  partitions  of  which,  when  seen 
through  the  more  transparent  parts  of  the  cortical  sheath,  give  to 


FIG.  762.— A,  small  hair  of  squirrel ;  B,  large 
hair  of  squirrel ;  C,  hair  of  Indian  bat. 


HAIK 


1031 


the  surface  of  the  latter  a  fluted  appearance.  The  hair  of  the  ornitho- 
rhynchus  is  a  very  curious  object  ;  for  whilst  the  lower  part  of  it 
resembles  the  fine  hair  of  the  mouse  or  squirrel,  this  thins  away  and 
then  dilates  again  into  a  very  thick  fibre,  having  a  central  portion 
composed  of  polygonal  cells,  inclosed  in 
a  flattened  sheath  of  a  brown  fibrous 
substance. 

The  structure  of  the  human  hair  is 
in  certain  respects  peculiar.  When  its 
outer  surface  is  examined,  it  is  seen  to 
be  traversed  by  irregular  lines  (fig1.  764, 
A),  which  are  most  strongly  marked  in 
foetal  hairs ;  and  these  are  the  indications 
of  the  imbricated  arrangement  of  the 
flattened  cells  or  scales  which  form  the  cuticular  layer.  This 
layer,  as  is  shown  by  transverse  sections  (C,  D),  is  a  very  thin 
and  transparent  cylinder ;  and  it  incloses  the  peculiar  fibrous  sub- 
stance that  constitutes  the  principal  part  of  the  shaft  of  the  hair. 
The  constituent  fibres  of  the  substance,  which  are  marked  out  by 
the  delicate  striae  that  may  be  traced  in  longitudinal  sections  of  the 
hair  (B),  may  be  separated  from  each  other  by  crushing  the  hair, 
especially  after  it  has  been  macerated  for  some  time  in  sulphuric 
acid ;  and  each  of  them,  when  completely  isolated  from  its  fellows, 
is  found  to  be  a  long  spindle-shaped  cell.  In  the  axis  of  this  fibrous 
cylinder  there  is  very  commonly  a  band  which  is  formed  of  spheroidal 


FIG.  763. — Transverse  section 
of  hair  of  peccary. 


FIG.  764. — Structure  of  human  hair  :  A,  external  surface  of  the  shaft,  show- 
ing the  transverse  striae  and  jagged  boundary  caused  by  the  imbrications  of 
the  cuticular  layer;  B,  longitudinal  section  of  the  shaft,  showing  the 
fibrous  character  of  the  cortical  substance,  and  the  arrangement  of  the 
pigmentary  matter;  C,  transverse  section,  showing  the  distinction  be- 
tween the  cuticular  envelope,  the  cylinder  of  cortical  substance,  and  the 
medullary  centre ;  D,  another  transverse  section,  showing  deficiency  of 
the  central  cellular  substance. 

cells  ;  but  this  '  medullary '  substance  is  usually  deficient  in  the  fine 
hairs  scattered  over  the  general  surface  of  the  body,  and  is  not 
always  present  in  those  of  the  head.  The  hue  of  the  hair  is  due 
partly  to  the  presence  of  pigmentary  granules,  either  collected  into 
patches  or  diffused  through  its  substance,  but  partly  also  to  the 
existence  of  a  multitude  of  minute  air-spaces,  which  cause  it  to 


1032  VEETEBKATED   ANIMALS 

appear  dark  by  transmitted  and  white  by  reflected  light.  The  cells 
of  the  medullary  axis  in  particular  are  very  commonly  found  to 
contain  air,  giving  it  the  black  appearance  shown  at  C.  The 
difference  between  the  blackness  of  pigment  and  that  of  air-spaces 
may  be  readily  determined  by  attending  to  the  characters  of  the 
latter  as  already  laid  down,  and  by  watching  the  effects  of  the 
penetration  of  oil  of  turpentine  or  other  liquids,  which  do  not  alter 
the  appearance  of  pigment  spots,  but  obliterate  all  the  markings 
produced  by  air-spaces,  these  returning  again  as  the  hair  dries.  In 
mounting  hairs  as  microscopic  preparations  they  should  in  the  first 
instance  be  cleansed  of  all  their  fatty  matter  by  maceration  in 
ether,  and  they  may  then  be  put  up  either  in  weak  spirit  or  in 
Canada  balsam,  as  may  be  thought  preferable,  the  former  menstruum 
being  well  adapted  to  display  the  characters  of  the  finer  and  more 
transparent  hairs,  while  the  latter  allow  the  light  to  penetrate  more 
readily  through  the  coarser  and  more  opaque.  Transverse  sections 
of  hairs  are  best  made  by  glueing  or  gumming  several  together  and 
then  putting  them  into  the  microtome ;  those  of  human  hair  may 
be  easily  obtained,  however,  by  shaving  a  second  time,  very  closely, 
a  part  of  the  surface  over  which  the  razor  has  already  passed  more 
lightly,  and  by  picking  out  from  the  lather,  and  carefully  washing, 
the  sections  thus  taken  off.1 

The  stems  of  feathers  exhibit  the  same  kind  of  structure  as  hairs, 
their  cortical  portion  being  the  horny  sheath  that  envelopes  the 
shaft,  and  their  medullary  portion  being  the  pith -like  substance 
which  that  sheath  includes.  In  small  feathers  this  may  usually  be 
made  very  plain  by  mounting  them  in  Canada  balsam  ;  in  large 
feathers,  however,  the  texture  is  sometimes  so  altered  by  the  drying 
up  of  the  pith  (the  cells  of  which  are  always  found  to  be  occupied 
by  air  alone)  that  the  cellular  structure  cannot  be  demonstrated 
save  by  boiling  thin  slices  in  a  dilute  solution  of  potass,  and  not 
always  even  then.  In  small  feathers,  especially  such  as  have  a 
downy  character,  the  cellular  structure  is  very  distinctly  seen  in  the 
lateral  barbs,  which  are  sometimes  found  to  be  composed  of  single 
files  of  pear-shaped  cells,  laid  end  to  end  ;  but  in  larger  feathers 
it  is  usually  necessary  to  increase  the  transparence  of  the  barbs, 
especially  when  these  are  thick  and  but  little  pervious  to  light, 
either  by  soaking  them  in  turpentine,  mounting  them  in  Canada 
balsam,  or  boiling  them  in  a  weak  solution  of  potass.  In  feathers 
which  are  destined  to  strike  the  air  with  great  force  in  the  act  of 
flight,  we  find  each  barb  fringed  on  either  side  with  slender  flattened 
filaments  or  '  barbules  ; '  the  barbules  of  the  distal  side  of  each  barb 
are  furnished  011  their  attached  half  with  curved  hooks,  whilst  those 
of  the  proximal  side  have  thick  turned-up  edges  in  their  median 
portion ;  as  the  two  sets  of  barbules  that  spring  from  two  adjacent 
barbs  cross  each  other  at  an  angle,  and  as  each  hooked  barbule  of 
one  locks  into  the  thickened  edge  of  several  barbules  of  the  other, 
the  barbs  are  connected  very  firmly,  in  a  mode  very  similar  to  that 

1  On  the  minute  structure  of  hair,  consult  Grimm's  Atlas  der  menscliliclieii  nnd 
tierischen  Haare  (Lahr,  1884,  4to,  with  a  preface  by  W.  Waldeyer). 


HORNS,   HOOFS,   CLAWS  1033 

in  which  the  anterior  and  posterior  wings  of  certain  hymenopterous 
insects  are  locked  together.  Feathers  or  portions  of  feathers  of 
birds  distinguished  by  the  splendour  of  their  plumage  are  very  good 
objects  for  low  magnifying  powers  when  illuminated  on  an  opaque 
ground ;  but  care  must  be  taken  that  the  light  falls  upon  them  at 
the  angle  necessary  to  produce  their  most  brilliant  reflection  into 
the  axis  of  the  [microscope ;  since  feathers  which  exhibit  the  most 
splendid  metallic  lustre  to  an  observer  at  one  point  may  seem  very 
dull  to  the  eye  of  another  in  a  different  position.  The  small  feathers 
of  humming-birds,  portions  of  the  feathers  of  the  peacock,  and 
others  of  a  like  kind,  are  well  worthy  of  examination ;  and  the 
scientific  microscopist,  who  is  but  little  attracted  by  mere  gorgeous- 
ness,  may  well  apply  himself  to  the  discovery  of  the  peculiar- 
structure  which  imparts  to  these  objects  their  most  remarkable 
character.1 

Sections  of  horns,  hoofs,  claws,  and  other  like  modifications  of 
epidermic  structure — which  can  be  easily  made  by  the  microtome, 
the  substance  to  be  cut  having  been  softened,  if  necessary,  by  soaking 
in  warm  water — do  not  in  general  afford  any  very  interesting 
features  when  viewed  in  the  ordinary  mode  ;  but  there  are  no  objects 
on  which  polarised  light  produces  more  remarkable  effects,  or  which 
display  a  more  beautiful  variety  of  colours  when  a  plate  of  selenite  is 
placed  behind  them  and  the  analysing  prism  is  made  to  rotate.  A 
curious  modification  of  the 
ordinary  structure  of  horn  is 
presented  in  the  appendage 
borne1  by  the  rhinoceros  upon 
its  snout,  which  in  many 
points  resembles  a  bundle  of 
hairs,  its  substance  being 
arranged  in  minute  cylinders 
around  a  number  of  separate 
centres,  which  have  probably 
been  formed  bv  independ- 
ent papillae  (fig.  765).  When 
transverse  sections  of  these 
cylinders  are  viewed  by  polar- 
ised light,  each  of  them  is 

seen  to  be  marked  by  a  cross,          FIG.  765.— Transverse  section  of  horn  of 
somewhat  resembling  that  of  rhinoceros  viewed  by  polarised  light, 

starch-grains  ;    and  the  light 

and  shadow  of  this  cross  are  replaced  by  contrasted  colours  when 
the  selenite  plate  is  interposed.  The  substance  commonly  but  erro- 
neously termed  whalebone,  which  is  formed  from  the  surface  of  the 
membrane  that  lines  the  mouth  of  the  whale,  and  has  no  relation 
to  its  true  bony  skeleton,  is  almost  identical  in  structure  with 
rhinoceros-horn,  and  is  similarly  affected  by  polarised  light.  The 
central  portion  of  each  of  its  component  threads,  like  the  medullary 

1   See  K.  S.  Wray,  '  On  .the  Structure  of  the  Barbs,  Barbules,  and  Barbicels  of  a 
typical  Pennaceous  Feather,'  in  the  Ibis  for  1887,  p.  420. 


1034 


VERTEBRATED  ANIMALS 


substance  of  hairs,  contains  cells  that  have  been  so  little  altered  as 
to  be  easily  recognised ;  and  the  outer  or  cortical  portion  also  may 
be  shown  to  have  a  like  structure  by  macerating  it  in  a  solution  of 
potass  and  then  in  water.  Sections  of  any  of  the  horny  tissues  are 
best  mounted  in  Canada  balsam. 

Blood. — Carrying  our  microscopic  survey,  now,  to  the  elementary 
parts  of  which  those  softer  tissues  are  made  up  that  are  subservient 
to  the  active  life  of  the  body  rather  than  to  its  merely  mechanical 
requirements,  we  shall  in  the  first  place  notice  the  isolated  floating 

cells   contained   in  the   blood, 
which  are  known  as  blood-cor- 
puscles.     These    are    of    two 
kinds :     the     '  red '     and     the 
'  white '  or   '  colourless.'      The 
red  present,  in  every  instance, 
the  form  of  a  flattened  disc, 
which  is  circular  in  man  and 
most  mammalia  (fig.  767),  but 
is  oval  in  birds,  reptiles  (fig. 
FIG.  766.— Red  corpuscles    of  frog's  blood:   766),   and  fishes,  as   also    in   a 
aa,  their  flattened  face ;  6,  particle  turned  few  mammals  (all  belonging  to 
nearly  edgeways ;  c,  colourless  corpuscle ;     ,  ,        ., x  x        T 

d,  red  corpuscles  altered  by  diluted  acetic  tne  camel   tribe).      In   the   one 

acid.  form    as   in   the   other,   these 

corpuscles  seem  to  be  flattened 
cells,  the  walls  of  which,  how- 
ever, are  not  distinctly  dif- 
ferentiated from  the  ground 
substance  they  contain,  as 
appears  from  the  changes  of 
form  which  they  spontaneously 
undergo  when  kept  by  means 
of  a  'warm  stage'  at  a  tem- 
perature of  about  100°  F.,  and 

*»»  *»»   ****»<*  F™  in 

rather  within  the  focus  of  the  microscope  ;   breaking   them   up.       The    red 
and  at  &,  as  they  appear  when  precisely  in  corpuscles     in      the      blood     of 

oviparous  Yertebrata  are  dis- 
tinguished by  the  presence  of  a 

central  spot  or  nucleus ;  this  is  most  distinctly  brought  into  view 
by  treating  the  blood-discs  with  acetic  acid,  which  causes  the  nucleus 
to  shrink  and  become  more  opaque,  whilst  rendering  the  remaining 
portion  extremely  transparent  (fig.  766,  d).  By  examining  un- 
altered red  corpuscles  of  the  frog  or  newt  under  a  sufficiently  high 
magnifying  power  the  nucleus  is  seen  to  be  traversed  by  a  network 
of  filaments,  which  extends  from  it  throughout  the  ground  sub- 
stance of  the  corpuscle,  constituting  an  intracellular  reticulation. 
The  red  corpuscles  of  the  blood  of  mammals,  however,  possess  no 
distinguishable  nucleus,  the  dark  spot  which  is  seen  in  their  centre 
(fig.  767,  b)  being  merely  an  effect  of  refraction,  consequent  upon 
the  double  concave  form  of  the  disc.  When  these  corpuscles  are 
treated  with  water,  so  that  their  form  becomes  first  flat  and  then 


BLOOD-CORPUSCLES  1 03  5 

double  convex,  the  dark  spot  disappears  ;  whilst,  on  the  other  hand, 
it  is  made  more  evident  when  the  concavity  is  increased  by  the 
partial  shrinkage  of  the  corpuscles,  which  may  be  brought  about 
by  treating  them  with  fluids  of  greater  density  than  their  own  sub- 
stance. When  floating  in  a  sufficiently  thick  stratum  of  blood 
drawn  from  the  body,  and  placed  under  a  cover-glass,  the  red 
corpuscles  show  a  marked  tendency  to  approach  one  another,  adher- 
ing by  their  discoidal  surfaces  so  as  to  present  the  aspect  of  a  pile 
of  coins  ;  or,  if  the  stratum  be  too  thin  to  admit  of  this,  partially 
overlapping,  or  simply  adhering  by  their  edges,  which  then  become 
polygonal  instead  of  circular.  The  size  of  the  red  corpuscles  is  not 
altogether  uniform  in  the  same  blood  ;  thus  it  varies  in  that  of  man 
from  about  the  ^o^th  to  the  T^Vo^h  of  an  inch.  But  we  generally  find 
that  there  is  an  average  size,  which  is  pretty  constantly  maintained 
among  the  different  individuals  of  the  same  species  ;  that  of  man  may 
be  stated  at  about  3oVoth  of  an  inch.  The  following  table  l  exhibits 

MAMMALS 

Man    ....  1-3200  I  Camel    .         .     1-3254,  1-5921 

Dog     .         .         .         .  1-3542  |  Llama    .         .     1-3361,  1-6294 

Whale         .         .         .  1-3099  |  Javan  chevrotain      .      1-12325 

Elephant     .         .         .  1-2745  j  Caucasian  goat         .       1-7045 

Mouse          .         .         .  1-3814  \  Two-toed  sloth          .       1-2865 

BIRDS 


Golden  eagle 
Owl 
Crow 
Blue-tit   . 
Parrot 

.  1-1812,  1-3832       Ostrich    . 
.  1-1830,  1-3400       Cassowary 
.  1-1961,  1-4000  j    Heron      . 
.  1-2313,  1-4128       Fowl 
.  1-1898,  1-4000  !    Gull 

.  1-1649,  1-3000 
.  1-1455,  1-2800 
.  1-1913,  1-3491 
.  1-2102  1-3466 
.  1-2097,1-4000 

REPTILES    AND    BATRACHIA 

Turtle      . 
Crocodile 
Green  lizard 
Slow-worm 
Viper       , 

.  1-1231,  1-1882       Frog 
.  1-1231,  1-2286      Water-newt 
.  1-1555,  1-2743       Siren 
.  1-1178,  1-2666      Proteus    . 
.  1-1274,  1-1800      Amphiuma 

.  1-1108,  1-1821 
.  1-8014,  1-1246 
.  1-420,  1-760 
.  1-400,  1-727 
.  1-345,  1-561 

FISHES 

Perch       . 
Carp 
Gold-fish 

.  1-2099,  1-2824      Pike 
.  1-2142,  1-3429       Eel  . 
.  1-1777,  1-2824      Gymnotus 

.  1-2000,  1-3555 
.  1-1745,  1-2842 
.  1-1745,  1-2599 

the  average  dimensions  of  some  of  the  most  interesting  examples  of 
the  red  corpuscles  in  the  four  classes  of  vertebrated  animals,  expressed 
in  fractions  of  an  inch.  Where  two  measurements  are  given  they 
are  the  long  and  the  short  diameters  of  the  same  corpuscles.  (See  also 
fig.  768.)  Thus  it  appears  that  the  smallest  red  corpuscles  known 
are  those  of  the  Javan  chevrotain  (Tragulus  javanicus),  whilst  the 
largest  are  those  of  that  curious  group  of  Batrachia  (frog  tribe)  which 

1  These  measurements  are  chiefly  selected  from  those  given  by  Mr.  Gulliver  in 
his  edition  of  Hewson's  Works,  p.  286  et  seq. 


1036 


VERTEBEATED   ANIMALS 


.00  O  f  '0 


retain  the  gills  through  the  whole  of  life  ;  one  of  the  oval  blood- discs 
of  the  Proteus,  being  more  than  thirty  times  as  long  and  seventeen 
times  as  broad  as  those  of  the  musk-deer,  would  cover  no 
fewer  than  510  of  them.  Those  of  the  Amphiuma  are  still  larger.1 
According  to  the  estimate  of  Vierordt,  a  cubic  inch  of  human 
blood  contains  upwards  of  eighty  millions  of  red  corpuscles  and 
nearly  a  quarter  of  a  million  of  the  colourless. 

The  white  or  '  colourless '  corpuscles  are  more  readily  distinguished 

in  the  blood  of  batrachians 
than  in  that  of  man, 
being  in  the  former  case 
of  much  smaller  size,  as 
well  as  having  a  circular 
outline  (fig.  766,  c) ;  whilst 
in  the  latter  their  size  and 
contour  are  so  nearly  the 
same  that,  as  the  red  cor- 
puscles themselves,  when 
seen  in  a  single  layer,  have 
but  a  very  pale  hue,  the 
deficiency  of  colour  does 
not  sensibly  mark  their 
difference  of  nature.  The 
proportion  of  white  to  red 
corpuscles  being  scarcely 
even  greater  (in  a  healthy 
man)  than  1  to  250,  and 
often  as  low  as  from  one 
half  to  one  quarter  of  that 
ratio,  there  are  seldom 
many  of  them  to  be  seen 
in  the  field  at  once  ;  and 
these  may  be  recognised 
rather  by  their  isolation 
than  their  colour,  espe- 

FIG.  768. — Comparative  sizes  of  red  blood  cor-  cially  if  the  glass  cover  be 
puscles :  1,  man;  2,  elephant;  3,  musk-deer;  TO™™/]  Q  li-H-lo  ,  n  the.  «lirlA 
4,  dromedary  ;  5,  ostrich ;  6,  pigeon  ;  7,  humming-  m°Ved  a  Mt  ™> 

bird ;  8,  crocodile ;  9,  python ;  10,  proteus ;  11,  SO  as  to  cause  the  red  cor- 
perch ;  12,  pike ;  13,  shark.  puscles  to  become  aggrega- 

ted into  rows  and  irregular 

masses.  It  is  remarkable  that,  notwithstanding  the  great  variations 
in  the  sizes  of  the  red  corpuscles  in  different  species  of  vertebrated  ani- 
mals, the  size  of  the  white  is  extremely  constant  throughout,  their  dia- 
meter being  seldom  much  greater  or  less  than  -^nnrth  of  an  inch  in  the 
warm-blooded  classes  and  ^Voth  in  reptiles.  Their  ordinary  form 
is  globular,  but  their  aspect  is  subject  to  considerable  variations, 
which  seem  to  depend  in  great  part  upon  their  phase  of  development. 


10 


1  A  very  interesting  account  of  the  '  Structure  of  the  Red  Corpuscles  of  the 
Amphiuma  tridactylum '  has  been  given  by  Dr.  H.  D.  Schmidt,  of  New  Orleans,  in 
the  Journ.  Hoy.  Microsc.  Soc.  vol.  i.  1879,  pp.  57,  97. 


BLOOD-CORPUSCLES  1037 

Thus,  in  their  early  state,  in  which  they  seem  to  be  identical  with 
the  corpuscles  found  floating  in  chyle  and  lymph,  they  seem  to  be 
nearly  homogeneous  particles  of  protoplasmic  substance,  but  in 
their  more  advanced  condition,  according  to  Dr.  Klein,  their  sub- 
stance consists  of  a  reticulation  of  very  fine  contractile  proto- 
plasmic fibres,  termed  the  '  intracellular  network/  in  the  meshes  of 
which  a  hyaline  interstitial  material  is  included,  and  which  is  con- 
tinuous with  a  similar  network  that  can  be  discerned  in  the  substance 
of  the  single  or  double  nucleus  when  this  conies  into  view  after  the 
withdrawal  of  these  corpuscles  from  the  body.  In  their  living  state, 
however,  whilst  circulating  in  the  vessels,  the  white  corpuscles, 
although  clearly  distinguishable  in  the  slow-moving  stratum  in 
contact  with  their  walls  (the  red  corpuscles  rushing  rapidly 
through  the  centre  of  the  tube),  do  not  usually  show  a  distinct 
nucleus.  This  may  be  readily  brought  into  view  by  treating 
the  corpuscles  with  water,  which  causes  them  to  swell  up, 
become  granular,  and  at  last 
disintegrate,  with  emission  of 
granules  which  may  have  been 

previously  seen  in  active  mole-  \  J        j 

cular    movement    within    the 
corpuscle.      When   the   white 
corpuscles  in  a  drop  of  freshly 
drawn     blood     are     carefully 
watched  for  a  short  time,  they 
may  be  observed   to    undergo 
changes    of    form,    and    even 
to  move  from    place  to  place, 
after  the  manner   of  Amcebce. 
When      thus       moving       they    FlG.  769.-Altered  white  corpuscle  of  blood 
engulf      particles      which       lie        an  hour  after  having  been  drawn  from  the 
in      their      course — such     as       finger, 
granules    of     vermilion     that 

have  been  injected  into  the  blood-vessels  of  the  living  animal — and 
afterwards  eject  these  in  the  like  fashion.1  Such  movements  will 
continue  for  some  time  in  the  colourless  corpuscles  of  cold-blooded 
animals,  but  still  longer  if  they  are  kept  in  a  temperature  of  about 
75°.  The  movement  will  speedily  come  to  an  end,  however, 
in  the  white  corpuscles  of  man  or  other  warm-blooded  animals, 

1  Metschnikoff  has  made  the  highly  interesting  and  important  observation  that  the 
immunity  of  certain  animals  to  certain  diseases  appears  to  be  due  to  the  power  that  the 
white  corpuscles  possess  of  acting  as  '  phagocytes,'  or  eating  the  germs  of  the  disease. 
Metschnikoff  found  that  the  virulent  rods  of  the  Bacillus  of  anthrax  '  when  intro- 
duced by  inoculation  into  an  animal  liable  to  take  the  fever,  such  as  a  rodent,  were 
absorbed  by  the  blood-cells  only  in  exceptional  instances.  They  were  readily  absorbed 
by  the  blood-cells  of  animals  not  liable  to  the  disease,  as  frogs  and  lizards,  when  the 
temperature  was  not  artificially  raised  (fig.  770),  and  then  disappeared  inside  the 
cells.  .  .  .  From  all  these  data  we  must  assume  with  Metschnikoff  that  the  Bacillus 
is  harmless  because  it  is  absorbed  and  destroyed  by  the  blood-cells,  and  injurious 
because  this  does  not  happen ;  or  at  least  that  it  becomes  harmless  if  the  destruction 
by  the  blood-cells  takes  place  more  rapidly,  and  to  a  greater  extent  than  the  growth 
and  multiplication  of  the  Bacillus,  the  converse  being  also  true '  (see  A.  de  Bary, 
On  Bacteria,  English  edition,  p.  136).  The  importance  of  phagocytes  is  becoming 
more  and  more  recognised  by  the  pathologist. 


1038  VERTEBRATE!)  ANIMALS 

unless  the  slide  is  kept  on  a  warm  stage  at  the  temperature 
of  about  100°  F.  A  remarkable  example  of  an  extreme  change  of 
form  in  a  white  corpuscle  of  human  blood  is  represented  in  fig.  769. 
Similar  changes  have  been  observed  also  in  the  corpuscles  floating  in 
the  circulating  fluid  of  the  higher  invertebrate,  as  the  crab,  which 
resemble  the  '  white '  corpuscles  of  vertebrated  blood,  rather  than 
its  '  red  '  corpuscles — these  last,  in  fact,  being  altogether  peculiar  to 
the  circulating  fluid  of  vertebrated  animals. 

In  examining  the  blood  microscopically  it  is,  of  course,  impor- 
tant to  obtain  as  thin  a  stratum  of  it  as  possible,  so  that  the  cor- 
puscles may  not  overlie  one  another. 
This  is  best  accomplished  by  selecting 
a  piece  °f  *hin  glass  of  perfect  flat- 
ness,  and   then,    having   received   a 
small    drop    of  blood    upon    a   glass 
slide>  to  lay  the  thin  glass  cover  not 
upon   this,    but   with  its    edge  just 
touching  the  edge  of  the  drop  :  for 
the  blood  will  then  be  drawn  in  by 
FIG.  no.-a,  blood-cell  of  a  frog  in  capillary  attraction,  so  as  to  spread 
the    act  'of    engulfing  a   rod   of  in  a   uniformly  thin  layer    between 
Bacillus  anthracis,  observed  in  the  the    two    glasses.        Such    thin     films 

S'ttS^fel  !S£  -y  be  preserved  in  the  liquid  state 
later.  (After  Metschmkoff ;  highly  by  applying  a  cover  glass  and  ce- 
magm'fied.)  menting  it  with  gold-size  before 

evaporation  has  taken  place  ;   but  it 

is  preferable  first  to  expose  the  drop  to  the  vapour  of  osmic 
acid,  and  then  to  apply  a  drop  of  a  weak  solution  of  acetate  of 
potass ;  after  which  a  cover  glass  may  be  put  on,  and  secured 
with  gold-size  in  the  usual  way.  It  is  far  simpler,  however, 
to  allow  such  films  to  dry  without  any  cover,  and  then  merely  to 
cover  them  for  protection ;  and  in  this  condition  the  general 
characters  of  the  corpuscles  can  be  very  well  made  out,  notwith- 
standing that  they  have  in  some  degree  been  shrivelled  by  the 
desiccation  they  have  undergone.  This  method  is  particularly  ser- 
viceable as  affording  a  fair  means  of  comparison,  when  the  assist- 
ance of  the  microscopist  is  sought  in  determining,  for  medico-legal 
purposes,  the  source  of  suspicious  blood-stains,  the  average  dimen- 
sions of  the  dried  blood-corpuscle  of  the  several  domestic  animals 
being  sufficiently  different  from  each  other,  and  from  those  of  man, 
to  allow  the  nature  of  any  specimen  to  be  pronounced  upon  with  a 
high  degree  of  probability.1 

Simple  Fibrous  Tissues. — A  very  beautiful  example  of  a  tissue  of 
this  kind  is  furnished  by  the  membrane  of  the  common  fowl's  egg ; 
which  (as  may  be  seen  by  examining  an  egg  whose  shell  remains 
soft  for  want  of  consolidation  by  calcareous  particles)  consists  of  two 
principal  layers,  one  serving  as  a  basis  of  the  shell  itself,  and  the 
other  forming  that  lining  to  it  which  is  known  as  the  membrana 

1  This  is  a  matter  which  has  given,  rise  to  much  discussion  among  experts.     See 
Proc.  Amer.  Micr.  Soc.  xiv.  (1893),  pp.  91-120 


FIBROUS   TISSUE 


1039 


putaminis.  The  latter  may  be  separated  by  careful  tearing  with 
needles  and  forceps,  after  prolonged  maceration  in  water,  into  several 
matted  lamella?  resembling  that  represented  in  fig.  771  ;  and  similar 
lamellae  may  be  readily  obtained  from  the  shell  itself  by  disserving 


FIG,  771. — Fibrous  membrane 
from  egg-shell. 


FIG.  772.— White  fibrous  tissue 
from  ligament. 


ftta 


away  its  lime  by  dilute  acid.  The  simply  fibrous  structures  of  the 
body  generally,  however,  belong  to  one  of  two  very  definite  kinds  of 
tissue,  the  '  white '  and  the  '  yellow,'  whose  appearance,  composition, 
and  properties  are  very  different.  The  white 
fibrous  tissue,  though  sometimes  apparently 
composed  of  distinct  fibres,  more  commonly 
presents  the  aspect  of  bands,  usually  of  a  flat- 
tened form,  and  attaining  the  breadth  of  3- ^th 
of  an  inch,  which  are  marked  by  numerous 
longitudinal  streaks,  but  can  seldom  be  torn  up 
into  minute  fibres  of  determinate  size.  The 
fibres  and  bands  are  occasionally  somewhat 
wavy  in  their  direction  ;  and  they  have  a  pecu- 
liar tendency  to  fall  into  undulations,  when  it  is 
attempted  to  tear  them  apart  from  each  other 
(fig.  772).  This  tissue  is  easily  distinguished 
from  the.  other  by  the  effect  of  acetic  acid, 
which  swells  it  up  and  renders  it  transparent, 
at  the  same  time  bringing  into  view  certain 
oval  nuclear  particles  of  'germinal  matter,' 
which  are  known  as  'connective  tissue  cor- 
puscles.' These  are  relatively  much  larger,  and 
their  connections  more  distinct,  in  the  earlier 
stages  of  the  formation  of  this  tissue  (fig.  773). 
It  is  perfectly  inelastic ;  and  we  find  it  in  such 
parts  as  tendons,  ordinary  ligaments,  fibrous 

capsules,  &c.  whose  function  it  is  to  resist  tension  without  yielding 
to  it.  It  constitutes,  also,  the  organic  basis  or  matrix  of  bone  ;  for 
although  the  substance  which  is  left  when  a  bone  has  been  macerated 
sufficiently  long  in  dilute  acid  for  all  its  mineral  components  to  be 


FIG.  773.— Portion  of 
young  tendon,  show- 
ing the  corpuscles 
of  '  germinal  matter,' 
with  their  stellate 
prolongations,  inter- 
posed among  its  fibres. 


1040 


ATERTEBKATED   ANIMALS 


removed  is  commonly  designated  as  cartilage,  this  is  shown  by 
careful  microscopic  analysis  not  to  be  a  correct  description  of  it, 
since  it  does  not  show  any  of  the  characteristic  structure  of  car- 
tilage, but  is  capable  of  being  torn  into  lamellae,  in  which,  if  suf- 
ficiently thin,  the  ordinary  structure  of  a  fibrous  membrane  can  be 
distinguished.  The  ydlow  fibrous  tissue  exists  in  the  form  of  long, 
single,  elastic,  branching  filaments,  with  a  dark  decided  border  ; 
which  are  disposed  to  curl  when  not  put  on  the  stretch  (fig.  774), 
and  frequently  anastomose,  so  as  to  form  a  network.  They  are  for 
the  most  part  between  ^Vo**1  an(*  rcro-offfcn  °f  an  inc^  in  diameter ; 
but  they  are  often  met  with  both  larger  and  smaller.  This  tissue 
does  not  undergo  any  change  when  treated  with  acetic  acid.  It 
exists  alone  (that  is,  without  any  mixture  of  the  white)  in  parts 
which  require  a  peculiar  elasticity,  such  as  the  middle  coat  of  the 
arteries,  the  '  vocal  cords,'  the  '  ligamentum  nuchae '  of  quadrupeds, 

the  elastic  ligament  which 
holds  together  the  valves  of 
a  bivalve  shell,  and  that  by 
which  the  clawrs  of  the  feline 
tribe  are  retracted  when 
not  in  use ;  and  it  enters 
largely  into  the  composition 
of  areolar  or  connective 
tissue. 

The  tissue  formerly 
known  to  anatomists  as 
;  cellular,'  but  now  more 
properly  designated  connec- 
tive or  areolar  tissue,  con- 
sists of  a  network  of  minute 
fibres  and  bands  which  are 

interwoven  in  every  direction,  so  as  to  leave  innumerable  areolce  or 
little  spaces  that  communicate  freely  with  one  another.  Of  these 
fibres  some  are  of  the  *  yellow  '  or  elastic  kind,  but  the  majority  are 
composed  of  the  *  white  '  fibrous  tissue  ;  and,  as  in  that  form  of  ele- 
mentary structure,  they  frequently  present  the  condition  of  broad 
flattened  bands  or  membranous  shreds  in  which  no  distinct  fibrous 
arrangement  is  visible.  The  proportion  of  the  two  forms  varies, 
according  to  the  amount  of  elasticity,  or  of  simple  resisting  power, 
which  the  endowments  of  the  part  may  require.  We  find  this  tissue 
in  a  very  large  proportion  of  the  bodies  of  higher  animals  ;  thus  it 
binds  together  the  ultimate  muscular  fibres  into  minute  fasciculi, 
unites  these  fasciculi  into  larger  ones,  these  again  into  still  larger 
ones  which  are  obvious  to  the  eye,  and  these  into  the  entire  muscle ; 
whilst  it  also  forms  the  membranous  divisions  between  distinct 
muscles.  In  like  manner  it  unites  the  elements  of  nerves,  glands, 
&c.,  binds  together  the  fat-cells  into  minute  masses  (fig.  780),  these 
into  large  ones,  and  so  on;  and  in  this  way  penetrates  and  forms 
part  of  all  the  softer  organs  of  the  body.  But  whilst  the  fibrous 
structures  of  which  the  '  formed  tissue '  is  composed  have  a  purely 
mechanical  function,  there  is  good  reason  to  regard  the  '  connective 


FIG.  774. — Yellow  fibrous  tissue  from  liga- 
mentum nuchfe  of  calf. 


SKIN 


IO4I 


tissue  corpuscles '  which  are  everywhere  dispersed  among  them,  as 
having  a  most  important  function  in  the  first  production  and  sub- 
sequent maintenance  of  the  more  definitely  organised  portions  of 
the  fabric.  In  these  corpuscles  distinct  movements,  analogous  rto 
those  of  the  sarcodic  extensions  of  rhizopods,  have  been  recognised 
in  transparent  parts,  such  as  the  cornea  of  the  eye  and  the  tail  of 
the  young  tadpole,  by  observations  made  on  these  parts  whilst  living. 
For  the  display  of  the  characters  of  the  fibrous  tissues  small  and 
thin  threads  may  be  cut  with  the  curved 
scissors  from  any  part  that  affords 
them  ;  and  these  must  be  torn  astfnder 
with  needles  under  the  simple  micro- 
scope, until  the  fibres  are  separated  to 
a  degree  sufficient  to  enable  them  to 
be  examined  to  advantage  under  a 
higher  magnifying  powrer.  The  differ- 
ence between  the  'white'  and  the 
'  yellow '  components  of  connective  tissue 
is  at  once  made  apparent  by  the  effect 
of  acetic  acid ;  whilst  the  '  connec- 
tive tissue  corpuscles'  are  best  dis- 
tinguished by  the  staining  process, 
especially  in  the  early  stage  of  the 
formation  of  these  tissues  (fig.  773). 

Skin ;  Mucous  and  Serous  Mem- 
branes.— The  skin,  which  forms  the  ex- 
ternal envelope  of  the  body,  is  divisible  FIG.  775.— Vertical  section  of  skin 
into  two  principal  layers  :  ihecutisvera  of  finger:  A,  epidermis,  the 
or  l  true  skin,'  which  usually  makes  up 
by  far  the  larger  part  of  its  thickness, 
and  the  'cuticle,'  'scarfskin,'  or  epi- 
dermis, which  covers  it.  At  the  mouth, 
nostrils,  and  the  other  orifices  of  the 
open  cavities  and  canals  of  the  body, 
the  skin  passes  into  the  membrane  that 
lines  these,  which  is  distinguished  as 
the  mucous  membrane,  from  the  pecu- 
liar glairy  secretion  of  mucus  by  which 
its  surface  is  protected.  But  those  great 
closed  cavities  of  the  body  which  surround  the  heart,  lungs,  intes- 
tines, &c.  are  lined  by  membranes  of  a  different  kind  ;  which,  as 
they  secrete  only  a  thin  serous  fluid  from  their  surfaces,  are  known 
as  serous  membranes.  Both  mucous  and  serous  membranes  consist, 
like  the  skin,  of  a  cellular  membranous  basis,  and  of  a  thin  cuticular 
layer,  which,  as  it  differs  in  many  points  from  the  epidermis,  is  dis- 
tinguished as  the  epithelium.  The  substance  of  the  *  true  skin  '  and 
of  the  '  mucous  '  and  '  serous  '  membranes  is  principally  composed  of 
the  fibrous  tissues  last  described  ;  but  the  skin  and  the  mucous  mem- 
branes are  very  copiously  supplied  with  blood-vessels  and  with  glan- 
dube  of  various  kinds  ;  and  in  the  skin  we  also  find  abundance  of 
nerves  arid  lymphatic  vessels,  as  well  as,  in  some  parts,  of  hair- 

3x 


surface  of  which  shows  depres- 
sions a  a,  between  the  emi- 
nences b  b,  on  which  open  the 
perspiratory  ducts  s ;  at  in  is 
seen  the  deeper  layer  of  the 
epidermis,  or  stratum  Malpighii. 
B,  cutis  vera,  in  which  are  im- 
bedded the  sweat-glands  d, 
with  their  ducts  e,  and  aggre- 
gations of  fat-cells/;  g,  arterial 
twig  supplying  the  vascular 
papillae  p ;  t,  one  of  the  tactile 
papillae  with  its  nerve. 


1042  VERTEBRATES   ANIMALS 

follicles.  The  general  appearance  ordinarily  presented  by  a  thin 
vertical  section  of  the  skin  of  a  part  furnished  with  numerous  sensory 
papillae,  is  shown  in  fig.  775  ;  where  we  see  in  the  deeper  layers 
of  the  cutis  vera  little  clumps  of  fat-cells,  /,  and  the  sweat- 
glands,  d  d,  whose  ducts,  e  e,  pass  upwards :  whilst  on  its  surface 
we  distinguish  the  vascular  papillae,  /?,  supplied  with  loops  of  blood- 
vessels from  the  trunk,  g,  and  a  tactile  papilla,  t,  with  its  nerve 
twig.  The  spaces  between  the  papillae  are  filled  up  by  the  soft 
'  Malpighian  layer,'  m,  of  the  epidermis,  A,  in  which  its  colouring 
matter  is  chiefly  contained,  whilst  this  is  covered  by  the  horny  layer, 
h,  which  is  traversed  by  the  spirally  twisted  continuations  of  the 
perspiratory  ducts,  opening  at  s  upon  the  surface,  which  presents 
alternating  depressions,  a,  and  elevations,  b.  The  distribution  of 
the  blood-vessels  in  the  skin  and  mucous  membranes,  which  is  one 
of  the  most  interesting  features  in  their  structure,  and  which  is  in- 
timately connected  with  their  several  functions,  will  come  under 
our  notice  hereafter.  In  serous  membranes,  on  the  other  hand, 
whose  function  is  simply  protective,  the  supply  of  blood-vessels  is 
more  scanty. 

Epidermic  and  Epithelial  Cell-layers.— The  epidermis  or  '  cuticle ' 
covers  the  whole  exterior  of  the  body  as  a  thin  semitransparent 
pellicle,  which  is  shown  by  microscopic  examination  to  consist  of 
a  series  of  layers  of  cells  that  are  continually  wearing  off  at  the 
external  surface,  and  being  renewed  at  the  surface  of  the  true  skin ; 
so  that  the  newest  and  deepest  layers  gradually  become  the  oldest 
and  most  superficial,  and  are  at  last  thrown  off  by  slow  desquamation . 
In  their  progress  from  the  internal  to  the  external  surface  of  the 
epidermis  the  cells  undergo  a  series  of  well- 
marked  changes.     When  we  examine  the 
innermost  layer,  we  find  it  soft  and  granu- 
lar, consisting  of  nucleated  cells  which  are 
flatter  in  the  upper  than  the  lower  strata, 
which  make  up  the  layer.     This  was  for- 
merly considered  as  a  distinct  tissue,  and 
was  supposed  to  be  the  peculiar  seat  of  the 
colour  of  the  skin  ;  it  received  the  desig- 
nation of  Malpighian  layer  or  rete  mucosum. 
FIG.  776.— Cells  from  the  pig-   The  change  in  form  is  accompanied  by  a 
mentumnigrumottheeye:    change  in  the  chemical  composition  of  the 
S&ttSSSffi*  t^ue.whichseemstobeduetothemeta.mor- 
nucleus.  phosis  of  the  contents  of  the  cells  into  a 

horny  substance  identical  with  that  of  which 

hair,  horn,  nails,  hoofs,  &c.  are  composed.  Mingled  with  the  epi- 
dermic cells  we  find  others  which  secrete  colouring  matter  instead 
of  horn  ;  these,  which  are  termed  '  pigment-cells,'  are  especially  to 
be  noticed  in  the  epidermis  of  the  negro  and  other  dark  races,  and 
are  most  distinguishable  in  the  Malpighian  layer,  their  colour  ap- 
pearing to  fade  as  they  pass  towards  the  surface.  The  most  remark- 
able development  of  pigment-cells  in  the  higher  animals,  however, 
is  on  the  inner  surface  of  the  choroid  coat  of  the  eye,  where  they 
have  a  very  regular  arrangement,  and  form  several  layers,  known  as 


EPIDERMIS 


1043 


the  p-igmentum  niyrum.  When  examined  separately  these  cells  are 
found  to  have  a  polygonal  form  (fig.  776,  «),  and  to  have  a  distinct 
nucleus  (b)  in  their  interior.  The  black  colour  is  due  to  the  accu- 
mulation, within  each  cell,  of  a  number  of  flat  rounded  or  oval 
granules,  of  extreme  minuteness,  which  exhibit  an  active  movement 
when  set  free  from  the  cell,  and  even  whilst  inclosed  within  it. 
The  pigment-cells  are  not  always,  however,  of  this  simply  rounded  or 
polygonal  form ;  they  sometimes  present  remarkable  stellate  pro- 
longations, under  which  form  they  are  well  seen  in  the  skin  of  the 
frog  (fig.  791,cc).  The  gradual  formation  of  these  prolongations 
may  be  traced  in  the  pigment-cells  of  the  tadpole  during  its  meta- 
morphosis (fig.  777).  Similar  varieties  of 
form  are  to  be  met  with  in  the  pigmentary 
cells  of  fishes  and  small  Crustacea,  which 
also  present  a  great  variety  of  hues ;  and 
these  seem  to  take  the  colour  of  the  bottom 
over  which  the  animal  may  live,  so  as  to 
serve  the  better  for  its  concealment. 

The  structure  of  the  epidermis  may  be 
examined  in  a  variety  of  ways.  If  it  be 
removed  by  maceration  from  the  true  skin, 
the  cellular  nature  of  its  under  surface  is  at 
once  recognised,  when  it  is  subjected  to  a 
magnifying  power  of  200  or  300  diameters, 
by  light  transmitted  through  it,  with  this 
surface  uppermost ;  and  if  the  epidermis  be 
that  of  a  negro  or  any  other  dark-skinned 


race,  the  pigment  cells  will  be  very  distinctly 
seen.  This  under-surface  of  the  epidermis 
is  not  flat  but  is  excavated  into  pits  and 
channels  for  the  reception  of  the  papillary  FIG. 


777.  —  Pigment  -  cells 


from  tail  of  tadpole :  a  a, 
simple  forms  of  recent 
origin ;  6  &,  more  complex 
forms  subsequently  as- 
sumed. 


elevations  of  the  true  skin  ;  an  arrangement 
which  is  shown  on  a  large  scale  in  the  thick 
cuticular  covering  of  the  dog's  foot,  the  sub- 
jacent papillae  being  large  enough  to  be  dis- 
tinctly seen  (when  injected)  with  the  naked 

eye.  The  cellular  nature  of  the  newly  forming  layers  is  best  seen 
by  examining  a  little  of  the  soft  film  that  is  found  upon  the  surface 
of  the  true  skin,  after  the  more  consistent  layers  of  the  cuticle  have 
been  raised  by  a  blister.  The  alteration  which  the  cells  of  the 
external  layers  have  undergone  tends  to  obscure  their  character ; 
but  if  any  fragment  of  epidermis  be  macerated  for  a  little  time  in  a 
weak  solution  of  soda  or  potass,  its  dry  scales  become  softened,  and 
are  filled  out  by  imbibition  into  rounded  or  polygonal  cells.  The 
same  mode  of  treatment  enables  us  to  make  out  the  cellular  struc- 
ture in  warts  and  corns,  which  are  epidermic  growths  from  the 
surface  of  papillae  enlarged  by  hypertrophy. 

The  epithelium  may  be  designated  as  a  delicate  cuticle,  covering 
all  the  free  internal  surfaces  of  the  body,  and  thus  lining  all  its 
cavities,  canals,  &c.  Save  in  the  mouth  and  other  parts  in  which 
it  approximates  to  the  ordinary  cuticle,  both  in  locality  and  in 

3x2 


1044 


VEKTEBRATED   ANIMALS 


nature,  its  cells  (fig.  778)  usually  form  but  a  single  layer ;  and  are 
so  deficient  in  tenacity  of  mutual  adhesion  that  they  cannot  be  de- 
tached in  the  form  of  a  continuous  membrane.  Their  shape  varies 
greatly.  Sometimes  they  are  broad,  flat,  and  scale-like,  and  their 
edges  approximate  closely  to  each  other,  so  as  to  form  what  is 
termed  a  *  pavement '  or  '  tessellated '  epithelium  :  such  cells  are 
observable  on  the  web  of  a  frog's  foot  or  on  the  tail  of  a  tadpole  ; 
for,  though  covering  an  external  surface,  the  soft  moist  cuticle  of 
these  parts  has  all  the  characters  of  an  epithelium.  In  other  cases 
the  cells  have  more  of  the  form  of  cylinders,  standing  erect  side  by 
side,  one  extremity  of  each  cylinder  forming  part  of  the  free  surface, 
whilst  the  other  rests  upon  the  membrane  to  which  it  serves  as  a 
covering.  If  the  cylinders  be  closely  pressed  together,  their  form  is 
changed  into  prisms ;  and  such  epithelium  is  often  known  as 
'  prismatic.'  On  the  other  hand,  if  the  surface  on  which  it  rests  be 
convex,  the  bases  or  lower  ends  of  the  cylinders  become  smaller  than 


FIG.  778. — Detached  epithelium-cells: 
a,  with  nuclei  b,  and  nucleoli  c, 
from  mucous  membrane  of  the 
mouth. 


FIG.  779. — Ciliated  epithelium  : 
a,,  nucleated  cells  resting  on 
their  smaller  extremities;  &, 
cilia. 


their  free  extremities ;  and  thus  each  has  the  form  of  a  truncated 
cone  rather  than  of  a  cylinder,  and  such  epithelium  (of  which 
that  covering  the  villi  of  the  intestine  is  a  peculiarly  good  ex- 
ample) is  termed  '  conical.'  But  between  these  primary  forms  of 
epithelial  cells  there  are  several  intermediate  gradations  ;  and  one 
often  passes  almost  insensibly  into  the  other.  Any  of  these  forms 
of  epithelium  may  be  furnished  with  cilia  ;  but  these  appendages  are 
more  commonly  found  attached  to  the  elongated  than  to  the 
flattened  forms  of  epithelial  cells  (fig.  779).  Ciliated  epithelium  is 
found  upon  the  lining  membrane  of  the  air-passages  in  all  air- 
breathing  Yertebrata ;  and  it  also  presents  itself  in  many  other 
situations,  in  which  a  propulsive  power  is  needed  to  prevent  an  ac- 
cumulation of  mucous  or  other  secretions.  Owing  to  the  very  slight 
attachment  that  usually  exists  between  the  epithelium  and  the 
membranous  surface  whereon  it  lies,  there  is  usually  no  difficulty 
whatever  in  examining  it,  nothing  more  being  necessary  than  to 
scrape  the  surface  of  the  membrane  with  a  knife  and  to  add  a  little 
water  to  what  has  been  thus  removed.  The  ciliary  action  will 
generally  be  found  to  persist  for  some  hours  or  even  days  after 
death  if  the  animal  has  been  previously  in  full  vigour  ;  and  the 
cells  that  bear  the  cilia,  when  detached  from  each  other,  will 


FAT  1045 

swim  freely  about  in  water.  If  the  thin  fluid  that  is  copiously  dis- 
charged from  the  nose  in  the  first  stage  of  an  ordinary  '  cold  in  the 
head'  be  subjected  to  microscopic  examination,  it  will  commonly 
be  found  to  contain  a  great  number  of  ciliated  epithelium-cells, 
which  have  been  thrown  off  from  the  lining  membrane  of  the  nasal 
passages. 

Fat. — One  of  the  best  examples  which  the  bodies  of  higher 
animals  afford,  of  a  tissue  composed  of  an  aggregation  of  cells,  is 
presented  by  fat,  the  cells  of  which  are  distinguished  by  their  power 
of  drawing  into  themselves  oleaginous  matter  from  the  blood.  Fat- 
cells  are  sometimes  dispersed  in  Jjhe  interspaces  of  areolar  tissue  ; 
whilst  in  other  cases  they  are  aggregated  in  distinct  masses,  con- 
stituting the  proper  adipose  substance.  The  individual  fat-cells 
always  present  a  nearly  spherical  or  spheroidal  form  ;  sometimes, 
however,  when  they  are  closely  pressed  together,  they  become  some- 
what polyhedral,  from  the  flattening  of  their 
walls  against  each  other  (fig.  780).  Their 
intervals  are  traversed  by  a  minute  network 
of  blood-vessels  (fig.  795),  from  which  they 
derive  their  secretion ;  and  it  is  probably 
by  the  constant  moistening  of  their  walls 
with  a  watery  fluid,  that  their  contents  are 
retained  without  the  least  transudation, 
although  these  are  quite  fluid  at  the  tem- 
perature of  the  living  body.  Fat-cells,  when 
filled  with  their  characteristic  contents,  have 
the  peculiar  appearance  which  has  been 
already  described  as  appertaining  to  oil- 
globules,  being  very  bright  in  their  centre, 
and  very  dark  towards  their  margin,  in  FIG.  780.— Areolar  and  adi- 
consequence  of  their  high  refractive  power  ;  ^os^  tlgS^lree:sa  ^  ^trTolar 
but  if,  as  often  happens  in  preparations  that  tissue. 
have  been  long  mounted,  the  oily  contents 

should  have  escaped,  they  then  look  like  any  other  cells  of  the  same 
form.  Although  the  fatty  matter  which  fills  these  cells  (consisting 
of  a  solution  of  stearine  or  margarine  in  oleine)  is  liquid  at  the 
ordinary  temperature  of  the  body  of  a  warm-blooded  animal,  yet  its 
harder  portion  sometimes  crystallises  on  cooling,  the  crystals  shoot- 
ing from  a  centre,  so  as  to  form  a  star-shaped  cluster.  Osmic  acid 
has  been  found  by  Dr.  B.  Solger  to  separate  a  more  fluid  central 
portion  from  a  firmer  peripheral  part.  In  examining  the  structure 
of  adipose  tissue  it  is  desirable,  where  practicable,  to  have  recourse 
to  some  specimen  in  which  the  fat-cells  lie  in  single  layers,  and  in 
which  they  can  be  observed  without  disturbing  or  laying  them 
open  ;  such  a  condition  is  found,  for  example,  in  the  mesentery  of 
the  mouse ;  and  it  is  also  occasionally  met  with  in  the  fat-deposits 
which  present  themselves  at  intervals  in  the  connective  tissues  of  the 
muscles,  joints,  &c.  Small  collections  of  fat-cells  exist  in  the  deeper 
layers  of  the  true  skin,  and  are  brought  into  view  by  vertical 
sections  of  it  (fig.  775,  /).  And  the  structure  of  large  masses  of  fat 
may  be  examined  by  thin  sections,  these  being  placed  under  water 


1046 


VEKTEBRATED   ANIMALS 


FIG.  781.— Cellular  cartilage  of 
mouse's  ear. 


in  thin  cells,  so  as  to  take  off  the  pressure  of  the  glass  cover  from 
their  surface,  which  would  cause  the  escape  of  the  oil-particles.  No 
method  of  mounting  (so  far  as  the  Author  is  aware)  is  successful  in 
causing  these  cells  permanently  to  retain  their  contents. 

Cartilage. — In  the  ordinary  forms  of  cartilage,  also,  we  have  an 
example  of  a  tissue  obviously  composed  of  cells  ;  but  these  are  com- 
monly separated  from  each  other  by 
an  '  intercellular  substance/  which  is 
so  closely  adherent  to  the  outer  walls 
of  the  cells  as  not  to  be  separable 
from  them.  The  thickness  of  this 
substance  differs  greatly  in  different 
kinds  of  cartilage,  and  even  in  dif- 
ferent stages  of  the  growth  of  any 
one.  Thus  in  the  cartilage  of  the 
external  ear  of  a  bat  or  mouse  (fig. 
781),  the  cells  are  packed  as  closely 
together  as  are  those  of  an  ordinary 
vegetable  parenchyma;  and  this  seems  to  be  the  early  condition 
of  most  cartilages  that  are  afterwards  to  present  a  different 
aspect.  In  the  ordinary  cartilages,  however,  that  cover  the  ex- 
tremities of  the  bones,  so  as  to  form  smooth  surfaces  for  the  work- 
ing of  the  joints,  the  amount  of  intercellular  substance  is  usually 
considerable ;  and  the  cartilage-cells  are  commonly  found  imbedded 
there  in  clusters  of  two,  three,  or  four  (fig.  782),  which  are  evidently 
formed  by  a  process  of  '  binary  subdivision.'  The  substance  of  these 

cellular  cartilages  is  entirely 
destitute  of  blood-vessels, 
being  nourished  solely  by 
imbibition  from  the  blood 
brought  to  the  membrane 
coveringtheirsurface.  Hence 
they  may  be  compared,  in 
regard  to  their  grade  of  or- 
ganisation, with  the  larger 
alga3,  which  consist,  like 
them,  of  aggregations  of  cells 
held  together  by  intercellular 
substance,  without  vessels  of 
FIG.  782.— Section  of  the  branchial  cartilage  of  any  kind,  and  are  nourished 
tadpole  :  a,  group  of  four  cells,  separating  by  imbibition  through  their 
from  each  other ;  &,  pair  of  cells  in  apposi-  -,  i  />  mi 

tion;  cc,  nuclei  of  cartilage-cells;  d,  cavity  whole  surface.  There  are 
containing  three  cells  (the  fourth  probably  many  cases,  however,  in 
behind).  which  the  structureless  inter- 

cellular substance  is  replaced 

by  bundles  of  fibres,  sometimes  elastic,  but  more  commonly  non- 
elastic ;  such  combinations,  which  are  termed  ^ro-cartilages,  are 
interposed  in  certain  joints,  wherein  tension  as  well  as  pressure  has 
to  be  resisted  ;  as,  for  example,  between  the  vertebrae  of  the  spinal 
column  and  the  bones  of  the  pelvis.  In  examining  the  structure 
of  cartilage  nothing  more  is  necessary  than  to  make  very  thin 


GLANDS 


1047 


sections,  preferably  with  the  microtome.  These  sections  may  be 
mounted  in  weak  spirit,  Goadby's  solution,  or  glycerin-jelly;  but 
in  whatever  way  they  are  mounted,  they  undergo  a  gradual  change 
by  lapse  of  time,  which  renders  them  less  fit  to  display  the  cha- 
racteristic features  of  their  structure. 

Structure  of  the  Glands.— The  various  secretions  of  the  body  (as 
saliva,  bile,  urine,  &c.)  are  formed  by  the  instrumentality  of  organs 
termed  glands ;  which  are,  for  the  most  part,  constructed  on  one 
fundamental  type,  whatever  be  the  nature  of  their  product.  The 
simplest  idea  of  a  gland  is  that  which  we  gain  from  an  examination 
of  the  *  follicles  '  or  little  bags  imbedded  in  the  wall  of  the  stomach, 
some  of  which  secrete  mucus  for  the  protection  of  its  surface  and 
other  gastric  juice.  These  little  bags  are  filled  with  cells  of  a 
spheroidal  form,  which  may  be  considered  as  constituting  their 
epithelial  lining ;  these  cells,  in  the  progress  of  their  development, 
draw  into  themselves  from  the  blood  the  constituents  of  the  par- 
ticular product  they  are  to  secrete ;  and  they  then  seem  to  deliver 
it  up,  either  by  the  bursting  or  by  the  melting  away  of  their  walls, 
so  that  this  product  may  be  poured  forth  from  the  mouth  of  the  bag 
into  the  cavity  in  which  it  is  wanted.  The  organ  which  is  generally, 
though  by  no  means  accurately,  called  the  liver  presents  this  con- 
dition  in  the  lowest  animals  wherein  it  is  found.  In  many  Polyzoa, 
compound  Tunrcata,  and  Annulata  the  cells  of  this  organ  can  be  seen 
to  occupy  follicles  in  the  walls  of  the  stomach ;  in  insects  these 
follicles  are  few  in  number,  but  are  immensely  elongated,  so  as  to 
form  tubes  which  lie  loosely  within  the  abdominal  cavity,  frequently 
making  many  convolutions  within  it,  and  discharge  their  contents 
into  the  commencement  of  the  intestinal  canal ;  whilst  in  the 
higher  Mollusca,  and  in  Crustacea,  the  follicles  are  vastly  multiplied 
in  number,  and  are  connected  with  the  ramifications  of  gland-ducts, 
like  grapes  upon  the  stalks  of  their  bunch,  so  as  to  form  a  distinct 
mass  which  now  becomes  known  as  the  liver.  The  examination  of 
the  tubes  of  this  organ  in  the  insect,  or  of 
the  follicles  of  the  crab,  which  may  be 
accomplished  with  the  utmost  facility,  is 
well  adapted  to  give  an  idea  of  the 
essential  nature  of  glandular  structure. 
Among  vertebrated  animals  the  salivary 
glands,  the  pancreas  (sweetbreads),  and 
the  mammary  glands  are  well  adapted  to 
display  the  follicular  structure  (fig.  783), 
nothing  more  being  necessary  than  to  FlG;  788.-Ultimate  follicles 
-,  ,.  ^j,  ,/.  of  mammary  gland,  with 

make  sections  of  these  organs  thin  enough       their  secreting  cells   a  a, 
to  be  viewed  as  transparent  objects.     The       containing  nuclei  b  b. 
kidneys  of  vertebrated  animals  are  made 

up  of  elongated  tubes,  which  are  straight,  and  are  lined  with  a 
pavement-epithelium  in  the  inner  or  '  medullary '  portion  of  the 
kidney,  whilst  they  are  convoluted  and  filled  with  a  spheroidal 
epithelium  in  the  outer  or  *  cortical.'  Certain  flask-shaped  dilata- 
tions of  these  tubes  include  curious  little  knots  of  blood-vessels, 
which  are  known  as  the  '  Malpighian  bodies '  of  the  kidney ;  these 


1048  VEKTEBRATED   ANIMALS 

are  well  displayed  in  injected  preparations.  For  such  a  full  and 
complete  investigation  of  the  structure  of  these  organs  as  the 
anatomist  and  physiologist  require,  various  methods  must  be  put 
in  practice  which  this  is  not  the  place  to  detail.  It  is  perfectly 
easy  to  demonstrate  the  cellular  nature  of  the  substance  of  the 
liver  by  simply  scraping  a  portion  of  its  cut  surface,  since  a  number 
of  its  cells  will  then  be  detached.  The  general  arrangement  of  the 
cells  in  the  lobules  may  be  displayed  by  means  of  sections  thin 
enough  to  be  transparent ;  whilst  the  arrangement  of  the  blood- 
vessels can  only  be  shown  by  means  of  injections.  Fragments  of 
the  tubules  of  the  kidney,  sometimes  having  the  Malpighian  cap- 
sules in  connection  with  them,  may  also  be  detached  by  scraping  its 
cut  surface  ;  but  the  true  relations  of  these  parts  can  only  be  shown 
by  thin  transparent  sections,  and  by  injections  of  the  blood-vessels 
and  tubuli.  The  simple  follicles  contained  in  the  walls  of  the 
stomach  are  brought  into  view  by  vertical  sections  ;  but'  they  may 
be  still  better  examined  by  leaving  small  portions  of  the  lining 
membrane  for  a  few  days  in  dilute  nitric  acid  (one  part  to  four  of 
water),  whereby  the  fibrous  tissue  will  be  so  softened  that  the 
clusters  of  glandular  epithelium  lining  the  follicles  (which  are  but 
very  little  altered)  will  be  readily  separated. 

Muscular  Tissue. — Although  we  are  accustomed  to  speak  of  this 
tissue  as  consisting  of  'fibres,'  yet  the  ultimate  structure  of  the 
'  muscular  fibre  '  is  very  different  from  that  of  the  '  simple  fibrous 
tissues'  already  described.  When  we  examine  an  ordinary 
muscle  (or  piece  of  ;  flesh ')  with  the  naked  eye,  we  observe  that  it 
is  made  up  of  a  number  of  fasciculi  or  bundles 
of  fibres  (fig.  784),  which  are  arranged  side  by 
side  with  great  regularity,  in  the  direction  in 
which  the  muscle  is  to  act,  and  are  united  by 
connective  tissue.  These  fasciculi  may  be 
separated  into  smaller  parts,  which  appear  like 
simple  fibres ;  but  when  these  are  examined  by 
the  microscope,  they  are  found  to  be  themselves 
fasciculi,  composed  of  minuter  fibres  bound 
together  by  delicate  '  filaments  of  connective 
tissue.  By  carefully  separating  these  we  may 
obtain  the  ultimate  muscular  fibre.  This  fibre 
exists  under  two  forms,  the  striated  and  the 

FIG.    784. Fasciculus  non  -striated.     The  former  is  chiefly  distinguished 

of   striated  muscular  by  the  transversely  striated  appearance  which 

Snsvtr^sfri^a^d  ^   Pr6Sents   (%'.  785)>   and   which   is   due   to   an 

at  6  its  junction' with  alternation  of  light  and    dark  spaces  along  its 

the  tendon.  whole   extent;     the    breadth   and    distance   of 

these  striae  vary,  however,  in  different   fibres, 

and  even  in  different  parts  of  the  same  fibre,  according   to  their 

state  of  contraction    or  relaxation.       Longitudinal    striae   are   also 

frequently  visible,  which   are  due  to  a  partial   separation  between 

the  component   fibrillae  into  which   the  fibre   may  be   broken   up. 

When  a  fibre  of  this  kind  is  more  closely  examined,  it  is  seen  to  be 

inclosed  within  a  delicate  tubular  sheath,  which  is  quite  distinct  on 


MUSCLE 


1049 


FIG.  785. — Striated  muscular  fibre,  separating 
into  fibrillset 


whilst  in 
inch,  and 


the 
the 


the  one  hand  from  the  connective  tissue  that  binds  the  fibres  into 
fasciculi,  and  equally  distinct  from  the  internal  substance  of  the 
fibre.  This  membranous  tube,  which  is  termed  the  sarcolemma,  is 
not  perforated  by  capillary  vessels,  which  therefore  lie  outside  the 
ultimate  elements  of  the  muscular  substance  ;  whether  it  is  pene- 
trated by  the  ultimate 
fibres  of  nerves  is  a  point 
not  yet  certainly  ascer- 
tained. The  diameter  of 
the  fibres  varies  greatly 
in  different  kinds  of  verte- 
brated  animals.  Its  ave- 
rage is  greater  in  reptiles 
and  fishes  than  in  birds 
and  mammals,  and  its  ex- 
tremes also  are  wider ;  thus 
its  dimensions  vary  in  the 
frog  from  y^oth  to  TT£nyth 

of  an  inch,  and  in  the  skate  from  -6l5th  to  -g^th ; 
human  subject  the  average  is  about  -f^th  of  an 
extremes  about  2^0  th  and  ^oth. 

The  substance  of  the  fibre,  when  broken  up  by  '  teasing '  with 
needles,  is  found  to  consist  of  very  minute  fibrilbe,  which,  when 
examined  under  a  magnifying  power  of  from  250  to  400  diameters, 
are  seen  to  present  a  slightly  beaded  form,  and  to  show  the  same 
alternation  of  light  and  dark  spaces  as  when 
the  fibrillfe  are  united  into  fibres  or  into 
small  bundles  (fig.  785).  The  dark  and  light 
spaces  are  usually  of  nearly  equal  length ; 
each  light  space  is  divided  by  a  transverse 
line,  called  *  Dobie's  line,'  while  each  dark 
space  is  crossed  by  a  lighter  band,  known  as 
'  Hensen's  stripe.'  It  has  been  generally 
supposed  that  these  markings  indicate  dif- 
ferences in  the  composition  of  the  fibre  ;  but 
Professor  J.  B.  Hay  craft  has  revived  an 
idea,  which  originated  with  Mr.  Bowman, 
that  they  are  the  optical  expressions  of  its 
shape.  The  borders  of  the  striated  fibre 
(he  truly  states)  present  wavy  margins,  in- 
dicative of  a  transverse  ridging  and  furrow- 
ing, the  whole  fibre  (or  a  single  fibril)  thus 
consisting  of  a  succession  of  convex  bead- 
like  projections  with  intermediate  concave 
depressions.  When  the  axis  of  the  fibre  is  in 
true  focus,  Dobie's  line,  D  (fig.  786),  crosses 

the  deepest  part  of  the  concavity,  while  Hensen's  stripe,  H,  crosses 
the  most  projecting  part  of  the  convexity,  and  it  can  be  shown,  both 
theoretically  and  experimentally,  that  this  alternation  of  lights  and 
shades  will  be  produced  by  the  passage  of  light  through  a  similarly 
shaped  homogeneous  rod  of  any  transparent  substance.  If,  on  the 


FIG.  786. — Diagram  of 
striated  fibrilla. 


1050  VERTEBRATED   ANIMALS 

other  hand,  the  surface  of  the  fibre  be  brought  into  focus,  the  convex 
ribbings  appear  light  and  intervening  depressions  dark,  which  is  the 
aspect  originally  represented  by  Bowman.  The  appearances  are  the 
same  in  the  extended  and  contracted  states  of  the  fibre ;  with  the 
exception  that  the  alternation  of  light  and  dark  striae  is  closer  in  the 
contracted  state,  while  the  breadth  (representing  the  thickness)  of 
the  fibre  is  correspondingly  increased.1  It  is  well  none  the  less  in 
the  present  state  of  our  knowledge  to  refrain  from  conclusions  as  to 
the  absolute  structure  of  the  striated  fibrillae.  It  ranges  itself,  from 
the  modern  m'icroscopist's  point  of  view,  with  other  striated  objects, 
and  will  require  the  possession  of  lenses  of  a  N.  A.  twice  or  thrice  that 
of  those  which  are  now  within  our  reach.  There  is  no  immediate  pro- 
spect of  these,  it  is  true ;  but  they  cannot  be  considered  impossible 
by  the  student  of  the  past  history  of  microscopy. 

In  the  examination  of  muscular  tissue  a  small  portion  may  be 
cut  out  with  the  curved  scissors ;  this  should  be  torn  up  into  its 
component  fibres;  and  these,  if  possible,  should  be  separated  into 
their  fibrillae  by  dissection  with  a  pair  of  needles  under  the  simple 
microscope.  The  general  characters  of  the  striated  fibre  are  admi- 
rably shown  in  the  large  fibres  of  the  frog  ;  and  by  selecting  a 
portion  in  which  these  fibres  spread  themselves  out  to  unite  with  a 
broad  tendinous  expansion,  they  may  often  be  found  so  well  dis- 
played in  a  single  layer  as  not  only  to  exhibit  all  their  characters 
without  any  dissection,  but  also  to  show  their  mode  of  connection 
with  the  *  simple  fibrous  '  tissue  of  which  that  expansion  is  formed. 
As  the  ordinary  characters  of  the  fibre  are  but  little  altered  by 
boiling,  recourse  may  be  had  to  this  process  for  their  more  ready 
separation,  especially  in  the  case  of  the  tongue.  Dr.  Beale  recom- 
mends glycerin  for  the  preparation,  and  glycerin  media  for  the 
preservation,  of  objects  of  this  class  ;  and  states  that  the  alternation 
of  light  and  dark  spaces  in  the  fibrillae  is  rendered  more  distinct  by 
such  treatment.  The  fibrillae  are  often  more  readily  separable  when 
the  muscle  has  been  macerated  in  a  weak  solution  of  chromic  acid. 
The  shape  of  the  fibres  can  only  be  properly  seen  in  cross-sections  ; 
and  these  are  best  made  by  the  freezing  microtome.  Striated  fibres, 
separable  with  great  facility  into  their  component  fibrillae,  are 
readily  obtainable  from  the  limbs  of  Crustacea  and  of  insects ;  and 
their  presence  is  also  readily  distinguishable  in  the  bodies  of  worms, 
even  of  very  low  organisation  ;  so  that  it  may  be  regarded  as  charac- 
teristic of  the  articulated  series  generally.  On  the  other  hand,  the 
molluscous  classes  are,  for  the  most  part,  distinguished  by  the  non- 
striation  of  their  fibre ;  there  are,  however,  some  exceptions,  such  as 
the  muscles  of  the  odontophore  in  the  snail  and  the  powerful  adductor 
muscle  of  Pecten.  Its  presence  seems  related  to  energy  and  rapidity 
of  movement,  the  non-striated  presenting  itself  where  the  move- 
ments are  slower  and  feebler  in  their  character. 

The  '  smooth '  or  non-striated  form  of  muscular  fibre,  which  is 

1  Quart.  Journ.  Microsc.  Sci.  n.s.  xxi.  p.  307.  More  recent  views  will  be  found  in 
Mr.  C.  F.  Marshall's  paper  in  vol.  xxviii.  of  the  same  journal,  and  in  the  memoirs 
cited  by  him.  The  subject  is  one  which  will  doubtless  long  occupy  the  attention  of 
the  histologist. 


MUSCLE  ;   NERVE 


1051 


especially  found  in  the  walls  of  the  stomach,  intestines,  bladder,  and 
other  similar  parts,  is  composed  of  flattened  bands  whose  diameter  is 
usually  between  ^uWth  anc^  ToVo^h  °f  an  incn  ;  and  these  bands  are 
collected  into  fasciculi,  which  do  not  lie  parallel  with  each  other,  but 
cross  and  interlace.  By  macerating  a  portion  of  such  muscular  sub- 
stance, however,  in  dilute  nitric  acid  (about  one  part  of  ordinary 
acid  to  three  parts  of  water)  for  two  or  three  days,  it  is  found  that 
the  bands  just  mentioned  may  be  easily  separated  into  elongated  fusi- 
form cells,  not  unlike  *  woody  fibre '  in  shape  (fig.  787,  a  a)  ;  each 
distinguished,  for  the  most  party  by  the  presence  of  a  long  staff- 
shaped  nucleus,  b,  brought  into  view  by  the  action  of  acetic  acid,  c. 
These  cells,  in  which  the  distinction  between  cell- wall  and  cell-con- 
tents can  by  no  means  be  clearly  seen,  are  composed  of  a  soft  yellow^ 
substance  often  containing  small  pale  granules,  and  sometimes  yellow 
globules  of  fatty  matter.  In  the  coats  of  the  blood-vessels  are  found 


FIG.  787. — Structure  of  non-striated 
muscular  fibre :  A,  portion  of 
tissue  showing  fusiform  cells  a  a, 
with  elongated  nuclei  b  b  ;  B,  a 
single  cell  isolated  and  more 
highly  magnified ;  C,  a  similar 
cell  treated  with  acetic  acid. 


FIG.  788.— Ganglion-cells  and  nerve- 
fibres  from  a  ganglion  of  lamprey. 


cells  having  the  same  general  characters,  but  shorter  and  wider  in 
form  ;  and  although  some  of  these  approach  very  closely  in  their 
general  appearance  to  epithelium-cells,  yet  they  seem  to  have  quite 
a  different  nature,  being  distinguished  by  their  elongated  nuclei,  as 
well  as  by  their  contractile  endowments. 

Nerve-substance. — Wherever  a  distinct  nervous  system  can  be 
made  out,  it  is  found  to  consist  of  two  very  different  forms  of  tissue, 
namely,  the  cellular,  which  are  the  essential  components  of  the 
ganglionic  centres,  and  the  fibrous,  of  which  the  connecting  trunks 
consist.  The  typical  form  of  the  nerve-cells  or  '  ganglion-globules 
may  be  regarded  as  globular ;  but  they  often  present  an  extension 
into  one  or  more  long  processes,  which  give  them  a  '  caudate '  or 
'  stellate  '  aspect.  These  processes  have  been  traced  into  continuity, 
in  some  instances,  with  the  axis-cylinders  of  nerve-tubes  (fig.  788)  ; 
whilst  in  other  cases  they  seem  to  inosculate  with  those  of  other 


IO$2 


VERTEBRATED  ANIMALS 


vesicles.  The  cells,  which  do  not  seem  to  possess  a  definite  cell- wall, 
are,  for  the  most  part,  composed  of  a  finely  granular  substance,  which 
extends  into  their  prolongations ;  and  in  the  midst  of  this  is  usually 
to  be  seen  a  large  well-defined  nucleus.  They  also  generally  contain 
pigment-granules,  which  give  them  a  reddish  or  yellowish-brown 
colour,  and  thus  impart  to  collections  of  ganglionic  cells  in  the 
warm-blooded  Vertebrata  that  peculiar  hue  which  causes  them  to  be 
known  as  the  cineritious  or  grey  matter,  but  which  is  commonly 
absent  among  the  lower  animals.  Each  of  the  tubular  nerve-fibres, 
on  the  other  hand,  of  which  the  trunks  are  made  up,  consists,  in  its 
fully  developed  form,  of  a  delicate  membranous  sheath,  within  which 
is  a  hollow  cylinder  of  a  material  known  as  the  '  white  substance  of 
Schwann,'  whose  outer  and  inner  boundaries  are  marked  out  by  two 
distinct  lines,  giving  to  each  margin  of  the  nerve-tube  what  is  de- 
scribed as  a  *  double  contour.'  The  contents  of  the  membranous 
envelope  are  very  soft,  yielding  to  slight  pressure  ;  and  they  are  so 
quickly  altered  by  the  contact  of  water  or  of  any  liquids  which  are 
foreign  to  their  nature  that  their  characters  can  only  be  properly 
judged  of  when  they  are  quite  fresh.  The 
centre  or  axis  of  the  tube  is  then  found  to 
be  occupied  by  a  transparent  substance 
which  is  known  as  the  '  axis  cylinder  ; ' 
and  there  is  reason  to  believe  that  this  last, 
which  is  a  protoplasmic  substance,  is  the 
essential  component  of  the  nerve-fibre, 
while  the  function  of  the  hollow  cylinder 
that  surrounds  it,  which  is  composed  of  a 
combination  of  fat  and  albuminous  matter, 
is  simply  protective.  The  diameter  of  the 
nerve-tubes  differs  in  different  nerves,  being 
sometimes  as  great  as  y^^th  of  an  inch, 
and  as  small  in  other  instances  as  T^rou^h- 
In  many  of  the  lower  invertebrata,  such  as 
Medusae  and  Comatulce,  we  seem  fully 
justified  by  physiological  evidence  in  re- 
garding as  nerves  certain  protoplasmic 
fibres  which  do  not  possess  the  characteristic  structure  of  '  nerve- 
tubes,'  and  fibres  destitute  of  the  '  double  contour '  are  found  also 
in  certain  parts  of  the  body  of  even  the  highest  vertebrates.  These 
fibres,  which  are  known  as  '  gelatinous,'  are  considerably  smaller 
than  the  preceding,  and  do  not  exhibit  any  differentiation  of  parts 
(fig.  789).  They  are  flattened,  soft,  and  homogeneous  in  their  ap- 
pearance, and  contain  numerous  nuclear  particles  which  are  brought 
into  view  by  acetic  acid.  They  can  sometimes  be  seen  to  be 
continuous  with  the  axis-cylinders  of  the  ordinary  fibres,  and  also 
with  the  radiating  prolongations  of  the  ganglion-cells ;  so  that  their 
nervous  character,  which  has  been  questioned  bv  some  anatomists, 
seems  established  beyond  doubt. 

The  ultimate  distribution  of  the  nerve -fibres  is  a  subject  on 
which  there  has  been  great  divergence  of  opinion,  and  one  which  can 
only  be  successfully  investigated  by  observers  of  great  experience. 


PIG.  789. — Gelatinous  nerve 
fibres,  from  olfactory  nerve. 


NEEVE-FIBRES 


1053 


The  Author  believes  that  it  may  be  stated  as  a  general  fact,  that  in 
both  the  motor  and  the  sensory  nerve-tubes,  as  they  approach  their 
terminations  in  the  muscles  and  in  the  skin  respectively,  the 
protoplasmic  axis-cylinder  is  continued  beyond  its  envelopes, 
often  then  breaking  up  into  very  minute  fibrillae,  which  inosculate 
with  each  other,  so  as  to  form  a  network  closely  resembling  that 
formed  by  the  pseudopodial  threads  of  Rhizopods.  Recent  observers 
have  described  the  nbrillae  of  motor  nerves  as  terminating  in 
'  motorial  erid-plates '  seated  upon  or  in  the  muscular  fibres ;  and 
these  seem  analogous  to  the  little  '  islets  '  of  sarcodic  substance  into 
which  those  threads  often  dilatte.  Where  the  skin  is  specially 
endowed  with  tactile  sensibility  we  find  a  special  papillary 
apparatus,  which  in  the  skin  may  be  readily  made  out  in  thin 
vertical  sections  treated  with  solution  of  soda  (fig.  790).  It  was 
formerly  supposed  that  all  the  cutaneous  papillae  are  furnished  with 
nerve-fibres,  and  minister  to  sensation  ;  but  it  is  now  known  that  a 
large  proportion  (at  any  rate)  of  those  that  are  furnished  with  loops 
of  blood-vessels  (figs.  775,  p,  798),  being  destitute  of  nerve-fibres, 
must  have  for  their  special 
office  the  production  of 
epidermis  ;  whilst  those 
which,  possessing  nerve- 
fibres,  have  sensory  func- 
tions, are  usually  destitute 
of  blood-vessels.  The 
greater  part  of  the  interior 
of  each  sensory  papilla 
(fig.  790,  c  c)  of  the  skin 
is  occupied  by  a  peculiar 
'  axile  body,'  which  seems 
to  be  merely  a  bundle  of 
ordinary  connective  tissue, 
whereon  the  nerve-fibre 
appears  to  terminate.  The 
nerve  -  fibres  are  more 
readily  seen,  however,  in 
the  '  fungiforin  '  papillae  of 

the  tongue,  to  each  of  which  several  of  them  proceed ;  these  bodies, 
which  are  very  transparent,  may  be  well  seen  by  snipping  off  minute 
portions  of  the  tongue  of  the  frog ;  or  by  snipping  off  the  papillae 
themselves  from  the  surface  of  the  living  human  tongue,  which  can 
be  readily  done  by  a  dexterous  use  of  the  curved  scissors,  with  no 
more  pain  than  the  prick  of  a  pin  would  give.  The  transparence 
of  these  papillae  also  is  increased  by  treating  them  with  a  weak 
solution  of  soda.  Nerve-fibres  have  also  been  found  to  terminate 
on  sensory  surfaces  in  minute  '  end-bulbs '  of  spheroidal  shape  and 
about  -(-jToth  of  an  inch  in  diameter,  each  of  them  being  composed 
of  a  simple  outer  capsule  of  connective  tissue,  filled  with  clear 
soft  matter,  in  the  midst  of  which  the  nerve-fibre,  after  losing  its 
dark  border,  ends  in  a  knob.  The  '  Pacinian  corpuscles,'  which  are 
best  seen  in  the  mesentery  of  the  cat,  and  are  from  ^th  to  jLth  of 


FIG.  790. — Vertical  section  of  skin  of  finger,  show- 
ing the  branches  of  the  cutaneous  nerves,  a,  b, 
inosculating  to  form  a  plexus,  of  which  the  ulti- 
mate fibres  pass  into  the  cutaneous  papillae,  c  c. 


1054  VERTEBRATED   ANIMALS 

an  inch  long,  seem  to  be  more  developed  forms  of  these  '  end- 
bulbs.' 

For  the  sake  of  obtaining  a  general  acquaintance  with  the 
microscopic  characters  of  these  principal  forms  of  nerve-substance, 
it  is  best  to  have  recourse  to  minute  nerves  and  ganglia.  The  small 
nerves  which  are  found  between  the  skin  and  the  muscles  of  the  back 
of  the  frog,  and  which  become  apparent  when  the  former  is  being 
stripped  off,  are  extremely  suitable  for  this  purpose  ;  but  they  are  best 
seen  in  the  Hyla  or  '  tree-frog,'  which  is  recommended  by  Dr.  Beale 
as  being  much  superior  to  the  common  frog  for  the  general  purposes 
of  minute  histological  investigation.  If  it  be  wished  to  examine  the 
natural  appearance  of  the  nerve-fibres,  no  other  fluid  should  be  used 
than  a  little  blood-serum  ;  but  if  they  be  treated  with  strong  acetic 
acid,  a  contraction  of  their  tubes  takes  place,  by  which  the  axis- 
cylinders  are  forced  out  from  their  cut  extremities,  so  as  to  be  made 
more  apparent  than  they  can  be  in  any  other  way.  On  the  other 
hand,  by  immersion  of  the  tissue  in  a  dilute  solution  of  chromic  acid 
(about  one  part  of  the  solid  crystals  to  two  hundred  of  water),  the 
nerve-fibres  are  rendered  firmer  and  more  distinct.  Again,  the  axis- 
cylinders  are  brought  into  distinct  view  by  the  staining  process, 
being  dyed  much  more  quickly  than  their  envelopes;  and  they 
may  thus  be  readily  made  out  by  reflected  light  in  transverse 
sections  of  nerves  that  have  been  thus  treated.  The  gelatinous 
fibres  are  found  in  the  greatest  abundance  in  the  sympathetic  nerves ; 
and  their  characters  may  be  best  studied  in  the  smaller  branches  of 
that  system.  So  for  the  examination  of  the  ganglionic  cells,  and  of 
their  relation  to  the  nerve-tubes,  it  is  better  to  take  some  minute 
ganglion  as  a  whole  (such  as  one  of  the  sympathetic  ganglia  of  the 
frog,  mouse,  or  other  small  animal)  than  to  dissect  the  larger 
ganglionic  masses,  whose  structure  can  only  be  successfully  studied 
by  such  as  are  proficient  in  this  kind  of  investigation.  The  nerves 
of  the  orbit  of  the  eyes  of  fishes,  with  the  ophthalmic  ganglion  and 
its  branches,  which  may  be  very  readily  got  at  in  the  skate,  and  of 
which  the  components  may  be  separated  without  much  difficulty, 
form  one  of  the  most  convenient  objects  for  the  demonstration  of  the 
principal  forms  of  nerve-tissue,  and  especially  for  the  connection  of 
nerve-fibres  and  ganglion-cells.  For  minute  inquiries,  however,  into 
the  ultimate  distribution  of  the  nerve-fibres  in  muscles  and  sense- 
organs,  certain  special  methods  must  be  followed,  and  very  high 
magnifying  powers  must  be  employed.  Those  who  desire  to  follow 
out  this  inquiry  should  acquaint  themselves  with  the  methods  which 
have  been  found  most  successful  in  the  hands  of  the  able  histologists 
who  have  devoted  themselves  to  it.1 

Circulation  of  the  Blood. — One  of  the  most  interesting  spectacles 
that  the  microscopist  can  enjoy  is  that  which  is  furnished  by  the 

1  For  further  information  regarding  the  nervous  system  the  memoir  of  F.  Nansen 
on  'The  Structure  and  Combination  of  the  Histological  Elements  of  the  Central 
Nervous  System '  in  Bergen's  Museums  Aarsberetning  for  1886  (1887),  p.  29,  should 
be  consulted.  An  excellent  summary  of  the  more  valuable  modern  methods  of 
staining  nerve-fibres  and  cells  was  given  in  1892  to  the  Koyal  Microscopical  Society 
by  Dr.  C.  E.  Beevor.  See  their  Journal,  1892,  p.  897, 


CIRCULATION   OF  BLOOD  1 05  5 

circulation  of  the  blood  in  the  capillary  blood-vessels  which  dis- 
tribute the  fluid  through  the  tissues  it  nourishes.  This,  of  course, 
can  only  be  observed  in  such  parts  of  animal  bodies  as  are  sufficiently 
thin  and  transparent  to  allow  of  the  transmission  of  light  through 
them,  without  any  disturbance  of  their  ordinary  structure ;  and  the 
number  of  these  is  very  limited.  The  web  of  the  frog's  foot  is  per- 
haps the  most  suitable  for  ordinary  purposes,  more  especially  since 
this  animal  is  to  be  easily  obtained  in  almost  every  locality ;  and  the 
following  is  the  simple  arrangement  preferred  by  the  Author  :  A 
piece  of  thin  cork  is  to  be  obtained,  about  nine  inches  long  and  three 
inches  wide  (such  pieces  are  prepared  by  cork-cutters,  as  soles),  and  a 
hole  about  |ths  of  an  inch  in  diameter  is  to  be  cut  at  about  the  middle 
of  its  length,  in  such  a  position  that,  when  the  cork  is  secured  upon  the 
stage,  this  aperture  may  correspond  with  the  axis  of  the  microscope. 
The  body  of  the  frog  is  then  to  be  folded  in  a  piece  of  wet  calico, 
one  leg  being  left  free,  in  such  a  manner  as  to  confine  its  move- 
ments, but  not  to  press  too  tightly  upon  its  body ;  and  being  then 
laid  down  near  one  end  of  the  cork-plate,  the  free  leg  is  to  be  ex- 
tended, so  that  the  foot  can  be  laid  over  the  central  aperture.  The 
spreading  out  of  the  foot  over  the  aperture  is  to  be  accomplished 
either  by  passing  pins  through  the  edge  of  the  web  into  the  cork  be- 
neath, or  by  tying  the  ends  of  the  toes  with  threads  to  pins  stuck 
into  the  cork  at  a  small  distance  from  the  aperture ;  the  former 
method  is  by  far  the  least  troublesome,  and  it  may  be  doubted 
whether  it  is  really  the  source  of  more  suffering  to  the  animal  than 
the  latter,  the  confinement  being  obviously  that  which  is  most  felt. 
A  few  turns  of  tape,  carried  loosely  around  the  calico  bag,  the  pro- 
jecting leg,  and  the  cork,  serve  to  prevent  any  sudden  start ;  and 
when  all  is  secure,  the  cork-plate  is  to  be  laid  down  upon  the  stage 
of  the  microscope,  where  a  few  more  turns  of  the  tape  will  serve  to 
keep  it  in  place.  The  web  being  moistened  with  water  (a  precaution 
which  should  be  repeated  as  often  as  the  membrane  exhibits  the 
least  appearance  of  dryness)  and  an  adequate  light  being  reflected 
through  the  web  from  the  mirror,  this  wonderful  spectacle  is  brought 
into  view  on  the  adjustment  of  the  focus  (a  power  of  from  75  to  100 
diameters  being  the  most  suitable  for  ordinary  purposes),  provided 
that  no  obstacle  to  the  movement  of  the  blood  be  produced  by 
undue  pressure  upon  the  body  or  leg  of  the  animal.  It  will  not  un- 
frequently  be  found,  however,  that  the  current  of  blood  is  nearly  or 
altogether  stagnant  for  a  time  ;  this  seems  occasionally  due  to  the 
animal's  alarm  at  its  new  position,  which  weakens  or  suspends  the 
action  of  its  heart,  the  movement  recommencing  again  after  the 
lapse  of  a  few  minutes,  although  no  change  has  been  made  in 
any  of  the  external  conditions.  But  if  the  movement  should  not 
renew  itself,  the  tape  which  passes  over  the  body  should  be  slackened  ; 
and  if  this  does  not  produce  the  desired  effect,  the  calico  envelope 
also  must  be  loosened.  When  everything  has  once  been  properly 
adjusted,  the  animal  will  often  lie  for  hours  without  moving,  or 
will  only  give  an  occasional  twitch  ;  and  even  this  may  be  avoided  by 
previously  subjecting  it  to  the  influence  of  ether  or  chloroform,  which 
may  be  renewed  from  time  to  time  whilst  it  is  under  observation. 


1056  VERTEBRATED   ANIMALS 

The  movement  of  the  blood  will  be  distinctly  seen  by  that  of  its 
corpuscles  (fig.  791),  which  course  after  one  another  through  the 
network  of  capillaries  that  intervenes  between  the  smallest  arteries 
and  the  smallest  veins ;  in  those  tubes  which  pass  most  directly 
from  the  veins  to  the  arteries  the  current  is  always  in  the  same 
direction ;  but  in  those  which  pass  across  between  these  it  may  not 
unfrequently  be  seen  that  the  direction  of  the  movement  changes 
from  time  to  time.  The  larger  vessels  with  which  the  capillaries  are 
seen  to  be  connected  are  almost  always  veins,  as  may  be  known 
from  the  direction  of  the  flow  of  blood  in  them  from  the  branches 
(b  b)  towards  their  trunks  (a) ;  the  arteries,  whose  ultimate  sub- 
divisions discharge  themselves  into  the  capillary  network,  are  for 
the  most  part  restricted  to  the  immediate  borders  of  the  toes.  When 
a  power  of  200  or  250  diameters  is  employed,  the  visible  area  is  of 
course  greatly  reduced  ;  but  the  individual  vessels  and  their  contents 
6  b 


FIG.  791.— Capillary  circulation  in  a  portion  of  the  web  of  a  frog's  foot : 
«,  trunk  of  vein ;  b,  b,  its  branches ;  c,  c,  pigment-cells. 

are  much  more  plainly  seen  :  and  it  may  then  be  observed  that  whilst 
the  '  red '  corpuscles  flow  at  a  very  rapid  rate  along  the  centre  of  each 
tube,  the  *  white '  corpuscles,  which  are  occasionally  discernible,  move 
slowly  in  the  clear  stream  near  its  margin. 

The  circulation  may  also  be  displayed  in  the  tongue  of  the  frog 
by  laying  the  animal  (previously  chloroformed)  on  its  back,  with  its 
head  close  to  the  hole  in  the  cork-plate,  and,  after  securing  the  body 
in  this  position,  drawing  out  the  tongue  with  the  forceps  and  fixing 
it  on  the  other  side  of  the  hole  with  pins.  So,  again,  the  circula- 
tion may  be  examined  in  the  lungs — where  it  affords  a  spectacle  of 
singular  beauty — or  in  the  mesentery  of  the  living  frog  by  laying 
open  its  body  and  drawing  forth  either  organ,  the  animal  having 
previously  been  made  insensible  by  chloroform.  The  tadpole  of  the 
frog,  when  sufficiently  young,  furnishes  a  good  display  of  the  capillary 
circulation  in  its  tail  ;  and  the  difficulty  of  keeping  it  quiet  during 


CIRCULATION   OF  BLOOD  1057 

the  observation  may  be  overcome  by  gradually  mixing  some  warm 
water  with  that  in  which  it  is  swimming  until  it  becomes  motion- 
less ;  this  usually  happens  wrhen  it  has  been  raised  to  a  temperature  of 
between  100°  and  110°  Fahr. ;  and,  notwithstanding  that  the  muscles 
of  the  body  are  thrown  into  a  state  of  spasmodic  rigidity  by  this 
treatment,  the  heart  continues  to  pulsate,  and  the  circulation  is 
maintained.1  The  larva  of  the  water-newt,  when  it  can  be  obtained, 
furnishes  a  most  beautiful  display  of  the  circulation,  both  in  its 
external  gills  and  in  its  delicate  feet.  It  may  be  inclosed  in  a  large 
aquatic  box  or  in  a  shallow  cell,  gentle  pressure  being  made  upon 
its  body,  so  as  to  confine  its  movements  without  stopping  the  heart's 
action.  The  circulation  may  also  be  seen  in  the  tails  of  small  fish, 
sucl i  as  the  minnoiv  or  the  stickleback,  by  confining  these  animals  in 
tubes,  or  in  shallow  cells,  or  in  a  large  aquatic  box  ;  but  although 
the  extreme  transparence  of  these  parts  adapts  them  well  for  this 
pin-pose  in  one  respect,  yet  the  comparative  scantiness  of  their 
blood-vessels  prevents  them  from  being  as  suitable  as  the  frog's  web 
in  another  not  less  important  particular.  One  of  the  most  beautiful 
of  all  displays  of  the  circulation,  however,  is  that  which  may  be  seen 
upon  the  yolk-bag  of  young  fish  (such  as  the  salmon  or  trout)  soon 
after  they  have  been  hatched  ;  and  as  it  is  their  habit  to  remain 
almost  entirely  motionless  at  this  stage  of  their  existence,  the  obser- 
vation can  be  made  with  the  greatest  facility  by  means  of  the 
zoophyte-trough.  The  store  of  yolk  which  the  yolk-bag  supplies 
for  the  nutrition  of  the  embryo  not  being  exhausted  in  the  fish  (as 
it  is  in  the  bird)  previously  to  the  hatching  of  the  egg,  this  bag 
hangs  down  from  the  belly  of  the  little  creature  on  its  emersion, 
and  continues  to  do  so  until  its  contents  have  been  absorbed  into 
the  body,  which  does  not  take  place  for  some  little  time  after- 
wards. And  the  blood  is  distributed  over  it  in  copious  streams, 
partly  that  it  may  draw  into  itself  fresh  nutritive  material,  and 
partly  that  it  may  be  subjected  to  the  aerating  influence  of  the 
surrounding  water. 

The  tadpole  serves,  moreover,  for  the  display,  under  proper 
management,  not  only  of  the  capillary,  but  of  the  general  circulation  ; 
and  if  this  be  studied  under  the  binocular  microscope,  the  observer 
not  only  enjoys  the  gratification  of  witnessing  a  most  wonderful 
spectacle,  but  may  also  obtain  a  more  accurate  notion  of  the  rela- 
tions of  the  different  parts  of  the  circulating  system  than  is  other- 
wise possible.  The  tadpole,  as  every  naturalist  is  aware,  is 
essentially  a  fish  in  the  early  period  of  its  existence,  breathing  by 
gills  alone,  and  having  its  circulating  apparatus  arranged  accord- 
ingly ;  but  as  its  limbs  are  developed,  and  its  tail  becomes  relatively 
shortened,  its  lungs  are  gradually  evolved  in  preparation  for  its 
terrestrial  life,  and  the  course  of  the  blood  is  considerably  changed. 
In  the  tadpole  as  it  comes  forth  from  the  egg  the  gills  are  external, 
forming  a  pair  of  fringes  hanging  at  the  sides  of  the  head  (fig.  792,  l) 
and  at  the  bases  of  these,  concealed  by  opercula  or  gill-flaps 

1  A  special  form  of  live-box  for  the  observation  of  living  tadpoles  &c.,  contrived 
by  Prof.  F.  E.  Schulze,  is  described  and  figured  iu  the  Quart.  Journ.  Microsc.  Sci. 
nls.  vol.  vii.  1867,  p.  261. 

o  Y 


1058 


VERTEBRATED   ANIMALS 


resembling  those  of  fishes,  are  seen  the  rudiments  of  the  internal 
gills,  which  soon  bagin  to  be  developed  in  the  stead  of  the  preceding. 


FIG.  792. — Circulation  in  the  tadpole. 

1.  Anterior  portion  of  young  tadpole,  showing  the  external  gills,  with  the  incipient 
tufts  of  the  internal  gills,  and  the  pair  of  minute  tubes  between  the  heart  and  the 
spirally  coiled  intestine,  which  are  the  rudiments  of  the  future  lungs. 
.  -.-  2.  More  advanced  tadpole,  in  which  the  external  gills  have  almost  disappeared  : 
a,  remnant  of  external  gills  on  the  left  side  ;  6,  operculum  ;  c,  remnant  of  external  gill 
on  the  right  side,  turned  in. 

8.  Advanced  tadpole,  showing  the  course  of  the  general  circulation :  a,  heart ; 
I  branchial  arteries ;  c,  pericardium ;  d,  internal  gill ;  e,  first  or  cephalic  trunk  ; 
/,' branch  to  lip ;  f/,  branches  to  head ;  h,  second  or  branchial  trunk;  i,  third  trunk, 
uniting  with  its  fellow  to  form  the  abdominal  aorta,  which  is  continued  as  the  caudal 
artery"  k,  to  the  extremity  of .  the  tail;  I,  caudal  vein;  m,  kidney;  n,  vena  cava ; 
o  liver  ;  p,  vena  portse  ;  q,  sinus  venosus,  receiving  the  jugular  vein,  r,  and  the  ab- 
dominal veins,  f,  u,  as  also  the  branchial  vein,  v. 

4  The  branchial  circulation  011  a  larger  scale  :  A,  B,  C,  three  primary  branches  of 
the  branchial  artery  ;  a.,  cartilaginous  arches  ;  b,  additional  framework  ;  c,  e,  twigs  of 
branchial  artery ;  d,  f,  rootlets  of  branchial  vein. 

5.  Origin  of  the  vessels  of  the  internal  gills,  #,  from  the  roots  of  those  of  the 
external. 

6.  The  heart,  systemic  arteries,  pulmonary  arteries  and  veins,  and  lungs,  in  the 
adult  frog,  the  heart  being  turned  up  in  the  right-hand  figure,  to  show  the  junction 
of  the  pulmonary  veins  and  their  entrance  into  the  left  auricle. 


CIRCULATION   IN  TADPOLE  1059 

The  external  gills  reach  their  highest  development  on  the  fourth  or 
fifth  day  after  emersion ;  and  they  then  wither  so  rapidly  (whilst 
being  at  the  same  time  drawn  in  by  the  growth  of  the  animal)  that 
by  the  end  of  the  first  week  only  a  remnant  of  the  right  gill  can  be 
seen  under  the  edge  of  the  operculum  (2,  c),  though  the  left  gill 
(b)  is  somewhat  later  in  its  disappearance.  Concurrently  with  this 
change  the  internal  gills  are  undergoing  rapid  development ;  and 
the  beautiful  arrangement  of  their  vascular  tufts,  which  originate 
from  the  roots  of  the  arteries  of  the  external  gills,  as  seen  at  g,  5,  is 
shown  in  4.  It  is  requisite  that  the  tadpole  subjected  to  obser- 
vation should  not  be  so  far  advanced  as  to  have  lost  its  early  trans- 
parence of  skin  ;  and  it  is  further  essential  to  the  tracing  out  of  the 
course  of  the  abdominal  vessels  that  the  creature  should  have  been 
kept  without  food  for  some  days,  so  that  the  intestine  may  empty 
itself.  This  starving  process  reduces  the  quantity  of  red  corpuscles, 
and  thus  renders  the  blood  paler ;  but  this,  although  it  makes  the 
smaller  branches  less  obvious,  brings  the  circulation  in  the  larger 
trunks  into  more  distinct  view.  '  Placing  the  tadpole  on  his  back,' 
says  Mr.  Whitney,  *  we  look,  as  through  a  pane  of  glass,  into  the 
chamber  of  the  chest.  Before  us  is  the  beating  heart,  a  bulbous- 
looking  cavity,  formed  of  the  most  delicate  transparent  tissues, 
through  which  are  seen  the  globules  of  the  blood,  perpetually,  but 
alternately,  entering  by  one  orifice  and  leaving  it  by  another.  The 
heart  (fig.  792,  3,  a)  appears  to  be  slung,  as  it  were,  between  two 
arms 'or  branches,  extending  right  and  left.  From  these  trunks  (b) 
the  main  arteries  arise.  The  heart  is  inclosed  within  an  envelope  or 
pericardium  (c),  which  is,  perhaps,  the  most  delicate,  and  is,  certainly, 
the  most  elegant  structure  in  the  creature's  organism.  Its  extreme 
fineness  makes  it  often  elude  the  eye  under  the  single  microscope, 
but  under  the  binocular  its  form  is  distinctly  revealed.  Then  it  is 
seen  as  a  canopy  or  tent,  inclosing  the  heart,  but  of  such  extreme 
tenuity  that  its  folds  are  really  the  means  by  which  its  existence  is 
recognised.  Passing  along  the  course  of  the  great  vessels  to  the 
right  and  left  of  the  heart,  the  eye  is  arrested  by  a  large  oval  body 
(cl)  of  a  more  complicated  structure  and  dazzling  appearance.  This 
is  the  internal  gill,  which  in  the  tadpole  is  a  cavity  formed  of  most 
delicate  transparent  tissue,  traversed  by  certain  arteries,  and  lined 
by  a  crimson  network  of  blood-vessels,  the  interlacing  of  which,  with 
their  rapid  currents  and  dancing  globules,  forms  one  of  the  most 
beautiful  and  dazzling  exhibitions  of  vascularity.'  Of  the  three 
arterial  trunks  which  arise  on  each  side  from  the  truncus  arteriosus, 
b,  the  first,  or  cephalic,  e,  is  distributed  entirely  to  the  head,  running 
first  along  the  upper  edge  of  the  gill,  and  giving  off  a  branch,/,  to 
the  thick  fringed  lip  which  surrounds  the  mouth ;  after  which  it 
suddenly  curves  upwards  and  backwards,  so  as  to  reach  the  upper 
surface  of  the  head,  where  it  dips  between  the  eye  and  the  brain. 
The  second  main  trunk,  A,  seems  to  be  chiefly  distributed  to  the  gill, 
although  it  freely  communicates  by  a  network  of  vessels  both  with 
the  first  or  cephalic  and  with  the  third  or  abdominal  trunk.  The 
latter  also  enters  the  gill  and  gives  off  branches  ;  but  it  continues 
its  course  as  a  large  trunk,  bending  downwards  and  curving  towards 

3Y2 


1060  VERTEBRATED   ANIMALS 

the  spine,  where  it  meets  its  fellow  to  form  the  abdominal  aorta,  ir 
which,  after  giving  off  branches  to  the  abdominal  viscera,  is  con- 
tinued as  the  caudal  artery,  k,  to  the  extremity  of  the  tail.  The- 
blood  is  returned  from  the  tail  by  the  caudal  vein,  I,  which  i> 
gradually  increased  in  size  by  its  successive  tributaries  as  it  passes 
towards  the  abdominal  cavity;  here  it  approaches  the  kidney,  in, 
and  sends  off  a  branch  which  incloses  that  organ  011  one  side,  while 
the  main  trunk  continues  its  course  on  the  other,  receiving  tributaries 
from  the  kidney  as  it  passes.  The  venous  blood  returned  from  the 
abdominal  viscera,  on  the  other  hand,  is  collected  into  a  trunk,  p. 
known  as  the  portal  vein,  which  distributes  it  through  the  substance 
of  the  liver,  o,  as  in  man  ;  and  after  traversing  that  organ  it  is  dis- 
charged by  numerous  fine  channels,  which  converge  towards  the 
great  abdominal  trunk,  or  vena  cava,  n,  as  it  passes  in  close  proximity 
to  the  liver,  onwards  to  the  sinus  venosus,  q,  or  rudimentary  auricle 
of  the  heart.  This  also  receives  the  jugular  vein,  r,  from  the  head, 
which  first,  however,  passes  downwards  in  front  of  the  gill  close  to 
its  inner  edge,  and  meets  a  vein  t,  coming  up  from  the  abdomen, 
after  which  it  turns  abruptly  in  the  direction  of  the  heart.  Two 
other  abdominal  veins,  u,  meet  and  pour  their  blood  direct  into  the 
sinus  venosus  ;  and  into  this  cavity  is  also  poured  the  aerated  blood 
returned  from  the  gill  by  the  branchial  vein,  v,  of  which  only  the 
one  on  the  right  side  can  be  distinguished.  The  lungs  may  be  de- 
tected in  a  rudimentary  state,  even  in  the  very  young  tadpole, 
being  in  that  stage  a  pair  of  minute  tubular  sacs,  united  at  the  upper- 
extremities,  and  lying  behind  the  intestine  and  close  to  the  spine. 
They  may  be  best  brought  into  view  by  immersing  the  tadpole  for  a 
few  days  in  a  weak  solution  of  chromic  acid,  which  renders  the 
tissue  friable,  so  that  the  parts  that  conceal  them  may  be  more 
readily  peeled  away.  Their  gradual  enlargement  may  be  traced 
during  the  period  of  the  tadpole's  transparence  ;  but  they  can  only 
be  brought  into  view  by  dissection  when  the  metamorphosis  lias 
been  completed.  The  following  are  Mr.  Whitney's  directions  for 
displaying  the  circulation  in  these  organs  :  '  Put  the  young  frog  into 
a  wineglass  and  drop  on  him  a  single  drop  of  chloroform.  This 
suffices  to  extinguish  sensibility.  Then  lay  him  on  the  back  on  a 
piece  of  cork  and  fix  him  with  small  pins  passed  through  the  web 
of  each  foot.  Remove  the  skin  of  the  abdomen  with  a  fine  pair  of 
sharp  scissors  and'  forceps.  Turn  aside  the  intestines  from  the  left 
side,  and  thus  expose  the  left  lung,  which  may  nowr  be  seen  as  a 
glistening  transparent  sac  containing  air-bubbles.  With  a  fine 
camel-hair  pencil  the  lung  may  now  be  turned  out,  so  as  to  enable 
the  operator  to  see  a  large  part  of  it  by  transmitted  light.  Unpin 
the  frog  and  place  him  on  a  slip  of  glass,  and  then  transmit  the 
light  through  the  everted  portion  of  lung.  Remember  that  the  lung 
is- very  elastic,  and  is  emptied  and  collapsed  by  very  slight  pressure. 
Therefore,  to  succeed  with  this  experiment,  the  lung  should  be 
touched  as  little  as  possible,  and  in  the  lightest  manner,  with  the 
brush.  If  the  heart  is  acting  feebly  you  will  see  simply  a  trans- 
parent sac,  shaped  according  to  the  quantity  of  air-bubbles  it  may 
happen  to  contain,  but  void  of  red  vascularity  and  circulation.  But 


INJECTED   PREPARATIONS 


IO6l 


•should  the  operator  succeed  in  getting  the  lung  well  placed,  full  of 
air,  and  have  the  heart  still  beating  vigorously,  he  will  see  before  him 
a  brilliant  picture  of  crimson  network,  alive  with  the  dance  and 
dazzle  of  blood-globules,  in  rapid  chase  of  one  another  through  the 
•delicate  and  living  lace-work  which  lines  the  chamber  of  the  lung.' 
The  position  of  the  lungs  in  relation  to  the  heart  and  the  great 
vascular  trunks  is  shown  in  fig.  792,  6. 

Injected  Preparations. — Next  to  the  circulation  of  the  blood  in 
the  living  body,  the  varied  distribution  of  the  capillaries  in  its 
several  organs,  as  shown  by 
means  of '  injections '  of  colour- 
ing matter  thrown  into  their 
principal  vessels,  is  one  of  the 
most  interesting  subjects  of 
microscopic  examination.  The 
.art  of  making  successful  pre- 
parations of  this  kind  is  one 
in  which  perfection  can  usually 
be  attained  only  by  long  prac- 
tice and  by  attention  to  a 
great  number  of  minute  par- 
ticulars ;  and  better  specimens 

may  be  obtained,  therefore,  FIG.  793.— Transverse  section  of  small  intes- 
from  those  who  have  made  it  tine  of  rat,  showing  the  villi  in  situ. 

a   business   to   produce  them 

than  are  likely  to  be  prepared  by  amateurs  for  themselves.  For 
this  reason  no  account  of  the  process  will  be  here  offered,  the  minute 
details  which  need  to  be  attended  to,  in  order  to  attain  successful 


FIG.  794. — Section  of  the  toe  of  a  mouse :  a,  a,  a,  tarsal  bones ;  6,  digital  artery ; 
c,  vascular  loops  in  the  papilla?  forming  the  thick  epidermic  cushion  on  the  under 
surface  ;  d,  distribution  of  vessels  in  the  matrix  of  the  claw. 

results,  being  readily  accessible  elsewhere  to  such  as  desire  to  put  it 
in  practice.1 

1  See  especially  the  article  'Injection'  in  the   Micrographic   Dictionary;    M, 


1062 


YEKTKBRATED   ANIMALS 


Many  anatomical  parts,  when  well  injected  and  mounted,  become 
objects  of  both  interest  and  instruction.  This  is  the  case  with  the 
villi  of  the  intestine,  seen  in  fig.  793,  which  presents  a  transverse 
section,  in  which  they  are  seen  in  situ.  A  thin  section  of  the  toe 
of  a  mouse  (fig.  794)  is  another  illustration  of  the  effectiveness  of 
this  mode  of  preparation. 

A  relation  may  generally  be  traced  between  the  disposition  of 
the  capillary  vessels  and  the  functions  they  subserve ;  but  that 
relation  is  obviously,  so  to  speak,  of  a  mechanical  kind,  the  arrange- 


FIG.  795. — Capillary  network 
around  fat-cells. 


FIG.  796. — Capillary  network  of 
muscle. 


ment  of  the  vessels  not  in  any  way  determining  the  function,  but 
merely  administering  to  it,  like  the  arrangement  of  water  or  gas 
pipes  in  a  manufactory.  Thus,  in  fig.  795,  we  see  that  the  capil- 
laries of  adipose  substance  are  disposed  in  a  network  with  rounded 
meshes,  so  as  to  distribute  the  blood  among  the  fat-cells ;  whilst  in 
fig.  796  we  see  the  meshes  enormously  elongated,  so  as  to  permit 
the  muscular  fibres  to  lie  in  them.  Again,  in  fig.  797,  we  observe 
the  disposition  of  the  capillaries  around  the  orifices  of  the  follicles 


FIG.  797. — Distribution  of  capil- 
laries in  mucous  membrane. 


FIG.  798. — Distribution  of  capil- 
laries in  skin  of  finger. 


of  a  mucous  membrane ;  whilst  in  fig.  798  we  see  the  looped 
arrangement  which  exists  in  the  papillary  surface  of  the  skin,  and 
which  is  subservient  to  the  nutrition  of  the  epidermis  and  to  the 
activity  of  the  sensory  nerves. 

In  110  part  of  the  circulating  apparatus,  however,  does  the 
disposition  of  the  capillaries  present  more  points  of  interest  than  it 
does  in  the  respiratory  organs.  In  bony  fishes  the  respiratory  surface 

Robin's  work,  Du  Microscope  et  des  Injections ;  Prof.  H.  Frey's  treatise,  Das  Mikro- 
skop  itnd  die  inikroskopische  Technik ;  Dr.  Beale's  How  to  ^vork  with  the  Micro- 
scope ;  the  Handbook  to  the  Physiological  Laboratory ;  and  Rutherford's  and 
Schafer's  treatises  on  Practical  Histology. 


RESPIRATORY   ORGANS 


1063 


is  formed  by  an  outward  extension  into  fringes  of  gills,  each  of  which 
consists  of  an  arch  with  straight  lamime  hanging  down  from  it,  and 
every  one  of  these  lamina?  (fig.  799)  is  furnished  with  a  double  row 
of  leaflets,  which  is  most  minutely  supplied  with  blood-vessels, 
their  network  (as  seen  at  A.) 
being  so  close  that  its  meshes 
(indicated  by  the  dots  in  the 
figure)  cover  less  space  than  the 
vessels  themselves.  The  gills  of 
fish  are  not  ciliated  on  their 
surface,  like  those  of  molluscs  » 
and  of  the  larva  of  the  water- 
newt,  the  necessity  for  such  a 
mode  of  renewing  the  fluid  in 
contact  with  them  being  super- 
seded by  the  muscular  apparatus 
with  which  their  gill- chamber  is 
furnished.  But  in  batrachians 
and  reptiles  the  respiratory  sur- 
face is  formed  by  the  walls  of 
an  internal  cavity,  that  of  the 
lungs :  these  organs,  however, 
are  constructed  on  a  plan  very 
different  from  that  which  they 
present  in  higher  Vertebrata, 
the  great  extension  of  surface 
which  is  effected  in  the  latter 
by  the  minute  subdivision  of 
the  cavity  not  being  here  neces- 
sary. In  the  frog  (for  example)  network  of  the  lamellae, 
the  cavity  of  each  lung  is  un- 
divided ;  its  walls,  which  are 
thin  and  membranous  at  the 
lower  part,  there  present  a 
simple  smooth  expanse ;  and  it 
is  only  at  the  upper  part,  where 
the  extensions  of  the  tracheal 
cartilage  form  a  network  over 
the  interior,  that  its  surface  is 
depressed  into  sacculi  whose 
lining  is  crowrded  with  blood- 
vessels (fig.  800).  In  this 
manner  a  set  of  air-cells  is 
formed  in  the  thickness  of  the 
upper  wall  of  the  lung,  which 
communicate  with  the  general 
cavity,  and  very  much  increase 

the  surface  over  whicli  the  blood  comes  into  relation  with  the  air  ; 
but  each  air-cell  has  a  capillary  network  of  its  own,  which  lies 
on  one  side  against  its  wall,  so  as  only  to  be  exposed  to  the  air 
on  its  free  surface.  In  the  elongated  lung  of  the  snake  the  same 


r.  799. — Two  branchial  processes  of  the 
gill  of  the  eel,  showing  the  branchial 
lamellae  :  A,  portion  of  one  of  these  pro- 
cesses enlarged,  showing  the  capillary 


FIG.  800. — Interior  of  upper  part  of 
lung  of  frcg. 


1064 


VERTEBRATED   ANIMALS 


general  arrangement  prevails  ;  but  the  cartilaginous  reticulation 
of  its  upper  part  projects  much  farther  into  the  cavity,  and  incloses 
in  its  meshes  (which  are  usually  square,  or  nearly  so)  several  layers 
of  air-cells,  which  communicate,  one  through  another,  with  the 
general  cavity.  The  structure  of  the  lungs  of  birds  presents  us  with 
an  arrangement  of  a  very  different  kind,  the  purpose  of  which  is  to 
expose  a  very  large  amount  of  capillary  surface  to  the  influence  of  the 
air.  The  entire  mass  of  each  lung  may  be  considered  as  subdivided 
into  an  immense  number  of '  lobules  '  or  'lunglets'  (fig.  801,  B),  each  of 


FIG.  801. — Interior  structure  of  lung  of  fowl,  as  displayed  by  a  section,  A 
passing  in  the  direction  of  a  bronchial  tube,  and  by  another  section  B 
cutting  it  across. 


FIG.  802. — Arrangement  of  the  capillaries  011  the  walls  of  the  air-cells  of 
the  human  lung. 

which  has  its  own  bronchial  tube  (or  subdivision  of  the  windpipe) 
and  its  own  system  of  blood-vessels,  which  have  very  little  com- 
munication with  those  of  other  lobules.  Each  lobule  has  a  central 
cavity,  which  closely  resembles  that  of  a  frog's  lung  in  miniature, 
having  its  walls  strengthened  by  a  network  of  cartilage  derived  from 
the  bronchial  tube,  A,  in  the  interspaces  of  which  are  openings  lead- 
ing to  sacculi  in  their  substance.  But  each  of  these  cavities  is  sur- 
rounded by  a  solid  plexus  of  blood-vessels,  which  does  not  seem  to  be 
covered  by  any  limiting  membrane,  but  which  admits  air  from  the 


LUNGS  1065 

central  cavity  freely  between  its  meshes  ;  and  thus  its  capillaries  are 
in  immediate  relation  with  air  on  all  sides — a  provision  that  is  ob- 
viously very  favourable  to  the  complete  and  rapid  aeration  of  the  blood 
they  contain.1  In  the  lung  of  man  and  mammals,  again,  the  plan  of 
structure  differs  from  the  foregoing,  though  the  general  effect  of  it  is 
the  same.  For  its  whole  interior  is  divided  up  into  minute  air-cells, 
which  freely  communicate  with  each  other,  and  with  the  ultimate 
ramifications  of  the  air-tubes  into  which  the  trachea  subdivides  ;  and 
the  network  of  blood-vessels  (fig.  802)  is  so  disposed  in  the  partitions 
between  these  cavities  that  the  blood  is  exposed  to  the  air  on  both 
sides.  It  has  been  calculated  tnat  the  number  of  these  air-cells 
grouped  around  the  termination  of  each  air-tube  in  man  is  not  less 
than  eighteen  thousand,  and  that  the  total  number  in  the  entire 
lung  is  six  hundred  millions. 

1  On  the  respiratory  organs  of  birds,  see  Campana,  La  Respiration  des  Oiseanx, 
Paris,  1875. 


io66 


CHAPTER     XXIII 

APPLICATION  OF   THE   MICROSCOPE   TO   GEOLOGICAL 
INVESTIGATION 

THE  utility  of  the  microscope  is  by  no  means  limited  to  the  deter- 
mination of  the  structure  and  actions  of  the  organised  beings  at 
present  living  on  the  surface  of  the  earth  ;  for  a  vast  amount  of 
information  is  afforded  by  its  means  to  the  geological  inquirer,  not 
only  with  regard  to  the  essential  nature  and  composition  of  the  rock- 
masses  of  which  its  crust  is  composed,  but  also  with  regard  to  the 
minute  characters  of  the  many  vegetable  and  animal  remains  that 
are  intombed  therein. 

The  systematic  employment  of  the  instrument  in  petrographies! 
research  dates  from  1858,  when  Dr.  H.  C.  Sorby,  F.R.S.,  published  his 
classical  paper  '  On  the  Microscopical  Structure  of  Crystals,  indicating 
the  Origin  of  Minerals  and  Rocks.' l  The  observations  in  this  paper 
were  based  upon  the  microscopical  examination  of  thin  sections  of 
rocks  and  minerals  ;  still,  although  Dr.  Sorby  was  the  first  to  apply 
this  manner  of  investigation  to  such  objects,  the  first  to  suggest  and 
arrange  the  method  of  preparing  thin  sections  appears  to  have  been 
William  Nicol.  A  description  of  his  method  is  given  by  H.  Witham 
(1831).2  Previous  to  1858  only  those  minerals  could  be  examined 
microscopically  which  possessed  the  necessary  degree  of  transparency, 
whilst  rocks  were  largely  closed  secrets.  Nevertheless  Cordier  (in 
1815)  was  able  to  determine  the  constituent  minerals  of  many  rocks 
by  the  study  of  the  powder  under  the  microscope  ;  a  procedure  which 
Fleurian  de  Bellevue  had  previously  recommended  in  1800,  and 
which  is  still  found  valuable  for  certain  purposes.  Seven  years  before 
Dr.  Sorby's  paper  appeared,  the  German  scholar  Oschatz  exhibited 
a  series  of  thin  sections  of  minerals  and  rocks  and  drew  attention 
to  their  important  bearing  upon  structural  studies,  but  the  collection 
was  regarded  more  as  a  curiosity  than  as  a  scientific  achievement.3 

That  paper,  however,  gave  an  enormous  impetus  to  geological 
research,  and  this,  in  the  hands  of  English  and  German  students, 
led  to  the  growth  of  a  *  micro-petrology.' 

In  order  to  examine  minerals  and  rocks,  sections  must  be  pre- 
pared thin  enough  to  permit  of  the  use  of  transmitted  light ;  for 

1  Quart.  Journ.  Geol.  Soc.  vol.  xiv.  1858,  pp.  453-500. 

2  Observations  on  Fossil  Vegetables,  Edinburgh  and  London,  1831. 

3  The  history  of  the  application  of  the  microscope  to  geology  has  been  sketched 
by  F.  Zirkel  in  his  paper  Die  Einfiilirung  des  Mikrosko2)S  in  das  mineralogiscli- 
geologische  Studium,  Leipzig,  1881. 


MICROSCOPIC   SECTIONS   OF  ROCKS  1067 

this  purpose  they  should  be  from  about  y^th  'to  r -^ Ttth  of  an  inch 
thick. 

A  chip  about  an  inch  square  is  struck  or  cut  off  the  specimen  to 
be  studied.  One  surface  of  this  is  then  ground  down  on  a  flat  cast- 
iron  plate  with  emery  and  Avater.  This  grinding  may  be  done  either  by 
hand  or  by  means  of  a  machine  specially  constructed  for  this  purpose 
(Chap.  VII).1  The  former  method  will  be  described  here.  When 
a  smooth  surface  is  at  last  obtained  the  specimen  is  well  washed  with 
water  and  then  polished  upon  a  slab  of  plate  glass  with  the  finest 
flour  emery  and  water.  When  all  inequalities  are  thus  removed  the 
fragment  is  again  well  cleansed  from  all  adhering  emery. 

The  next  process  is  to  cement  it  with  Canada  balsam  upon  a  slab 
of  glass  about  two  inches  square  and  about  an  eighth  of  an  inch  in 
thickness.  The  Canada  balsam  is  first  heated  over  a  spirit  lamp  in 
an  iron  spoon,  care  being  taken  not  to  allow  it  to  burn.  This  is  the 
most  difficult  part  of  the  whole  process,  and  only  experience  can  teach 
how  long  the  balsam  must  be  heated  in  order  to  possess,  on  cooling, 
the  necessary  hardness.  If  it  be  heated  too  long  it  will  crack  upon 
cooling.  The  right  point  appears  to  be  that  in  which  large  air-bubbles 
force  themselves  through  the  viscous  mass. 

A  small  quantity  of  the  warm  balsam  is  poured  upon  the  slab  of 
glass,  and  the  smooth  surface  of  the  rock-fragment,  being  pressed  into 
the  balsam,  is  held  down  upon  the  glass  till  the  balsam  hardens.  The 
slab  is  then  examined  from  its  under  side  to  see  that  no  air-bubbles 
have  been  included  between  the  glass  and  the  stone.  Should  they  be 
present  in  any  quantity,  the  whole  process  must  be  repeated.  When 
the  balsam  has  quite  hardened,  the  other  side  of  the  fragment  is 
ground  down  with  coarse  emery  and  water  on  the  iron  plate.  Upon 
the  section  commencing  to  become  transparent,  the  grinding  with  the 
coarse  emery  must  cease.  The  stone  is  then  thoroughly  cleansed 
with  water,  and  the  final  grinding  is  conducted  upon  the  plate-glass 
slab  with  flour  emery  and  water. 

The  slide  is  then  placed  under  a  stream  of  water  in  order  to 
remove  all  traces  of  the  emery  powder  from  the  minute  pores  of  the 
rock.  This  is  now  the  time  to  employ  chemical  tests  to  the  com- 
ponent minerals,  if  such  a  course  be  deemed  advisable.  If  the  rock 
is  of  a  fragile  nature,  it  is  well  to  mount  the  section  as  it  is  ;  but  in 
most  cases  it  is  possible  by  delicate  manipulation-  to  remove  it  to  a 
mounting  more  suited  to  optical  work.  This  transference  is  effected 

1  F.  G.  Cuttell  (61  Camden  Eoad,  N.W.),  T.  Eiley  (18  Burnfoot  Avenue, 
Fulham,  S.W.),  and  J.  Ehodes,  Museum  of  Geology,  Jermyn  Street,  S.W.,  prepare 
good  sections ;  and  the  principal  petrological  opticians  can  generally  recommend 
efficient  operators.  Voigt  and  Hochgesang  (Gb'ttingen,  Rothe  Str.  13)  and  R.  Fuess 
(Berlin,  S.W.,  108  Alte  Jacob  Str.)  do  also  most  excellent  work.  German  craftsmen 
are  more  skilful  in  overcoming  difficulties  (e.g.  with  soft  rocks)  than  English,  and 
can  make  thinner  slices.  Hence,  it  is  better  to  send  specimens  to  Germany  when 
thinness  is  desired  ;  but  when  the  size  of  the  slice  is  important,  to  have  the  work  done 
in  England.  In  a  very  thin  slice  the  colour  phenomena  are  less  conspicuous,  so 
that  reduction  in  thickness  beyond  a  certain  limit  is  not  all  gain ;  but  in  rocks  of 
an  opaque  character,  or  in  the  study  of  very  minute  structures,  it  is  hardly  possible 
to  err  on  the  side  of  thinness,  and  slices  '  made  in  Germany '  are  much  the  better. 
If  a  student  is  purchasing  ready  made  specimens  from  a  dealer,  he  will  find  the 
following  rough  test  useful.  Look  through  the  slice  at  a  window  with  a  clear  sky 
beyond  ;  it  is  too  thick  when  the  bar  cannot  be  distinctly  seen. 


1068       THE   MICROSCOPE   IN   GEOLOGICAL   INVESTIGATION 

by  the  application  of  a  gentle  heat  to  the  slab  until  the  balsam 
becomes  liquefied,  when  the  section  can  be  pushed  with  a  piece  of 
wire  on  to  a  suitable  slide  of  glass.  Obviously  a  drop  of  balsam 
should  be  poured  upon  the  latter  before  the  section  is  transferred. 
The  slide  is  then  warmed  until  the  balsam  becomes  liquid,  when  the 
superfluous  quantity  is  drawn  over  the  upper  surface  of  the  section. 
"When  the  section  is  completely  covered  with  the  balsam,  a  thin 
clean  cover-glass  is  held  for  a  moment  over  the  spirit  flame  and  laid 
upon  the  section.  Gentle  pressure  is  then  applied  to  the  surface  to 
bring  it  close  down  to  the  section  and  to  remove  all  air-bubbles. 
The  slide  is  then  allowed  to  become  quite  hard,  when  it  may  be 
cleansed  with  turpentine  or  alcohol  and  ether. 

Very  porous  rocks  must  first  be  treated  with  Canada  balsam,  in 
order  to  give  them  the  consistency  necessary  for  the  preparation  of 
thin  sections.  Isolated  mineral  grains  and  sands  can  be  mounted 
by  means  of  Canada  balsam  dissolved  in  chloroform.  The  slide  must 
not  be  heated,  but  evaporation  allowed  to  take  place.  Another 
method  is  described  by  Thoulet ;  1  whilst  very  soft  or  decomposed 
rocks  should  be  mounted  according  to  Wichmann's  proposal.2 

In  the  application  of  the  microscope  to  petrological  and  minera- 
logical  research  the  employment  of  polarised  light  is  constantly  re- 
quired, and  various  means  and  appliances  are  needful  for  its  most 
advantageous  application,  which  are  not  required  by  the  ordinary 
microscopist.  Considerable  pains  have  been  bestowed  by  both 
English  and  Continental  makers  to  fulfil  the  requirements,  and  good 
instruments  are  now  plentiful.3 

An  instrument  designed  by  Mr.  Allan  Dick  has  been  brought 
out  by  Messrs.  J.  Swift  and  Son.  As  this  combines  all  that  experi- 
ence has  led  petrologists  to  consider  desirable  for  mineralogical 
and  petrological  investigation,  a  brief  account  of  it  is  subjoined.  It 
is  specially  adapted  to  the  study  of  the  optical  properties  of  minerals 
generally,  and  particularly  to  that  of  the  thin  plates  of  minerals  seen 
in  ordinary  sections  of  rocks  prepared  for  microscopical  examination. 
The  microscope  is  shown  in  fig.  803,  but  since  the  engraving  was 
made  one  or  two  improvements  as  to  matters  of  detail  have  been 
introduced.4 

The  eyepiece  tube  is  slotted  at  E  to  receive  the  micrometer  scale 
(shown  detached  at  F),  and  to  the  tube  is  hinged  the  analyser  B'. 
which  is  capable  of  independent  rotation  in  the  usual  manner. 
Upon  the  eyepiece  tube  is  mounted  a  toothed  wheel,  which  gears 
into  another  toothed  wheel  mounted  on  one  end  of  a  rod  formed  of 
pinion  wire.  The  stage,  in  the  newest  forms,  is  fitted  with  a  scale 
of  rectangular  divisions  inserted  to  act  as  a  finder,  and  with  a  roller 
object-clip  (patented  by  the  makers)  in  place  of  the  usual  sliding  bar. 
Below  the  stage,  which  has  neither  sliding  nor  rotatory  movements. 

1  Annales  de  Chimie  et  de  Physique  (5),  xx.  pp.  362-482. 

2  Tschermak's  Miner  cdogische  und  Petrogr.  Mitt.  Bd.  v.  1882,  p.  83. 

5  Mr.  J.  Swift,  of  Tottenham  Court  Road,  Mr.  Watson,  of  Holborn,  London,  and 
Messrs.  Henry  Crouch,  Limited,  make  suitable  instruments.  Those  constructed  by 
Zeiss,  of  Jena ;  Nachet,  of  Paris ;  Voigt  and  Hochgesang,  of  Gb'ttingen ;  Fuess,  of 
Berlin;  and  Hartnack,  of  Potsdam,  can  also  be  recommended. 

4  The  instrument  is  protected  by  letters  patent. 


PETKOLOGICAL   MICROSCOPE 


1069 


is  mounted  the  polariser,  B.  capable  of  independent  rotation  like  the 
analyser,  and  upon  the  tube  of  the  polariser  is  mounted  a  toothed 


FIG*  803.— Swift's  petrological  microscope. 

wheel  of  the  same  size  as  that  upon  the  analyser ;  this  wheel  gears 
into  a  wheel  carried  by  a  tube  which  forms  a  telescopic  extension  of 


1070       THE   MICROSCOPE   IN   GEOLOGICAL   INVESTIGATION 

the  pinion  wire,  the  object  being  to  allow  of  the  raising  or  lowering 
of  the  body  of  the  microscope  for  focussing.  The  analyser  and  the 
polariser  may  thus  be  rotated  synchronously  without  disconnecting 
their  toothed  wheels.  The  polariser,  in  the  latest  form  of  the 
instrument,  is  mounted  on  a  crank  arm,  so  that,  if  not  required,  it 
may  be  thrown  out  of  the  axis  of  the  stand.  Now,  in  the  microscopes 
usually  constructed  for  petrological  work  the  rotation  of  a  small 
crystal  on  the  stage  between  the  polarising  and  the  analysing  prisms 
is  liable  to  put  •  it  out  of  position  in  regard  to  the  cross-threads  in 
the  eyepiece,  as  the  centring  of  the  objective  is  scarcely  ever  so 
perfect  as  not  to  produce  some  displacement ;  and,  if  the  centring 
be  adjusted  so  as  to  be  perfect  for  one  objective,  it  is  likely  to  be 
faulty  for  another.  (By  a  small  crystal  is  meant  a  crystal  under  the 
ToVoth  of  an  inch  in  diameter,  and  of  such  thickness  as  one  finds  at 
the  edges  of  petrological  sections.)  Hence,  by  the  arrangement 
described  above,  centring  is  dispensed  with,  and  the  object  is  made  to 
rotate  between  the  two  prisms  of  the  polarising  apparatus  without 
changing  its  position  beneath  the  objective.  To  a  petrol ogist  who 
is  accustomed  to  a  rotating  stage  and  fixed  cross-wires,  a  familial- 
section  appears  strange  when  first  looked  at  on  a  fixed  stage  with 
movable  cross- wires, -but  after  a  few  hours'  work  with  the  instrument 
the  feeling  of  strangeness  passes  and  that  of  the  solid  advantage  of 
a  perfect  centring  remains. 

On  the  polariser  tube,  above  the  toothed  wheel  and  below  the 
stage,  is  fitted  a  goniometer,  D,  which,  in  combination  with  crossed 
lines  in  the  eyepiece,  will  permit  of  the  measurement  of  the  angles  of 
crystals  without  necessitating  the  shifting  of  the  object  when  once 
adjusted  in  the  field.  0  is  a  set  screw  by  which  the  polarising 
apparatus  and  goniometer  may  be  fixed  in  any  desired  position. 
Both  the  analysing  and  polarising  prisms  are  divided  to  every  45°,  a 
spring  catch  marking  the  extinction  point.  The  opening  between 
the  upper  lens  of  the  eyepiece  and  the  analysing  prism  B;  (fig.  803) 
is  for  the  purpose  of  placing  such  plates  as  the  ^-undulation  plate  K 
in  position. 

The  great  value  of  the  instrument  is  in  the  facility  with  which 
studies  in  convergent  light  can  be  performed.  G  is  a  slide  fitted 
with  a  double  convex  lens  which  may  be  used  for  showing  the 
optical  figures  of  crystals,  and  H  is  a  similar  slide  carrying  a  lens 
and  a  diaphragm  of  small  aperture  used  for  showing  optical  pictures 
in  minute  crystals.  The  polariser  is  fitted  with  two  convergent  lenses, 
which  work  in  conjunction  with  the  lens  A.  on  the  slide  of  the  stage, 
when  great  convergence  is  required.  This  slide  may  be  pushed  in 
without  disturbing  the  object  upon  the  stage.  The  achromatic  con- 
denser, A,  shown  at  the  foot  of  the  figure,  also  works  in  conjunction 
with  the  sliding  lens,  A,  when  the  highest  angular  aperture  is  required.1 

1  In  the  latest  made  instruments  a  new  achromatic  convergent  system  is  intro- 
duced over  the  polariser.  It  gives  a  N.A..  of  TOO,  and  an  aplanatic  cone  0'92.  When 
used  as  an  immersion  condenser,  these  are  increased  respectively  to  T12  and  1'05. 
It  is  fitted  with  an  iris  diaphragm  placed  above  the  polarising  prism.  A  milled 
collar  actuates  the  focussing  of  the  lower  portion  of  the  condenser.  The  fine  adjust- 
ment is  the  differential- screw  form,  which  is  sufficiently  delicate  and  accurate  to 
determine  the  refractive  index  of  minerals  by  the  difference  between  the  focus -taken 


CORRODED   CRYSTALS  1071 

When  convergent  light  is  required  the  slide  on  the  stage  and 
either  C4  or  H  are  pushed  in,  and  the  eyepiece  covered  with  the 
analyser  B'.  The  optical  figures  of  the  crystal  then  appear  with 
almost  ideal  clearness.  If  this  simple  method  is  compared  with  that 
previously  in  use,  the  superiority  of  the  instrument  will  be  im- 
mediately recognised.  It  is  in  fact  the  most  perfect  petrological 
microscope  yet  issued,  and  is  one  which  will  suit  equally  the  minera- 
logical  and  petrological  student. 

The  microscopical  investigation  of  rock  sections  has  almost  re- 
volutionised petrology.  Although  the  geologist  has  no  difficulty  in 
determining  by  his  unaided  eye  wtth  the  use  of  simple  chemical  tests 
the  mineral  components  of  rocks  of  coarse  texture,  the  case  is 
different  with  those  of  extremely  fine  grain  ;  still  more  with  such  as 
present  an  apparently  homogeneous,  compact,  or  glassy  character. 
The  study  reveals  facts  of  the  most  striking  significance,  and  wel- 
come light  has  been  thrown  upon  the  question  of  the  order  and 
method  of  formation  of  rock  constituents.1 

The  material  which  issues  from  a  volcano  during  an  eruption 
is  rarely  in  a  state  of  complete  fusion.  In  most  cases  it  contains 
crystals  and  parts  of  crystals  which  have  formed  before  the  arrival 
of  the  fluid  mass  at  the  surface  of  the  earth.  Such  crystals  are 
usually  of  large  size  and  can  generally  be  recognised  with  the  naked 
eye.  But  sometimes  these  have  undergone  other  changes  before  the 
final  consolidation  of  the  rock.  They  may  have  been  formed  under  high 
pressure,  for  the  pressure  lowers  the  melting-point  of  most  substances. 
Accordingly,  as  the  pressure  is  relieved  upon  the  lava  getting  at  or 
near  the  surface,  the  crystals  which  are  floating  in  the  fused  mass 
at  the  time  are  liable  to  become  corroded  or  redissolved.  Again,  some 
subterranean  change  may  produce  a  distinct  rise  in  the  temperature 
of  the  mass,  or  an  access  of  heated  water  may  increase  the  solvent 
power  of  the  molten  portion.  Instances  of  corrosion  from  one  or 
more  of  these  causes  are  numerous.  The  quartzes  of  the  quartz- 
through  the  substance  and  its  outside  measure,  the  milled  head  being  divided  to  50, 
and  each  division  equalling  one  thousandth  of  a  millimetre.  A  wheel  of  small  aper- 
tures is  fitted  to  the  upper  Bertrand  lens  of  the  microscope  for  the  purpose  of  show- 
ing optical  pictures  in  minute  crystals  of  various  sizes. 

1  The  reader  is  referred  to  the  following  works  treating  of  the  microscopical  charac- 
ters of  minerals  and  rocks  : — F.  Fouque*  et  Michel  Le*vy,  Mineralogie  micrographique, 
Paris,  1878 ;  E.  Hussak,  Anleitung  zumBestimmendergesteinsbildenden  Mineralien, 
Leipzig,  1885;  E.  Kalkowsky,  Elements  der  Lithologie,  Heidelberg,  1886;  A.  V. 
Lasaulx,  Elemente  der  Petrographie,  Bonn,  1875,  and  Einfiihrung  in  die  Gtsteins- 
lehre,  Breslau,  1886  (also  edition  in  French)  ;  Levy  et  Lacroix,  Les  Mineraux  des 
Bodies,  Paris,  1888  ;  F.  H.  Eosenbusch,  Mikroskopische  Physiographic,  2nd  edition, 
vol.  i.  '  Die  Mineralien '  (translated  into  English  by  Iddings),  vol.  ii.  '  Die  massigen 
Gesteine ; '  Hulfstabellen  zur  mikroskopischen  Mineralbestimmung  in  Gesteinen 
(translated  into  English  by  F.  H.  Hatch) ;  and  Elemente  der  Gesteinlehre,  1898 ; 
F.  Rutley,  The  Studij  of  Rocks,  3rd  edition,  1884,  and  Bock-forming  Minerals,  1888  ; 
J.  J.  H.  Teall,  British  Petrography,  1888 ;  F.  Zirkel,  Lehrbuch  der  Petrographie, 
2  vols.  2nd  edition,  1893  ;  Basalt  gesteine,  Bonn,  1870  ;  Die  mikroskopische  Beschaf- 
fenheit  der  Mineralien  und  Gesteine,  Leipzig,  1873 ;  Microscopical  Petrography 
(U.S.  Geol.  Exploration  of  40th  parallel),  Washington,  1876 ;  A.  Harker,  Petrology 
for  Students,  1895  (1st  edition).  The  English  student  will  find  much  valuable  infor- 
mation and  useful  directions  in  G.  A.  J.  Cole's  Aids  to  Practical  Geology.  But  the 
literature  is  now  so  voluminous  that  it  is  practically  impossible  to  give  anything  like 
a  complete  list ;  for  important  papers  will  be  found  in  almost  every  periodical  deal- 
ing with  geology,  among  which  those  published  in  the  United  States  must  not  be 
forgotten. 


10/2       THE   MICROSCOPE   IN   GEOLOGICAL  INVESTIGATION 


porphyries  have  this  corroded  appearance  ;  whilst  the  porphyritic 
constituents  of  the  basic  rocks  (hornblende,  olivine,  &c.)  not  in- 
frequently show  the  same  alteration  (vide  fig.  804 ;  the  dotted  line 
marks  the  original  outline).  In  the  case  of  the  hornblende  the 
dissolved  portions  usually  give  rise  to  the  formation  of  small  grains 
of  augite  and  magnetite,  which  are  then  found  encircling  the 
'  mother-crystal.'  Biotite  is  somewhat  similarly  affected,  and  some- 
times the  whole  crystal  in  either  mineral  may  be  rendered  almost 
opaque  by  the  separation  of  minute  grains  of  magnetite. 

The  movement  of  the  igneous  mass  may  cause  fracture  of  the 
crystals  owing  to  strain  or  to  mutual  pressure.  The  pieces  of  such 
broken  crystals  may  often  be  found  in  one  and  the  same  section, 
sometimes  at  no  great  distance  from  each  other.  As  the  magma 
solidifies,  a  further  development  of  crystals  occurs.  The  products  of 
this  period  constitute  the  '  ground-mass  '  of  the  rock  and  are  usually 
small  in  size,  the  microscope  being  frequently  required  for  their 
detection  and  determination. 

A  glass  is  sometimes  produced  in  the  last  stage  of  consolidation, 


FIG.  804. — Corroded  olivine  in  basalt 
of  Kilimanjaro,  East  Africa. 


FIG.  805.— Microlites.   (After  Zirkel.) 


and  appears  as  a  base  or  *  setting'  to  the  previously  formed  minerals. 
This,  however,  is  usually  studded  by  minute  mineral  products  endea- 
vouring to  crystallise  under  unfavourable  circumstances.  Generally 
speaking,  these  products  are  present  in  tw.o  stages  of  development. 
The  less  perfectly  developed  forms  of  these  are  known  as  crystallites. 
They  occur  in  a  variety  of  forms — hair-like,  spherical,  &c. — and  the 
smaller  forms  appear  to  be  optically  inactive.  In  some  instances, 
such  as  those  termed  '  globulites,'  they  may  be  minute  segrega- 
tions of  a  glassy  nature  ;  in  others  crystalline  aggregates,  in  which 
from  the  extreme  minuteness  of  the  constituents  and  their  mutual 
interference  the  usual  tests  fail ;  in  other  cases  they  may  be  desig- 
nated embryonic  crystals. 

The  bodies  belonging  to  the  higher  stage  of  development  are  called 
microlites  or  microliths  (fig.  805).  They  differ  from  the  crystallites 
in  possessing  the  internal  structure  of  true  crystals  and  in  acting  on 
polarised  light.  The  position  of  the  microlites  with  reference  to 
each  other  or  to  the  large  crystals  is  frequently  an  indicator  of  the 
movements  of  the  original  fluid  mass.  When  streams  of  microlites 
are  seen  lying  with  their  long  axes  in  one  direction,  this  direction  is 


STRUCTURES   OF   CRYSTALS 


1073 


equivalent  to  that  of  the  flow,  and  where  such  streams  encounter 
large  crystals  they  sweep  round  them  in  graceful  curves  :  this 
appearance  in  a  rock  is  known  as  fluxion-structure. 

In  certain  glassy  rocks  microlites  are  collected  into  more  or 
less  spherical  masses,  exhibiting  a  radial  structure,  called  spheru- 
lites ;  commonly  these  are  not  bigger  than  a  pea,  but  sometimes 
they  are  one  or  two  inches  in  diameter  ;  they  are  then  less  regular 
in  shape  and  structure  and  are  often  named  for  distinction  pyro- 
merides.  Chemical  analysis  often  shows  that  they  differ  slightly 
in  composition  from  the  base.  Crystalline  rocks  also  sometimes 
exhibit  a  similar  structure,  e.g.  the  orbicular  diorite  of  Corsica. 
A  spherulitic  structure  can  be  produced  in  a  compact  rock  by  subse- 
quent heating,  short  of  melting,  and  many  glassy  rocks  in  lapse  of 
time  become  *  devitrified '  by  setting  up  an  obscure  confused  crys- 
talline structure.1 

Masses  of  molten  material  may,  however,  consolidate  at  a  con- 
siderable depth  beneath  the  surface  of  the  earth  ;  in  such  cases  the 
distinction  between  the  first  and  second  periods 
of  crystallisation  is  not  generally  so  well 
marked. 

A  crystal  is,  in  one  respect,  like  an 
organism — it  is  affected  by  its  environment. 
The  crystal  modifies  its  surroundings,  and 
is  in  turn  modified  by  them  ;  there  is  action 
and  reaction  between  it  and  its  environment. 
This  remarkable  property  of  all  crystalline 
bodies  is  well  shown  by  the  microscope. 
Crystals  are  constantly  found  built  up  of 
different  layers  or  zones  of  material  slightly 
unlike  in  their  optical  characters,  and  thus 
dissimilar  in  chemical  constitution.  This  is 

the  so-called  zonal  structure,  and  is  common  in  the  felspars  and 
augites — in  short,  in  nearly  all  minerals  which  admit  of  isomor- 
phic  replacement  in  their  constituents  (fig.  806).  Its  presence  in  the 
case  of  the  augites  is  often  indicated  by  a  difference  in  colour.  This 
structure  may  be  experimentally  produced  by  placing  an  artificial 
crystal  in  a  solution  of  a  substance  isomorphic  with  that  of  the 
crystal. 

The  microscope  has  rendered  another  great  service,  inasmuch  as 
it  has  enabled  the  petrologist  to  draw  conclusions  as  to  the  physical 
condition  of  the  fused  mass  or  magma  at  the  time  crystallisation 
commenced.  All  chemists  are  aware  that  when  crystals  are  deposited 
from  solutions  at  ordinary  temperatures  they  usually  contain  small 
cavities  full  of  the  mother -liquor.  Now,  the  growth  of  crystals  in 
igneous  rocks  is  exactly  analogous  to  that  in  a  supersaturated  saline 
solution.  Portions  of  the  fused  mass  become  entangled,  which  on 
cooling  remain  in  a  glassy  condition,  or  '  become  stony,  so  as  to 
produce  what  may  be  called  glass-  or  stone-cavities.'  2  When  formed 

1  This  subject  is  discussed  in  Quart.    Journ.    Geol.  Soc.   1885    (Presidential 
address). 

2  Sorby,  Quart.  Journ.  Geol.  Soc.  1858,  p.  242. 

3  z 


Zirkel. 


1074       THE   MICKOSCOPE    IN  GEOLOGICAL  INVESTIGATION 

under  great  pressure  by  the  combined  influence  of  liquid  water  and 
fused  mineral  matter  the  crystals  will  contain  glass-cavities  and  also 
fluid  inclusions. 

Glass-inclusions  are  very  abundant  in  the  porphyritic  crystals 
of  volcanic  rocks,  and  represent  to  some  extent  the  composition  of 
the  fused  mass  at  the  period  of  inclosure.  The  glass  composing  the 
inclusions  is  often  darker  in  colour  than  the  glass  forming  the  base 
of  the  rock.  This  is  perhaps  due  to  the  presence  in  the  glass  of 
the  inclusions  of  a  greater  amount  of  iron  and  the  bases  usually 
associated  with  it.  The  glass  often  contains  crystallites  and  micro- 
lites,  due  sometimes  to  inclosure  at  the  same  time,  sometimes  to  a 
subsequent  crystallising  action  set  up  by  the  glass.  Gas  bubbles 
are  also  inclosed. 

The  existence  of  fluid  inclusions  in  crystals  has  long  been  kno\vn  ; 
but  not  until  Dr.  Sorby  directed  his  attention  to  the  subject  was 
their  universal  distribution  in  rock- constituents  imagined,  or  their 
bearing  upon  geological  problems  recognised.  They  are  often  very 
minute,  being  frequently  less  than  ro^oijth  of  an  inch  in  diameter. 
They  are  rare  or  absent  in  rocks  of  the  volcanic  group,  but  are 
especially  characteristic  of  the  plutonic  rocks,  such  as  granite, 
gabbro,  diorite,  &c.  Where  glass-inclusions  are  common,  fluid 
inclusions  are  rare  or  wanting. 

The  forms  of  such  inclusions  vary,  but  sometimes  they  are 
bounded  by  planes  corresponding  to  the  external  faces  of  the  crystals, 
in  which  case  they  are  termed  '  negative  '  crystals. 

Sometimes  the  fluid  inclusions  are  so  numerous  in  the  quartzes 
of  the  granites  as  to  be,  according  to  Dr.  Sorby,1  '  not  above  the 
j^yoth  of  an  inch  apart.  This  agrees  with  the  proportion  of  a 
thousand  millions  to  a  cubic  inch,  and  in  some  cases  they  must  U- 
more  than  ten  times  as  many.' 

An  intimate  relation  usually  exists  between  the  number  of 
cavities  in  a  crystal  and  the  rate  at  which  it  was  formed. 
Generally  speaking,  it  may  be  said  that  the  more  rapid  the  growth, 
the  more  numerous  the  inclusions. 

Not  infrequently  the  cavities  contain  bubbles  varying  from 
r^iRFoth  *o  s^jy^oth  °f  an  incn  in  size.  These  bubbles  sometimes 
possess  an  apparently  spontaneous  movement,  at  other  times  heat 
must  be  applied  to  produce  a  change  of  position. 

According  to  Dr.  Sorby's  experiments,  the  bubbles  arise  in  con- 
sequence of  the  contraction  of  the  liquid  on  cooling  from  the  high 
temperature  at  which  the  cavities  were  filled. 

The  nature  of  the  inclosed  fluid  has  been  determined  with  some 
accuracy.  Generally  the  liquid  is  a  solution  of  water  charged  with 
salts ;  but  occasionally  it  is  sufficiently  concentrated  to  cause  the  de- 
position in  the  cavities  of  little  cubes  of  salt.  The  presence  has  also 
been  established  of  liquid  carbonic  dioxide,  the  bubble  of  which  dis- 
appeared at  about  32°  C.,  the  critical  point  for  this  gas.2 

The  discovery  in   the  mineral  components  of  plutonic  rocks  of 

1  Sorby,  Quart.  Journ.  Geol.-  Soc.  1858,  p.  486. 

2  The  application  of  the  burning  end  of  a  cigar  to  the  section  is  usually  sufficient 
to  cause  the  bubble  to  disappear. 


INCLUSIONS   IN  MINERALS  1075 

these  fluid  inclusions  is  manifestly  of  high  importance.  Daubree's 
experiments  have  shown  the  enormous  mineral -forming  powers 
possessed  by  greatly  heated  water,  while  the  presence  of  liquid  carbonic- 
dioxide  testifies  to  the  enormous  pressure  under  which  plutonic 
rocks,  such  as  granite  and  diorite,  have  consolidated. 

Inclusions  of  gaseous  matter  are  also  common  ;  and  it  is  self- 
evident  that  the  occurrence  of  one  mineral  in  another  is  no  rarity ; 
the  included  mineral  being  generally  the  older.  To  such  microscopic 
inclusions  of  crystalline  bodies  is  due  the  remarkable  colour  of  some 
minerals.  In  fact,  so  numerous  and  so  minute  are  the  inclusions  in 
some  minerals  that  even  with  high  powers  the  minerals  appear  to 
be  charged  with  the  finest  dust.  Leucite  sometimes  affords  a  good 
instance  of  this  (fig.  807).  Not  unfrequently,  as  with  it,  the  included 
microlites  are  so  arranged  as  to  outline  a  crystal  of  the  mineral. 

The  foregoing  allows  us  to  conclude  that  an  absolutely  pure 
mineral  is  exceptional.  All  such  mineral  bodies  contain  inclosures 
of  foreign  matter  which  have  become  entangled  during  their  forma- 
tion ;  when  they  contain  glass-inclusions  they  have  been  precipitated 
out  of  a  mass  in  the  condition  of  igneous  fusion.  It  follows,  therefore, 
that  the  presence  of  amorphous  glass,  either  as 
a  glassy  residue  or  as  glass-inclusions,  is  a 
frequent  characteristic  of  igneous  rocks.  Still, 
the  absence  of  such  material  does  not  always 
demonstrate  a  non -igneous  origin,  for  plutonic 
rocks,  such  as  granite,  do  not  possess  this  feature, 
having  become  solid  under  circumstances  which  FlG  go?.— Leucite  from 
brought  about  complete  crystallisation  of  the  Kilimanjaro,  East 
materials.  Glass-inclusions  are  certainly  re-  Africa, 
ported  by  Sigmund l  to  be  present  in  the  quartzes 
of  the  granites  of  the  Monte  Mulatto,  near  Predazzo,  in  South 
Tyrol,  but  V.  Chrustschoff  considers  them  products  of  contact- 
metamorphism. 

We  have  dealt  hitherto  more  especially  with  igneous  masses,  but 
the  sedimentary  rocks  demand  some  attention. 

The  microscope  enables  us  to  recognise  to  some  extent  the  sources 
whence  the  materials  composing  clastic2  rocks  were  derived.  For 
instance,  the  presence  of  quartzes  containing  numerous  fluid  inclusions 
(especially  those  of  carbonic  dioxide)  and  hair-like  crystals  of  rutile 
leads  us  to  conclude  they  are  derived  from  granites  or  similar  rocks. 
The  cemented  material  can  also  be  studied  and  its  nature  determined. 
In  certain  loose  sands  and  sandstones  there  has  sometimes  occurred  a 
curious  process  which  the  microscope  first  brought  under  notice. 
This  is  the  precipitation  on  the  outer  surface  of  rounded  quartz- 
grains  of  a  greater  or  less  amount  of  silica,  which  has  been  deposited 
in  crystalline  continuity  with  that  of  the  original  nuclei  (fig.  808). 
The  phenomenon  is  like  that  which  happens  when  an  irregular  frag- 
ment of  a  crystal  is  placed  in  a  concentrated  solution  of  the  same 

1  '  Petrographische  Studien  am  Granit  von  Predazzo,'  Jahrb.  Jc.  k.  geol.  Reiclis- 
anstalt,  Bd.  xxix.  1879,  pp.  305-316. 

3  Greek  K\a(Trbs  =  broken.     See  on  this  subject  T.  G.  Boiiney,  Presidential  ad- 
dress to  Section  C,  Brit.  Assoc.  Reports  (Birmingham),  1886. 

3x2 


1076       THE   MICEOSCOPE   IN   GEOLOGICAL  INVESTIGATION 

salt  slowly  evaporating.  Restoration  of  the  broken  angles  first  takes 
place ;  then  deposition  goes  011  over  the  whole  exposed  surface,  in 
perfect  optical  and  crystalline  continuity,  so  as  to  change  a  broken 
fragment  into  a  definite  crystal.  A  similar  process  frequently  takes 
place  in  limestones  which  are  not  absolutely  pure.1  Sometimes  this 
secondary  deposit  is  carried  so  far  on  the  grains  of  a  clean  sandstone 
that  the  interstices  are  completely  filled  up  and  the  rock  is  converted 
into  a  quartzite. 

By  the  microscopical  examination  of  volcanic  dust  or  ashes  it  is 
possible  to  determine  the  constitution  of  the  igneous  mass  whose 
eruption  gave  rise  to  such  material.  Thus  the  ashes  and  dust  which 
fell  at  various  places  after  the  great  Krakatoa  eruption  in  1883  were 
found  to  belong  to  an  acid  lava,  a  pyroxene  andesite.2 

Further,  glacial  boulders  can  be  satisfactorily  identified  with  rocks 
in  situ  by  a  microscopical  examination  of  their  thin  sections.     Thus 
Norwegian   rocks  have   been  shown  to   occur   as  boulders   in   the 
Eastern  Counties,  while  Swedish  and  Finnish 
rocks   are   common   in   the   drift   of  North 
Germany  and  Saxony. 

We  now  come  to  the  discussion  of  the 
metamorphism  to  which  all  rock-masses  are 
liable.  The  metamorphism  caused  by  atmo- 
spheric agencies  results  in  decomposition  and 
disintegration.  The  constituents  are,  of 
course,  very  differently  affected,  but  rapidity 
of  disintegration  demands  the  decomposition 
of  one  of  the  principal  constituents.  Such  a 
eonstituentisfelspar^hichdecoinposesxmder 
posited  on  the  surface  the  influence  of  water  charged  with  carbonic 
(After  Dr.  Sorby.)  add  into  kaolin  ;  while  the  products  of  the 

decomposition  of  non-aluminous  minerals  arc 

carbonates,  ferric  oxide,  and  quartz.  The  minute  accessory  con- 
stituents, such  as  the  titanium  oxides,  are  not  affected  by  these 
agencies,  and  hence  are  to  be  found  in  all  clays  and  sands.3  At 
greater  depths  from  the  surface  disintegration  is  replaced  by  the 
formation  of  new,  especially  hydrous,  minerals.  Thus  serpentine 
is  formed  from  olivine,  and  sometimes  from  suitable  varieties  of 
augite  or  hornblende  ;  chlorite  from  biotite ;  epidote  from  suitable 
minerals,  and  so  on. 

Thermal  waters  charged  with  various  substances  are  common  in 
all  volcanic  districts  and  play  their  part  in  the  metamorphosis  of 
rocks.  In  this  way  a  volcanic  rock  may  become  silicified  through 
the  percolation  of  such  solutions;  and  microscopical  examination  has 

1  E.  Wethered,  Quart.  Journ.  Geol.  Soc.  xlviii.  (1892),  p.  377. 

2  See  J.  Murray  and  A.  Renard  on  '  Volcanic  Ashes  and  Cosmic  Dust '  in  Nature, 
1884,  vol.  xxix.  p.   585 ;  also  J.  W.  Judd,  Krakatoa  Report,  published  by  the  Royal 
Society. 

5  W.  M.  Hutchins,  however,  is  of  opinion  that  rutile  is  produced  as  a 
secondary  mineral  in  certain  slates,  though  he  would  not  dispute  its  occurrence  as 
stated  above  (Geol.  Mag.  1890,  p.  264).  A  series  of  papers  bearing  on  the  subject 
which  he  has  published  since  that  date  in  the  same  periodical  are  all  worthy  of 
careful  study. 


METAMORPHLSM   OF  ROCKS  1077 

shown  that  in  portions  of  the  Roche  Castle  rock,  in  Pembrokeshire, 
the  porphyritic  felspars  have  been  replaced  by  quartz.  The 
tourmaline,  gilbertite,  and  other  minerals  often  found  at  or  near 
the  junction  of  granite  and  sedimentaries  (e.g.  in  parts  of  Cornwall 
and  Devon)  are  probably  results  of  hydrothermal  metamorphism,  and 
in  this  way  many  metallic  ores  may  be  deposited  ;  while  the  conver- 
sion of  peridotites  into  serpentines,  sandstones  into  quartzites  (not 
to  mention  other  instances),  are  results  of  the  action  of  water, 
probably  with  some  slight  increase  of  pressure  and  temperature. 

The  intrusion  of  an  igneous  Jrock  generally  has  an  important 
influence  on  the  structure  and  mineralogical  composition  of  the 
surrounding  mass,  portions  of  which  it  can  include  and  partially 
dissolve  (contact-metamorphism).  Sections  from  the  junction  of  an 
igneous  rock  with  one  of  sedimentary  origin  are  highly  interesting. 
The  metamorphism  is  found  to  consist  largely  in  the  development  of 
new  minerals,  such  as  chiastolite,  andalusite,  brown  and  white  mica, 
garnets,  staurolite,  etc. ;  the  first  and  third  of  these  appear  to  form  most 
readily,  andalusite  after  a  time  replacing  chiastolite ;  while  the  last 
three  require  high  temperatures.  Gradually  the  original  sedi- 
mentary structure  disappears  from  a  rock  affected  by  contact- 
metamorphism,  and  one  truly  crystalline  is  set  up,  which,  however, 
has  characters  of  its  own.1  Limestone  becomes  crystalline,  fossils 
disappearing,  and  minerals  such  as  wollastonite,  idocrase,  &c.,  are 
formed  from  impurities.  Occasionally  the  heat  is  so  intense  as  to 
fuse  at  least  the  matrix  of  sandstones  into  a  brownish  glass. 

The  microscope  has  also  proved  most  useful  in  studying  questions 
relating  to  dynamic  metamorphism,  or  that  due  to  '  earth-stresses.' 
The  deformation  by  movement  has  sometimes  been  so  great  as  to 
obliterate,  partially  or  even  wholly,  the  original  structure  of  a  rock.2 
The  intense  pressures  must  produce  some  elevation  of  tempera- 
ture and  increase  the  solvent  action  of  water,  so  that  the  original 
constituents  of  the  rock  are  destroyed,  partially,  if  not  wholly,  and 
at  a  later  stage  new  minerals  are  produced.  It  has  been  shown  that 
many  gneisses  and  schists  (though  not  all)  have  been  formed  by 
crushing  or  shearing  from  igneous  rock,  e.g.  gneiss  from  granite, 
hornblende  schist  from  dolerite.  In  the  former  case,  the  crushing 
of  the  felspar,  the  formation  of  white  mica  and  free  quartz  from 
its  dust,3  the  effects  produced  on  the  other  minerals,  can  all  be 
studied  under  the  microscope  ;  and  in  the  latter  the  conversion 
of  augite  into  hornblende.  This,  however,  maybe  brought  about  by 
more  than  one  cause,  and  each  probably  produces  effects  which  can  be 
distinguished.  These  questions,  however,  on  which  many  experienced 
petrologists  have  been  engaged  for  at  least  fifteen  years,  are  much 
too  difficult  and  technical  to  be  discussed  in  a  book  of  this  character ; 
enough  to  say  that  heat,  pressure,  and  water,  singly  and  conjointly, 
produce  important  changes  in  rocks,  many  of  which  can  now  be 
identified. 

1  Bonney,  Quart.  Journ.  Geol.  Soc.  xliv.  (1888),  p.  11. 

2  Tresca,  '  Flow  of  Solids,'  Proc.  Inst.  Mech.  Eng.  1878,  p.  301. 

3  A   minute   hydrous   mica,   often  called  sericite,  seems  to  form  readily  in  an 
argillaceous   rock   under  pressure.      The   silky-looking  slates  (to  which  the  name 
phyllite  is  restricted  by  some  authors)  are  largely  composed  of  it. 


1078      THE   MICROSCOPE   IN   GEOLOGICAL  INVESTIGATION 

The  optical  methods  now  in  use  enable  the  petrologist  to  determine 
the  constituents  of  rock-masses  with  great  success.  The  colour 
of  the  mineral  in  transmitted  light,  the  crystallographic  outlines, 
the  direction  of  the  cleavage  planes,  the  polarisation  tints,  the  posi- 
tion of  the  axes  of  elasticity,  as  also  of  the  optical  axes,  all  these, 
with  other  minor  properties,  render  his  determinations  of  real  value. 
In  certain  cases  pleochroism  is  a  valuable  test ;  this  is  well  deve- 
loped in  such  minerals  as  hornblende,  biotite,  tourmaline,  etc. 

Yery  important  service  has  been  rendered  by  the  microscope  in 
the  study  of  the  phenomena  known  as  optical  anomalies.  There 
exist  a  large  number  of  minerals  which  show  in  thin  sections  optical 
properties  which  do  not  agree  with  those  of  the  crystal  system  to 
which  they  belong.  Experiment  has  proved  that  compression, 
strain,  or  other  mechanical  distortion,  may  cause  amorphous  bodies, 
like  glass,  and  crystals  belonging  to  the  regular  system  to  become 
double-refracting,  and  a  uniaxial  crystal  becomes  biaxial  by  the  appli- 
cation of  pressure  at  right  angles  to  its  optical  axis. 

Mention  may  well  be  made  here  of 
the  anomalies  presented  by  the  mineral 
leucite,  which  is  a  most  important  con- 
stituent of  the  lavas  of  Vesuvius  and 
the  neighbourhood  of  Rome.  It  crystal- 
lises apparently  in  icositetrahedra  (fig. 
809),  and  thus  to  belong  to  the  regular 
system  it  should  remain  dark  under 
crossed  nicols,  that  is,  be  isotropic.  The 
small  crystals  certainly  behave  in  this 
manner,  but  the  large  ones  display  more 
or  less  double  refraction  with  decided 
FIG.  809.— Leucite  showing  twin-  traces  of  twin-lamellae  (fig.  809).  This 

striation  under  crossed  nicols.    anomaly  was  for  a  lon~  time  inexplicable, 
(After  Zirkel.)  , .,,     T^/  .         ,  ,  ,    & 

till    Klein  showed  l  that  such    crystals 

revert  when  heated  to  500°  C.  to  a  condition  of  perfect  isotropv. 
which  property  they  again  lose  upon  becoming  cool.  The  conclusion 
to  be  drawn  from  his  classical  investigation  is  that  the  leucite 
originally  crystallised  in  the  regular  system  and  that  its  present 
optical  condition  is  owing  to  molecular  change  due  to  strains  set  up 
as  the  temperature  falls  during  and  after  solidification.  It  is 
worthy  of  notice  that  MM.  Fouque  and  Michel  Levy  have  syn- 
thetically produced  a  leucite  rock,  the  leucites  of  which  possessed 
the  optical  anomalies  described  above. 

The  relation  between  optical  characters  and  chemical  constitu- 
tion has  received  some  degree  of  attention,  and  in  the  case  of  the 
felspar  group  has  been  accurately  determined .  Only  the  '  quantitative ' 
portion  of  the  subject  can  be  dealt  with  here,  and  we  must  abstain 
from  the  discussion  of  those  minerals  whose  microscopical  appearance 
leads  the  trained  petrologist  to  draw  qualitative  conclusions.  By 
employing  convergent  light,  a  slice  of  a  mineral,  cut  in  the  right 
direction,  can  be  examined  and  an  'optical  picture'  obtained. 

1  For  a  description  of  the  so-called  '  Erhitzungs-Mikroskop,'  see  Groth's  Pliysi- 
kalische  Krystallographie,  Leipzig,  1885,  p.  631. 


OPTICAL   EXAMINATION   OF  MINEEALS  1079 

Inferences  may  be  drawn  from  the  presence  or  absence  of  this  on  the 
surface  of  easiest  cleavage  in  a  flake.  In  a  slice  from  a  rock  the 
minerals  may  be  cut  in  any  direction,  and  are  often  too  small  for 
proper  study  ;  nevertheless  important  inferences  may  be  drawn  from 
the  shadows  seen  to  sweep  over  them  as  the  stage  is  rotated  between 
crossed  nicols.1  Even  if  only  parallel  rays  be  used,  with  the  ordinary 
apparatus,  minerals  often  may  be  identified  with  practical  certainty 
from  their  optical  characters.  Minerals  of  the  regular  system,  like 
colloids,  being  isotropic,2  produce  no  effect  on  the  polarised  rays,  and 
thus  remain  dark  between  crossed  ^nicols.  So  do  all  slices  cut  from 
a  uniaxial  mineral  perpendicular  to  the  principal  axis  (that  of 
symmetry),  for  they  are  isotropic  to  light  passing  in  that  direction. 
The  same  property  exists  in  all  biaxial  minerals  in  two  directions 
(called  the  optic  axes).  But  in  passing  through  slices  cut  in  any 
other  directions  from  doubly  refracting  minerals,  the  polarised  ray  is 
divided  into  two  rays,  vibrating  in  directions  perpendicular  to  each 
other  and  coincident  with  three  lines  called  the  axes  of  elasticity, 
i.e.  the  directions  of  greatest,  least,  and  mean  elasticity.  When  the 
slice  is  turned  into  such  a  position  that  two  of  these  correspond 
with  the  vibration  planes  of  the  crossed  nicols,  it  becomes  dark.  If 
extinction  (of  light)  occurs  parallel  with  the  trace  of  a  pinacoid  or 
prism  face  (or  with  a  corresponding  cleavage  plane)  in  a  section 
through  the  vertical  axis,  or  with  the  trace  of  the  former  in  a  section 
perpendicular  to  it,  this  is  called  *  straight  extinction,'  but  if  not,  it 
is  said  to  be  oblique.  Thus  in  a  uniaxial  crystal  every  slice  cut 
parallel  with  the  principal  axis  gives  straight  extinction.  In  the 
orthorhombic  system,  the  axes  of  elasticity  correspond  with  the 
crystallographic  axes,  so  minerals  belonging  to  it  also  extinguish 
straight.  In  the  monoclinic  system  the  orthodiagonal  axis  is  an  axis 
of  elasticity,  hence  the  extinction  angle  is  at  a  maximum  in  clino- 
diagonal  sections,  and  is  zero  in  the  zone  containing  the  ortho-  and 
basal  pinacoids.  In  the  triclinic  system  there  is  no  relation  between 
the  two  sets  of  axes.  Of  this  system,  however,  oscillatory  twinning, 
producing  alternately  banded  colours,  is  a  frequent  characteristic. 
Measurements  of  the  extinction  angle  are  of  much  value  for  dis- 
tinctive purposes.  Thus  a  rhombic  pyroxene  can  at  once  be  dis- 
tinguished from  a  monoclinic  by  its  straight  extinction.3  Again  the 
maximum  extinction  angle  in  a  hornblende  falls  short  of  20°  ;  in  an 
augite  it  may  exceed  40°.  The  magnitude  of  this  angle  is  affected 
by  changes  in  the  chemical  composition  of  a  mineral :  for  instance,  it 
is  very  small  in  soda-hornblendes,  such  as  glaucophane  and  riebeckite. 
It  varies  in  the  felspar  group,  and  is  very  useful  in  distinguishing 
the  several  species.4  But  as  the  minerals  in  a  rock-section  seldom 
chance  to  lie  in  the  right  positions  for  accurate  measurement,  better 

1  See  for  a  full  account  of  this,  with  illustrations,  F.  Fouque  and  M.  Levy,  Minera- 
logic  Micro  graphique,  1879,  pp.  101-3.  Also  F.  Rutley,  Hock-forming  Minerals,  p.  84. 

2  That  is,  having  the  ether  equally  elastic  in  all  directions. 

5  Obviously,  more  than  one  observation  is  needed,  because,  as  intimated  above,  a 
monoclinic  mineral,  if  cut  in  certain  directions,  also  gives  straight  extinction. 

4  Levy,  Determination  des  Felspaths  (1894)  p.  31.  Summaries  of  results  will  be 
found  in  Rutley,  Rock-forming  Minerals,  pp.  204,  221,  and  Cole,  Aids  in  Practical 
Geology  (see  '  Felspar'  for  the  references). 


1080    THE   MICKOSCOPE   IN  GEOLOGICAL  INVESTIGATION 

results  are  generally  obtained  by  crushing  up  a  small  fragment  of 
the  rock  itself  and  mounting  a  few  selected  flakes,  which  can  readily 
be  arranged  for  examination.  Indeed,  the  study  of  a  little  powdered 
rock  is  often  valuable  as  an  adjunct  to  that  of  a  section,  and  when  we 
have  some  special  purpose  in  view,  or  specimens  do  not  promise  to  be 
interesting,  it  may  even  obviate  the  necessity  of  cutting  slices. 

The  researches  of  the  late  Max  Schuster  have  established  the  im- 
portant fact  that  in  the  normal  plagioclase  felspars,  which  may  be 
considered  as  isomorphous  mixtures  of  albite  (Na2(Al2)Si6O16)  and 
anorthite  (Ca(Al2)Si2O8),  the  optical  and  chemical  characters  stand 
in  the  closest  possible  relations  to  each  other.  Hence,  given  the 
extinction  angle  on  a  known  surface,  the  chemical  constitution  is 
known  and,  roughly  speaking,  the  specific  gravity. 

Another  optical  test  of  importance  is  the  refractive  index  of  a 
mineral.  The  methods  of  measuring  this  are  described  in  most  of 
the  larger  text-books,  but  much — often  enough  for  all  practical  pur- 
poses— can  be  done  in  a  rough  and  ready  way.  For  instance, 
minerals  with  a  high  refractive  index,  such  as  diamond,  garnet, 
zircon,  appear  to  stand  out  conspicuously  on  the  slide.  When  they 
occur  in  sand  or  the  powder  of  a  rock  this  is  even  more  marked,  and 
internal  reflection  due  to  the  large  critical  angle  gives  to  the  grain 
a  strong  dark  outline.  Again,  if  a  mineral  with  a  high  refractive 
index  be  in  apposition  (as  in  a  slice  from  a  rock)  with  another  having 
a  lower  one,  or  with  Canada  balsam,  and  a  quarter-inch  objective  be 
used  (with  a  plane  reflector)  and  focussed  on  the  top  of  the  first 
mineral,  a  thin  bright  line  is  seen  just  within  its  edge  ;  but  when 
the  focus  is  changed  to  the  bottom,  this  appears  without  the  edge. 

The  importance  of  pleochroism  has  been  already  mentioned.  It 
is  not  seen  in  colourless  minerals,  or  in  slices  so  cut  as  to  be  isotropic 
in  the  plane  at  right  angles  to  the  path  of  the  transmitted  beam. 
In  augite  it  is  generally  weak,  though  visible  in  some  green 
varieties ;  but  in  hornblende  strong,  especially  in  certain  varieties. 
Glaucophane  exhibits  a  violet  blue  and  a  reddish  purple ;  riebeckite 
turns  almost  black  ;  biotite,  chlorite,  amblystegite,  and  tourmaline 
show  it  well,  but  in  iolite  it  can  be  seen  only  in  thick  slices.  The 
student  should  note  the  results  as  the  polarised  beam  vibrates  parallel 
with  each  axis  of  elasticity ;  these  facts,  however,  as  a  rule,  are  more 
important  to  the  petrographer  than  to  the  petrologist,  and  the  latter 
will  not  find  it  worth  his  while  to  spend  time  in  determining  them. 

The  polarisation  tints  of  a  mineral,  i.e.  those  seen  with  crossed 
nicols,  depend  to  some  extent  on  the  thickness  of  the  slices,  as  has 
been  already  stated,  but  they  are  often  variable  even  in  the  same  mineral . 
Hence,  though,  as  a  rule,  the  student  will  find  each  species  gives  a 
certain  group  of  tints  in  the  order  of  the  chromatic  scale,  he  must 
be  prepared  for  abnormalities.  For  instance,  quartz,  when  it  occurs 
in  a  granite,  usually  gives  high  tints,  but  in  a  trachyte  they  are 
rather  low.  At  first  the  student  must  be  cautious  in  drawing 
inferences  from  polarisation  tints,  but  after  a  certain  amount  of 
practice  he  may  do  this  with  more  confidence,  though  he  will  rely  more 
on  the  'quality'  than  on  the  'quantity'  of  the  colour.  For  in- 
stance, though  both  augite  and  olivine  usually  afford  rich  colours,  an 


EXAMINATION   OF    MINERALS  IOSl 

experienced  eye  can  generally  tell  the  difference,  for  the  latter 
appears  more  diaphanous  than  the  former.  In  petrology,  as  in 
medicine,  a  cautious  empiricism,  which  signifies  experience  concen- 
trated and  regulated  by  common  sense,  is  sometimes  even  more 
valuable  than  any  amount  of  printed  rules. 

On  this  account  the  student  may  be  glad  to  have  a  few  general 
directions  as  to  the  best  method  of  studying  a  rock  slice.  First, 
look  at  it  with  a  rather  strong  pocket  lens,  especially  if  it  be  crystal- 
line or  fragmented,  so  as  to  get  a  good  idea  of  its  general  structure, 
which  is  sometimes  less  easily  seei>  under  the  microscope,  because  the 
field  of  view  at  any  one  time  is  small,  and  high  magnification  may 
make  it  '  hard  to  see  the  wood  for  the  trees.'  Then  place  it  on  the 
stage  and  examine  first  with  transmitted,  next  with  reflected  light. 
The  former  shows  what  minerals  are  colourless,  and  the  natural 
tints  of  the  coloured,  bringing  out  well  slight  differences  of  structure, 
especially  any  due  to  incipient  decomposition.1  The  latter  enables 
him  to  distinguish  the  opaque  minerals,  e.g.  pyrite  from  magnetite, 
sometimes  the  latter  from  other  iron  oxides ;  to  identify  native  iron, 
awaruite,  and  gold  ;  perhaps  also  graphite,  but  it  is  better  to  verify 
the  last  by  powdering  a  little  of  the  rock,  when  the  streak  is  easily  ob- 
tained. Sometimes  we  are  helped  in  distinguishing  even  transparent 
minerals  by  the  different  way  in  which  they  reflect  light.  Next, 
put  on  the  polariser  and  examine  pleochroism ;  and  lastly,  insert  the 
analyser,  for  the  general  study  of  the  tints  produced  and  especially 
of  the  extinction  angles  of  certain  of  the  minerals.  When  a  mineral 
gives  very  low  polarisation  tints,  especially  in  the  case  of  certain 
aggregates,  or  we  are  searching  for  a  glassy  base  in  a  slice  crowded 
with  microliths,  we  may  be  helped  by  inserting  a  selenite  or  quartz 
plate  (better  just  below  the  slide)  to  obtain  a  coloured  field,2  for  the 
eye  can  be  more  sure  of  a  difference  of  tint  than  of  a  very  faint 
glimmer  of  light. 

In  dealing  with  rocks  apparently  clastic  we  have  to  determine 
whether  the  structure  is  original,  or  has  been  superinduced  (by  crushing 
or  shearing) ;  also  what  amount  of  mineral  change  has  subsequently 
occurred,  and  of  what  this  is  significant — investigations  which, 
though  of  the  highest  interest,  are  often  by  no  means  easy,  so  that 
the  most  experienced  worker  may  occasionally  be  baffled.  One  final 
piece  of  advice :  before  adopting  a  conclusion,  look  at  it  all  round, 
to  see  how  it  fits  in  with  previously  acquired  knowledge  and  the 
probabilities  in  the  particular  case. 

The  micro-spectroscope  has  not  at  present  been  so  much  used 
by  petrologists  as  it  might  have  been.  It  has  been  employed 
by  Professor  Orville  Derby  in  the  determination  of  the  pre- 
sence of  monazite  in  Brazilian  sands.3  This  mineral  contains  a 
large  percentage  of  didymium,  and  accordingly  gives  the  bands 

1  Holes  in  the  slice  and  bubbles  in  the  balsam,  which  often  perplex  beginners, 
are  now  most  readily  detected.     Also  a  mineral  of  easy  cleavage  is  sometimes  slightly 
ruptured  in  the  grinding,  producing  diffraction  tints  (as  in  calcite).     These,  between 
crossed  nicols,  might  be  mistaken  for  oscillatory  twinning ;  but  at  the  present  stage 
their  true  nature  is  obvious. 

2  This  method  can  also  be  used  to  enhance  a  weak  pleochroism. 

3  American  Journal  of  Science,  vol.  xxxvii.  1889,  p.  109. 


1082       THE   MICROSCOPE   IN   GEOLOGICAL   INVESTIGATION 

characteristic  of  that  element.  The  test  afforded  by  studying  the 
colour  of  the  flame  when  a  small  fragment  is  acted  upon  by  the  blow- 
pipe is  often  valuable — but  this,  of  course,  hardly  forms  part  of 
microscopy.1 

The  discovery  of  the  presence  of  foreign  inclusions  in  all  minerals 
has  led  to  a  remarkable  revolution  in  mineral-chemistry.  In  earlier 
days  it  was  customary  to  analyse  a  mineral  without  questioning  its 
purity.  Hence  the  early  analyses  and  the  formulae  developed  there- 
from express  the  actual  constitution  plus  the  inclusions.  Methods 
have  now  been  invented  by  which  the  foreign  matter  can  be  removed. 
Advantage  is  taken  of  the  difference  that  is  usual  between  the 
specific  gravity  of  the  mineral  and  that  of  its  inclusions,  the  so-called 
*  heavy  solutions  '  being  employed  for  the  separation.2  Most  satis- 
factory results  have  been  obtained  by  such  means.  In  cases  where 
the  greatest  accuracy  is  necessary,  the  apparatus  designed  by  Dr. 
P.  Mann  had  better  be  employed.3  It  is  well  microscopically  to 
examine  the  isolated  substance  before  executing  the  analysis,  for 
the  optical  test  with  polarised  light  is  so  sensitive  as  to  detect  the 
smallest  impurities.  Also,  in  the  case  of  ordinary  bulk  analyses  of 
rocks,  it  is  advisable  to  follow  the  same  course,  as  by  doing  so  one 
is  often  enabled  to  make  a  qualitative  analysis  with  the  microscope 
alone. 

A  valuable  adjunct  to  petrology  is  to  be  found  in  micro- 
chemistry.4  Instances  sometimes  occur  where  a  mineral  cannot 
be  satisfactorily  determined  by  its  optical  characters,  and  in  such 
cases  micro- chemical  methods  are  resorted  to.  Let  us  suppose  it  is 
desirable  to  see  whether  any  of  the  rock -components  are  silicates 
containing  soda  and  soluble  in  acids.  The  cover-glass  is  accordingly 
removed  and  the  balsam  dissolved  in  alcohol.  A  weak  solution  of 
hydrochloric  acid  is  then  poured  over  the  surface,  when,  if  soluble 
silicates  are  present,  gelatinisation  will  take  place.  Upon  allowing 
the  gelatinous  mass  to  evaporate  little  squares  of  salt  will  form  if 
such  a  silicate  is  present.  Sometimes  colouring  substances  may  be 
used  for  the  same  purpose.  By  the  treatment  of  a  slide  with  nitric 
acid  a  silicate  like  nepheline  becomes  porous  and  permeable  to 
anilin  blue,  fuchsin,  &c.  In  the  case  of  nepheline  the  colouring 
matter  cannot  be  washed  out,  and  hence  *  staining '  proves  a  delicate 
test. 

Where  such  a  course  is  possible,  minute  pieces  of  the  question- 
able minerals  should  be  isolated  and  treated  singly.  There  are  two 

1  It  was  suggested  by  Professor  Szabo  and  is  well  described  in  G.  A.  J.  Cole,  Aids 
in  Practical  Geology,  Part  ii.  ch.  viii. 

2  For  their  mode  of  preparation  see  Rosenbusch,  Mikroskopische  Physiographic, 
p.  206  et  seq.     (English  edition  by  Iddings.) 

5  Neues  Jahrbuchfiir  Mineralogie,  &c.  Bd.  ii.  1884,  p.  172. 

4  The  following  works  can  be  consulted  on  this  subject:— E.  Boricky,  Elemente 
einerneuenchemisch-mikroskopischen  Mineral-  und  Gesteinsanalyse  ,Prague,  1877 ; 
T.  H.  Behrens,  Mikrochemische  Methoden  zur  Miner alanalyse,  Amsterdam,  1881 ; 
Haushofer,  Mikroskopische  Beactionen,  Braunschweig,  1885  ;  Klement  et  Renard, 
Reactions  microchimigues  a  cristaux,  &c.,  Bruxelles,  1886  ;  Rosenbusch,  Mikro- 
skopische Phyeiographie\  vol.  i.  1885,  pp.  195-238  (English  edition  by  Iddings) ; 
F.  Rutley,  Rock-forming  Minerals,  London,  1888.  A  useful  summary  of  a  number 
of  microchemical  investigations  is  given  by  C.  A.  McMahon,  Mineralog.  Magazine, 
vol.  x.  p.  79. 


PALEONTOLOGY  1083 

methods  in  use  for  testing  such  particles  micro-chemically.  The 
first  is  that  proposed  by  Boricky,  who  employed  pure  hydro-fluo- 
silicic  acid  (H2SiF6),  which  attacks  almost  all  rock-forming  minerals. 
The  mineral  particle  is  placed  upon  a  glass  object-holder  protected 
from  the  action  of  the  acid  by  a  covering  of  Canada  balsam,  and  the 
acid  allowed  to  attack  the  mineral.  After  evaporation  an  examina- 
tion under  the  microscope  reveals  the  presence  of  delicate  crystals 
of  the  silico-fluorides  of  the  metals  present  in  the  mineral.  The 
nature  of  the  crystals  may  then  be  determined  microscopically. 

The  second  method  is  that  proposed  by  Behrens,  and  mostly 
follows  the  usual  method  of  chemical  analysis.  The  isolated  particle 
is  heated  in  a  small  platinum  crucible  with  ammonium  fluoride,  the 
mass  then  evaporated  with  sulphuric  acid  and  dissolved  in  hot  water. 
A  small  quantity  of  the  solution  is  then  evaporated  and  examined. 
If  calcium  is  present  in  the  mineral  small  crystals  of  gypsum  will 
form.  Other  quantities  are  treated  writh  the  ordinary  reagents. 
The  crystalline  products,  which  are  the  result,  can  be  identified  by 
optical  methods.  It  is  possible  by  Behrens's  tests  to  detect  the 
presence  of  O0005  mgr.  CaO  in  a  grain. 

In  all  cases  it  is  advisable  to  protect  the  objective  during  the 
microscopical  examination  with  a  thin  sheet  of  white  mica. 

The  microscope  has  always  played  an  important  part  in  the 
science  of  Palaeontology.  The  great  work  on  '  Micro-geology,' 
published  in  1855  by  Professor  Ehrenberg,  testifies  to  the  influence 
it  had,  even  at  that  period,  upon  research  of  this  nature. 

The  result  of  the  microscopic  examination  of  lignite  or  fossilised 
wood  and  of  ordinary  coal  is  a  good  example  of  the  value  of  the 
instrument  in  this  interesting  department.  Specimens  of  fossil 
wood  in  a  state  of  more  or  less  complete  preservation  are  found  in 
numerous  strata  of  very  different  ages.  Generally  speaking,  it  is 
only  when  the  wood  is  found  to  have  been  penetrated  by  silica  that 
its  organic  structure  is  well  preserved  ;  but  instances  occur  every  now 
and  then  in  which  penetration  by  carbonate  of  lime  has  proved  equally 
favourable.  In  either  case  transparent  sections  are  needed  for  the 
full  display  of  the  organisation.  Occasionally,  however,  it  has  hap- 
pened that  the  infiltration  has  filled  the  cavities  of  the  cells  and 
vessels,  without  consolidating  their  walls ;  and  as  the  latter  have 
undergone  decay  without  being  replaced  by  any  cementing  material, 
the  lignite,  thus  composed  of  the  internal '  casts '  of  the  woody  tissues, 
is  very  friable,  its  fibres  separating  from  each  other  like  those  of 
asbestos  ;  and  laminae  split  asunder  with  a  knife,  or  isolated  fibres 
separated  by  rubbing  down  between  the  fingers,  exhibit  the  characters 
of  the  woody  structure  extremely  well  when  mounted  in  Canada 
balsam.  Generally  speaking,  the  lignites  of  the  Tertiary  strata 
present  a  tolerably  close  resemblance  to  the  woods  of  the  existing 
period :  thus  the  ordinary  structure  of  dicotyledonous  and  monocotyle- 
donous  stems  may  be  discovered  in  such  lignites  in  the  utmost 
perfection  ;  and  the  peculiar  modification  presented  by  coniferous 
wood  is  also  most  distinctly  exhibited.  As  we  go  back,  however, 
through  the  strata  to  the  Secondary  period,  we  more  and  more  rarely 
meet  with  the  ordinary  dicotyledonous  structure  ;  and  the  lignites  of 


1084       THE   MICEOSCOPE   IN   GEOLOGICAL  INVESTIGATION 

the  earliest  deposits  of  these  series  are,  almost  universally,  either 
gymnosperms  l  or  palms. 

Descending  into  the  palaeozoic  series,  we  are  presented  in  the 
vast  coal  formations  of  our  own  and  other  countries  with  an  extra- 
ordinary proof  of  the  prevalence  of  a  most  luxuriant  vegetation  in  a 
comparatively  early  period  of  the  world's  history.  The  determina- 
tion of  the  characters  of  the  Ferns,  Sigillarice,  Lepidodendra,  Cala- 
mites,  and  other  kinds  of  vegetation  whose  forms  are  preserved  in 
the  shales  or  sandstones  that  are  interposed  between  the  strata  of 
coal,  has  been  hitherto  chiefly  based  on  their  external  characters  ; 
since  it  is  seldom  that  these  specimens  present  any  such  traces 
of  minute  internal  structure  as  can  be  subjected  to  microscopic 
elucidation.  But  persevering  search  has  brought  to  light  numerous 
examples  of  coal-plants  whose  internal  structure  is  sufficiently  well 
preserved  to  allow  of  its  being  studied  microscopically ;  and  the 
careful  researches  of  Professor  W.  C.  Williamson  have  shown  that 
they  formed  a  series  of  connecting  links  between  Cryptogainia 
and  flowering  plants,  being  obviously  allied  to  Equiselacece,  Lyco- 
podiacece,  &c.,  in  the  character  of  their  fructification,  whilst  their 
stem-structure  foreshadowed  both  the  '  endogenous  '  and  '  exogenous ' 
types  of  the  latter.2  Notwithstanding  the  general  absence  of  any 
definite  form  in  the  masses  of  decomposed  vegetable  matter  of  which 
coal  itself  consists,  the  traces  of  structure  revealed  by  the  microscope 
are  often  sufficient — especially  in  the  ordinary  '  bituminous '  coal — 
not  only  to  determine  its  vegetable  origin,  but  in  some  cases  to 
justify  the  botanist  in  assigning  the  character  of  the  vegetation 
from  which  it  must  have  been  derived ;  and  even  where  the  stems 
and  leaves  are  represented  by  nothing  else  than  a  structureless  mass 
of  black  carbonaceous  matter,  there  are  found  diffused  through  this 
a  multitude  of  minute  resinoid  yellowish-brown  granules,  which  are 
sometimes  aggregated  in  clusters  and  inclosed  in  sacculi ;  and  these 
may  now  be  pretty  certainly  affirmed  to  represent  the  spores,  while 
the  sacculi  represent  the  sporangia,  of  gigantic  Lycopodiacece  of  the 
Carboniferous  flora.3 

Lime-secreting  alga?  are  now  known  to  have  often  played  an 
important  part  in  the  formation  of  calcareous  rocks.  Those 
organisms  called  coccoliths  and  rhabdoliths,  which  though  so 
minute  are  important  constituents  in  chalk  and  some  other  lime- 
stones, are  referred  to  these  plants  (?  to  the  class  Floridece),  and  a 
tiny  tubular  organism  named  Girvanella  which  occurs  in  various 
palaeozoic  and  later  limestones  is  now  generally  regarded  as  an 
alga.  According  to  Mr.  E.  Wethered  4  it  plays  an  important  part 
in  the  formation  of  pisolitic  and  oolitic  grains.  Moreover 
calcareous  algae,  such  as  Lithothamnion,  are  sometimes  important 
constituents  in  Tertiary  limestones,  as  for  instance  in  the  Leitha- 

1  Under  this  head  are  included  the  Cycadece,  along  with  the  ordinary  Conifera, 
or  pine  and  fir  tribe. 

2  See  his  memoirs  on  the  coal-plants  published  in  the  volumes  of  the  Phil.  Trans., 
which  are  now  being  continued  by  Dr.  D.  H.  Scott. 

3  For  notes  upon  methods  to  be  employed  in  making  preparations  of  coal,  see 
Rutley,  Study  of  Bocks,  1884,  p.  71. 

4  Quart.  Journ.  Geol.  Soc.  xlvi.  (1890),  p.  270,  xlviii.  p.  377,  xlix.  p.  236. 


MINUTE   ORGANISMS  AS  ROCK-MAKERS  1085 

kalk  of  Europe.  They  have  also  been  identified  in  rocks  of  Secondary 
and  even  of  Palaeozoic  age.  It  is  an  admitted  rule  in  geological 
science  that  the  past  history  of  the  earth  is  to  be  interpreted,  so  far 
as  may  be  found  possible,  by  the  study  of  the  changes  which  are 
still  going  on.  Thus,  when  we  meet  with  an  extensive  stratum  of 
fossilised  Diatomacece  in  what  is  now  dry  land,  we  can  entertain  no 
doubt  that  this  silicious  deposit  originally  accumulated  either  at  the 
bottom  of  a  fresh-water  lake  or  beneath  the  waters  of  the  ocean ; 
just  as  such  deposits  are  formed  at  the  present  time  by  the  produc- 
tion and  death  of  successive  generations  of  these  bodies,  whose 
indestructible  casings  accumulate  in  the  lapse  of  ages,  so  as  to  form 
layers  whose  thickness  is  only  limited  by  the  time  during  which 
this  process  has  been  in  action.  In  like  manner,  when  we  meet 
with  a  limestone  rock  entirely  composed  of  the  calcareous  shells  of 
Foraminifera,  some  of  them  entire,  others  broken  up  into  minute 
particles  (as  in  the  case  of  the  Fusulina  limestone  of  the  Carboni- 
ferous period,  and  the  Nunvniulitic  limestone  of  the  Eocene),  we 
interpret  the  phenomenon  by  the  fact  that  the  dredgings  obtained 
from  certain  parts  of  the  ocean-bottom  consist  almost  entirely  of 
remains  of  existing  Foraminifera,  in  which  entire  shells,  the  animals 
of  which  may  be  yet  alive,  are  mingled  with  the  clebris  of  others 
that  have  been  reduced  to  a  fragmentary  state.  Such  a  deposit, 
consisting  chiefly  of  Orbitolites,  is  at  present  in  process  of  formation 
on  certain  parts  of  the  shores  of  Australia,  as  Dr.  Carpenter  was 
informed  by  Mr.  J.  Beete  Jukes,  thus  affording  the  exact  parallel  to 
the  stratum  of  Orbitolites  (belonging,  as  his  own  investigations  have 
led  him  to  believe,  to  the  very  same  species)  that  forms  part  of  the 
'  calcaire  grossier '  of  the  Paris  basin.  So  in  the  fine  white  mud 
which  is  brought  up  from  almost  every  part  of  the  sea-bottom  of  the 
Levant,  where  it  forms  a  stratum  that  is  continually  undergoing  a 
slow  but  steady  increase  in  thickness,  the  microscopic  researches  of 
Professor  W.  C.  Williamson  !  have  shown,  not  only  that  it  contains 
multitudes  of  minute  remains  of  living  organisms,  both  animal  and 
vegetable,  but  that  it  is  entirely  or  almost  wholly  composed  of  such 
remains.  Amongst  these  are  about  twenty-six  species  of  Dia- 
tomacese  (silicious),  eight  species  of  Foraminifera  (calcareous),  and  a 
miscellaneous  group  of  objects  (fig.  810),  consisting  of  calcareous  and 
silicious  spicules  of  sponges  and  Gorgonice,  and  fragments  of  the 
calcareous  skeletons  of  echinoderms  and  molluscs.  A  collection  of 
forms  strongly  resembling  that  of  the  Levant  mud,  with  the  exception 
of  the  silicious  Diatomaceae,  is  found  in  many  parts  of  the  '  calcaire 
grossier '  of  the  Paris  basin,  as  well  as  in  other  extensive  deposits  of 
the  same  early  Tertiary  period. 

It  is,  however,  in  regard  to  the  great  chalk  formation  that  the 
information  afforded  by  the  microscope  has  been  most  valuable. 
Mention  has  already  been  made  of  the  fact  that  a  large  proportion 
of  the  North  Atlantic  sea-bed  has  been  found  to  be  covered  with  an 
'  ooze  '  chiefly  formed  of  the  shells  of  Olobigerince  ;  and  this  fact,  first 
determined  by  the  examination  of  the  small  quantities  brought  up 
by  the  sounding  apparatus,  has  been  fully  confirmed  by  the  results  of 

1  Memoirs  of  the  Manchester  Literary  and  Philosophical  Society,  vol.  vii. 


1086       THE   MICROSCOPE   IN    GEOLOGICAL  INVESTIGATION 

the  more  recent  explorations  of  the  deep-sea  with  the  dredge  ;  which, 
bringing  up  half  a  ton  of  this  deposit  at  once,  has  shown  that  it  is 
not  a  mere  surface-film,  but  an  enormous  mass  whose  thickness  cannot 
be  even  guessed  at.  '  Under  the  microscope,'  says  Professor  Wyville 
Thomson  l  of  a  sample  of  1^  cwt.  obtained  by  the  dredge  from  a  depth 
of  nearly  three  miles,  '  the  surface-layer  wras  found  to  consist  chiefly 


FIG.  810.— Microscopic  organisms  in  Levant  mud:  A,  C,  D,  silicious 
spicules  of  Tethya  ;  B,  H,  spicules  of  Geodia  ;  E,  calcareous  spicule  of 
Grantia.]  F,  G,  M,  O,  portions  of  calcareous  skeleton  of  EcMnodermata  ; 
I,  calcareous  spicule  of  Gorgonia  ;  K,  L,  N,  silicious  spicules  of  sponges  ; 
P,  portion  of  prismatic  layer  of  shell  of  Pinna. 

of  entire  shells  of  Globigerina  bulloides,  large  and  small,  and  of  frag- 
ments of  such  shells  mixed  with  a  quantity  of  amorphous  calcareous 
matter  in  fine  particles,  a  little  fine  sand,  and  many  spicules,  portions 
of  spicules,  and  shells  of  Radiolaria,  a  few  spicules  of  sponges,  and  a 
few  frustules  of  diatoms.  Below  the  surface-layer  the  sediment  be- 
comes gradually  more  compact,  and  a  slight  grey  colour,  due  probably 

i  The  Depths  of  the  Sea,  p.  410.     See  also  Voyage  of  Challenger,  ch.  in.,  and 
Challenger  Reports,  especially  Deep  Sea  Deposits  (Murray  and  Benard.) 


MINUTE   ORGANISMS   AS   ROCK-MAKERS  1087 

to  the  decomposing  organic  matter,  becomes  more  pronounced,  while 
perfect  shells  of  Globigerina  almost  disappear,  fragments  become 
smaller,  and  calcareous  mud,  structureless,  and  in  a  fine  state  of 
division,  is  in  greatly  preponderating  proportion.  One  can  have  no 
doubt,  on  examining  this  sediment,  that  it  is  formed  in  the  main  by 
the  accumulation  and  disintegration  of  the  shells  of  Globigerina  ;  the 
shells  fresh,  whole,  and  living  in  the  surface-layer  of  the  deposit  ; 
and  in  the  lower  layers  dead,  and  gradually  crumbling  down  by  the 
decomposition  of  their  organic  cement,  and  by  the  pressure  of  the 
layers  above.'  This  white  calcareous  mud  also  contains  in  large 
amount  the  'coccoliths'  and  *  coccospheres '  formerly  mentioned. 
Now  the  resemblance  which  this  Globigerinct-inud,  when  dried,  bears 


FIG.  811. — Microscopic  organisms  in  chalk  from  Gravesend :  a,  6,  c,  d, 
Text ul aria  globulosa;  e,  e,  e,  Rotalia  aspera]  f,  Textularia  aculeata; 
g,  Planularia  hexas',  h,  Navicula. 

to  chalk  is  so  close  as  at  once  to  suggest  the  similar  origin  of  the 
latte?- ;  and  this  is  fully  confirmed  by  microscopic  examination.  For 
many  samples  of  it  consist  in  great  part  of  the  minuter  kinds  of 
Foraminifera,  especially  Globigerince,  whose  shells  are  imbedded  in  a 
mass  of  apparently  amorphous  particles,  many  of  which,  nevertheless, 
present  indications  of  being  the  disintegrated  fragments  of  similar 
shells,  or  of  larger  calcareous  organisms.  In  the  chalk  of  some 
localities  the  disintegrated  prisms  of  Pinna,  or  of  other  large  shells 
of  the  like  structure  (as  Inoceramus),  form  the  great  bulk  of  the 
recognisable  components  ;  whilst  in  other  cases,  again,  the  chief  part 
is  made  up  of  the  shells  of  Cytherina,  a  marine  form  of  entomo- 
stracous  crustacean.  Different  specimens  of  chalk  vary  greatly  Tn 


IO88       THE   MICROSCOPE   IN   GEOLOGICAL  INVESTIGATION 

the  proportion  which  not  only  the  distinctly  organic  remains  bear  to 
the  amorphous  residuum,  but  also  the  different  kinds  of  the  former 
bear  to  each  other ;  and  this  is  quite  what  might  be  anticipated  when 
we  remember  how  one  or  another  tribe  of  animals  predominates 
in  the  several  parts  of  a  large  area ;  but  it  may  be  fairly  concluded, 
from  what  has  been  already  stated  of  the  amorphous  component  of 
the  Globigerina-rnud,  that  the  amorphous  constituent  of  chalk  like- 
wise is  the  disintegrated  residuum  of  foraminiferal  shells,  or  at  any 
rate  of  some  small  calcareous  organism.  But,  further,  the  Globigerina- 
mud  now  in  process  of  formation  is  in  some  places  literally  crowded 
with  sponges  having  a  complete  silicious  skeleton  ;  and  some  of  them 
bear  such  an  extraordinarily  close  resemblance,  alike  in  structure 


FIG.  812. — Microscopic  organisms  (chiefly  f ora  mini f era)  in  chalk  from  Meudon, 
seen  partly  as  opaque,  and  partly  as  transparent  objects. 

and  in  external  form,  to  the  Ventriculites  which  are  well  known  as 
chalk  fossils,  as  to  leave  no  reasonable  doubt  that  these  also  were 
silicious  sponges  living  on  the  bottom  of  the  cretaceous  sea.  Finally 
(as  was  first  pointed  out  by  Dr.  Sorby)  the  coccoliths  and  cocco- 
spheres  at  present  found  on  the  sea-bottom  are  often  to  be  discovered 
by  the  microscopic  examination  of  chalk.1  All  these  correspondences 
show  that  the  formation  of  chalk  took  place  under  conditions 
essentially  similar  to  those  under  which  the  deposit  of  Globigerina- 
mud  is  being  formed  over  the  Atlantic  sea-bed  at  the  present  time. 
In  examining  chalk  or  other  similar  mixed  aggregations,  whose 

1  'On  the  Organic  Origin  of  the  so-called  "  Crystalloids  "  of  Chalk  '  in  Ann.  Nat. 
Hist.  ser.  iii.  vol.  viii.  1861,  pp.  193-200.  Murray  and  Renard,  Deep  Sea  Deposits 
(Challenger  Reports),  p.  257. 


CHALK,    FLINT,    AND    CHERT  1089 

component  particles  are  easily  separable  from  each  other,  it  is  de- 
sirable to  separate,  with  as  little  trouble  as  possible,  the  larger  and 
more  definitely  organised  bodies  from  the  minute  amorphous  particles  ; 
-and  the  mode  of  doing  this  will  depend  upon  whether  we  are  operat- 
ing upon  the  large  or  upon  the  small  scale.  If  the  former,  a  quantity 
of  soft  chalk  should  be  rubbed  to  powder  with  water  by  means  of  a 
soft  brush  ;  and  this  water  should  then  be  proceeded  with  according 
to  the  method  of  levigation  already  directed  for  separating  the 
Diatomacece.  It  will  usually  be  fojmd  that  the  first  deposits  contain 
the  larger  Foraminifera,  fragments  of  shell,  etc.,  and  that  the  smaller 
Foraminifera  and  sponge-spicules  fall  next,  the  fine  amorphous  par- 
ticles remaining  diffused  through  the  water  after  it  has  been  standing 
for  some  time,  so  that  they  may  be  poured  away.  The  organisms 
thus  separated  should  be  dried  and  mounted  in  Canada  balsam.  If 
the  smaller  scale  of  preparation  be  preferred,  as  much  chalk  scraped 
fine  as  will  lie  on  the  point  of  a  knife  is  to  be  laid  on  a  drop  of  water 
on  the  glass  slide,  and  allowed  to  remain  there  for  a  few  seconds  ; 
the  wrater,  with  any  particles  still  floating  on  it,  should  then  be  re- 
moved ;  and  the  sediment  left  on  the  glass  should  be  dried  and 
mounted  in  balsam.  For  examining  the  structure  of  flints  such 
chips  as  may  be  obtained  with  a  hammer  will  commonly  serve  very 
well,  a  clear  translucent  flint  being  first  selected,  and  the  chips  that 
are  obtained  being  soaked  for  a  short  time  in  turpentine  (which  in- 
-creases  their  transparence) ;  those  which  show  organic  structure, 
whether  sponge-tissue  or  xanthidia,  are  to  be  selected  and  mounted 
in  Canada  balsam.  The  most  perfect  specimens  of  sponge-structure, 
however,  are  only  to  be  obtained  by  slicing  and  polishing. 

The  study  of  thin  slices  of  flint  and  chert  during  late  years 
lias  thrown  much  light  on  their  origin  and  on  the  structure  of 
fossil  sponges.  Spicules  are  often  found  to  be  extremely  abundant 
as  in  the  chert  (Upper  Greensand)  from  the  quarry  by  Ventnor 
station  (Isle  of  Wight),  where  they  can  be  detected  by  the  naked  eye. 
The  radiolaria  from  the  Tertiary  marl  of  Barbadoes  have  long  been 
known  to  microscopists,  but  these  organisms  more  recently  have  been 
detected  in  cherts.  In  Britain  such  cherts  have  been  described 
from  the  Ordovician  rocks  of  Mullioii  Island,  Cornwall,  and  of  south 
Scotland,  and  the  Carboniferous  of  south-west  England.1 

There  are  various  other  deposits,  of  less  extent  and  importance 
than  the  great  chalk-formation,  which  are,  like  it,  composed  in  great 
part  of  microscopic  organisms,  chiefly  minute  Foraminifera  ;  2  and  the 
presence  of  these  may  be  largely  recognised,  by  the  assistance  of  the 
microscope,  in  sections  of  calcareous  rocks  of  various'  dates,  whose 
other  materials  were  fragments  of  corals,  crinoid-stems,  or  the  shells 
of  molluscs.  In  the  formation  of  the  Coralline  Crag  (Tertiary)  of  the 
eastern  coast  of  England,  polyzoaries  had  the  greatest  share ;  but 

1  On  the  former  subject  see  G.  J.  Hinde,  British  Museum   Catalogue  of  Fossil 
Xponyes ;  on  the  latter,  the  same,  Quart.  Journ.  GeoL  Soc.  vols.  xlvi.  xlix.  li. 

2  For  illustrations  of  fossil  foraminifera,  see  Carpenter,  Introduction  to  Study  of 
Foraminifera    (Ray    Society),    and    the    publications    of    the    Palaeontograpliical 
Society;  Crag  Foraminifera  (T.  Rupert  Jones,  &c.) ;  Carboniferous  and  Permian 
Foraminifera  (H.  B.  Brady),     The  series   also  contains  volumes  upon  the   Crag 
Polyzoa  and  various  small  Entomostraca  of  different  ages. 

4A 


1090       THE   MICKOSCOPE   IN   GEOLOGICAL  INVESTIGATION 

the  Tertiary  limestone  of  which  Paris  is  chiefly  built  consists  almost 
exclusively  of  the  shells  of  Miliolida>,  and  is  thus  known  as  miliolite 
(millet-seed)  limestone.  In  the  vast  stratum  of  nummulitic  lime- 
stone which  was  formed  in  the  earlier  part  of  the  Tertiary  period 
the  microscope  enables  us  to  see  that  the  matrix  in  which  the  large 
entire  nummulites  are  imbedded  is  itself  composed  of  comminuted 
fragments  and  young  shells  of  the  same,  together  with  minuter 
Foramihifera.  Similar  organisms,  with  fragments  of  crinoids. 
mollusca,  coral,  &c.,  are  abundantly  present  in  the  Jurassic  lime- 
stones in  this  country,  in  those  of  Secondary  age  generally  in 
Europe,  as  well  as  in  the  Carboniferous  and  other  Palaeozoic  lime- 
stones ;  in  fact,  wherever  subsequent  changes  have  not  rendered  the 
structure  of  the  original  constituents  indistinguishable.  Thus  in 
the  great  plains  of  Russia  there  are  certain  bands  of  limestone  of 
this  epoch,  varying  in  thickness  from  fifteen  inches  to  five  feet,  and 
frequently  repeated  through  a  vertical  depth  of  two  hundred  feet 
over  very  wide  areas,  which  are  almost  entirely  composed  of  the 
extinct  genus  Fusulina.  Again,  those  parts  of  the  Carboniferous 
limestone  of  Ireland  which  have  undergone  least  disturbance  can  be 
plainly  shown,  by  the  examination  of  microscopic  sections,  to  consist 
of  the  remains  of  Foraminifera,  Polyzoa,  fragments  of  corals,  tfcc. 
And  where,  as  not  unfrequently  happens,  beds  of  this  limestone  are 
separated  by  clay  seams,  these  are  found  to  be  loaded  with  '  microzoa ' 
of  various  kinds,  particularly  Foraminifera  (of  which  the  Saccamina 
has  come  down  to  the  present  time),  and  the  beautiful  polyzoaries 
known  as  '  lace-corals.' 

Mention  has  been  already  made  of  Professor  Ehrenberg's  very 
remarkable  discovery  that  a  large  proportion  (to  say  the  least)  of  the 
green  sands  which  p'resent  themselves  in  various  stratified  deposits, 
from  the  Silurian  period  to  the  Tertiary  era,  and  in  that  called 
the  Upper  Greensand,  is  composed  of  the  casts  of  the  interior  of 
minute  shells  of  Foraminifera  and  Mollusca,  the  shells  themselves 
having  entirely  disappeared.  The  mineral  material  of  these  casts 
has  not  merely  filled  the  chambers  and  their  communicating 
passages,  but  has  also  penetrated,  even  to  its  minutest  ramifications, 
the  canal-system  of  the  intermediate  skeleton.  The  precise  parallel 
to  these  deposits  presents  itself  in  certain  spots  of  the  existing  sea- 
bottom,  such  as  the  Agulhas  bank,  near  the  Cape  of  Good  Hope, 
where  the  dredge  comes  up  laden  with  a  green  sand,  which  on 
microscopic  examination  proves  to  consist  almost  entirely  of 
'internal  casts'  of  existing  Foraminifera.1 

It  is,  however,  in  the  case  of  the  teeth,  the  bones,  and  the  dermal 
skeleton  of  vertebrate  animals  that  the  value  of  microscopic  inquiry 
becomes  most  apparent ;  since  their  structure  presents  so  many 
characteristics  which  are  subject  to  well-marked  variations  in  their 
several  classes,  orders,  and  families  that  a  knowledge  of  these 
characters  frequently  enables  the  microscopist  to  determine  the 

1  See  Challenger  Reports  ;  Deep  Sea  Deposits  (Murray  and  Kenard),  p.  378, 
&c.  The  same  volume  describes  and  figures  the  microscopic  structure  of  remarkable 
manganese  concretions,  dredged  at  great  depths  in  the  ocean,  and  often  associated 
with  organisms. 


DETERMINATION   OF   FOSSIL   TEETH   AND   BONES        1 09 1 


nature  of  even  the  most  fragmentary  specimens.  It  was  in  regard 
to  teeth  that  the  possibility  of  such  determinations  was  first  made 
clear  by  the  laborious  researches  of  Professor  Owen ; l  and  the 
following  may  be  given  as  examples  of  their  value  : — A  rock- 
formation  extends  over  many  parts  of  Russia  whose  mineral 
diameters  might  justify  its  being  likened  either  to  the  Old  or  to  the 
Xew  Red  Sandstone  of  this  country,  and  whose  position  relatively  to 
other  strata  is  such  that  there  is  great  difficulty  in  obtaining 
evidence  from  the  usual  sources  as  to  its  place  in  the  series.  Hence 
the  only  hope  of  settling  this  question  (which  was  one  of  great 
practical  importance,  since,  if  the  formation  were  New  Red,  coal 
might  be  expected  to  underlie  it,  whilst  if  Old  Red,  no  reasonable 
hope  of  coal  could  be  entertained)  lay  in  the  determination  of  the 
organic  remains  which  this  stratum  might  yield  ;  but  unfortunately 
these  were  few  and  fragmentary,  consisting  chiefly  of  teeth,  which 
are  seldom  perfectly  pre- 
served. From  the  gigan- 
tic size  of  these  teeth, 
together  with  their  form, 
it  was  at  first  inferred 
that  they  belonged  to  sau- 
rian reptiles,  in  which 
case  the  sandstone  would 
have  been  considered  as 
Xew  Red ;  but  micro- 
scopic examination  of 
their  intimate  structure 
unmistakably  proved 

them  to  belong  to  a 
genus  of  fishes  (Dendro- 
due)  which  is  exclusively 
palaeozoic,  and  thus  de- 
cided that  the  formation 
must  be  Old  Red.  So, 


FIG.  813. — Section  of  tooth  of  Ldbyrinthodon. 


again, 


the 


microscopic. 


examination  of  certain  fragments  of  teeth  found  in  a  sandstone  of 
Warwickshire  disclosed  a  most  remarkable  type  of  tooth-structure 
(shown  in  fig.  813),  which  was  also  ascertained  to  exist  in  certain  teeth 
that  had  been  discovered  in  the  '  Keupersandstein '  of  Wiirtemberg  ; 
and  the  identity  or  close  resemblance  of  the  animals  to  which  these 
teeth  belonged  having  been  thus  established,  it  became  almost 
certain  that  the  Warwickshire  and  Wiirtemberg  sandstones  were 
equivalent  formations.  The  next  question  arising  out  of  this  discovery 
was  the  nature  of  the  animal  (provisionally  termed  Labyrinthodon, 
a  name  expressive  of  the  most  peculiar  feature  in  its  dental  structure) 
to  which  these  teeth  belonged.  They  had  been  referred,  from  external 
characters  merely,  to  the  order  of  saurian  reptiles  ;  but  it  is  now 
clear  that  they  were  gigantic  salamandroid  Amphibia,  having  many 
points  of  relationship  to  Ceratodus  (the  Australian  « mud-fish '), 
which  shows  a  similar,  though  simpler,  dental  organisation. 

1  See  his  Odontography. 

4  A  2 


1092        THE   MICROSCOPE   IN   GEOLOGICAL  INVESTIGATION 

The  researches  of  Professor  Quekett  on  the  minute  structure  of 
bone  l  have  shown  that  from  the  average  size  and  form  of  the  lacunae, 
their  disposition  in  regard  to  each  other  and  to  the  Haversian 
canals,  and  the  number  and  course  of  the  canaliculi,  the  nature  of 
even  a  minute  fragment  of  bone  may  often  be  determined  with  a 
considerable  approach  to  certainty,  as  in  the  following  examples, 
among  many  which  might  be  cited  : — Dr.  Falconer,  the  distinguished 
investigator  of  the  fossil  remains  of  the  Himalayan  region,  and  the 
discoverer  of  the  gigantic  fossil  tortoise  of  the  Sivalik  hills,  having 
met  with  certain  small  bones  about  which  he  was  doubtful,  placed 
them  for  minute  examination  in  the  hands  of  Professor  Quekett, 
who  informed  him,  on  microscopic  evidence,  that  they  might  certainly 
be  pronounced  reptilian,  and  probably  belonged  to  an  animal  of  the 
tortoise  tribe ;  and  this  determination  was  fully  borne  out  by  other 
evidence,  which  led  Dr.  Falconer  to  conclude  that  they  were  toe- 
bones  of  his  great  tortoise.  Some  fragments  of  bone  were  found, 
many  years  since,  in  a  chalk-pit,  which  were  considered  by  Professor 
Owen  to  have  formed  part  of  the  wing-bones  of  a  long-winged  sea- 
bird  allied  to  the  albatross.  This  determination,  founded  solely  on 
considerations  derived  from  the  very  imperfectly  preserved  external 
forms  of  these  fragments,  was  called  in  question  by  some  other 
palaeontologists,  who  thought  it  more  probable  that  these  bones 
belonged  to  a  large  species  of  the  extinct  genus  Pterodactylus,  a  flying 
lizard  whose  wing  was  extended  upon  a  single  immensely  prolonged 
digit.  No  species  of  pterodactyle,  however,  at  all  comparable  to- 
this  in  dimensions,  was  at  that  time  known  ;  and  the  characters 
furnished  by  the  configuration  of  the  bones  not  being  in  any  degree 
decisive,  the  question  would  have  long  remained  unsettled  had  not 
an  appeal  been  made  to  the  microscopic  test.  This  appeal  was  so 
decisive,  by  showing  that  the  minute  structure  of  the  bone  in  ques- 
tion corresponded  exactly  with  that  of  pterodactyle  bone,  and  differed 
essentially  from  that  of  every  known  bird,  that  no  one  who  placed 
much  reliance  upon  that  evidence  could  entertain  the  slightest  doubt 
on  the  matter.  By  Professor  Owen,  however,  the  validity  of  that 
determination  was  questioned,  and  the  bone  was  still  maintained  to 
be  that  of  a  bird,  until  the  question  was  finally  set  at  rest,  and  the 
value  of  the  microscopic  test  triumphantly  confirmed,  by  the  discovery 
of  undoubted  pterodactyle  bones  of  corresponding  and  even  of  greater 
dimensions  in  the  same  and  other  chalk  quarries. 

The  microscopic  examination  of  the  sediments  now  in  course  of 
deposition  on  various  parts  of  the  great  oceanic  area,  and  especially 
of  the  large  number  of  samples  brought  up  in  the '  Challenger  'sound- 
ings, has  led  to  this  very  remarkable  conclusion — that  the  detritus 
resulting  from  the  degradation  of  continental  land-masses  is  not 
carried  far  from  their  shores,  being  entirely  absent  from  the  bottom 
of  the  ocean-basins.  The  sediments  there  found  were  not  of 
organic  origin,  but  mainly  consist  of  volcanic  debris  and  of  clay  that 
seems  to  have  been  produced  by  the  disintegration  of  masses  of  very 

1  See  his  memoir  on  the  '  Comparative  Structure  of  Bone  '  in  the  Trans.  Microsc. 
Soc.  ser.  i.  vol.  ii. ;  and  the  Catalogue  of  the  Histological  Museum  of  the  Hoy.  Coll. 
of  Surgeons,  vol.  ii. 


ORIGIN   OF   OCEANIC   AREAS  1093 

vesicular  Liva,  which,  after  long  floating  and  dispersion  by  surface- 
drift  or  ocean-currents,  have  become  water-logged  and  have  sunk  to 
the  bottom.  As  no  ordinary  silicious  sand  is  found  anywhere  save 
in  the  neighbourhood  of  continents  and  continental  islands,  and  as 
almost  all  oceanic  islands  are  either  of  volcanic  origin  or  coral  atolls, 
this  almost  universal  absence  of  any  trace  of  submerged  continental 
land  over  the  great  oceanic  area  affords  strong  confirmation  to  the 
belief  that  the  sedimentary  rocks  which  form  the  existing  land  were 
deposited  in  the  neighbourhood  of  pre-existing  land,  whose  degrada- 
tion furnished  their  materials  ^  and  suggests  that  the  original 
disposition  of  the  great  continental  and  oceanic  areas  was~  not  very 
different  from  what  it  now  is.1  Further,  the  microscopic  examination 
of  these  oceanic  sediments  reveals  the  presence  of  extremely  minute 
particles,  which  seem  to  correspond  in  composition  to  meteorites,  and 
which  there  is  strong  reason  for  regarding  as  *  cosmic  dust '  pervading 
the  interplanetary  spaces.  Thus  the  application  of  the  microscope 
to  the  study  of  these  deposits  brings  us  in  contact  with  the  greatest 
questions  not  only  of  terrestrial,  but  also  of  cosmical  physics,  and 
furnishes  evidence  of  the  highest  value  for  their  solution. 

1  See  Sir  A.  Geikie  on  '  Geographical  Evolution,'  Proc.  Roy.  Geog.  Soc.  July  1879  ; 
and  for  detailed  results  '  Preliminary  Keport  of  Cruise  of  "  Challenger " '  (Wyville 
Thomson),  Proc.  Roy.  Soc.  vol.  xxiv.  (1876)  p.  463,  and  '  Challenger '  Reports  (Murray 
and  Renarcl),  Deep  Sea  Deposits,  p.  327. 


1094 


CHAPTER  XXIY 

MICROCRYSTALLISATION.    OPTICAL  PROPERTIES  OF  CRYSTALS. 
MOLECULAR  COALESCENCE.    MICRO-CHEMICAL   ANALYSIS. 

ALTHOUGH  by  far  the  most  numerous  and  most  important  applica- 
tions of  the  microscope  were  formerly  those  by  which  the  structure 
and  actions  of  organised  beings  are  made  known  to  us,  yet  the  in- 
creased attention  which  has  been  paid  during  recent  years  to  tin* 
use  of  the  microscope  in  elucidating  the  internal  structure  of 
crystalline  substances,  whether  of  natural  or  artificial  origin,  lias 
made  this  instrument  as  indispensable  to  the  crystallographer  ;md 
the  mineralogist  as  it  formerly  was  to  the  physiologist.  Solid  sub- 
stances are  almost  invariably  found  in  nature  or  obtained  as  labora- 
tory products  in  the  form  of  individual  fragments,  each  bounded  by 
plane  surfaces  which  are  inclined  at  such  angles  that  the  whole 
figure  is  possessed  of  a  greater  or  lesser  .degree  of  geometrical 
symmetry.  Such  solid  bodies  are  termed  crystals,  and,  although 
formerly  the  regularity  of  external  shape  constituted  the  only  avail- 
able means  of  recognising  them,  it  is  now  demonstrated  that  the 
external  form  is  only  the  result  of  the  so-called  homogeneous 
internal  structure  of  the  crystal.  This  homogeneity  of  structure 
consists  in  the  arrangement  of  the  smallest  characteristic  particles 
or  units  of  the  structure  being  the  same  about  every  unit  of  the 
structure.  The  different  kinds  of  possible  homogeneous  arraniiv 
ments  of  points  in  space  have  been  investigated  by  Bravais,  Sohncke, 
and  others,1  and  on  classifying  them  according  to  their  symmetry 
they  fall  into  thirty-two  classes  identical  with  the  thirty-two  known 
crystalline  systems.  These  thirty-two  types  of  structure  differ  in 
their  symmetry,  and  this  difference  is  expressed  in  the  symmetry  of 
the  external  form  ;•  the  external  form,  however,  is  very  liable  to 
distortion,  in  consequence  of  a  lack  of  uniformity  in  the  conditions 
prevailing  during  the  growth  of  the  crystal,  and  so  is  at  best  but  an 
untrustworthy  guide  to  the  symmetry  of  the  internal  structure.  The 
optical  properties  of  the  solid  structure,  also  themselves  expressions 
of  the  symmetry,  and  consequently  of  the  crystalline  system,  are 
not  disturbed  by  casual  influences  to  nearly  so  great  an  extent  as  is 
the  regular  external  form  ;  the  symmetrical  variation  of  the  optical 
properties  of  crystalline  structures  in  accordance  with  the  symmetry 

1  See  A.  Sclioenflies,  Krystallsysteme  und  KrystaUstntctiir,  Leipzig,  1891. 


FORMATION   OF   CRYSTALS 


1095 


of  arrangement  of  the  structural  units  gives  rise  to  the  phenomena 
of  double  refraction,  circular  polarisation,  pleochroism,  &c.,  observed 
with  crystalline  bodies.  The  important  results  to  be  anticipated 
from  the  microscopic  examination  of  crystalline  preparations  such  as 
rock  sections,  etc.,  was  pointed  out  by  H.  C.  Sorby  in  1858  ;  the  micro- 
scopic methods  as  at  present  applied  to  pure  crystallography  have 
been  fully  described  by  P.  Groth  1  and  by  Th.  Liebisch,2  whilst  their 
applicability  to  the  identification  of  the  crystalline  constituents  of 
rocks  has  been  exhaustively  treated  by  H.  Rosenbusch.3 

The  study  of  crystalline  materials  in  such  minute  crystals  as  are 
appropriate  subjects  for  observation  by  the  microscope  is  not  only 
.a  very  interesting  application  of  its  powers,  but  is  capable  of 
affording  some  valuable  hints  to  the  designer.  This  is  particu- 
larly the  case  with  crystals  of  snow,  which  belong  to  one  of  the 
;  hexagonal  systems,'  the  basis  of  every  figure  being  a  hexagon  of 
six  rays ;  for  these  rays  ;  become  incrusted  with  an  endless  variety 
of  secondary  formations  of  the  same 
kind,  some  consisting  of  thin  lamina' 
alone,  others  of  solid  but  translucent 
prisms  heaped  one  upon  another,  and 
others  gorgeously  combining  lamina 
and  prisms  in  the  richest  profu- 
sion,' 4  the  angles  by  which  these 
figures  are  bounded  being  invari- 
ably 60°  or  120°.  Beautiful  ar- 
borescent forms  are  not  unfrequeiitly 
produced  by  the  peculiar  mode  of 
aggregation  of  individual  crystals  ; 
of  this  we  have  often  an  example  on 
a.  large  scale  on  a  frosted  window ; 
but  microscopic  crystallisations  some- 
times present  the  same  curious  phe- 
nomenon (fig.  814).  Avanturine, 
lapis  lazuli,  crystallised  silver,  &c. 
make  very  good  specimens ;  whilst 

thin  sections  of  granite,  gabbro,  and  other  crystalline  rocks,  also  of 
agate,  aragonite,  piedmontite,  the  zeolites,  and  other  minerals,  are 
very  beautiful  objects  for  the  polar iscopc. 

The  actual  process  of  the  formation  of  crystals  may  be  watched 
under  the  microscope  with  the  greatest  facility,  all  that  is  necessary 
being  to  lay  on  a  slip  of  glass,  previously  warmed,  a  saturated  solu- 
tion of  the  substance,  and  to  incline  the  stage  in  a  slight  degree,  so  that 
the  drop  shall  be  thicker  at  its  lower  than  at  its  upper  edge.  The 
crystallisation  will  speedily  begin  at  the  upper  edge,  where  the  pro- 
portion of  liquid  to  solid  is  most  quickly  reduced  by  evaporation,  and 
will  gradually  extend  downwards.  If  it  should  go  on  too  slowly, 


FIG.  814.— Crystallised  silver. 


1  Physikalische  Krystallographie,  Leipzig,  189,5. 

2  Grundriss  der  physiJcalischen  Krystallographie,  Leipzig,  1896. 

5  Microscopical  Physiography  of  the  Bock-making  Minerals,  London,  1895. 
4  Glaisher  on  '  Snow-crystals  in  1855,'  Quart.  Journ.  Microsc.  Sci.  vol.  iii.  1855, 
p.  179.     See  also  C.  A.  Bering,  Zeits.  f.  Kryst.  Bd.  xiv.  1888,  p.  250. 


1096  MICROCKYSTALLISATION.    ETC. 

or  should  cease  altogether,  whilst  a  large  proportion  of  the  liquid 
still  remains,  the  slide  may  be  again  warmed,  so  as  to  re-dissolve 
the  part  already  solidified,  after  which  the  process  will  recom- 
mence with  increased  rapidity.  This  interesting  spectacle  may 
be  watched  under  any  microscope,  but  the  instrument  specially 
designed  by  O.  Lehmaim  1  is  particularly  adapted  to  studies  of  this 
kind.  The  degree  of  heat  can  be  varied  at  will.  The  phenomena 
become  far  more  striking,  however,  when  the  crystals,  as  they  come 
into  being,  are  made  to  stand  out  bright  upon  a  dark  ground,  by 
the  use  of  the  spot  lens,  the  paraboloid,  or  any  other  form  of  black- 
ground  illumination  ;  still  more  beautiful  is  the  spectacle  when  the 
polarising  apparatus  is  employed,  so  as  to  invest  the  crystals  with 
the  most  gorgeous  variety  of  hues. 

By  chemically  precipitating  crystalline  products  under  the  micro- 
scope we  can  obtain  a  still  deeper  insight  into  the  crystallisation 
process.  One  of  the  earliest  workers  at  this  subject  was  Link,'2 
who  observed  that  precipitates  first  separate  in  the  form  of  very 
minute  liquid  .globules,  and  that  these  subsequently  coagulate  to 
form  an  undoubtedly  crystalline  precipitate.  Later  investigation 
of  the  subject  by  Fraiikenheim,  and  then  by  Vogelsang,3  led  to  the 
conclusion  that  during  the  passage  of  a  substance  from  the  dissolved 
to  the  crystalline  state  it  passes  through  a  whole  series  of  inter- 
mediate stages.  On  allowing  sulphur  to  crystallise  very  slowly  from 
a  carbon  bisulphide  solution  thickened  with  Canada  balsam,  the 
liquid  globules,  which  first  separate  gradually,  solidify  to  small 
isotropic  spheres  termed  globulites ;  these  embryonic  forms  then 
coalesce,  yielding  regular  aggregates  known  as  crystallites.  The 
latter  subsequently  arrange  themselves  in  rows  as  tnargarites, 
several  of  which  then  amalgamate,  forming  longulites,  and  the 
process  of  aggregation  proceeds  until  at  last  the  crystalloids — the 
first  product  in  which  the  structure  of  the  crystal  itself  is  traceable 
— are  obtained.  The  separate  existence  of  so  many  transition  forms 
has  been  disputed,  notably  by  Behrens  ;  4  but  their  mention  serves 
the  purpose  of  indicating  that  the  formation  of  crystalline  bodies  is 
really  an  operation  of  considerable  complexity. 

Upon  the  temperature  maintained  during  crystallisation  depends 
the  size  and  arrangement  of  the  crystals.  Thus  santonin,  when 
crystallising  rapidly  on  a  very  hot  plate,  forms  large  crystals 
radiating  from  centres  without  any  undulations ;  when  the  heat  is 
less  considerable  the  crystals  are  smaller,  and  show  concentric 
waves  of  very  decided  form  (fig.  815);  but  when  the  slip  of  glass 
is  cool  the  crystals  are  exceedingly  minute.  In  the  case  of  cupric 
sulphate,  Mr.  B.  Thomas 5  succeeded,  by  keeping  the  slide  at  a 
temperature  of  from  80°  to  90°,  in  obtaining  most  singular  and 
beautiful  forms  of  spiral  crystallisation,  such  as  that  represented  in 

1  Molekularphysik,  2  vols.  Leipzig,  1888  and  1889. 

2  Pogg.  Ann.  Bd.  xlvi.  1839,  p.  258.  3  Die  Krystalliten,  Bonn,  1875. 

4  Die  Krystalliten,  Kiel,  1874. 

5  See  his  paper  '  On  the  Crystallisation  at  various  temperatures  of  the  Double 
Salt,  Sulphate  of  Magnesia  and  Sulphate  of  Zinc,'  in  Quart.  Journ.  Microsc.  8ci.  u.s. 
vi.  pp.  137,  177.      See  also  H.  N.  Draper  on  'Crystals  for  the  Micro-polariscope,' 
in  Intellectual  Observer,  vol.  vi.  1865,  p.  437. 


OPTICAL  PEOPEKTIES  OF  CRYSTALS         1 097 

fig.  816.  Mr.  Slack  has  shown  that  a  great  variety  of  spiral  and 
curved  forms  can  be  obtained  by  dissolving  metallic  salts,  or  saliciii. 
santonin,  <tc.,  in  water  containing  3  or  4  per  cent,  of  colloid 
silica.  The  nature  of  the  action  that  takes  place  may  be  under- 
stood by  allowing  a  drop  of  the  silica  solution  to  dry  upon  a  slide ; 
the  result  of  which  will  be  the  production  of  a  complicated  series  of 
cracks,  many  of  them  curvilinear.  When  a  group  of  crystals  in  for- 
mation tend  to  radiate  from  a  centre,  the  contractions  of  the  silica 
will  often  give  them  a  tangential  pull.  Another  action  of  the 
silica  is  to  introduce  a  very  slight  curling  with  just  enough  eleva- 
tion above  the  slide  to  exhibit  fragments  of  Newton's  rings,  when  it 
is  illuminated  with  Powell  and  Lealand's  modification  of  Professor 


FIG.  815. — Radiating  crystallisation  of  santonin.  * 

Smith's  dark-ground  illuminator  for  high  powers,  and  viewed  with 
a  ^th  objective.  With  crystalline  substances  these  actions  add  to 
the  variety  of  colours  to  be  obtained  with  the  polariscope,  the 
best  slides  exhibiting  a  series  of  tertiary  tints.1  Yery  interesting 
results  may  often  be  obtained  from  a  mixture  of  two  or  more  salts, 
•and  some  of  the  double  salts  give  forms  of  peculiar  beauty.  O.  Leh- 
mann  has  done  excellent  work  in  this  department ;  but  reference 
must  be  had  to  his  previously  mentioned  work  on  '  Molekularphysik  ' 
for  a  description  of  the  phenomena  such  mixtures  exhibit.  The 
following  list  specifies  the  salts  and  other  substances  whose  crystalline 
forms  are  most  interesting.  When  these  are  viewed  with  polarised 
light  some  of  them  exhibit  a  beautiful  variety  of  colours  of  their 
own,  whilst  others  require  the  interposition  of  the  selenite  plate  for 

1  '  On  the  Employment  of  Colloid  Silica  in  the  preparation  of  Crystals  for  the 
Polariscope,'  in  Monthly  Microsc.  Journ.  v.  p.  50. 


1098  MICKOCRYSTALLISATION,   ETC. 

the  development  of  colour.  The  substances  marked  d  are  distin- 
guished by  possessing  the  curious  property  termed  pleocliroisin, 
which  was  first  noticed  by  Dr.  Wbllaston  and  carefully  investigated 
by  Sir  D.  Brewster.  This  property,  to  which  was  previously  applied 
the  misnomer  dichroism,  consists  in  the  exhibition  by  these  crystals 
of  colours  varying  with  the  direction  in  which  they  are  examined ; 
thus,  the  cube-shaped  crystals  of  magnesium  platinocyanide  reflect 
light  of  a  deep  red  colour  from  two  parallel  faces,  whilst'  light  of  a 
vivid  beetle-green  is  reflected  from  the  other  four  faces.  Pleochroism 
is  only  exhibited  by  doubly  refracting  substances,  and  is  caused  by 
the  fact  that  the  two  plane  polarised  rays  into  which  a  ray  passing 
into  the  crystal  is  decomposed,  are  absorbed  selectively — that  is  to 
say,  the  crystalline  medium  absorbs  light  of  certain  colours  from  the 
one  polarised  ray,  whilst  absorbing  quite  differently  coloured  com- 
ponents from  the  second  ray.  Pleochroic  substances  are  most  easily 


FIG.  816. — Spiral  crystallisation  of  copper  sulphate. 

recognised  by  the  fact  that  they  change  in  colour  when  rotated  011 
the  microscope  stage  in  plane  polarised  light — namely,  when  only  one 
Nicol  prism  is  interposed  between  the  eye  and  the  lamp.  It  not 
unfrequently  happens  that  a  remarkably  beautiful  specimen  of 
crystallisation  develops  itself  which  the  observer  desires  to  keep 
for  display.  In  order  to  do  this  successfully,  it  is  necessary  to 
exclude  the  air  ;  and  Mr.  "Warrington  recommends  castor  oil  as  the 
best  preservative.  A  small  quantity  of  this  should  be  poured  on 
the  crystallised  surface,  a  gentle  warmth  applied,  and  a  thin  glass 
cover  then  laid  upon  the  drop  and  gradually  pressed  down ;  and 
after  the  superfluous  oil  has  been  removed  from  the  margin  a  coat 
of  gold-size  or  other  varnish  is  to  be  applied.  Although  most  of  the 
objects  furnished  by  vegetable  and  animal  structures,  which  are 
advantageously  shown  by  polarised  light,  have  been  already  noticed 
in  their  appropriate  places,  it  will  be  useful  here  to  recapitulate  the 
principal,  with  some  additions. 


SALTS   FOE    CRYSTALLISATION 


1099 


Alum 

Ammonium  Borate 
Chloride 

Hydrogen  Tartrate 
Nitrate 
Oxalate 
Oxalurate 
Phosphate 
Platinocyanide,  d 
Sulphate 
Urate 
Asparagine 
Aspartic  Acid 
Barium  Chloride 

„       Nitrate 
Bismuth     „ 
Boracic  Acid 
Cadmium  Sulphate 
Calcium  Carbonate   (from  urine  of 

horse) 
Calcium  Hydrogen  Tartrate 

Oxalate 
Cholesterin 

Chromic  Ammonium  Oxalate,  d 
,,        Oxalate 

„         Potassium  Oxalate,  d 
,,  „          Binoxalate 

Cinchonidine 
€itric  Acid 
Cobalt  Chloride 
•Cupric  Acetate,  d 

Ammonium  Chloride 
„  Sulphate 

Magnesium         „ 
Potassium  „ 

Nitrate 
Sulphate 
Ferrous  Cobalt  Sulphate 

„         Sulphate 
Hippuric  Acid 
Lead  Phosphate,  d 
Magnesium  Ammonium  Phosphate 

(from  urine) 
Magnesium  Sulphate 
Manganese  Acetate 
Mannitol 
Margarine 
Mercuric  Chloride 
Cyanide 
Murexide 
Nickel  Sulphate 
Oxalic  Acid 
Potassium  Arsenate 
Carbonate 
Chlorate 
Chromate 
Bichromate 
Ferricyanide 
Ferrocyanide 


Potassium  Hydrogen  Carbonate 
„  „         Tartrate 

,,          Iodide 
,,          Nitrate 
„          Oxalate 
„          Permanganate 
„          Sulphate 
Quinidine 

Quinine  Hydriodide 
Salicin 
Saligenin 
Santonin 
Sodium  Acetate 

Borate  (borax) 
Carbonate 
Chloride 
Nitrate 
Oxalate 
Phosphate 
Sulphate 
Tartrate 
Urate 
Stearin 

Strontium  Nitrate 
Sugar 

Tartaric  Acid 
Thallium  Platinichloride 
Uranium  Nitrate 
Uric  Acid 
Zinc  Acetate 
,,      Sulphate 

Vegetable 
Cuticles,   Hairs,  and    Scales,   from 

Leaves 

Fibres  of  Cotton  and  Flax 
Raphides 

Spiral  cells  and  vessels 
Starch -grains 
Wood,     longitudinal     sections     of, 

mounted  in  balsam 

Animal 

Fibres  and  Spicules  of  Sponges 
Polypidoms  of  Hydrozoa 
Spicules  of  Gorgoni® 
Polyzoaries 
Tongues   (Palates)    of   Gasteropods 

mounted  in  balsam 
Cuttle-fish  bone 
Scales  of  Fishes 
Sections  of  Egg-shells 
„  Hairs 

Quills 
,,  Horns 

of  Shells 
Skin 
Teeth 
„  Tendon,  longitudinal 


Molecular   Coalescence. — Remarkable  modifications  are  shown 


I  100 


MICROCRYSTALLISATION,   ETC. 


in  the  ordinary  forms  of  crystallisable  substances,  when  the  aggre- 
gation of  the  inorganic  particles  takes  place  in  the  presence  of  certain 
kinds  of  organic  matter;  and  a  class  of  facts  of  great  interest  in 
their  bearing  upon  the  mode  of  formation  of  various  calcified  struc- 
tures in  the  bodies  of  animals  was  brought  to  light  by  the  ingenious 
researches  of  Mr.  R-ainey,1  whose  method  of  experimenting  essentially 
consisted  in  bringing  about  a  slow  decomposition  of  the  calcium  salts 
contained  in  gum-arabic  by  the  agency  of  potassium  hydrogen  car- 
bonate. The  result  is  the  formation  of  spheroidal  concretions  of  calcium 
carbonate,  which  progressively  increase  in  diameter  at  the  expense  of 
an  amorphous  deposit  which  at  first  intervenes  between  them,  two 
such  spherules  sometimes  coalescing  to  produce  '  dumb-bells,'  whilst 
the  coalescence  of  a  larger  number  gives  rise  to  the  mulberry-like 
body  shown  in  fig.  817,  b.  The  particles  of  such  composite  spherules 
appear  subsequently  to  undergo  rearrangement  according  to  a  definite 
plan  of  which  the  stages  are  shown  at  c  and  d ;  and  it  is  upon  this 
plan  that  the  further  increase  takes  place,  by  which  such  larger  con- 
cretions as  are  shown  at  a,  a> 
are  gradually  produced .  The 
structure  of  these,  especially 
when  examined  by  polarised 
light,  is  found  to  correspond 
very  closely  with  that  of  the 
small  calculous  concretions 
which  are  common  in  the 
urine  of  the  horse,  and 
which  were  at  one  time 
supposed  to  have  a  matrix 
of  cellular  structure.  The 
small  calcareous  concretions 
termed  otoliths,  or  ear- 
stones,  found  in  the  audi- 
tory sacs  of  fishes,  present  an 
arrangement  of  their  par- 
ticles essentially  the  same. 

Similar  concretionary  spheroids  have  already  been  mentioned  as 
occurring  in  the  skin  of  the  shrimp  and  other  imperfectly  calcified 
shells  of  Crustacea ;  they  occur  also  in  certain  imperfect  layers  of 
the  shells  of  Mollusca ;  and  we  have  a  very  good  example  of  them 
in  the  outer  layer  of  the  envelope  of  what  is  commonly  known  as  a 
'  soft  egg,'  or  an  '  egg  without  shell,'  the  calcareous  deposit  in  the 
fibrous  matting  already  described  being  here  insufficient  to  solidify 
it.  In  the  external  layer  of  an  ordinary  egg-shell,  on  the  other 
hand,  the  concretions  have  enlarged  themselves  by  the  progres- 
sive accretion  of  calcareous  particles,  so  as  to  form  a  continuous 
layer,  which  consists  of  a  series  of  polygonal  plates  resembling  those 
of  a  tessellated  pavement.  In  the  solid  '  shells  '  of  the  eggs  of  the 

1  See  his  treatise  '  On  the  Mode  of  Formation  of  the  Shells  of  Animals,  of  Bone, 
and  of  several  other  structures,  by  a  process  of  Molecular  Coalescence,  demonstrable 
in  certain  artificially  formed  products,'  1858;  and  his  'Further  Experiments  and 
Observations  '  in  Quart.  Journ.  Microsc.  Sci.  n.s.  vol.  i.  1801,  p.  23. 


FIG.  817.— Artificial  concretions  of 
carbonate  of  lime. 


HARTING'S    CALCO-GLOBULDsE  HOI 

ostrich  and  cassowary  this  concretionary  layer  is  of  considerable 
thickness  ;  and  vertical  as  well  as  horizontal  sections  of  it  are  very 
interesting  objects,  showing  also  beautiful  effects  of  colour  under  polar- 
ised light.  And  from  the  researches  of  Professor  W.  C.  Williamson 
on  the  scales  of  fishes,  there  can  be  no  doubt  that  much  of  the 
calcareous  deposit  which  they  contain  is  formed  upon  the  same  plan. 
This  line  of  inquiry  has  been  contemporaneously  pursued  by 
Professor  Harting,  of  Utrecht,  who,  working  on  a  plan  funda- 
mentally the  same  as  that  of  Mr.  Rainey  (viz.  the  slow  precipitation 
of  insoluble  calcium  salts  in  the  presence  of  an  organic  'colloid'), 
has  not  only  confirmed  but  greatly  extended  his  results,  showing 
that  with  animal  colloids  (such  as  egg-albumen,  blood-serum,  or  a 
solution  of  gelatine)  a  much  greater  variety  of  forms  may  be  thus 
produced,  many  of  them  having  a  strong  resemblance  to  calcareous 
.structures  hitherto  known  only  as  occurring  in  the  bodies  of  animals 
of  various  classes.  The  mode  of  experimenting  usually  followed  by 
Professor  Harting  was  to  cover  the  hollow^  of  an  ordinary  porcelain 
plate  with  a  layer  of  the  organic  liquid  to  the  depth  of  from  O4  to 
0*6  of  an  inch,  and  then  to  immerse  in  the  border  of  the  liquid, 
but  at  diametrically  opposite  points,  the  solid  salts  intended  to  act 
on  one  another  by  double  decomposition,  such  as  calcium  chloride, 
nitrate,  or  acetate,  aiid  potassium  or  sodium  carbonate;  so  that, 
being  very  gradually  dissolved,  the  two  substances  may  come  slowly 
to  act  upon  each  other,  and  may  throw  down  their  precipitate  in  the 
midst  of  the  '  colloid.1  The  whole  is  then  covered  with  a  plate  of  glass, 
and  left  for  some  days  in  a  state  of  perfect  tranquillity  ;  when  there 
begins  to  appear  at  various  spots  on  the  surface  minute  points  re- 
flecting light,  which  gradually  increase  and  coalesce,  so  as  to  form  a 
crust  that  comes  to  adhere  to  the  border  of  the  plate  ;  whilst  another 
portion  of  the  precipitate  subsides,  and  covers  the  bottom  of  the 
plate.  Round  the  two  spots  where  the  salts  are  placed  in  the  first 
instance  the  calcareous  deposits  have  a  different  character  :  so  that 
in  the  same  experiment  several  very  distinct  products  are  generally 
obtained,  each  in  some  particular  spot.  The  length  of  time  requisite 
is  found  to  vary  with  the  temperature,  being  generally  from  two  to 
eight  weeks.  By  the  introduction  of  such  a  colouring  matter  as 
madder,  logwood,  or  carmine,  the  concretions  take  the  hue  of  the 
one  employed.  When  these  concretions  are  treated  with  dilute 
acid,  so  that  their  calcareous  particles  are  wholly  dissolved  out, 
there  is  found  to  remain  a  basis  substance  which  preserves  the  form 
of  each  ;  this,  which  consists  of  the  '  colloid '  somewhat  modified,  is 
termed  by  Harting  calco-globuline.  Besides  the  globular  concretions 
with  the  peculiar  concentric  and  radiating  arrangement  obtained 
by  Mr.  Rainey  (fig.  817),  Professor  Harting  obtained  a  great 
variety  of  forms  bearing  some  resemblance  to  the  following  :  1.  The 
*' discoliths '  and  *  cyatholiths '  of  Huxley.  2.  The  tuberculated 
'spicules'  of  Alcyonaria,  and  the  very  similar  spicules  in  the 
mantle  of  some  species  of  Doris.  3.  Lamella?  of  'prismatic  shell - 
substance,'  which  are  very  closely  imitated  by  crusts  formed  of 
flattened  polyhedra,  found  on  the  surface  of  the  '  colloid.'  4.  The 
spheroidal  concretions  wrhich  form  a  sort  of  rudimentary  shell  within 


1 102  MICBOOKYSTALLISATION,   ETC. 

the  body  of  Limax.  5.  The  sinuous  lamellae  which  intervene  between 
the  parallel  plates  of  the  ;  sepiostaire'  of  the  cuttle-fish,  the  imitation 
of  this  being  singularly  exact.  6.  The  calcareous  concretions  that 
give  solidity  to  the  *  shell'  of  the  bird's  egg,  the  semblance  of  which 
Professor  Harting  was  able  to  produce  in  situ  by  dissolving  away 
the  calcareous  component  of  the  egg-shell  by  dilute  acid,  then  im- 
mersing the  entire  egg  in  a  concentrated  solution  of  calcium  chloride, 
and  transferring  it  thence  to  a  concentrated  solution  of  potassium 
carbonate,  with  which,  in  some  cases,  a  little  sodium  phosphate 
was  mixed.1  Other  forms  of  remarkable  regularity  and  definite- 
ness,  differing  entirely  from  anything  that  ordinary  crystallisation 
would  produce,  but  not  known  to  have  their  parallels  in  living  bodies, 
have  been  obtained  by  Professor  Harting.  Looking  to  the  relations 
between  the  calcareous  deposits  in  the  scales  of  fishes  and  those  by 
which  bones  and  teeth  are  solidified,  it  can  scarcely  be  doubted  that 
the  principle  of  ;  molecular  coalescence '  is  applicable  to  the  latter, 
as  well  as  to  the  former  ;  and  that  an  extension  and  variation  of  this 
method  of  experimenting  would  throw  much  light  on  the  process  of 
ossification  and  tooth  formation.  The  connection  of  these  results 
with  the  work  of  Vogelsang  (p.  1096)  on  globulites  and  other 
embryonic  crystalline  forms  is  obvious.  The  inquiry  has  been 
further  prosecuted  by  Dr.  W.  M.  Ord,  with  express  reference  to  the 
formation  of  urinary  and  other  calculi.2 

Micro-chemical  Analysis. — The  methods  which  serve  for  the 
qualitative  analysis  of  chemical  substances,  and  which  are  based 
upon  the  reactions  shown  by  such  substances  when  treated  with 
solutions  of  various  reagents,  have  been  applied  by  numbers  of 
workers  to  the  identification  of  the  constituents  of  a  material  by  i  IK- 
aid  of  chemical  reactions,  the  results  of  which  are  traced  upon  the 
microscope  stage.  Thus  a  very  complete  scheme  has  been  worked 
out  by  H.  Behrens  for  the  detection  of  the  constituents  of  inorganic 
compounds,3  and  a  somewhat  similar,  although  naturally  less  com- 
prehensive, scheme  has  been  given  by  the  same  author  for  the 
identification  of  organic  compounds.4  The  analytical  methods  arc 
intended  primarily  to  serve  for  identifying  the  components  of  a 
material  available  only  in  small  quantities ;  but  in  many  cases  the 
micro-chemical  method  is  more  rapidly  applied,  and  is  more 
accurate  in  its  results,  than  the  ordinary  processes  of  qualitative 
analysis.  In  applying  the  microscope  for  this  purpose  the  substance 
to  be  examined  is  placed  upon  a  watch-glass  or  glass  slide,  either  in 
the  solid  state  or  in  the  form  of  a  solution  ;  the  various  crystalline 
forms  which  make  their  appearance  as  a  result  of  the  addition  of 
different  reagents  are  then  noted,  -and  from  the  information  thus 
obtained  a  knowledge  of  the  constituents  of  the  original  substance  is 
deduced.  A  very  important  application  of  micro-chemical  analysis 

1  See  Prof.  Harting's  Eecherches  (le  Morphologic  synthetique  sur  la  production 
artificielle  de  quelques  Formations  Calcaireslnorganiques,publiees  par  I'Acadcni  ic 
Royale  Neerlandaise  des  Sciences,  Amsterdam,  1872;  and  Quart.  Joiirn.  Microsc. 
Scl  xii.  p.  118. 

2  See  his  treatise  On  the  Influence  of  Colloids  upon  Crystalline   F«rm   and 
Cohesion,  London,  1879. 

5  Anleitung  zur  inikrochemischoi  Analyse,  Hamburg,  1895. 

4  Mikrochemischen  Analyse  der  organischen  VerHndungen,  Hamburg-,  Lsur>. 


MICEO-CHE3IICAL  ANALYSIS 


I  103 


has  been  made  in.  connection  with  the  detection  of  poisons,  and  by 
a  judicious  combination  of  microscopical  with  chemical  research, 
the  application  of  reagents  may  be  made  effectual  for  the  de- 
tection of  poisonous  or  other  substances  in  quantities  far  more 
minute  than  have  been  previously  supposed  to  be  recognisable. 
Thus  it  is  stated  by  I)v.  Wormley  l  that  micro-chemical  analysis 
enables  us  by  a  very  few  minutes'  labour  to  recognise  with  un- 
erring certainty  the  reaction  of  the  iooVootn  Part  °f  a  grain  of 
either  hydrocyanic  acid,  mercury,  or  arsenic  ;  and  that  in  many 
other  instances  we  can  easily  detect  by  its  means  the  presence  of 
very  minute  quantities  of  substances,  the  true  nature  of  which 
could  only  be  otherwise  determined  in  comparatively  large  quantity,, 
and  by  considerable  labour.  This  inquiry  may  be  prosecuted,  how- 
ever, not  only  by  the  application  of  ordinary  chemical  tests  under 
the  microscope,  but  also  by  the  use  of  other  means  of  recognition 
which  the  use  of  the  microscope  affords.  Thus  it  has  been  shown  that 
by  the  careful  sublimation  of  arsenic  and  arsenious  acid,  the  subli- 
mates being  deposited  upon  small  discs  of  thin  glass,  these  are  dis- 
tinctly recognisable  by  the  forms  they  present  under  the  microscope 
(especially  the  binocular)  in  extremely  minute  quantities  ;  and  that 
the  same  method  of  procedure  may  be  applied  to  the  volatile  elements* 
mercury,  cadmium,  selenium,  tellurium,  and  some  of  their  compounds, 
and  to  some  other  volatile  bodies,  as  sal-ammoniac,  camphor,  and 
sulphur.  The  method  of  sublimation  was  afterwards  extended  to  the 
vegetable  alkaloids,  such  as  morphine,  strychnine,  veratririe,2  &c. 
And  subsequently  it  was  shown  that  the  same  method  could  be  further 
extended  to  such  animal  products  as  the  constituents  of  the  blood 
and  of  urine,  and  to  volatile  and  decomposable  organic  substances 
generally  By  the  careful  prosecution  of  micro-chemical  inquiry, 
especially  with  the  aid  of  the  spectroscope  (where  possible),  the 
detection  of  poisons  and  other  substances  in  very  minute  quantity 
can  be  accomplished  with  a  facility  and  certainty  such  as  were 
formerly  scarcely  conceivable. 

1  Micro-chemistry  of  Poisons,  New  York,  1857. 

a  See  Wynter  Blyth,  Poisons,  their  Effects  and  Detection,  London,  1895. 


APPENDICES    AND    TABLES 

USEFUL    TO    THE    MICROSCOPIST 


4  B 


1 107 

APPENDIX    A 

TABLE   OF  NATURAL   SINES 


o 

0' 

15'=  J° 

30'=i° 

45'=f° 

0 

0' 

15'=J° 

30'  =  \° 

45'=s° 

0 

•0000 

•0044 

•0087 

•0131 

46 

•7193 

•7224 

•7254 

•7284 

1 

•0175 

•0218 

•0262 

•0305 

47 

•7314 

•7343 

•7373 

•7402 

2 

•0349 

•0393 

•0436 

•0480 

48 

•7431 

•7461 

•7490 

•7518 

3 

•0523 

•0567 

•0610 

•0654  '" 

49 

•7547 

•7576 

•7604 

•7632 

4 

•0698 

•0741 

•0785 

•0828 

50 

•7660 

•7688 

•7716 

•7744 

5 

•0872 

•0915 

•0958 

•1002 

51 

•7771 

•7799 

•7826 

•7853 

6 

•1045 

•1089 

•1132 

•1175 

52 

•7880 

•7907 

•7934 

•7960 

1 

•1219 

•1262 

•1305 

•1349 

53 

•7986 

•8013 

•8039 

•8064 

8 

•1392 

•1435 

•1478 

•1521 

54 

•8090 

•8116 

•8141 

•8166 

9 

•1564 

•1607 

•1650 

•1693 

55 

•8192 

•8216 

•8241 

•8266 

10 

•1736 

•1779 

•1822 

•1865 

56 

•8290 

•8315 

•8339 

•8363 

11 

•1908 

•1951 

•1994 

•2036 

57 

•8387 

•8410 

•8434 

•8457 

12 

•2079 

•2122 

•2164 

•2207 

58 

•8480 

•8504 

•8526 

•8549 

13 

•2250 

•2292 

•2334 

•2377 

59 

•8572 

•8594 

•8616 

•8638 

14 

•2419 

•2462 

•2504 

•2546 

60 

•8660 

•8682 

•8704 

•8725 

15   -2588 

•2630 

•2672 

•2714 

61 

•8746 

•8767 

•8788 

•8809 

1C,   -2756 

•2798 

•2840 

•2882 

62 

•8829 

•8850 

•8870 

•8890 

17 

•2924 

•2965 

•3007 

•3049 

63 

•8910 

•8930 

•8949 

•8969 

18 

•3090 

•3132 

•3173 

•3214 

64 

•8988 

•9007 

•9026 

•9045 

19 

•3256 

•3297 

•3338 

•3379 

65 

•9063 

•9081 

•9100 

•9118 

20 

•3420 

•3461 

•3502 

•3543 

66 

•9135 

•9153 

•9171 

•9188 

21 

•3584 

•3624 

•3665 

•3706 

67 

•9205 

•9222 

•9239 

•9255 

22 

•3746 

•3786 

•3827 

•3867 

68 

•9272 

•9288 

•9304 

•9320 

23 

•3907 

•3947 

•3987 

•4027 

69 

•9336 

•9351 

•9367 

•9382 

24 

•4067 

•4107 

•4147 

•4187 

70 

•9397 

•9412 

•9426 

•9441 

25 

•4226 

•4266 

•4305 

•4344 

71 

•9455 

•9469 

•9483 

•9497 

26 

•4384 

•4423 

•4462 

•4501 

72 

•9511 

•9524 

•9537 

•9550 

27 

•4540 

•4579 

•4617 

•4656 

73 

•9563 

•9576 

•9588 

•9600 

28 

•4695 

•4733 

•4772 

•4810 

74 

•9613 

•9625 

•9636 

•9648 

29 

•4848 

•4886 

•4924 

•4962 

75 

•9659 

•9670 

•9681 

•9692 

30 

•5000 

•5038 

•5075 

•5113 

76 

•9703 

•9713 

•9724 

•9734 

31 

•5150 

•5188 

•5225 

•5262 

77 

•9744 

•9753 

•9763 

•9772 

32 

•5299 

•5336 

•5373 

•5410 

78 

•9781 

•9790 

•9799 

•9808 

33 

•5446 

•5483 

•5519 

•5556 

79 

•9816 

•9825 

•9833 

•9840 

34 

•5592 

•5628 

•5664 

•5700 

80 

•9848 

•9856 

•9863 

•9870 

35 

•5736 

•5771 

•5807 

•5842 

81 

•9877 

•9884 

•9890 

•9897 

36 

•5878 

•5913 

•5948 

•5983 

82 

•9903 

•9909 

•9914 

•9920 

37 

•6018 

•6053 

•6088 

•6122 

83 

•9925 

•9931 

•9936 

•9941 

38 

•6157 

•6191 

•6225 

•6259 

84 

•9945 

•9950 

•9954 

•9958 

39 

•6293 

•6327 

•6361 

•6394 

85 

•9962 

•9966 

•9969 

•9973 

40 

•6428 

•6461 

•6494 

•6528 

86 

•9976 

•9979 

•9981 

•9984 

41 

•6561 

•6593 

•6626 

•6659 

87 

•9986 

•9988 

•9990 

•9992 

42 

•6691 

•6724 

•6756 

•6788 

88 

•9994 

•9995 

•9997 

•9998 

43 

•6820 

•6852 

•6884 

•6915 

89 

•9998 

•9999 

1-0000 

1-0000 

44 

•6947 

•6978 

•7009 

•7040 

90 

1-0000 

45  -7071 

•7102 

•7133 

•7163 

Note.— The  sine  of  any  given  angle  is  the  length  of  the  perpendicular  opposite  the  given  angle  i 
right-angled  triangle  which  contains  the  given  angle  divided  by  the  length  of  the  hypotenuse.    T 
above  table  is  constructed  on  the  principle  that  the  hypotenuse  is  always  equal  to  unity,  by  which 
means  the  fraction  is  got  rid  of,  as  the  denomiaator  may  be  left  out.    Thus, 


na 
The 


hypotenuse 


=.5 


4B2 


io8 


APPENDICES   AND   TABLES 


APPENDIX   B 

TABLE   OF  REFRACTIVE  INDICES 


Substance 


Refractive  Index 


ftp 

Water M  D  1'334  54-7 

Saliva ME  1-339 

Sea-water ME  1-343 

Human  blood ME  1-354 

Alum  (sat.  sol.) M  D  1-457 

Ether  (60°  Fahr.) M  »  1*357  84-9 

Albumen '  .        .         .  M  D  1-350 

Absolute  Alcohol M  D  1-364  58-6 

Oil  of  Ambergris M  E  1-368 

Salt  (sat.  sol.) ME  1'375 

Fluor  Spar M  D  1-4338          97'3 

Diatom  Silex M  D  1-434 

Spermaceti M  »  1*503 

Bees-wax M  »  1*553 

Oil  of  Olives  (sp.  gr.  0-913) M  D  1-476  54-7 

Borax M  D  1*515  60-6 

Naphtha ME  1-475 

Oil  of  Turpentine  (sp.  gr.  -885)    .        .         .         .  M  D  1-474  46-5 

Oil  of  Linseed  (sp.  gr.  -932) M  D  1'485 

Castor  Oil     . M  D  1-490 

Chloride  of  Tin /t  D  1-503 

Oil  of  Cinnamon M  D  1-619  14-3 

Oil  of  Cedar M  D  1*510 

Gum  Arabic ME  1-512 

Dammar M  »  '520 

Oil  of  Cloves M  D       '533 

Sugar M  D       '535 

Felspar ft  D       '764 

Cedrene M  D       '539 

Canada  Balsam M  D  1-526  41-5 

Oil  of  Fennel M  D  1-544 

Eock  Crystal M  D  1-545  70-0 

Eock  Salt  (sp.  gr.  2-143) M  »  1'555 

Nitro-benzene       .         .        .        .        .        .         .  M  D  1'558 

Styrax M  D  1'582 

Meta-cinnamene M  D  1*597  29-8 

•Quinidine M »  1*602  24-1 

Benzylaniline M  D  1*611 

Methyldiphenylamine M  D  1*616 

Balsam  of  Tolu ME  1*618 

Bisulphide  of  Carbon  .         .        .        .        ..         .  M  D  1*630  18-3 

Oil  of  Cassia M  D  1*578  17 -0 

Quinoline M  D  1*633 

Tourmalin  (ordinary  ray) M  »  1*668 

Kreasote /* »  1*538  29-9 

Petroleum M  D  1*457  15-3 

Phenyl-thiocarbimide M  »  1*654  18-7 

Iceland  Spar  (ordinary  ray) M  i>  1-657  49-0 


USEFUL  TO   THE  MICROSCOPIST 


1109 


Substance 

Monobromonaphthalene 
Pipeline  and  Balsam 
Naphthyl-phenyl-ketone 
Bromide  of  Antimony  . 
Pipeline 

Methylene  di-iodide 
Sulphur  in  methyle 
Zircon 

Carbonate  of  Lead 
BorateofLead     . 
Phosphorus  in  meth 
Sulphur  (melted)  . 
Phosphorus 
Diamond  (sp.  gr.  3-4) 
Chromate  of  Lead 
Eealgar  (artificial) 


Refractive  Index 

19^9 

.        . 

.        . 

AtD 

1-657 

ne 

11  D 

1-669 

17-6 

(approximately) 

AtD 

1-680 

. 

. 

. 

At  D 

1-681 

9-88 

f 

t 

t 

A*  D 

1-743 

21-2 

di-iodide 

.        . 

. 

AtD 

1-778 

— 

. 

. 

. 

AtD 

1-950 

— 

. 

. 

. 

AtD 

1-81  to  2-08 

— 

*•• 

. 

AtA 

1-866 

— 

sne  di-iodide  (equal  weights) 

AtD 

1-944 

17-1 

.  • 

. 

. 

At  E 

2-148 

— 

. 

. 

. 

AtD 

2-224 

— 

.        . 

.        . 

. 

At  D 

2-47 

— 

tt  D 

2-50  to  2-97 

___ 

r*   •* 

At  E 

2-549 



Glass 


Substance                                                                     Refractive  Index  ^ — 

Crown            n  D     1-51  to  1-56  59-0  to  46-0 

Plate  /j.  D     1-516 

Extra  Light  Flint           .        .        .        .  ^  D     1'541  49-2 

Light  Flint A* »     1'574  41-0 

Dense  Flint            .                                      A*  D     1'622  36-5 

Extra  Dense  Fluid         .                             A*  D     1'650  34-2 

Double  Extra  Dense  Flint                          A*  D     1'710  30-0 

Boro-silicate  Crown                             A4 »     1'51  64-0 

Phosphate  Crown                                 A*  D     1*51  to  1-56  70'0  to  67'0 

Barium  Silicate  Crown                        A* »     1'54  „   1-60  59-0  „  55-0 

Boro-silicate  Flint                                A*  D     1*55  „   1-57  49-0  „  47'0 

Borate  Flint             .                             A*  D     1'55  „   1-68  55-0  „  33-0 

Barium  Phosphate  Crown                   A*  D     1'58  65-2 

Very  heavy  Silicate  Flint                    A*  D     1-963  19'7 
Glass  of  Antimony        .        .                   A*  D    2-216 

The  extraordinary  dispersion  of  the  alkaloid  Piperine  will  be  noticed.     Its 

refractive  index  is  less  than  that  of  Chance's  Double  Extra  Dense  Flint,  yet 
Piperine  has  three  times  its  dispersion. 


JIIO 


APPENDICES  AND  TABLES 


APPENDIX  0 


TABLE   OF  ENGLISH  MEASURES  AND   WEIGHTS,   WITH    THEIR 
METRICAL  EQUIVALENTS 

The  following  are  calculated  from  the  values  of  the  metre,  determined 
in  1896,  and  the  kilogramme  in  1883,  by  the  order  of  the  Board  of  Trade. 

LENGTH 

Inch =  2-539998  Centimetres. 

Foot  =  12  inches =  3-047997  Decimetres. 

Yard  =  3  feet =   '914399  Metre. 

Fathom  =  2  yards     ......  =  1-828798      „ 

Pole  =  5$  yards =  5-029196  Metres. 

Chain  =  4  poles =  2-011678  Decametres. 

Furlong  =  10  chains =  2-011678  Hectometres. 

Statute  Mile  =  8  furlongs  =  5,280  feet  =  1-609343  Kilometre. 

Geographical  Mile  =  6,087-23  feet  .  =  1-855386 

Knot  =  6,080  feet =1-853182         „ 


SUPERFICIES 


Square  Inch 

„        Foot 
Yard 


144  Sq.  Inches     = 
9  Feet   = 


Perch  =  30£     „      Yards 
Kood  =  40  Perches      . 
Acre  =  4  Eoods      .     . 
Square  Mile 


6-45159  Square  Centimetres. 
•00645  MiUiare. 
•92903 

8-36126  Milliares. 
•83613  Centiare. 
2-52928  Declares. 
=   10-11712  Ares. 
=   40-46849     „ 
=  258-99836  Hectares. 


VOLUME 

Cubic  Inch =  16-387  Cubic  Centimetres. 

„      Foot.     .      =1728  Cubic  Inches  =   2-83168  Centisteres. 
„      Yard.     .      =27          „     Feet     =   7-64553  Decisteres. 

CAPACITY 
Apothecaries1 

Minim,  Ul    .     *    .     =       -05919  Cubic  Centimetre  or  Millilitre. 
Drachm,  f  5  =  60  11^  =     3-5515       „       Centimetres  or  Millilitres. 
Ounce,  fj     =8f5    =   28-4123       „  „  =  2-84123  Centilitres. 

Pint,  O    .     =  20  f  ^  =  568-245         „  „  =  5-68245  Decilitres. 

Gallon,  C      =80=     4-54596     „       Decimetres,  Millisteres,  or  Litres. 


Gill .     . 
Pint      . 

Quart  . 
Gallon. 
Peck  . 
Bushel 
Quarter 


Imperial 

.     .     .     =  142-061     Cubic  Centimetres  =  1*42061  Decilitre. 
=  4  gills       =568-245         „  „  =  5-68245  Decilitres. 


=  2  pints 
=  4  quarts  = 
=  2  gallons  = 
=  4  pecks  = 
=  8  bushels  = 


1-13649 

4-54596 

9-09193 

3-63677  Decalitres. 

2=90942  Hectolitres. 


Decimetre,  Millistere,  or  Litre. 
Decimetres,  Millisteres,  or  Litres. 


USEFUL  TO   THE   MICROSCOPIST 


IIII 


WEIGHT 

Apothecaries' 

Grain,  gr =  6'479892  Centigrammes. 

Scruple,  3 =   20  gr.  =  1-29598    Gramme. 

Drachm,  5  .     .     .     =  3  3  =   60  gr.  =  3-88794    Grammes. 
Ounce,  5     .     .     .     =85  =  480  gr.  =  3-11035    Decagrammes. 

Avoirdu/pois 

6-479892    Centigrammes. 

1-77185      Gramme. 

2-83495      Decagrammes. 

4-5359243  Hectogrammes. 

6-35029      Kilogrammes. 
12-70059 

Hundredweight,  cwt.  =   4  qr =  50-80235  =  '50802  Quintal. 

Ton  =  20  cwt.    .  .     =   1-01605      Tonne. 


Drachm,  dr.      .     . 
Ounce,  oz.    .     .     . 
Pound,  Ib.    .     .     . 
Stone,  st.      ... 
Quarter,  qr.       .     . 

=  16  dr. 
=  16  oz. 
=  14  Ib. 
=  28  Ib. 

=  27-34375  gr.  = 
437-5  gr.  = 
7000  gr.= 

1  Ib.  Avoirdupois  •• 
1  Ib.  Troy  or  Apothecaries  = 


•822857  Ib.  Troy  or  Apothecaries'. 
1-2152?  Ib.  Avoirdupois. 


TABLE    OF   METRIC    MEASURES   AND    WEIGHTS,   WITH    THEIR 
ENGLISH  EQUIVALENTS 


The  metre  was  originally  intended  to  be  the  TtfW&rnnith  Part  of  tne 
distance  from  the  pole  of  the  earth  to  the  equator,  measured  along  a 
certain  meridian,  but  owing  to  an  error  its  length  is  too  short.  The 
metre  is  therefore  the  length  of  a  definite  standard  in  Paris. 


LENGTH 


Micron,  i.e.  , 
Millimetre 
Centimetre 
Decimetre    , 
METRE    .     . 

Decametre  , 
Hectometre 
Kilometre  . 


Milliare  . 
Centiare  . 
Deciare  . 
Are  =  Unit 
Hectare  . 


=  Unit 


Millimetre . 
Centimetre 
Decimetre . 
Metre  .  . 


10  Metres  .  .  . 
10  Decametres  . 
10  Hectometres . 


=    -00003937  Inch. 

=    -03937 

=    -39370 

=  3-93701  Inches. 

=  3-28084  Feet. 

=  1-093614  Yard. 

=  1-98839  Pole. 

=  4-97097  Chains. 

=  4-97097  Furlongs. 

=   -6213716  Statute  Mile. 

=   -5389714  Geographical  Mile. 

=   -5396124  Knot. 


SUPERFICIES 

=  10  Sq.  Decimetres  =     1-07639  Sq.  Ft.  =  155-0006  Sq.  In. 

=    1    „    Metre  =      1-19599  Square  Yard. 

=  10   „    Metres          =    11-95992       „       Yards. 

=   1  „    Decametre    =119-59921 

=    1    ,     Hectometre  =     2-47106  Acres. 


VOLUME 

Millistere     .  .  =     1  Cubic  Decimetre  =   61-0239  Cubic  Inches. 

Centistere    .  .  =10      „     Decimetres  =  610-239        „  „ 

Decistere     .  .  =100      „              „          =     3-531476,,  Feet. 

Stere-Unit  .  =      1      „         Metre       =      1'30795    „  Yard. 

Decastere    .  .  =10      „         Metres     =   13-07954    „  Yards. 

Hectostere  .  =    10  Decasteres           =  130-7954      „  „ 


II  12 


APPENDICES   AND   TABLES 


CAPACITY 

Millilitre    -  Cubic  Centimetre =  -007039  Irnpr.  Gill. 

Centilitre  =  10  Cubic  Centimetres =    '07039 

Decilitre    -100     „  „  =    -7039 

Litre    .      =  Millistere -1-7598  Pint. 

Decalitre  - 10  Litres =2-19975          Gals. 

Hectolitre  =  10  Decalitres =  2-74969          Bush. 

Kilolitre    =  10  Hectolitres  =  1  Stere  =  1  Cubic  Metre  =  3-43712          Qrs. 


Milligramme 

Centigramme 

Decigramme 

Gramme .     . 

Decagramme 

Hectogramme 

Kilogramme 

Myriagramrne 

Quintal  .     . 

Tonneau 


WEIGHT 

=  ^j  Centigramme 
=  3^  Decigramme 
=  ^3  Gramme 
=  Unit 

=  10  Grammes 
=  10  Decagrammes 
— 10  Hectogrammes 
=  10  Kilogrammes 
=  10  Myriagranirnes 
=- 10  Quintals 


Avoirdupois 
=     -01543  Grain. 
=      -15432       „ 
=    1-54324       „ 
=  15-432356  Grains. 
=    5-64383  dr. 
=   3-5274  oz. 
-   2-204622  Ib. 
=  22-04622     „ 
=   1-96841  cwt. 
=      -98421  ton. 


The  legal  equivalent  of  the  metre  is  39-37079  inches,  and  of  the  kilo- 
gramme 15432-34874  grains.  In  the  above  tables  the  values  obtained  in 
1883  and  1896  by  the  order  of  the  Board  of  Trade  have  been  adopted  as 
being  the  toore  accurate.  In  1893  the  metre  was  measured  by  Eogers, 
who  found  it  equal  to  39-370155  inches. 

Weights  can  be  more  accurately  compared  than  either  lengths  or 
capacities.  The  actual  weight  of  the  standard  kilogramme  in  Paris  is 
15432-35639  grains,  and  the  English  avoirdupois  pound  is  equal  to 
453-5924277  grammes. 


USEFUL  TO   THE   MICROSCOPIST 


III3 


CONVERSION  OF  BRITISH  AND  METRIC  MEASURES 

Computed  by  Mr.  E.  M.  Nelson  from  the  New  Coefficient  obtained  by  Order  of 
the  Board  of  Trade  in  1896. 


LINEAL. 

Metric  into  British. 

u. 

ins. 

mm. 

ins. 

mm. 

ins. 

1 

•000039 

1 

•039370 

51 

2-007876 

2 

•000079 

2 

•078740 

52 

2-047246 

3 

•000118 

3 

•118110 

53 

2-086616 

4 

•000157 

4 

•157480 

54 

2-125986 

5 

•000197 

5 

•196851 

55 

2-165356 

6 

•000236 

6 

*  -236221 

56 

2-204726 

7 

•000276 

7 

•275591 

57 

2-244096 

8 

•000315 

8 

•314961 

58 

2-283467 

9 

•000354 

9 

•354331 

59 

2-322837 

10 

•000394 

10 

•393701 

60 

2-362207 

11 

•000433 

11 

•433071 

61 

2-401577 

12 

•000472 

12 

•472441 

62 

2-440947 

13 

•000512 

13 

•511811 

63 

2-480317 

24 

•000551 

14 

•551182 

64 

2-519687 

15 

•000591 

15 

•590552 

65 

2-559057 

IS 

•000630 

16 

•629922 

66 

2-598427 

17 

•000669 

17 

•669292 

67 

2-637798 

IS 

•000709 

18 

•708662 

68 

2-677168 

19 

•000748 

19 

•748032 

69 

2-716538 

20 

•000787 

20 

•787402 

70 

2-755908 

21 

•000827 

21 

•826772 

71 

2-795278 

22 

•000866 

22 

•866142 

72 

2-834648 

23 

•000906 

23 

•905513 

73 

2-874018 

24 

•000945 

24 

•944883 

74 

2-913388 

25 

•000984 

25 

•984253 

75 

2-952758 

26 

•001024 

26 

1-023623 

76 

2-992129 

27 

•001063 

27 

1-062993 

77 

3-031499 

28 

•001102 

28 

1-102363 

78 

3-070869 

29 

•001142 

29 

1-141733 

79 

3-110239 

30 

•001181 

30 

1-181103 

80 

3-149609 

31 

•001220 

31 

1-220473 

81 

3188979 

32 

•001260 

32 

1-259844 

82 

3-228349 

33 

•001299 

33 

1-299214 

83 

3-267719 

34 

•001339 

34 

1-338584 

84 

3-307089 

35 

•001378 

35 

1-377954 

85 

3-346460 

36 

•001417 

36 

1-417324 

86 

3-385830 

37 

•001457 

37 

1-456694 

87 

3-425200 

38 

•001496 

38 

1-496064 

88 

3-464570 

39 

•001535 

39 

1-535434 

89 

3-503940 

40 

•001575 

40 

1-574805 

90 

3-543310 

41 

•001614 

41 

1-614175 

91 

3-582680 

42 

•001654 

42 

1-653545 

92 

3-622050 

43 

-001693 

43 

1-692915 

93 

3-661420 

44 

•001732 

44 

1-732285 

94 

3-700791 

45 

•001772 

45 

1-771655 

95 

3-740161 

46 

•001811 

46 

1-811025 

96 

3-779531 

47 

•001850 

47 

1-850395 

97 

3-818901 

48 

•001890 

48 

1-889765 

98 

3-858271 

49 

•001929 

49 

1-929136 

99 

3-897641 

60 

•001969 

50 

1-968506 

6O 

•002362 

70 

•002756 

decim.                          ins. 

8O 

•003150 

1                            3-9370113 

9O 

•003543 

2                           7-8740226 

100 

•003937 

3                          11-8110339 

2OO 

•007874 

4                            15-7480452 

3OO 

•011811 

5                           19-6850565 

400 

•015748 

<;                          23-6220678 

5OO 

•019685 

7                          27-5590791 

600 

•023622 

8                          31-4960904 

7OO 

•027559 

9                          35-4331017 

8OO 

•031496 

9OO 

•035433 

1  metre       3-2808428  ft. 

1OOO 

(=1  mm.) 

1-09361425  yd. 

APPENDICES  AND   TABLES 


British  into  Metric. 


in. 
1 

mm. 
25-399978 

2 

50-799956 

3 

76-199934 

4 

101-599912 

5 

126-999890 

6 

152-399868 

7 

177-799846 

8 

203-199824 

9 

228-599802 

10 

253-999780 

11 

279-399758 

1ft. 

304-799736 

lyd. 

914-399208 

in. 

mm. 

i 

12-699989 

-L 

8-466659 

2 

16-933319 

i 

6-349994 

£ 

19-049983 

I 

5-079996 

| 

10-159991 

F 

15-239987 

2 

20-319982 

i 

4-233330 

21-166648 

( 

3-628568 

j 

3-174997 

• 

9-524992 

15-874986 

|  . 

22-224980 

1 

2-822220 

i 

2-539998 

IB 

7-619993 

To 

17-779985 

TO 

22-859980 

in. 


2-309089 

2-116665 

10-583324 

14-816654 

23-283313 

1-953844 

1-814284 

1-693332 

1-587499 

4-762496 

7-937493 

11-112490 

14-287487 

17-462485 

20-637482 

23-812479 

1-494116 

1-411110 

1-336841 

1-269999 

1-209523 

1-154544 

1-104347 

1-058332 

1-015999 

•846666 

•725714 

•634999 

•564444 

•508000 

•461818 

•423333 

•390769 

•362857 

•338666 

•317500 


in. 


mm. 

•298823 
•282222 
•267368 
•254000 
•169333 
•127000 
•301600 
•084667 
•072571 
•063500 
•056444 
•050800 
•046182 
•042333 
•039077 
•036286 
•033867 
•031750 
•029882 
•028222 
•026737 


25-399978 
12-699989 
8-466659 
6-349994 
5-079996 
4-233330 
3-628568 
3-174997 
2-822220 
2-539998 
1-693332 
1-269999 
1-015999 


USEFUL  TO   THE   MICKOSCOPIST 


III5 


TABLE  FOB  THE   CONVERSION  OF  FRACTIONAL  PARTS  OF  AN 
ENGLISH  INCH  INTO  METRICAL  LINEAR  MEASURE. 


1  -^ 

mm. 

i  -f- 

Micra. 

*"  1  -T- 

Micra. 

1  * 

Micra. 

2 

12-70 

33 

770 

66 

385 

99 

256 

3 

8-47 

34 

747 

67 

379 

100 

254 

4 

6-35 

35 

726 

68 

374 

105 

242 

5 

5-08 

36 

706 

69 

368 

110 

231 

6 

4-23 

37 

686 

70 

363 

115 

221 

7 

3-63 

38 

668 

71 

358 

120 

212 

8 

3-17 

39 

651 

72 

353 

125 

203 

9 

2-82 

40 

635 

73 

348 

130 

195 

10 

2-54 

41 

619 

74 

343 

135 

188 

11 

2-31 

42 

605 

75 

339 

140 

181 

12 

2-12 

43 

591 

76 

334 

145 

175 

13 

1-95 

44 

577 

77 

330 

150 

169 

14 

1-81 

45 

564 

78 

326 

155 

164 

15 

1-69 

46 

552 

79 

321 

160 

159 

16 

1-59 

47 

540 

80 

317 

165 

154 

17 

1-49 

48 

529 

81 

314 

170 

149 

18 

1-41 

49 

518 

82 

310 

175 

145 

19 

1-34 

50 

508 

83 

306 

180 

141 

20 

1-27 

51 

498 

84 

302 

185 

137 

21 

1-21 

52 

488 

85 

299 

190 

134 

22 

1-15 

53 

479 

86 

295 

195 

130 

23 

MO 

54 

470 

87 

292 

200 

127 

24 

1-06 

55 

462 

88 

289 

205 

124 

25 

1-02 

56     454 

89 

285 

210 

121 

57 

445 

90 

282 

215 

118 

Micra. 

58 

438 

91 

279 

220 

115 

26 

977 

59 

430 

92 

276 

225 

113 

27 

941 

60 

423 

93 

273 

230 

110 

28 

907 

61 

416 

94 

270 

235 

108 

29 

876 

62 

410 

95 

267 

240 

106 

30 

847 

63 

403 

96 

265 

245 

104 

31 

819 

64 

397 

97 

262 

250 

102 

32 

794 

65 

391 

98 

259 

in6 


APPENDICES   AND   TABLES 


Lines 
per  inch 

Lines 
in  mm. 

Lines 
per  inch 

Lines 
in  mm. 

Fractions 
of  an  inch 

j* 

5,000 

197 

200,000 

7,874 

l-5,000th 

5-08 

10,000 

394 

210,000 

8,268 

10,000 

2-54 

15,000 

591 

220,000 

8,661 

20,000 

1-27 

20,000 

787    . 

230,000 

9,055 

30,000 

•847 

25,000 

984 

240,000 

9,449 

40,000 

•635 

30,000 

1,181 

250,000 

9,843 

50,000 

•508 

35,000 

1,378 

260,000 

10,236 

60,000 

•423 

40,000 

1,575 

270,000 

10,630 

70,000 

•363 

45,000 

1,772 

280,000 

11,024 

80,000 

•317 

50,000 

1,968 

290,000 

11,417 

90,000 

•282 

55,000 

2,165 

300,000 

11,811 

100,000 

•254 

60,000 

2,362 

350,000 

13,780 

110,000 

•231 

65,000 

2,559 

400,000 

15,748 

120,000 

•212 

70,000 

2,756 

450,000 

17,717 

130,000 

•195 

75,000 

2,953 

500,000 

19,685 

140,000 

•181 

80,000 
85,000 

3,150 
3,346 

25,399-98 

Krt  QAA 

Lines  in  p 
1 

150,000 
160,000 

•169 
•159 

90,000 

3,543 

oU,oUU 

fjf*  OAA 

170,000 

•149 

95,000 
100,000 

3,740 
3,937 

1  D,  &\)\) 

101,600 
127,000 

4 
5 

180,000 
190,000 

•141 
•134 

110,000 
120,000 
130,000 
140,000 
150,000 

4,331 
4,724 

5,118 
5,512 
5,906 

152,400 
177,800 
203,200 
228,600 
254,000 

6 

7 
8 
9 
10 

200,000 
250,000 
300,000 
350,000 
400,000 

•127 
•1016 

•0847 
•0726 
•0635 

160,000 

6,299 

450,000 

•0564 

170,000 

6,693 

500,000 

•0508 

180,000 

7,087 

190,000 

7,480 

USEFUL  TO  THE  MICROSCOPIST  1117 

It  is  often  necessary  in  the  examination  of  a  photo-micrograph  of 
diatomic  or  other  periodic  structures  to  determine  at  what  rate  per  inch 
or  per  mm.  the  structure  is  in  the  original  object,  the  amplification  of  the 
photo-micrograph  being  known. 

Example :  In  a  photo-micrograph  of  a  diatom  amplified  735  diains. 
12  dots  can  be  counted  in  -3  of  an  inch.  At  what  rate  per  inch  is  the 
structure  in  the  diatom  ? 

-,  magnifying  power  x  number  counted  . 

space  counted 

735  x  12  =  29,400  per  inch. 
•3  inch 

(2)  If  the  answer  is  required  in  rate  per  mm.,  the  space  in  which 
the  number  is  counted  being  in  inches  as  before,  then,  because  1  inch 
=  25*4  mm. 

735JL12-.  =1157-5  per  mm. 
•3  inch  x  25-4 

(3)  Suppose  a  rule  divided  in  mm.  is  used  to  determine  the  space  in 
which  the  number  on  the  photo-micrograph  is  counted,  and  the  rate  per 
inch  is  required ;  if  twelve  dots  can  be  counted  in  7  mm.,  then,  because 
1  inch  =  25 -4  mm. 

735  x  12  x  25-4 


7  mm. 


=  32,004  per  inch. 


III8  APPENDICES  AND  TABLES 


APPENDIX   D 

COMPARISON  OF  THE  SCALES  OF  FAHRENHEIT'S,   THE 
CENTIGRADE,  AND  REAUMUR'S  THERMOMETERS 

THESE  three  thermometers  are  graduated  so  that  the  range  of  temperature 
between  the  freezing  and  boiling  points  of  water  is  divided  by  Fahrenheit's 
scale  into  180°  (from  32°  to  212°),  by  the  Centigrade  into  100°  (from  0°  to 
100°),  and  by  that  of  Reaumur  into  80°  (from  0°to  80°)  portions  or  degrees. 
Hence  we  derive  the  following  equivalents  : — 

A  degree  of  Fahrenheit  is  equal  to  *5  of  the  Centigrade,  or  to  '4  of 
Reaumur's  ;  a  degree  of  the  Centigrade  is  equal  to  1-8  of  Fahrenheit's,  or 
to  *8  of  Reaumur's ;  and  a  degree  of  Reaumur's  is  equal  to  2*25  of 
Fahrenheit's,  or  to  1/25  of  the  Centigrade. 

To  convert  degrees  of  Fahrenheit  into  the  Centigrade  or  Eeaumur's, 
subtract  32  and  multiply  the  remainder  by  f  for  the  Centigrade,  or  £  for 
Reaumur's. 

To  convert  degrees  of  the  Centigrade  or  Reaumur's  into  Fahrenheit's, 
multiply  the  Centigrade  by  f ,  or  Reaumur's  by  f ,  as  the  case  may  be, 
and  add  32  to  the  product. 

EXAMPLE 

Let  F,  C,  and  R  =  the  number  of  degrees  Fahrenheit,  Centigrade,  and 
Reaumur  respectively.  Then 


..ifiT-M,  B_40, 

F  =  C  +  R  +  32. 

This  last  formula  is  of  use,  because  in  England  thermometers  are 
usually  graduated  in  Fahrenheit  and  Centigrade.  Reaumur  may  be  found 
by  inspection  by  subtracting  the  Centigrade  from  the  Fahrenheit  and 
taking  32  from  the  remainder.  On  the  Continent  thermometers  are 
generally  graduated  in  Reaumur  and  Centigrade.  Fahrenheit  can  be  found 
by  adding  Reaumur  and  Centigrade  and  32. — Example :  If  the  thermometer 
reads  8  Reaumur  and  10  Centigrade,  the  Fahrenheit  will  be 
8  +  10  +  32  =  50  F. 


USEFUL  TO  THE  MICHOSCOPIST  I  I  19 


APPENDIX  E 

OPTICAL  FORMULA 

To  find  C,  the  optical  centre  of  a  lens  :  Let  A  and  B  be  the  vertices,  let 
the  radius  of  the  curve  A  =  r,  and  tjiat  of  B  =  s,  t  —  thickness  of  the  lens 
and  p.  the  refractive  index.  Then 

~r-s'  ~r^s 

Example  explaining  the  method  of  treating  the  signs :  First,  it  should 
be  particularly  noticed  that  all  curves  which  are  convex  to  the  left  hand 
have  positive  radii,  and  those  turned  the  other  way  negative  radii. 

In  a  biconvex  let  r  =  2,  s  =   —  3,  and  t  =  1 ;  then  by  (i) 

AC-    2xl       .  J    =2-    BC-   ~8xl  =-^=  -3 
2-(-3)     2  +  3     5'  2-(-3)     2  +  3          5' 

The  point  C  is  measured,  therefore,  to  the  right  hand  from  A,  and  to 
the  left  from  B.  In  a  plano-concave  let  r  =  -  2,  s  =  oo  ,  and  t  =  1 ;  then 

Ar,  -2x1  n<  -pp  00     X  1  00  -.  /;v 

A  L/  = =  U  I        -D  U  =  — =  — =    —  1          »      .      ( 1 1 

-2-  oo  -2-00       -oo 

c  is  therefore  coincident  with  A. 
The  principal  points  D  and  E  may  be  found  thus : 


-;     BE  =.- 
r  —  s  i    r  —  s 


1  o 

Example  :  In  a  meniscus  r  =  —  3,s  =    -  2,  t  =  -,  and  p.  =  -  ;  concavities 

facing  the  left  hand. 

_3     1  3  _3 

AD         I      _  J_  .  ?       _L  =   2  -_J   =   2     3   =   !     (ii) 
3     _3-(-2)         3  '  -3  +  2        3    ~        3*4        2 

2 

D  is  measured  £  inch  to  the  right  from  A. 

-2.!  -1 

2  2  l      l 


_  . 

-  5'  _3_(_2)    =   3  ' 
2 

E  is  measured  ^  inch  to  the  right  from  B. 

If  the  meniscus  is  turned  round  so  that  its  convexities  face  the  left 
hand,  r  =  2,   s  =  3,    *  =  i,   /*-?; 


Similarly  B  E  «-  —  -.     Both  are  therefore  measured  to  the  left.     The- 


1  1  20  APPENDICES  AND  TABLES 

formulae  (ii)  are  approximations,  sufficiently  accurate  for  general  practical 
purposes,  but  in  cases  of  importance  the  following,  longer  but  more 
accurate,  formulas  should  be  used  : 


. 

Plano-convex  Lens.  —  Let  /=the  principal  focal  point  and  y  =  the 
semi-aperture  ;  then  if  parallel  rays  are  incident  on  A,  the  plane  side  of 
the  lens,  r  =  oo,  and  by  (ii)  B  E  =  0.  The  principal  point  is  therefore  at 
the  vertex  B,  and  the  focal  length 


B/=^;       E/=B/ 
p-1 

The  spherical  aberration 


Q 

Thus  when   /*  =   , 
SB 


8f=  -4-5-     .........    (v) 


If  the    parallel    rays    are    incident   on    the   convex  side  A,   s  =  oo  , 

h 

(vi).       E/=    '          .....     (vii) 


B  E  =  --  (ii),  and  the  focal  length 


fl-l       f* 

The  spherical  l  aberration 


When  /i  =  1-516  (plate  glass) 

§/=  -Mlj!      ........     (viii) 

When  p  =  l-62  (flint  glass) 

8/=  --8042^    ........     (viii) 

To  find  the  radius  of  a  plano-convex  lens,  the  ref.  index  and  focus 
E/  being  given  : 

r=/0*-l)   .     I     ........  (vii) 

To  find  the  radius  of  a  plano-convex  lens,  the  ref.  index,  the  thickness, 
and  the  focus  B/  being  given  : 


A  plano-concave  lens  follows  a  plano-convex;  /  will  be  negative, 
which  shows  that  the  focus  is  virtual.  Concaves  being  thin,  t  is  usually 
neglected. 

Equi-convex  and  equi-concave  generally: 


Equi-convex  more  accurately  : 

1  Heath's  Geometrical  Ojptics,  1887. 


USEFUL  TO   THE   MICROSCOPIST  II2I 

Equi-convex  more  accurately : 


Spherical l  aberration 

8f=  -^""J^y  •  yf (xi) 

In  an  equi-convex  lens  when  /z  =  1  516 

To  find  the  radius  of  either   an   equi-convex  or  equi-concave  lens, 
generally,  the  ref.  index  and  the  focus  B/ being  given  : 

To  find  the  radius  of  an  equi-convex  lens,  the  ref.  index,  the  thickness, 
and  the  focus  B  /  being  given  : 


2/i 
Bi-convex  and  bi-concave,  generally  : 


/LI  —  1     s-r 
Correction  for  thickness  : 


Bi-concave  t  may  be  neglected  B/=  E/  practically. 

Bi-convex  more  accurately,  and  converging  and  diverging  menisci 


8 
B/- 


When  the  light  is  travelling  from  right  to  left 
r/fc-i)*+t\ 

A/=  —  lr_^  —  L^  ......  (xiv> 

(M-  l)|r-»-0*  -1)-} 
Spherical  aberration  : 


o 

Example  :  Let  r  =  2,  s  =  -  3,  t  =  1,  and  /z  =    ;  then  by  (xiv) 
1  Heath's  Geometrical  Optics,  1887. 


4c 


I  I  22  APPENDICES   AND    TABLES 


t-ltt-2 


g-i)|i-<-«-(S-: 

L)|| 

>             l(^ 

3-1.2) 
2     3/ 

-"   "I 

5 

4. 

1     14 

7 

7 

2  '  ¥ 

3 

A  f  '  -       2 

2 

A/    -        ^ 

7' 

1 

144 

iii)           Bf     12          ^ 

25 

12      6 

4 

^      5           3 

A 

5     25 

2  25 

B/- 


(xiv) 
Similarly  A/  =  -2* (xiv) 

By  (xii)  and  (xiii) 

This  is  larger  by  ^g  inch  than  the  result  obtained  by  (xiv). 
The  following  is  an  example  worthy  of  note.     Suppose 

r-s<t  and  >(>-!)-. 
Thus  let  r  =  5  -.  s  =  5,  t  =  1,  p  =|. 

5^1_11\       _155 
Then  by  (xiv)  B/  =  — — ^  =       -2-  =    -310. 

2\2~3/  12 

It  will  be  observed  that,  although  this  meniscus   is  thickest  in  the 
middle,  it  has,  however,  a  large  negative  focus. 

The  principal  points  of  a  sphere  are  at  its  centre. 
The  focus  of  a  sphere,  measured  from  the  centre  : 

E  f  =       ^r  (xvi) 

J       20* -1) 

The  focus  of  a  sphere  measured  from  its  surface  : 

B'-;-|^T>- ^ 

The  focus  of  a  hemisphere  measured  from  the  plane  surface,  the  light 
being  incident  on  the  convex  surface : 

B/=    ^ (vi) 

But  when  the  light  is  incident  on  the  plane  surface,  the  lens  being 
turned  round : 


B/ 


When  p.  =  1'5  the  focus  of  a  sphere  measured  from  the  surface  =  £  the 
radius. 

The  focus  of  a  hemisphere  measured  from  the  plane  side  =  1^  the 
radius,  and  when  measured  from  the  convex  side  the  focus  =•  2  radii. 

In  a  cylindrical  lens  the  principal  points  cross  over. 

To  find  the  radii  r  and  s  of  a  crossed  lens  of  minimum  aberration  for 
parallel  rays  : 


USEFUL  TO   THE   MICKOSCOPIST  1  1  23 


For  boro-silicate  glass  /u,  =  1-51  ;  r  =  -5898/;  and  s  =    -  3'769/  ;  (xvii) 
S/=   -1-042  ^       ......... 

For  flint  glass  /tt  -  1-62  ;  r  =  -G53/;  ands=  -12'06/;  (xvii) 


.........  (XV) 

Critical  angle.  —  Let  &  be  the  critical  angle  for  a  ray  passing  out  of  a 
denser  medium  into  a  rarer  one. 

Then  sin  8  =  -     .........    (xviii) 

A1 

When  M  =  1-333,  6  =  48°  36f  ;  /*  =  |  ,  0  =  41°48£  ;  /z  =  l-52  0  =  41°  8f  ; 

^  =  1-62,  0  =  38°  7'. 

Let  /be  the  principal  focus,  and  _p  =  the  distance  from  the  object  to 
the  optical  centre  of  the  lens,  p'  =  the  distance  from  the  optical  centre  of 
the  lens  to  the  conjugate  image. 

Then  f-*t*        P=4^/,        /--££>    .....  (xix) 

P-f  P"f  P+P 

Let  v  be  the  distance  from  the  object  to  /,  and  w  be  the  distance  from 
/on  the  other  side  of  the  lens  to  the  conjugate  image. 
Then 

v  =  p-f;  w=p'-f\  p  =  v+f;  p'  =  w+f;  and  vw=f*\   v  =  -L 

w 
f- 
w=J  -    .....................      (xx) 

If  o  be  the  size  of  the  object  and  i  the  size  of  its  conjugate  image 


/        v       p      p-f         f 
iv  =  if^ip^   if    =i(p-f). 
f      w     p'     p'-f          f 


^  o  ^  +  o       +  o       i 

Examples:  With  an  objective  of  J-inch  focus  it  is  required  to  project 

an  image  of  a  diatom  -03  long,  so  that  it  may  be  1'5  inch  on  the  screen, 

what  must  be  the  distance  of  the  screen  from  the  optical  centre  of  the  lens  ? 

,_/(i  +  o)_ -5(1-5 +  -(M)_25.g_ 
o  -03 

Therefore  p' =  25-  inches,  the  distance  required (xxi) 

2 

Conversely,  if  the  image  of  a  diatom  projected  by  a  £-inch  objective 
measures  2  inches  on  the  screen  at  40£  inches  from  the  optic  centre 
what  is  the  size  of  the  diatom  ? 

..           2x1 
o=    '1J     =  f_— =  -0125  ,     .     .     .      (xxi) 

P'-f   40  1-1"80 
4     4 

the  size  of  the  diatom  required. 

4  c  2 


I  1 24  APPENDICES  AND   TABLES 

The  last  formula  of  (xxi)  is  very  convenient  for  finding  the  focus  of 
an  objective  ;  w  must,  of  course,  be  large  in  proportion  to  the  focus ; 
o  may  be  a  stage  micrometer. 

As  the  posterior  focus,  /,  is  in  ordinary  microscope  objectives  of 
1-inch  focus  and  upwards,  near  the  back  lens,  the  distance  w  may  be 
measured  from  there. 

Example  :  The  image  of  '01  inch  on  a  stage  micrometer  projected  by 
an  objective  is  2-4  inches  on  a  screen,  distant  5  feet  from  the  back  lens ; 
required  the  focus  of  the  objective. 

f_ow_  -01x60  _'6   _1 
f~  "    -*4 2^4-4 

To  find  F,  the  equivalent  focus  of  two  lenses  in  contact : 

F  =  j^ (xxii) 

where  /,  is  the  focus  of  one  lens  and  f  that  of  the  second. 

Example  :  It  is  required  to  make  a  combination  of  two  plano-convex 
lenses,  the  focus  of  one  lens,  /,  being  twice  /',  that  of  the  other,  and  whose 

Q 

combined  focus  F  =  '6,  /u  =  -  ;    find  their  radii  (see  figs.  4,  6,  8,  and  9). 

Then/=2/'. 

F_ 

3 


/  =  UL  =  £?  =  -9;  and/  =2/  =  l'8  .     .     .     .     (xxii) 
r  =  (^  =  l)  /=  /?  _  i\  1-8  =  -9  ;  similarly  r'  =  -45  .     .     (vil) 
The  focus  for  three  lenses  follows  that  for  two,  thus : 

f=  *»%/,'.  *.  •     (xxii> 


which  may  be  written  —  =  2  -. 

F        / 

To  find  F,  the  equivalent  focus  of  two  lenses,  not  in  contact,  generally, 
F  to  be  measured  from  the  last  principal  point  (E')  of  the  second  lens  ; 
Let  d  =  ihe  distance  between  the  lenses  : 


More  accurately,  let  D  E  be  the  principal  points  of  the  first  lens  and 
D'  W  those  of  the  second,  A  B  and  A7  B'  being  the  respective  vertices, 
d  =  the  distance  from  E  to  D' ;  then  G  and  G',  the  principal  points  of 
the  combination,  are  : 


and  F=_-^__ (xxvi) 


F  is  measured  from  one  of  the  principal  points  of  the  combination.  An 
example  will  be  of  interest.  Let  parallel  rays  fall  on  the  convex  face  of 
the  field  lens  of  a  Huyghenian  eyepiece  ;  find  their  focus. 

Let  /,  the  focus  of  the  field  lens  •-=  3,  and  that  of  the  eye  lens  /'  =  1 ; 


USEFUL  TO   THE   MICKOSCOPIST  1125 

q 

ft  =  -,  and  the   distance  between  the  surfaces,  that  is  B  A7,  =  1-8;  t  the 
2 

3  3 

thickness  of  the  field  lens  =  -  -  ;  and  t'  that  of  the  eye  lens  =  —  ;  A  .D  =  0 

(ii);  BE  =  -*  =  --2  (ii).      Similarly  A'D'  =  0;    B'E'=-i  =  -  1  (ii)  ; 

fl  \i  10 

=  -2  4-1-8  =  2.     Now 

'' 


3x1        3 


We  see,  therefore,  that  the  equivalent  focus  is  1^  inch,  but  the 
principal  point  G',  from  which  the  focus  is  measured,  is  1  inch  to  the  left 
from  E'  ;  therefore  the  focal  point  is  ^  inch  to  the  right  from  E'.  Now  as 
E'  is  ^  inch  to  the  left  of  B,  the  plane  surface  of  the  eye  lens,  it  follows 
that  F,  the  focal  point,  is  ^  inch  to  the  right  of  the  plane  surface  of  the 
eye  lens.  If  this  problem  is  worked  by  the  simpler  formula  (xxiii),  the 
answer  will  be  -44  from  the  plane  surface  of  the  eye  lens  ;  this  is  only  an 
error  of  -04  in  excess. 

This  explains  '  the  microscope  objective  of  10-ft.  focus.' 
The  equivalent  focus  of  the  objective  was  10  ft.,  but  the  principal  point 
G'  from  which  that  focus  was  measured  was  9  ft.  11£  inches  from  the 
objective,  which  would  give  £  inch  as  the  working  distance  of  the  lens. 
The  objective  in  question  has  a  double  convex  back  lens  and  a  plano- 
concave front  ;  a  small  decrease  in  the  distance  between  the  lenses,  such 
as  a  ^  inch,  has  the  effect  of  causing  the  principal  point  G'  to  recede 
many  feet,  and  of  causing  a  great  increase  in  the  equivalent  focus. 

With  regard  to  the  tube  length,  which  is  equal  to  d  in  (xxvi),  the 
position  of  the  principal  points  of  a  combination  plays  an  important  part. 
Suppose  the  Huyghenian  eyepiece,  in  the  preceding  example,  were 
mounted  as  an  objective;  the  tube  length  would  have  to  be  measured 
from  the  first  principal  point  of  the  eyepiece,  wherever  that  might  be,  to  the 
second  principal  point  of  the  objective,  which  in  the  example  before  us  is 

.....  (xxiv) 


G  is  therefore  measured  3  inches  to  the  right  from  the  point  D  ;  D  is, 
as  we  have  seen,  coincident  with  A,  the  convex  vertex  of  the  field  lens. 
So  anyone  measuring  the  tube  length  from  the  field  lens,  which  is  the 
posterior  lens  of  our  supposed  objective,  or  from  the  middle  of  the 
combination,  would  be  1^  or  3  inches  in  error.  The  correct  point  from 
which  the  measurement  should  be  made  lies  one  inch  in  front  of  the  eye 
lens,  which  is  the  front  lens  of  our  supposed  objective. 

The  importance  of  this  cannot  be  over-estimated,  as  the  optical  tube 
length  has  a  direct  bearing  on  the  power.  If  Q=the  distance  of  vision 
(say  10  inches),  M  =  the  magnifying  power,  F  =  the  equivalent  focus  of  the 
eyepiece,  F'  =  the  equivalent  focus  of  the  objective,  O  =  Prof.  Abbe's 
'  Optical  Tube  length,'  viz.  the  denominator  in  the  fraction  in  formula 
(xxvi)  ;  then 


If  0  -  the  focal  length  of  the  entire  microscope,  N.A.  =  the  numerical 
aperture,  and  e  =  the  diameter  of  the  eye-spot,  then 


Journal  B.M.S. 


1126  APPENDICES   AND   TABLES 

(xxix) 


2  9 

If  X  •=  the  number  of  waves  per  inch  of  light  of  a  given  colour,  L  the 
limit  of  resolving  power  of  any  objective  with  an  illuminating  beam  of 
maximum  obliquity  is 

L  =  2A(N.A.)  ........    (xxx) 

But  with  a  solid  f  axial  cone  and  white  light  the  resolving  limit  is 
equal  to  the  N.A.  multiplied  by  70,000.  When  Gilford's  screen  is  used,  or 
photography  employed,  the  limit  is  raised  to  the  N.A.  multiplied  by  80,000. 

The  aberration  for  non-parallel  rays.  —  It  is  a  little  more  troublesome 
to  find  the  aberration  of  rays  other  than  parallel,  but  if  the  following 
instructions  are  carefully  attended  to  the  problem  merely  becomes  the 
simplification  of  a  vulgar  fraction.  Let  P  and  P'  be  the  distances  of  the 
point  and  its  image  from  the  lens.  First  find  a,  by  either  (xxxi)  or 
(xxxii)  : 

«.  =  ?/-!  ......  (xxxi);       a  =  l-^      .....  (xxxii) 

Next  find  x  by  (xxxiii)  or  (xxxiv)  : 

g.=  2Qx-l)/_1  ^     (xxxiii);      ^i+fcl)/.     .  (xxxiv) 

r  s 

Then  find  a>  by  (xxxv)'  : 
a,  =        -J  {  £±|ar«  +  40*  +  1)  a  x  +  (3/i  +  2)  (/*  -  1)  a*  +  -^   }   (xxxv) 

Spfa  -I)/3  L/i-1  /*-U 

F=7~F+i(a;-^2-472+a)2/2-          (xxxvi) 

The  aberration  8  P'  =    -o>P'2#2     .     .     .     (xxxvii) 

To  find  the  aberration  of  two  lenses  in  contact.  Let  Q  and  Q'  be  the 
object  and  its  conjugate  at  the  second  lens,/'  be  the  focus  of  the  second 
lens,  and  F  the  focus  of  the  combination  ;  then  P'  =  —  Q. 


for  the  first  lens,  ~  =  -  -  _  +  o>  y-      .     .  .  (xxxvi) 

for  the  second  lens,        =      +  -_  +  to'  y~     .  .  .  (xxxvi) 

for  both  lenses,  —  =  -.  +  -  -  _  +  («  +  w')  z/-  .  (xxxviii) 

Therefore,  for  n  lenses,  -  „  =  2--  —  +2o>2/"  .  (xxxviii) 


The  aberration  8  Q'  =    -  2  &>  Q'2£/2  and  S  F  =    -  2  o>  F2«/-      .      (xxxix) 

Example  :  Two  plano-convex  lenses  of  equal  foci  have  their  convex 
surfaces  in  contact  (fig.  7)  ;  find  the  aberration  for  parallel  rays.     Then 


M: 

For  the  first  lens  r  =00  ;  therefore  x=    -1  (xxxiii) ;  P  =  oo  ;  therefore 
i=   - 1  (xxxi)  ;  and  o> --          (xxxv) 

For  the  second  lens  s  =  oo  ;  therefore  x  =  1  (xxxiv) ;   ^  =    -  -  ;  there- 
1  Journal  B.M.S. 


USEFUL   TO  THE   MICKOSCOPIST  II2./ 

fore   a  =  -  3   (xxxi)  ;     «'  =  —  -  (xxxv)  ;        a>  +  a/  =  2  a>  =  —  —  -  ; 

9fk 

/=  2  F  (xxii)  ;        therefore  2  a>  =  ^       ^  ; 

XT?         20  F  V          5     y2 

—%4W=    ~6'F     '     ' 

This  is  half  the  aberration  of  an  equi-convex  lens  (fig.  1)  of  the  same 
focal  length  as  the  combination  where 

v-.      ........  » 


If  the  front  lens  of  the  combination  be  turned  round  so  that  its  convex 
surface  faces  the  incident  light  the  aberration  is 


or  half  what  it  was  before  (fig.  5). 

This  is  nearly  a  third  of  the  aberration  of  a  plano-convex  in  the  best 
position  (fig.  2),  which  is 

*-  -«  •    • 


The  following  figures  pictorially  illustrate  spherical  aberration  in 
single  lenses  and  in  various  combinations  of  two  plano-convex  lenses,  all 
having  the  same  focus  F,  the  same  aperture,  and  the  same  refractive 
index  f  .  The  dot  nearer  the  lens  is  the  focal  point  for  the  marginal,  and 
that  farther  away  the  focal  point  for  the  central  rays;  the  distance 
between  the  dots  is  the  spherical  aberration  8  F. 


PIG.  1.  FIG.  2. 

Fig.  1.  An  equi-convex,  r  =  F  ; 

SF=   -1-612=    -'113     .......     (xi) 

x1 

F 

Fig.  2.  A  plano-convex,  r  =    -> 


8F=    -1-16=    --121     .......  (viii) 

17 

- 

a 


n  17 

Fig.  3.  A  crossed  convex,  r  =  _i-  F  ;    s=    --F     ......  (xvii) 


8F=    -1-07^=  -'111 

X1 


Fig.  7.  A  combination  of  two  pianos  with  their  convex  faces  in  con- 
tact, the  focus  /  of  the  first  lens  being  equal  to  /,  that  of  the  second. 

The  focus  of  the  combination  F  =  ^      ..........  (xxii) 

SF=   -'833  2/!  =  --087   ......    (xxxix) 


Fig.  4.  The  same,  only  2/=/  ; 


fe-  -*168  ,     .....    (xxxix) 
F 


1128 


APPENDICES  AND   TABLES 


Fig.  6.  The  same,  only/=2/'; 

8F=  --5  |2 -052 (xxxix) 

Fig.  5.  The  first  lens  inverted,  /=/' ; 

8F=--416|2  =  --043 (xxxix) 


Pig.  8.  The  same,  only  2/=// ; 
8F=  --62( 
Fig.  9.  The  same,  only/=2/; 


=  --065 
.r 


.     .     .     (xxxix) 
-•039 (xxxix) 


FIG.  4. 


FIG.  5. 


FIG.  6. 


i   i 


I  I 


FIG.  7. 


FIG.  8. 


FIG.  9, 


We  see,  therefore,  that  with  the  same  focal  length  F  the  aberration  of 
fig.  1  is  the  greatest,  and  that  of  fig.  9  the  least.  We  also  see  in  the  com- 
binations that  by  decreasing  up  to  a  certain  point  the  focus  of  the  first 
lens  the  aberration  is  increased,  and  vice  versa.  The  best  form  of  a 
combination  of  plate  glass,  p,  =  1*516,  for  parallel  rays  similar  in  arrange- 

5  ff 

ment  to  fig.  9  is  when  /=  -£-. 
o 

The  Aplanatic  Meniscus. — A  spherical  refracting  surface  has  two 
aplanatic  foci,  such  that  if  converging  rays,  which  have  their  focus  at  P', 
meet  a  convex  spherical  refracting  surface,  whose  centre  of  curvature 
is  r,  and  if  the  distance  between  the  points  P'  and  r  =ju,r,  then  those 
rays  will  be  refracted  aplanatically  to  some  other  point,  say  P,  which  will 
lie  on  the  same  side  of  the  surface  as  P'.  This  fact  is  of  great  service, 
because  it  enables  an  aplanatic  meniscus  to  be  constructed ;  thus,  if  we 
make  r  the  radius  of  the  curve  A,  we  can  make  s,  the  radius  of  the  curve 
B,  a  radius  from  the  point  P.  If,  then,  P  is  a  radiant,  the  light  travelling 
from  left  to  right  will  pass  through  the  curve  B  without  refraction,  because 
P  is  the  centre  of  the  curve  B.  The  light  will  then  pass  on  unchanged 
to  the  curve  A,  and  will  by  it  be  refracted  aplanatically,  as  if  it  had  come 
irom  P'.  P  will  be  negative  and  P'  positive. 

The  formulae  for  finding  r  and  P'  when  P  is  given  are  : 


and  those  for  finding  r  and  P  when  P'  is  given  are  : 

V 


(xli) 


USEFUL  TO   THE   MICEOSCOP1ST  I  I  29 

An  excellent  combination,   suitable  for  a  bull's-eye,  can  be  made  of 
boro-silicate  glass,  refractive  index  1-51,  i>  =  64'0 

1st  lens1  crossed  r  =   +    2-359),.  „- 

s=  +  15-078  faiam< 

2nd  lens1  meniscus  r  =  +  1*280  i  ,. 

8=  -  3-434  Jdiameter  ! 

Distance  between  lenses,  -05  ;  equivalent  focus,  2-0  ;  back  focus,  1-55  1 
total  aberration,  -  '103  ;  clear  aperture,  2-0  ;  angle,  62°. 
This  combination  is  eminently  suitable  for  photo-micrography,  and  for 
those  cases  where  a  bull's-eye  is  necessary. 

A  simpler  form  of  bull's-eye  cafti  be  made  of  two  pianos,  using  the 
same  glass  ;  see  fig.  9,  p.  1128. 

1st  lens,  radius  +  3*0,  diameter  2'1 
2nd    „         „        +1-8         „          1-9 
Distance  between  lenses,  -05  ;  equivalent  focus,  2£. 

To  find  the  radii  r  and  s  of  a  lens  which  will  refract  light  from  a  point 
jp  to  point  p'  with  minimum  aberration. 


Let  /3  be  the  coefficient  of  ^-  in  formulae  v,  viii,  xi,  and  xv,  then  for 

parallel  rays  in  each  particular  case  the  lateral  aberration  =  ^--  /3  .     .  (xlv) 

1  y3 
Diameter  of  least  circle  of  aberration  =  „  j%  &    ......      (xlvi) 

Distance  of  least  circle  of  aberration  from  focus  =    —  -  ^-  0     .    (xlvii) 
'  When  the  rays  are  not  parallel 
(xlv)  =  a>j2/2/3  (xlvi)  =  -up'y3  (xlvii)  =  -|a>.pV 

It  is  interesting  to  note  that^-  =2(/u  —  1)^     .......  (xlviii) 

Therefore,  when  ^  =  .  ,        y~  =  i. 
*         J 

To  find  m,  the  magnifying  power  of  simple  lenses  or  magnifying 
glasses.  Let  d  be  the  least  distance  of  distinct  vision  apart  from  the  lens, 
and  /  be  the  principal  or  solar  focus  of  the  lens.  Then,  when  the  eye  is 
held  close  to  the  lens, 

m  =  1  +  A-    .     .     .     .....     (xlix) 

When  the  eye  is  held  at  the  back  principal  focus  of  the  lens,  subtract 
one  from  this  quantity.  For  real  images  projected  upon  a  screen,  the 
distance  of  the  screen  being  d,  subtract  two. 

It  may  be  of  interest  to  note  that  formula  (xix)  on  this  page  may  be  used 
to  determine  the  focus  of  spectacles  required  to  bring  the  abnormal  focus 

1  In  this  formula  the  convention  used  with  regard  to  the  signs  is  that  of  manu- 
facturing opticians,  and  not  that  employed  in  the  rest  of  the  appendix. 


I  1  30  APPENDICES   AND   TABLES 

of  either  a  presbyopic  or  myopic  person  to  a  normal  focus.  Make  p  the 
abnormal,  and^/  the  normal  focus  ;  then/  will  be  the  focus  of  the  spectacles 
required. 

In  both  cases  p  is  a  negative  quantity,  because  it  is  on  the  same  side 
of  the  lens  as  p'  ;  it  is  usual  to  make  pf  10  or  12  inches. 

Achromatism 

Let  /LI  be  the  refractive  index  of  a  mean  ray  (D  line  nearly)  for  a 
certain  material,  p.v  that  for  a  blue  ray,  and  /ur  that  for  a  red  ray  ;  the  dis- 

persive power  of  the  material  is  /ig~jir;  this  is  usually  written  —  ^-,  or  -or. 

PjT  *  f*  ~  1 

The  formula  for  achromatism  is 

dp        1  ftp'       1. 

/i-r?    jT^r/' 

that  is,  ^  +  5'  =  °      .........       (1) 

The  foci  of  the  two  lenses  are  therefore  directly  as  their  dispersive 
powers,  and  the  focus  of  one  will  be  negative. 

An  achromatic  effect,  which  is  not  achromatism  in  the  strict  mean- 
ing of  the  term,  can  be  obtained  with  two  lenses  of  the  same  kind  of  glass 
by  making  d  the  distance  between  the  lenses  : 


If  p  is  large,/  in  the  denominator  may  be  neglected  ;  this  will  make 
d  half  the  sum  of  the  foci,  which  is  the  formula  for  both  the  Huyghenian 
and  Kamsden  eyepieces  ;  but  when  p  =  /,  d  is  the  sum  of  the  foci. 

Formulce  relating  to  Spherical  Mirrors 

Let  p  -  one  focus,  p'  =  its  conjugate,  /  =  principal  focus,  and  r 
=  radius  of  curvature  ;  then  in  concave  mirrors 


„- 


-  =  -  ..... 

P+P  P         / 

To  find#  interchange^?  and^?'. 

If  o  is  the  size  of  an  object,  and  i  the  size  of  its  image,  and  v  the  dis- 
tance of  the  image  from  the  principal  focal  point,  then 


In  convex  mirrors  prefix  a  negative  sign,  thus:  r=  —  2/,  and  so  on 
with  the  other  formulae. 

The  formulae  for  mirrors  may  be  derived  from  those  of  lenses  by  sub- 
stituting -  1  for  p;  thusr=  -2/(vii). 

Let  y  =  the  semi-aperture  ;  then  the  spherical  aberration 

«/=   ~\  '  yj          .....     (v)  or  (viii). 

A  mirror  to  be  aplanatic  for  parallel  rays  must  have  a  parabolic  curve. 

A  mirror  to  reflect  rays  diverging  from  a  point  p,  so  that  they  may 
converge  aplanatically  to  another  point  p',  must  be  elliptical,  having  p 
and  p'  for  its  foci. 


USEFUL  TO  THE  MICROSCOPIST         1131 

Formula  relating  to  Prisms 

Lot  i  ==  the  refracting  angle  of  the  prism,  $  the  angle  of  incidence  on 
the  first  surface,  <£'  the  angle  of  refraction  at  the  first  surface,  ^  the  angle 
of  incidence  in  the  prism  at  the  second  surface,  and  \//  the  angle  of  re- 
fraction on  emergence  ;  then  the  total  deviation 


When  the  ray  passes  through  the  prism  symmetricaUy  the  deviation 
is  at  a  minimum  :    <£  =  >//,  <j>'  =  \js  —  -,  and 


sinl 

by  which  formula  the  refractive  indices  of  media  can  be  found,  because 
both  i  and  D  are  capable  of  accurate  measurement. 

Formula  relating  to  Conic  Sections 
Ellipse.  —  Let  A  =  major  axis  ;  a  =  minor  axis.     Then 

Focus  =  A-N/A2-^  ........    (lh) 

Parabola.—  Let  A  =  height;  a=  -  base.    Then 

2 

Focus  =  -^-  .........     (lv) 

4:  A. 

Hyperbola.  —  Let  A  =  major  axis  ;  a  —  minor  axis.     Then 

Focus  =  Vg^-A     .......     (Ivi) 

Works  consulted:  —  Coddington,  Camb.  1830;  Parkinson,  Camb.;  'Ency- 
clopaedia Britannica'  ;  '  Journal  E.M.S.  '  ;  Heath,  Camb.  1887,  &c.  It  will  be 
seen  that  several  of  the  formulse  have  been  entirely  reset,  while  some 
appear  now  for  the  first  time. 


1132  APPENDICES  AND  TABLES 


APPENDIX   F 

EXAMPLES   USEFUL   TO   THE  MICBOSCOPI8T 


Square  £  inch  .     .     .      =  10-08061  square  millimetres. 

„       A     »  •    •     .     -  6-45159 

„      A     „  .     .     .     =  4-48027 

>,     rfo    >•  .•..'-  -06452 


=  64515-9 
=     645-159 


Square  centimetre     .....  =  15-50006  square  ^  inch. 

„      miUimetre     .....  =  15-50006       „      T^     „ 

„      100  /i    .......  =15-50006       „     T^   » 

„      lO^i      .......  =     -15500      „         „      „ 

•  -      /*      .....     .     .     .  =      -00155       „         „      „ 

Multiples  of  the  above  may  be  found  by  multiplying  the  values  given 
by  the  square  of  the  multiplier. 

Thus,  square  &  inch  =  A  x  4  ;  the  square  of  4  =  4  x  4  =  16,  and  6-45159 
x  16  =  103-2254  square  millimetres,  the  answer  required. 

Cubic  £  inch     .     .     .     .     =       32-00589  cubic  millimetres. 
„      A    »       .---...'-       16-38702     „ 
»      A    ».       .     .     .     .      =         9-48323     „ 
„      rfo  ,  .....     =  '01639     „ 

»     nAnr,»       ....     =16387-02          „  11 

Cubic  centknetre  .....  =  61-0239         cubic  ^  inch. 

„      miUimetre  .....  =  61-0239  „    T^     „ 

„      100  p.      ......  =  61-0239  „  T^7    „ 

„      10  /*   .......  =      -061023         „      „       „ 

t,    -p    ........  =      '000061023  „      „       „ 

Multiples  of  the  above  may  be  found  by  multiplying  the  values  given 
by  the  cube  of  the  multiplier. 

Thus,  2  cubic  millimetres  :  2  cubed  =2x2x2  =  8,  and  61-0239  x  8 
=  488-1912  cubic  jfa  inch,  the  answer  required. 

Areas  of  Circles 

$  inch  diameter  =  1-22718        sq.  A  inch  =         7-91726  sq.  millimetres 
A    «  »          =    -78539816   „    „      „     =         5-06706    „ 

A    „  ,,  '545415       „    „      „     =         3-51879    „ 

T**  »  >i         =    '78540          „  jfa    „     =  -05067    „ 

=  50670-6  „          p. 

=    '78540  ,,»»     =     506-7 


1  millimetre  diam.  =  -78539816  sq.  mm.  =  12-17372      sq.  Tfo  inch. 

100  /x     ....     =  7854-0                „      M    -  12-17372 

.     .     .     .     =  78-54              „       „     =      -12174 

=  -7854          „       „    =      -0012174 


USEFUL  TO  THE  MICKOSCOPIST          1135 

Multiples  of  any  of  the  above  may  be  obtained  in  the  same  manner  as 
in  the  preceding  example. 

Thus,  if  the  diameter  of  the  circle  =  ^ ^  inch,  then  the  square  of  3  being 

9  and  '7854  x  9  =  7'0686  sq.  ^  inch  and  -05067  x  9  =  -45603  sq.  millimetre, 
is  the  area  required. 

Volumes  of  Spheres 

$      in.  diameter.  =  1-02266  cubic  ^  inch  =      16-75835  cubic  millimetres. 

ft     „          „       .  =    -52360      „  „    =       8-58024      „ 

TV      »  »  -30301      „  „    =       4-96543      „  „ 

T*TT    „  i»       •  -    '52360      „     ^    „    =         -00858      „ 

TT&iT"          »       •=   '52360      „    T$nr  »    =8580-24  „     /t 

1  mm.  diam.  .     .     .  =  -52360  cubic  mm.  =  31-952  cubic  T£u  inch. 

100^       „       ...  =523600-0  „         p.     =31-952     „     TTJW    „ 

10  n        „       .     .     .  =       523-60          „         „     =     -03195,,         „       „ 
M             „       .     .     .  =  -52360     „         „     =     -00003195      „      „ 

Multiples  of  any  of  the  above  follow  the  preceding  example  of  cubic 
measures.  Thus,  if  the  diameter  of  the  sphere  =  30  /*,  then  the  cube  of  3- 
being  27  and  523-6  x  27  =  14137-2  cubic  ^  and  -03195  x  27  =  -86265  cubic  T^. 
inch,  is  the  volume  required. 


1 1 34  APPENDICES   AND    TABLES 


APPENDIX   G 

USEFUL  NUMBERS  AND  FORMULA 

Paris  line  =  -088813783  inch. 

Cubic  foot  of  water  weighs  62-2786  Ib.  avoirdupois  at  62°  Fahr. 

Cubic  inch  of  water  weighs  252-286  grs.  at  62°  Fahr. 

Gallon  of  water  weighs  10  Ib.  avoirdupois  at  62°  Fahr. 

1  gallon  =  277-27384  cubic  inches. 

Cubic  foot  of  sea  water  weighs  63'96  Ib. 

Weight  of  sea  water  =  1-027  weight  of  fresh  water. 

1  inch  of  rainfall  =  100  tons  per  acre. 

Pressure  of  water  in  Ib.  per  sq.  inch  =  '433  head  of  water. 

Expansion  of  water  between  32°  Fahr.  and  212°  Fahr.  =  -04775. 


Dip  of  horizon  (including  refraction)  in  nautical  miles  =  1-16  <v/height. 

Marks  on  hand  lead-line  for  sea  soundings  1,  2,  and  3  fathoms,  1,  2, 
and  3  tags  of  leather  respectively  ;  5  and  15  fathoms  white  rag  ;  7  and 
17  fathoms  red  rag  ;  10  fathoms  leather  with  hole  in  it  ;  13  fathoms  blue 
rag  ;  20  fathoms  2  knots  :  30  fathoms  3  knots,  &c.  A  small  knot  is  placed 
at  intermediate  5  fathoms  after  20  fathoms  —  viz.  at  25,  35,  45,  &c. 

Pressure  of  wind  in  Ib.  per  sq.  foot  =  0'002288  (velocity  in  feet  per 
second)2. 

Areas  and  Volumes 

Area  of  triangle  =  base  x  £  perpendicular. 

Volume  of  wedge  =  area  of  base  x  £  perpendicular  height. 

Volume  of  cone  or  pyramid  =  area  of  base  x  ^  perpendicular  height. 

Surface  of  side  of  cone  =  circumference  of  base  x   £  length  of  side. 

Area  of  parabola  =  base  x  f  height. 

Velocity  of  light  =  186,377  statute  miles  per  second.1 

Wave-length  of  yellow  light  -=  -  -  --    inch. 

4olOO 

Number  of  vibrations  per  second  508,961,293,000,000. 

Falling  Bodies 

S,  space  fallen  in  feet  ;  V,  velocity  in  feet  per  second  ;  g  =  32-2  ;  t,  time 
in  seconds. 

2  /S  =  8-025  A/S. 


Arithmetical  Progression 

A,  first  term  ;  B,  last  term  ;  S,  sum  ;  d,  difference  between  terms  ;  nt 
number  of  terms. 

1  Latest  determination  by  Prof.  Newcomb,  of  Washington. 


USEFUL  TO   THE   MICEOSCOPIST  1135 


Geometrical  Progression 
m,  multiplier  or  divisor. 

A  =      B  B=Vm  — 

w'"-1'*  m-l  ' 

Properties  of  Circles  and  Spheres  <£c. 
77  =  3-141592653589793  +  log  77  =  0-4971498727 

77- =  9-8696  A/77~=  1-77245  =  -31831 

A  ='10132  --=-56419 

772  A/77  4 

^  =  -5236  .  A/2-1-41421  v/2«  =  8-02496 

6 

Circumference,  C.     Area,  A.      Radius,  r.      Diameter,  d.     Volume,  V. 
Surface,  S. 

"    6  *         "77" 

Area  of  sector  of  circle  -  de*rees  in  wo_x_area  _ofdrde 

360 

Length  of  arc  =  number  of  degrees  x  -017453  r. 
Unit  of  circular  measure  =  57°'29578. 
Side  of  square  of  equal  area  to  a  circle  =  r  \/TT. 
Side  of  inscribed  square  =  r  A/2. 
Area  of  ellipse  =  i  major  axis  x  i  minor  axis  x  77. 

Volume  of  spheroid  =  polar  axis  x  (equatorial  axis)2  x  !T. 

6 

Number  of  Threads  per  inch  in  Whitworth's  Standard  Screws 

Sixes  -fa  .         .  No.  of  threads  150  Sixes  ±    .  .      No.  of  threads  20 

„      A  •  „                80 

,f     &  •  00 

.,£•••  5,                40  „  j7ff  .  14 

„      &  .  „               32  „  i   .  .              „                12 

„      A  -  5,                24  „  I    .  „               11 

Convenient  Approximations  for  rapid  Calculations 

6  knots  =  7  miles,  more  correctly  13  knots  =  15  miles. 

8  kilometres  =5       „          „  „         50  kilometres    =  31      „ 

1  metre  =  3  ft.  3£  in.  „  „         32  metres          =  35  yards. 

5  centimetres  =  2  inches     „  „         33  centimetres  •=  13  inches. 

3  millimetres  =  ^  inch         ,,  .,         5  millimetres    =  £  inch. 

1  pole  =  5  metres  ;  1  furlong  =  2  hectometres. 

;V  =  oth) <r  inch  ;  T^u  inch  =  5-  mm. ;  T^joWtf  inch  =  ^  /i. 

2  are  -  239  sq.  yds  ;  1  rood  =  10  are ;  2  acres  =  81  are  ;   100  hectare 
-  247  acres  ;  3  cubic  yards  =  23  decisteres  ;  1  decastere  =  13  cubic  yards ; 
2  decilitres  =  7  /  §  (ounces);  4  litres  =  7  pints;  2  grammes  =  31  grains; 


I  136  APPENDICES  AND  TABLES  USEFUL  TO  THE  MICROSCOPIST 

4  grammes  =  15  (drachm)   (apothecaries')  ;  7  grammes  =  4  dr.  (drachms) 
(avoirdupois). 

5  kilogrammes  =  11  Ib.  (avoirdupois). 

50  kilogrammes  -  1  cwt. 

Nobert's  19  Band  Test  Plate 
Band          Lines  per  inch  Band        Lines  per  inch 


1  11259-5 

5  33778-5 


15  90076-1 

19  112595-1 


10  61927-3 

Difference  between  each  band  =  5629*75. 

NoberVs  last  20  Band  Test  Plate 
Band          Lines  per  inch  Band         Lines  per  inch 


1  11259-5 


5  56297-6 

10  112595-1 

Difference  between  each  band  =  11259'S. 


15  168892-7 


225190-3 


Convenient  Formula  for  Lantern  Projection  or  Enlargement  and 
Reduction. 

Let  D  be  the  distance  of  the  screen,  and  d  the  distance  of  the  object 
from  the  optical  centre  of  the  lens,  F  the  equivalent  focus  of  the  lens,  M 
the  magnifying  power  or  '  number  of  times  '  for  enlargement  or  reduction, 
then  — 


Example  :  It  is  required  to  project  by  a  lens  of  6  inches  equivalent 
focus  a  slide  having  a  3-inch  mask  so  that  it  may  give  a  10-ft.  disc,  what 
must  be  the  distance  of  the  screen  ?  Here  M  the  magnification  will  be 
40  times.  D  =  F  (M  +  1)  =  6  (40  +  1)  =  246  inches  =  20£  feet. 

Note,  in  a  double  combination  the  optical  centre  may  be  assumed  to 
be  half  way  between  the  lenses.  To  reduce,  interchange  the  object  and  the 
screen. 

Royal  Microscopical  Society's  Gauges 

Substage  1-527  inch  =  38-786  mm. 

Eyepieces,  No.  1  -9173     „  =  23*300 

„    2  1-04        „  =26-416 

„    3  1-27         „  -32-258 

„    4  1-41         „  =35-814 

For  the  screw  of  the  objective  and  nose-piece  the  Society  supply,  at 
cost  price,  a  sizing  die  and  plug,  also  screw  chasers.  They  do  not,  how- 
ever, supply  standard  screw  gauges.  Full  particulars  regarding  the 
screw  will  be  found  in  the  «  E.M.S.  Journal,'  1896,  p.  389. 


INDEX 


ABB 


Abbe  (Prof.),  on  amplifying  power  of  lens, 
26 ;  on  homogeneous  immersion,  28 ; 
on  .improvement  of  optical  glass,  32 ; 
on  classification  of  eye-pieces,  34  ;  on  ! 
principle  of  microscopic  vision,  43,  44, 
45 ;  on  definition  of  aperture,  44 ;  on 
aperture,  48  note ;  on  radiation,  57 ; 
on  angle  of  aperture,  60,  61,  62 ;  on 
diffraction,  63-75 ;  on  '  intercostal 
points,'  73 ;  on  '  penetration,'  82  ;  on 
over-amplification,  90;  on  stereoscopic 
vision,  90,  94  ;  on  '  aplaiiatic  system,' 
94  ;  on  orthoscopic  effect,  95  ;  on  Rams- 
den's  circles,  106 ;  on  solid  cones  of 
light,  418 

—  his  stereoscopic  eye-piece,  102 ;  camera 
lucida,     281-284 ;     apertometer,     307, 
390-396;    chromatic   condenser,    308- 
309 ;    achromatic   condenser,  311-312, 
385 ;  condenser,  iris-diaphragm   fitted 
to,  312;  diffraction  theory  and  homo- 
geneous   immersion,   364 ;    compensa- 
tion eye-piece,  378 ;  method  of  testing 
object-glasses,    384-387;     test    plate, 
387-390 :    experiments    in    diffraction 
phenomena,  434 

Aberration,  19 ;  positive,  21,  360  note ; 
negative,  21,  27,  360  note;  chromatic, 
81 ;  spherical,  31,  301,  306,  388 ;  errors 
of  spherical  and  chromatic,  corrected 
by  Ross,  857 

Abies  balsamea,  443 

Abiogenesis,  761 

Abraham's  prism,  401 

Absorption  or  dioptrical  image,  64 

—  and  diffraction  images  due  to  diffrac- 
tion, 65  note 

~  of  light  rays,  Angstrom's  law,  323 

—  bands,  323-327 
Abstriction  of  spores,  633 
Acalephce,  sexual  zob'ids  of  polypes,  862  ; 

relationship  to  hydroids,  872  ;  develop- 
ment of,  874  ;  medusan  phase  of,  877 

Acantliometra  xiphicantha,  850  ;  echi- 
noides,  852 

Acanthometrina,  846,  852  ;  central  cap- 
sule of,  852 

Acarina,  eggs  of,  1004-1006 ;  anatomy  of, 


ACT 

1009-1012  ;  larvae  of,  1009  ;  nymph  of, 
1009;  integument  of,  1010;  legs  of, 
1010;  eyes  of,  1011;  classification  of, 
1012 

Accommodation,  of  the  eye,  88  ;  depth,  89 
Acetabulama,  563 ;  pileus  of,  563 
Acetic  acid,  as  a  test  for  nuclei,  517 
Acheta,  987 

—  campestris,  eggs  of,  1005 

Achlya,  zoospores  of,  564 ;  obspores  of, 
565  ;  zobsporanges  of,  640 

—  prolifera,  563  and  note,  563 
Achnanthece,  characters  of,  615 
Achnanthes,  frustules  of,  588,  615 ;  stipe 

of,  588,  615 ;  '  stauros  '  of,  616 ;  struc- 
ture of  frustule,  615 

Achnanthes  longipes,  615 

Achromatic,  comparison  of,  with  chro- 
matic and  apochromatic  lenses,  368 

—  condenser,  Abbe's,  260,  308-312,  314  ; 
Powell  and  Lealand's,  301, 311 ;  for  ob- 
servation of  pycnogonids,  959 

—  doublet,  Fraunhofer's,  148  ;  meniscus, 
876 

—  lenses,  Charles's,  148  ;  Marzoli's,  353  ; 
Selligue's,  354 

—  objectives,  19,  32  ;  Tully's,  354 ;  Wen- 
ham's,  361,  862;  cover  slips  for  use 
with,  439 

—  oil  condenser,  Powell  and  Lealand's,. 
310,  311 

Achromatism,  17,  19,  150 ;  in  photo- 
micrography, 34 ;  rise  of,  147 ;  in- 
augurated, 365 ;  imperfect,  causing 
yellowness,  417 

Acineta,  783 

'  Acinetiform  young  '  of  Ciliata,  782  note 

Acinetina,  783 ;  food  of,  783 

'  Acorn '  monad,  759 

'  Acorn-shells,'  967 

Actinia,  reproduction  from  fragments> 
863 

—  Candida,  thread-cells  of,  879 

—  crassicornis,  thread-cells  of,  879 
Actinocyclus,  588,  610,  620 
Actinomma  inerme,  850,  851 
Actinophrys,  846 

—  form  of  Microgromia,  736 

—  sol,  737-742 
Actinoptychus,  588,  610,  611 

4    D 


113! 


INDEX 


ACT 

Actinosphcerium  Eicliornii,  741 

Actinotrocha,  950 

ACTINOZOA,  863,  877-883 

Actius,  on  myopy,  118 

Actuarius,  on  myopy,  118 

Adams'  variable  microscope,  142;  non- 
achromatic  microscope,  148 

*  Adder's  tongue '  fern,  (379 ;  sporanges  of, 
675 

Adipose  substance,  1045 

Adjusting  objectives,  Ross's,  357,  360 

Adjustment,  coarse,  159-162;  Swift's 
diagonal  rack,  161 ;  Nelson's  stepped 
rack,  161 ;  Wale's,  224-226 

—  fine,  162-175 

-  Ross's,  153, 155, 175  ;  to  Pritchard's 
microscope,  153 ;  Watson's  long 
lever,  162,  172,  175 ;  '  Continental ' 
type,  162 ;  Swift's  vertical  side 
lever,  16?,  173,  174;  Campbell's 
differential  screw,  164,  165,  175, 
236;  Zeiss's,  166,  175;  Reichert's 
new  lever,  171,  175 ;  Powell's,  174  ; 
short  side  lever,  174 ;  speeds  of, 
175 ;  to  the  sub-stage,  Nelson's, 
185  ;  for  Powell  and  Lealand's  sub- 
stage,  186 

—  screw  collar,  360 

^cidiospore  generation  of  Puccinia, 
637 

JEcidium  berberidis,  relation  to  Puc- 
cinia, 637 

—  tussilaginis,  638 

JEthaUum  septicum,  plasmode  of,  634 
AgaricuS)  647 

—  campestris,  648 
Agate,  1095 

Agave,  leaf  of,  686  ;  raphides  of,  696 
Agrion,  987 

—  pulchellum,  wing  of,  as  test  for  defi- 
nition, 426 

—  puella,  pupa  of,  994  ;  wing  of,  994 
Air-angle,  50,  78 

Air-bubbles,  microscopic  appearance  of, 

429,  430 

Air-chamber  of  leaves,  716 
Alse  of  Surirella,  606 
'  Alar  prolongations  '  in  Fusulina,  826  ; 
in  NummitUtes,  827,  831;  of  Calca- 
rina,  830 
Albite,  1080 
Albuminous  substances,  tests  for,   516, 

517 

Alburnum,  704,  708 
Alcohol,  as  solvent  for  resins,  &c.,  517; 

as  hardening  agent,  484 
Alcyonaria,   877,   879;    spines  of,   imi- 
tated, 1101 

Alcyonian,  resembled  by  polyzoan,  908 
Alcyonidium,  908  ;  polyzoary  of,  909 
—  gelatinosum,  calcareous   spicules  in, 

908  note 
Alcyonium  digitatum,  879;  spicules  of, 

880 
Alder,  on  branchial  sac  of  Corella,  912 

note 
Alexander,  on  myopy  and  presbyopy,  118 


ANA 


,    preparation    of,    514;    included 
under  general  term  of  '  thallophytes,' 
540  ;  symbiotic  in  radiolarians,  848 
—  lime  secreting,  1084 
Algal  constituents  of  lichens,  650 
Alkaloids,  micro-chemical  examination  of, 

1103 
Allman's  experiments  on    luminosity  of 

Noctiluca,  7t>9 

Allman,  on  Polyzoa,  909  ;  on  the  '  Haus  ' 
of  Appendicularia,    918  ;   on  Myrio- 
thela,  863  note 
Aloe,  raphides  of,  696 
Alternation  of  generations  in  Batracho- 
spermum,  575  ;  in  Fungi,  634  ;  in  ferns, 
680;  in  Medusce,  877 
Althcea  rosea,  pollen-grains,  721 
Alveolina,  804  ;  resembled  by  Loft  until, 

818;  resembled  by  Fusulina,  825 
Amaranthacece,  pollen-grains,  721 
Amaranthus  hypochondriacus,  seeds  of, 

724  '   - 

Amaroucium  proliferum,  as  example  of 

compound  ascidian,  912 
Amici  suggests  oil  for  immersion  lenses, 

29 

Amici's  invention  of  immersion  system, 
27  ;  horizontal  microscope,  148  ;  camera 
lucida,   279;    objectives,   355;    triple- 
back  objectives,  361  ;  water-immersion 
objectives,  362;    oil-immersion  objec- 
tives, 364 
Ammodiscus,  814 
Ammothea  pycnogonoides,  958 
Amoeba,  733,  742-747,  1018 
Amceba-ph&se  of  Monas,  756 
A  mceba  proteus,  742 
—  radiosa,  experiments  on,  743 
Amcebcp,   cells   of    sponges    resembling, 

855 

Amoeboid  phase  of  Tetramitus,  761 
Amphibians,  plates  in  skin  of,  1026 
Amphioxus,  affinities  with  ascidians,  917 

note 

Amphiplenra  pelliicida,    with    oblique 

illumination,  59,  75  ;  resolution  of,  85  ; 

markings    measured,    274;    markings 

on,  592 

Amphistegina,   827  ;    internal    cast    of, 

841 

Amphitetras,  614 

Amphiuma  ,  red  blood-corpuscle  of,  1036 
Amphonyx,  haustellium  of,  992 
Amplification,  88-90 

—  linear,  26,  39  ;  of  images,  45 
Ampullaceous  sacs  of  sponges,  856,  857 
Anabcena,  548 

Anacharis,  528 

—  alsinastrum,  cyclosis  in,  689  ;  habitat, 
690 

Anagallis,  raphides  of,   696;    seeds  of, 
724 

—  arvensis,  petals  of,  719 
Anal  plate  of  Antedon,  903 
Analgesinfp,  1013,  1014 
Analyser,  318 
Analysing  nose-piece,  294 


INDEX 


1139 


ANA 


APF 


Analysis,  micro-chemical,  1102  ;  method 

of,  1102 

Ajiaraphidece,  599 
Anchor-like  plates  of  Synapta,  895 
Andalusite,  1077 
Androspore  of  (Edogonium,  572 
Anemones,  863.     See  ACTINOZOA 
Anemophilous  flowers,  722 
Anethum  graveolens,  seeds  of,  724 
Angle,  extinction,  1079 
-    of    incidence,    3;    of   refraction,  3; 

of  aperture,  61 
Angles    of    aperture,    air,    balsam,    oil, 

water,  83-87 
Angstrom's  law   for    the   absorption    of 

light  rays,  323 
Anguillula  aceti,  945 

—  fluviatilis,  945 

—  glutinis,  945 
Anguillulce,  945 
Angular  aperture,  395 

—  of  dry  objective,  391 ;  of  oil  immer- 
sion, 391 

-  of  aperture,  resolution  dependent 
on,  44 

—  of  water  immersion,  891 
Angular  distribution  of  rays,  56 ;   grip, 

61 ;  semi-aperture,  77 

AngulifercE,  characters  of,  613 

Animal  kingdom,  two  divisions  of,  727 

Animalcule  cage,  346 

Animalcules,  753.  See  KOTIFEBA,  Infu- 
soria, KHIZOPODA,  &c. 

Animals  and  plants,  differences  between, 
530 

Anisochelae  of  sponges,  860 

Anisonema,  765 

Annual  layers  in  trees,  704 

Annular  cell,  Weber's,  350 

—  ducts  of  Phanerogams,  698 

—  illumination  and  false  images,  419 

—  illumination  for  examining  perforated 
membrane  of  diatom,  419 

Annulata  (Annelids),  larvae  of,  collecting, 
529  ;  marine,  947  ;  appendages  of,  949 ; 
jaws  of,  949  ;  development  of,  949 ; 
eggs  of,  950  ;  fresh-water,  955  ;  lumi- 
nosity of,  955 ;  bibliography,  956 ; 
'  liver '  of,  1047 

Annulus  of  sporange  of  fern,  676 

Anodon,  pearls  in,  923 ;  glochidia  of, 
933  ;  for  observation  of  ciliary  motion, 
940 

Anomia,  prismatic  layer  in,  924 

Anopla  (Nemertines),  951 

Anoplophrya  circulans,  774 

Anorthite,  1080 

Antedon,  food  of,  771 ;  pentacrinoid 
larva  of,  900-902 ;  pseudembryo  of,  903 

Antennae  of  insects,  987  ;  preparation  of, 
988,  989  note 

Antherid  of  Vaucheria,  563 ;  of  Chara, 
577,  578;  of  Fucacece,  627,  628;  of 
Floridece,  631 ;  of  Peronosporece,  638  ; 
of  Marchantia,  665,  667  ;  of  mosses, 
^>70 ;  of  Sphagnacece,  673 ;  of  ferns, 
677 ;  tapetal  cells  in,  678 


Antherozoids,  536,  540 ;  of  Volvox,  555 ; 
of  Vaucheria,  563 ;  of  Sphceroplea, 
572;  of  (Edogonium,  572;  of  Batra- 
chospermum,  574 ;  of  Chara,  579 ;  of 
Phceosporece,  627;  of  Fucacece,  628; 
of  ferns,  678 ;  of  Rhizocarpece  681 

Anthers,  719 

Anthony  (Dr.),  on  pseudo-trachea  of  fly's 
proboscis,  990  note 

Anthophysa,  765 

Antirrhinum  majus,  seed  of,  724 

Apertometer,  195,  390  ;  Abbe's,  307,  394  ; 
Tolles',  390  ;  use  of,  394 

Aperture,  in  microscopic  objectives,  33, 
42-50,  60 ;  how  obtained,  45  ;  Abbe  on 
definition  of,  45,  48  note 

—  angular,  49  note,  53,  395 

—  numerical,  theoretical    maximum   of, 
30 

—  numerical,  49  note,   53,  76,  390 ;  for 
dry  objective,  50 ;  for  oil  immersion, 
50  ;  for  water  immersion,  50 

—  numerical,    of    Zeiss's  apochromatic 
series  of  objectives,  371 

—  of  objective,  390 

—  relation  of,  to  power,  82,  83,  363  ;  as- 
certained by  vertical  illumination,  338 

—  table,  Royston-Pigott's,  30 

—  numerical,  table  of,  84-87 
Apertures,  relative,  49 
Aphanizomenon,  548 
Aphanocapsa,  547 

Aphides,  wings  of,  998,  999 ;  agamic  re- 
production in,  1006 
Aphodius,  antennae  of,  988 
Apidce,  987 

Apis  mellifica,  mouth-parts  of,  990 
Aplanatic  system,  20, 23 

—  objective  use  of,  21 

—  cone,  307 

—  aperture,  309,  315 

—  foci,  Lister's  discovery,  355 
Apochromatic    objectives,    19,     30,    33, 

35,  80,  258 ;  advantages  of,  33,  34 ; 
objective,  Zeiss's,  365-371 ;  dry,  368  ; 
comparison  of,  with  chromatic  and 
achromatic  lenses,  368 ;  homogeneous 
objectives,  value  of,  in  study  of 
monads,  762;  objective,  use  with 
various  test  scales,  976 

—  condenser,  Powell  and  Lealand's,  302 
Apochromatism,  366,  369 
Apocynacecv,     laticiferous     tissue     of, 

695 

Apogamy  in  ferns,  680 
Apospory  in  ferns,  680 
Apotheces  of  lichens,  650,  651 
Apparent  creation  of  structure,  68 
Appendicularia,   911,  917;    pharyngeal 

sac  of,  917 ;  tail  of,  917  ;  cotochord,  918 ; 

1  Haus '  of,  918 

Apple,  raphides  in  bark  of,  696 
Apposition,  growth  by,  533 

—  mode  of  growth  of  starch,  695 
Apus,  959,  962 ;  parthenogenesis  of,  964 

note 

—  cancriformis,  carapace  of,  962 

4  «  2 


1 140 


INDEX 


AQU 

Aquarium  microscopes,  260-269 ;  J.  W. 

Stephenson's,    267-268;     Rousselet's, 

268-269 

Aquatic  microscope,  147 
ARACHNIDA,  1008 

—  eggs  of,  1005  ;  related  to  Pycnogonida, 
959  note ;  reproductive  organs  of,  1011 

Arachnoidiscus,  588,  612 

Arachnospluerd  obligacanthn,  850,  852 

Aragonite,  1095 

-in  shell  of  Pholas,  924 

Aralia  papyrifera,  parenchyme  of,  687 

Araneida,  1008 

Arcella,  746 

Archegones  of  Va  ucheria,  563 ;  of  Chara, 
577,  578  ;  of  Marchantia,  665,  668 ;  of 
mosses,  670,  671 ;  of  Sphagnacece, 
673;  of  ferns,  677,  678;  of  Lyco- 
podiece,  681 ;  of  Bhizocarpece,  681 

Archer,  on  amcebiform  phase  of  Stepha- 
nosphcera,  557  note  ;  on  desmids,  579 
note ;  classification  of,  585 ;  on 
Clathrulina,  742  note 

Archerina  Boltoni,  730 

Arctium,  stem  of,  709 

Arcyriaflava,  sporanges  of,  635 

Arenacea,  810-814 

Arenaceous  character  of  Textularinice, 
823 

—  Foraminifera,  varying  size  of  particles 
in  test  of,  818 

—  test  of  Foraminifera,  810 
Arenicola,  948 

Areolee  of  frustule  of  Coscinodiscus,  591 

Areolar  connective  tissue,  1040,  1045 

Argas,  bite  of,  1012 

ArgasidcB,  1012 

Argonauta,  929 

Argosince,  1008 

Argulus  foliaceus,  966 

Aristolochia,  stem  of,  709 

'  Aristotle's  lantern  '  of  echinids,  890 

Aristotle  on  myopy  and  presbyopy,  118 

Arsenic,  micro-chemical  analysis  of,  1103 

Artemia,  962 

—  salina,  movement  of,  960  ;  habitat  of, 
963 

Arteries,  1056 
Arthrodesmus  incus,  568 
ARTHROPODA,  957-1016 ;  smallest  of,  1008 ; 
eye  of,  983 

—  limbs  of    Pedalion  compared   with 
those  of,  792 

Arthrosporous  Bacteria,  657 

Artificial  light,  417 

'Artificial  lightning,'  682 

Ascaris  lumbricoides,  944 

Asci  of  Ascomycetes,  642 ;  of  lichens,  650 

Ascidians,  diatoms  in  stomach  of,  614, 

623;    solitary,  911 ;  branchial  sac  of, 

912,  913,  915  ;  circulation  in,  912,  915  ; 

compound,  912 ;  cloaca  of,  913 ;  stolons 

of,   914;    bibliography   of,    914   note; 

social,  914  ;  general  structure  of,  916  ; 

development  of,  916 ;  tadpole  of,  917 ; 

affinities  with  Amphioxus,  917  note 
Asclepiadece,  pollinium  of,  722 


BAC 

Ascogone  of  Ascomycetes,  643 ;  of  lichens,. 
651 

Ascomycetes,  642-645 ;  as  fungus-con- 
stituents of  lichens,  651 

Ascopores  of  Ascomycetes,  642;  of 
lichens,  650 

Asellus  aquaticus,  ciliated  parasite  in 
blood  of,  774 

Asilus,  eye  of,  987 

Aspergillus,  fermentation  by,  647 

Asphalte  for  cells,  446 

—  varnish,  443 

Aspidisca,  a  phase   in  development  of 

Trichoda,  781 

Aspidium,  indusium  of,  675  ;  sori  of,  675 
AsplancJina,  in  confinement,  528 
Astasia,  545  ;  mouth  in,  765 
Asteroidea,  skeleton  of,  891 ;  spines  of,. 

891 ;  larva  of,  897 
Asterolampra,  595,  610 
Aster omphalus,  610 
Astromma,  849 
Astrophyton,  spines  of,  891 
Astrorhiza,  811,  815 
Astrorhizida,  812 
Athecata,  868 
Athyrium  Filix-fcemina,  apospory  in,, 

680 

Atrium  of  Noctiluca,  766 
Auditory  vesicles  of  Mollusca,  941 
Audouin,  on  '  muscardine,'  645 
Augite,  1072 ;  zonal  structure  in,  1073 
Aulacodiscus,  612 

—  Kittonii,  markings  on,  591 

—  Sturtii,  markings  on,  592 
Autofission  of  diatoms,  594 
Auxospore,  594-601 
Avanturine,  1095 
Avicularia  of  Polyzoa,  910,  911 
'  Awns  '  of  Chcetocerece,  614 
Axile  body  of  tactile  papilla,  1053 
Axinella  paradoxa,  858 

Axis  cylinders  of  nerve-tube,  1051,  1052" 


P, 


Bacillariacece  of  Kiitzing,  587 
Bacillaria    varadoxa,    movements    of, 

602,  606 

Bacilli,  form  of,  653 
Bacillus, '  granular  spheres '  of,  660  note 

—  anthracis,  656 ;  spores  live  in  abso- 
lute alcohol,  660 

—  megaterium,  655 

—  of  anthrax,  1037  note 

—  of   tuberculosis,   modes   of    staining,. 
515,  516 

—  subtilis,  656,  657,  658  ;  spores  of,  660 
'  Bacon-beetle,'  980 

Bacon  (Roger),  inventor  of  simple  micro- 
scope, 126 

Bacteria,  use  of  large  and  small  cones  in 
examining,  421 ;  photo-micrographs, 
423;  as  test  for  definition,  426 ;  staining, 
514-516;  (see  Schizomycetes),  651;. 
affinities  to  Algce,  651;  to  Flagellata, 


INDEX 


II4I 


BAC 


BIK 


•651 ;  to  Nostocacece,  652 ;  movements 

of,  652 ;  mode  of  multiplication,  652 ; 

forms  of,  653 ;    classification   of,  655  ; 

nutrition    of,    658;     flagella    of,    659; 

germinating  power  of,  660 ;  spores  of, 

660 

Bacteriastrum  furcatum,  614 
Bacteriology,  662 
Bacterium,      lineola,    compared      with 

Cercomonas,  651 

—  lineola,  658 

—  termo,   flagellum   of,  72,  658;  move-'' 
ment  of,  653  ;  zoogloea  of,  659  ;  germi- 
nation of,  660 

Bailey,  on  internal  casts  of  Foramini- 
fera,  828  note 

Bailey's  method  of  isolating  diatoms, 
624 

Baker,  on  Cuff's  microscope,  142 

Baker's  microscopes,  202,  218-221,  230, 
246,  251 ;  mechanical  stage,  181 ;  sub- 
stage,  188, 189  ;  achromatic  condenser, 
306  ;  fitting  for  condenser,  312 

—  lamp,  407 
Balanidce,  967 

Balanus  balanoides,  967  ;  disc  of,  968 
Balsam  angle,  50,  78 

—  refi'active  index  of,  77 
Banksia,  stomates  of,  716 
Barbadoes  earth,  846,  849 
Bark,  700,  702,  708 

Barker's  Gregorian  telescope,  145 

Bar  movement,  262 

'  Barlow  lens '  applied  to  a  microscope, 
149 

Barnacles,  967.     See  Cirripedia 

Basals  of  Antedon,  901 

Basidiomycetes,  647;  as  fungus  consti- 
tuent of  lichens,  651 

Basidiospores  of  Basidiomycetes,  647 ; 
of  Hymenomycetes,  647 

Basids  of  Puccinia,  637  ;  of  Basidiomy- 
cetes, 647 

Bast,  710 

Bat,  parasite  of,  1012;  hair  of,  1030; 
cartilage  in  ear  of,  1046 

'  Bathybius,'  747 

Bdtrachia,  red  blood-corpuscles,  1035; 
lungs  of,  1063 

Batrachospermece,  574 

Batrachospermum  moniliforme,  575 

—  protoneme  of,  575 

'  Battledore  scale  '  of  Lyccenidce,  975 

Bausch  and  Lomb's  microscopes,  212-214, 
222,  239,  247,  252 ;  mechanical  stage, 
183,  184;  sub-stage,  212;  chemical 
microscope,  263 ;  camera  lucida,  285 ; 
objectives,  375 

Bdella,  maxillary  palps  of,  1010 

Bdcllidce,  1013 

Bdelloida,  791 

Bead-moulds,  645 

Beale's  camera,  279, 288  ;  glycerin  method 
of  preserving,  520 

Beale,  on  organic  structure,  1017 

Beck's  microscopes,  228,  233 ;  mechani- 
cal stage,  184  ;  rotatory  nose-piece,  291 ; 


condensers,  304,  305 ;  side  reflector, 
333 ;  vertical  illuminator,  337  ;  disc- 
holder,  339 ;  compressor,  347 ;  rings 
for  locking  coarse  adjustment,  352 ; 
objectives,  375 ;  lamp,  405-407  ;  achro- 
matic binocular  magnifier,  456  note ; 
disc-holder  for  examination  of  Fora- 
minifera,  845 

Beck  (R.),  on  markings  of  Podura  scale, 
978 

Bee,  hairs  of,  980 ;  head  of,  982  ;  wing  of, 
994,  998  ;  sting  of,  1003 

Beeldsnyder's  achromatic  objective,  147 

Beetles.     See  Coleoptera 

Beggiatoa,  form  of,  652,  653 

—  alba,  653-655 
Begonia,  seeds  of,  724 

Behrens'  method  of  analysing  minerals, 

1083;  of  micro-chemical  analysis,  1102 
Bell  (Jeffrey),  on  the  spines  of  Cidaris, 

889 

Bell's  cements,  443,  479 
Beneden  (Ed.   Van),  on  Gregarina  gi- 

gantea,   749  note;    on   movement  of 

gregarines,  750 
Benzol,  uses  of,  517 
Bergh,  on  Flagellata,  764 
'  Bergmehl,'  622 
Berkeleya,  602 
Bermuda  earth,  608,  611 
Beroe,  collecting,  529 

—  Forskalii,  881,  882 

—  ovatus,  Eimer  on,  882  note 
Bicellaria  ciliata,  910 

Biconvex  lens,  formulae  relating  to,  21 
Biddulphia,  612 

—  cyclosis  in,  587 ;  chains  of,  588,  596 ; 
structure  of  frustule,  590  note 

Biddulphiece,  character  of,  612 

Biflagellate  monad,  759 

Bignonia,  seed  of,  724 

Bignoniacea,  winged  seeds,  724 

Biloculina,  802 

Binary  subdivision  of  cell,  535,  536 

Binocular  eye-piece,  Tolles',  101 ;  Abbe's, 
102 

Binocular  magnifier,  Beck's  achromatic, 
456  note 

Binocular  microscope,  61,  97 

Biddell's,  96  ;  Nachet's,  98 ;  stereo- 
scopic, Wenham's,  98 ;  Stephenson's, 
100;  Stephenson's  erecting,  100; 
stereoscopic,  for  study  of  opaque  ob- 
jects, 103 ;  non-stereoscopic,  105 ; 
Powell  and  Le^land's  high-power,  105, 
106;  Rousselet's  portable,  245;  Ste- 
phenson's for  dissection,  248,  401,  456  ; 
spectrum  microscope,  327 

Biology,  530 

'  Bipinnaria,'  resemblance  of  Actvno- 
trocha  to,  950 

Bipinnaria  asterigera,  897 

Birch,  pollen-grains  of,  722 

BIRD,  parasite  of,  1012,  1014  ;  lacunae  in 
bone  of,  1022;  epidermic  appendages 
of,  1029  ;  red  blood-corpuscles  of,  1034, 
1035  ;  lungs  of,  1064 


1 142 


INDEX 


BIR 


BUT 


Bird's  egg,  concretions  on  shell  imitated, 

1102 
*  Bird's  head  processes,'  see  Avicularia, 

910 

Bismarck  brown,  489 
Bivalves,  structure  of  ligament  in,  1010 
'  Black  dot,'  Nelson's,  277 
'  Bladderwrack,'  627 
Blanchard  (R.),  on  Sporozoa,  749  note 
Blatta,  antennae  of,  988 

—  orientalis,  eggs  of,  1005 
Blenny,  scales  of,  1027 

Blood,  colourless  corpuscles,  1018;  method 
of  mounting,  1038 ;  circulation  of,  1054  ; 
flow  of,  1055  ;  micro-chemical  examina- 
tion, 1103 

—  of   insects,   circulation   of,    993,   994 ; 
of  Vertebrata,  1034 

Blood-corpuscle,  relation  of  size  to  that 

of  bone  lacunae,  1022 
Blood-corpuscles  of  Vertebrata,  1034 
Blowfly's   maxillary    palpus,    hairs    on, 

examination  of,  422 
Blowfly,  proboscis  of,  examination  with 

apochromatic,   371 ;   hairs  on,  as  test 

for  definition,  426 

—  development  of,  1007  ; '  imaginal  discs ' 
of,  1007 

1  Blue  mould,'  643 

Bodo,  545 

Body  of  the  microscope,  157 

'  Bog-mosses,'  673 

Boletus,  647 

Bombyx,  987 

—  mori,  eggs  of,  1005 

Bonannus's  microscope,  132;  his  hori- 
zontal microscope,  133,  134  ;  his  com- 
pound condensers,  298,  299 

Bone,  1020  ;  structure  of,  1020-1023 ; 
preparation  of,  1023 ;  matrix  of,  1039  ; 
decalcification  of,  512 

Bones,  fossilised,  1090-1092 

1  Bony  pike,'  scale  of,  1022 

Borax  carmine,  490 

Bordered  pits  in  the  trachei'des  of  coni- 
fers, 698,  703 

Boscovich,  on  chromatic  dispersion,  42 

Botryllians,  914 

Botryllus  violaceus,  915 

Botryocystis,  545 

Botrytis  bassiana,  645 

Botterill's  growing  slides,  340 ;  his 
zoophyte  trough,  348 

Bouguet,  on  uniform  radiation,  51 

Bowerbank,  on  sponge  spicules,  859  note  ; 
on  structure  of  molluscan  shells,  921, 
928 

BowerbanJcia,  gizzard  of,  905;  stem  of, 
908 ;  polyzoaries  of,  909 

'  Box-mite,'  1012 

Brachinus,  antennae  of,  988 

Brachionus  rubens,  787-790,  791 ;  male 
of,  790 

BKACHIOPODA,  shells  of,  919,  925-927  ; 
relation  of  shell  to  mantle,  926  ;  affini- 
ties to  Polyzoa,  927 

Brachyurous  decapods,  young  of,  969 


Brady  (H.B.),  on  Foraminifera,  810;  on 
arenaceous  Foraminifera,  811 ;  on 
affinity  of  Carpenteria,  823 

Brady  and  Carpenter,  on  fossil  Lituolce> 
817 

'  Brake-fern,'  675.     See  Aspidiiuii 

Bran,  725 

Branchiae  of  annelids,  948,  949 

Branchiopoda,  959  ;  divisions  of,  961 

Branchipus,  movement  of,  960 

—  stagnalis,  962,  963 
Branchiura,  965  note,  966 

Brandt  (K.),  on  artificial  division  of 
Actinosph&rium,  741  note  ;  on 
zooxanthellse,  848 

Braun,  on  Pediastrum,  567 

Brewster,  his  hand  magnifier,  37  on 
modification  of  stereoscope,  91  on 
'  lens  '  from  Sargon's  palace,  119  hi& 
'  Treatise  on  the  Microscope,'  120  ;  on 
achromatic  condensers,  299,  300 

Bright-line  spectro-micrometer,  325 

Brightwell,  on  Triceratium,  613  note  ; 
on  Chcetocerea,  614  note 

Brilliancy  of  image,  382 

'  Brimstone  moth,'  eggs  of,  1005 

Brine  shrimp,  960,  963 

1  Brittle  stars,'  891.     See  Ophi/iroidea 

Brooke's  nose-piece,  291 

Brownian  movement,  experiments,  481,, 
432 

Browning's  bright-line  spectro-micro- 
meter, 325 

Briicke  lens,  88 

Brunswick  black  as  a  black  'ground/ 
444  ;  for  cells,  446 

Bryacece,  678 

Bryobia,  1013 

Bryony,  cells  of  pollen-chambers,  720 

Bryozoa,  904.     See  POLYZOA 

Bryuni  intermedium,  peristome  of., 
672 

Bubbles  in  cavities  of  crystals,  1074 

Buccinum,  987 

—  undatum,   palate   of,   980,   932,  938  j 
nidamentum  of,  934 

Buchner's    experiment     011     spores     of 

Bacteria,  660 
Buckthorn,  stem  of,  708 
Bug,  mounting  medium  for,  973 
Bugula,  polyzoary  of,  909 

—  avicularia,  910,  911 
Built-up  '  cells,'  449 
Bulbils  of  Nitella,  577 

Bulloch's   modification   of    Zentmayer's 

microscope,  204 
Bull's-eye,   300;    use   of,  329-333,  407- 

420 ;  with  high  power,  331 ;  for  use  in 

study  of  saprophytic  organisms,  833  ? 

Powell  and  Lealand's,  338 
Bull's-eye  stand,  248 
Bundle-sheath,  711 
Burdock,  stem  of,  709 
Biitschli,  on  mouth  of  Astasia,   765  ;  on 

Vorticellce,  773  note 

—  and  Engelmann,  on  conjugating  vorti- 
cellids,  782 


INDEX 


1143 


BUT 

Butterflies,  wing  of,  994 
Butterfly.     See  Lepidoptera 


Cabbage-butterfly,  eye  of,  983 ;  number 
of  facets  in,  983  ;  eggs  of,  1005 

Caberea  Boryi,  vibracula  of,  907  note 

Cabinet  for  slides,  523  ;  arrangement  of, 
523  , 

Cactus,  cells  of  pollen-chambers,  720 

Cactus  senilis,  raphides  of,  696;  brittle- 
ness  of,  696 

Cacti maria  crocea,  development  of,  900 
note 

Calamites,  1084 

Cahithus,  antennae  of,  988 

Calcarina,  825,  830  ;  compared  with 
Eozoon,  838 

CalcispongifB,  spicules  of,  859 

Calcite  in  shells,  924 

Calco-globuline,  1101 

CaUi-thamnion,  630 

Calosanthes  indica,  winged  seed  of,  724 

Calotte  diaphragms,  297 

Calycanthus,  bark  of,  709 

Calycine  monad,  760 

Calycles  of  hydroids,  868 

Calypter  of  mosses,  671 

Calyx  of  Flagellata,  764 

Cambium,  710 

—  in  Exogens,  697 

—  layer,  708 

Cambridge  rocking  microtome,  469 
Camera  lucida,  277  ;  Soemmering's,  278  ; 
Wollaston's,  27H;  Amici's,   279;    Nel- 
son's,   279,    280;    Beale's,    279,    288; 
Cooke's,   280,   281;    Abbe's,   281-284; 
Swift's   modification   of  Abbe's,   284  ; 
Bausch   and   Lomb's   modification   of 
Abbe's,  285  ;  Schroder's,  285,  286 
Campani's  microscope,  128 ;  eye-piece,  876 
Campanula,  pollen-grain  of,  721 
Camj}anularia,  870 

—  gelatinosa,  865 
Ccunpanulariida,   870;  zob'phytic  stage 

of,  877 

Campbell's  differential  screw,  162,  164, 
165,  174,  202,  230 

Campylodiscus,  587,  588,  595 ;  move- 
ments of,  602;  structure  of  frustule, 
606 

—  clypeus,  607 

—  spiralis,  cyclosis  in,  587 
Canada  balsam,  443 

capped  jars  for,  477 ;  as  mounting 

medium,  480,    521 ;   as   a  preserv- 
ative medium,  518 ;   mode  of   pre- 
paration, 518  ;  refractive  index,  521 ; 
for  mounting  insects,  973 
Canal    system    of    Calcarina,    825 ;    of 
Polystomella,   827;    of  Nummulites, 
827 

Canaliculi  of  bone,  1019,  1021 
Cancellated  structure  of  bone,  1020 
Cancer  pagurus,  skeleton  of,  968 


CBL 


Canna,  starch-grains  of,  695 
Cannocchiale,  125 
Capacity  of  object-glass,  382 
Capillaries,  1056.  1062 
Capillitium  of  Myxomycetes,  636 
Capsule,  central,  of  Badiolaria,  847 

—  of  mosses,  670  ;  of  Purpura,  934 

—  silicious,  of  Clathrulina,  742 
Carapace  of  Copepoda,  960  ;  of  Clado- 

cera,  961 
Carbon  bisulphide  as  a  solvent  for  oils 

&c.,  517 
Carboniferous  epoch,  vegetation  of,  681 

—  limestone,  1090 
Carchesium,  collecting,  527 
Carcinus     mcenas.    metamorphosis    of, 

970 

Carnation,  parenchyme  of,  688 
Carnivora,  arrangement  of  enamel   in, 

1025 

Carp,  scales  of,  1027 
Carpenter  (H.  P.),  on  crinoids,  903  note 
Carpenter  ( W.  B.),  on  stereoscopic  vision, 
90-93  ;  on  classification  of  Foramini- 
fera,  799 ;  on  Eozoon,  838  ;   on  alter- 
nation of  generation  in  Medusa,  877  ; 
on   the   so-called   excretory    pores    of 
Ctenophora,  882  note ;  on  development 
of  Antedon,  903  note  ;  on  structure  of 
molluscan  shells,  921 
Carpenteria,  822 ;  mode  of  growth  com- 
pared with  Eozoon,  838 

—  rhaphidodendron,  823 
Carpogone  of  Floridece,  632  ;  of  Ascomy- 

cetes,  643 

Carpospores  of  Floridece,  632 
Carrot,  seeds  of,  724 
Carter  (H.  J.),  on  affinity  of  Carpenteria, 

823 

Cartilage,  1046  ;  mounting.  1047 
Carum  carui,  seeds  of,  724 
Caryophyllia,  lamellae  of,  878 

—  Smithii,  thread-cell  of,  879 

Cascarilla,  raphides  of,  696 

Cassowary,  egg-shell  of,  1101 

Castracane,  on  beaded  structure  of  di- 
atoms, 593  ;  on  Pfitzer's  auxospores, 
595 ;  on  sporangial  frustules  of 
diatoms,  595 ;  on  reproduction  of  di- 
atoms, 597  ;  on  diatoms,  598 

Cat,  Pacinian  corpuscles  of,  1053 

Caterpillars,  '  pro-legs  '  of,  1002  ;  feet  of, 
1002 

Cathcart's  freezing  microtome,  474,  475 

Catoptric  form  of  microscope,  145,  146 

Caulerpa,  563 

Cauterisation  by  focussing  the  sun's  rays 
(Pliny),  117 

Cedar,  stem  of,  705 

Cell,  contents  of,  533-535  ;  binary  sub- 
division of,  535 

Cell-division  and  nucleus,  1018, 1019  and 
note 

'  Cell '  of  Polyzoa,  904 

Celloidin  imbedding  method,  503-506 

—  staining  and  mounting,  sections,  506 
Cell- sap,  534 


1 144 


INDEX 


CEL 


CHE 


•*  Cells '  for  examining  Infusoria,  &c., 
349 ;  for  dry  mounting,  445  ;  of  cement, 
446  ;  paraffin,  446  ;  paper,  446 ;  ring- 
cells,  446;  of  plate-glass,  for  zoo- 
phytes, &c.,  448  ;  built  up,  449  ;  sunk, 
449 ;  mounting  in,  482-484 ;  of  bone, 
483  ;  of  tin,  483 

Cells  of  plants,  532 ;  multi-nucleated,  534  ; 
primordial,  536 ;  of  vertebrates,  1018 

Cell-structure,  Strasburger  on,  537 

Cellular  cartilage,  1046 

—  parenchyme,  688 
Cellulose,  533 

—  tests  for,  516,   517  ;  envelope  of  des- 
mids,  580;  in  Dinoflagellata,  770;  in 
zoocytium  of  Ophrydium,  778 

Cell-wall,  533 ;  mode'  of  growth  of,  533 ; 
apposition,  533  ;  intussusception,  533 

Cell-wall  of  Phanerogams,  692 

Cement-cells,  446 

•Cements,  442;  liquid,  442;  Bell's,  443, 
479 ;  japanner's  gold  size,  443  ;  Bruns- 
wick black,  444  ;  glue  and  honey,  444  ; 
shellac,  444 ;  Hollis's  liquid  glue,  444, 
479 ;  Venice  turpentine,  444 ;  marine 
glue,  445  ;  Heller's  porcelain,  521 

Cementum  of  teeth,  1025,  1026 

Centipedes.     See  Myriopoda 

Central  capsule  of  Badiolaria,  734 

•Centring,  382,  389 

Centring  nose-piece,  293;  as  sub-stage, 
230 

Centro-dorsal  plate  of  Antedon,  902 

Cephalolithis  sylvina,  847 

Cephalophorous  mollusca,  palates  of, 
930-983 

CEPHALOPODA,  929 

—  organs  of  hearing  in,  941 ;  chromato- 
phores  of,  942 

CeramiacecB,  630 
•Ceramium,  630 
Ceratium,  111 

—  furca,  771 

—  tripos,  771 
Ceratodus,  1091 

Cercomonas  typica,  compared  with  Bac- 
teria, 651 

•Cereals,  seeds  of,  starch  in,  694 
Centra  vinula,  eggs  of,  1005 
Cestoid,  943 

Cetonia,  antennae  of,  988 
Chcetocerece,  affinities  of,  614 

—  '  awns '  of,  614 

—  occurrence  in  marine  animals,  614 
Chcetoceros  Wighamii,  614 
Chatophoracece,  573 

'  Chaff-scales,'  silex  in,  715 

Chalk,  microscopic  constituents  of,  1085, 

1087 ;     resemblance     to     Globigerina 

ooze,   1087 ;  mode  of  preparation  for 

examination,  1088 
Chama,  prismatic  layer  in,  924 
Chamberlets  in  Foraminifera,  798,  803, 

804  ;  of  Parkeria,  817 ;'  in  Fusulina, 

825  ;  of  Cycloclypeus,  835 
•Chamidce,  Foraminifera     attached  to 

shells  of,  845 


Changes  of  form  of  white  corpuscles,  1087 

Chantransia  and  Batrachospermum,  575 

Chara,  576,  669  ;  antherozoids  of,  667 

CharacecB,  575-579 

Charles's  achromatic  lenses,  148 

Cheese-mite,  1008,  1 013 

Cheilostomata,  characters  of,  909  ;  ex- 
amples of,  910 

Cheirocephalus,  962,  963 

Chemical  tests  for  biological  work,  516, 
517 

Cherry-stone,  section  of,  693 

Chert,  1089 

Cherubin  d'Orleans,  his  binocular  micro- 
scope, 130,  131 ;  his  compound  micro- 
scope, 180 

Chevalier,  on  Charles's  achromatic 
lenses,  148 

Chevalier's  combination  of  lenses,  38 ; 
achromatic  microscope,  148,  150 ;  mo- 
difications of  Selligue's  lenses,  354 ; 
objectives,  Lister's  note  on,  354,  355 

Cheyletiy  1013 

Cheyletidce,  tracheae  of,  1011 

Cheyletus,  hairs  of,  1010;  legs  of,  1010; 
mouth  parts  of,  1010 

Chickweed,  petals  of,  719 

Chicory,  adulteration  of,  725  note 

CMlodon,  mouth  of,  774 

—  cucullulus,  binary  division  of,  777,  779 
Chilognatha,  981 

Chirodota  violacea,  '  wheels  '  of,  896 

Chitin,  in  test  of  Arcella,  746 ;  of  insects' 
skin,  974 

Chitinous  substances,  mounting,  481 

Chiton,  shell  structure,  928 ;  eyes  on 
shells  of,  941 

Chlamydomonas,  545 

Chlamydomyxa,  affinity  with  Monero- 
zoa,  727 

Chlamydospores,  of  Mucorini,  641 ;  of 
gregarines,  750 

Chloral  hydrate  as  a  preservative  me- 
dium, 519 

Chloroform,  uses  of,  517 

Chlorophyll  corpuscles,  534,  535 

Chlorosporece,  625 

CHORDATA,  911.     See  VERTEBRATA 

Choroid  coat  of  eye,  pigment-cells  in, 
1042 

Chromatic,  comparison  of,  with  achro- 
matic and  apochromatic  lenses,  368 

—  aberration,  16,  17,  31 

—  condenser,  Abbe's,  308,  309 ;  811,  312  ; 
38-5 

—  Powell  and  Lealand's,  301-303 

—  correction,  test  for,  388 

—  dispersion,  diminished  by  Huyghens' 
objective,  42 

Chrumatophores  of  Peridinium,  770 ;  of 

Cephalopods,  942 
Chromatoplasm,  537 
Chrodcoccacece,  characters  of,  547 
Chroococcus,  547  ;  as  gonid  of  lichen,  651 
Chroolepus,  as  gonid  of  lichen,  651 
Chrysaora,   874,   876;    development  of, 

876 


INDEX 


1145 


CHY 


COL 


€hyle,  corpuscles  in,  1037 

Chytridiacece,  636 

Cicadce,  wings  of,  998,  999 

Cichoriacece,  pollen- grains  of,  721 

Cicindela,  987 

Cidaris,  spine  of,  885,  888 

—  metularia,mo(Le  of  formation  of  spines 
in,  889 

Cienkowski,  on  decaying  cells  of  Nitella, 
579  note;  on  parasitic  plasmode  in 
Nitella,  579  note ;  on  reproduction  of 
Noctiluca,  769  »>, 

Cilia,  532,  1044  ;  of  Infusoria,  771 ;  use 
of,  in  Ciliata,  773 ;  of  Turbellaria,  946 

-Ciliary  action,  772 

—  motion  on  gills  of  Mollusca,  940 

—  movement  in  protophyes,  535 
Ciliata,  771-783 ;  ciliary  action  of,  772, 

774;  'shield'  of,  773;  lorica  of,  773; 
myophan- layer,  773 ;  trichocysts  of, 
773 ;  ento-parasitic  forms,  774  ;  mouth 
of,  774  ;  foot-stalk  in,  774  ;  impression- 
able organs  of,  775 ;  '  eye-spots  '  of, 
775 ;  food  of,  775 ;  artificial  feeding, 
776 ;  contractile  vesicles  of,  776 ;  mul- 
tiplication of,  777  ;  conjugation  of,  777, 
782  ;  encystment  of,  778-782  ;  disper- 
sion of,  781 ;  desiccation  of,  781 ;  Stein 
on  acinetiform  young  of,  782  note 

•Ciliate  Infusoria,  general  structure  of, 
754 

Ciliated  epithelium,  1044 

Ciliobrachiate  zoophytes,  905 

Cilio-flagellata,  770 

Cilium  of  Noctiluca,  766  note 

Ciinex  lectularius,  eggs  of,  1005 

Cinchona,  raphides  of,  696 

Cinclidium  arcticum,  peristome  of,  672 

Cineraria,  pollen-grains  of,  722 

Cineritious  matter,  1052 

Circulation  in  ascidians,  912,  915 

—  of  blood,  1054 

Circumambient  chamber  in  Orbitolites, 

806 

Cirrhi  of  Cirripedia,  968 
Cirripedia,  967 
Cladocera,  961 
Cladococcus  viminalis,  851 
Cladonia  furcata,  650 
Cladophora  glomerata,  574  ;  cell  division 

of,  569,  574 

Cladorhiza  inversa,  860 
Claparede    and  Lachmann,   on   Lieber- 

kuehnia,  731 ;  on  '  rolling  '  movement 

of  Amoeba,  744 

Clark  (James),  on  Flagellata,  764 
Clastic  rocks,  1075 
Clathrulina  elegans,  742 
Clausius  on  emission  of  light,  54 
Clavelinidce,  gemmation  of,  911 ;  stolons 

of,  914 

Claviceps  purpurea,  644 
Clavicornia,  antennae  of,  987 
Claws,  1029,  1033 
•Clay,  1092 

Cleanliness,  importance  of,  522 
Clematis,  stem  of,  702 


'  Closed  '  bundles,  710 

Closterium,  cyclosis  in,  581 ;  '  swarming 
of  granules '  in,  581 ;  binary  division 
in,  582 ;  two  zygospores  in,  584  note ; 
zygospore  of,  584 ;  form  of  cell,  585 

Clostridia,  form  of,  653 

'  Clothes-moth,'  999 

Clove-pink,  seed  of,  723 

'  Club-mosses,'  681 

Clypeaster,  spines  of,  889 

Coal,  '  bituminous,'  1084 

Coal-plants,  1083 

Coarse  adjustment,  159  ;  '  stepped '  rack- 
work  for,  161 ;  arrangements  for  '  lock- 
ing,' 352 

Cobcea,  testa  of  seeds  of,  725 

—  scandens,  pollen-grains  of,  721 
Coccidia,  752 

Coccidiidce,  749 

Coccidium  oviforme,  752 

Coccoliths,  747-749  ;  in  chalk,  1084, 1088 

Cocconeidece,  characters  of,  614 

Cocconeis,  615 

Cocconema,  602,  616 

—  fusidium,  621 

Coccospheres,  747-749  ;  in  chalk,  1088 
Cockchafer,  antennae  of,  974.     See  Melo- 

lontha 

1  Cockle  '  in  wheat,  945 
Cockroach.     See  Blatta 
Cocoa-nut,  725 

—  shell  of,  693 
Cocos-wood,  704 
Coddington  lens,  37 
Codium,  563 

Codonella,  silicious  shell  of,  773 
Codosiga  umbellata,    fission    of,    764 ; 

arborescent  colonies  of,  765 
CCELENTEBATA,  862-883  ;  bibliography  of, 

883;  permanent  gastrula- stage  of,  72(5 

—  See  ZOOPHYTES 
Coeloplana,  883 

Coenosarc,  of  hydroids,  867,  870 

Ccenurus,  944 

Cohn,  on  sexual  generation  of  Volvox, 
555 ;  on  movements  in  Oscillator  ia,  548 ; 
on  reproduction  of  Spharoplea,  570 

Coleochtztacece,  575 ;  zoospores  of,  575  ; 
trichogyne  of,  575 

Coleochcete,  575 

Coleoptera,  973  ;  dermo-skeleton  of,  974  ; 
scales  of,  975  ;  elytra  of,  981 ;  eyes  of 
983,987;  antennae  of,  987,  988  ;  mouth- 
parts  of,  989 ;  winsrs  of,  999 ;  leg  of, 
1000 

Coleps,  food  of,  776      . 

Collar  correction,  358 

Collared  cells  of  sponges,  856 

'  Collars  '  of  Flagellata,  764 

'  Collateral '  bundles,  710 

Collection  of 'microscopic  objects,  appara- 
tus for,  526-529 

Collembola,  977 

Colletonema,  602 

Collins's  condenser  with  rotating  sub- 
stage,  386 

Collomia  testa  of  seeds  of,  725 


1 146 


INDEX 


COL 

Collomia  grandiflora,   spiral  fibres   in 

seeds  of,  693 
Collozoa,  852 
Colonial  Acinetina,  784 
Colonies,  in  Codosiga,  764 ;  of  Radiola- 

rians,  849 ;  of  Polyzoa,  924 
Columel  of  Sphagnacece,  674 
Comatula,  900,  901 ;  nerves  of,  1052 
'  Comb-bearers,'  881.     See  Ctenophora 
Commensalism,  in  lichens,  650 
Compensating  eye-pieces,  34,  273,  378 
Composite,  laticiferous  tissue  of,  695 
Compound  condenser,  sub-stage,  134 

—  microscope,   construction  of,  39;  in- 
vention of,  Govi  on,  120 

Compression  of  light  rays,  57 

Compressor,  Rousselet's,  346 ;  Davis's, 
347 ;  Beck's,  347 

Compressorium,  346 

'  Concentric  '  bundles,  710 

Conceptacles  of  Fucacece,  627  ;  of  Mar- 
chantia,  666 

Conchifera,  shell  of,  919 

Concretionary  spheroids,  1100 

Condensers,  190,  298-316 

Kellner  eye-piece  used  as,  196 ; 
Gillett's,  204,  300  ;  Hartsoeker's,  298  ; 
Bonaiinus's  compound,  298,  299; 
Powell  and  Lealand's,  301,  302,  310; 
apochromatic,  802;  Swift's,  302,  305; 
immersion,  303,  805;  Watson's,  308, 
304;  Beck's,  304,  305;  Zeiss's,  305, 
308,  309;  Baker's,  306;  Webster's, 
308;  Abbe's,  808,  309;  fittings  for, 
312-314 ;  Swift's,  for  use  with  polari- 
scope,  314 

—  total  aperture  of,  307 

—  tabular  list  of,  315 

—  achromatic,   300,  304,  305,   306,  811 ; 
Brewster  on,  299 

—  chromatic,  308,  309,  311 

—  compound,  134 

—  cone  of  light  with,  190 
Conferva,  557 

Confervacece,  569,  570 ;  binary  division 
of,  569  ;  zoospores  of,  570 ;  resemblance 
of  Melosirece  to,  608 

Conferva*,  945,  960 

Conical  epithelium,  1044 

Conids,  of  Ascomycetes,  643 ;  of  Basidio- 
mycetes,  647 

Conifera,  684 ;  woody  cells  of,  697 

Coniferous  wood  fossilised,  705,  1083 

Conjugates,  affinities  of,  549 

Conjugate  foci,  13  ;  focus,  24  ;  image,  24 

Conjugating  cells,  540 

Conjugation,  a  sexual  act,  537 

Conjugation  of  Mesocarpus,  549 ;  of 
Spirogyra,  549;  of  Ulothrix,  557;  of 
Hydrodictyon,  565 ;  of  Desmidiacece, 
584  ;  of  diatoms,  599  ;  of  Phceosporece, 
627  ;  of  Myxomycetes,  634  ;  of  Arcella, 
746 ;  (zygosis)  of  Gregarina,  751 ;  of 
Heteromita,  760  ;  of  Tetramitus,  761 ; 
of  Noctiluca,  769;  of  Glenodinium, 
770;  of  Podophrya,  785;  of  Ciliata, 
782  ;  of  VorticeU'a,  782 


COS 

Connective  tissue,  1019 ;  fibrous,  1038  - 
corpuscles  of,  1039, 1040 ;  areolar,  1040- 
Contact  metamorphism,  1077 
Continental  correctional  collar,  359 

—  microscopes,  objections  to,  162 

—  model,  254-261 ;  criticism  on,  259 
Continuity  of  protoplasm,  538  ;  in  Flori- 

dea?,  630 
Contractile  vacuole  in  Volvox,  552 

-    vesicle,    of    Actinophrys,    737 ;     of 

Microgromia,   737;   of  Amoeba,   743; 

of    Infusoria,    function    of,    754 ;    in 

Flagellata,  764  ;  of  Paramecium,  776  ; 

of  Ciliata,   776;   of  Stentor,   777;   of 

Botifera,  789 
Convergence  of  light,  18 
Convergent  light  in  petrology,  1070, 1078- 
Conversion  of  relief  in  spectroscope,  92 
Convolvulacece,  laticiferous  tissue  of,  695 
Convolvulus,  pollen-grains,  721 
Copepoda,  960 ;  classification  of,  965  note 
Copeus  cerberus,  791 
Copper  sulphate,  crystallisation  of,  1096 
Coquilla-nut,  725 

—  section  of,  692 

Coralline  crag,  microscopic  constituents 

of,  1089 
Corallines,  960 

—  conceptacles  of,  632  ;  ostiole  of,  632 

—  (sertularids),  870 

Corals,  section  of   hard  and  soft  parts,. 
510 

—  red,  877  ;  stony,  878  ;  mushroom,  878 
Corella  parallelogram  ma,  branchial  sac 

of,  912 

Coreopsis  tinctoria,  seeds  of,  724 
Cork,  708 

Corky  layer  of  bark,  708 
Cormophytic  type,  668 
Cormorant,  parasite  of,  1010 
Corneules  of  arthropod  eyes,  983 
Corn-grains,  husk  of,  725 
Cornuspira,  801,  803 
'  Corpuscle  '  of  gymnospenns,  685 
Corpuscles,  white,  1037;  change  of  form 

of,    1038;  of   connective   tissue,    1039,. 

1041 ;  of  blood,  flow  of,  1056 
Corrected  lenses,  382 
Correction  collar,  21,  30,  50,  274  ;  English,. 

857 ;  Continental,  359 
Corroded  crystals,  1071 
Corrosive  sublimate,  as  a  fixative,  484 
Corynactis  Allmanni,  thread-cell  of,  879" 
Coscinodiscece,  characters  of,  608 
Coscinodiscus,  588,  620 

—  cyclosis  in,  587  ;  frustules  of,  589,  590  ; 
markings  on  frustule  of,   591 ;  areolae 
of,  591 

—  asteromphalus,  591 ;  for  testing  lenses,. 
889 

—  oculus  iridis,  609 

—  punctatus,     fossil,     with     embryonal 
form,  598 

Cosmarium,   division   of,   582;  form    of 
cell,  585 

—  botrytis,  zygospore  of,  584 
!    Cosmic  dust,  1093 


INDEX 


COS 

Costae  of  Campylodiscus,  607 
Cotyledons,  685 
Cover-glass,  439 

—  consequence  of  using,   19 ;  as  section 
lifter,  478 

—  tester,  440;  Zeiss's,  440;  Ross's,  440; 
Smith's  (J.  Ciceri),  441 

—  varying    thicknesses    of,     439 ;     with 
achromatic    objectives,  489;    cleaning 
them,  442 

Cox  (J.  D.)f  on  structure  of  frustule  in 
Istlimia,  590  note 

Crab,  957 ;  metamorphosis,  969 ;  blood- 
corpuscles  of,  1038 ;  '  liver '  of,  1047 

Crabro,  leg  of,  974 

Crane-fly.     See  Tipula 

Craterium  pyriforme,  1009 

Crayfish,  957 ;  young  of,  969 

Creation  of  structure  by  diaphragms,  68 

( '  ribrillna  figularis,  906 

Cricket,  gizzard  of,  993 ;  wings  of,  999 ; 
sound-producing  apparatus,  999.  See 
Acheta 

Crinoidea,  skeleton  of,  892;  larva  of, 
898 

Cn'xia,  909 

Crisp  (F.I,  on  'aperture,'  44;  on  radia- 
tion, 57 ;  on  collection  of  microscopes, 
117 

Cristatella,  909 

Cristellaria,  shell  of,  798,  819 

Critical  angle,  6,  7;  image,  30,  299; 
images,  287;  mode  of  obtaining,  409, 
410 

Crocus,  pollen-grains  of,  722 

Crouch's  adapter  for  parabolic  speculum, 
333 

'  Crow  silk,'  569 

Crown  glass,  refractive  index  of,  5 ;  com- 
position of,  32 

Grusta  petrosa  of  teeth,  1025,  1026 

CBUSTACEA,  957-971 

—  larvae  of,  collecting,  529 
CKUSTACEA,  suctorial,  965 

—  collecting,  970  ;  preserving,  971 ;  com- 
pound eyes  of,  982;  pigment-cells  of, 
1043  ;  '  liver  '  of,  1047  ;  concretionary, 
spheroids  in  shells  of,  1100 

CKYPTOGAMIA,  530-683 

—  preparation  of,  514 ;  structure  of,  532- 
535  ;  reproduction  of,  535-549 ;  litera- 
ture, 683 ;  passage  to  PHANEBOGAMIA,    \ 
684  and  note 

Cryptoraphidece,  599 

Crystalline  forms,  list  of,  for  microscope, 

1099 
Crystallisation,  microscopic  examination 

of,  1095-1098 

—  effect  of  temperature  on,  1096 

—  preservation  of  specimens  of,  1098 
Crystallisation,  process  of,  1096 
Crystallites,  1072,  1096 

—  in  glass  cavities,  1074 
Crystalloids,  1096 

Crystals,  corroded,  1071 ;  in  lava,  1071 ; 
zonal  markings  in,  1073 ;  cavities  in, 
1073  ;  inclusions  in,  1074,  1075  ;  micro- 


CYP 

scopical  structure  of,  1075  ;  optical  pro- 
perties and  chemical  constitution, 
1078,  1079;  as  microscopic  objects, 
1094 ;  of  snow,  1095 ;  as  objects  for 
polariscope,  1097 

Crystals,  their  homogeneous  structure, 
1094 

—  types  of  structure,  1094 

—  optical  properties,  1094 

—  variations  in  symmetry,  1094 
Ctenaria  ctenophora,  877  note 
Ctenoid  scales,  1028 

Ctenophora,    877,   881,    883;    excretory 

pores  of,  882  note 
Ctenostomata,  characters  of,  909 
Cucurbitacece,  pollen-grains  of,  721 
Cuff's  micrometer,  142  ;  microscope,  142 
Culicidce,  antennae  of,  988 ;  larvae,  blood 

of,  994 
Curculio,  antennae  of,  988 

—  imperialis,  scales  of,  975 ;  elytra  of, 
981 

Curculionidce,  981 ;  foot  of,  1000 ;  suekers. 
on  foot  of,  1002 

Currant,  parenchyme  of  fruit,  685 ;  pollen- 
tubes  of,  723 

Curvature  of  the  field,  388 

'  Cushion- star,'  891.     See  Goniaster 

Cuticle,  1041,  1042 

—  of  leaves,  713  ;  of  Ciliata,  773 
Cutin,  713 

Cutis  vera,  1041 

Cutleria,  conjugation  of,  627 

Cuttle-fish,  929,  942.     See  Sepia 

—  '  sepiostaire '  of,  structure,  929  ;  imi- 
tated, 1102 

'  Cuttle-fish  bone,'  structure  of,  929 

Cyancea  capillata,  ephyrae  of,  874,  875 ; 
scyphistoma  of,  875  ;  strobila,  875 

Cyanthus  minor,  seed  of,  724 

Cyatholiths,  748,  749;  artificially  pro- 
duced, 1101 

Cycadece,  684 

Cycas,  raphides  of,  696 

Cyclammina  cancellata,  816,  818 

Cyclical  mode  of  growth  in  shell  of 
Foraminifera,  798 

Cycloclypeus,  829  ;  shell  of,  798 

—  compared  with  Orbitolites,  801,  835 
Cycloid  scales,  1028 

Cyclops,  eye  of,  960  ;  larva,  968 

—  quadricornis,   961 ;    number   of    off- 
spring of,  964 

Cyclosis,  534 ;  in  Chara,  576 ;  in  des- 
mids,  581 ;  in  Diatomacece,  587 ;  in 
Phanerogam  cells,  688;  in  plant  hairs, 
690 ;  in  Lieberkuehnia,  732 ;  in  Acine- 
tina,  783 

Cyclostomata  (Polyzoa),  characters  of, 
909 

Cydippe,  collecting,  529 

—  pileus,  882 
Cymbella,  602 
Cymbellece,  affinities  of,  616 
Cynipidce,  ovipositor  of,  1003 
Cyprcsa,  shell  of,  928 
Cypris,  960 


1148 


INDEX 


CYS 

Cyst,  of  Protococcus,  544,  551 ;  of  Proto- 
myxa,  728 ;  of  Clathrulina,  742 ;  of 
gregarines,  751 ;  of  Dallingeria,  759  ; 
of  Polytoma,  760 

Cystic  Entozoa,  relation  to  cestoids,  944 
Cysticercus,  relation  to  cestoids,  944 
Cystids  of  Hymcnomycetes,  648 
Cystocarp  of  Floridece,  632 ;  of  Batra- 

chospermum,  574 
Cystopus  Candidas,  640 
Cythere,  960,  961 

Cytherina,  shells  of,  in  chalk,  1087 
Cytodes,  contrasted  with  plastid,  727 
Cytoplasm,  537 


Dallinger  and  Drysdale's  moist  stage, 
341 ;  tripod,  402 ;  on  life-history  of 
monads,  756-768 ;  on  effects  of  tempe- 
rature on  monads,  761 

Dallinger  (W.  H.),  on  Navicula,  &c.,  as 
test  objects,  600  note ;  on  nucleus  of 

•  monads,  762 

Dallinger's  thermo-static  stage,  344-346 
Dallingeria  Drysdali,   life-history  and 

structure  of,  758  ;  nucleus  of,  762 
Dalyell  (J.  G.),  on  Hydra  tuba,  874 
Damceus  geniculatus,  proventriculus  of, 

1011 
Dammar,  as  a  preservative  medium,  518  ; 

as  a  mounting  medium,  521 
Dandelion,   laticiferous    tissue   of,   695 ; 

-  pollen-grains  of,  721 

Daphnia,   eye  of,   960 ;    eggs    of,   964 ; 

ephippial  eggs  of,  964 
Daphnia  pulex,$§% 
Darwin  (Charleston  Cirripedia,QQ7 
Datura,  seeds  of,  724 
Davis,  on   desiccation   of   Motif  era,  791 

note 
Dawson  (W.),  on  foraminiferal  nature  of 

Eozoon,  837 
'  Day-fly.'     See  Ephemera. 

*  Dead-man's  toes,'  879.     SeeAlcyonium 
Dean's   medium   for   mounting   insects, 

973 
De   Bary,  on   fungi,    &c.,   634  note ;   on 

potato-disease,  640 ;  on  alternation  of 

generations  in  ferns,  680 
Decalcification,    512  ;     of    echinoderms, 

512 ;  of  bones,  512  ;    of   teeth,  512  ;   of 

Foraminifera,  513  ;  of  Eozoon,  513 
Decapoda,    957 ;    exoskeleton    of,    968 ; 

macrurous,  969  ;  brachyurous,  969 
Decomposition,  produced  by  Bacteria, 

661 

—  of  rock-masses,  1076 
Defining  power,  425  ;  tests  for,  426 
Definition  of  image,  882 
Degeneration  in  Tunicata,  911 
Dehydration,  487 
Dellebarre's  microscope,  144 
Delphinium,  seeds  of,  724 
Demodex,  legs  of,  1010 

—  folliculorum,  1014 


DIA 

De  Monconys,  his  compound  microscope, 

128 
Dendritina,  a  varietal  form  of  PeneropHs, 

803 

Dendrodus,  teeth  of,  1091 
Dendrosoma,  784 
Dentine,  1019,  1023-1026 

—  resemblance   of   cuticle   of   crabs   to, 
969  ;  in  placoid  scales,  1028 

Deparia,  indusium  of,  675 

—  prolifera,  676 

Depth  of  focus,  83,  89  ;  of  vision,  88,  89, 

90  ;  perception  of,  94,  95 
Dermal  skeleton  of  Vertebrata,  1026 
Dermaleichi,  1008, 1014 
Dermanyssus,  1012 

—  larva  of,  1009 
Dermestes,  hair  of  larva,  980 
Descartes'  simple  microscope  with  reflec- 
tion, 126 

Desiccation  of  rotifers,  791 

Desiderata  in  a  microscope,  261-263 

Desilicification,  513 

DESMTDIACE^E,  549,  579-587 ;  connection 
with  Pediastrece,  566  ;  sutural  line  X)f, 
580 ;  cellulose  envelope,  580  ;  mucila- 
ginous sheath,  580  ;  primordial  utricle, 
580  ;  endochrorre,  580  ;  movements  of, 
580  ;  cyclosis  in,  581 ;  binary  division  of, 
582  ;  sexual  reproduction,  584  ;  classi- 
fication of,  585 ;  habitat  of,  586 ;  mode 
of  collecting,  586 

—  Hantzsch's  glycerin  method  of  pre- 
serving, 520 

Desmidiece,  945 

—  conjugation  of,  584 ;  zygospore  of,  584 
Desmidium,  binary  division,   582 ;    fila- 
ments of,  583 

Desmids.     See  Desmidiacece 

Deutovium  of  Acarina,  1008 

Deutzia  scabra,  stellate  hairs  of,  714  ; 
epiderm  of,  715 

Development  of  Hydra,  866,  867  ;  of  hy- 
droids,  868  ;  of  embryo  in  Gastropoda, 
919;  of  molluscs,  933;  of  Annelida, 
949 ;  of  Tomopteris,  953 ;  of  insects, 
1007 

Deviation,  9 

'  Diamond  Beetle,'  975 

Dianthus,  seed  of,  723 

—  caryophyllceus,  parenchyme  of,  688 
Diaphragm,  261,  297,  306,  308,  310,  812, 

313,  314 

—  with  two  openings  for  double  illumina- 
tion, 104 

—  .Zeiss's  iris,  297;  calotte,  297  ;  in  eye- 
pieces, 376-379,  381 ;   for  use  in  test- 
ing object-glasses,  385,  386 

—  in  Tully's  microscope,  149 
Diatoma,  588 ;  frustules  of,  588,  605 

—  viilgare,  chains  of,  605 
DIATOMACE.E,  549,  587-625 

—  perforated   membrane    of,    examined 
with  annular  illumination,  419;  mode 
of  examination  of,  419  ;  mounting,  481 
silicious  coat,  refractive  index  of,  521 
stipes  of,  588  ;  beaded  appearance,  592 


INDEX 


1149 


DIA 


DYT 


markings  of,  593  ;  binary  division 
of,  594-597  ;  reproduction  of,  594-601 ; 
placochromatic,  598 ;  coccochromatic, 
598  ;  conjugation  of,  599  ;  zygospores 
of,  599  ;  gonids  of,  599  ;  movements  of, 
601 ;  classification  of,  602  ;  habits  of, 
619  ;  habitats  of,  620  ;  distribution  of, 
621 ;  fossil  forms  of,  622 ;  used  as 
food,  622 ;  collecting,  622 ;  cleaning, 
623,  624  note  ;  mounting,  624  ;  as  food 
of  Ciliata,  775  ;  in  mud  of  Levant, 
1085 

Diatom-frustules  in  ooze,  1086 

Diatomin,  587 

Diatoms  in  stomach  of  ascidians,  Holo- 
thurice,  &c.,  614,  623 

Diatoms.   See  DIATOMACE.& 

Dichroism,  1098.    See  PLEOCHBOISM 

Dickiea,  602 

Dicotyledonous  stems,  fossilised,  1083 

DICOTYLEDONS,  700 ;  stem  of  medullary 
rays  of,  702  ;  epiderm  of,  712 

Dictyocalyx  pumiceus,  861 

Dictyochya  fibula,  620 

Dictyocysfal)  silicious  shell  of,  773 

Dictyoloma  peruviana,  winged     seed, 
724 

Dictyospyris  clathrus,  847 

Dictyota,  ob'spheres  of,  627 

Didemnians,  914 

Didymium  serpula,  plasmode  of,  635 

Differential  screw,   Campbell's  fine  ad- 
justment, 162,  164,  165,  174,  202,  230 

Differential  staining,  493 

Differentiation  of  cell,  533 

Difflugia,  746;  test  of,  746 

.Diffraction,  62 

—  Abbe's  theory   of,    and  homogeneous 
immersion,  363 

—  Fraunhofer's  law,  57 

—  rays  are  image-forming,  59 

—  spectra,  28,  67  ;    phenomena,  62,   64 ; 
image,  64,  72  ;  experiments,  66-70  ;  fan 
of  isolated  corpuscles,  72  ;  problem,  73 ; 
pencil,  74,  75 ;  hypothesis  of  Abbe,  74  ; 
fan,  75  ;  theory,  application  of,  76,  78  ; 
bands,  277  ;  phenomena,  Abbe's  experi- 
ments, 434  ;  ghost,  435 

Digestive  vesicles  of  Ciliata,  776 

Digitalis,  seeds  of,  724 

Dimorphism  in  Foraminifera,  802 

Dinobryon,  765 

Dinoflagellata,  770 

Diiiomastigophora,  770  note 

Dioptric   investigations   by  Gauss,  106- 

110 

Dioptrical  image,  30,  72 
Diorite,  fluid  inclusions  in,  1074 
Dipping  tubes,  350 
Diptera,  973  ;  eyes  of,  987  ;  antennae  of, 

988  ;  mouth-parts  of,   990  ;    wings  of, 

998  ;  ovipositor  of,  1003  ;  imaginal  discs 

of,  1007 

Direct  division  of  nucleus,  538 
'  Directive  vesicles '  of  egg  of  Purpura, 

937 
Disc-holder,  Beck's,  339 


Discida,  849 

Discoliths,  748,  749  ;  artificially  produced, 

1101 
Discorbina,  824 

—  globularis,  798 
Disintegration  of  rock-masses,  1076 
Dispersion,  9,  17  ;  in  glass,  31 

—  and  desiccation  of  encysted  Ciliatcu 
781 

Dispersive  power,  2,  9,  18  ;  of  flint  glass. 

10 
Dissecting  apparatus,  455 

—  microscope,    Greenough's    binocular, 
248;     Stephenson's     binocular,     248; 
Huxley's,     251 ;     Zeiss's,     251,     253 
Bausch  and  Lomb's,  252 

Distance  of  projection  of  image,  26,  27 
Distinct  vision,  26 
Distoma,  life-history  of,  946 

—  hepaticum,  945 
Divergence  of  light,  18 

Divini's  compound  microscope,  129 
Division,  binary,  of  cells,  535  ;  of  desmids, 
582 

—  artificial,  of  Actinosphcerium,  741  note 

—  of  naiads,  955 
Dobie's  line,  1049 
Dog-fish,  scales  of,  1028 

D'Orbigny,  on  plan  of  growth  of  Fora- 
minifera, 799 

Doris,  spicules  in  mantle,  928, 929  ;  nida- 
mentum  of,  934  ;  eggs  of,  942 ;  spines 
of,  imitated,  1101 

—  bilamellata,    development    of,    935- 
937 

—  pilosa,  palate  of,  931 

—  tuberculata,  palate  of,  931 

Double  illumination,  Stephenson's  me- 
thod, 105 

Doublet,  Wollaston's,  36,  153 
Dragmata,  of  sponges,  860 
Dragon-flies,  wings  of,  998 
Dragon-fly,  facets  in  eyes  of,  983 

—  See  Libellula 
Draparnaldia  glomerata,  574 
Draw-tube  of  microscope,  157 
Drebbel's     modification     of      Keplerian 

telescope,  121 

Dredge,  528 

Drepanidium  ranarum,  752 

Drone-fly.     See  Eristalis. 

Dropping-bottle,  476  ;  German,  477  ;  ex- 
pansion, 477 

Drosera,  glands  of,  714;  seeds  of,  724 

Dry-mounting,  Smith's  '  cells  '  for,  446 

Ducts  of  Phanerogams,  698 

Dudresnaya,  fertilisation  in,  632 ;  ferti- 
lising tubes,  632 

Dujardin,  on  '  sarcode,'  530  note 

—  separates    Amoeba    from    Infusoria, 
733 

Dunning's  zoophyte  trough,  348 
Duramen,  704 

Dwarf-male  of  CEdogonium,  572 
Dytiscus,  eye  of,  987 ;  antennae  of,  988 ; 

spiracle  of,  996 ;  trachea  of,  996 ;  foot 

of,  1001,  1002 


1 150 


INDEX 


EAR 

E 


EPI 


Earth-stresses,  1077 
Earwig.     See  Forficula 
Eccremocarpus  scaber,  winged  seeds  of, 

724 
Echinoderm     larvae,     collecting,      900 ; 

preparing,  900  ;  mounting,  900 

—  skeletons  in  mud  of  Levant,  1085 
ECHINODEBMATA,    larvae    of,    collecting, 

529 

—  884-903 ;    skeleton  of,  884,   891,   892, 
894;    spines    of,    885-889,  891;    pedi- 
cellarise  of,  889;   teeth   in,    890,    892; 
preparation   of    skeleton    spines,    &c., 
892  ;  internal  skeleton,  894  ;  larvae  of, 
896 

Echinoderms,  decalcification  of,  512 
Echinoidea,  skeleton  of,  884 ;  spines  of, 

885 ;    pedicellariae    of,   889 ;    larva   of, 

898  ;  direct  development  in,  900  note 
Echinometra,  spine  of,  886,  892  ;  colour 

of  spines,  888 
.Echinus,  shell  of,  885,  886;    spines   of, 

885  ;  teeth  of,  890 

—  lividus,  coloured  spines  of,  887 
Ectocarpacece,  626 

Ectocarpus  siliculosus,  conjugation  of, 
627 

Ectoderm,  726 

Ectoplasm,  535 

Ectoprocta,  909 

Ectosarc,  534 ;  in  Rhizopoda,  733 ; 
experiments  on,  743 ;  of  Ciliata,  773 

Edentata,  cement  in  teeth  of,  1026 

Edible  crab,  metamorphosis  of,  970 

Edwards  (A.  M.),  on  supposed  '  swarm- 
spores  '  of  Amoeba,  744 

Eel,  scales  of,  1027 

'  Egg  without  shell,'  concretionary  sphe- 
roids in,  1100 

Egg-capsule  of  Cyclops,  961 

Egg-sacs  of  Lerncea,  966 

Egg-shell  membrane,  1038 

Eggs  of  Sepiola,  Doris,  942  ;  of  Acarina, 
1005  ;  of  insects,  1005 

Ehrenberg,  on  eye-spot  in  Protococcus, 
543 ;  on  Volvox,  551 ;  on  structure  of 
frustules,  590;  on  rapidity  of  repro- 
duction of  Paramecium,  111 ;  on 
internal  casts  of  Foraminifera,  827 
note  ;  on  fossil  Radiolaria,  854  note 

Elceagnus,  raphides  in  pith  of,  696 ; 
peltate  scales  of,  714 

Elastic  ligament  of  bivalves,  structure  of, 
1040 

Elater,  antennae  of,  988 

Slaters  of  Marchantia,  668;  of  Equi- 
setacece,  680 

Elatine,  seeds  of,  724 

Elder,  pith  of,  687 

Ellis's  aquatic  microscope,  147 

Elm,  raphides  of,  696 

Elodea  canadensis,  cyclosis  in,  689 

Elytra  of  Coleoptera,  981,  999 

Embryo  of  Phanerogams,  723 

—  cell  of  fern,  development  of,  679 


Embryo-sac,  685 

—  of  ovule  in  Phanerogams,  534 ;    free- 
cell  formation  in,  536 

Emission    of    light,    power   of,    51,    54 ; 

unequal,  52 

Emitted  light,  unequal  intensity  of,  51 
Empusa  muscce,  642 
Enamel  of  teeth,  1025 

—  of  teeth  of  Echinus,  891 

—  on  ganoid  scales,  1028 
Encephalartos,  raphides  of,  696 
Encrinites,  892 

End-bulbs,  1053 

Endochrome,  533;   of  Palmoglcea,  541; 

of   Spirogyra,   550 ;    of    Volvox,   551, 

552,  554  ;  of  desmids,  580 
Endoderm,  726 
Endogenous  spores  of  Mucorini,  (540 

—  stems,  700-712 
Endogens,  spiral  vessels  of,  698 
Endonema,  602 
Endophlceum,  708 
Endoplasm,  533 

Endosarc,  533;  in  Rhizopoda,  733;  of 
Ciliata,  773 

Endosperm,  685 

Endospores  of  mosses,  672  ;  in  ferns,  677  ; 
of  Volvox,  556 ;  of  Hymenoiiiycetes, 
648 

Endosporous  Bacteria,  655 

Enock's  metallic  ring  for  mounting,  482 

Entomophilous  flowers,  722 

Entomophthorece ,  642 

Entomostraca,  957,  959-965 ;  desicca- 
tion of,  963 ;  agamic  reproduction  of, 
963;  eggs  of,  964;  development  of, 
965;  eye  of,  982;  non -sexual  repro- 
duction, 1006 

—  collecting,  529 

—  Rotifera  upon,  787 
Entomostracan  eggs  as  food  of  Ciliata, 

775 

Entoprocta,  909 

Entosphcerida,  850 

Entozoa,  943 

Eolis,  nidamentum  of,  934 

Eozoo'n,  837  ;  mounting,  481 ;  mode  of 
growth  of,  compared  with  that  of 
Polytrema,  824;  canal  system  com- 
pared with  Calcarina,  825 ;  affinities 
of,  838 ;  intermediate  skeleton,  839  ; 
nummuline  layer,  839 ;  internal  cast 
of,  840 ;  asbestiform  layer,  841 ;  pseu- 
dopodia  of,  841 ;  young  of,  842 

—  canadense,  837 

—  decalcification,  513 

Epe'ira,  foot   of,    1015  ;  silk  threads  of, 

1015 
Ephemera,  branchiae  of  larva,  997 

—  marginaia,  larva  of,  973  ;  circulation 
of  blood  in  larva  of,  994 

Ephippial  eggs  of  Rotifera,  790 
Ephyrae  of  Cijancea,  875  ;  of  Chn/saom, 

876 

Epiblast,  726  note 
Epiderm  of  leaves,  712 
Epidermic  appendages,  1029 


INDEX 


II5I 


EPI 


PER 


Epidermis,  1041,  1042 ;  method  of  ex- 
amining, 1043 

Epidote,  1076 

Epilobium,  emission  of  pollen-tubes,  722 

Epipactis,  pollen-tubes  of,  723 

Epiphlceum,  708 

Epispore  of  Mucorini,  642 

Epistome  of  Polyzoa,  909;  of  Actino- 
trocha,  950 

Epistylis,  collecting,  527 

Epithelium,  1043,  1044 

Epithemia,  conjugation  of,  599;  zygo-*- 
spores  of,  599 

—  turgida,  604 
Equiconcave  lens,  22 
Equilucent  zones  of  light,  368 
Equisetacece,  680  ;  in  coal,  1084 
EquisetitmJ  spores  and  elaters  of,  681 ; 

epiderm  of,  715  ;  silex  in,  715 
Equitant  leaves  of  Iris,  &c.,  717 
Erecting  binocular,  Stephenson's,  100 

—  prism,  Stephenson's,  101 
Ergot,  644 

Erica,  seeds  of,  724 

Eristalis,  eye  of,  987  ;  antennae  of,  988 

Error  of  centring,  389 

Er/jtJiropsis  agilis,  eye-spot  of,  775 

Eschara,  calcareous  polyzoaries  of,  909  ; 

extension  of  perivisceral  cavity,  927 
Ether  as  a  solvent,  517 
Ether-freezing  microtome,  Hayes's,  472  ; 

Cathcart's,  474 

Ethmosphcera  siphonophora,  850,  851 
Eucalyptra  vulgaris,  669 
Eucopepoda,  965  note 
Eucyrtidium  elegans,  847,  852 

—  Mongolfieri,  847 

—  tubulus,  847 

Eudorina,  sexual  process  of,  557 

Euglena,  545,  765 

Euglypha  alveolata,  reproduction  of, 
746 

Euler's  microscope,  148 

Euler  on  achromatic  microscopes,  147 

Eunotia,  604 

Eunotiece,  characters  of,  604 

Euphorbiacece,  laticiferous  tissue  of,  695 

Euphrasia,  micropyle  of,  723 

Euplectella  aspergillum,  860  note 

Eupodiscecs,  characters  of,  612 

Eurotiiim  repens,  643 

Evening  primrose,  emission  of  pollen- 
tubes,  722 

'  Exclamation  markings  '  on  scales,  978 

Excretory  organ  of  Rotifera,  789, 790 

Exner  (».),  on  the  image  in  eye  of 
Lampyris,  984 

Exogenous  stems,  700 

—  stem,  structure  of,  708 

—  and  endogenous  stems  contrasted,  709, 
710 

Exogens,  nbro-vascular  bundles,  697, 
698  ;  medullary  sheath  of,  698  ;  spiral 
vessels  in,  698 

Exoskeleton  of  decapods,  968 

Exospores  of  mosses,  672  ;  of  ferns,  677  ; 
of  Hymenomycetes,  648 


Extinction,  straight,  1079 

angle,  measurement  of,  1079 

Extine  of   pollen-grains,  720;  markings 

on,  720 
Eye,  accommodation  of,  88 

—  of  Pecten,  940;  of  Onchidium,  941; 
of  slug,  941 ;  of  snail,  941 ;  of  arthro- 
pod, structure  of,  983 

Eye-glass  of  compound  microscope,  36 
39 

Eye-lens,  376 

Eye-piece,  375-381 ;  Abbe's  compensa- 
tion, 40,  378 ;  Huyghenian,  40  ;  Kell- 
ner's,  42,  376;  Eamsden's,  43,  378; 
Campani's,  376  ;  Huyghens',  376  ;  Nel- 
son's new  Huyghenian,  377 ;  Watson's 
Holoscopic,  379 

—  binocular,  Tolles',  101 ;  Abbe's,  102 

—  Kellner's,  as  condenser,  196 

—  micrometer,  271-277,  380 ;  orthoscopic, 
376 ;  projection,  380,  381 ;  index,  381 ; 
pointer  in,  381 ;  diaphragms  in,  381 

—  stereoscopic,  Abbe's,  102 
Eye-pieces,  classification  of,  by  Abbe,  34  ; 

compensating,  34,  35,  378 ;    negative, 
376,  377;    positive,   377;    solid,    378; 
searcher,  working,  projection,  378 
Eyes  on  Chiton  shells,  941 

—  compound,  of  insects,  982,  983 

—  compound,  982-987  ;  simple,  982,  JW6  ; 
preparing,  986  ;  mounting,  986 


F 


Faber,  inventor  of  the  name  microscope 

124,  125 

Falciform  young  of  Goccidia,  752 
False  images,  419 

Farrants's  medium,  478,  520  ;  for  mount- 
ing insects,  973 
Farre  (A.),  on  structure  of  Polyzoa,  908 

note 

Farrella,  polyzoaries  of,  909 
Fat,  1045 

Fat-cells,  1018,  1040,  1042,  1045 ;  capil- 
lary network  around,  1062 
Fats,  solvents  for,  517 
Feathers,  1029,  1032 
'  Feather-star,'  900.     See  Autedon 
Feeding,  mode  of,  in  Actinophrys,  7^«s ; 

in  sponges,  856 
Feet  of  insects,   1000-1002;    of   spiders, 

1014 

Felspar,  decomposition  of,  1076,  1077 
Felspar  rock,  effect   of    dynamic   meta- 

morphism  on,  1077 
Felspars,  zonal  structure  in,  1073 
'  Female  '  plants  of  Polytrichum,  671 
Fermentation  of   alcohol  by  yeast,  646; 

by  Penicillium,  Mucor,  &c.,  647 
—  putrefactive,  661 
Fermentative  action  of  Fungi,  532 
Ferns  (see  Filices),  674  ;  in  coal,  1084 
Fertilisation  of  Phanerogams,  722 
Fertilisation-tubes  of  Peronosporece,  63.S 
Fertilising  tube  of  Dudresnaya,  632 


1152 


INDEX 


FES 


FOK 


Festuca  pratenns,  paleoe  of,  715 
Fibres   and   cells   of   Vertebrates,   1018, 

1019 

Fibro-cartilage,  1019,  1046 
Fibro-vascular  bundles,  697,  708,  710 

—  of  ferns,  074  ;  in  the  '  veins  '  of  leaves, 
697  ;  of  Exogens,  697,  698  ;  of  Phane- 
rogams, 700 

Fibrous  tissues  of  Vertebrates,  1019 

—  tissue,  1038  ;  white,  1039, 1040  ;  yellow, 
1040 

Field  of  eye-pieces,  879 

Field-glass,  40 

Field-lens,  376;  applied  to  eye-lens  by 
de  Moncoiiys,  128,  376  ;  by  Hooke,  128, 
376 

FILICES,  674-680;  stem,  structure  of, 
674  ;  fructification  of,  675  ;  prothallium 
of,  677  ;  antherids  of,  677  ;  archegones 
of,  677  ;  development  of,  679 ;  apospory 
in,  680 ;  apogamy  in,  680 ;  alternation 
of  generations  in,  680 

'  Filiferous  capsules.'     See  Thread-cells 

Finder,  295  ;  Maltwood's,  296 

Fine  adjustment,  162-175 

—  applied  to  the  stage  by  Powell, 
155  ;  by  moving  the  whole  body,  162  ; 
by  simply  moving  the  nose-piece,  162, 
173  ;  continental,  162-164 ;  Campbell's 
differential  screw,  164;  Zeiss's,  166; 
Eeichert's,  171 ;  Watson's  lever,  172 ; 
Swift's  vertical  side-lever,  173 ; 
Powell's,  174 

Fire-fly,  antennae  of,  987 

'  Fire-fly,'  984,  988.     See  Lampyris 

Fish,  circulation  in  tail  of,  1057 ;  on 
yolk-sac,  1057 

'  Fish-louse,'  966 

Fish-scales,  concretions  in,  1101 

Fishes,  lacunae  in  bone  of,  1022  ;  dentine 
of,  1023 ;  cement  of  teeth  in,  1026 ; 
plates  in  skin  of,  1026;  red  blood- 
corpuscles  of,  1034,  1035;  pigment- 
cells  of,  1043  ;  muscle  fibre  of,  1049 ; 
gills  of,  1063 

Fission  in  LieberkueJinia,  733;  of 
Monas,  756;  of  Monosiga,  764;  of 
Codosiga,  764 ;  of  planarians,  947 

Fissipennes,  wings  of,  999 

Fixation,  484-487 

Fixing  agents :  alcohol,  484 ;  corrosive 
sublimate,  484 ;  osmic  acid,  485 ;  picric 
acid,  485 

Flabella  of  Licmophora,  605 

Flagella,  532 ;  of  Bacteria,  652,  658,  659 

Flagellata,  755-771 

—  experiments  on,  761 ;  nucleus  in,  762 ; 
karyokinesis  in,  763 ;    colonial   forms, 
764 

—  collared,  resembling  cells  of  sponges, 
855 

Flagellate    chambers    of    sponges,   856, 

857 

Flagellum  of  Noctiluca,  766  note 
Flat  bottle  for  collecting,  527 
Flatness  of  field,  425 
Flea,  presumed  auditory  organ  of,  422 ; 


hairs  on  pygidium  of,  as  a  test,  421 

mounting  medium  for,  973 
'  Flesh,'  1048 
Flint,  derivation  of,  622 

—  glass,  refractive  index  of,  5  ;   disper- 
sive power  of,  10  ;  composition  of,  32 

—  implements   found   with    Orbitolince, 
824 

Flints,  preparation  of,  1089 

Floral  envelope,  718 

Floridece,  630-632  ;  affinities  of,  630 

Flosculariadcs,  791 

Floscules  in  confinement,  528 

'  Flowering  fern,'  sporanges  of,  676 

'  Flowering  plants,'  684.  See  PHANERO- 
GAMIA 

Flowers,  718-723 ;  Inman's  method  of 
preparation,  719 

'  Flowers  of  tan,'  634 

Fluid  inclusions  in  crystals,  1074 

1  Fluke,'  945 

Fluorite  lenses  for  apochromatic  objec- 
tive, 85,  366 

Flustra,  mode  of  growth  in,  904 ;  gem- 
mation in,  906 ;  number  of  polypides, 
908  ;  polyzoaries  of,  909  ;  extensions  of 
perivisceral  cavity  in,  927 

'Flustrella  concentrica,  847 

Fly,  various  instructive  organs  to  be  ob- 
tained from,  972 ;  eye  of,  facets  in, 
983;  proboscis  of,  989;  circulation  in 
wing  of,  994  ;  spiracle  of,  996  ;  areolse 
on  wings  of,  998  ;  foot  of,  1000 

Focal  alteration  and  form  of  objects,  421 

—  depth,  38 

—  distances,  by  feeling,  177 

—  length  of  a  plano-convex  lens,  15 
Focke  on  Navicula  and  Surirella,  602 

note 

Focus,  virtual  conjugate,  14,  25  ;  princi- 
pal, 16  ;  mean,  17 ;  virtual,  22 ;  conju- 
gate, 24  ;  depth  of,  83,  89 

—  of  lenses,  13,  21,  22  ;  chromatic,  16 
Focussing  arrangements,  159-175 
Fontinalis  antipyretica,  671 

Food  of  Hydra,  685 

Foraminifera,  733,  795-846 

—  study  of,  by  means  of  Beck's  disc- 
holder,  339;  examination  of,  423; 
wooden  slides  for  mounting,  450; 
method  for  sectionismg,  508  note ;  de- 
calcification  of,  513  ;  structure  of,  795  ; 
chamberlets  in,  798,  803,  804,  806  ; 
cyclical  mode  of  growth  in,  798  ;  plans 
of  growth,  798,  804 ;  porcellanous 
shells,  799 ;  vitreous  shells,  799 ;  tubu- 
lation  of  shell  in,  799,  800;  rotaline 
type,  800 ;  nummuline  type,  800 ;  in- 
termediate skeleton  of,  801 ;  canal  sys- 
tem of,  801 ;  Porcellanea,  801 ;  fos- 
silised forms  of,  801,  804,  812,  824,  837  ; 
dimorphism  in,  802 ;  secondary  septa 
in,  803 ;  Arenacea,  810 ;  sandy  iso- 
morphs,  814 ;  nodosarine  type,  815  ; 
Vitrea,  819;  internal  casts,  82:5,  827 
note ;  nummuline  series,  826 ;  alar 
prolongations,  830,  831 ;  interseptal 


INDEX 


1153 


FOR 


GEL 


canals,  830  ;  marginal  cord  in,  830,  834  ; 
collecting,  843 ;  method  of  separating 
from  sand  .&c.,  844 ;  mounting,  845 ; 
tubuli  of,  compared  with  those  of  den- 
tine, 1020 ;  in  mud  of  Levant,  1085 ; 
in  rocks,  1085  ;  internal  casts  of,  1090 

Forbes,  on  reproduction  of  Sertulariida, 
870 

Forceps,  351 

—  slide,  453 

—  stage,  339  * 
Forficida,  antennae  of,. 988 
Forficulidce,  wings  of,  999 

Form   of   objects  and  focal   alteration, 

421 

Formation  of  microscopic  images,  43 
'Formed    material,'    1018;     of    fibrous 

tissue,  1019  ;  of  dentine,  1020 
Fossil  coniferous  wood,  705,  1083 

—  crinoids,  892  ;  echinids,  892 

—  Cypridce,  960 

—  Foraminifera,  801,  824-826 

—  Lituolce,  816 

—  Badiolaria,  846,  854  note 

—  Saccammina,  812 

—  sponges,  1089 

—  wood,  705,  706 

Fossilised  Foraminifera  (Eozob'n),  837 

—  wood,  sections  of,  712 
Fragilaria,  605 

Fragilariece,  characters  of,  605 
Fragmentation  of  nucleus,  538 
Fraunhofer's  law  of  diffraction,  57 
— •  achromatic  doublet,  148 

-  lines,  323-326 
Fredericella,  collecting,  528 
Free-cell  formation,  535,  719 

in  embryo-sac,  534,  536 

Freezing  apparatus  for  Thoma's  (Jung's) 
microtome,  467 

—  microtome,  Hayes's,  472  ;  Cathcart's, 
474 

—  imbedding  by,  505 

Fresnel,  on  Selligue  and  Adams's  micro- 
scope, 148  ;  on  range  of  magnification, 
149 

Freyana  heteropus,  legs  of.  1010 

Fripp's  method  of  testing  object-glasses, 
386 

Frog,  blood- corpuscles  of,  1034,  1035; 
muscle  fibre  of,  1049 ;  papillae  on  tongue 
of,  1053 ;  circulation  in  mesentery  of, 
1056  ;  circulation  in  tongue  of,  1056  ; 
lung  of,  1063 

Frog's  bladder,  histology  of,  as  seen  with 
apochromatic,  372 

—  foot,  epithelium  of  web  of,  1044  ;  cir- 
culation in  web  of,  1055 

Frond  of  Phceospcrecz,  626 
Fructification,  gonidial,  541 ;  sexual,  541 

—  of  thallophytes,  540  ;  of  Ascomycetes, 
642 ;  of  lichens,  649 ;  of  mosses,  670 ; 
of  ferns,  675 ;  of  Equisetacece,  680 

Frustules  of  Diatomacece,  588:  shapes 
of,  588,  589;  structure  of,  589,  590 
note ;  girdle,  589 ;  ostioles  in,  590  ; 
markings  on,  591 ;  character  of,  as  basis 


of  classification,  602 ;  of  Coscinodiscusy 

609 

FucacecB,  627 ;  conceptacles  of,  627 
Fuchsia,  pollen-grains  of,  722 
Fucus,  626 
Fucus  platycarpus,  627,  628 

—  vesiculosus,  629 
Fulgoridfe,  wings  of,  999 
Funaria  hygrometrica,  669 

—  sporange  of,  671 
FUNGI,  540,  633-664 

—  preparation  of,  514  ;  zymotic  action  of, 
532 ;     alternation    of     generations    in 
classification  of,  634;  parasitic  on  in- 
sects, 642 

Fungia,  lamellae  of,  878 

Fungiform  papillae,  1053 

Fungus-cellulose,  633 

Fusion  in  Dallingeria,  759 

Fuss's  description  of  a  microscope,  147 

Fusulina,  825,  826,  1090 

Fusulina -limestone,  825,  1085 


Gabbro,  1095 

—  fluid  inclusions  in,  1074 
Gad-fly,  ovipositor  of,  1004 
— •  See  Tabanus 
Gaillonella  procera,  621 

—  granulata,  621 

—  biseriata,  621 

Galileo,  inventor  of  the  compound  micro- 
scope, 120-125 ;  Viviani's  life  of,  120 ; 
his  invention  of  compound  microscope, 
Wodderborn  on,  121 ;  his  occhialino, 
121,  124;  his  occhiale,  122,  124;  his 
microscope,  127 

'  Gall-flies,'  ovipositor  of,  1003 

Galley-worms.     See  Myriopoda 

Gamasidd,  legs  of,  1010;  integument  of, 
1010 ;  Malpighian  vessel  of,  1011 ; 
heart  of,  1011 ;  tracheae  of,  1011 ;  cha- 
racters of,  1012;  reproductive  organs 
of,  1012 

Gamusus  terribilis,  mandibles  of,  1009 

Ganglion-globules  (cells),  1051 

Ganglionic  cells,  1054 

Ganoid  scales,  1028 

Garlic,  raphides  of,  696 

Garnets,  1077 

Gas  bubbles  in  glass  cavities,  1074 

Gaseous  inclusions  in  crystals,  1075 

Gastropoda,  palates  of,  mounting,  481 ; 
palate  of,  919 ;  development  of,  919 ; 
shell  structure  of,  928 ;  embryonic 
development  of,  934-940;  organs  of 
hearing  in,  941 

Gastrula,  726;  -stage  in  Coelenterata, 
726;  formation  of,  726  note ;  of  zoo- 
phytes, 862  ;  of  Gastropoda,  935  ;  of 
blowfly,  1007 

Gauss's  optical  investigations,  106-110; 
his  dioptric  investigations,  106-110  ;  his 
system,  practical  example  of,  111-1 J  6 

Gelatinous  nerve-fibres,  1052 

in  sympathetic,  1054 

4    E 


1 1 54 


INDEX 


GEM 

Gemellaria,  polyzoary  of,  909 

Gemmae  of  Marchantia,  666,  667;  of 
Salpingceca,  764 ;  of  Suctoria,  784  ;  in 
Foraminifera,  798 ;  of  Polyzoa,  906 

Gemmation  and  shape  of  shell  in  Fora-    \ 
mini/era,  796 

Gemmules  of  Noctiluca,  769  ;  of  sponges, 
857 

Gentiana,  seeds  of,  724 

Geodia,  spicules  of,  859,  1086 

Gephyrean  worm,  950 

Geranium,  glandular  hairs  of,  714  ;  cells 
of  pollen-chambers,  720  ;  pollen-grains, 
720 

Germ-cells  of  Volvox,  555  ;  of  Marchan- 
tia, 668  ;  of  mosses,  671 ;  of  ferns,  679 ; 
of  Phanerogams,  685  ;  of  sponges,  857  ; 
of  Hydra,  866 

'Germinal  matter,'  1018;  of  fibrous  tis- 
sue, 1019  ;  of  dentine,  1020 

Gesneria,  seeds  of,  724 

Ghostly  diffraction  image,  Nelson  on,  72 
note 

Gibbes  (Heneage  ,  on  staining  Bacteria, 
515 

Gifford's  screen,  321 

Gill  (C.  Haughton),  on  the  'dots'  of 
Navicula,  593 

Gillett's  condenser,  204,  300 

Gills  of  tadpole,  1057,  1059 

Giraudia,  conjugation  of,  627 

Girvanella,  1084 

'  Gizzard '  of  insects,  993 

Glanders,  661 

Glands,  structure  of,  1047 

—  of  Drosera,  714 

Glass-cavities  in  crystals,  1074 ;  gas 
bubbles  in,  1074 

'  Glass-crabs,'  968 

Glass  inclusions  in  crystals,  1074 

—  rings  for  cells,  446-448 
Glaucium  luteum,  cyclosis  in,  691 
Glenodinium    cinctum,   conjugation  of, 

770 
Globigerina,   shell  of,   798;   mud,   811; 

pseudopodia  of,  821 ;  mode   of  life  of, 

821 ;  Wyville  Thomson's  views  on,  821 ; 

Carpenter's  views  on,  822 
Globigerina  bulloides,  820 ;  in  the '  ooze,' 

1086 

—  conglobata,  821 

—  ooze,  820,  1085  ;  resemblance  to  chalk, 
1087 

—  rubra,  colour  of,  799 

Globigerine    shell,    sandy  isomorph    of, 

814 

Globigerinida,  820 
Globule  of  Chara,  577,  578 
Globulites,  1096 
Glochidia  of  Anodon,  933 
Glceocdpsa,  547  ;  as  gonid  of  lichen,  651 
Glow-worm,  984  ;  antennee  of.  988 
Glue  and  honey  cement,  444 
Gluten  of  grass  seeds,  725 
•Glycerin,   as  preservative   medium,  518, 

520;  Hantzsch's  method,  520;  Beale's 

method,  520 


GEE 

Glycerin-jelly,  Lawrence's  mounting  in, 
480,  519  ;  solvent  for  CaCO5,  520  ;  for 
mounting  insects,  973 ;  for  mounting 
cartilage,  1047 

Glyciphagus  Krameri,  1013 

—  'valmifer,  1008 

—  platygaster,  1013 

—  plumiger,  1008  ;  hairs  of,  1010 
Gnathostomata  (Crustacean),  965  note 
Goadby's  solution  for  mounting  cartilage, 

1047 

Goes  (Dr.\  on  affinity  of  Carpenteria,  823 

Goette,  on  development  of  Antedon,  903 

Gold  size,  443 

Gomphonema,  stipe  of,  588,  616 ;  move- 
ments of,  602;  attacked  by  Vampyrella, 
730 

—  geminatum,  616 ;  stipe  of,  616 

—  gracile,  621 

Gomphonemece,  characters  of,  616 
Goniaster  equestris,  spines  of,  891 
Gonid ial  cells,  541 

—  fructification,  541 

—  layer  of  lichens,  649 
Gonidiophores  of  Peronosphorece,  639 
Gonids,  or  non-sexual  spores  of  Crypto- 
gams,  541   note ;  of  Vaucheria,   562 ; 
of  Podosphenia,  597 ;  of  Floride<e,6Sl ; 
of  Fungi,  633 ;  of  Peronosporece,  639 

Goniocidaris  florigera,  spine  of,  888 
Gonium,  545 

Gonothecse  of  Campanulariida,  870 
Gonozoid  of  hydroids,  868 ;  of  Syncoryne, 

869 ;  of  Tubularia,  869 
Gonozoids  of  Sertulariida,  870 
Gordius,  944,  945 
Gorgonia,  spicules  in,  929 

—  guttata,  spicules  of,  880 
Gorgonics,   877  ;  spicules  of,  in  mud  of 

Levant,  1085 
Goring  (Dr.),  on  magnification  of  objects, 

43 
'  Gory  dew,'   due  to  Palmella  cruenta, 

558 
Govi,    on    invention    of    microscope   by 

Galileo,  120 

Graduated  rotary  stage,  395 
Grammatophora,  chains  of,  588,  607 

—  angulosa,  620 

—  marina,  607 
Grammatophora parallela,  620 

—  serpentina,  607 

—  subtilissima,  607 
Granite,  1095 

—  fluid  inclusions  in,  1074 
Grantia,  857,  861 ;  spicule  of,  1086 
Grasses,  nodes  of,  701 ;  silex  in  epiderm 

of,  715  ;  palese  of,  715  ;  seed  of,  725 
Grasshopper,  gizzard  of,  993 ;  wings  of, 

999 

Green  glass  for  softening  light,  321,  417 
Greensands,  microscopic  constituents  of, 

1090 

Gregarina,   characters   of,    749 ;    move- 
ment of,  750 

—  gigantea,  in  lobster,  749  note 

—  Scenuridis,  751 


INDEX 


H55 


GEE 


Gregarinida,  749 

Gregory  (J.  W.),  on  Eozoon,  843  note 

Gregory  (W.),  on  species  of  diatoms,  600 

note 
Greville,    on   Spatangidium,     610;    on 

Triceratiioii,  613  note 
Grey  matter,  1052 
Griffith's  turn-table,  451 
Griffitlisia,  630 
Grinding   sections   of   hard   substances, 

506  , 

Grindl's  microscope,  132 
Gromia,  734-736,  796 

—  and   Arcella,   pseudopodia    of,    con- 
trasted, 746 

Ground-mass  of  rocks,  1072 
Groundsel,  pollen-grains  of,  722 
Growing    slides,   Botterill's,    340 ;  Mad- 

dox's,  341 ;  Lewis's,  341 
Guard-cells,  715 
'Gulf-weed,'  630 
Gum  and  glycerin,  520  ;  and  syrup,  as  a 

preservative  medium,  519 

—  imbedding  for  vegetable  substances, 
514 

—  arabic,  formula,  445  ;  for  freezing,  505 

—  resins,  latex  of,  695 

—  styrax,  as  a  mounting  medium,  521  ; 
index  of  refraction,  521 

Gyges,  545 
Gymnochroa,  868 
Gymnolcemata,  909 
Gymnosperms,  fossilised,  1084 
—   generative   apparatus   in,    compared 

with  Cryptogams,  684 
Gypsina,  824 


H 


Haddon,  on  budding  in  Polyzoa,  907  note 
Haeckel    (E.),  on  Eadiolaria,   846  ;  on 

Hydrozob'n  affinity  of  Ctenophora,  877 

note 
-    and    Hertwig,   on    classification    of 

radiolarians,  849  note 
HcsmamoebidfB,  752  and  notr 
Hcematococcus,    red    phase    of     Proto- 

coccus,  543 
—  sanguineus,  558 
Haematoxylin,  solutions,  491,  492 
Hcemionitis,  sori  of,  675 
HamoeporicKa,  749 
Haime   (Jules),  on  development  of  Tri- 

choda,  780 
'  Hair-moss,'  671 
:  Hair-worm,'  944 
Hairs  of  leaves,  714 ;  of  insects,  980 ;  of 

Acarina,  1010 ;  of  mammals,  1029 
Halicaridce,  1013 
Haliomma  Sumboldtii,  851 

-  hystrix,  848 
Haliotis  (diatom),  613 

-  (mollusc),   shell    structure    of,   928; 
palate  of,  931 

Haliphysema,  814 ;   sponge-spicules  in, 


Haller,  on  auditory  organs  of  Acarina, 

1010 

Halteres  of  Diptera,  1000 
Hand-magnifier,  Brewster's,  37 
Hansgirg,    on    movement    of    Oscillato- 

rlacece,  548 
Hantzsch's  glycerin  method  for  desmids, 

520 
Haplophragmium,  814 

—  globigeriniforme,  813 

Hardening  agents,  484-487;  corrosive 
sublimate,  484 ;  alcohol,  484 ;  osmic 
acid,  485  ;  picric  acid,  485 

Hardy's  flat  bottle  for  collecting,  527 

Harpalus,  antennse  of,  988 

Harting,  on  Janssen's  microscope,  120 ; 
his  experiments  on  formation  of  con- 
cretions, 1101 

Hartnack,  on  immersion  system,  27 

Hartnack's  model,  256 

Hartsoeker's  simple  microscope,  134;  his 
condenser,  134,  298 

'  Hart's-tongue,'  675.  See  Scolopen- 
drium 

'  Harvest-bug,'  1013 

'  Haus  '  of  Appendicularia,  918 

Haustellate  mouth,  992 

Haustellium,  992 

Haversian  canals  in  bone,  1021 

Haycraft  (J.  B.),  on  structure  of  striated 
muscle  fibre,  1049 

Hayes's  ether  freezing  microtome,  472; 
minimum  thickness  of  sections  there- 
with, 478 

Hazel,  peculiar  stem  of,  704 ;  pollen- 
grains  of,  722 

Hearing,  organs  of,  in  Gastropoda,  941 ; 
in  Cephalopoda,  941 

Heart  of  ascidians,  912  ;  of  Acarina,  1011 

Heartsease,  pollen-tubes  of,  723 

'  Heart-wood,'  704 

Heating-bath,  Mayer's,  453 

Heliopelta,  588,  611 

Heliozoa,  characters  of,  734 ;  examples 
of,  737-742 

Helix pomatia,  teeth  of,  930 

—  hortensis,  palate  of,  930 
Heller's  porcelain  cement,  521 
Helmholtz  on  aperture,  47 
Hemiaster  cavernosus,  development  of, 

900  note 
Hemiptera,  eyes  of,  987 ;  wings  of,  999  ; 

suctorial  mouth  of,  1000 
Hensen's  stripe,  1049 
Hepaticce,   665;    thalloid,   668;    foliose, 

668 ;  elaters  of,  compared  with  spiral 

cells,  &c.,  of  pollen-chamber,  720 
Herbivora,   arrangement   of    enamel   in 

teeth  of,  1025  ;  cement  in  teeth  of,  1026 
Herring,  scales  of,  1028 
Herschelan  doublet,  309 
Hertel's  compound  microscope,  137,  139 
Hertwig's  research  on  Microgromia,  735 

note',  on  Actinia,  877  note 
Heterocentrotus,  spine  of,  885 

—  mammillatus,  spine  of,  887 
Heterocysts  of  Nostoc,  549 

4E2 


1156 


INDEX 


HET 


HYP 


Heteromita  uncinata,  life-history  of,  760 

Heterostegina,  834 

Heurck  (Van\  on  markings  of  diatoms, 

593 

Hexarthra,  792 
Hicks,  on  amoebiform  phase  of  Volvox, 

556 ;  on  preparation  of  insect  antennae, 

989  note ;  on  structure  of  halteres  and 

elytra,  1000 
Himantidium,  604 
Hipparchia  janira,  eggs  of,  1005 
Hippopus,  613 
Hippothoa,  909 
Holland's  triplet,  37 
Hollis's  liquid  glue,  444 
Hollyhock,  pollen-grains  of,  721,  722 
Holothuria  botellus,  plates  of,  895 

—  edulis,  plates  of,  895 

—  inhabilis,  plates  of,  895 

—  vagabunda,  plates  of,  895 
Holothurice,  diatoms  in  stomach  of,  614, 

623 

Holothurioidea,  skeleton  of,  894 ;  pharyn- 
geal  skeleton  of,  895  note ;  plates  in    j 
skin  of,  895 ;  preparation  of  calcareous 
plates,  896 ;  abbreviated  development 
in,  900  note 

Holtenia  Carpenteri,  861 

Homeocladia,  602 

Homogeneous,  word  first  applied  to 
lenses,  30 

—  immersion,  364  ;  Abbe's  combination, 
365 

—  immersion  lenses  of  Powell  and  Lea- 
land,  30 ;  of  Zeiss,  29 

—  objectives,  value  of,  in  study  of  monads, 
762 

—  system,  28 

Homoptera,  wings  of,  998,  999 
Hood  of  mosses,  671 

Hoofs,  1029,  1033 

—  sections  of,  mounting,  481 ;  for  polari- 
scope,  481 

Hooke's  adoption  of  field-lens  to  eye- 
lens,  128,  376 

—  compound  microscope,  128 
Hooked  monad,  760 

Hooker  (J.  D.),  on  diatoms  of  antarctic 

circle,  621 

Hooklets  on  wings  of  Hymenoptera,  999 
Hoplothora,  1012 

—  maxillae  of,  1010 

Hormogones  of  Oscillatoriacece,  547;  of 
Bivulariacece,  548 ;  of  Scytonemacece, 
548 ;  of  Nostoc,  549 

Hormosina  globulifera,  813.  815 

—  Carpenteri,  815 
Hornblende,  1077 

—  corroded  cr j  stals  of,  1072  ;  pleochroism 
in,  1078 

Hornet,  wing  of,  999 ;  sting  of,  1003 

Horns,  1029,  1033 

Horny   substances,   chemical   treatment 

of,  517 

'  Horse-tails,'  680.     See  Equisetacece 
Hosts  of  parasitic  plants,  532 
House-fly.     Sea  Musca 


Hudson,  on  the  functions  of  contractile 

vesicle  of  rotifers,  789  note 
Hudson   and  Gosse,  on  classification  of 

rotifers,  790 
Human  blood-corpuscles,  1034 

—  hair,  1031 

Husk  of  corn-grains,  725 

Huxley,  on  the  ectosarc  of  Amoeba,  743 
note ;  on  coccoliths,  747 ;  on  Bathybius, 
747  ;  on  Collozoa,  853  note ;  on  struc- 
ture of  molluscan  shells,  922 ;  on  pul- 
villus  of  cockroach,  1000  note ;  on  agamic 
reproduction  of  Aphis,  1006 

Huxley's  simple  dissecting  microscope, 
251,  252 

Huyghenian  eye-piece  and  spherical 
aberration,  42 

Hyacinth,  raphides  of,  696;  cells  of 
pollen-chambers,  720 ;  pollen-grains  of, 
722 

Hyaline  shells  of  Foraminifera,  799 

Hyalinia  cellaria,  palate  of,  931 

Hyalodiscus  subtilis,  608 

Hyaloplasm,  537 

Hydra,  collecting,  527 ;  intracellular 
digestion  in,  863 ;  thread-cells  of,  864  ; 
structure  of,  864 ;  reproduction  of,  866 ; 
gemmation  of,  866 

—  fusca,  863,  865 

—  viridis,  863,  867 

—  vulgaris,  863 

'  Hydra  tuba '  of  Chrysaora,  874,  876 
Hydrachnidce,  1008  ;' mandible  of,  1009; 

eyes    of,    1011;     reproductive   organs 

of,  1012 ;  characters  of,  1013 
Hydrangea,  number  of  stomates  in,  716 ; 

seeds  of,  724 
Hydrodictyon,  557,  566 

—  reticulatum,  565 
Hydroida,  classification  of,  868 
Hydroids,  compound,  867  ;  structure  of, 

867  et  seq. ;  Medusae  of,  868  ;  planulae 
of,  868,  871;  habitats  of,  871;  ex- 
amination  of,  871 ;  mounting,  871 ; 
polariscope  with,  872 ;  preservation  of, 
872 

Hydrophilns,  antennae  of,  987,  988 

Hydrozoa,  863-877 

Hydrozoa  and  marine  mites,  1013 

Hyla,  nerves  of,  1054 

Hymenium  of  Ascomycetes,  642 ;  of  Basi- 
diomycetes,  647;  of  Hymenomycetes, 
648 

Hymenomycetes,  647 ;  pileus  of,  648 ; 
stipe  of,  648 

Hymenoptera,  973;  eyes  of,  987;  mouth- 
parts  of,  990  ;  wings  of,  998  ;  sting  of, 
1002,  1003 ;  ovipositor  of,  1002,  1003 

Hyoscyamus,  spiral  cells  of  pollen- 
chambers  of,  720  ;  seeds  of,  724 

Hypericum,  seeds  of,  724 

Hyphae  ot  fungi,  633 

Hypnospore  of  Hydrodictyon,  565 

Hypnospores,  meaning  of,  541  note 

Hypoblast,  726  note 

Hypopial  stage  of  Tyroglyphidce,  1013 

Hypopus,  1013 


INDEX 


1157 


ICE 


IRI 


'Ice-plant,'  epiderm  of,  714 
Ichneumonidte,  ovipositor  of,  1003 
Illuminating  power,  425 

—  power   of   objectives,    54;    compared 
with  penetrating  power,  393 

Illumination  for  dissection,  401 

—  for  opaque  objects,  149 

—  oblique,  190,  191,  388 

—  of  objects,  Ross  on,  300  ;  monochro- 
matic,  321-323  ;  Gifford's  screen    for? 
321 ;  Meithe's  filter  for,  322  ;  Nelson's 
apparatus  for,  323  ;  by  reflection,  329- 
338 ;  opaque,  329 ;  from  the  open  sky, 
412;    by   diffused    daylight,   412;   for 
dark  ground,  413  ;  experiments  in,  414  ; 
monochromatic,   means   of    obtaining, 
417,  418 ;   annular,    419  ;   colour,  423  ; 
double,  objects   for   study  with,   423 ; 
with  small  cones,  as  cause  of  errors  in 
interpretation,  427 

Illuminator,  oblique,  190;  white  cloud, 
194;  parabolic,  316-317;  Swift's  sub- 
stage,  319;  Smith's  vertical,  386; 
Powell  and  Lealand's,  337;  Beck's, 
337 ;  for  examination  of  metals,  337 

Image,  real,  14  note  ;  virtual,  14  note,  876 ; 
conjugate,  24 ;  inverted  conjugate,  24 ; 
absorption  or  dioptrical,  64  ;  diffrac- 
tion, 64;  negative,  64;  positive,  64; 
solid,  95 ;  real  object,  375 ;  definition 
of,  382  ;  formed  by  compound  eye,  984, 
985 

Images,  by  diffraction,  dioptric  and 
interference,  72 

Imaginal  discs  in  larva  of  blowfly,  1007 

Imbedding  processes,  495-506  ;  paper 
trays  for,  497 ;  in  paraffin,  metal  case 
for,  498  ;  orienting  bottle  for,  499 

paraffin  method,  409-503  ;  in  gum, 

475,  505,  506 ;  celloidin  method,  503- 
505 

—  by  coagulation  or  freezing,  505,  506 
Immersion  lenses  and  vertical  illumina- 
tors, 387,  338 

homogeneous,  outcome  of  Abbe's 

theory  of  diffraction,  364 

water,  Zeiss's,  370 

Amici's,  362 ;  Powell  and  Lea- 

land's,  362,  364 ;  Prazmowski  and  Hart- 

nack's,362;  Tolles',  362 

—  objectives,  28  ;  examination  of,  387 

—  system,   27-29 ;    invented   by   Amici, 
27 

Imperfect  achromatism,  cause  of  yellow- 
ness, 417 

*  Impressionable  organs '  in  Ciliata,  775 
Incidence,  angle  of,  3 
Incident  ray,  2 
Incus  of  Rotifera,  788 
Index  eye-piece,  381 

—  of  visibility,  521 

Indian  corn,  epiderm  of,  712  ;  stomates 

of,  715 

Indirect  division  of  nucleus,  538 
Indusium  in  ferns,  675 


Inflection  of  diverging  rays,  62 
Infusoria,  754-785  ;  as  food  of  Actino- 

phrys,  739  ;  Ehrenberg's  work  on,  758  ; 

ciliate,  754,  772  ;  unicellular  nature  of, 

755  note  ;  character  of,  772 
Infusorial  earth,  607,  608,  611,  613,  617, 

620-622  ;  from  Barbadoes,  846,  849 
Injected  preparations,  1061 
Inoceramus,  portions  of  shell  of,  in  chalk, 

1087 
INSECTS,  972-1007 

—  mounting  media  for,  973  ;  integument 
of,  974;   tegumentary  appendages  of, 
974  ;  scales  of,  975-980  ;  hairs  of,  980 ; 
parts    of   head,   982 ;    eyes,    982-987 ; 
antennae  of,  987  ;  mouth-parts  of,  989  ; 
circulation  of  blood,  993 ;  alimentary 
canal,  993;  wings  of,   994,   998-1000; 
trachea    of,    994;    stigmata  of,   995; 
sound-producing  apparatus,  999 ;  organ 
of  smell,  1000;  organ  of  taste,  1000; 
feet    of,   1000-1002 ;    stings  of,   1002, 
1003 ;  ovipositors  of,  1002,  1003 ;  eggs 
of,  1004  ;  agamic  reproduction  of,  1006 ; 
embryonic     development     of,     1007; 
'  liver '  of,  1047 

—  parasitic  fungi  in,  642,  645 

—  parts  of,  wooden  slides  for  mounting, 
450 

Insect  work,  dark-ground  illumination 
for,  423 ;  polarised  light  for,  423  . 

Integument  of  insects,  974 ;  of  Acarina, 
1010 

Integuments  of  ovule,  685 

Intensity  of  light,  necessaries  for,  417 

Intercellular  substance,  1019;  in  carti- 
lage, 1046 

Intercostal  points,  Stephenson  on,  73; 
not  revelation  of  real  structure,  73 

Interference,  62 

—  image,  72 

Intermediate  skeleton  in  Foraminifera, 
801 ;  of  Globigerinida,  820 ;  of  Calca- 
rina,  825 ;  otttotalia,  825 ;  of  Nummu- 
lites.  826 ;  of  Eozoon,  839 

Internal  casts  of  Textularia,  823;  of 
Rotalia,  824;  of  Eozoon,  840  ;  of  wood, 
1083  ;  of  shells  in  greensand,  1090 

Interpretation,  errors  of,  427 

'  Interseptal  canals '  of  Calcarina,  830 

Intestine,  cells  of  villi  in,  1044 

Intine  of  pollen-grains,  720 

Intracellular  digestion  in  zoophytes,  863 

Intussusception,  533 

—  mode  of  growth  of  starch,  694 
Invagination,  726 

Invertebrata,  blood  corpuscles  of,  1038 

Inverted  conjugate  image,  24 

Iodine,  as  a  test  for  starch,  &c.,  516 

Ipomcea purpurea,  pollen-grains  of,  721 

Iridescent  scales  of  insects,  975 

Iris,  epiderm  of,  712 ;  leaves  of,  717 ;  cells 

of  pollen-chambers,  720 
Iris-diaphragm,  297, 313  ;  fitted  to  Abbe  s 

condenser,  312 
Iris  germanica,  epiderm  and  stomates  ot, 

715,  716 


INDEX 


IKR 

Irrationality  of  spectrum,  19,  305 

Isochelae  of  sponges,  860 

Isoetece,  682 

Isotropism,  1079 

Isthmia,  chains  and  frustules  of,  588,  612  ; 
structure  of  frustules,  590  note ;  divi- 
sion of,  596 

—  nervosa,  613 

—  areolations  in,  592 
Italian  reed,  stem  of,  699 
'Itch-mites,'  1013 
Ivory,  1024 

Ixodes,  heart  of,  1011 

Ixodidce,    1008;    integument    of,    1010; 

auditory  organ,  1011 ;  tracheae  of,  1011 ; 

characters  of,  1012 


Jackson's  modification  of  Ross  model, 
199  ;  his  eye-piece  micrometer,  276 

Janssen  (H.  and  J.),  inventors  of  first 
microscope,  120 ;  their  compound 
microscope,  120 

Jars,  capped,  for  Canada  balsam,  477 

Jelly-fish.     See  Acalephce  and  Medusce 

Jones's  compound  microscope,  144,  145 

Jungermannia,  668 

Jung's  (Thoma's)  microtome,  461-469 


K 

Kaolin,  1076 

Karop,  his  fine  adjustment  to  sub-stage, 
187 

Karop  and  Nelson  on  fine  structure  of 
diatoms,  591  note 

Karyokinesis  in  monads,  763 

Kellner's  eye-piece,  42,  376 ;  as  a  con- 
denser, 196 

Kent  (Saville),  on  contractile  vacuoles  of 
Volvox,  552  note  ;  on  Flagellata,  764 

Keplerian  telescope,  Drebbel's  modifica- 
tion as  a  microscope,  121 

Keramosphtera  Murrayi,  810  note 

Keratose  network  of  sponges,  855 ;  pre- 
paration of,  857 

Kidneys  of  Vertebrata,  1047 

King-crab,  957 

Kirchner,  on  the  obspores  of  Volvox, 
556 

Klebahn,  on  formation  of  auxospores  of 
diatoms,  601 

Klebs,  on  mucilaginous  sheath  of  des- 
mids,  580 ;  on  movement  of  desmids, 
580 

—  and  Biitschli,  on  the  '  cilia '  of  Dino- 
flagellata,  770 

Klein,  on  Volvox,  556  note 

Knife,  special,  for  microtome,  462 

Koch's  method  of  sectionising  corals, 
878 

Kowalevsky,  on  development  of  ascidians, 
917  note 

Krukenberg,on  digestion  in  sea-anemones, 


LE& 

Kiitzing,  on  Palmodictyon,  559 ;  on  struc- 
ture of  frustules  of  diatoms,  590 ;  his 
classification  of  diatoms,  (>(>:'> 


Labarraque's  fluid  for  bleaching  vege- 
table substance,  514 

Labels,  permanent,  523 

Labyrinthic  structure  of  CyclamtniiHi, 
816;  olFarkeria,  818 

LabyrintJiodon,  tooth  of,  1091 

Lacunae  and  canaliculi  of  bone,  misinter- 
pretation of,  428 

—  of  bone,  1019-1022  ;  dimensions  of,  in 
various  animals,  1022 

—  relation  of  size  to  that  of  blood  cor- 
puscle, 1022 

Lagena,  796,  819 
Lagenida,  819 
Laguncula,  906,  908,  950 

—  stolon  of,  904  ;  polypides  of,  compared 
with  Clavellinidce,  914 

—  repens,  anatomy  of,  904,  905 
'  Lamella.'  of  corals,  878 

—  of  Hymenomycetes,  648 
Lamellibranchiata,  shell  of,  919 
Lamellicornia,  antennae  of,  988 
Laminaria,  626,  627 
Laminariacece,  627 

Lamna,  tooth  of,  1024 

Lamp,     Nelson's.     404;     Beck's      406; 

Baker's,  407 
Lampyris,  antennae  of,  988 

—  splendidula,  photograph  through  eye 
of,  984 

Land-crab,  young  of,  969 

Lankester  (E.  Ray),  on  Bacteria,  652 ;  on 
movement  of  gregarines,  750 ;  on 
Hcemamcebidce,  752  note ;  on  iiitra- 
cellular  digestion  in  Limnocodium, 
868 

Lantern-flies,  wings  of,  999 

Lapis  lazuli,  1095 

Larva  of  Echinodermata,  896 ;  of  As- 
teroidea,  898  ;  of  Echinoidea,  898 ;  of 
Ophiurvidea,  898  ;  of  Crinoidea,  900 ; 
of  ascidians,  916 ;  of  fly,  1007 ;  of 
Acarina,  1009 

Latex  of  Phanerogams,  695 

Lathrcea  squamaria,  embryo  of,  728 

Laticiferous  tubes,  free-cell  formation  in, 
534 

—  tissue  of  Phanerogams,  695 
Laurentian  rocks,  837,  842 

'  Laver,'  or  green  seaweed,  559 

Lawrence's  glycerin  jelly,  519 

Leaves,  epiderm  of,  712 ;  internal  struc- 
ture of,  716 ;  mode  of  preparation  for 
examination  of,  718 

Leech,  956 

Leeuwenhoek's  simple  microscope,  132 

Legg's  method  of  selecting  Foraminifera, 
844 

Legs  of  insects,  1000,  1002 ;  of  Acarina, 
1008,  1010 


INDEX 


LEG 

Leguminosce,  seeds  of,  685 

Leiosoma palmacinctum,  1008  ;  hairs  of, 

1010 
Leitz's  microscopes,  206,  227 

—  bull's  eye,  330 

—  objectives,  374 

Lens,  spherical,  12 ;  biconvex,  12,  13 ; 
plano-concave,  13;  diverging  meniscus, 
13 ;  plano-convex,  13,  15,  22,  37 ;  con- 
verging meniscus,  13  ;  biconcave,  13  ; 
plano-convex,  focal  length  of,  15 ; 
crossed  biconcave,  16;  crossed  bicon- 
vex, 16  ;  equiconvex,  16,  22  ;  Stanhope, 
37  ;  Coddington,  37  ;  Briicke,  38 

—  from  Sargon's  palace,  119 

—  invention  of,  119,  120 

—  achromatic,  Charles's,  148 ;  Barlow's, 
149 

Lenses,  refraction  by,  10,  25 

—  homogeneous  immersion,  of    Powell 
and  Lealand,  30 ;  of  Zeiss,  29 

—  fluorite ;  for  apochromatic  objectives, 
35 

—  combination  of,  37 

—  resolving  power  of,  64,  382  ;  amplify- 
ing power  of,  25,  26 

—  testing  by  Diatoms,  389 
Lepadidce,  967 
Lepidium,  seeds  of,  724 
Lepidocyrtus  curvicollis,  scales  of,  979 
Lepidodendra,  682,  1084 
Lepidoptera,  scales  of,  975,  976 ;  wings 

of,  981,  999  ;  scales  of,  mounting,  981, 

982 ;  eyes   of,  987 ;  antennae  of,  988 ; 

mouth-parts,  992;  eggs  of,  1005 
Lepidosteus,  bony  scale  of,  1022, 1028 
Lepidostrobi,  682 

Lepisma  saccharina,  scales  of,  976,  977 
Lepismidce,  979 
Lepralia,  909  ;  mode  of  growth  in,  904  ; 

extension    of    perivisceral    cavity    of, 

927 

Leptodiscus  (ally  of  Noctiluca),  769  note 
Leptogonium  scotinum,  649 
Leptothrix,  form  of,  653 
Leptus  autumnalis,  1013 
Lerncea,  965  note,  966 
Lessonia,  627 

Lettuce,  laticiferous  tissue,  695 
Leucite,    mineral    inclusions    in,    1075 ; 

anomalies  in,  1078 
Lever    of    contact,    Ross's,    for    testing 

covers,  440 
Libelhda,  eye  of,  983,  987;  respiratory 

apparatus    of    larva,    997 ;    wings   of, 

998 

Liber,  or  inner  bark,  708 
LICHENS,   648-651;    fungus-constituents 

of,  651 
Licmopkora,  stipe  of,  588,  604  ;  flabella 

of,  605 

—  flabellata,  588,  604 
Licmophorece,  616 

—  characters  of,  604 ;  vittae  of,  604 
Lieberkuehnia,  movement  of,  732 

—  paludosa,  733 

—  Wagneri,  731 


LOM 

Lieberkiihn's  microscope,  139  ;  his  specu- 
lum, 334-336 

'  Ligamentum  nuchae,'  structure  of,  1040 
Light;  refraction  of,  2;  recomposition 
of,  by  prisms,  18 ;  convergence  of,  18 ; 
path  of,  through  compound  microscope, 
40;  quantity  of,  50,  51,  54;  emission 
of,  51,  54 ;  quantity  of,  and  aperture, 
54  note  ;  cone  of,  190 ;  monochromatic, 
321,  417,  418  ;  intensity  of,  necessaries 
for,  416 

—  convergent,  in  petrology,  1070,  1078 
Lignified  tissue,  test  for,  517 
Lignites,  1083 

Lignum  vitce,  wood  of,  704 

Lilac,  pith  of,  687 

Lilium,  experiments  with  pollen-grains 

of,  721 

'  Lily-stars,'  900.     See  Crinoidea 
Limax  maximus,  palate  of,  930 

—  shell  of,  imitated,  1102 

—  rufus,  shell  structure  of,  928 
Lime,  raphides  of,  696 

—  secreting  Algae,  1084 
Limestone,  metamorphism  of,  1077 

—  rocks,  1084, 1085 

Limnceus  stagnalis1  nidamentum  of, 
934 ;  velum  of,  936 

LimnocaridcB,  characters  of,  1013 

Limnocharis,  seeds  of,  724 

Limnocodium,  intracellular  digestion  in, 
863 

Limpet.     See  Patella 

Limulus,  957 

Linaria,  seeds  of,  724 

Lister's  struts  for  support  of  body,  149  ; 
his  influence  on  improvement  of  Eng- 
lish achromatic  object-glasses,  150 ; 
his  zoophyte  trough,  348 ;  his  discovery 
of  two  aplanatic  foci,  355  ;  his  note  on 
Chevalier's  objectives,  355 ;  his  influ- 
ence on  microscopical  optics,  356  ;  his 
triple-front  combination,  360 

Listrophorus,  1008 

Lithasteriscus  radiatus,  620 

Lithistid  sponges,  spicules  of,  859' 

Lithocyclia  ocellus,  847 

Lithothamnion,  1084 

Lituola,  814 

Lituolce,  large  fossil  forms  of,  816 

Lituolida,  814 

Live-box,  346 

Liver,  1047 

Liver-cells,  1048 

'  Liverworts,'  665.     See  Hepaticce 

Lobosa,  characters  of,  734 ;  examples  of, 
742-747 

Lobster,  957;  metamorphosis  of,  969 

'  Lob-worm,'  948 

Loculi,  of  anthers,  720 

Locust,  gizzard  of,  993;  ovipositors  of, 
1004 

Locusta,  eye  of,  987 

Loftusia,S18 

Loligo,  pigment-cells  of,  942 

Lomas  (J.),  on  calcareous  spicules  in 
Alcyoniditim,  908  note 


ii6o 


INDEX 


LON 

'  London  Pride,'  parenchyme  of,  688 
Longicornia,  antennae  of,  988 
Longulites,  1096 
Lopkophore  of  Polyzoa,   905,    950  ;  of 

fresh-water  Polyzoa,  909 
Lophopus,  collecting,  528 
Lophospermum  erubescens,  winged  seed 

of,  724 

Lophyropoda,  959 
Lorica  of  Ciliata,  773  ;  of  Acineta,  783 ; 

of  Rotifera,  787 
Loup-holders,  248 

—  for  tank  work,  Steinheil's,  268 
Loups,  Reichert's,  38 ;  Steinheil's,  38, 378 ; 

Steinheil's  aplanatic,  248  ;  Zeiss's,  268 
Louse,  mounting  media  for,  973 
Loven,  on  classificatory  value  of  palates 

in  Gastropoda,  932 
Loxosoma,  lophophore  of,  909 
Lubbock,  on  Thysanura,  977 ;  on  Podura 

scale,  979 

Lucanus,  eye  of,  987  ;  antennae  of,  988 
Luminosity  of  Noctiluca,  765 ;  of  Cteno- 

phora,  883 ;  of  annelids,  955 
Lungs,  circulation  in,  1056,  1062-1065 
Lychnis,  seeds  of,  724 
Lychnocanium  falciferum,  847 

—  lucerna,  847 

Lycoperdon,  647  ;  hymenium  of,  647 

Lycopodiacece,  681 ;  in  coal,  1084 

Lycopodiece,  681 

Lyminas,  collecting,  527 

Lymph,  corpuscles,  1037 

Lysigenous  spaces  in  Phanerogams,  688    i 


Maceration   of   vegetable   tissues,   700 ; 

Schultz's  method,  700 
Machilis  polypoda,  scale  of,  978 
Machines  for  cutting  hard  sections,  511, 

512 

Macrocystis,  627 
Macrospores     of    Polytoma,     760;     of 

sponges,  857 

Macrurous  Decapoda,  young  of,  969,  970 
Madder,  cells  of  pollen- chambers,  720 
'  Madre,'  Acanthometra,  occurring  in,  852 
Madrepores,  878 
Magma,  1073 
Magnetite,  1072 
Magnification,    range    of,    of   Selligue's 

microscope,  149 
Magnifying  power,  testing  of  objectives, 

425  ;  determination  of,  288 
Mahogany,  size  of  ducts  of,  699  ;  stem  of, 

706 

Malacostraca,  968 
'  Male '  plants  of  Polytrichum,  671 
Mallei  of  Rotifera,  788 
Mallow,  pollen -grains  of,  721,  722 
Malpighian  vessel  of  Gamasidce,  1011 

—  layer  of  skin  in  mammals,  1042 

—  bodies  in  vertebrate  kidney,  1047 
Maltwood's  finder,  296 

Malva  sylvestris,  pollen-grains  of,  721 


MEC 

Malvacece,  pollen-grains  of,  721 

MAMMALIA  :  lacunae  in  bones  of,  1022 ; 
plates  in  skin  of,  1026  ;  epidermic  ap- 
pendages of,  1029 ;  red  blood-corpuscles 
of,  1034,  1035;  epidermis  of,  1042; 
muscle  fibre  of,  1049  ;  lungs  of,  1065 

Mammary  glands,  1047 

Man,  arrangement  of  enamel  in  teeth  of, 
1025  ;  cement  in  teeth  of,  1026  ;  hair  of, 
1031 ;  muscle  fibre  of,  1049 ;  lung  of, 
1065 

Mandibulate  mouth,  989 

Manganese  concretions,  1090 

'  Mantle  '  and  growth  of  shell  in  Mollusca, 
925 

Marchantia,  665-668 ;  archegones  of,  665, 
668 ;  stomates  of,  666 ;  elaters  of,  668 

—  androgyna,  665  note 

—  polymorpha,  665-668 
Margaritacece,  919 ;  nacreous  layer  of, 

922  ;  prismatic  layer  of,  923 
Margarites,  1096 
'  Marginal  cord '  of  Operculina,  830 

—  of  Nummulites,  834 
Marine  forms,  collecting,  528 

—  glue  for  forming  '  cells,'  445 

—  mites,  1018 

—  work,  tow-net  for,  528 ;  dredge  for,  528 ; 
stick-net  for,  529 

Marshall's  compound  microscope,  135, 
136,  138,  139 

Marsipella  elongata,  818 

Martin's  '  pocket  '  reflecting  microscope, 
140;  his  large  microscope,  140;  his 
improvements  in  optical  and  mechani- 
cal arrangements,  142  ;  his  achromatic 
microscope,  147 ;  his  reflecting  micro- 
scope, 147;  his  achromatic  objective,  147 

Marzoli's  achromatic  lenses,  353 

Masonella,  811 

Mastax  of  Eotifera,  787 

Mastigophora  Hyndmanni,  906 

Mastogloia,  stipe  of,  588,  619  ;  gelatinous 
sheath  of,  588,  619 ;  development  of, 
597  ;  range  of  variation  in,  618 

—  lanceolata,  619 

—  Smithii,  619 

Matthews's  method  of  sectionising  hard 

substances,  507 
May  all,  on   history  of  microscope,  117  ; 

on  Divini's  microscope,  130 
Mayall's  removable  mechanical  stage,  183 
Mayer's  heating  bath,  453 
'  Meadow-brown,'  eggs  of,  1005 
'  Measly  pork,'  due  to  Cysticerciis,  944 
'  Mechanical  finger'     for     selecting   di- 
atoms, 625 

—  movements  of  the  stage   in   Lister's 
(Tully's)  microscope,  149 

—  stage,  175 

-  Turrell's,  176 ;  Watson's,  177  ; 
Nelson's,  179,  181;  Zeiss's,  179,  183; 
Swift's,  180  ;  Allen's,  180  ;  Mayall's  re- 
movable, 183 ;  Reichert's,  183  ;  Bausch 
and  Lomb's,  183,  184  ;  Beck's,  184 
Continental,  179 

—  tube-length  of  microscope,  158 


INDEX 


MED 

Medullary  rays,  705 

—  in  dicotyledons,  702 
'  Medullary  sheath  '  of  Exogens,  698  ;  of 

dicotyledons,  703 
Medusa  of  fresh-water,  863 
Medusa  t  mounting,   448 ;    of  Hydroids, 

868 ;    naked-eyed,    868 ;    development 

of,  874  ;    alternation  of  generations  in, 

877  ;  nerves  of,  1052 
Medusoids,  collecting,  529 
Megalopa,  970 

Megaloscleres,  859  f 

Megasphere    of    certain   Foraminiferat 

802 
Megaspores    of    Rhizocarpece,    681 ;    of 

carboniferous   trees,  682;    of  Isoetece, 

682  ;  of  Selaginellece,  682 
Megatherium,  teeth  of,  1026 
Megatricha  of  Ehrenberg,  a  phase  in 

development  of  Suctoria,  785  ;  Badcock 

on,  785 
Megazoospores  of  Ulothrix,  557;  of  Ulva, 

561 ;  of  Scenedesmus,  566 
Megerlia  lima,  shell  of,  927 
Melanosporece,  625 
Meleagrina,  919,  922 

—  margaritifera,  923 

Melicerta,  collecting,  527  ;  in  confine- 
ment, 528 

Melicertidce,  791 

Melolontha,  eye  of,  987 ;  antennae  of, 
988 ;  spiracle  of  larva,  996 

—  vulgaris,  eye  of,  983 

Melosira,  frustules  of,  588,  594 ;  auxo- 
spores  of,  595,  600  ;  sporules  of,  597  ; 
zygospore  of,  600 

—  ochracea,  608 

—  subflexilis,  594,  595 

—  varians,   594,   595 ;     endochrome   of, 
598 

Melosirece,  characters  of,  608;  resem- 
blance to  Confervacece,  608 

Membrana  putaminis,  1032 

Membranipora,  908,  909 

Membraniporidte,  908 

Mercury  nitrate  as  a  test  for  albuminous 
substances,  517 

Meridiece,  604,  616 

—  characters  of,  604 
Meridian  circular e,  588,  604 
Merismopedia,  547 

'  Mermaid's   fingers,'   879.      See   Alcyo- 

nium 
Mesembryanthemujn,  seeds  of,  724 

—  crystallinum,  epiderm  of,  714 
Mesocarpus,  conjugation  of,  549;  zygo- 
spore of,  550 

Mesogloea  of  Hydra,  &c.,  864  note 
Mesophlceum,  708 
Metal  case  for  imbedding,  498 
Metamorphism,  dynamic,  1077 
Metamorphism    of     rock-masses,     1076, 

1077  ;  of  limestones,  1090 
Metamorphosis    of     Lerncea,    966 ;     of 

Cirripedia,    967 ;    of     Malacostraca, 

969 
Metazoa,  727,  855 


MIC 

Meteorites  in  oceanic  sediments,  1093 

Metschnikoff,  on  acinetan  character  of 
Erythropsis,  775  ;  on  intracellular  di- 
gestion, 863  ;  on  phagocytes,  1037  note 

Mica,  1077 

Michael's  (A.)  opalescent  mirror,  194 

Micrasterias  denticulata,  binary  divi- 
sion of,  583  ;  form  of  cell  of,  585 

Micro-chemical  analysis,  1102 

-  method  of,  1102 

Micro-chemistry  in  petrology,  1082,  1083  ; 
of  poisons,  1103 

Micrococci,  form  of,  653 

Microcysts  of  Myxomycetes,  636 

Microgromia  socialis,  735 

Microlites,  1072 ;  in  glass-cavities,  1074 

Micrometer,  Cuff's,  142 

—  use  of,  274 

—  eye-piece,  271 

—  Nelson's  new,  271, 272,  273 ;  Zeiss's, 
272;  Jackson's,  276 

Micrometers,  270-277 

Micrometry  by  photo-micrography,  277 

Micron,  a,  82  note,  460 

Micro-petrology,  1066 

'  Microplasts '  of  Bacterium  rubescens, 
660  note 

Micropyle  in  ovule,  685 ;  of  Euphrasia, 
723  ;  in  orchids,  &c.,  723 

Microscleres,  859,  860 

Microscope,  Mayall  on  the,  117  ;  history 
and  evolution  of  the,  117-269 ;"  inven- 
tion of,  120  ;  inventor  of  the  name,  124  ; 
essentials  in,  157-194  ;  adjustments  in, 
159-175 ;  stage  of,  175-184 ;  sub-stage 
of,  184-191 ;  mirror  of,  191-194  ;  desi- 
derata in,  261-263 ;  preservation  of, 
436 

—  Galileo's,  127  ;  Campani's,  128  ;  Prit- 
chard's,  with  Continental  fine  adjust- 
ment,   153;    Boss's    'Lister'    model, 
153  ;     Powell's     (H.),     155  ;      James 
Smith's,  155 

—  achromatic,  Euler  on,  147 ;    Martin's, 
147;  Chevalier's,  148,  150;  Selligue's, 
148;  Tally's,  149;    Ross's  early  form 
of,  152 

—  aquarium,  266-269 

-  binocular,   Eiddell's,    97;    Nachet's, 
98;  Wenham's  stereoscopic,  98;    Ste- 
phenson's,  100,  248,  455  ;  Greenough's, 
102,250;  Powell  and  Lealand's,  105; 
Cherubin  d' Orleans',  130  ;  Ross's,  196  ; 
Ross-Zentmayer's,    199;    Rousselet's, 
245  ;  Sorby's  spectrum,  327 

—  chemical,  Bausch  and  Lomb's,  263,  264 

—  compound,  36,  39-42,  120,   125  ;  con- 
struction of,  39 ;  path  of  light  through, 
40;  Rezzi  on  invention  of,  125 ;  Jans- 
sen's,   120 ;    Hooke's,   128 ;    de    Mon- 
cony's,  128  ;  Divini's,  129  ;  Marshall's, 
135;  Hertel's,  139;  Joblot's,  139;  Cul- 
peper   and    Scarlet's,  140;    Martin's, 
140;      Adams's    variable,     142,    148; 
Jones's,  144,  148 

-  comparison  of    English   and    Conti- 
nental models,  254-261 


LI  62 


INDEX 


MIC 

Microscope,  concentric,  191, 199 

—  dissecting,    Greenough's,     102,   250; 
Stephenson's  binocular,  248;  Baker's 
(Huxley's),  251;  Bausch  and  Lomb's 
(Barnes),  252 ;  Zeiss's,  253 

—  horizontal,  Bonannus's,  134;  Amici's, 
148 

—  petrological,  1068 

—  photographic,  257,  258 

—  radial,  191,  199 ;  Ross-Weiiham's,  199 
-^-  reflecting,  Newton's,  132 ;   Martin's, 

140,  147  ;  Smith's,  145  ^ 

—  simple,  36,  126,  248 ;    path  of    light 
through,  25;  inventor  of ,  126 ;  Bacon's, 
126;  Descartes',  126 ;  Bonannus's,  132  ; 
Muschenbroek' s,132 ;  Leeuwenhoek's, 
132  ;  Hartsoeker's,  134  ;  Wilson's,  140 

—  spectrum  binocular,  327 

—  three  great  types  of,  174,  199 
Microscopes,  for  chemical  purposes,  263, 

264 

—  for  examination  of  metals,  264-266 

—  modern,   194-269 ;    Powell   and   Lea- 
land'-s,  194,  218,  237  ;  Ross's,  196,  230 ; 
Watson's,    199,    218,    224,    234,    237; 
Baker's,  202,   218,  230;    Swift's,  203, 
224,228,233,  1068;  Leitz's,  206,  237; 
Reichert's,    206,    224,   241,   242,   264; 
Zeiss's,   206,   237,    250;    Bausch    and 
Lomb's,  212,  222,  239,  252,  263 ;  Spen- 
cer Lens  Company's,  214  ;  Beck's,  228, 
233 

—  portable,  245-247;  Powell  and  Lea- 
land's,  245;  Swift's,  245;  Rousselet's, 
245;  Baker's,  246;  Bausch  and  Lomb's, 
247 

Microscopic  and  macroscopic  vision,  62 

—  determination  of  geological  formations, 
1090 

—  dissection,  single  lenses  for,  38 

—  investigation  of  rocks,  &c.,  1066 

—  vision,  principles  of,  43 
Microscopical  optics,  principles  of,  1 
Microscopist's  work-table,  398-403 
Microscopy,  definition  of,  397 
Microsomes,  531,  537 

Micro  -  spectroscope,  Sorby  -  Browning, 
823-327;  Swift's,  325  note ;  Hilger's, 
325  note 

—  method  of  using,  328 ;   in  petrology, 
1083 

Microsphere  of  certain  Foraminifera,  802 
Microspores  of  Sphagnacece,  674;  of 
Mhizocarpece,  681  ;  in  carboniferous 
trees,  682  ;  of  Isoetece,  682 ;  of  Selagi- 
nellece,  682;  of  Polytoma,  760;  of 
sponges,  857 

Microtome,  458-475  ;  Ryder's,  401 ;  sim- 
ple, 458-460;  Thoma's  (Jung's),  461- 
469 ;  freezing  apparatus  for,  467 ;  Mi- 
not's,  472;  Strasser's,  472;  Gudden's, 
472 

—  Cambridge  rocking,  469-472  ;  advan- 
tages of,  472 

—  freezing,  Hayes's,  472  ;  minimum 
thickness  of  sections  with,  473  ;  Cath- 
cart's,  474 


MON 

Microzob'spores    of     UlotJirix,    557 ;     of 

Ulva,  561 ;  of  Hydrodictyon,  565 
'  Mildew,'  637.     See  Uredinece 
Miliola,  shell  of,   799;   encrusted  with 

sand,  810 
Miliolee,  802 

Miliolida,  801 ;  in  limestone,  1090 
Miliolina,  802 

Milioline  Foraminifera,  fossils  of,  801 
Miliolite  limestone,  1090 
Millepore,  resemblance  of  Polytrema  to, 

824 
Millon's  test  for  albuminous  substances> 

517 
Mineral  nature  of  Eozoon,  843 

—  sections,  where  to  get  made,  1067 
Minerals  and  rocks,  bibliography  of,  1071 

note 

—  optic  axes  of,  1079 

—  refractive  index  of,  1080 

—  chemical,    spectroscopic    and    micro- 
scopic testing  of,  1078-1083 

Minnow,  circulation  in  tail  of,  1057 
Mirror,  191-194 

—  opalescent,  as  a  substitute  for  polaris- 
ing prism,  194 

—  replaced  by  rectangular  prism,  192 
Mites,  1008.     See  Acarina 

Mobius,  on  mineral  nature  of  Eozoon,  843 
Mohl  (Von),  on  protoplasm,  530  note 
Moist-stage,   Dallinger  and    Drysdale's, 

341-344 

Molecular  coalescence,  1099-1102 
Molgula,  development  of,  917 
MOLLUSCA,  larvae  of,  collecting,  529 

—  shells  of,  919  ;  shell-structure  of,  919- 
925 ;  colour  of  shell,  921 ;  mantle  and 
shell-growth,  925  ;  palate  of,  930  ;  de- 
velopment  of,   983 ;    ciliation  of  gills, 
940  ;  organs  of  sense  in,   940 ;  biblio- 
graphy, 942  ;  resemblance  of  barnacles 
to,  967 ;  '  liver '  of,  1047 ;  muscle  fibre  of, 
1050  ;  internal  casts  of,  1090 ;  concre- 
tionary spheroids  in  shells  of,  1100 

Molluscan  shells  in  mud  of  Levant,  1085 
Monad-form  of  Microgromia,  737 
Monadince,    life-histories    of,    755-763 ; 
saprophytic,   affinities   of,    756 ;  effect 
of  temperature  on,  761 ;  nucleus  in,  762 
Monads,  755.     See  Monadince 
Monas,  575 

—  Dallingeri,  life-history  of,  756 

—  lens,  755 

Monaxonida,  spicules  of,  859 
Monazite,  1081 

Monconys  (De)  devises  microscope  with 

field-lens,  128 
Monerozoa,  727-733 
Monocaulus,  871 
Monochromatic  light,  321,  417,  431 

—  illumination,  means  of  obtaining,  417, 
418 

!    MONOCOTYLEDONS,   700 ;    stem   of,   700 ; 

nodes  of,  701 ;  epiderm  of,  712 
Monocotyledonous  stem,  fossilised,  1083 
Monocular,  Powell  and  Lealand's,  194, 

195 


INDEX 


I  163 


MON 


MYX 


Monocystis  agilis,  cyst  of,  750 
Monophyes,  digestion  in,  863  note 
Monosiga,  fission  of,  764 
Monothalamous  Foraminifera,  796 
Monotropa,  seeds  of,  724 
Moracece,  laticiferous  tissue  of,  695 
Mordella  beetle,  eye  of,  facets  in,  983 
Mormo,  scales  of,  980 
Morpho  Menelaus,  scales  of,  976 
Morula  of  higher  animals  compared  with 

'  multicellular  '  Protozoa,  726 
Morula  of  Gastropoda,  935  % 

Moseley  (H.  N.^,  on  skeleton  of  pharynx 

of  holothurian,  895  note ;  on  Chiton's 

eyes,  941 
Mosses,  669-674 

—  capsules  of,  wooden  slides  for  mount- 
ing, 450 

'  Mother-of-pearl,'  922 

Moths.     See  Lepidoptera 

Motion,  spiral,  433,  434 

Motor  nerves,  1053 

Motorial  end-plates,  1053 

'  Moulds,'  640,  643 

Moults  of  Entomostraca,  964,  965 

'Mountain-flour,'  622 

Mounted  objects,  keeping,  523 ;  labelling, 

523 ;  arrangement  of,  524 
Mounting  plate,  452 

—  instrument,  James  Smith's,  454 

—  thin  sections,  477 

—  in   natural  balsam,  480 ;  in   aqueous 
liquids,  481 ;  in  deep  cells,  482 

—  diatoms,   481,  624;  Ophiurida,   481; 
Polycystince,     481 ;      sponge- spicules, 
481 ;  chitinous  substances,  481 ;  palates 
of  gastropods,  481 ;  sections  of  horns, 
&c.,    481;    Lepidoptera    scales,    982; 
hairs  of  insects,  982 ;  eyes  of  insects, 
986 ;  blood,  1038 

—  media,  517-522  ;  camphor  water,  518 ; 
salt  solution,  519 ;  white  of  egg,  519 ; 
syrup,  519;  Ripart  andPetit's  fluid,  519  ; 
glucose   media,    519 ;  chloral   hydrate, 
519 ;    gum   and   syrup,   519 ;    glycerin 
jelly,   519 ;    F arrant 's    medium,    520 ; 
glycerin  and  mixtures  of,  520  ;  Canada 
balsam,    521 ;    Dammar,   521 ;    Styrax, 
521 ;  monobromide  of  naphthalin,  521 ; 
phosphorus,  521 

Mouse,  hair  of,  1030-1031;  cartilage  in 

ear  of,  1046 

Mouse's  intestine,  villi  of,  1062 
Mouth,  suctorial,  of  Hemiptera,  999 

—  of  Acarina,  1009 
Mouth-parts  of  insects,  989 
Movement,  interpretation  of,  431-434 

—  of   Lieberkuehnia,   732 ;    of  Amoeba, 
744;    of    Dallingeria,   758;    of  plana- 
rians,  946  ;  of  Artemia,  960 ;  of  JBran- 
chipus,  960  ;  of  fly  on  smooth  surface, 
1001 ;  of  white  corpuscles,  1037  ;  of  con- 
nective   tissue    corpuscles,    1041 ;     of 
OscillatoriacecE,  547  ;  of  desmids,  580  ; 
of  diatoms,  601 ;  of  Bacteria,  652 ;  of 
Ciliata,  774 

Mucilaginous  sheath  of  desmids,  580 


Mttcor,  fermentation  by,  647 

—  mucedo,  641 

Mucorini,  640  ;  spores  of,  640  ;  epispores 

of,  642 
Mucous  membrane,  1041  ;  capillaries  in, 

1062 
Mud  of  Levant,  microscopic  constituents 

of,  1085 

Mulberry,  laticiferous  tissue  of,  695 
Mulberry-mass,  726 
Miiller  (J.),  on  the  Badiolaria,  846  ;  pn 

larva  of  Nemertines,  951 
Miiller's   (Fr.)  'Common   Nervous  Sys- 

tem '  in  Polyzoa,  907  and  note 
Multicellular  organisms,  726 
Multiplication   of   Palmoglcea,   541;    of 

Protococcus,  543  ;  of  Volvox,  555  ;  of 

Palmella,   558  ;   of  Bacteria,  652  ;  of 

Microgromia,   736  ;  of  Amoeba,   744  ; 

of  Dallingeria,   758  ;  of   Heteromita, 

760;  of  Tetramitus,  760;  of  Noctiluca, 

769  ;  of  Peridinium,  770  ;  of  Suctoria, 

784  ;  of  Ciliata,  777 
Multiplying  power  of  eye  -piece,  290 
Munier  Chalmas  and   Schlumberger,  on 

dimorphism  of  Foraminifera,  802 
Munier-  Charles,  on  certain  fossil  Fora- 

minifera, 564 

Muricea  elongata,  spicules  of,  880 
Musca,  eye  of,  987  ;  antennas  of,  988 

—  vomitoria,  eggs  of,  1006 
'  Muscardine,'  645 
Musci,  670-674 
Muscinece,  673 
Muscle-cells,  1051 

Muscular  fibre,  1048  ;  structure  of,  1049  ; 

capillary  network  in,  1062 
Muscular  tissue,  preparation  of,  1050 
Mushroom,  647 

—  spawn  of,  647 
Musk-deer,  hair  of,  1030 
Musschenbroek's  simple  microscope,  132 
Mussels.     See  Unionidce  and  Mytilacece 
Mya  arenaria,  hinge  tooth  of,  924 
Mycele  of  Fungi,  633  ;  of  U  stilaginece  ,  636 
Mycetozoa,  634 

MyUobates,  tooth  of,  1025 

Myobia,  1008  ;  legs  of,  1010  ;  maxillae  of, 

1010 

Myobiidce,  1013 
Myocoptes,  legs  of,  1010 
Myophan-layer  '  of  Vorticella,  773 


Myopy,  118 
Mrio 


yriophyllum  a  good  weed  to  collect,  527 
MYBIOPODA,  hairs  of,  980 
Myriothela,  intracellular    digestion   in, 

863 

Mytilacece,  sub-nacreous  layer  in,  924 
Mytilus,  for  observation  of  ciliary  motion, 

940 

Myxamcebce,  634 
Myxogastres,  634 
Myxomycetes,  579  note,  634;  develop- 

ment of,  634,  636  ;  spores  of,  634,  636  ; 

swarm-spores    of,    634;    affinity    with 

Monerozoa,  727 
Myxosporidia,  749,  752 


1164 


INDEX 


NAC 

N 

Nachet,  on  '  immersion  system,'  27 ;  his 
binocular,  97,  98,  99 ;  his  changing 
nose-piece,  293 

Nacreous  layer  in  molluscan  shells,  919, 
922,  924 

Naegeli  and  Schwendener,  on  microscopi- 
cal optics,  67 

Nageli's  theory  of  formation  of  starch, 
695 

Nails,  1029,  1033 

Nais,  955 

Naphthalin,  monobromide  of,  as  a  mount- 
ing medium,  521 ;  refractive  index  of, 
521 

Narcissus,  spiral  cells  of  pollen-chambers 
in,  720 

Nassula,  mouth  of,  774 

Nauplius,  compared  with  Pedalionidce, 
792 

Nautiloid  shell  of  Foraminifera,  797 

Nautilus,  929 

Navicula,  590,  597,  617;  markings  on, 
593  ;  cysts  of,  597  ;  zygospores  of,  597  ; 
zobzygospores  of,  597 

—  bifrons,  presumed  relation  to  Suri- 
rella  microcora,  602  note 

—  in  chalk,  1087 

—  lyra,  as  test  for  definition,  426 

—  rhomboides,  markings  on,  592  ;  as  test 
for  definition,  426 

Naviculece,  frustule  of,  589 ;  ostioles  in, 
590 

—  characters  of,  616 
Nebalia,  carapace  of,  962 

Needles  for  dissection,  their  mode  of  use, 

457 
Negative  aberration,  27,  360  note 

—  crystals,  1074 

—  eye-pieces,  376,  377,  878 

Nelson,  on  the  sub-stage  condenser,  72 
note  ;  on  ghostly  diffraction  images,  72 
note',  his  model,  with  Swift's  fine- 
adjustment  screw,  172  ;  his  horse-shoe 
stage,  179,  228 ;  his  fine  adjustment  to 
the  sub-stage,  185 ;  his  screw  micro- 
meter eye-piece,  271 ;  his  new  micro- 
meter eye-piece,  272  ;  his  '  black  dot,' 
277 ;  his  plan  for  estimating  edges  of 
minute  objects,  277  ;  his  changing  nose- 
piece,  294 ;  his  revolving  nose-piece, 
295  ;  on 'rings  and  brushes,  319,  320 ; 
his  means  of  obtaining  monochromatic 
illumination,  323  ;  his  lamp,  404 

Nelson  and  Karop,  on  fine  structure  of 
diatoms,  591  note 

Nemalion  multifidum,  631 

Nematodes,  desiccation  of,  945 

Nematoid  worms,  944 

Nemertine  larva,  951 

Nepa,  tracheal  system,  995 ;  wings  of, 
1000 

—  ranatra,  eggs  of,  1005 
Nepenthes,  spiral  fibre-cells  of,  698 
Nereides,  948 

Nereocystis,  627 


NUC 

Nerve-cells,  1051 

Nerve-fibres,  1052 

Nerve-substance,  1051 ;  mode  of  prepara- 
tion, 1054 

Nerve-tubes,  1051 

Nervures  of  wing  of  Agrion,  994 

Nettle,  hairs  of,  714 

Neuroptera,  973 ;  eyes  of,  987 ;  circula- 
tion in  wings  of  pupa,  994  ;  wings  of, 
998 

Newt,  red  blood-corpuscles  of,  1034  ;  cir- 
culation in  gills  of  larva,  1057 

Newton's  reflecting  microscope,  132 
-    suggestion  of   reflecting   microscope, 
145 

—  rings,  1097 
Nicol  prisms,  318 

Nicol's  analysing  prism,  294  ;  for  resolv- 
ing striae,  381 

Nicotiana,  seeds  of,  724 

'  Nidamentum '  of  Gastropoda,  934 

Nitella,  576 

Nitric  acid  as  a  test  for  albuminous  sub- 
stances, 517 

Nitrogenous  substances,  test  for,  517 

Nitzschia,  602 

—  scalaris,  cyclosis  in,  587 

—  sigmoidea,  606 
Nitzschiece,  606 

Noctiluca,  collecting,  529 ;  tentacle 
(flagellum)  of,  766,  768 ;  cilium  of,  766 
note ;  protoplasmic  network  of,  767 ; 
reproduction  of,  769 

—  miliaris,  765-769 
Noctuina,  antennae  of,  988 
Nodes  of  monocotyledons,  701 
Nodosaria,  819 
Nodosarince,  shell  of,  797 
Nodosarine   shell,   sandy   isomorphs    of, 

815 
Nonionina,  829 

—  shell  of,  797,  798 

Nonionine  shell,  sandy  isomorph  of,  814 
Non-stereoscopic  binoculars,  105 
Non-striated  muscle,  1048,  1050 
Nose-pieces,  291-295  ;   centring,  used  as 
sub-stage,  228;  Brooke's,  291;  Beck's 
rotating,  291 ;   Powell  and  Lealand's, 
291 ;  Watson's  dustproof,  292  ;  Zeiss's 
calotte,  292;    centring,   293;  Nachet's 
changing,  293 ;  analysing,  294 ;  Vogan's, 
294  ;  Nelson's  revolving,  295 
Nosema  bombycis,  cause  of  pebrine,  661 
Nostoc,   548,  549;    as   gonid   of    lichen, 
651 ;  resemblance   of    Ophrydium  to, 
778 
Nostocacece,  548 ;  affinities  with  Bacteria 

and  Myxomycetes,  652 
•    Notochord  in  Tunicata,  911 ;  of  Appen- 

dicularia,  918 

Notonecta,  987  ;  wings  of,  1000 
Nucellus,  685 
Nuclear  stains,  491-494 

—  spindle,  538  ;  plate,  538 
Nuclein,  537 

Nucleoli,  534 
Nucleoplasm,  537 


INDEX 


1165 


xuc 


oos 


Nucleus,  534 

—  action  of  acetic  acid  on,  517 ;  its  im- 
portance to  cell,  535  ;  division  of,  538 ; 
fragmentation  of,  538 ;    presumed  ab- 
sence of,  in  some  forms,  727  ;  initiative 
action  in  monads,  762 

—  and  cell  division,  1019  note 
Nucule  of  Chara,  577,  579 
Nudibranchs,  nidameiitum  of,  934 ;  em- 
bryos of,  936 

Numerical  aperture,  29,  53,  60,  390, 
425  ;  formula  for,  390 ;  problems  on/1 
391 

of  dry  objective,  391  ;  of  water- 
immersion,  391  ;  of  oil-immersion,  391 

and  resolving  power  of  objective, 

393 

—  apertures,  table  of,  84-87 
Nummuline  layer  of  Eozoon,  840 

—  plan  of  growth,   Parker   and  Rupert 
Jones  on,  827  note 

Nummulinidce,  826 
Nummulites,  826,  827,  831 

—  distans,  832 

—  garansensis,  832 

—  Icevigata,  832 

—  striata,  internal  cast  of,  834 

—  tubuli  in  shell  of,  800 
Nummulitic   limestone,   831,    835,   1085, 

1090 
Nuphar  lutea,  parenchyme,  687  ;  stellate 

cells  of,  687 
Nymph  of  Acarina,  1009 ;  of  Oribatidte, 

1009 


O 


Oak,  size  of  ducts  in,  699 

-  galls,  1003 

Oberhiiuser's  spiral  fine  adjustment,  153 

Object-glass  of  compound  microscope, 
36,  39 ;  of  long  focus,  40 ;  of  short 
focus,  40  ;  capacity  of,  382 

Object-glasses,  power  of,  44 

-  testing,  381;  Abbe's  method  of 
testing,  384-387 :  diaphragms  for  use 
in  testing,  385 ;  Fripp's  method  of 
testing,  386 

Object-holder  for  Thoma's  (Jung's)  mi- 
crotome, 464, 465,  466 

—  changer,  Zeiss's,  293 

Objectives,  achromatic,  19,  32;  aplanatic, 
19 ;  apochromatic,  19,  30,  34,  80 ;  cor- 
rected, 20,  21 ;  immersion,  28,  34,  58 ; 
aperture  of,  43,  65,  390;  maximum 
aperture  of,  44 ;  comparison  of,  46 ; 
illuminating  power  of,  54  note ;  im- 
mersion v.  dry,  54,  79  ;  dry,  with  balsam 
mounted  objects,  55  ;  dry,  58;  dry,  for 
study  of  life-histories,  81 ;  penetrating 
power  of,  83,  393 ;  sliding  plate  with, 
290 ;  rotating  disc  with,  290 ;  of  wide 
aperture,  369 ;  of  small  aperture,  ex- 
amination of,  388  ;  tests  for,  388,  394  ; 
resolving  power  of,  and  numerical  aper- 
ture, 393 


Objectives,  triple-back,  361;  Wenham's 
single  front,  361 ;  duplex  front,  362 ; 
Leitz's,  374;  Reichert's,  374;  adjust- 
ing, 357,  360 

—  achromatic,  Martin's,  147  ;   Marzoli's, 
353;     Tully's,   354;     Selligue's,     354; 
Amici's,  355 ;  Ross's,  356,  360 ;  Powell's, 
356,  361  ;  Smith's,  356,  360  ;  Wenham's, 
361 ;  covers  for  use  with,  439 

—  apochromatic,  366,  370,  371-375 

—  homogeneous  immersion,  introduction 
of,  364 

—  '  semi-apochromatic,'  35, 374,  375 

—  oil-immersion,  Powell  and  Lealand's, 
30 ;      Amici's,     364 ;      Tolles',      364 ; 
Zeiss's,  370;  Leitz's,  374;   Reichert's, 
374;       Swift's,     375;       Beck's,    375; 
Bausch   and  Lomb's,  375;    Watson's, 
375 

—  water-immersion,    Powell    and     Lea- 
land's,  362,  365 ;  Prazmowski  and  Hart- 
nack's,  362  ;  Zeiss's,  370 

Oblique  illumination,  190,  191,  387 

—  illuminator,  190 

Obliteration  of  structure  by  diaphragms, 
68 

Occhiale,  Galileo's,  122,  123 

Occhialino,  Galileo's,  121,  124 

Oceanic  sediinents,  microscopic  examina- 
tion of,  1092 

Ocelli  of  planarians,  947  ;  of  insects,  982, 
986 

Ocellites  of  compound  eye,  982 

Ocular,  40,  375  ;  spectral,  327 

(Edogoniaoece,  572 

(Edogonium  ciliatum,  573 

(Enothera,  pollen-grain,  721;  emission 
of  pollen-tubes  of,  722 ;  embryo  of,  723 
j  Oil  for  immersion  lenses,  suggested  by 
Amici,  29 

—  of  cedar-wood,  for  immersion  objec- 
tives, 29 

Oil-globules,  429-431 
Oil-immersion,  29 

-   objectives.      See   Objectives,    oil- 
immersion 
j    Oils,  solvents  for,  517 

Okeden,    on    isolation   of    diatoms,   624 

note 
Oleander,  epiderm  of,  714 ;  stomates  of, 

716 

Olivine,  corroded  crystals  of,  10,72 
Onchidium,  eyes  of,  941 
'    Oncidium,  spiral  cells  of,  693 
j    Onion,  raphides  of,  696 

Oogones  of   Vaucheria,  563 ;  of  Sphere- 
plea,   572 ;    of    (Edogonium,   572 ;    of 
Chara,  577 ;  of  Fucacece,  627,  628 ;  of 
Peronosporece,  638 
|     Oolitic  grains,  1084 

Oophyte  in  ferns,  680 

|  Obspheres,  use  of  the  term,  537  note ;  of 
Volvox,  556;  of  Vaucheria,  563;  of 
Sphceroplea,  570 ;  of  (Edogonium,  572 ; 
of  Chara,  577  ;  of  Phceosporece,  627  ; 
of  Fucacece,  628  ;  of  Marchantia,  668  ; 
of  ferns,  679 


1 1 66 


INDEX 


oos 

Ob'spores,  540 ;  of  Volvox,  556 ;  of  Vau- 
cheria,  563;  of  Achlya,  565;  of 
Sphceroplea,  572 ;  of  (Edogonium,  573  ; 
of  Chara,  579 ;  of  Fucacece,  628 

Ooze,  Globigerina,  organisms  in,  811, 
813,  820  ;  compared  with  chalk,  1085 

Opalescent  mirror  as  a  substitute  for 
polarising  prism,  194 

Opalina,  774 

Opaque  illumination  by  side  reflector,  333 

—  mounts,  336 

'  Open  '  bundles,  710 

Operculina,  830  ;  and  Nummulites  com- 
pared, 834 

Opercule  of  mosses,  671 

Ophiacantha  vivipara,  development  of, 
900  note 

Ophioglossacece,  development  of  pro- 
thallium  of,  679 

Ophioglossum,  sporanges  of,  676 

Ophiothrix  pentaphyllum,  spines  of, 
891 ;  teeth  of,  892 

Ophiurida,  mounting,  481 

Ophiuroidea,  skeleton  of,  891 ;  spines  of, 
891;  teeth  of,  892;  larva  of,  898; 
direct  development  in,  900  note 

Ophrydia,  quantities  of,  777 

Ophrydium,  cellulose  in  zoocytium  of,  778 

—  versatile,  effect  of  light  on,  775 
Ophryodendron,  784 

Opium  poppy,  latex  of,  695 

Optic  axis  of  Powell  and  Lealand's  No.  1, 

194 
Optical  anomalies  in  petrology,  1078 

—  centre,  24 

—  tube-length  of  microscope,  158,  159 
Orals  of  Ahtedon,  901 
Orbiculina,  803,  804,  808 

—  compared  with  Heterostegina,  834 
Orbitoides,  835 

—  and  Cycloclypeus  compared,  835 

—  Fortisii,  836 
Orbitolina,  824 

Orbitolincs,  occurring  with   flint  instru- 
ments, 824 
Orbitolites,  804-810 

—  shell  of,  798 ;    range  of  variation  in, 
810  ;  structure  of  Parkeria  resembling, 
817 ;  deposits  of,  1085 

—  and  Cycloclypeus  compared,  801 

—  complanata,  animal  of,  807-809 

—  italiacv,  806  note,  808 

—  tenuissima,  808 
Orbulina,  820 

Orbuline  shell,  sandy  isomorph  of,  815 

Orchidece,  pollinium  of,  722 

Orchids,  mycropyle  of,  723 

Orchis,  pollen- tubes  of,  723;  seeds  of, 
724 

Organised  structure  and  living  action, 
530 

Organs,  533 

'  Organs  of  sense '  in  Ciliata,  775  note 

Oribatidce,  nymph  of,  1009  ;  mouth-parts 
of,  1009 ;  legs  of,  1010  ;  integument  of, 
1010  ;  auditory  organ,  1011 ;  reproduc- 
tive organs,  1011 ;  supercoxal  glands 


of,  1011 ;  tracheae  of,  1011 ;    characters 

of,  1012 
Orienting  small  objects  for  sectionising, 

499 

Origanum  onites,  seeds  of,  724 
Ornithorhynchus,  hair  of,  1081 
Orobanche,  seeds  of,  724 
Orthoptera,  eyes  of,  987  ;    antennae    of, 

988;  wings  of,  999  ;  nymph  of,  1009 
Orthoscopic  effect,  95  ;   with  Ramsden's 

circles,  106 

—  eye-piece,  376 

Orthosira  Dickiei,    sporangial    frustule 

of,  595 

Oscillatoria,  movement  of,  547 
Oscillatoriacece,  547 

—  movements  of,  433 
Oscula  of  sponges,  856 

Osmic  acid  and  fatty  structures,  517 
Osmunda,  sporanges  of,  676 

—  regalis,  prothallium  of,  679  note 
Ossein,  of  bone,  1023 

Ostiole  of  conceptacle  of  corallines,  632 

Ostioles  of  Naviculacece,  590 ;  of  Cijni- 
bellete,  590 

Ostracoda,  960 

Ostreacece,  shell  of,  923 

Ostrich,  egg-shell  of,  1101 

Otoliths  compared  with  artificial  concre- 
tions, 1100 

—  of  Mollusca,  941 
Ovarium  of  Polyzoa,  907 
Over-amplification,  88 
Over-corrected  objective,  20 
Over-correction,  358-360 
Overton,  on  Volvox,  556  note 
Ovipositor  of  Oribatidce,  1012 
Ovipositors  of  insects,  1002-1004 
Ovule  of  Phanerogams,  684 

—  suspensor  of,  534 

—  structure  of,  684-685  ;  development  of , 
722 

Ovum  of  Hydra,  866 

Oxytricha,  a  phase  in   development   of 

Trichoda,  780 
Oxyuris  vermicularis,  944 
Oysters,  shell  of,  923 


Pacinian  corpuscles,  1053 
Palaeontology,  use  of  microscope  in,  1083 
'  Palate  '  of  Gastropoda,  919,  930 ;  classi- 

ficatory  value  of,  932  ;  preparation  of, 

932 ;    viewed   with    polariscope,   933  ; 

bibliography,  933 
Paleee  of  grasses,  silex  in,  715 
Palisade-parenchyma  of  leaves,  716 
Palm,  stem  of,  701 
Palmella,  as  gonid  of  lichen,  651 
—  cruenta,  558 

Palmellacece,  557  ;  frond  of,  558 
Palmodictyon,  559  ;  zoospores  of,  559 
Palmogloea  macrococca,  life-history  of, 

541,  542 
Palpicornia,  antennae  of,  988 


INDEX 


1167 


PAL 


PHA 


Paludina,  infested  by  Distoma,  946 
Pancreas,  1047 
Pandorina,  545 

—  morum,  generative    process    of,  557; 
swarm-spores  of,  557 

Papaveracece,  laticiferous  tissue  of,  695 

Paper-cells,  446 

Parabolic    illuminator,    316;    speculum, 

333;  reflector  (Sorby's),  334 
Paraboloid  illuminator,  316 
Paraffin,  solvents  for,  496 

—  imbedding  method,  496-503 

—  for  imbedding,  melting  point  of,  500 

—  mounting,  sections,  501 

—  cells,  446 

Paramecium,  Conn's  experiments  on, 
743 ;  contractile  vesicles  of,  776 

Paraphyses  of  Puccinia,  688 ;  of  lichens, 
650  ;  of  mosses,  671 

Parasites,  nourishment  of,  532 

Parasitic  Crustacea,  965 

—  Fungi,  633 
Parietal  utricle,  533 
Parker  (T.  J.),  on  Hydra,  863 
Parkeria,   817;    a    possible    Stromato- 

poroid,  817  note 
Parnassia,  seeds  of,  724 
Parthenogenesis,  1007  note 

—  in  SaprolegnicB,  640 

Passiflora  coerulea,  pollen-grains  of,  721 
Passiflorece,  pollen-grains  of,  721 
Paste-worm,  945 
Pasteur's  solution  for  growing  yeast,  646 

note ;  his  experiments  with  Bacteria, 

660, 661 
Patella,  shell  structure,  928 ;   palate  of, 

931 
Path  of  ray  of  light  through  a  compound 

microscope,  40 
Pathogenic  bacteria,  658 
Pavement  epithelium,  1044 
Pear,  constitution  of  fruit,  693 
'  Pearl  oyster.'     See  Meleagrina 
Pearls,  923 

'  P^brine '  in  silkworms,  661 
Peccary,  hair  of,  1030 
Pecten,  prismatic   layer  in,  924  ;  pallial 

eyes  of,  940  ;  fibres  of  adductor  muscle, 

1050 

Pectinibranchiata,  937 
Pectinidce,  sub-nacreous  layer  in,  924 
Pedalion,  792 
Pedalionidce,  792 
Pedesis,  431 ;  experiments  in,  432 
Pediastrece,  566 ;  affinities  of,  566 
Pediastrum,     zob'spores,      567 ;     micro- 

zoospores,  567 

—  Ehrenbergii,  568 

—  granulatum,  566-568 

—  pertusum,  568 

—  tetras,  568 

Pedicellariae   of    echinids   and   asterids, 

889 

Pedicellina,  lophophore  of,  909 
Pedicularis  palustris,  723 

—  sylvatica,  embryo  of,  723 
Pedunculated  cirripeds,  967 


Pelargonium,  petal  of,  7i9  ;  pollen-grain, 

721 

Pelomyxa palustris,  744 
Peneroplis,  801 

—  variation  in  shape   of   shell  in,   797; 
shell  of,  799  ;  varietal  forms  of,  803 

Penetrating  power,  425 
in  objectives  83  ;  of  objective,  com- 
pared with  illuminating  power,  393 
Penetration,  38,  82, 83 
Penicillium,  fermentation  by,  647 

—  glaucum,  643 

Pentacrinus  asterius,  skeleton  of,  892 
I    Pentatoma,  wings  of,  1000 
i    Peony,  starch  in  cells  of,  694 
i    '  Pepperworts,'  681 

Perception  of  depth,  94 
1    Perch,  scales  of,  1028 

Perforated  shells  of  Bracliiopoda,  926 

Perforation  of  shell  in  Foraminifera,  799, 
800 

Perianth,  718 
;    Perichlamydium  prcetextum,  851 

Peridinia,  770.  771 

Peridinium  uberrimum,  770 

Perigoiie  of  mosses,  670 

Periodic  structures,  74 

Periostracum  of   molluscan  shells,   922 ; 
of  brachiopod  shells,  926 

Peripatus,  tracheae  of,  1011 

Peritheces  of  lichens,  650 

PeronosporecB,  638-640 

Perophora,  respiratory  sac  of,  915 ;  cir- 
culation of,  915 

'  Perspicillum,'  Wodderborn's,  125 

Petals,  718 

Petrobia  lapidum,  eggs  of,  1009 

Petrological  microscope,  Swift's,  1068 

Petrology  :  micro-spectroscope  in,  1081 ; 
micro-chemistry  in,  1082 

Pettenkofer's  test,  517 
I    Petunia,  seeds  of,  724 

Peziza,  botrytis-iorm  of,  645 
i    Pfitzer,  on  reproduction  of  diatoms,  594 
I    Phceodaria,  852 
j    Phceosporece,  625-627 
i    Phagocytes,  1037  note 
j    Phakellia  ventilabrum,  858 
I    Phallus,  647 

j    PHANEROGAMIA,  woody  structures,  pre- 
paration of,  514 

—  embryo-sac  of,  free-cell  formation  in, 
534-536 

—  relation  of,  to   Cryptogams,  682,  684 
and  note  ;  structure  of  stems,  &c.,  685, 
700 ;  structure  of  cells,  686-688  ;  inter- 
mediate   lamella,    688 ;     intercellular 
spaces,  688  ;  cell-wall  of,  692  ;  sclerogen, 
693 ;  spiral  cells  in,  693 ;   laticiferous 
tissue  of,  695 ;  mineral  deposits  in  cells 
of,  696  ;  woody  fibre  in,  696  et  seq.;  fibro- 
vascular  bundles,  697 ;  root,  structure 
of,   700  ;    epiderm  of  leaves,  712-718  ; 
flowers  of,  718  ;  pollen-grains  of,  719  ; 
fertilisation  of,   722;  ovules   of,    722; 
seeds  of,  723 

Phanerogams.     See  PHANEROGAMIA 


ii68 


INDEX 


PHI 

PhilontJms,  antennae  of,  988 

Phloem,  710 

—  of  Exogens,  697 

Pholas,  shell  of,  924 

Phoronis,  950 

Phosphorescence  of  sea,  AueioNoctiluca, 

765 

Phosphorus,  as  a  mounting  medium,  521 
Photographic    microscope,   Zeiss's,   257, 

258 
Photometrical    equivalent    of     different 

apertures,  50 
Photo-micrograph  through  eye  of  Lam- 


pyris,  984 
>ho 


Photo-micrography  for  micrometry,  277 ; 
projection  eye- pieces  for,  380 

—  Campbell's  differential  screw  used  in, 
165 

Phryganea,  eye  of,  983 

Phycocyanin  in  Chroococcacecs,  547 

Phyco-erythrin,  631 

Phycomyces  nitens,  641 

Phycophaein,  626 

Phylactolcemata,  909 

Phyllite,  1077  note 

Phyllopoda,  962 

Pliyllosomata,  skeleton  of,  968 

Physarum  album,  development  of,  635 

Physcia  parietina,  650 

Physma  chalaganum,  650 

Phytelephas,  endosperm  of  seed  of,  693 

Phytophthora  infestans,  639,  640 

Phytopti,  mouth-parts  of,  1010 

Phytoptidce,  1008  ;  characters  of,  1014 

Phytoptus,  larva  of,  1009 

Picric  acid,  485 

Picro-carmine,  489 

Piedmontite,  1095 

Pieridce,  scales  of,  975 

Pigment-cells  of  cuttles,  942 ;  of  ver- 
tebrate skin,  1042 ;  of  fishes,  1043 ;  of 
Crustacea,  1043 

Pigmentum  nigrum,  of  eye,  1043 

Pike,  scales  of,  1028 

Pileorhiza,  710 

Pileus  of  Acetabularia,  563 

Pilidium  gyrans,  950 

Pilulina  Jeffreysii,  812 

Pimpernel,  petals  of,  719 

Pines,  pollen-grains,  showers  of,  722  note 

Pinna,  structure  of  shell  of,  919-922; 
prisms  of  shell  of,  in  Globigerina  ooze, 
1086  ;  prisms  of,  in  chalk,  1087 

—  nigrina,  colour  of  shell  of,  921 
Pinnularia,  617 

—  dactylus,  621 

—  nobilis,  621 
Pinus  canadensis,  443 
Pipette,  351,  476 
Pisolithic  grains,  1084 
Pistil,  722 

Pitcher-plant,  spiral  fibre-cells  of,  698 
Pith,  arrangement  of,  700,  762 
Pitted  ducts  of  Phanerogams,  699 
Placoid  scales,  1028 
Plagioclase  felspar,  1080 
Planaria,  stomach  of,  946 


POL 

Planar  ice,  946  ;  movement  of,  946 ;  fis- 
sion of,  947  ;  ocelli  of,  947 ;  intracellular 
digestion  in,  863 

Planarians.     See  Planarice 

—  allied  to  Ctenophora,  883 
Plano-concave  lens,  13 
Plano-convex  lenses,  13,  15,  22,  37 
Planorbulina,  824 

Plantago,  'Plantain,'  cyclosis  in,  691 
Plants  and  animals,  differences  between, 

531 

Planulse,  868 

Planularia  liexas,  in  chalk,  1087 
Plasmode  in  cells  of  Nitella,  579  note ; 

of  jEthalium,  634 ;  of  Myxomycetes, 

635 
Plasmodium  of  Protomyxa  aurantiaca, 

729 

Plastid,  contrasted  with  cytode,  727 
Plastidules,  flagellated,   of  Protomyxa, 

729 
Plates,    calcareous,   of    Holothurioidea, 

895 

Pleochroism,  1078,  1098 
Pleochroism,  variations  of,  1080 
Pleurosigma,  588,  617 

—  diffraction  image  of,  71 

—  angulatum,   69-71 ;  as  test  for  defi- 
nition, 426 ;  markings  on,  592,  593 

—  formosum,  as  test  for  definition,  426 

—  Spencerii,  sporules  of,  597 

Pliny,  on  cauterisation  by  focussing  sun's 

rays,  117 ;  on  sight,  118 
Ploima,  791,  792 
PlumateUa,  collecting,  528 
'  Plumed-moth,'  wings  of,  999 
Plumule  of  Pieridce,  975 
Plutarch,  on  myopy,  118 
Pluteus  larva  of  echinoids,  897-899 
Podocyrtis  cothurnata,  847 

—  mitra,  847,  852 

—  Schomburgkii,  849,  852 
Podophrya  quadripartita,  784 ;  imma- 
ture form,  785 

—  elongata,  785 
Podosphenia,  sporules  of,  597 
Podura   scale   as  test  for  high  powers, 

389 

1  Podura  scales,'  976,  979 

Poduridce,  979 

Pointer  in  eye-piece,  381 

Poisons,  micro-chemistry  of,  1103 

Polarisation  tints,  1080     . 

Polariscope,  condensers  for  use  with, 
314 ;  for  examination  of  gastropod 
palates,  933;  crystals  for  use  with, 
1097 ;  list  of  objects  for,  1099 

Polarised  light,  rings  and  brushes  of  mine- 
rals under,  319,  320 ;  for  insect  work, 
423  ;  use  of,  in  micro-petrology,  1068 

Polariser,  318,  319;  achromatic  conver- 
gent for,  1070  note 

Polarising  apparatus,  317-319  ;  condenser 
for,  314  ;  Swift's  illuminating  and,  319 

Polarising  prism,  substitution  of  opales- 
cent mirror  for,  194 

'  Polierschiefer,'  617 


INDEX 


1169 


POL 


PRI 


Polishing  ground  sections,  511 

—  sections  of  hard  substances,  506 

—  -slate,  617 

—  -stones,  508,  617 

Polistes   (wasp^,   with   attached    mould, 

642 
Pollen- chambers  of  anthers,  720 

—  -grain  and  tube,  684 

—  -grains,   719 ;    form   of,   720 ;    experi- 
ments with,  721 

—  mass,  of  orchids,  722 

—  tube,  721 

—  tubes,  traced  through  the  style,  723 
Pollinium  of  orchids  and  asclepiads,  722 
Pollinoids  of  Floridece,  632 ;  of  lichens, 

650 

Polyaxial  spicules,  859 
Polycelis  levigatus,  947 
Poiyclinid.ee,  913 
Polycystina,  846,  851 
Polycystina,   as   test    for    low    powers, 

389 ;  mounting,  481 
Polydesmiclce,  981 
'  Polygastrica,'     Ehrenberg's     erroneous 

views  on,  753 

Polygonum,  pollen-grains  of,  721 
Polymorphina,  820 
Polyommatus  Argus,  scales  of,  976 
Polyparies  of  zoophytes,  862 
Polypary  of  hydroids,  867 
Polypes,  863.     See  Hydrozoa 
Polypide,  of  Polyzoa,  906  ;  formation  of 

buds  from,  907 
Polypidom  of  zoophyte,  904 
Polypite,  of  hydroids,  867 
Poly  podium,  sori  of,  675 
Polyporus,  647 

Polystichum  angulare,  apospory  in,  680 
Polystomella,  shell  of,  797 

—  craticulata,  827,  829 

—  crispa,  827,  829 
Polythalamous  Foraminifera,  796 
Polytoma  uvella,  life-history  of,  759 
Polytrema,  824;    mode  of  growth  com- 
pared with  Eozofin,  838 

—  miniaceum,  colour  of,  799 
Polytrichum  commune,  670,  671 
Polyxenus  lagurus,  hair  of,  981 

—  hair  of,  as  test  for  objectives,  389  ; 
as  test  for  definition,  426 

Polyzoa,  collecting,  527,  528;  keeping 
alive,  528  ;  '  cell '  of,  904  ;  structure 
of,  904 ;  gemmae  of,  906 ;  muscular 
system,  907 ;  sexual  reproduction  of, 
907 ;  '  colonial  nervous  system,'  907 
and  note;  fresh- water,  lophophore  of, 
909 ;  epistome  of,  909 ;  classification 
of  the  group,  909;  bibliography  of, 
910;  relation  to  Brachiopoda,  927; 
'  liver '  of,  1047 

Polyzoaries  in  coralline  crag,  1090 

Polyzoary,  904 

Pond-stick,  526 

Poplar,  pollen-grains  of,  722 

Poppy,  laticiferous  tissue,  695 ;  seed  of, 
723 

Porcellanea,  801-810 


Porcellanous  shells  of  Foraminifera, 
799 ;  of  Gastropoda,  928 

—  and  vitreous  Foraminifera,  difference 
in,  799-801 

Porcupine,  hair  of,  1030 

Pores  of  sponges,  856 

Porpliyra,  trychogyne  of,  632 

Porphyritic  crystals,  glass  inclusions  in, 
1074 

'  Portable  '  microscope,  245-247 ;  Powell 
and  Lealand's,  245 ;  Rousselet's  bino- 
cular, 245 ;  Swift's,  245  ;  Baker's,  246  ; 

**    Bausch  and  Lomb's,  247 

Portunus,  skeleton  of,  968 

Positive  aberration,  360  note 

—  eye-piece,  43 

—  eye-pieces,  377,  378 

Potash,   caustic,   action   on    horny   sub- 
stances, 517 
Potato-disease,  640 

—  starch-grains  of,  695 

—  tubers,  starch  in,  694 

Powell  (T.),  formula  for  objective,  34 
Powell  and  Lealand's  homogeneous  im- 
mersion objective,  30 ;  fluorite  lenses, 
35 ;  high-power  binocular,  105 ;  sub- 
stage,  186, 195, 196  ;  their  microscopes, 
194,  218,  237;  portable  microscope, 
245  ;  rotating  nose-pieces,  291 ;  achro- 
matic condenser,  301 ;  achromatic  oil 
condenser,  302;  apochromatic  con- 
denser, 302  ;  dry  achromatic  condenser, 
809;  chromatic  oil  condenser,  310; 
condenser  for  polariscope,  314 ;  bull's- 
eye,  333;  vertical  illuminator,  337; 
protecting  ring  for  coarse  adjustment, 
352 ;  water-immersion  objectives,  362, 
364  ;  ^-inch  objective,  for  observation 
of  cyclosis,  689;  objectives  for  study 
of  monads,  762 

Powell's  (H.)  microscope,  155 ;  fine  ad- 
justment applied  to  the  stage,  155 

—  lenses,  361 

—  fine  adjustment,  174 
Prasmowski   and    Hartnack's   water-im- 
mersion objectives,  362 

Prawn,  skeleton  of,  pigment  of,  969 
Preparation  of  vegetable  tissues,  514 
Presbyopy,  118 
Preservative  media,  517r522 
Primary  tissues  of  Vertebrata,  1017 
Primordial  ceUs,  535,  536 

—  utricle,  533  ;  of  desmids,  580  ;  of  Pha- 
nerogam cells,  688 

—  chamber  in   Foraminifera,   798;   of 
Orbitolites,  806 

Primrose,  cells  of  pollen-chambers,  720 
'  Prince's  feather,'  seed  of,  723 
Principle  of  microscopic  vision,  43 
Principles  of  microscopical  optics,  1 
Pringsheim,    on    generative   process    of 

Pandorina,  557 ;  on  Vaucheria,  563 
Prism,  refraction   by,  8,  9;    Wenham's, 

98 ;  Stephenson's  erecting,  100 

—  polarising,  substitution  of  opalescent 
mirror  for,  194 

—  rectangular,  in  place  of  mirror,  192 

4   F 


1170 


INDEX 


PRI 


BAY 


Prism,  Nicol's,  318;  Nicol's  analysing, 
for  resolving  striae,  381 ;  Abraham's, 
401 

—  refracting  angle  of,  9,  18 
Prismatic  epithelium,  1044 

—  layer  in  molluscan  shells,  919-925 

—  layer  of  shells  compared  with  anamel, 
920,  1025 

—  shell- substances  imitated,  1102 
Prisms,  recomposition  of  light  by,  18 
Pristis,  tooth  of,  1024 
Pritchard's  doublets,  298 

—  microscope  with  Continental  fine  ad- 
justment, 153 

Privet  hawk-moth,  eggs  of,  1005 
Problems  on  refractive  index,  5 
Procarp,  of  Floridece,  632 
Projection  eye-piece,  880 
Promycele  of  Puccinia,  637 
Prosenchymatous  tissue,  696 
Proteus,  red  blood-corpuscle  of,  1036 
Prothallium    of    Sphagnacece,    674;    of 
ferns,  677 ;   of  Equisetacece,   681 ;   of 
Rhizocarpece,  681;   of  Lycopodiacece, 
681 

Protococcus,  as  gonid  of  lichens,  651 
-  pluvialis,    543-547  ;    life-history  of, 
543 ;  multiplication  of,  544 ;  zob'spores 
of,  544  ;  mobile  and  still  forms  of,  545- 
547  ;  encysted,  551 
Protomyxa  aurantiaca,  727-729 
Protoneme  of  Batrachospermum,  575 
Protophytes,  530,  651,  726 

—  mounting,  518  ;  mode  of  nourishment 
of,  532 ;  movement  by  cilia  and  con- 
tracting vacuoles  of,  535 

Protoplasm,  530  ;  vital  attributes  of,  531 ; 
continuity  of,  538,  630 ;  of  Rhizopoda, 

733  ;  of  Noctiluca,  767 
Protoplasmic  substance  in   Vertebrata, 

1017 
PBOTOZOA,  726-785 

—  mode  of  nourishment  of,  532 

'  Pseudembryo '  of  Antedon,  903 

Pseudo-navicellae,  751 

Pseudo-parenchyme  of  Fungi,  633 

Pseudopodia  of  Protomyxa,  728;  of 
Vampyrella,  730 ;  of  Lieberkuehnia, 
731;  of  Rhizopoda,  733;  of  Reticu- 
laria,  734 ;  of  Heliozoa,  734 ;  of  Lobosa, 

734  ;  of  Gromia,  735  ;  of  Micro gromia, 
736 ;  of  Actinophrys,  738 ;  of  Amoeba, 
743  ;  of  Arcella,  &c.,  746 ;  in  Amceba- 
phase  of  monad,  757  ;  of  Eozoon,  841 ; 
of   Globigerina,  822;   of  Radiolavia, 
847 ;  of  endoderm  cells  in  zoophytes, 
862 

Pseudoraphidecp,  599 

Pseudoscope,  Wheatstone's,  92 

Pseudoscopic  effects,  95 

• —  effect  with  Ramsden's  circles,  106 

—  vision,  92 

Pseudo-scorpions,  1008 

Pseudo-stigmata    of     Oribatidce,    1011, 

1012 
Pseudo-tracheae,   on  fly's  proboscis,  990 

note 


'  Psorosperms,'  752 

Pteris,  sori  of,  675  ;  indusium  of,  675 

—  serrulata,  apogamy  in,  680 
Pterocanium,  852 
Pterodactylus,  bones  of,  1092 
Pterophorus,  wings  of,  999 
Pteroptus,  1012 

Ptilota,  630 

Puccinia  graminis,  637 

Puff-ball,  647 

Pulvilliof  insects,  1001 ;  cockroach,  1000 

note 
Pupa  of  Neuroptera,  circulation  in,  994 

—  stage  of  fly,  1007 
'  Purple  laver,'  632 

Purpura,  method  of  examination  of  egg- 
capsules  of,  939  ;  supplemental  yolk  of, 
938,  1007 

—  lapillus,  nidamentum  of,  934 ;  develop- 
ment of  yolk-segments  of,  937 

'  Puss-moth,'  eggs  of,  1005 
Pycnogonida,  957 ;  related  to  Arachnid  a, 

959  note 

Pyrola,  seeds  of,  724 
Pyroxene,  aridesite,  1076 


q 


Quadrula  symmetrica,  747 

Quartz-porphyries,  1072 

Quartzite,  1077 

Quekett   (E.),  on   Martin's    microscope, 

140 ;  on  production  of  raphides,  696  ; 

on  preparation  of  tracheae  of  insects, 

997 ;    on   minute    structure  of    bone, 

1092 

1  Quills '  of  porcupine,  1030 
Quinqueloculina,  802 


Radials  of  Antedon,  901 

Radiating  crystallisation,  1097 

Radiation  of  light  in  different  media,  58- 
58  ;  in  air  and  balsam,  55-57 

Radiolaria,  collecting,  529;  fossilised 
forms  of,  846,  854  note ;  central  cap- 
sule of,  847;  skeleton  of,  848-854;  zoo- 
xanthellae  in,  848  ;  bibliography  of,  853 

—  colonies  of,  848 ;  distribution  of,  H53- 
854 ;  mounting,  854 

Radiolarian,  shells  in  '  ooze,'  1086 

Rainey,  on  presumed  cause  of  cattle 
plague,  752  ;  on  molecular  coalescence, 
1100 

Ralfs,  on  British  desmids,  579  note ; 
classification,  585;  on  Nitzschia  and 
Bacillaria,  606 

Ramsden  circles,  106 

Ramsden's '  screw  micrometer  eye-piece,' 
272;  positive  eye-piece,  42,  378,  380 

Raphidece,  599 

Raphides  of  Phanerogams,  696 ;  of  plants 
and  sponge-spicules  compared,  860 

Rays,  scales  of,  1028 


INDEX 


II7I 


REA 


EOS 


Eeagents,  mode  of  labelling  bottles,  402 
Real  image,  14  note',  formation  of,  23,  24 

—  object  image,  375 
Recomposition  of  light  by  prisms,  18 
Red  ant,  integument  of,  974 

—  blood-corpuscles  of   Vetebrata,  1084  ; 
size  of,  in  various  Vertebrata,  1035 ; 
relative  sizes  of,  in  various  Vertebrata, 
1036 

—  coral,  877 

—  corpuscles,  flow  of,  1056 

'  Red   snow,'  due  to  Palmella  cntentcf, 

558 

'  Red  spider,'  1013 
Red  spots  in  Infusoria,  775 
Reflector,  Sorby's  parabolic,  334 
Refracted  ray,  2 

Refracting  angle  of  a  prism,  9,  18 
Refraction,  57 

—  angle  of,  3 

—  of  light,  laws  of,  2,  3 

—  by  plane  surface,  3,  4 ;  by  curved  sur- 
face, 5  ;  by  prisms,  8,  9  ;  by  lenses,  10- 
25 

Refractive  index,  absolute,  2 ;  of  water, 
3 ;  relative,  4,  5  ;  of  crown  glass,  5  ; 
of  flint  glass,  5  ;  of  balsam,  77  ;  of  gum 
styrax,  521;  of  Canada  balsam,  521; 
of  monobromide  of  naphthalin,  521 ;  of 
phosphorus,  521 

—  of  silicious  coat  of  diatoms,  521 

—  indices  of  air,  of  cedar  oil,,  of  water,  60 
Reichert's  loups,  38 ;  his  lever  fine  ad- 
justment,  171;   his   microscopes,  206, 
210,  224,  241,  242,  264-266 ;  his  objec- 
tives, 374,   375 ;  his  thermo-regulator, 
453 

Reindeer,  hair  of,  1030 

Reproduction  in  Actinophrys,  740;  of 
Actinosphcerium,  741 ;  of  Clathrulina, 
742 ;  of  Euglypha,  746 ;  of  sponges, 
857  ;  of  Campanulariida,  870 ;  sexual, 
of  Polyzoa,  907  ;  agamic,  of  Entomo- 
straca,  963 ;  agamic,  of  Aphides,  &c., 
.1006 

Reproductive  organs  of  Acarina,  1011 

REPTILES,  lacunae  in  bone  of,  1022 ; 
cement  in  teeth  of,  1026  ;  plates  in  skin 
of,  1026 ;  epidermic  appendages  of, 
1029;  red  blood-corpuscles  of,  1034, 
.1035;  muscle-fibre  of,  1049;  lungs  of, 
1063 

Reseda,  seeds  of,  724 

Residuary  secondary  spectrum,  365 

Resins,  solvents  for,  517 

Resolving  power  of  objectives,  83,  425 

—  of  object-glasses,  44 ;  of  lenses,  64  ; 
of   objective   and  numerical  aperture, 
75,  393 

Respiration  of  insects,  apparatus  of,  994 
Respiratory  organ  of  spiders,  1014 
Bete  mucosum,  1042 
Retepora,  calcareous  polyzoaries  of,  909 
Reticidaria,  733  ;  characters  of,  733  ;  ex- 
amples of,  734-737 

Reticulated  ducts  of  Phanerogams,  698 
Retinulse,  983 


Revolving  nose-piece,  Nelson's,  295 
Rezzi,  on  invention  of  compound  micro- 
scope, 125 
Rhabdammina,  813 

—  abyssorum,  815 
RJiabdolithus pipa,  847 

—  sceptrum,  847 

Rhabdoliths,    in    chalk    and   limestone, 

1084 

Rhabdom,  983 
Rhabdopleura,  909 
Rhamnus,  stem  of,  703 
Rheophax  sabulosa,  815 

—  scorpiurus,  815 
Rhinoceros,  horn  of,  1033 
Rhizocarpece,  681 
Rhizoids  of  mosses,  669 
Rhizome  of  ferns,  675 
RHIZOPODA,  733-747 

—  protoplasm  of,  531 ;  ectosarc  of,  534 ; 
skeletons  of,   795;    sarcode  of,   1018; 
pseudopodial  network  of,  1053 

Rhizosolenia,  614 

—  cyclosis  in,  587 
Rhizostoma,  874,  876 
Rhizota,  790,  791 

Rhododendron,  pollen-grains  of,  722 
Rhodospermete,  574 
Rhodospermin,  631 
Rhodosporece,  625 
Rhopalocanium  ornatum,  85?. 
Rhubarb,  stellate  raphides  of,  696 ;  spiral 

ducts  of,  699 

Rhynchonellida,  shell  structure  of,  927 

Ribbons  of  sections,  464,  469 

Ribes,  pollen-tubes  of,  723 

Rice,  silicified  epiderm  of,  715 

'  Rice-paper,'  687 

Rice- starch,  695 

Ridd ell's  binocular  microscope,  97 

Ring-cells,  446-448 

Rivalto  (Giordano  da),  on  invention  of 
spectacles,  118 

RivulariacecB,  hormogones  of,  548 

Roach,  scales  of,  1028 

Rochea  falcata,  epiderm  of,  714 

Rock,  ground-mass  of,  1072 ;  fluxion- 
structure  of,  1073 

—  sections,  method  of  examining,  1081 
Rocks,   method  of   making  sections   of, 

1066-1068  ;  metamorphism  of,  1076 

Rodents,  hair  of,  1030 

Root  of  Phanerogams,  structure  of,  700 
et  seq. 

Root-cap,  710 

Rosalina  varians,  798 

Rose,  glandular  hairs  of,  714 

Ross  (Andrew),  on  correction  of  object- 
glass,  19-21 ;  his  early  form  of  achro- 
matic microscope,  152;  mechanical 
movements  of  his  stage,  153 ;  his  fine 
adjustment,  153,  173  ;  on  illumination 
of  objects,  300 ;  his  arrangement  for 
locking  coarse  adjustment,  352 ;  his 
achromatic  objectives,  356-358 ;  his 
lever  of  contact  for  testing  covers, 
440 

4  r2 


1 1?2 


INDEX 


EOS 


SCH 


Boss's  (Andrew) '  Lister  '  microscope,  153 
Ross,  model,  197 

—  and  Co.'s  microscopes,  196,  230-233  ; 
camera  lucida,  285,  286 

Ross-Jackson  model,  199 
Ross-Wenham's  radial  microscope,  199 
Ross-Zentmayer  model,  198 
Rot  alia,  824 ;  intermediate  skeleton  of, 
825 

—  aspera,  in  chalk,  1087 
— -  Beccarii,  shell  of,  797 

—  Schroeteriana,  825 
Rotalian  series,  823 
Rotaliince,  colour  of  shell,  799 
Rotaline  shells-of  Foraminifera,  797 

—  shell,  sandy  isomorph  of,  814 
Rotating  disc  of  objectives,  290 
Rotatoria,  753.    See  ROTIFEBA 
Rotifer  vulgaris,  787 

ROTIFEEA,  collecting,  527;  keeping  alive, 
528 ;  a  food  of  Actinophrys,  739 ;  de- 
scription of,  786-792  ;  habitats  of,  786  ; 
structure  of,  787-790 ;  mastax  of,  787 ; 
lorica  of,  787 ;  contractile  vesicle  of, 
789;  males  of,  790;  eggs  of,  790;  clas- 
sification of,  790,  791 ;  desiccation  of, 
791 ;  bibliography  of,  792 ;  preparation 
and  preservation  of,  793,  794 ;  wheel 
apparatus  of,  compared  with  velum  of 
gastropods,  936,  939;  winter  eggs  of, 
964 ;  non-sexual  reproduction  of,  1006 

Rotten-stone,  617 

'  Round  worm,'  944 

Rousselet's  binocular  portable  micro- 
scope, 245 ;  his  tank  microscope,  268  ; 
his  compressorium,  346 ;  his  live-box, 
346  ;  his  method  of  preparing  rotifers, 
793 

Royston-Piggott  constructs  first  aperture 
table,  30 

Rugosa,  877 

Rumia  cratcegata,  eggs  of,  1005 

Rush,  stellate  tissue  in,  687 

Rutile  in  clastic  rocks,  1075  ;  a  secondary 
mineral  in  slates,  1076  note 

Ryder's  microtome,  401 


Sabellaria,  tubes  of,  948 
Sable,  hair  of,  1030 
Saccammina  in  limestone,  1090 

—  Carteri,  812 

—  spherica,  812 
Saccharomyces  cerevisice,  645 
Saccharomycetes,   645;    zymotic  action 

of,  645 ;  endospores  of,  646 
Saccolabium  guttatum,  spiral  cells  of, 

693 

Sachs,  on  Chara,  579  note 
Sago,  starch-grains  of,  695 
Salivary  glands,  1047 
Salmon,  scales  of,  1028 

—  disease,  640 

Salpce,  diatoms  in  stomach  of,  614,  623 
Salpidce,  911 


Salpingceca,  calyx  of,  764 

Salt  solution  as  a  preservative  medium, 

519 
Salter  (J.),  on  the  'teeth'  of  Echinus, 

890 
Salvia  verbenaca,  spiral  fibres  in  seeds 

of,  693 

Sand-grains  surrounded  by  silica,  1075 
'  Sand-stars.'     See  Ophiuroidea 
'  Sand-wasp,'  974 
Sandy  isomorphs  (Foraminifera),  814 

—  tests  of  Lituolida,  814 
Santonine,  crystallisation  of,  1096 
Sap-wood,  704 

Saprolegnia,  alliance  with  Achlya,  564 
note 

—  ferax,  640 
Saprolegnice,  640 
Saprophytic,  Bacteria,  658 

—  fungi,  633,  642,  647 
Sarcocystids,  752 

Sarcode,  530  note,  531 ;   of  Bliizopoda, 

733 

Sarcolemma,  1049 
Sarcoptes  scabiei,  1018 
Sarcoptidce,  mandibles  of,  1009 ;  maxillse 

of,  1010 ;  hairs  of,  1010  ;  legs  of,  1010  ; 

characters  of,  1013 
Sarcoptince,  1013 
Sarcosporidia,  749 
Sargassum  bacciferum,  630 
Sarsia  (Medusa  of  Syncoryne),  869 
'  Saw-flies,'  ovipositor  of,  1003 
Saxifraga,  seeds  of,  724 

—  umbrosa,  parenchyme  of,  688 
Saxifrage,  cells  of  pollen-chambers,  720 
Scalariform  ducts  of  ferns,  674 ;  as  modi- 
fied spiral  ducts,  699 

'  Scales,'  covering  epiderm  of  leaves,  714  ; 
of  Elceagnus,  714 

—  of  Lepidoptera,  975,  976 ;    of  Coleo- 
ptera,  975  ;  of  Curculio  imperialis,  975 ; 
of  Lyccenidce,  975,  977 ;    of  Pieridce, 
975 ;     as    tests    for    objectives,    976 ; 
of    insects,    markings    of.     976 ;     of 
Thysanura,  977  ;  on  wing  of  Lepido- 
ptera, 999  ;  of  fishes,  1026  ;  of  reptiles, 
1026,  1029 

Scallops.     See  Pecten  ' 

Scarabcei,  antennae  of,  988 

'  Scarfskin,'  1041 

Scatophaga  stercoraria,  eggs  of,  1005 

Scenedesmus,  megazoospores  of,  566 

Schists,  1077 

Schizogenous   spaces   in    Phanerogams, 

688 

Schizomycetes,  651-664 
Schizonema,  602,  617 
r-  Gremllii,  618 

—  gelatinous  sheath  of,  588,  617 
Schizonemece,  character  of,  617 
Schnetzler,  on  movement  of  Oscillatoria, 

548 
Schott    (Dr.)   and  the  improvement   of 

object-glasses,  32 
Schroder  on  binocular  vision,  105 ;  his 

camera  lucida,  285 


INDEX 


H73 


SCH 


SIL 


Schultz's  method    of    macerating  vege- 
table tissues,  700 
Schultze  (Prof.    Max),    on    identity    of 

'  sarcode  '  and  '  protoplasm,'  530  note ; 

on  cyclosis  in  Diatomacece,   587 ;  on 

affinity  of  Carpenteria,  823 
Schulze   (Prof.  P.   E.),  on  soft  parts  of 

Euplectella,  860  note 
Schwendener,  on  lichens,  648 
Scirtopoda,  791,  792 
Scissors,  spring,  457 
Sclerenchyme  of  fernp,  674 
Sclerogen,  693 
Sclerotesin  Fungi,  633 ;  of  Myxomycetes, 

636 
Scolopendrium,  indusium  of,   675 ;  sori 

of,  675  ;  sporanges  of,  676 
Scorpions,  957,  1008 
Screw-collar  adjustment,  358 
Scrophularia,  seeds  of,  724 
'  Scyphistoma  '  of  Cyanea,  875 
Scytonema,  as  gonid  of  lichen,  651 
Scytone?nacece,  548 ;  hormogones  of,  548 
Scytosiphon,  conjugation  of,  627 
Sea-anemone.     See  Actinia 
Sea-anemones,  intracellular  digestion  in, 

863 

Sea-fans,  877.     See  Gorgonice 
'  Sea-jellies,'  853 
Sealing-wax  varnish,  444 
1  Sea-mats,'  908.    See  Flustra  and  Mem- 

branipora 

Searcher  eye-pieces,  378 
'  Sea-slugs.'     See  Doris,  Eolis 
1  Sea-urchin,'  884.     See  Echinus 
Sea-weeds,  625-632 

—  continuity  of  protoplasm  in,  538,  630 

—  red,  630 

Secondary   spectrum,   19,   31;  overcome 

by  Abbe's  objectives,  365 
Section  lifters,  477 ;  cover-glass  as,  478 

—  mounting,  477,  501,  506 

Sections,  ribbons  of,  464,  469 ;  of  hard 
substances,  506 ;  of  bones,  506,  510 ; 
of  coral,  506,  510 ;  of  enamel,  506 ;  of 
fossils,  506;  of  shells,  506;  of  teeth, 
506,  510 ;  of  hard  and  soft  substances 
together,  510 ;  of  Phanerogam  tissues, 
699 

Sedum,  pollen-grains  of,  721 ;  seeds  of, 
724 

Seeds,  685,  723 

Segmentation  of  Gastropoda  egg,  935  ; 
of  annelid  body,  948 

Seiler's  solution  for  cleaning  slides,  439 

Selaginella,  archegone  of,  homology  of, 
685 

Selaginellece,  682 

Selenite  plates,  318 

—  blue  and  red,  319 

—  stage,  319 

• —  with  mica  film,  319 

Selligue's  achromatic  microscope,  148, 

150 ;  objectives,  354 
Semi-apochromatic  objectives,  35 ;  of 

Leitz,  374 ;  of  Reichert,  374  ;  of  Swift, 

375 


Sempervivum,  seeds  of,  724 

Seneca,  on  magnifying  by  water,  118 

Sense,  organs  of,  in  Mollusca,  940 

Sensory  nerves,  1053 

—  organs  of  sponges,  856 

Sepals,  718 

Sepia,  pigment-cells,  942 

Sepiola,  eggs  of,  942 

'  Sepiostaire  '  of  cuttle-fish,  structure  of, 

924 ;  imitations  of,  1102 
Septa  in  shell  of  Foraminifera,  796,  803, 

804 
Serialaria,  presumed  nervous  system  in, 

907 

Serous  membrane,  1041,  1042 
Serpula,  tubes  of,  948 
Serricornia,  antennae  of,  987 
Sertularia  cupressina,  871 
Sertulariida,  gonozooids  of,  870;  zoo- 

phytic  stage  of,  877 
Sessile  cirripeds,  967 
Seta  of  Tomopteris,  953 
'  Sewage  fungus,'  653 
Sexual    fructification   of    Thallophytes. 

540 

—  generation  of  Volvox,  555 
Shadbolt,  on  structure  of  Arachnoidiscus, 

612 

Shadbolt' s  turn-table,  451 
Shadow  effects,  61 
Shark,  dentine  of,  1023 
Sharks,  scales  of,  1028 
Sheep-rot,  945 
Shell,  bivalve,  of  Ostracoda,  960 

—  calcareous,  of   Eeticularia,   733 ;    of 
Microgromia,  736 

—  silicious,  of  Dictyocysta,  Codonella, 
773 

—  of  Foraminifera,  796-801 ;  of  Lamel- 
libranchiata,  919 ;   of    Brachiopoda, 
919 

Shellac  cement,  protection  against  cedar 

oil,  444 

'  Shell-fish,'  919.  See  Mollusca 
Shells  of  Mollusca,  nacreous  layer  of, 
919,  922,  923,  924 ;  prismatic  layer  of, 
919,  920,  921 ;  colour  of,  921 ;  an  ex- 
cretory product,  922;  sub-nacreous 
layer  of,  923,  924 

—  of  Brachiopoda,  925  ;  periostracum 
of,  926  ;  perforations  of,  926 

—  of  Gastropoda,  structure  of,  928 

—  of  Cirripedia,  968 

'  Shield '  of  Ciliata,  773 

Shrimp,  concretionary  spheroids  in  skin 

of,  1100 

Shrimps,  skeleton  of,  969 
Side  reflector,  333 

—  lever,  short,  fine  adjustment,  174 
Swift's   vertical  fine   adjustment, 

173 
Siebold,  on  agamic  reproduction  in  bees, 

1006 

Sieve-plates,  710 

Sieve-tubes,  710 ;  in  Exogens,  697 
Sigillarice,  682,  1084 
Silene,  seeds  of,  724 


ii74 


INDEX 


SIL 

Silex  in  Equisetacece,  680 ;  in  epiderm  of 

grasses,  715 

Silk  glands  of  spiders,  1015 
*  Silk-weeds,'  569 
«  Silkworm,'  eggs  of,  1005   , 
Silkworm  diseases,  645,  661 
Silpha,  antennae  of,  988 
Simple  magnifier,  37 

—  microscope,  248 
Sines,  law  of,  3 

Siphonacece,    562-564 ;    Munier-Charles 

on  fossil  forms  of,  564 
Siphonostomata,  965  note 
Siricidce,  ovipositor  of,  1003 
Sirodot,  on  alternation  of  generations  in 

Batracliospermum,  575 
Skate,  muscle  fibre,  1049 
Skeleton,  dermal,  of  Vertebrata,  1026 ; 

fossilised,  1090 

—  fibrous,  of  sponges,  857 

—  silicious,  of  Heliozoa,  734  ;  of  Eadio- 
laria,  846 

—  of  sponges,  855;   of  zoophytes,  862; 
of    Echnoidea,    884;    of    Asteroidea, 
891 ;  of  Ophiuroidea,  891 ;  of  Crinoidea, 
892 ;  of  Holothurioidea,  894 ;  of  Ante- 
don,  901 ;  of  Vertebrata,  structure  of, 
1020 

Skin,  1041 ;  pigment-cells  in,  1042 ;  capil- 
laries in,  1062 
Skip-jack,  antennae  of,  987 
Slack,  on  the  costse  of  Pinnularia,  617 
Slack's  optical  illusion,  428 
Slide-forceps,  453 
Slides,  glass  for,  438 
Slides  for  cultures,  340,  841 

—  Seiler's  solution  for  cleansing,  439 
Sliding-plate  of  objectives,  290 
Sloths,  fossil,  teeth  of,  1024 

Slug.     See  Limax 

Slug's  eye,  941 

Slugs,  Motif  era  in,  787 

Smell,  organ  of,  in  insects,  1000 

Smith's  Cassegrainian  microscope,  145, 
146 

Smith  (H.  L.),  on  Tolles'  binocular  eye- 
piece, 101 ;  his  vertical  illuminator, 
886 ;  on  classification  of  diatoms,  599 

Smith  (James),  his  microscope,  155 ;  on 
use  of  bull's-eye  with  high  powers, 
881;  his  achromatic  lenses,  356 ;  his 
separating  lenses,  860 ;  his  mounting 
instrument,  454 

Smith  (T.  F.),  on  markings  of  diatoms, 
593 

Smith  (W.),  011  cyclosis  in  Diatomacece, 
587 ;  on  species  of  diatoms,  600  note ; 
on  habits  of  diatoms,  619 

Smith  (W.  H.),  on  structure  of  frustules, 
590  note ;  on  movements  of  diatoms, 
602 

Snail,  930 ;  eye  of,  941.     See  Helix 

—  muscle  of  odontophore,  1050 

Snake,  lung  of,  1063 

Snapdragon,  seed  of,  723 

Snell's  '  Law  of  Sines,'  49 

Snow,  crystals  of,  1095 


SPH 

Suowberry,  parenchyme  of  fruit  of,  688 

Snowdrop,  pollen-grains  of,  722 

Soda,  caustic,  action  on  horny  substances, 
517 

Soemmering's  simple  camera,  278 

Sole,  scales  of,  1026,  1027,  1028 

Solen,  prismatic  layer  in,  924 

Solid  cones  of  light  for  minute  observa- 
tion, 419 

—  eye-pieces,  378 

—  image,  95 

-  objects,  delineation  of,  88;  correct 
appreciation  of,  88 

—  vision  and  oblique  illumination,  61 
Sollas,  on  sponges,  855  note ;  on  the  ex- 
tensions of  the  perivisceral  cavity  in 
Polyzoa,  927 

Sorby  (H.  C.),  on  microscopic  structure 

of  crystals,  1066 
Sorby' s  parabolic  reflector,  334 
Sorby-Browning's      micro-spectroscope, 

323 

Soredes  of  lichens,  649 
Sori  of  ferns,  675 
Sound-producing  apparatus  of  crickets, 

999 

Spatangidium,  610 
Spatangus,  spines  of,  889 
'  Spawn  '  of  mushroom,  647 
Spectacles,  invention  of,  118 
Spectra,  diffraction,  67 

—  artificial,  324 
Spectral,  ocular,  Zeiss's,  827 
Spectro-micrometer,  bright-line,  325 
Spectroscope  in  micro-chemical   opera- 
tions, 1103 

Spectroscopic  test,  324 
Spectrum,  19 ;  irrationality  of,  19 

—  binocular,  microscope,  327 

—  map,  325 

—  natural,  824 

—  of  dark  lines,  323  ;  of  bright  lines,  323 
Speculum,  parabolic,  333;  Lieberkuhn's, 

334-336;  in  Smith's  illuminator,  336 
Spencer   Lens   Company's   Microscopes, 

214,  215  - 
Spermathecse   of    Gamasidce,   1012 ;    of 

TyroglypMdce,  1012 

Spermatia  oiPuccinia, 638 ;  of  lichens,650 
Sperm-cells  of    Thallophytes,   536;    of 

Volvox,  555  ;  of  ferns,  678  ;  of  sponges, 

857 ;  of  Hydra,  866 ;  of  Polyzoa,  907 
Spermogones    of     Puccinia,     638 ;     of 

lichens,  651 
Sphacelaria,  626 
Sphacele,  626 

Sphceria  in  caterpillars,  645 
Sphceroplea  annulina,  570-572 
Sphcerozosma,  rows  of  cells  in,  583 
Sphcerozoum  ovodimare,  853 
Sphagnacece,  673 
Sphagnum,  leaf  of,  673 
Sphenogyne  speciosa,  winged  seed  of,  724 
Spherical  aberration,  14,  15,  31,  299,  301, 

306,  387 
diminished  by  Huyghens'  objective, 

42 


INDEX 


1175 


SPH 


STA 


Spheroidal   concretions  of    carbonate  of 

lime,  1100 

Sphingidce,  antennae  of,  988 
Sphinx,  eye  of,  987 ;  antennsie  of,  988 

—  ligustri,  eggs  of,  1005 
Spicules  of  alcyonarians,  880 

—  of  sponges,  857  ;  their  names,  859-860 

—  silicious,  of  sponges,  857 

—  calcareous,  of  sponges,  857 
Spiders,    1008,    1014-1016 ;    microscopic 

objects  furnished  by,  1014 ;  spinning 
apparatus,  1015  f 

Spindle  fibres,  538 

Spinnerets  of  spiders,  1015 

Spiny  lobster,  metamorphosis,  969 

Spiracles  of  insects,  995,  996 

Spiral  cells  in  Phanerogams,  693  ;  mode 
of  preparation  of,  694 

—  crystallisation,  1096 

—  focussing  arrangement  for  projection- 
lens,  380 

—  vessels  of  Phanerogams,  698 ;  obser- 
vation of,  in  situ,  719  ;  of  plants  com- 
pared with  tracheae  of  insects,  995 

Spiriferidcs,    perforation    in    shells    of, 

927 

Spiriferina  rostrata,  shell  of,  927 
Spirillina,  819 

—  sandy  isomorph  of,  814 
Spirillum,  movement  of,  433  ;  granular 

spheres  of,  660  note 

—  undula,  659 

—  volutans,  movement  of,  652,  653,  659 
Spirit,  dilute,  as  a  preservative  medium, 

518 

Spirochcste,  653 
Spirogyra,  549,  550  ;  attacked  by  Vampy- 

rella,  730 
Spirolina,  a  varietal  form  of  Peneroplis, 

803 

Spiroloculina,  802 
Spirula,  929 

—  shells  of,  bearing  Protomyxa,  727 
Spirulina,  movement  of,  548 
Splachnum,  sporange  of,  669 

Splenic  fever  due  to  Bacillus  anthracis, 

656,  661 
Sponae-spicules,  857-860 

—  mounting,  481 

—  in  Carpentaria,  822 ;  in  mud  of  Le- 
vant, 1085 

Sponges,  855-862  ;  skeleton  of,  structure 
of,  855,  856;  reproduction  of,  857; 
habitat  of,  861 ;  preparation  of,  861 ; 
bibliography  of,  862  note 

—  fossil,  1089 
Spongilla,  861 
Spongolithis  acicularis,  620 
Spongy  parenchyma  of  leaves,  716 
Spontaneous  generation,  761 
Sporange  of  Fungi,  633;    of  Myxomy- 

cetes,  636  ;    of  Marchantia,  665,  668 ; 

of  mosses,  671 ;  of  Sphagnacece,  673  ; 

of  ferns,  675  ;  of  Equisetacece,  680 
Sporangia  of  Lycopodiacece  in  coal,  1084 
Sporangiophores  of  Mucorini,  640 
Spore,  use  of  the  term,  537  note 


Spores  of  Nostoc,  549  ;  of  Myxomycetes, 
634,  636;  of  Peronosporece,  639;  of 
Bacteria,  655,  657,  660;  of  Mar- 
chantia, 668  ;  of  mosses,  670  ;  of  ferns, 
676 ;  of  ferns,  method  for  studying 
development  of,  679  note ;  of  Equise- 
tacece, 680 ;  of  Lycopodiece,  682 ;  of 
gregarines,  751 ;  of  Monas  Dallingeri, 
757  ;  of  Lycopodiacece  in  coal,  1084 

—  different  kinds  of,  541  note 

—  resting,  of  Chcetophoracece,  574 
Sporids  of  Ustilaginece,  636 ;  of  Puccinia, 

638 

Sporocarp  of  Ascomycetes,  644 
Sporogone  of  mosses,  672 
Sporophores  of  Myxomycetes,   636 ;  of 

Peronosporece,   639;  of  Ascomycetes, 

643 

Sporophyte  in  ferns,  680 
Sporozoa,  749-752 
Sporules  of  Melosira,   597 ;  of  Pleuro- 

sigma,  597 ;  of  Podosphenia,  597 
Spot-lens,  316 
Spring-clip,  453 

—  press,  453 

—  scissors,  457 

'  Spring-tails,'  979.     See  Poduridcs 

Squid,  942 

Squirrel,  hair  of,  1030 

Stag-beetle,  antennas  of,  988 

Stage,  horse-shoe,  Nelson's,  179,  228 ;  of 
the  microscope,  175-184;  qualities 
needful  in  a,  177  ;  concentric,  rotatory 
motion  of,  179  ;  in  the  '  Continental ' 
model,  259 ;  graduated  rotary  for  use 
with  apertometer,  395 

—  attachable,  simple  form,  180  ;  Swift's, 
180  ;  Allen's  (Baker's),  181 ;  Keichert's, 
183;    Bausch  and   Lomb's,  183,  184; 
Mayall's,  183 ;  Zeiss's,  183  ;  Beck's,  184 

forceps,  338 

—  -micrometer,  270,  274,  288,  290 

—  moist,  341 

plate,  glass,  340 

—  thermostatic,  344-346 

—  Turrell's,  176  ;  Watson's,  177  ;  Zeiss's, 
179 ;  Tolles',  204 

vice,  339 

'  Staggers '  of  sheep,  due  to   Ccenurus, 

944 

Stahl,  on  movement  of  desmids,  581 
Staining,  488 

—  regressive,  491 

—  Bacteria,  514-516 

—  flagella,  516 

Stains,  intra-vitam,  488,  489 

—  for  unfixed  tissues,  489 

—  for  fixed  tissues,  490,  491 

—  nuclear,  491-494 

—  plasma,  494,  495 

Stains,  solutions  of,  methylen  blue,  488  ; 
Bismarck  brown,  489  ;  Congo  red,  489  ; 
methyl-green,  489 ;  neutral-red,  489 ; 
alcoholic  borax-carmine,  490 ;  alum- 
cochineal,  490  ;  carmalum,  490  ;  heema- 
lum,  490 ;  alcoholic  cochineal,  491 ; 
iron-huematoxylin,  492  ; '  Kernschwarz,' 


1176 


INDEX 


STA 


SUP 


492  ;  safranin,  493  ;  acid-fuchsin,  494  ; 

basic-fuchsin,   494  ;  Lyons   blue,   494 ; 

picric     acid,     494 ;     water-blue,    494 ; 

thionin,  494 
Stanhope  lens,  37 
Stanhoscope,  38 
Staphylimis,  antennae  of,  988 
Star-anise,  tissue  of  testa  of,  692 ;  testa 

of  seeds  of,  725  niNI 

Starch,  tests  for,  517 ;  formation  of,  694^ 

—  grains,    534,   535 ;   mode    of   growth, 
694;  hilum  of,  695;  in  Canna,  695;  in 
potato,   695 ;  in   wheat,    695 ;   in   rice, 
695 

'  Star-fish,'  891.     See  Asteroidea 
Statospore  of  Prptomyxa,  728 
Staurastrum,   binary   division   of,   582; 
form  of  cell,  585 

—  dejectum,  568 
Stauroneis,  617 

'  Stauros '  of  Achnanthes,  616 

Steenstrup  on  alternation  of  generations, 
877 

Stein,  on  affinities  of  Volvox,  551  note ', 
on  contractile  vacuoles  of  Volvox,  552 
note ;  on  Flagellata,  764 ;  on  Nocti- 
luca,  769  note ;  on  Acinetina,  785  note 

Steinheil's  loups,  38 ;  his  combination  of 
lenses,  38  ;  his  aplanatic  loup,  249 ;  his 
loup  for  tank  work,  268 ;  his  formula 
for  combination  of  lenses,  316 ;  his 
triple  loups,  378 

Stellaria,  seeds  of,  724 

—  media,  petals  of,  719 

Stem  of  mosses,  669 ;  of  Sryacece,  673 ; 
of  Sphagnacece,  673 ;  structure  of,  in 
Phanerogams,  700;  of  Phanerogams, 
development  of,  709 ;  treatment  of,  for 
examination  of  their  structure,  711 

Stemmata  of  insects,  986 ;  of  spiders, 
1014 

Stentor,  collecting,  527 ;  contractile 
vesicle  of,  774 ;  impressionable  organs 
of,  775  ;  conjugation  of,  7S2 

Stephanoceros,  collecting,  527;  in  con- 
finement, 528 

Stephanolithis  spinescens,  847 

—  nodosa,  847 

Stephanosphcera     pluvialis,     amoeboid 

phase  of,  557  note 
Stephenson,  on  Pleurosigma  angulatum, 

70  ;  on  '  intercostal  points,'  73 

—  his  suggestion   on  homogeneous  im- 
mersion, 28 

—  on  Coscinodiscus,  609 
Stephenson' s  stereoscopic  binocular,  100; 

its  erecting  arrangement,  101,  102 ;  as 

a  dissecting  microscope,  248,  456  ;  his 

tank  microscope,  267 
Stereocaulon  ramulosus,  650 
Stereoscope,  91 ;  Brewster's  modification 

of,  91 
Stereoscopic   binocular,  Wenham's,   98 ; 

for  study  of  opaque  objects,  103-105 

—  eye-piece,  Tolles's,  101 ;  Abbe's,  102 

—  vision,  90-97 
Sterigmata  of  Puccinia,  637 


Sterile  cells  of  Volvox,  555 
Stichopus  Kefersteinii,  895 
Stick-net  for  marine  work,  529 
Stickleback,  parasite  of,  966  ;  circulation 

in  tail  of,  1057 
Stigmata  of  insects,  995,  996 
'  Stinging  hairs '  of  nettle,  714 
Stings  of  insects,  1002,  1003 
Stipe  of  diatoms,  588 ;   of  Lidnophontf 

604 ;  of  Gomphonema,  616 
Stolon  of  Foraminifera,  796 ;  of  Eozo&n, 

839  ;  of  Laguncula,  904 ;  of  ascidians, 

914 

Stomach,  follicles  of,  1047 
Stomates,  715 

—  of  Marchantia,  666 
Stomopneustes  variolaris,  spines  of,  888 
Stone-cavities  in  crystals,  1073 
Stone-mite,  eggs  of,  1009 

Stones  of  fruit,  preparing  sections  of, 
699 

—  of  stone  fruit,  constitution  of,  693 
Stone-wort,  576 

Stony  corals,  resembled  by  polyzoaries, 
904 

Stop,  introduction  of,  37;  in  the  eye- 
piece, 42  ;  use  of,  812,  316 

'  Straight  extinction,'  1079 

Strawberry,  parenchyme  of  fruit,  688 

Streptocaulus pulcherrimus,  871 

Striated  muscle,  1048 ;  size  of  fibres  in 
different  groups,  1049 

Striatella  unipunctata,  598 

Striatellece,  characters  of,  607 

'  Strobila'  of  Cyanea,  875 

Stromatopora,  doubtful  character  of, 
842 

Stromatoporoids,  817  note 

Strophomenidce ,  perforations  in  shells  of, 
927 

Stylodyctya  gracilis,  851 

Suberous  layer  of  bark,  708 

Sub-nacreous  layer  in  molluscan  shells, 
923,  924 

Sub-stage,  184-191,  262;  Nelson's  fine 
adjustment  to,  185;  Powell  and  Lea- 
land's,  186;  Karop's  fine  adjustment 
to,  187;  Watson's,  187;  Baker's,  188; 
centring  nose-piece  used  as,  230 

'  Sub-stage  condenser,'  Nelson  on,  72 
note]  compound,  134 

—  illumination,  298-316 

—  simplest  form  of,  313 
Succulent  plants,  stomates  in,  716 
Sucker  on  legs  of  Sarcoptidce,  1010 
Suckers  on  foot  of  Dytiscus,  1001 ;  of 

Curculionidte,  1002 
Suctoria  (Protozoa),  783-785 

—  (Crustacea),  965,  966 

'  Sugar-louse,'  977.     See  Lepisma 
Sulphuric  acid,  as  a  test,  517 
'  Sun-animalcule,'  737 
'  Sundew,'  glands  of,  714 
Sunk-cells,  449 
Super-amplification,  33 
Super- stage,  see  attachable  mechanical 
stage,  180 


INDEX 


1177 


SUP 


THA 


Supplemental  yolk  in  Purpura,  938,  939, 

1007 
Surirella,  588,  606 ;  conjugation  of,  599 ; 

zygospores  of,  599 ;  movements  of,  602  ; 

frustule  of,  606 

—  biseriata,  cyclosis  in,  587 

—  caledonica,  621 

—  constricta,  606 

—  craticula,  621 

—  plicata,  621 
Surirettece,  606 

Suspensor  of  ovule  of  Phanerogams,  534 
Sutural  line  of  desmids,  580  f 

Swarm-spores,   536 ;    meaning   of    term, 
537  note ;  of  Pandorina,  557  ;  of  Cut- 
leria,  627;  of  Clathrulina,  742;  pre- 
sumed, of  Pelomyxa,  745 
Sweat-glands,  1042 
'  Sweetbread,'  1047 

Swift's   side-lever,    162;     vertical    side- 
lever  fine  adjustment,  173, 174;  attach- 
able stage,  180 ;  microscopes,  203,  224, 
228,   233;    portable   microscope,   245; 
condenser,    302,    305 ;     condenser    for 
polariscope,    314 ;     microspectroscope, 
325  note ;  objectives,  375 ;  petrological 
microscope,  1068 
Symbiosis  in  lichens,  650 
Symbiotes  tripilis,  hairs  of,  1010 
Symbiotic  algae  in  radiolarians,  848 
Sympathetic  nerves,  1054 
Symphytum  asperrimum,  seeds  of,  724 
Synalissa  symphorea,  650 
Synapta  digitata,  '  anchors  '  of,  895 

—  inhcerens,  '  anchors  '  of,  895 
Synaptce,  rotifers  in,  787 
Syncoryne  Sarsii,  gonozooids  of,  868 
Syncrypta,  545 

Synedra,  606 

Syringammina,  811 

Syringe    for    catching    minute    aquatic 

objects,  351 
Syrup,  as  a  preservative  medium,  519 

—  and  gum,  as  a  preservative  medium, 
519 


Tabanus.  eyes    of.    987;    ovipositor  of, 

1004 

Tabellaria  vulgaris,  621 
Table  of  numerical  apertures,  84-87 

—  for  microscopists,  398-402 ;    for    dis- 
secting and  mounting,  399 

Tactile  papillse  of  skin,  1042;  nerve  to, 
1053 

Tadpole,  pigment-cells  of,  1043;  circu- 
lation in  tail  of,  1056 ;  general  circula- 
tion in,  1057;  blood-vessels  of,  1059, 
1060 

—  of  ascidians,  917 

Tadpole's  tail,  epithelium  of,  1044 

Tasnia,  943 

Tank  microscopes,  266-269 

Tannin,  test  for,  517 

Tapetal  cells  in  fern  antherid,  678 

'  Tape-worm,'  943 


Tardigrada,  desiccation  of,  945 
Tarsonemidce,  1013 
Taste,  organs  of,  in  insects,  993,  1000 
Teeth,  decalcification  of,  512 

—  fossilised,  1090 

—  in  palate  of  Helix,  930 ;    of  Limax, 
930 ;  of  Buccinum.  930  ;  of  Mollusca, 
930 

—  preparation  of,  1023  and  note 

—  of  Echinus,  890  ;  of  Ophiothrix,  892 ; 
of  Vertebrata,  1023 

—  of  elephant,  Eolleston  on  enamel  in, 
928 ;  of  Bhodentia,  Tomes  on  enamel 
in,  928 

Tegeocranus  cepheiformis,  1008 

—  dentatus,  1008 

Tegumentary  appendages  of  insects,  974 
Telescope,  Barker's  Gregorian,  145 
Teleutospore    generation    of    Puccinia, 

637 
Temperature,    effect    of,      on     various 

monads,  761 
Tendon,  1019 

Tentacle  of  Noctiluca,  766,  768 
'  Tentacles '  of  Drosera,  714 ;  of  Suctoria, 

785  ;  of  Hydra,  864  ;  of  annelids,  949 
Tenthredinid(B,  ovipositor  of,  1003 
Terebella,  tubes  of,  948 ;  gills  of,  949 

—  conchilega,  948 
Terebratula  bullata,  shell  of,  927 
Terebratulcz,  shells  of,  925,  926 
Terpsinoe  musica,  608 
Terpsinoece,  character  of,  607 
Tertiary  tints  in  crystalline  bodies,  1097 
Tessellated  epithelium,  1044 

Test  of  Gromia,  735;    of  Arcella,  746; 

of  Difflugia,  746 
Testa  of  seeds,  725 
Testaceous  amcebans,  746,  747 
Testing  object-glasses,  381 ;    diaphragm 

for  use  in,  385  ;  Fripp's  method,  386 ; 

Abbe's  method,  384-387 
Test-plate,  Abbe's,  387 
Tests,  sandy,  of  Lituolida,  814 
Tethya,  spicules  of,  1086 
Tetramitus    rostratus,    life-history    of 

760  ;  nucleus  of,  763 
Tetranychi,  1013 
Tetranychus,  mandibles  of,  1009 
Tetraspores  of  Floridece,  631 ;  of  Vam- 

pyrella,  730 
Textularia,  823 

—  aculeata,  in  chalk,  1087 

—  ylobulosa,  in  chalk,  1087 
Textularian  form  of  shell,  798 

—  series,  823 
Textulariidce,  811 

Textularinice,  arenaceous   character  of, 

828 

Thalassicolla,  846,  853 
Thallophytes,  530-632 
Thallophytic    type,    passage    to   cormo- 

phytic,  668 
Thallus  of   Ulva,  560;   of  Phceosporea, 

626 ;  of  lichens,  649 
Thaumantias  Eschscholtzii,  873 
-  pilosella,  873 


INDEX 


THE 

<  Theca  '  of  mosses,  671 
Thecaphora,  868 
Thecata,  868,  870 

—  zob'phytic  stage  of,  877 
Thermo-regulator,  Reichert's,  458 
Thermostatic    stage,    Dallinger's,    344- 

346 

Thoma's  (Jung)  microtome,  461-469 

Thompson  (J.  Vaughan),  on  pentacrinoid 
larva  of  Antedon,  901 ;  on  Cirripedia, 
967 

Thomson  (Wyville),  on  development  of 
Antedon,  903 

Thread-cells  of  Ciliata,  773 ;  of  Hydra, 
864  ;  of  Zoantharia,  878,  879  ;  of  pla- 
narians,  947 

'  Thread-worm,'  944 

Threads  of  spiders'  webs,  1015 

Thurammina  papillata,  815 

Thwaites,  on  conjugation  of  Epithemia, 
599  ;  of  Melosira,  560 

Thysanura,  scales  of,  977 

Ticks,  1008.     See  Acarina 

Tineidce,  wings  of,  999 

Tinoporus  baculatus,  824 

Tipula,  spiracle  of,  976 ;  eye  of,  987  ; 
antennae  of,  988 

Tolles'  binocular  eye-piece,  101 ;  his  me- 
chanical stage,  204;  his  immersion 
objectives,  362,  364  ;  his  apertometer, 
390 

Tomes  (Charles),  on  teeth,  1025 

Tomopteris  onisciformis,  952,  953 ;  de- 
velopment of,  954 

—  quadricornis,  954 

'  Tongue'  of  Gastropoda.     See  Palate 

'  Tortoiseshell  butterfly,'  eggs  of,  1005 

Torula  cerevisics,  645 

Total  reflexion,  6,  7 

Tourmaline,  pleochroism  in,  1078 

Tow-net,  528 

Tow-nets  of  Challenger  Expedition,  529 

note 
Tracheae   of  insects,    994;    of  Acarina, 

1011 
Trachei'des  of  ferns,  674  ;  of  conifers,  698, 

703 

Trachelomonas,  545 
Tradescantia    virginica,     cyclosis     in 

hairs  of,  691 
Tragulusjavanicus,  red  blood-corpuscle 

of,  1035 

Trematodes,  945 
Triceratium,  588,  613 

—  as  test  for  illumination,  415,  416 

—  favus,  593,  613 

—  fimbriatum,     as     test     for    medium 
powers,  389 

Trichocysts  of  Ciliata,  773 

Trichoda  lynceus,  crawling  of,  774 ;  re- 
production of,  780,  781 

Trichodina  grandinella,  a  phase  in  de- 
velopment of  Vorticella,  780 

Trichogyne  of  Coleochcete,  575 

—  of  Floridece,  632  ;  in  lichens,  650 
Trichonymplia,  774 
Trichophore  of  Floridea;,  632 


TING 

Trichophrya,  a  phase  in  development  of 

Suctoria,  785 

Trigonia,  prismatic  layer  in,  924 
Triloculina,  802 
Triple-backed  objectives,  361 
Triplet,  Holland's,  37 
Triplex  front  to  objectives,  370 
Tripoli  stone,  617 
Trochus  zizyphinus,  palate  of,  931 
Trombidiidce,  1008,  1009 ;  legs  of,  1010  ; 

hairs  of,  1010 ;  eyes  of,  1011 ;  tracheae  of, 

1011 ;  characters  of,  1012 
Trombidium,  maxillae  of,  1010  ;  larvae  of, 

1013 

—  holosericum,  1013 
Trophi  of  Botifera,  788 
Truncatulina  rosea,  colour  of,  799 
'  Tube-cells,'  cements  for,  442 
Tube-length,  English   and  Continental, 

158, 159 
Tuberculosis,  bacillus  of,  661 ;  methods  of 

staining,  515,  516 

Tubifex  rivulorum,  gregarine  of,  751 
Tubipora,  877 
Tubularia,  gonozooids  of,  869 

—  indivisa,  869 

Tubuli  in  Nummulites,  827  ;  of  dentine, 
1024 

Tubulipora,  909 

Tulip,  raphides  of,  696 

Tully's  (Lister's)  achromatic  microscope, 
149  ;  his  live-box,  345  ;  his  triplet,  354  ; 
his  achromatic  objective,  354 

'  Tunic  '  of  Tunicata,  911 

TUNIC  ATA,  904,  911-918  ;  zoological  posi- 
tion of,  911;  bibliography  of,  918; 
'liver 'of,  1047 

Turbellaria,  946, 947 

—  larvae  of,  collecting,  529 
Turbinoid  shell  of  Foraminifera,  797 
Turbo,  shell  structure  of,  928 
Turkey-stone,  use  of,  508  ;  constituents  of, 

617 
Turn-table,   Shadbolt's,  451;   Griffith's, 

451 

Turpentine,  uses  of,  444,  518 
Turrell's  mechanical  stage,  176 
Twin  lamellae  in  leucite,  1078 
Tylenchus  tritici,  945 
Tympanum  of  cricket,  999 
Tyroglyphi,  nymph  of,   1009;   legs  of, 

1010 
Tyroglyphidce,  reproductive  organs  of, 

1012  ;  characters  of,  1013 


U 


Ulothrix,  conjugation  of,  557 
Ulva,  560,  561 
Ulvacece,  559-561 
Umbelliferous  plants,  seeds  of,  724 
Umbonula  verrucosa,  906 
Under-corrected  objective,  20,  21 
Under-correction,  355-360 
Unger,  on  the  zob'spores  of  Vaucheria, 
563 


INDEX 


1179 


UNI 


WAR 


Unicellular  plants,  538 

Unio,  pearls  in,  923  ;  glochidia  of,  933 

—  occidens,  formation  of  shell  in,  925 

Unionidce,  nacreous  layer  of,  923 

Unit  (standard)  for  microscopy,  460 

Uredinece,  636-638 ;  alternation  of  gene- 
rations in,  636 

Uredo-form  of  Puccinia,  638 

Uredospores  of  Puccinia,  638 

Urinary  calculi  and  molecular  coales- 
cence, 1102 

Urine,  micro-chemical  examination,  110$ 

Urochordata,  911 

Uropoda,  tracheae  of,  1011 

'  Urticating  organs.'     See  Thread-cells 

Ustilaginece,  636 

Uvella,  545 


Vacuoles  in  vegetable  cell,  534 
—  contractile,   in   protophytes,   535;    of 
Volvox,  552 

—  of  Actinophrys,  737 
Vagine  of  mosses,  671 

Vallisneria,  habitat,  689;    mode  of  de- 
monstration of  cyclosis,  689,  690 
Valvulina,  shell  of,  798 
Vampyrella,  729,  730 

—  gomphonematis,  729 

—  spirogyrce,  729 

Vanessa,' eye  of,  987 ;  haustellium  of,  992 

—  urticce,  eggs  of,  1005 
Variation,  range  of,  in  Astromma,  849 
Varley's  live-box,  346 

Varnish,  test  for,  443 ;  asphalte,  443 

Varnishes,  442-445 ;  sealing-wax  in  alco- 
hol, 444  ;  red,  445  ;  white,  445  ;  various 
colours,  445 

'  Vascular  Cryptogams,'  links  with  Pha- 
nerogams, 682 

Vascular  papillae  of  skin,  1042 

Vaucheria,  562,  563 

—  Botifera  in,  787 

'  Vegetable  ivory,'  endosperm  of,  693 

Vegetable  substance,  preparation  of,  514  ; 
gum-imbedding  for,  514;  bleaching  of, 
514 

Veins  of  vertebrates,  1056 

Velum,  in  gastropod  larva,  936 

Venice  turpentine  cement,  for  glycerin 
mounts,  444 

Vent  r  indites,  861,  1088 

Venus'  flower  basket,  859,  860 ;  spicules 
of,  860 

Verbena,  seeds  of,  724 

Vertebrata,  1017-1065;  bone  of,  1020; 
teeth  of,  1023;  dermal  skeleton  of, 
1026;  blood  of,  1034;  red  blood-cor- 
puscles, 1034;  white  blood-corpuscles, 
1036 ;  fibrous  tissues,  1038 ;  skin,  mu- 
cous and  serous  membrane,  1041 ;  dis- 
tribution of  ciliated  epithelium,  1044 ; 
fat,  1045;  cartilage,  1046;  glands  of, 
1047;  muscle,  1048;  nervous  tissue, 
1051 ;  circulation,  1054 ;  respiration, 
1063 


Vertebrated  animals,  1017.  See  Verte- 
brata 

Vertical  illuminator,  336-338;  how  to 
use,  337 ;  for  examination  of  metals, 
337  ;  for  ascertaining  '  aperture,'  338 

Vespidce,  eye  of,  987 

Vibracula  of  Polyzoa,  910,  911 

Vibrio,  movement  of,  433 

—  rugula,  659 

'  Vibriones,'  as  applied  to  certain  nema- 

todes,  945 

Vibriones,  form  of,  653,  659 
Vigelius,  on  tentacular  cavity  of  Polyzoa, 

905  note 

Vine,  size  of  ducts  of,  699 
Viola  tricolor,  pollen-tubes  of,  723 
Violet,  cells  of  pollen-chamber,  720 
Virginian  spider-wort,  cyclosis  in,  691 
Virtual  image,  14  note,  24,  25,  376 
Vision,  depth  of,  88,  89,  90 ;  stereoscopic, 

89 

Visual  angle,  27 
Vitrea  (Foraminifera),  819 
Vitreous  cells  (arthropod  eye),  983 

—  optical  compounds,  31 

—  shells  of  Foraminifera,  799 

'  Vittae  '  of  Licmophorece,  604  ;  of  seeds 

of  umbellifers,  724 
Vocal  cords,  structure  of,  1040 
Vogan's  changing  nose-piece,  294 
Volcanic  ashes  and  dust,  microscopical 

examination  of,  1076 
Volvocinece,  550-557 
Volvox  associated  with  Astasia,  765 

—  vegetable  nature  of,  556  note',  amce- 
biform  phase  of,  556  ;  Botifera  in,  787 

—  aureus,  cellulose  in,  552 ;   starch  in, 
552 

—  globator,  550-557  ;  flagellate  affinities 
of,  551  note ;  contractile  vacuoles  in, 
552;  endochrome  of,  552 ;  development 
and  reproductive  cells  of,  554-556 

!  Vorticella,  foot-stalk  of,  773;  contrac- 
tion of  foot-stalk,  774,  775 ;  fission  of, 
777  ;  encystment  of,  778 ;  classification 
of,  782  ;  gemmiparous  reproduction  of, 
782 ;  conjugation  of,  782 

—  microstoma,  779 


W 


Waldheimia  australis,  shell  of,  926 
Wale's  model,  224;   his  limb,  224;   his 
coarse   adjustment,  226;    his  fine  ad- 
justment, 226 

I    Wallflower,  pollen-grains  of,  722 
|    Wall-lichens,  649 

I    Wallich,  on  structure  of  diatom  frustule, 

590  note',  on  Triceratium,  613  note; 

on  Chcetocerece,  614  note;   on  cocco- 

spheres,  747 ;  on  Polycystina,  852  note 

—  his  plan  for  sectioning  a   number  of 

hard  objects,  508  note 
j    '  Wanghie  cane,'  stem  of,  701 

'  Warm-stage '   for   observing  blood-cor- 
puscles, 1034 


ii8o 


INDEX 


WAK 

Warmth,  mode  of  applying,  for  cyclosis, 

692 

Wasps,  wings  of,  998,  999  ;  sting  of,  1003 
Water,  refractive  index  of,  8,  7 

—  distilled,  for  mounting  Protophytes, 
518 

—  milfoil,  collecting,  527 
Water-angle,  50 
Water-bath,  452 
Water-boatman,  wings  of,  1000 
'  Water-fleas,'  959,  962 
Water-globules  in  oil,  429,  430 
Water-immersion  objectives,  362 ;  Zeiss's, 

370 

Water-lily,  leaf-structure  of,  717  ;  cells 
of  pollen-chambers,  720 

'  Water-mites,'  1013 

'  Water-net,'  or  Hydrodictyon,  565 

Water-of-Ayr  stone,  508 

Water-scorpion,  995.     See  Nepa 

'Water-snail.'     See  Limnceus 

Water- vascular  system  of  Tcenia,  943 

Watson's  microscopes,  199-202,  218,  224, 
234,  237  ;  coarse  adjustment,  161,  202 ; 
fine  adjustment,  162,  172,  174,  175; 
mechanical  stage,  177  ;  sub-stage,  187 ; 
nose-piece,  292  ;  condensers,  303,  304  ; 
objectives,  375  ;  eye-pieces,  379 

Wavellite  in  Mya,  924 

Web  of  spiders,  1015 

Weber's  annular  cells,  350 

Webster  condenser,  308 

Weismann,  on  development  of  Diptera. 
1007 

Wenham,  on  binocular  vision,  105;  on 
cyclosis  of  Vallisneria,  690 

Wenham's  suggestion  of  homogeneous 
immersion,  29 ;  his  stereoscopic  bino- 
cular, 98,  99 ;  his  prism,  98 ;  his  para- 
boloid, 316-317  ;  his  achromatic  objec- 
tive with  single  front,  361 ;  his  duplex 
front  objective,  362 

West,  on  Chcetocerece,  614  note 

'Whalebone,'  1033 

Wheat,  starch-grains  of,  695 

Wheatstone's  stereoscope,  91 ;  his  pseudo- 
scope,  92 

'Wheel-animalcules,'     753,     786.        See 

KOTIFERA 

Wheel-like  plates  of  Chirodota,  896 
'  Wheels '  of  Rotifera,  787 
Whelk.     See  Buccinum 
'  White  ant,'  ciliate  parasite  of,  774 
White  blood-corpuscles  of    Vertebrata, 
1036 ;  flow  of,  1056 

—  fibrous  tissue,  1038-1041 

—  of  egg,  as  a  preservative  medium,  519 
Whitney's  directions  for  examination  of 

frog's  circulation,  1060 
Wild  clary,  spiral  fibres  of,  693 
Williamson  (W.  C.),  on  Volvox,  556  note ; 

on   structure  of  fish-scales,  1027;   on 

structure  of  coal-plants,  1084 
Willow-herb,    emission    of  pollen-tubes, 

722 

Wing  of  Agrion,  circulation  in,  994 
Winged  seeds,  724 


ZOO 

Wings  of  insects,  998-1000;  of  Ptero- 
phorus,  999;  venation  of,  in  Neuro- 
ptera,  998 

Wodderborn,  on  Galileo's  invention  of 
compound  microscope,  121,  125 

Wodderborn's  '  perspicillum,'  125 

Wollaston's  doublets,  36, 153  ;  his  camera 
lucida,  278 

Wood,  arrangement  of,  700,  702  ;  concen- 
tric rings  of,  703 ;  fossilised,  705,  1083 

Wooden  slides  for  opaque  objects,  450 

Woody  fibre,  696 

—  tissue  of  ferns,  674 

Working  eye-pieces,  378 

Worms,  943-956 


X 


Xylem  of  Exogens,  697,  698,  710 
Xylol-balsam  as  a  preservative  medium, 
518,  521 


Yeast,  646 ;  fermentation  due  to,  646 
Yellow  cells,  in  Actinice,  848 ;  in  radio- 

larians,  848 

—  fibrous  tissue,  1039,  1040 
Yolk-bag  of  young    fish,    circulation  on, 

1057 
Yucca,  epiderm  of,  712 ;  guard-cells  of 

stomates  in,  715,  716 


Zanardinia,  swarm-spores  of,  627 

Zea  Mais,  epiderm  of,  712 ;  stomates  of, 
715 

Zeiss's  oil-immersion  objectives,  29  ;  his 
eye-pieces  and  objectives,  34 ;  his 
photographic  microscope,  178, 257,  258  ; 
his  mechanical  stage,  179,  183 ;  his 
latest  microscope,  206,  237;  his  dis- 
secting microscope,  248,  253 ;  his  apla- 
natic  loup,  249,  268  ;  his  calotte  nose- 
piece,  292 ;  his  sliding  objective 
changer,  293  ;  his  iris-diaphragm,  297  ; 
his  spectral  ocular,  327 ;  his  apochro- 
matic  objective,  366-374 ;  his  water- 
immersion,  370  ;  his  apochromatic,  for 
resolving  diatom  markings,  592 ;  his 
apochromatic  for  study  of  monads,  762 

Zeiss-Steinheil's  loups,  249,  268 

Zentmayer's  microscope,  204 ;  swinging 
sub-stage  in,  204 

Zeolite,  1095 

Zinc,  chlor-iodide  of,  as  a  test,  516 

—  cement,  Cole's,  445 ;  Zeigler's,  445 

Zoantharia,  877 

Zoea,  970 

Zonal  structure  in  crystals,  1073 

Zoochlorellse  of  Heliozoa,  734 

Zobcytium  of  Ophrydium,  chemical  com- 
position of,  778 


INDEX 


zoo 

Zoogloea  of  Beggiatoa,  653 
Zobglcese,  655,  657 
ZOOPHYTES,  862-883 

—  cells  for  mounting,  448,  449 

—  non-sexual  reproduction  of,  1006 
Zoophyte  troughs,  348-350 
Zob'sporange  of  Volvox,  554,  555 
Zobsporanges  of  Phceosporece,  626 
Zoospores,  536 ;  of  Protococcus,  544,  545  ; 

of  Palmodictyon,  559  ;  of  Ulva,  560  ; 
of  Vaucheria,  562 ;  of  Achlya,  565 ; 
development  of,  565  ;  of  Hydrodictyow, 
566;  of  ConfervacecB,  570;  of  (Edo- 
gonium,  572 ;  of  Chtftophoracea,  574  ; 
of  Coleochcetacece,  575 ;  of  Phceosporece, 
626 ;  of  Fungi,  633 ;  of  radiolarians, 
849 
Zoothamium,  collecting,  527 


ZY3I 

Zobxanthellae  in  radiolarians,  848 
Zobzygospores  of  Navicula,  597 
Zukal,  on  movement  of  SpiruUna,  548 
Zygnemacece,  characters  of ,  549;  habitats 

of,  549  ;  conjugation  of,  549 
Zygosis  in  Actinophrys,  740 ;  of  Amoeba, 

744  ;  of  gregarines,  751 
Zygospore,   537;    formation   of,   540;    of 

Hydrodictyon,  565 ;  in  Desmidiacece-, 

584,  585 
Zygospores  of  Palinoglcea,  542 ;  of  Meso- 

carpus,   550 ;    of   Spirogyra,  550 ;    of 

Pandorina,    557 ;   of    Ulva,    561 ;   of 

Navicula,   597 ;    of   diatoms,   599 ;    of 

Mucorini,  641 
Zygote  of  Glenodinium,  770 
Zymotic  or  fermentative  action  of  Funai, 

633 


DATE  DUE  SLIP 

UNIVERSITY  OF  CALIFORNIA  MEDICAL  SCHOOL  LIBRARY 

THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


3m-10,'34 


QH205  Carpbnter,  W.B. 
C29      Thb  microscope 


